Rice Quality: A guide to rice properties and analysis (Woodhead Publishing Series in Food Science, Technology and Nutrition)

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Rice quality

© Woodhead Publishing Limited, 2011

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Related titles: Cereal grains: assessing and managing quality (ISBN 978-1-84569-563-7) The quality of cereal products is dependent to a large extent on the suitability of the cereal grains processed. Therefore it is essential that cereals producers and handlers understand grain quality requirements for different end uses. Grain suppliers and users must also be able to assess grain end-use quality rapidly and accurately and use this information to direct their grain quality management activities. This book provides a convenient and comprehensive overview of academic research and industry best practice in these areas. Chapters review quality aspects of different cereals and also specific aspects of grain quality analysis and management. Kent’s technology of cereals: an introduction for students of food science and agriculture (ISBN 978-1-85573-361-9) This well-established textbook provides students of food science with an authoritative and comprehensive study of cereal technology. Kent compares the merits and limitations of individual cereals as sources of food products as well as looking at the effects of processing treatments on the nutritive value of the products. The fourth edition of this classic book has been thoroughly updated with new sections including extrusion cooking and the use of cereals for animal feed. Cereals processing technology (ISBN 978-1-85573-561-3 Cereals processing is one of the oldest and most important of all food technologies. Written by a distinguished international team of contributors, this collection reviews the range of cereal products and the technologies used to produce them. It is designed for all those involved in cereals processing, whether raw material producers and refiners needing to match the needs of secondary processors manufacturing the final product for the consumer, or secondary processors benchmarking their operations against best practice in their sector and across cereals processing as a whole. Details of these books and a complete list of Woodhead’s titles can be obtained by: ∑ visiting our web site at www.woodheadpublishing.com ∑ contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 832819; tel.: +44 (0) 1223 499140 ext. 130; address: Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: francis. [email protected]). Please confirm which subject areas you are interested in.


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Woodhead Publishing Series in Food Science, Technology and Nutrition: Number 219

Rice quality A guide to rice properties and analysis Kshirod R. Bhattacharya

Oxford

Cambridge

Philadelphia

New Delhi


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Published by Woodhead Publishing Limited, 80 High Street, Sawston, Cambridge CB22 3HJ, UK www.woodheadpublishing.com Woodhead Publishing, 1518 Walnut Street, Suite 1100, Philadelphia, PA 19102-3406, USA Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com First published 2011, Woodhead Publishing Limited © Woodhead Publishing Limited, 2011 The author has asserted his moral rights. Every effort has been made to trace and acknowledge ownership of copyright. The publisher will be glad to hear from any copyright holders whom it has not been possible to contact. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials. Neither the author nor the publisher, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Control Number: 2011930019 ISBN 978-1-84569-485-2 (print) ISBN 978-0-85709-279-3 (online) ISSN 2042-8049 Woodhead Publishing Series in Food Science, Technology and Nutrition (print) ISSN 2042-8057 Woodhead Publishing Series in Food Science, Technology and Nutrition (online) The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elemental chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Replika Press Pvt Ltd, India Printed by TJI Digital, Padstow, Cornwall, UK


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Contents

Author contact details ........................................................................ Woodhead Publishing Series in Food Science, Technology and Nutrition ...................................................................................... Preface .............................................................................................. Acknowledgements .............................................................................

ix xi xxi xxiii

1

An introduction to rice: its qualities and mysteries .............. 1.1 Rice in history .................................................................. 1.2 More rice paradoxes ......................................................... 1.3 Rice data and the tales they tell ...................................... 1.4 Rice quality ...................................................................... 1.5 References ........................................................................

1 2 8 12 18 25

2

Physical properties of rice ........................................................ 2.1 Introduction ...................................................................... 2.2 Grain appearance .............................................................. 2.3 Density and friction .......................................................... 2.4 Effect of moisture content ................................................ 2.5 Miscellaneous properties .................................................. 2.6 Chalky grains ................................................................... 2.7 References .......................................................................

26 27 32 40 46 50 53 57

3

Milling quality of rice ............................................................... 3.1 Milling of rice .................................................................. 3.2 Grain cracking or fissuring at or around harvest ............. 3.3 Drying of rice ................................................................... 3.4 Why the rice grain fissures............................................... 3.5 Miscellaneous factors that affect milling quality of rice ............................................................................... 3.6 Fundamental cause of rice breakage: interrelationship and synergy between different factors ............................. 3.7 References ........................................................................

61 62 65 72 73 84 90 95


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vi

Contents

4

Degree of milling (DM) of rice and its effect ......................... 4.1 Milling paddy grain and how much to mill ..................... 4.2 Effect of degree of milling (DM) on rice quality ............ 4.3 References ........................................................................

100 100 106 114

5

Ageing of rice ............................................................................. 5.1 Introduction ...................................................................... 5.2 Consumers’ perception of changes in rice behaviour during storage ................................................................... 5.3 Changes in physiochemical properties of rice during ageing as measured in the laboratory............................... 5.4 Theories of rice ageing: relation to individual constituents ....................................................................... 5.5 Some final rice paradoxes ................................................ 5.6 Can the ageing process be hastened or retarded? ............ 5.7 References ........................................................................

116 117

6

7

8

119 121 135 153 155 159

Cooking quality of rice ............................................................. 6.1 Introduction ...................................................................... 6.2 Absorption of water by rice during cooking at or near the boiling temperature .................................................... 6.3 Hydration at lower temperatures ...................................... 6.4 Loss of solids during cooking .......................................... 6.5 Effect of presoaking in ambient water on cooking .......... 6.6 Other changes/events occurring during cooking .............. 6.7 Laboratory cooking of rice for various tests .................... 6.8 References ........................................................................

164 164

Eating quality of rice ................................................................ 7.1 Introduction ...................................................................... 7.2 The initiation: the water-uptake paradigm ....................... 7.3 The period of data accumulation: the amylose paradigm ........................................................................... 7.4 Exploration at the molecular level: the amylopectin paradigm ........................................................................... 7.5 Rheology of rice-flour paste ............................................. 7.6 Other factors that affect the eating quality of rice ........... 7.7 Testing for rice quality ..................................................... 7.8 References ........................................................................

193 193 195

Effect of parboiling on rice quality ........................................ 8.1 Introduction: parboiled rice .............................................. 8.2 Changes brought about in the rice grain and its constituents during the parboiling process ....................... 8.3 Properties of parboiled rice .............................................. 8.4 Effect of rice variety on properties of parboiled rice ...... 8.5 Products from parboiled rice ............................................ 8.6 References ........................................................................

247 247

166 178 180 181 182 183 189

196 213 224 229 234 238

252 273 291 293 293


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Contents 9

vii

Product-making quality of rice ................................................ 9.1 Introduction ..................................................................... 9.2 Table rice .......................................................................... 9.3 Rice flour and products thereof ........................................ 9.4 Rice breakfast cereals and snacks made from wholegrain rice ................................................................. 9.5 Other rice products ........................................................... 9.6 References ........................................................................

298 298 301 307

10 Speciality rices ........................................................................... 10.1 Introduction ..................................................................... 10.2 Aromatic rices .................................................................. 10.3 Basmati ............................................................................. 10.4 Jasmine ............................................................................. 10.5 Other speciality rices ........................................................ 10.6 References ........................................................................

337 337 339 348 361 364 372

11

377 378 378 383 394 397

Nutritional quality of rice ......................................................... 11.1 Introduction: why do we have to eat? .............................. 11.2 Nutritive value of rice is not a small matter .................... 11.3 The perspective of nutrition of the poor rice-eater .......... 11.4 Wholegrains versus refined grains: two views................. 11.5 No, wholegrains confer untold benefits ........................... 11.6 The perspective of the nutritive value of individual rice constituents ....................................................................... 11.7 Biotechnological approach to upgrade the nutritive value of rice...................................................................... 11.8 References ........................................................................

12 Rice 12.1 12.2 12.3

319 330 332

401 405 407

breeding for desirable quality.......................................... Introduction ...................................................................... Plant characteristics for optimum harvest ........................ Physical and morphological properties of the paddy grain that affect the quality of the final product .............. Susceptibility of the rice grain to cracking ..................... End-use quality................................................................. Conclusions ...................................................................... References ........................................................................

410 410 413

13 Analysis of rice quality ............................................................. 13.1 Introduction ...................................................................... 13.2 Sample preparation ........................................................... 13.3 Estimation of moisture ..................................................... 13.4 Milling quality .................................................................. 13.5 Physical properties ........................................................... 13.6 Degree of milling (DM) of rice ....................................... 13.7 Hydration and cooking quality of rice .............................

431 431 433 440 446 450 461 474

12.4 12.5 12.6 12.7

416 423 425 428 429


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viii

Contents 13.8 13.9 13.10 13.11 13.12 13.13 13.14 13.15 13.16 13.17 13.18

Alkali digestion score....................................................... Gel mobility test ............................................................... Estimation of amylose content ......................................... Hot-water-insoluble amylose content ............................... Pasting characteristics ...................................................... Gelatinisation temperature (GT) ...................................... Tests for eating quality of rice ......................................... Testing for aroma of scented rice varieties ..................... Various constituents ........................................................ Tests for parboiled rice..................................................... References ........................................................................

482 486 487 491 492 504 507 512 514 514 521

Appendix: some selected rice quality test procedures ................... A.1 Physical properties ........................................................... A.2 Milling of rice, and degree of milling (DM) ................... A.3 Estimation of moisture ..................................................... A.4 Hydration and cooking parameters .................................. A.5 Amylose content............................................................... A.6 Alkali digestion score....................................................... A.7 Gelatinisation temperature (GT) ...................................... A.8 Gel mobility test ............................................................... A.9 Pasting behaviour: Brabender viscography ...................... A.10 Instrumental measurement of cooked-rice texture using the TA.HDi Texture Analyser ........................................... A.11 Aroma in scented rice ...................................................... A.12 Tests for parboiled rice..................................................... A.13 References ........................................................................

531 531 536 542 545 549 552 553 556 557

Index

563

559 561 561 562


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Author contact details

Kshirod R Bhattacharya, Advisor Rice Research and Development Centre (R&D unit of Tilda Riceland Pvt. Ltd) Tilda Riceland Pvt. Ltd 42 Km Stone Delhi-Jaipur Highway Gurgaon-122 001 Haryana India E-mail: [email protected]


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Woodhead Publishing Series in Food Science, Technology and Nutrition

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xv

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xvii

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201 Protective cultures, antimicrobial metabolites and bacteriophages for food and beverage biopreservation Edited by C. Lacroix 202 Separation, extraction and concentration processes in the food, beverage and nutraceutical industries Edited by S. S. H. Rizvi 203 Determining mycotoxins and mycotoxigenic fungi in food and feed Edited by S. De Saeger 204 Developing children’s food products Edited by D. Kilcast and F. Angus 205 Functional foods: concept to profit Second edition Edited by M. Saarela 206 Postharvest biology and technology of tropical and subtropical fruits Volume 1: Fundamental issues Edited by E. M. Yahia 207 Postharvest biology and technology of tropical and subtropical fruits Volume 2: Açai to citrus Edited by E. M. Yahia 208 Postharvest biology and technology of tropical and subtropical fruits Volume 3: Cocona to mango Edited by E. M. Yahia 209 Postharvest biology and technology of tropical and subtropical fruits Volume 4: Mangosteen to white sapote Edited by E. M. Yahia 210 Food and beverage stability and shelf life Edited by D. Kilcast and P. Subramaniam 211 Processed meats: improving safety, nutrition and quality Edited by J. P. Kerry and J. F. Kerry 212 Food chain integrity: a holistic approach to food traceability, safety, quality and authenticity Edited by J. Hoorfar, K. Jordan, F. Butler and R. Prugger 213 Improving the safety and quality of eggs and egg products Volume 1 Edited by Y. Nys, M. Bain and F. Van Immerseel 214 Improving the safety and quality of eggs and egg products Volume 2 Edited by Y. Nys, M. Bain and F. Van Immerseel 215 Feed and fodder contamination: effects on livestock and food safety Edited by J. Fink-Gremmels 216 Hygiene design of food factories Edited by J. Holah and H. L. M. Lelieveld 217 Technology of biscuits, crackers and cookies Fourth edition Edited by D. Manley 218 Nanotechnology in the food, beverage and nutraceutical industries Edited by Q. Huang 219 Rice quality: a guide to rice properties and analysis K. R. Bhattacharya 220 Meat, poultry and seafood packaging Edited by J. P. Kerry 221 Reducing saturated fats in foods Edited by G. Talbot 222 Handbook of food proteins Edited by G. O. Phillips and P. A. Williams 223 Lifetime nutritional influences on cognition, behaviour and psychiatric illness Edited by D. Benton


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Preface

I had three vaguely felt objectives in my mind when I embarked upon writing this book. The first was straightforward – to write about rice quality. Rice-grain science is a relatively young science. Having been associated for historical reasons with a relatively undeveloped region of the world, rice grain attracted scientists much later than, for instance, wheat. Nonetheless rice science has now developed or is developing. Scholarly books have been written, some of them excellent (especially the rice bible, the American Association of Cereal Chemists’ monograph Rice Chemistry and Technology). However, these books do not specifically deal with rice quality, although rice quality is necessarily mentioned within their discourse. The quality of rice, i.e., its characteristics, plays a crucial role in the long chain of steps from the field to the plate that rice has necessarily to travel. A knowledge of the quality features and their consequences is thus vital for choosing the most appropriate varieties for appropriate uses and the best processing steps for a given variety or a product – thus ensuring the best use-value, hence the greatest economic value, of the produce. This knowledge can also be of much value to the breeder. When the seeds that the breeder supplies to the farmer have the potential of a greater use-value, the breeder, the farmer, the processor and the consumer all benefit. Systematic collation and critical evaluation of the relevant knowledge and presenting it specifically from the standpoint of quality would thus be of much value. A second objective has been the desire to complete the story of this fascinating grain. Having been born and brought up in a small-town middle-class home in Bengal (now in Bangladesh), and hence exposed to earthy rice culture (especially parboiled-rice culture); watching backyard parboiling of paddy in the village; watching cooking of rice in a pot over an open fire at least twice every day (occasionally helping grandmother at this job) and knowing what rice ‘cooking’ was ; then coming to Mysore in south India after post doctorate work to join the fledgling rice group in the Central Food Technological Research Institute (CFTRI) in 1960, and thus being professionally too hooked to rice for life; being exposed to raw-rice


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Preface

(nonparboiled rice) culture at Mysore for the first time, thereby also viewing parboiled rice now from a new vantage point; devoting my entire professional life to rice science and technology, not only in the lab, but also doing a good deal of field work on rice drying, milling and parboiling; finally joining a modern basmati rice group as an advisor after leaving CFTRI, thus adding a new dimension to my rice experience in terms of aromatic rice, rice trade, and rice R, D and P (Research, Development and Production) – having been through all this, rice is in my bones and I feel thrilled by its tantalising story. And I see some deficiency in this area. The majority of the books on the rice grain, excellent as many of these are in terms of facts, are rather twodimensional. They have the facts, but they often lack the depth provided by the dimension of time, the flow of the facts, in short, the story of the rice grain. Scientists live in society, are funded by society. And society has a history. Rice too has a history (perhaps, more aptly, her story!). Rice science would be richer if one knew what historical situation affected what practice or technology and gave impetus to what studies and how. It has been my second objective to try to bring out this hidden story while narrating the science of rice quality. A third objective has been the desire to rectify some records. Scientists are humans. Science is recorded by humans and to err is human. There is therefore always scope for improvement of records. Similarly, as T. S. Kuhn (The Structure of Scientific Revolutions, University of Chicago Press, 1962) has discussed, scientists are often, unknown to their conscious knowledge, heavily influenced and held back by the prevailing paradigms. Years, even decades, have often to pass after a prevailing paradigm has been mortally damaged by new evidence before it can be replaced by a new one. Advancement of knowledge is thus often held up. There are several interesting examples of this type in the field of rice-grain science. It may not be amiss to point out a few of them as useful lessons in sociology of science. Kshirod R. Bhattacharya


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Acknowledgements

It is now my most pleasant duty to acknowledge my debt to those whose direct or indirect contributions played a key role in my taking up this labour of love. I must first of all thank my lucky stars that circumstances in 1960 forced me to join, most reluctantly, the then tiny rice group in CFTRI. It was touch and go, for I initially thought it was nothing less than committing hara-kiri for a dreaming biochemist to descend to the level of agreeing to work on a grain that we cook and eat every day! Next I must thank the two giants, V. Subrahmanyan and H. S. R. Desikachar, whose daring imagination in the one and ground-level pragmatism in the other I came in due course to gape with wonder at, and which actually inspired me to take up my quest in rice. Then there are two younger colleagues, M. Mahadevappa and T. Srinivas, who opened new frontiers of studies on rice and encouraged me to delve more into the mysteries of this elusive grain, and whose professionalism, dedication and skill I watched with admiration. M. K. Bhashyam continued the tradition of T. Srinivas and the attendant inspiration. Finally I come to my actual associates and students whom I had the privilege to advise in their doctoral or master’s level work and which actually created the knowledge base that helped connect the story of rice and made me wish to write about it. It is a pleasure to name them – C. M. Sowbhagya, S. Zakiuddin Ali, the late Y. M. Indudhara Swamy, K. R. Unnikrishnan, Rangan Chinnaswamy, M. R. Sandhya Rani, Gurunathan Murugesan, Charu Lata Mahanta, K. Radhika Reddy, Manoharan Ramesh, Ananda Prakash Pradhan (from Nepal), Abdul Halim (from Indonesia), Prem Narayan Maheswari, I. Mohan Reddy, Anuradha Bhat Sondi, Botcha Manohar Kumar, Md. Shamsud-Din (Bangladesh) and Sudhir Deshpande. Whatever knowledge I have created in rice has been done jointly with them, and each of them I have had the pleasure to publish at least one paper with, all, as a principle, with their respective names as the first author. In addition I have had the pleasure to do some joint work with P. V. Subba Rao and Vasudeva Singh. To all of them I am most thankful.


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xxiv Acknowledgements I am also thankful to Ms Satoko Musumi, United Nations University Fund Coordinator, Yokohama, to Professor Toshinori Kimura, Graduate School of Agriculture, Hokkaido University and to Dr Kiyoko Saio, now retired from National Food Research Institute, Tsukuba, Ibaraki, Japan for much technical and general information about rice in Japan. My debt to Ms Patcharee Tungtrakul, Director, Institute of Food Research and Product Development, Kasetsart University, Bangkok can hardly ever be redeemed. I knew next to nothing about jasmine rice. Most of the information on this wonder rice in Chapter 10 was furnished by her or gathered with her generous help. Dr Rusty Bautista sent me much information and literature about rice work in the University of Arkansas, Fayetteville, for which I am indebted to him, as also to Professor J. Samuel Godber of the Food Science Department, Louisiana State University, Baton Rouge, LA for some references. Professor Keni’chi Matsumoto, International School of Economics and Business Administration, Reitaku University, fired my imagination by his theory of ‘mud culture’ of Asia and I had many delightful exchanges with him through mail which I acknowledge with great pleasure. I am also extremely grateful to my young colleagues in the Rice Research and Development Centre (RRDC) at Mysore who helped me in ways too numerous to list. Much of the library work was done by one or the other of them. Photographs, scanning, some line drawings, gathering of some information or some data or some references were mostly done by them, especially by S. Suresh – and above all by K. Radhika Reddy. A good deal of information, data, and references were collected from the Internet; since I am computer-illiterate, this was almost entirely done by her. Wherever I had some doubt, Radhika was there to give her advice and Suresh too to some extent his. The heaviest burden of responsibility and work was borne by my young secretary-colleagues whose debt I can never properly redeem. Since, as I said, I am computer-illiterate, the entire work of typing, reproduction, printout, forwarding and receiving of e-mails, keeping track of chapters, figures, tables, files, papers, references, reprints and books were done by them – especially by N. Kavitha, but also at one time or other by N. Sangeetha and M. Abhilasha. Like zealous mothers, these young ladies protected and nurtured this project. I must also acknowledge my gratitude to the Directors of the Tilda Riceland Private Limited, especially Rashmi, Vipul and Shilen Thakrar and S. Sheshadri for their generosity in permitting me to devote my time in writing this book and providing all facilities for it, including typing, computer work and extensive correspondence. Finally I come to my publisher who has been a pleasure to work with. Initially I was a bit apprehensive when I first read their terms and conditions and officialese. But later it has been a revelation. Their courtesy, understanding, patience and encouragement have been outstanding. I express my deepest gratitude to them – especially to Sarah Whitworth, Senior Commissioning


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Editor, initially Bonnie Drury, Project Editor and later Cathryn Freear, Senior Project Editor. Last, but surely not the least, I express my deep debt of gratitude to my wife, Shibani. Let me say, in a way, this work and all the work I have done have been a joint endeavour. For if I have had the opportunity and time to roam freely and gather the fruits from the country around, that was only because she held the fort for me and gave me a free run.


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To my associates and students in CFTRI and RRDC whose dedicated work built the knowledge platform from which to view rice-grain science with confidence.


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1 An introduction to rice: its qualities and mysteries

Abstract: Contrariness is the other name for rice. It is both concentrated and dispersed, a subsistence crop and a high-value one, it feeds the world but is associated with a relatively undeveloped area, has precise properties but is often unpredictable, it is a food for survival and a food for culture. Some 90% of the world’s rice is grown in a small area (‘rice country’) in Asia. Yet rice feeds the largest number of people in the world. At the same time rice is very flexible and grows practically all over the world. The ‘rice country’ represents only about 14% of land area, but 25% of arable land and carries 54% of the population of the world. Another characteristic of rice is its great diversity, first, among the three zones of the ‘rice country’ and, second, from variety to variety. Key words: rice in history, rice paradoxes, rice country, rice quality, variability in rice properties.

‘Rice is a unique crop of great antiquity and akin to progress in human civilization.’ Chang (2003) ‘Rice helps feed almost half the planet on a daily basis, employs tens of millions in jobs they cannot live without, and has an enormous impact on our environment … rice production has been described as the world’s single most important economic activity.’ International Year of Rice 2004 World Rice Research Conference (WRRC), Tokyo, First announcement


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Rice quality

This chapter is an introduction to rice from two angles. Firstly, we present some unconventional facts about the common cultivated rice (Oryza sativa L.) It is an attempt to understand how rice played its part in history. Secondly, we present a summary of what we understand by rice quality and why it is important. Also presented is a glimpse of the extraordinary diversity in rice quality.

1.1

Rice in history

1.1.1 Beginning of agriculture Hominids, i.e. early human-like species belonging to the genus Homo, evolved or appeared on Earth some three million years ago. Humans, Homo sapiens, evolved approximately 100–120 thousand years ago (Strait et al. 1997). Once hominids came down from the trees to the ground, progress was rapid (comparatively speaking). They could now travel long distances in search of, or hunting for food. This was made possible by their being now bipedal. They could use their freed hands, so tool-making came naturally and group action could evolve. All of these factors promoted the development of language, and the brain started to expand. After spending a million years and more thus as hunter-gatherers, and crossing many a milestone on the way, partly by chance and partly by necessity, they finally stumbled into settled agriculture some 10–12 thousand years ago. The discovery of agriculture was a watershed in human evolution, for it enabled humankind to settle down in one place. It ushered in the process of social and cultural evolution. The discovery and domestication of ‘dry’ crops within agriculture, viz. cereals, pulses or legumes, oilseeds and nuts, was another milestone. The crucial importance of these grains was that they were in equilibrium with the ambient atmosphere and hence ‘dry’, so these crops could be stored for long periods without spoilage. These grains enabled humankind to grow their main food once a year, yet serve them for the entire year. It could even leave a surplus that allowed the development of organisation and leisure for play of power, art and culture. Apparently barley, wheat, peas, lentils and flax were the first to be domesticated and grown. Cereals obviously were, and continue to be, pre-eminent among these crops because of their value as a supplier of energy and nutrition for sustenance.

1.1.2 Enter the ‘frail but economically mighty grass’ It is generally believed that agriculture first started around the Mesopotamian region in the valley between Euphrates and Tigris, or in the ‘fertile crescent’ spanning the Nile through Palestine to the confluence of Euphrates and Tigris (Storck and Teague 1952). If so, rice was probably not among the first cereals to be domesticated and cultivated. These are likely to have been © Woodhead Publishing Limited, 2011


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An introduction to rice: its qualities and mysteries

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barley and wheat. However, rice would not have been lagging far behind. Archaeological evidence in China, southeast Asia and the Indus Valley suggests that rice must be at least eight thousand years old, perhaps more (Grist 1959, Chang 2003). What is noteworthy is that despite possibly being marginally late in its domestication and sustained cultivation, this ‘frail but economically mighty grass’ (Chang 2003) has come to occupy such a preeminent position in human history.

1.1.3 Concentration and spread of rice cultivation There are many surprising facts about rice. One of these is related to the extraordinary concentration of rice production in a small part of the world. A glance at Fig. 1.1 brings out this paradox. Approximately 90% or more of the world’s rice is produced in the relatively tiny area marked in the figure in south, southeast and northeast Asia – which we will refer to as the ‘rice countries of Asia’ or simply the ‘rice country’. And yet, as will be mentioned in many statements quoted below, roughly half the world’s population is said to depend on rice as their staple. The reason for the extraordinary concentration of rice production has been hinted at by the following statement in the Rice Almanac brought out by the International Rice Research Institute (IRRI) in collaboration with WARDA, CIAT and FAO of the United nations as a ready reckoner of rice facts to help the international rice research community (Maclean et al. (2002)): ‘Rice occupies an extraordinarily high portion of the total planted area in South, Southeast, and East Asia. This area is subject to an alternating wet and dry seasonal cycle and also contains many of the world’s major rivers, each with its own vast delta. Here, enormous areas of flat, lowlying agricultural land are flooded annually during and immediately 120°

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Fig. 1.1 World map highlighting ‘the rice countries of Asia’.



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Rice quality following the rainy season. Only two major food crops, rice and taro, adapt readily to production under these conditions of saturated soil and high temperatures.’

D. H. Grist, an authority on rice of yesteryears, who wrote many editions of his well-known book Rice, wrote (Grist 1959): ‘The great rice areas of the Far East, such as the deltas of the Irrawaddy, Bhramputra, Mekong and the greater part of the Gangelia plain and the Krishnia areas are the results of erosion. Without erosion there would be far less land suitable for paddy. It is probable that paddy is grown because there is no other cereal which can grow under such high monsoon rainfall. Rice has enabled the populations of Asia to survive and indeed increase, because paddy checks – but does not entirely prevent – erosion. Had the people of Asia attempted to live by any other cereal they could not possibly have maintained their high density population for thousands of years. To prove the truth of this assertion one has only to compare the population density in countries of large rice production with those of other tropical countries where rice is not produced as the staple crop. Growing paddy necessitates water conservation and this in turn ensures soil conservation. Some of the terraced fields in Indonesia, the Philippines and south China are over two thousand years old and are typically conservation projects. This more than any other factor accounts for the predominance of rice as the staple food in southeast Asia, in countries of high rainfall.’ Despite the above extraordinary concentration of rice production in a relatively small area of the world, however, the other paradox is that rice is also extremely adaptable. It can be grown under a wide range of climatic conditions. It is today grown in every continent other than Antartica and in at least 100 countries from 45°N (under certain conditions up to even 53°N) to 40°S latitude, from sea level to approximately an altitude of 3000 metres and from being submerged under one to two metres of water (deep water rice) to dry upland areas. The Rice Almanac (Maclean et al. 2002) states: ‘Rice is produced in a wide range of locations and under a variety of climatic conditions, from the wettest areas in the world to the driest deserts. It is produced along Myanmar’s Arakan Coast, where the growing season records an average of more than 5,100 mm of rainfall, and at Al Hasa Oasis in Saudi Arabia, where annual rainfall is less than 100 mm. Temperatures, too, vary greatly. In the Upper Sind in Pakistan, the rice season averages 33° C; in Otaru, Japan, the mean temperature for the growing season is 17° C. The crop is produced at sea level on coastal plains and in delta regions throughout Asia, and to a height of 2,600 m on the slopes of Nepal’s Himalaya. Rice is also grown under an extremely broad range of solar radiation, ranging from 25% of potential during the main rice season in portions of Myanmar, Thailand, and India’s Assam state to approximately 95% of potential in southern Egypt and Sudan.’



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1.1.4 Rice as food and as a source of employment The importance of rice lies in many spheres – as food, as a source of income and employment (economy), as well as in social development and culture. The Rice Almanac (Maclean et al. 2002) contains the following among the large number of startling statements about the extraordinary importance of rice in history: •

• • • • •

Domestication of rice ranks as one of the most important developments in history. Rice has fed more people over a longer period than any other crop. Rice is the staple food for the largest number of people on Earth. Rice is eaten by nearly half the world’s population. Rice farming is the largest single use of land for producing food. Rice is the most important economic activity on Earth. Rice is the single most important source of employment and income for rural people.

These statements, along with the WRRC 2004 theme declaration quoted at the beginning of this chapter, give a vivid image of the importance of rice not only as a source of nourishment but also as a source of economic sustenance for humankind.

1.1.5 Rice is not simply food or economics No matter how important the position of rice is as a source of food and economic sustenance, rice is much more than that. It is also a part of community life, social organisation and culture. In most languages of the region, the words for rice and food are synonymous. When people are invited home, they are invariably offered rice, meaning food. The activities of the rural people in the rice-producing regions of Asia revolve round the seasonal activities related to rice production. Every phase of the production of rice, including ploughing and land preparation, sowing, transplantation, weeding, harvesting, threshing, transportation and storage, is modulated with the progress of the season. Each phase is connected to a particular season, rituals, celebration, community activity and religious function. Rice/paddy has the character of sacredness. One blesses a newly married couple with grains of paddy or rice, one offers cooked rice to the spirits of the dead and one offers rice as a symbol of auspiciousness in religious ceremonies. Not only that. In most rice-producing communities, rice and everything connected with rice are considered to be endowed with spirits which need to be always protected, respected and propitiated. The grain, as preserved as seed, is said to have the spirit of rice which has to be protected and propitiated before sowing to result in a bounty of harvest. Every aspect of rice production cycle has to be preceded by proper ritual customs. These rituals invoke and propitiate the corresponding spirits to see that one is rewarded with a bountiful harvest and also for a peaceful and happy community life.



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Rice quality

The UCLA (University of California Los Angeles campus) Fowler Museum of Cultural History arranged a travelling museum and brought out a multiauthored, beautifully illustrated volume under the name The Art of Rice (Hamilton 2003). The object was to bring out the inherent features of the ‘spirit and sustenance in Asia’: ‘There are many beloved rice deities, especially goddesses, who create and protect the sacred grain, rice deities are associated directly with a bountiful crop as well as with prosperity more generally. Indeed, rice is so fundamental to the many Asian people who grow and eat it that it has become synonymous with life itself . . . A key tenet of rice culture is that rice is a sacred food divinely given to humans that uniquely sustains the human body in a way no other food can.’ The discussion goes on that the Asian rice region covers great religious diversity: ‘Hinduism is the majority religion in India, Islam in Indonesia, Buddhism in Thailand, Roman Catholicism in the Philippines, and so on. Many countries have mixed traditions, such as Shinto and Buddhism in Japan, or Taoism and Confucianism in China. Besides, minorities in each country follow their own faiths. Yet rice culture is expressed in all of this diversity like an underlying current. Clearly these aspects of rice culture, as related especially to rice spirit beliefs, would have been established before the development and spread of the major world religions.’ A question that arises is: Is this relationship specific to rice or to Asia? In this connection Hamilton (2003) states that there are many places in the world where rice is economically important or even the staple, but where the characteristic rice culture is not found. Examples are southwest Asia, west Africa, the Carribbean, Madagascar, Italy, Spain, the United States and Australia. The emotional and spiritual connection of the rice producer to rice may not be specific to rice per se. Is it then related to rice in Asia? Keni’chi Matsumoto, Professor of International School of Economics and Business Administration, Reitaku University, presented a very interesting paper entitled ‘The power of settled life – rice farming as a lifestyle’ in the WRRC 2004 in Tokyo (Matsumoto 2004a). In this paper he quotes a Japanese folklorist to say that the Japanese can be described as rice farmers on an island. That is, Japan is defined by insularity and rice cultivation; also by settled life rooted in a community and a plot of land: ‘Now let us focus on settled rice farming as an ethnic lifestyle. The reward of your life as a farmer is to sustain your paddies and paddy terraces passed down from your ancestors. Issho-kenmei, a Japanese idiom for “do your best” literally means “maintain a piece of land for all your worth”. This idea had been supported as an ethnical ethos of Japanese. This describes



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why Japanese pay more respect to the process of “doing your best” than to getting good results or high profits. Sharing the ethos of issho-kenmei, people living in an area work together to dig drainage ditches, build irrigation systems and weed the whole village. The working group unavoidably encourages the development of a community system. In these community work systems, innovations and processes to increase productivity develop in order to improve the ancestral farming land.’ Rice production in Japan is not just food or economics, it provides the core values of life and community. Professor Matsumoto in another context has written about ‘mud civilisation’, ‘sand civilisation’ and ‘stone civilisation’ (Matsumoto 2004b). He defines the rice area in Asia as representing mud civilisation, the area around western Asia is the centre of sand civilisation and the modern European and American countries are the region of stone civilisation. The skyscrapers in the West that seem to dominate the surroundings symbolise the stone civilisation. Sand civilisation, he says, is characterised by lack of water, making it difficult for life to emerge and survive. Monotheism is characterised by one God and also a father. This civilisation is characterised by austerity but extraordinary networking. Mud, on the other hand, is conducive to life. Hence mud civilisation is characterised by polytheism as well as by gods of both genders, especially mother god, as a symbol of fecundity and promoter and protector of life. Another doubt may then arise. One may wonder that the sentimental perception of rice may not be specific either to Asia or to rice but is perhaps a preindustrial perception. In that logic, the attitude of say the Californian rice farmer – who has no sentimental or spiritual attachment to rice and to whom rice is no more than a profitable crop to be replaced by another if that is more profitable – would be considered a postindustrial attitude. That is plausible. The question then would be, why do the Japanese – surely an industrial society for half a century – still have an emotional bond to rice, no matter if it may not be perhaps as visceral as say 50 years ago? Professor Matsumoto (personal communication) in this context has something very significant to say. He quotes the Japanese creation myth ‘Kojiki’, according to which the Japanese land was created by making the rice fields. The Japanese value of ‘do your best’ came from that myth. Significantly, even the industrial community in Japan, Professor Matsumoto says, uses the same phrase when imploring or promising that one must do one’s best! Listen also to Ms Satoko Musumi (United Nations University Fund Coordinator, Yokohama, Personal communication): ‘RICE is NO commodity for business for Japanese, even for the young alien-like generation. RICE is our blood, even its annual consumption dropped from 120 kg+ (in the 1950s) down to 58 kg today,



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Rice quality It’s still not a mere calorie-, nutrition- or biological stomach-filling resource. Much, much more than cultural/ritual/historical. Japan’s presence exists on RICE as much as on Toyota, Sony, Camera, Anime industries!’ (emphasis in original)

There may be more to rice and Asia than economics can explain. Here is a task for anthropologists to explore.

1.2

More rice paradoxes

We have already seen a few paradoxes in rice, but there is no end to the puzzles and paradoxes that rice throws up. Here are some more.

1.2.1 Rice feeds half the world This statement is among the most enduring paradoxes of rice. As is clear from Fig. 1.1, a little over 90% of the world’s rice is grown in a region which is but a tiny part of the world. What is more, some 90% of the world’s rice is also consumed here. Yet every author mentions that approximately half the world’s population depends on rice as their staple or that rice feeds the largest number of people. There is more to it. Cereals are the most important food of humankind. Now regardless of its importance, rice is surely not the only or even the principal cereal grown. Rice (paddy), wheat and maize are the three major cereals of the world and their annual production is, roughly speaking, close to each other – around 600 million tonnes (Mt) each (2010). In addition to these three, there are a number of other minor cereals (barley, sorghum, rye, millets) which together may be considered to make up roughly an equal proportion to the three above. In other words rice can be considered to form roughly one-quarter of the available food energy source of the world. The obvious paradox then is, how does one-quarter of the available food feed half the people? Is it then a myth? There is no straightforward answer. Nor do scholars who regularly proclaim the universal nourisher role of rice generally deal with such paradoxes. We can hypothesise as follows: •

•

Rice is too expensive and precious a cereal to be used as anything other than food. Besides rice is by and large the poor man’s food, who has little else to eat. So rice is never wasted, never used for anything other than food. On the other hand other cereals (maize, wheat) are widely used either as feed or for industrial purposes. Rice is even today to a substantial extent a subsistence crop. That is, rice is by and large produced for self-consumption or consumption around the place where it is produced. This is testified by the fact that © Woodhead Publishing Limited, 2011


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• •

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hardly 5–6% of world’s rice production enters international trade (Childs 2004). Wheat, on the other hand, is widely internationally traded (about 20% of production). It is used not only in the baking industry but for other industrial uses (gluten, starch, industrial products) and to some extent even as feed. Maize and sorghum are used as feed as well as widely traded. Poor people make do with what they have, while others may have food to spare. Although the rice countries of Asia are small in terms of land area, their arable land area as well as population density are very high, as we will see below.

Combining all these facts, what emerges may provide some idea of why it is widely believed that rice feeds, if not half the world, without doubt the largest number of people in the world.

1.2.2 Rice and poverty seem to be related Another matter to marvel at is this extraordinary contrast: on the one hand it is said that rice feeds half the world, yet on the other hand it is plain that rice is strongly associated with some of the poorest regions of the world. At any rate the ‘rice country’ in south, southeast and east Asia used to be so until a few decades ago. How does one explain this association? Again it is not easy to find an answer to this uncomfortable association in the extensive literature on rice emanating from scholars or from the large number of organisations that deal with rice. We can offer some tentative hypotheses. • • •

The rice countries of Asia constitute, as we shall see later below, the most densely populated region in the world. As a result, a large part of the world’s population resides in this area. Having been outside the centre where industrialisation first emerged (Europe), and soon subjected to colonialism precisely by these industrialising countries, this then-flourishing region of Asia missed the liberating and wealth-creating opportunity of industrialisation for over 200–300 years. At the same time the population continued to multiply rapidly partly precisely because of this relationship with Europe. As a result the region sank more and more into poverty for a long time.

Sachs et al. (2001) have proposed a very interesting hypothesis regarding the incidence of wealth and poverty in the world, which may be relevant. They have provided extensive data to suggest that poverty and wealth in the world are strongly associated with the tropical or temperate location of a country and to its distance from the ocean. Subtropical and temperate regions, more so those countiguous to the ocean (Europe, Japan in Asia), have progressed in wealth creation. Tropical regions, particularly large land masses with poor access to the ocean, have continued to remain poor. That a © Woodhead Publishing Limited, 2011


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Rice quality

large number of the Asian rice countries do partly meet the latter description may not be without significance.

1.2.3 Rice yield is proportional to latitude Rice is basically a tropical crop but its production also extends to subtropical and temperate regions. One may broadly consider the humid tropics as the home of rice and the temperate areas as its nontraditional regions. Agriculturists have long noted that rice yield was greater in northern countries (in the Mediterranean countries such as Italy and Spain, in California in the USA, in Japan in Asia, and in Egypt in Africa) than in the original home countries of rice in Asia. The significance that the former were mostly in subtropical and temperate regions while the latter were in tropical regions was seen but not generally noticed. The fact that the former were by and large industrialised and the latter were not, made the latitudinal association difficult to judge. The question of latitude was raised by Grist in successive editions of his book. For instance he gave the data shown in Table 1.1. The mean yield of paddy in the region from latitude 0° (equator) up to approximately 20° latitude was substantially lower than that in the region beyond 20°N latitude. Irrespective of whether there is any biological reason for the above difference, there is no doubt that this difference does exist. Even within the rice countries of Asia today, yield in Japan, Korea and China (6–7 tonnes/ hectare, t/ha) is definitely more than those in south (3.0–3.5 t/ha) and southeast (2.5–4.5 t/ha) Asia (Maclean et al. 2002). The validity of this rule can be seen even within a single country. For instance, within India the nontraditional areas of rice, in the northern states of Punjab and Haryana, definitely show a higher yield of rice (4.0–6.0 t/ha) than those in the traditional areas in eastern and southern India (2.0–4.0 t/ha). Even though this issue is generally not openly or comprehensively discussed by scholars and hence no clearcut scientific explanation has emerged, some hints are available here and there. Based on these, the above difference may perhaps be explained on the following basis:

Table 1.1

Yield of paddy in different latitudes

Latitude (degrees)

Average yield (lb per acre)a

0−10 11−20 21−30 Over 30

982 1010 1289 1888

Reproduced, with permission, from Grist (1959). a 1 lb = 0.45 kg.



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An introduction to rice: its qualities and mysteries •

• •

•

11

because of the monsoon climate, there is substantial and continuous cloud cover during the main rice season in the tropical rice countries of Asia, which significantly reduces the photosynthesis; day length is shorter in smaller latitudes and more in higher latitudes in the growing season, again resulting in a difference in photosynthesis; the warm and humid weather during the main growing season in the tropical regions favours extensive diseases and pests for the rice plant, which drastically lowers the yield; and the subsistence farmer’s fields in the intensely cultivated and highly populated regions of tropical Asia are too small, and the farmers too poor, to permit necessary and efficient use of inputs, which do not promote optimum yield.

1.2.4 Rice is a typical Asian staple It is a fact that some 90% or more of world’s rice is grown and consumed in south, southeast and northeast Asia (the ‘rice countries’) (Fig. 1.1). How it came about is a matter of history. The reason for this association was explored in the discussion referred to above about the concentration and spread of rice cultivation (Section 1.1.3). Both Maclean et al. (2002) and Grist (1959) wrote about the concentration of rice in this relatively small area of Asia. As they explained, the vast flat lands, the presence of several mighty rivers (Huang Ho, Yangze Kiang, Mekong, Irrawaddy, Chao Phraya, Brahmaputra, Ganga) and their extensive deltas combined with the concentrated heavy rainfall in the monsoon climate may have resulted in rendering the region mainly suitable for growing rice. No doubt the great versatility of rice enables it to be grown extensively all over the world, but the above concentration caused by agroclimatic factors may have resulted in the obvious association of rice and Asia. Another association of rice, closely related to the above, is with high population density. The same factors mentioned above may have also resulted in a relatively large proportion of the respective land areas to be rendered arable (see below) and made use of to grow rice. This fact combined with the initial bounty of rice, followed by colonial stagnation and the resulting high fertility, have probably contributed to the high population density of the rice countries on the one hand and the association of rice with poverty on the other.

1.2.5 Is ‘mud civilisation’ waking up? While on this subject, it may not be out of place to mention one interesting phenomenon: the economic stirring currently being seen in these countries. As the twentieth century was coming to a close, there were clear signs that the ‘rice countries of Asia’ were beginning to grow economically. Japan had already changed completely since the1950s. Then came the ‘Asian



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Rice quality

tigers’ – South Korea, Taiwan–China, Hong Kong–China, Singapore. And now Vietnam, Indonesia, Malaysia, Thailand and India are clearly showing signs of being on the go. Does it mean that there has been something in the ‘mud civilisation’ that made it flourish in the past, then decline, and now be rising again?

1.3

Rice data and the tales they tell

For any proposition, data are essential. Data illustrate – and authenticate – what the author says. Data are sacrosanct; they do not lie. The point is, data by themselves do not necessarily tell the truth, at any rate the whole truth, either. Data tell their tale only when coaxed to do so. Data are of two kinds – passive and active. Mere reproduction of serial figures is passive. For instance, every textbook or discussion on rice presents figures of area and production of rice as well as perhaps yield (production/ area) and time trends. One may also get figures of export and import by country, by year and so on. These data are of course valuable, for they act as a ready reckoner of the trend of past and present production and trade. (The fact that some of figures are often faulty in the sense that the authors may have quoted data from more than one source without realising that some figures were for paddy and some for ‘clean rice’ (taken as 66.7% of paddy) – a not uncommon phenomenon at any rate in Indian agricultural and commercial literature – is another matter!) However, such data by themselves may not be enough to provide insight into what has been happening in history and how we arrived at our current position. It is only when data are juxtaposed and their relations explored that they come alive. Then they may provide insight and explain seeming paradoxes. An example is the brilliant exercise done by Sachs et al. (2001) mentioned above – regardless of whether one agrees with the conclusions. Not much of an exercise of this kind is available in the extensive literature on rice. Tables 1.2–1.5 present an attempt. Table 1.2 shows that the ‘rice countries of Asia’ are characterised by two significant trends. First, regardless of their total land area, the extent of their arable land area as a proportion of their total land area is very high – usually in the range of 15–25%, going up in some cases to about 50% or more (India, Bangladesh), the mean value being 21%. Contrast this figure with that of the same ratio for the rest of the world (world excluding the Asian rice countries) (Table 1.3). Most of the values here are in single digits, even as low as 2−3%, the mean value being about 10% after including a few high arable-area countries (the USA, Mexico, Nigeria, Turkey). Interestingly, if we consider traditional i.e. Western Europe (Table 1.4), the arable area as a proportion of total land area here again is high, of the order of 23%. Would it imply that a high level of civilisation was associated with a high proportion of land area being made arable? © Woodhead Publishing Limited, 2011


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100.00

623.26

19.41 17.12 3.55 2.28 1.31 0.98 0.78 1.46 1.85 2.69 0.71 0.21 0.42 0.33 0.31 0.09 0.41 0.34 54.25 100.00

1330.14 1173.11 242.97 156.12 89.57 67.09 53.41 99.90 126.80 184.40 48.64 14.45 28.95 22.76 21.51 6.37 28.27 23.02 3717.50 6853.06

8.80 9.22 1.28 0.47 0.42 0.89 0.62 0.36 0.28 1.21 0.10 0.23 0.15 0.17 0.06 0.06 0.11 0.05 24.48 100.00

14.86 48.83 11.03 55.39 20.14 27.54 14.92 19.00 11.64 24.44 16.58 20.44 16.07 22.40 13.96 4.01 5.46 24.00 20.95 12.02

138.59 145.18 20.15 7.42 6.55 14.09 9.81 5.67 4.36 19.03 1.63 3.61 2.30 2.70 0.90 0.93 1.79 0.77 385.49 1574.93

7.12 2.27 1.39 0.10 0.25 0.39 0.50 0.23 0.29 0.59 0.07 0.13 0.11 0.09 0.05 0.18 0.25 0.02 14.05 100.00

13097.82

% of the world

932.64 297.32 182.64 13.39 32.54 51.18 65.77 29.82 37.47 77.87 9.82 17.65 14.32 12.04 6.47 23.08 32.86 3.23 1840.11

Million

% of the arable area of the world

Mha

% of land area of country

Population

Arable area

Source: Paddy area and production – http://beta.irri.org/solutions/index.php?option=com_content&task=view&id=250 (USDA, PSD Online. accessed 16 April 2010). Total land area and arable area – https://www.cia.gov/library/publications/the-world-factbook/index.html (accessed 16 November 2010). Population – https://www.cia.gov/library/publications/the-world-factbook/index.html (accessed 18 November 2010) All data except population are for 2005; population data as latest in the website. M = mega = million = 106; t = tonne; ha = hectare. a Excluding Antarctica b This year was exceptional. China’s rice production is usually more, over 30% of the world, making the total of Asia over 90%.

28.98b 22.09 8.70 6.92 5.54 4.42 2.89 2.42 1.82 1.34 1.03 0.96 0.69 0.50 0.50 0.41 0.36 0.24 89.79

180.59b 137.70 54.20 43.14 34.50 27.58 18.00 15.11 11.34 8.32 6.44 5.99 4.29 3.11 3.09 2.57 2.22 1.47 559.64

% of the worlda

Mha

Mt

% of world

Total land area

Paddy production

Paddy production, arable area and population of rice countries of Asia

China India Indonesia Bangladesh Vietnam Thailand Myanmar Philippines Japan Pakistan South Korea Cambodia Nepal North Korea Sri Lanka Laos Malaysia Taiwan-China Rice countries total World (excl. Antarctica)

Country

Table 1.2



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Arable area Mha 117.43 165.01 41.56 58.60 46.85 27.45 22.11 7.55 6.49 3.59 19.56 13.56 24.35 1.81 1.18 14.48 3.53 3.30 14.76 11.21 2.09 3.76 2.88 2.91 30.07 3.75

Total land area % of the worlda 12.50 7.00 6.94 6.46 5.82 2.09 2.04 1.82 1.73 1.64 1.53 1.53 1.47 1.34 1.19 0.97 0.96 0.95 0.93 0.85 0.79 0.76 0.76 0.76 0.70 0.68

Mha

1637.77 916.19 909.35 845.65 761.79 273.67 266.98 238.17 226.76 214.97 200.00 200.00 192.30 175.95 155.47 126.67 125.92 124.67 121.99 111.97 103.87 100.00 100.00 99.55 91.08 88.60

7.17 18.01 4.57 6.93 6.15 10.03 8.28 3.17 2.86 1.67 9.78 6.78 12.66 1.03 0.76 11.43 2.80 2.65 12.10 10.01 2.01 3.76 2.88 2.92 33.02 4.23

7.46 10.48 2.64 3.72 2.97 1.74 1.40 0.48 0.41 0.23 1.24 0.86 1.55 0.12 0.08 0.92 0.22 0.21 0.94 0.71 0.13 0.24 0.18 0.18 1.91 0.24

% of land area % of arable area of country of the world

% of the world 2.03 4.53 0.49 2.93 0.31 0.60 0.23 0.50 1.03 0.38 1.12 0.64 1.64 0.09 0.05 0.23 0.15 0.19 0.72 1.28 0.65 0.20 0.44 1.17 2.22 0.61

Million 139.39 310.23 33.76 201.10 21.52 41.34 15.46 34.59 70.92 25.73 76.92 43.94 112.47 6.46 3.09 15.88 10.54 13.07 49.11 88.01 44.21 13.80 29.91 80.47 152.22 41.89

Population

Arable area, paddy area and population of major countries of the world excluding rice countries of Asia

Russia United States Canada Brazil Australia Argentina Kazakhstan Algeria Congo Saudi Arabia Iran Sudan Mexico Libya Mongolia Niger Chad Angola South Africa Ethiopia Colombia Mali Peru Egypt Nigera Tanzania

Country

Table 1.3

0.40 0.15 0.37 1.02 0.72 0.14

0.44

1.86 0.16 0.19

1.62

% of the worldb

Paddy production



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0.59 85.95 100.00

77.08 11257.71

13097.82

1574.93

22.98 1189.44

Source: as in Table 1.2. a = as in Table 1.2. b Only values of at least 0.1% of the world are mentioned. c 233−27 = 206 countries.

Turkey World minus rice countriesc World (excl. Antarctica) 12.02

29.81 10.57 100.00

1.46 75.52 6853.06

77.80 3135.56 100.00

1.14 45.75 100.00

0.10 10.21



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Source: as in Table 1.2. Footnotes as in Table 1.2.

64.005 49.954 41.093 34.922 30.744 29.402 24.159 9.195 8.244 6.889 4.239 3.977 3.388 310.213

0.489 0.381 0.314 0.267 0.235 0.224 0.184 0.070 0.063 0.053 0.032 0.030 0.026 2.369

21.416 13.578 2.437 11.570 0.830 7.765 5.612 1.590 1.368 1.159 2.230 0.394 0.744 70.691

Mha

Mha

% of the world

Arable area

Total land area

Arable area and population of traditional European countries

France Spain Sweden Germany Norway Italy United Kingdom Portugal Austria Ireland Denmark Switzerland Netherlands Total

Country

Table 1.4

33.460 27.180 5.930 33.130 2.700 26.410 23.230 17.290 16.590 16.820 52.590 9.910 21.960 22.788

1.360 0.862 0.155 0.735 0.053 0.493 0.356 0.101 0.087 0.074 0.142 0.025 0.047 4.489

% of land area of % of arable area country of the world 64.768 46.506 9.074 82.283 4.676 58.091 62.348 10.736 8.214 4.623 5.516 7.623 16.783 381.242

Millions

Population

0.945 0.679 0.132 1.201 0.068 0.848 0.910 0.157 0.157 0.067 0.080 0.111 0.245 5.563

% of the world

An introduction to rice: its qualities and mysteries Table 1.5

17

Arable area and population by continents

Continent

Arable land Population Population/arable (Mha) (million) land ratio

Asia South America North and Central America Africa Europe Oceania World

504.5 112.6 251.8 219.2 277.5 45.6 1411.1

4029.3 380.6 530.8 964.7 730.9 34.5 6670.8

8.0 3.4 2.1 4.4 2.6 0.8 4.7

Source: Compiled from http://faostat.fao.org/site/550/DesktopDefault. aspx?PageID=550#ancor (accessed 26 November 2010). Courtesy: Food and Agriculture Organisation of the United Nations

Incidentally one may note in passing the extraordinary position of India with respect to arable land (Table 1.2). Barring Bangladesh – which, geographically speaking, is an extension of the Indian Gangetic plain – India has the highest proportion of arable land in the world as a fraction of her total land area. Even in absolute terms, the extent of arable land of India (145 Mha) is only slightly lower than that of the top country, the USA (165 Mha) (Table 1.3), whose total land area is over three times that of India. Let us now come to another crucial index, viz. population. The population of the rice countries (Table 1.2) as a proportion of the total world population is a staggering 54%. Note that the total land area of these countries is hardly 14% of the world, but this 14% land area carries a population of roughly four times the proportion (54%). Even the arable land area of the region is only about 25% of the world. In other words, the rest of the world (Table 1.3) with over 85% of total land area and over 75% of the arable land area of the world carries a population of roughly only 45% of the world. The extraordinary pressure on land in the rice countries is clear enough and the ability of this land, i.e., rice, to provide sustenance is also clear. Even when we come to traditional Europe (Table 1.4), the pressure on land is not so high. Here roughly 2.4% of world’s total land area and 4.5% of arable land carries 4.5% of world’s population. Taking a broader view, and considering by continents, the data in Table 1.5 again support the above trend. Even the continent of Asia as a whole shows an exceptionally high population density, nearly double the mean population density of the world. If the several paradoxes mentioned earlier are now considered in the backdrop of these data, many of these tend to get clarified. How one-quarter of the food production feeds half of the world becomes, speaking very roughly, clear from the fact that Asian rice countries represent 25% of the world’s arable land but 54% of the world’s population. No doubt an appreciable proportion of the people in both China and India depend on grains other than rice as a staple, but that is partly compensated by rice-eaters outside



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the region. The high population density and the relatively low land area may be illustrative of the comparative poverty. The importance of rice as food and as a source of employment in the Asian rice countries is demonstrated. The extreme concentration of rice production in the Asian rice countries automatically explains the association of rice with Asia. To conclude, rice may be frail looking but is indeed a mighty grass and has played a giant role in history.

1.4

Rice quality

We have just reviewed how rice is one of the very few major food sources of the world. Clearly then, understanding its identity, behaviour and usevalue deserves careful consideration. A little over 600 million tonnes of paddy (rough rice) or its equivalent in milled rice is every year harvested, transported, dried, milled, stored, processed, converted into diverse products, and marketed and consumed in various forms in the world. The different quality characteristics of the grain play a decisive role in determining the efficiency or suitability of every step of this long chain, the palatability of the end products, and the selection of varieties, products and processing conditions for the different end uses. Quality of rice and varietal and other differences in it have necessarily been touched upon in all discourses on the chemistry and technology of rice, but no publication specifically devoted to this subject has appeared. A large literature exists, however, scattered over a wide area of subject-matters. Systematic collation and a critical review of this literature will give a new fillip to this critical area of cereal chemistry and technology.

1.4.1 Overview of rice quality What is rice quality? Quality is the other name of properties. It is by the properties of something or someone (appearance, behaviour,…) that we know it or them. If something is round or red, then round shape or red colour is one of its qualities. The degree of redness will become another quality, and so on. Considering the range of rice qualities, the first variable is its variety or cultivar. Rice varieties come in thousands. They differ greatly in their size, shape, colour, aroma, structure, morphology, histology, and macro- and micro-chemical make-up. Secondly, additional differences are introduced by drying, storage, milling, parboiling and other processing of the grain. All these differences affect the handling, processing, marketing, product-making, cooking and organoleptic properties and use-value of rice in general. Special factors arise as rice is handled, processed and consumed mostly in whole grain form. These diverse criteria, appropriately classified and codified in terms of the various use-values of the grain, constitute its quality. © Woodhead Publishing Limited, 2011


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Rice quality may be considered under the following headings: • • • • • • • • • •

physical properties; milling quality including as affected by its drying; degree of milling; ageing; cooking quality; physicochemical properties and eating quality; parboiling and its effect; product-making quality; aromatic and other speciality rice; and nutritional quality.

Physical properties Since rice is handled, processed and used mostly in whole grain form, its physical and structural properties constitute an essential aspect of its quality. These properties include its dimensions, density, bulk density, frictional properties and porosity, as well as their interrelations on the one hand, and the effect of varietal difference, moisture content and degree of milling on them on the other. Needless to say, these properties affect handling and processing. Dimensional classification of rice for marketing and its grading based on external grain features also follow from these properties. Thermal properties such as specific heat and heat transfer coefficient, also diffusion of moisture, affect rice’s hydration and drying behaviour and hence its processing. Various morphological features of the grain, such as the content and interlocking of the lemma and palea (husk); the content and adhesion of the embryo to the endosperm; and grain chalkiness affect its marketing, processing, milling and use-value. Milling quality The fact that rice is milled and cooked mostly in whole grain form plays a decisive role in its milling. Anything that militates against the grain integrity during milling would be considered undesirable. Cracks or fissures in the grain are probably the greatest concern in rice milling. Anything that promotes grain fissuring is a potential hazard to be avoided. In addition, husk content, tightness of husk interlocking; presence of ridges on the endosperm; presence of immature, infested and chalky grains; types of cracks; size, shape and thickness of the grain; its moisture content; degree, i.e. the extent, to which the rice is desired or proposed to be milled – all affect its milling results. In particular, drying before milling and storage are essential parts of rice technology. Drying, under certain circumstances, may lead to rice cracking, which profoundly affects milling quality by promoting grain breakage. Rice drying, for this reason, plays a crucial role in the economy of rice milling, as does bad storage. Any wetting also causes similar crack in rice and is inherently harmful. © Woodhead Publishing Limited, 2011


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Rice quality

Degree of milling (DM) of rice The bran layer, including the germ, that lies between the husk and the endosperm of paddy, may constitute perhaps 6–8% by weight of the grain. Preferences for the extent to which this layer is removed during milling – viz. the degree of milling, DM – varies between different ethnic groups and market systems. The DM not only affects the aesthetic and nutritional value of the resulting rice but also profoundly influences its storage (infestation, fat hydrolysis, fat autoxidation), packing and flow behaviours (bulk density, porosity, angle of repose). Chemical constituents, rate of cooking, and breakage during milling of rice are also more or less affected. The DM is thus an important criterion of rice quality. Ageing of rice Age of rice after harvest profoundly affects its organoleptic and eating quality. Rice cooks to a soft and sticky texture soon after harvest, but progressively yields firmer and free-flowing cooked grains as it ages. Reasons for this change are not yet fully understood, although the subject has been investigated for decades. Various theories have been put forward, including enzyme action, starch changes, fat oxidation, changes in nonstarchy polysaccharides and cell walls, and changes in protein. Despite much effort, this remains one of the dark areas of rice chemistry – in fact the last frontier of rice research. Since ageing profoundly affects consumer acceptance and processing behaviour of rice, continued work to understand this phenomenon is necessary. Methods to prevent, delay or accelerate ageing of the grain need to be developed. Cooking quality Cooking of rice may mean different things to different people. The method of cooking, the rice–water ratio, the cooking system, the duration of cooking all differ. All of these affect the degree of softening, the grain elongation, curling, segmentation and breakage of the grain after cooking. They may also affect the water uptake and solids loss during cooking. The variety, the size and shape of the grain, any cracks and chalky areas in it, its gelatinisation temperature, amylose content and protein content may affect the cooking behaviour and the grain texture. Not much is known about these properties and more work is needed. Physicochemical properties and eating quality of rice This may be the most important aspect of rice quality, as the test of the pudding is ultimately in its eating. The underlying physicochemical factors also profoundly affect the processing and product-making quality of rice. The thousands of rice varieties differ in their eating quality, especially in their texture after cooking. (This difference is distinct from and in addition to the effect of rice age on its eating quality.) This is an aspect that attracted the attention of scientists for close to three-quarters of a century and is only recently being properly understood. Much research has been done. After



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a long journey involving amylose starch, amylopectin, starch structure, gelatinisation temperature, protein content, rheology of rice paste and its alkali digestion, the matter has now been largely attributed to the branch structure of the amylopectin starch. The protein content is also implicated. Effect of parboiling on rice quality Parboiling of rice is an ancient process of wide prevalence. It profoundly changes every aspect of rice property and quality. There are different systems as well as degrees of parboiling, which affect rice properties in different ways. A wide range of rice properties and quality can thus be obtained by combining varietal and age differences of rice with different methods of parboiling. A thorough understanding of the variables in the parboiling process, and their precise effects on different tissues, chemical constituents and properties of the rice grain, is thus important. Fortunately this area has been extensively investigated and an impressive science of parboiled rice has been built up. How parboiling modifies the state of the starch in rice, how the starch varies with the processing conditions, and how these states affect the product quality, are becoming understood. Parboiling also affects every other constituent, including protein, fat and sugars, and all these affect the rice quality in various ways. Product-making quality Although consumed primarily as boiled (cooked) rice, a fair proportion of rice is also converted into various products, especially snack items. Varieties differ greatly in this respect. Some varieties are suitable for making one product, other varieties for others. It is seen that manufacturers or users traditionally use only certain varieties for making specific products, based on age-old empirical experience. Only recently many of these aspects have been scientifically investigated and criteria established. An enunciation of these factors constitute an important aspect of rice quality. An interesting aspect of rice products is that the type of the products differs among the three broad subregions of rice – south, southeast and northeast Asia. While mostly heated, flaked, puffed and parched forms of products are made in south Asia, cooked forms and cakes are more generally made in the other two – more often from low-amylose rice in southeast and from waxy rice in northeast Asia. It is important to understand the quality of these products and their relation to the raw material rice quality. Speciality rice There are certain scented or aromatic rice varieties which are highly valued for their specific aroma. Basmati rice in particular has recently shot into fame as an important commodity in international trade, as has jasmine rice from Thailand. These rices have many unique features and are now playing a big role in international trade. Characterising and understanding these features



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Rice quality

are important. Other scented varieties too may become important in future. Tests for identifying scented lines and the specific cultivar are important. There are other speciality rices, viz. waxy rice, very low-amylose rice, coloured rice and so on, which have many unusual properties, including medicinal or physiological properties as claimed, and hence unconventional use. Understanding these properties and the suitability for use-value are important. Nutritional quality Being a food, and a staple food of vast millions of people, the ultimate value of rice lies in its nutritional quality. There are three aspects to this. One is the comparative nutritional quality of rice as a staple as compared with those of other foodgrains and other foods. The second is the varietal difference in nutritional value. The role of protein, lysine, B vitamins and other micronutrients, possible imbalance in certain nutrients such as calcium and phytin, and certain toxic constituents are relevant in this context. The third is the effect on the nutritional value of rice as a result of its processing, such as milling, curing, parboiling, puffing and cooking.

1.4.2 Inherent variability in rice Three zones and three broad categories of rice As we shall see in the following pages, rice shows a surprisingly wide range of properties, or quality characteristics. Other than variations originating from handling and processing and from environmental factors, much of it is inherent in the varietal, i.e. genetic, difference (Juliano and Pascual 1980). Interestingly, a large part of this genetic variation follows a broad geographical pattern. When viewed in the backdrop of the world as a whole, the ‘rice country’ undoubtedly looks like a rather small, compact place tucked away in the ‘eastern margin of the world’ (Fig. 1.1). Yet neither the rice nor the ‘rice country’ is really compact and homogeneous. The rice country can be conceived of as a crescent stretching from south Asia at one end to northeast Asia at the other (Fig. 1.2). There are three fairly distinct zones of rice within this crescent – south Asia, southeast Asia, and northeast and east Asia. The rice types in these three zones are distinct in terms of grain size and shape, amylose content, texture and eating quality after cooking, and product-making quality. The preferences of the people within the three zones for these qualities too became adjusted accordingly and so differ from each other. In south Asia, at one end of the crescent (India, Bangladesh, Sri Lanka, Nepal, Pakistan, perhaps part of Myanmar, Malaysia and Thailand too) (Fig. 1.2), the preference is for one kind of rice. These have rather smallish, longish and slender grains, and a high amylose content (≥26% on dry basis, db). These types yield a firm, dry and nonsticky texture after cooking. Interestingly, another clear preference here is for well-aged rice. In fact



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23

Each dot represents 10 000 ha

Fig. 1.2 Map of the rice country. Over 90% of the world’s rice is grown and consumed here. It can be depicted as a crescent stretching from south Asia at one end to northeast Asia at the other. The original map is reproduced from IRRI (2007). The crescent sign has been superimposed on the map by the author.

there is a preference here for even still more hard and free-flowing texture of cooked rice achieved by parboil-processing of the rice in parts of the region. Another preference in this region is more for flaked, puffed and parched whole grain rice products made from common rice than for cakes or cooked and formed rice products made from speciality rice. In northeast and east Asia at the other end of the crescent, the preference is for the opposite characteristics – short, round, glossy grains; low amylose (≤ 20%, db); soft and sticky texture when cooked; rice fresh after harvest (without ageing); and cakes and cooked and formed products made more often than not from waxy rice. The most dramatic, and amusing, contradiction between these two zones is in people’s preference for fresh or aged rice. If the people of south Asia wait expectantly for months after harvest time for their rice to age, those in Japan and Korea sigh that time is an enemy of the ‘freshness’ of their rice. In southeast Asia, in the belly of the crescent, everything is intermediate between these two extremes. If the location is intermediate, so by and large are all the characteristics of the rice and the preferences of the people.



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24

Rice quality

Subspecies classification of rice Geneticists, agriculturists and breeders have known for a long time that rice is not one single thing, so they classified rice into indica and japonica classes, adding another class later, javanica. Now indica is what we have discussed as the rice of south Asia and also southeast Asia in general. Broadly speaking, indica grains are small or long, generally somewhat slender, having a rather high amount of amylose (over 20%, db), and cooking into somewhat firm and nonsticky texture. Japonica, on the other hand, is the rice of the opposite zone, i.e., northeast and east Asia. This rice is broadly rather short and round, low in amylose and cooking to somewhat soft and sticky texture. Javanica is a class of rice found in the equatorial zone of Indonesia. It is of course good to remember that the above description may or may not hold in full. This is particularly true of indica, which has very wide variability, ranging from short and round to long and slender grains and has a very low to very high amylose content and corresponding eating quality. As an example, rice in the northeast and northwest mountainous regions of India, though indica, have the appearance and cooking-eating properties of japonica rice (Bhattacharya et al. 1980). To generalise further, it can be said that indica is the original rice belonging to the tropics. Japonica is the form rice adopted when it had to adapt to the temperate region (e.g. rice in Japan, Korea, northern China, Egypt, Italy, Spain, California, Australia). The above classification was later on superseded by another one propounded by Glaszmann (1987). He divided rice into six isozyme polymorphic groups. According to this new scheme, typical japonica rice (temperate japonica) belongs to Group VI. Javanica rice in this scheme is considered as tropical japonica and belongs to the same Group VI. The bulk of indica rice belongs to Group I. Groups II, III and IV – previously all classified as indica – are some special early-maturing, summer (Aus) and deep-water rices. The aromatic rice of the Indo-gangetic and sub-Himalayan regions belongs to Group V. One question arises for which there seems to be no answer. It is said japonica (low-amylose) rice is the form rice adopted when adapting to the temperate region. Why or how did the species get fairly neatly segregated into high-amylose indica in south Asia, intermediate-amylose indica in southeast Asia and tropical japonica (javanica) in equatorial Asia (Indonesia)? Did it happen simply by random human selection at the beginning? Or did some unknown factors in geography play a part? Rice cannot be easily pinned down to any single rule Having said all that, it is good to remember that contradictions and paradoxes are part of the nature of rice. There is contradiction between concentration (‘rice country’) and dispersal (whole world) of rice production, between a subsistence crop and a high-value export, between a grain that feeds the world but which is associated with poverty, between a frail but economically mighty grass, between food for survival and food for thought. That is rice.



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25

If one finds one rule that neatly fits rice in general, there would surely be one lot or one variety or one group that does not. Contrariness is the other name for rice. It is for these reasons that one can hardly ever make a watertight rule for any property of rice. There is a saying in a classical Sanskrit text that even the gods do not know the mind of a woman, not to speak of mere mortals. It is immaterial whether this statement has any basis, we quote it here only for its flavour. Well, rice is surely like that. To see it in another way, rice is born in ‘mud’, and mud is slippery. Being a child of mud, rice too is slippery. If one tries to pin it down here, it tends to slip out there. It is our duty to understand, classify and codify rice, but one need never be too smug of what one finds in one place or at one time or even several times. Rice will almost always surprise one in another place or at another time. To study rice, one must be respectful, patient and flexible.

1.5

References

bhattacharya k r, sowbhagya c m and indudhara swamy y m (1980), ‘Quality of Indian rice’, J Food Sci Technol, 17, 189−193. chang t-t (2003), ‘Origin, domestication, and diversification’, in Smith C W and Dilday R H (Eds) Rice: Origin, History, Technology, and Production, Hoboken, NJ, John Wiley & Sons, 3−25. childs n w (2004), ‘Production and utilization of rice’, in Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 1−23. glaszmann j c (1987), ‘Isozymes and classification of Asian rice varieties’, Theor Appl Genet, 74, 21−30. grist d h (1959), Rice, 3rd edn, Longman Group, London. hamilton r w (2003), The Art of Rice: Spirit and sustenance in Asia, Los Angeles, CA, UCLA Fowler Museum of Cultural History, University of California. irri (2007), Annual Report, Los Baños, Laguna, Philippines, International Rice Research Institute. juliano b o and pascual c g (1980), Quality Characteristics of Milled Rice Grown in Different Countries, IRRI Research Paper Series, International Rice Research Institute, Los Baños, Laguna, Philippines, No., 48, 1−25. maclean j l, dawe d c, hardy b and hettel g p (2002), Rice Almanac, 3rd edn, Los Baños (Philippines), International Rice Research Institute; Bouaké (Côte d’Ivoire), West Africa Rice Development Association; Cali (Colombia), International Center for Tropical Agriculture; Rome (Italy), Food and Agricultural Organisation. matsumoto k (2004a), ‘The power of settled life – Rice farming as a lifestyle’, plenary lecture, World Rice Research Conference, Tokyo, 5 November. Matsumoto K (2004b), ‘The civilization of mud’, Reitaku J Interdisciplinary Studies, 12 (2), 17−27. sachs j d, mellinger a d and gallup j l (2001), ‘The geography of poverty and wealth’, Scientific American, March, 70−75. storck j and teague w d (1952), A History of Milling: Flour for man’s bread, Minneapolis, University of Minnesota Press. strait d s, grine f e and moniz m a (1997), ‘A reappraisal of early hominid phylogeny’, J Hum Evolution, 32, 17-82.



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2 Physical properties of rice

Abstract: Physical properties of rice include grain dimensions, hardness, grain friction, density and thermal aspects. Knowledge of these properties is essential in handling, storage and processing of rice. Physical properties differ with variety, moisture content and degree of milling. Dimensional classification of rice is best done based on surface area per unit weight or normalised grain weight. Density of milled rice is constant at about 1.45 g/ml but that of paddy varies from 1.16 to 1.24 g/ml due to varying air space inside the husk. Bulk density is affected by grain shape: the more slender the grain, the more the porosity and less the bulk density. With increasing moisture, density and bulk density decrease in milled rice but increase in paddy. The latter is caused by the presence of empty space within the husk. Coefficient of friction increases with increasing moisture. White belly (chalkiness on ventral side) is caused by excessive grain width (>2.35 mm). Key words: rice dimensions, white belly, rice density, frictional coefficient, physical properties of rice.

‘Knowledge of the physical and mechanical properties of the rice grain is used in the planting, harvesting, drying, storing, milling, and processing of rice……. [These] data begin to reach their potential values only when they are put to use.’ Kunze et al. (2004)



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Physical properties of rice

2.1

27

Introduction

The physical properties of any grain, including rice, include all of its external or integral characteristics, such as its appearance (size, shape, smoothness, colour), weight, hardness, volume, flow properties and so on. It should also be understood that physical properties of grains include the properties both of an individual grain as well as of the grains in bulk or in a mass. For instance a grain has individual size, shape, density and hardness, but a mass of grains would have these in a different way, especially so with reference to such properties as density and thermal properties. Then again an individual grain may slide down an incline at a particular angle, but the same grains in bulk or mass may flow or slide at a different angle. Both of these behaviours constitute the grain’s physical properties. There is one more important point with respect to physical properties of rice. There are many properties which are strictly physical, in the sense that they are not chemical, yet are not usually considered among those properties which are normally categorised under the heading of physical properties. These include properties such as volume expansion and solids loss during cooking and texture of the cooked rice. Although these properties are strictly physical in nature, they are considered as part of either cooking properties or physicochemical properties. The same applies to the viscosity of a rice-flour paste, the hardness of a sample of cooked rice, the degree of elongation of a rice grain during cooking, or its volume expansion during cooking. Even milling behaviour or milling quality is strictly a physical property, but is considered separately. In other words the properties of rice that are included under the category of its physical properties are decided as much by exclusion as by inclusion. Thus a broad way of defining physical properties of rice would be to say that it includes those properties of the rice grain, either individually or in a mass, which are physical in nature but are not specifically included among other categories such as cooking properties, chemical properties and physicochemical properties. Physical properties of rice are of paramount importance in all activities of production, preservation and utilisation of rice. A knowledge of the physical properties of rice is necessary in all activities from harvesting, drying, handling and storage to milling, packaging, marketing, cooking, product-making and utilisation. It is needed in the design of all relevant equipments required for all the above activities, in transportation, storage, handling, conveying (from one equipment to another), in any type of processing including milling and cooking, and in packaging and marketing. Physical properties of grains, including rice, are thus important components of their quality. To illustrate, knowledge is needed of the size, shape and density of the individual grains to design a screen or aspiration system for its cleaning; of its frictional properties to design the angle of the chute or hopper through which the grain has to flow during its processing; of the density of the rice in mass (i.e. the density of a quantity of grains as they are in a bulk, in other words the bulk density) to design the receiving bin or the storage bin; of the © Woodhead Publishing Limited, 2011


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28

Rice quality

porosity of its mass to know how much water will be needed per unit weight of the material for its wet processing; of the surface friction and shape and size of the grain in mass to design a vibrating system to separate its two grain forms, viz. paddy and brown rice; and so on. Similarly the thermal properties of the rice in mass must be known if the rice is to be processed, for instance, for parboiling or preparing quick-cooking rice. Thus although we generally tend to take the physical properties of rice, or any grain, rather casually or more or less for granted, their precise knowledge is quite important in its handling and processing. We can say in another way that we are using these properties, or quality attributes, at all times often without being really conscious of it. While discussing physical properties of rice, and grains in general, it may not be out of place to point out another interesting issue. The properties that are considered as physical properties of grains are traditionally considered more a preserve of the engineer (agricultural engineer in particular) than that of the scientist (especially the cereal chemist). It is not clear why this distinction has arisen. Probably, by tradition, measurement of physical properties involve use of concepts and gadgets – and equations – which are usually more the preserve of the engineer. Probably the engineer is thought to measure something in the grain as such (dry lab) while the cereal chemist treats the grain with water or something (wet lab). But obviously these distinctions are professional, rather than substantive. There is another aspect to it. There is a subtle difference in the approach of the two professions. Engineers are more doers, hence they need data. They cannot design equipment or a gadget or a facility without solid, usable data. So when gathering information on physical properties, engineers traditionally give importance to individual pieces of data – for instance, the actual length or angle or shape or temperature or weight of a specified variety of a specified crop, even when these properties may differ or change either with time or among a group of varieties – without giving too much thought to find out why or in what way these may differ or change. The scientist, on the other hand, is by nature more a theoriser, and therefore by inclination gives more importance to the relation between data, the reasons or patterns of their changing nature, rather than on the actual data themselves. This distinction – perhaps exaggerated, if somewhat amusing – is often visible if one compares the treatment of the subject in an article or treatise by an engineer on the one hand and by a scientist on the other: one tends to remain in the specific, the other tends to raise it to the general. But all these distinctions are unnecessary and meaningless since both approaches are needed in real life. If one cannot do without actual, usable data for here and now, one cannot go forward and predict or invent things for tomorrow if one does not theorise on their relation. It will be the endeavour of the author – more inclined towards the second approach by training – to keep both the approaches in mind in the presentation below. The difficulty continues in the literature as well. Agricultural engineers,



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29

who are the ones who mostly carry out this type of work, mostly publish their results in agricultural engineering journals and transactions which are, alas, often not easily accessible to grain chemists. Conversely grain chemists generally publish their work in cereal science and food science journals which, by virtue of their profuse number, agricultural engineers often find it beyond their time or ambit to consult. So unfortunately the two sciences tend to develop in parallel without crossing each other’s path too often. The problem is compounded further by the fact that rice chemists do not generally, by virtue of the nature of their training, spend too much time in studying the physical properties of rice. These problems are real in the case of physical properties of rice. A majority of the studies by agricultural engineers in this field are presented in journals and meetings that are not easily accessible to us. Fortunately these have been extensively reviewed by Kunze et al. (2004) which, for that reason, will be repeatedly referred to in the following presentation. In fact a few illustrative data are quoted in Table 2.1 from their review and will be referred to in appropriate places. Conversely studies of physical properties of rice by chemists are indeed few and far between. One of the few, a rather comprehensive but old study, and covering aspects not usually covered by engineers, happens to be by the present author and his colleagues (Bhattacharya et al. 1972). This study too will be quoted often in the following; some key data from the study are presented in Table 2.2.

2.1.1 Range of physical properties of rice The following may be considered as a fairly comprehensive list of properties that would fall under the rubric of physical properties of rice: • • • • • • • • •

grain size and shape (dimensions), mass (weight); density; bulk density, porosity; colour; angle of repose, coefficients of static and dynamic friction; hygroscopic properties (equilibrium moisture content (EMC), hygroscopic conductivity and diffusivity); thermal properties (specific heat, conductivity, coefficient of expansion, diffusivity); mechanical properties (tensile, bending and compressive strength, hardness); and grain chalkiness.

These are not self-contained, self-sufficient properties. Most of these are affected by other variables, viz. • • •

variety; moisture content; degree of milling; © Woodhead Publishing Limited, 2011


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Rice quality

Table 2.1

Some physical properties of ricea

Property

Grain dimensions: Length (mm) Breadth (mm) Thickness (mm) Volume (mm3) Density (g/ml) Bulk density (g/ml) Coefficient of linear expansion (brown rice)

Value Bluebonnet 50 paddy, at mc

Sunbonnet brown rice, at mc

13.6

21.9

10.5

23.5

9.68 2.59 1.90 18.36

10.03 2.69 1.98 19.66

7.06 1.98 1.62 11.83

7.42 2.04 1.69 14.17

1.365

1.381

1.442

1.379

0.587

0.616

0.674

0.663

0.00405/percent moisture content (db)

mc (% db)

Coefficient of friction 12.6 on sheet steel (paddy) 21.4 36.2

Normal load Static (N) friction

89 267 89 267 89 267

0.200 0.182 0.255 0.239 0.302 0.277

Specific heat

1.0509 + 0.03835 M J/g °C 1.2686 + 0.02834 M J/g °C 1.2477 + 0.02797 M J/g °C

Conductivity

0.0894 + 0.000958 M W/m °C 0.10102 + 0.00308 M W/m °C

Coefficient of cubic thermal expansion (milled rice)

2.403 ¥ 10–4/°C below 53 °C 3.367 ¥ 10–4/°C above 53 °C Moisture content (% db)

Tensile and 6.0 compressive strengths 11.7 (Bluebell 16.2 brown rice) 19.3

Dynamic friction at speed (cm/s) 1.91

4.45

0.176 0.158 0.207 0.182 0.306 0.276

0.186 0.177 0.218 0.191 0.253 0.275

Breaking force (N) under Tension

Compression

19.154 16.659 11.819 7.736

237.97 187.43 121.68 92.84

a Compiled from various data quoted by Kunze et al. (2004) from various sources. Used by permission. mc, M = moisture content (dry basis, db%)



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Physical properties of rice Table 2.2

Summary of various properties of paddy and rice

Parametera No. of Paddy and unit samples Range L, mm B, mm T, mm L/B B/T w, mg D, g/ml

DB, g/ml P, % VS, ml/g VB, ml/g AR,°

31

21 21 21 21 21 21 4b 16c 1d 21 21 21 21 14

6.14−10.84 2.28−3.50 1.59−2.26 2.00−3.96 1.33−1.59 14.4−32.7 1.174−1.194 1.207−1.229 1.196 0.563−0.642 46.2−54.2 0.804−0.852 1.556−1.777 34.0−38.25

No. of Milled rice samples Range

Mean ± S.D.

– – – – – – 1.182 ± 0.010 1.224 ± 0.009

23 23 23 23 23 23 21

3.99−7.66 1.71−2.85 1.43−2.01 1.57−3.50 1.19−1.47 11.00−23.9 1.445−1.456

– – – – – – 1.452 ± 0.004

– – – – 36.5 ± 1.5

23 23 23 23 14

0.777−0.847 41.5−46.4 0.687−0.692 1.180−1.287 37.0−38.25 35.0−37.6

– – 0.689 37.5 ± 0.5 36.5

Mean ± S.D.

Adapted, with permission, from Bhattacharya et al. (1972) John Wiley and Sons. a L = length, B = breadth (width), T = thickness, w = grain weight, D = density, DB = bulk density, P = porosity, VS = specific volume (= l/D), VB = bulk volume (= l/DB), AR = angle of repose. b Short and round grains (L/B, 1.99−2.12). c All other grains (L/B, 2.26−3.96). d IR 8 variety (L/B, 2.84).

• •

temperature; and sometimes also by the age of the rice after harvest.

In other words these are not independent constants, but are dependent on the circumstances in which the grains exist. Thus parameters such as dimensions, density, hardness, friction and mechanical properties not only vary from variety to variety but are also affected by the moisture content of the grain and its degree of milling, and also to a small extent by temperature. Further, all these properties have to be considered in terms of the three grain forms, viz. • • •

paddy (rough rice); brown rice (dehusked rice); and milled rice.

While paddy is handled mostly in and around the area of its production or storage, brown rice is rarely handled except during the course of processing. Trade is mostly in the form of milled rice. The question of degree of milling becomes relevant in the latter, because brown rice is not necessarily fully milled in all countries but is often undermilled in many markets and, rarely, also sometimes overmilled. The effect of degree of milling on various properties will be discussed in Chapter 4.



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32

2.2

Rice quality

Grain appearance

2.2.1 Grain size and shape The length (L), the breadth (B) (we prefer to use the term breadth rather than width, because we wish to reserve ‘w’ for grain weight and ‘W’ for water uptake) and thickness (T) of rice grains have been measured and reported by numerous workers in connection with diverse studies. All the parameters vary widely among varieties. While considering the dimensions of world’s rice, one must realise that the standardised dimensions of the American varieties, most widely studied and reported in the literature for obvious reasons, have been deliberately bred into in their produce and have no relevance to the randomly prevailing dimensions of rice in the world in general. As will be discussed later in more detail, the few American varieties that are grown and traded at any point of time are so bred as to have their dimensions and combinations of dimensions within strict limits (Adair et al. 1973, Webb 1985). In general exporting countries such as Thailand (Bhattacharya 1987) too rigorously control the attributes of their exportable grains. So do basmati rice exporters, India and Pakistan, with respect to their basmati exports (Kamath et al. 2008). But the thousands of varieties of rice grown in the world other than these ‘privileged’ few present a completely different picture. Their grain length, breadth and thickness dimensions do not follow the deliberately bred and selected, standardised pattern of the USA or other exported rice but come in all sorts of combinations. India, the second largest producer of rice in the world, but never an exporter until the basmati rice trade from the last two decades of the twentieth century, has traditionally had her huge produce come in random combinations of grain size and shape (and also chemical make-up). In that sense Indian rice is a reasonably representative microcosm of world’s rice (specially Indian indica rice, her main produce) and their dimensions give a fair idea about the range of these properties encountered in really existing rice varieties. In a careful study of 23 varieties (including two japonica [ponlai], two dwarf indica and one indica ¥ japonica cross, besides the traditional Indian indica), Bhattacharya et al. (1972) found the range of their various physical properties as shown in Table 2.2. The combinations of the three grain dimensions, including grain mass, were quite random. In a later study of 172 varieties (which comprised 129 traditional [pregreen revolution of late 1960s and early 1970s] Indian indica rice, 28 modern semidwarf Indian indica crosses, and the rest japonica, javanica and US varieties), the range of grain dimensions and mass for milled rice encountered (Bhattacharya and Sowbhagya 1980, Bhattacharya et al. 1982, Sowbhagya et al. 1984) were: L: 4.0–7.7 mm L/B ratio: 1.57–4.35 w: 7.7–27.8 mg



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33

In a recent study of basmati milled rice (Kamath et al. 2008) the highest grain length and the most slender grain shape (L/B ratio) encountered were: L: L/B:

7.63 mm, 4.48

As a consultant to a Government of India Committee set up to advise on rice grain classification, the present author had the opportunity some 25 years ago to broadly examine the L and B dimensions and grain weight (w) data of several hundreds of Indian varieties (unpublished). The range of values of the parameters was again high and L, B and w values came in all combinations. Juliano et al. (1964) in their study of rice from southeast Asia, the other heartland of rice, had measured the dimensions of 55 southeast Asian rice varieties of paddy. Converting these dimensions approximately to those of the corresponding milled rice (see below), the values showed that the pattern of their dimensions was fairly similar to that of the Indian varieties mentioned above (Bhattacharya and Sowbhagya 1980). The range of the values were smaller but broadly similar: 4.0–7.5 mm 1.75–4.15 11.2–19.6 mg

L: L/B: w:

While the range of the dimensions as also of their combinations in world’s rice was thus large, it is necessary to emphasise that some of these variations cannot be entirely random. After all, all varieties of rice belong to a species (referring here to Oryza sativa L.) and hence are bound by certain genotypic limitations. In fact the above study (Bhattacharya et al. 1972) showed that the certain dimensions of the rice grain had some interesting interrelations. To start with the grain length, it was an independent variable not related to the other parameters – except that the least and the highest values encountered were 4.0 and 7.7 mm (milled rice), respectively. So broadly was grain weight, except that it was obviously related to the total dimension and volume and density. But the breadth (B) was well correlated with thickness (T) both in paddy and in rice, the approximate relations being TP = 0.44 BP + 0.70

2.1

TR = 0.54 BR + 0.45

2.2

where the subscripts P and R represent paddy and rice (milled) respectively. Similarly all dimensions of milled (or brown) rice were closely related to those of the corresponding paddy: LR = 0.78 LP – 0.80

2.3

BR = 1.06 BP – 0.72 (slender grains)

2.4

BR = 0.81 BP (broad and round grains)

2.5



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34

Rice quality TR = 1.02 TP – 0.31

2.6

(L/B)R = (L/B)P – 0.35

2.7

(B/T)R = 0.85 (B/T)P + 0.11

2.8

wR = 0.73 wP

2.9

Clearly the dimensions of the internal kernel are largely set by, logically, those of the outer grain (paddy). The relation (2.6) implied that most rice grains were just over 0.3 mm thinner than the corresponding paddy grain. Similarly relation (2.7) showed that the L/B ratio of a rice kernel was usually a value of 0.35 lower than that of the corresponding paddy grain. Relation (2.9), of course, meant that the average milled rice outturn from paddy was 73%. The breadth of paddy and that of milled (or brown) rice too were similarly mutually interrelated. In this case, interestingly, there was a break in the regression line at the kernel breadth of 2.3 mm, thus producing two relations, one below and the other above this point. Relatively slender grains having a kernel breadth of 2.3 mm or less showed the relation (2.4), while the broader grains were governed by the relation (2.5). A little calculation will show that the difference in grain breadth between paddy and milled rice, i.e., the huskkernel gap, in either case thereby comes to about 0.55−0.60 mm, which is probably a structural requirement. For relatively slender rice, equation (2.5) would provide too little husk–kernel gap, and so would equation (2.4) for broader grains, which may be morphologically incompatible. This kernel B value of 2.3 mm may be a significant dividing line for more than one reason. There is one other property which also showed a distinct difference below and above this kernel breadth. Namely, grain white belly, where brown rice grains over 2.35 mm broad almost invariably had white belly, while those below usually had none (Bhashyam and Srinivas 1981, Murugesan and Bhattacharya 1994); this aspect will be discussed in greater detail later. In each case a few samples departed from the indicated relations, showing that these relations were general and not invariable. Two or three of these were common exceptions in all or most relationships, showing that these varieties had unusual dimensions; but these were not confined to any grain size (L, w) or shape (L/B). A recent study involving 13 varieties/lines of basmati rice and its crosses (all exceptionally long and slender) (some 100 samples), carried out at the Rice Research and Development Centre, Mysore (RRDC), revealed some interesting relationships between the dimensions of paddy, brown rice and milled rice (Kamath et al. 2008). These data are shown in Table 2.3.

2.2.2 Dimensional classification of rice This diversity of dimensions in world’s rice mentioned above emphasises the need to evolve systems of dimensional classification of rice. There are two ways to go about it.



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Table 2.3 Proportionality among some physical properties of paddy, brown rice and milled ricea Property

Paddy

Brown rice

Milled rice

Grain length Grain breadth Grain thickness L/B Grain weight Bulk density Grain hardness

100 100 100 100 100 100 100

74 83 89 89 81 143 52

70 79 85 89 74 149 55

Adapted, with permission, from Kamath et al. (2008) John Wiley and Sons. a In 13 basmati rice varieties/lines (~100 samples).

One is to classify each dimension, especially length (L) and shape (L/B). Such systems exist, as prescribed by the United States Department of Agriculture (USDA), the International Rice Research Institute (IRRI) and the Food and Agriculture Organisation (FAO), which have been reviewed by Bhattacharya and Sowbhagya (1980). These involve the classification of grain length into extra long, long, medium and short classes and shape (L/B) into slender, medium, bold and round classes (Table 2.4). But this classification is of limited use. It can be used mainly for research purposes or for providing certain grade specifications for marketing. The classifications already existing are quite satisfactory for this purpose, although a survey of over 200 varieties showed some lacuna, based on which a somewhat improved one was proposed (Bhattacharya and Sowbhagya 1980) (Table 2.4). The more important need is to classify not dimensions but grain types, i.e. varieties, for marketing. How does one classify the thousands of varieties of world’s rice with random combinations of dimensions and grain weight into a few classes to indicate their market rating in terms of dimensions? US rice, as already mentioned, is classified into three classes, long, medium and short grain, but this classification is only notionally based on grain length. The classification is based not on the length dimension alone – as often wrongly understood by the not so knowledgeable from the notional nomenclature – but on the combination of a whole lot of characteristics, viz. grain length, shape, amylose content, alkali score and so on (Adair et al. 1973, Webb 1985) (Table 2.5). In other words this classification is a comprehensive quality indicator not only in terms of the appearance but also of the cooking and processing qualities of the rice. Although not applicable to their domestic rice in general, many exporting countries often employ the same or similar nomenclature for the export portion of their rice. However it should be remembered that this practice of applying the same nomenclature to non-US export rice is often in a sense a misnomer, for the names ‘long grain’, etc. apply at best to their dimensions – perhaps only to their length dimension – and not at all necessarily to their cooking and processing qualities.



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Brown rice

Milled rice

Brown rice

Milled rice

Adair et al. (1973)

FAO (1972)

IRRI (1975)

Proposed

Extra long Long Medium Short

EL L M S

– – – –

–

Short


– – –

EL L M S

Extra long Long Medium


Over 7.0 6.0−7.0 5.0−5.99 Less than 5.0

Over 7.50 6.61−7.50 5.51−6.60 5.5 or less

Less than 5.0

7.0 or more 6.0−6.99 5.0−5.99

Over 7.5 6.61−7.5 5.51−6.6 5.5 or less

Slender Quasi-slender Bold Round

Slender Medium Bold Round

Slender Bold Round

Slender Medium Bold

Class

Specification

Class

Abbr

Shape (L/B)

Length (mm)

Reproduced from Bhattacharya and Sowbhagya (1980).

Grain form

Classification of size and shape of rice

Reference

Table 2.4

s q b r

– – – –

– – –

– – –

Abbr

Over 3.0 2.4−3.0 2.0−2.39 Less than 2.0

Over 3.0 2.1−3.0 1.1−2.0 1.1 or less

Over 3.0 2.0−3.0 Less than 2.0

Over 3 2.1−3 Upto 2.1

Specification

Abbr

Giant Big Small Tiny

G B S T

Extra heavy – Heavy – Moderately – heavy

Class

Grain weight (mg)

Over 23 18.1−23 12−18 Less than 12

Over 25 20−25 Under 20

Specification

Physical properties of rice Table 2.5

37

Characteristics of US rice

Characteristics

Grain type

Lengtha (mm) L/B ratioa Grain weighta (mg) Amylose content (%) Alkali spreading value, average Gelatinisation temperature (°C) Water uptake at 77 °C, ml/100 g Parboiling-canning stability, solids loss %

Long

Medium

Short

6.61−7.5 3.1 and over 15−20 23−26 3–5 71−74 121−136 18−21

5.51−6.6 2.1−3.0 17−24 15−20 6–7 65−68 300−340 31−36

Up to 5.5 2.0 and less 20−24 18−20 6–7 65−67 310−360 30−33

Compiled from data quoted by Webb (1985), used with permission. a Refers to brown rice; the remaining characteristics are for milled rice.

Be that as it may, the above system clearly leaves perhaps 90% or more of world’s rice outside a prescribed system of classification. As we have seen above, the world’s rice has in general diverse combinations of dimensions, because of which if applied to these rices, the above classification would be meaningless even in terms of dimensions alone, not to talk of cooking and processing qualities. What scheme is then one to adopt for such rices? This problem has been confronted especially in India for a very long time and various attempts have been made to evolve a valid system of classification for marketing. Although this effort has not necessarily been overly successful, there is a long history of this effort which is discussed in some detail below, for it may have some relevance for other countries too or for other grains or granular material. In India there is a preference for ‘fine-grained’ as opposed to ‘coarsegrained’ rice, and the former commands a premium in the market. So there have been several attempts to classify the hundreds or perhaps thousands of prevailing rice varieties into a few well-defined classes, viz. superfine, fine, medium (or common) and coarse on some specified basis. These efforts have been reviewed by Bhattacharya et al. (1982). The tasks were assigned to successive committees appointed by the Government (comprising mostly tenured and nontenured civil servants) over many decades but their outcomes were by definition never perceived to be wholly satisfactory, for otherwise there would have been no need to have successive committees! Mostly the committees could hit upon nothing better than the L/B ratio for the classification, one perhaps specifying, say, a ratio of 3.00 as the dividing line between fine and common classes, and the next committee, faced with vociferous protests from affected groups, revising it, after protracted discussion by the perhaps half a dozen members over several months over lists of hundreds of varieties, down to 2.5! The Ramiah Committee, the only committee to be headed by a scientist rather than a tenured administrator, for the first time realised that shape



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alone could not be a valid basis for classification (we can compare a walking stick and a needle, both with a very high L/B ratio, yet entirely different in their perception). So the Ramiah Committee decided to include length into their scheme in addition to shape. Unfortunately, having limited its technical personnel to only agricultural scientists, the Ramiah Committee could devise nothing better than to just juxtapose the two parameters of lengths and shapes to each other, to end up with five classes (long slender, short slender, medium slender, long bold and short bold) (Table 2.6), which were not only unwieldy but also inconsistent and unusable. For the system of juxtaposition failed to provide a continuous scale that moved smoothly from one class to another but jumped along instead with starts, stops and about-turns (see discussion in Bhattacharya et al. 1982). Finally a Shukla Committee was appointed in 1979 which consulted for the first time a nonagricultural rice scientist, a chemist-technologist, in the form of the present author. On examination it was clear to us that to evolve a continuous single scale of fineness or coarseness for a granular material of diverse size and shape, it was necessary to integrate both the parameters of size (length and breadth, or grain weight) and shape (L/B) into a single entity or number. Mathematically there was only one parameter which did that, viz. surface area per unit weight (S, cm2/g). This was logical too, for, first, the smaller a particle, the greater its S (consider a piece of chalk being broken into pieces). Second, a sphere had the least S, the surface area increasing as it was being deformed (consider a dough ball being rolled into a flat bread). In fact the degree of departure of a particle from the shape of a sphere, viz.sphericity, can be calculated from its surface area (Brown et al. 1950): sphericity =

surface f area off a sphere off equivallent volum me surface f area off the parrticle

Clearly this integrated parameter S could take care of both the grain size and grain shape of rice varieties. The formula of Husain et al. (1968) Table 2.6 Classification of rice varieties as per Ramiah Committee Class

L (mm)

L/B ratio

Long Slender Short Slender Medium Slender

≥6 6 or < 4.5 ≥6 20% of area (Ikehashi and Khush 1979). Other workers have used different scoring systems depending on the work at hand (e.g., Indudhara Swamy and Bhattacharya 1982). There is a clear distinction between the origin of white core and white belly. White core chalkiness seems to be genetically controlled, since some varieties habitually have white core grains while others do not. For instance, basmati rice and their derivatives generally have roughly 50% of the grains with a small white core. In fact that is considered one of the characteristic properties of basmati rice (Kamath et al. 2008).

2.6.1 White belly In contrast to white core above, white belly seems to be entirely dependent on grain breadth (width). This conclusion was repeatedly demonstrated from the1980s from the Central Food Technological Research Institute (CFTRI), Mysore, India (Bhashyam and Srinivas 1981, Srinivas et al. 1984, 1985, Raju and Srinivas 1991), but one curiously not generally often seen referred to in the literature. The CFTRI scientists showed that white belly, i.e., chalky area on the ventral side, was entirely dependent on grain width: All grains over 2.8 mm in breadth invariably had white belly; among grains with 2.0−2.8 mm width some had but others did not; and varieties with width < 2.0 mm never had white belly (Fig. 2.6). Srinivas et al. (1985) speculated that as the entry of nutrients into the caryopsis for laying down the grain substance was through the pigment strand on the dorsal side, the nutrient had to cross the entire grain width to lay down grain material at the ventral side. So wherever the grain width was too large, the nutrient supply got blocked or exhausted by the time it reached the ventral boundary, resulting in the white belly. Be that as it may, there is no doubt that the white belly property is related to the rice grain width. As a matter of fact Raju and Srinivas (1991) observed in Jaya variety, one notorious for its large white belly, that when the caryopsis in a part of a panicle was constrained by applying aluminium rings to reduce the grain width to 2.2 mm from its usual 2.7 mm, the white belly was absent in these grains.



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55

Fig. 2.6 Photograph showing relatively slender milled rice kernels with no white belly on the left, and wide milled kernels with prominent white belly on the right. 100

White belly grain, %

80

60

40

20

0 1.7

Fig. 2.7

2.1 2.5 Breadth of brown rice, mm

2.9

Dependence of white belly chalkiness on grain breadth in rice. Reproduced, with permission, from Murugesan and Bhattacharya (1994).

This conclusion about white belly in rice being related to grain width was confirmed later in a study of popped rice by Murugesan and Bhattacharya (1991, 1994). They observed that white belly in rice was detrimental to its popping. Interestingly when the incidence of white belly among varieties was plotted against their grain breadth, a brown rice breadth of 2.35 mm emerged as a distinct dividing line. Grains broader than this threshold value had high incidence of white belly, while those narrower had little or none (Fig. 2.7).



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2.6.2 Relation of chalkiness to growing temperature Apart from the above proximate origin of the two types of grain chalkiness, grain chalkiness is undoubtedly related to the environmental conditions in which the crop is grown. High night temperatures during grain filling has been repeatedly shown from as early as the 1960s (Stansel et al. 1961) to favour chalkiness. Bangwaek et al. (1994) grew three varieties in controlled day/night temperature regimes of 22/15, 25/15, 30/20 and 35/20 °C after flowering. Grain chalkiness was minimal in the first three regimes but very high in the last. Similarly Lisle et al. (2000) grew three cultivars at low (26/15 °C) and high (38/21 °C) day/night temperatures. Rice grown at the high temperature contained more chalky grains; and grains at inferior position had more chalkiness than in superior position. Starch granules were loosely packed in the chalky region as compared to the tight packing in the translucent region. Resurreccion and Fitzgerald (2007) observed in several varieties that high temperatures induced chalk. They thought that high temperatures affected the translocation of nutrients to the grain. Counce et al. (2005) tested two varieties, growing them in controlled climate conditions at high (24 °C) and low (18 °C) temperatures between midnight and 5.00 am. High temperatures reduced grain width and both total and head rice yields, and increased the amount of amylopectin chains having 13−24 glucose units. Finally Cooper et al. (2008) grew several varieties at 18, 22, 26 and 30 °C between midnight and 5.00 am. As the night temperature increased, grain chalkiness increased, head rice decreased, grain dimensions generally decreased and amylose content tended to decrease.

2.6.3 Effect of grain chalkiness It has been well demonstrated that grain chalkiness is related to loose starch-granule packing at the chalky region leaving some air spaces; the loosely packed granules scatter light and hence cause the appearance of opacity, i.e., chalkiness. Accordingly chalky grains are clearly softer than translucent ones (Table 2.7); besides, grains with appreciable chalkiness have been found to have lower grain density than vitreous kernels (RRDC unpublished). Chalky grains also have other grain characteristics, viz. slightly lower amylose content (Sandhya Rani and Bhattacharya 1989, Kim et al. 2000) and a higher room-temperature water absorption (Bhattacharya et al. 1979, Kim et al. 2000). In one particular basmati crop (1998 crop year), when due to environmental factors the night temperature remained somewhat higher than normal throughout the flowering and post-flowering period, the entire crop had unusually large areas of chalkiness and also other unusual properties, including lower amylose content (RRDC, unpublished). Kim et al. (2000) examined chalky and vitreous kernels in one variety. The former were slightly smaller than the latter in all dimensions, besides having a slightly lower amylose content and a slightly higher water absorption index



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57

(WAI) compared to the latter. The effect of grain chalkiness on various quality attributes of rice has also been investigated. Patindol and Wang (2003) studied the fine structure and physicochemical properties of rice starch from chalky and translucent kernels in six varieties. (Their definition of chalky kernels were those that had > 50% chalky area; and kernels having < 50% chalky area were considered as translucent. They also included a waxy rice in the study and some less opaque grains among these were considered as translucent.) Starch from chalky kernels had less amylose (amylose defined as low-molecular weight fraction from chromatographically separated starch); more very short-chain and less long-chain branches in their amylopectin; greater crystallinity (in X-ray diffractograms); more breakdown and less setback in their pasting pattern; and marginally higher gelatinisation temperature and enthalpy (in their differential scanning calorimetric thermogram). The authors also quoted a few earlier (especially Japanese and Chinese) works reporting somewhat similar data. Lisle et al. (2000), in their work mentioned above, also showed that chalky and translucent grains differed in their starch composition and structure as well as in cooking properties. As mentioned before, Sandhya Rani and Bhattacharya (1989) too found difference in the amylose content, slurry viscosity and some other properties between chalky and translucent kernels. Effect of grain chalkiness on rice milling quality will be discussed in the next chapter.

2.7

References

adair c r, bollich c n, bowman d h, jodon n e, johnston t h, webb b d and atkins j g (1973), ‘Rice breeding and testing methods in the United States’, in Rice in the United States: Varieties and Production. United States Department of Agriculture, Handbook 289 (revised), 22−75. anonymous (2004), Medicinal and Aromatic Rices: Compendium of Papers, August 2004, Kerala Agricultural University, Thrissur, Kerala, India. bandyopadhyay s and roy n c (1977), ‘Studies on swelling and hydration of paddy by hot soaking’, J Food Sci Technol, 14, 95−98. bangwaek c, vergara b s and robles r p (1994), ‘Effect of temperature regime on grain chalkiness in rice’, IRRN (International Rice Research Newsletter), 19 (4), 8. bautista r c, siebenmorgen j and cnossen a g (2000), ‘Characteristics of rice individual kernel moisture content and size distribution at harvest and during drying’, in Proceedings of International Drying Symposium (IDS2000), Amsterdam, Elsevier Science, paper 325. bhashyam m k and srinivas t (1981), ‘Studies on the association of white core with grain dimension in rice’, J Food Sci Technol, 18, 214−215. bhattacharya k r (1987), ‘How Thailand maintains her rice quality: lessons for India’, Indian Food Ind, 6, 171−175. bhattacharya k r (2004), ‘Parboiling of rice’, in: Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 329–404.



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bhattacharya k r and sowbhagya c m (1980), ‘Size and shape classification of rice’, Riso, 29, 181−185. bhattacharya k r, sowbhagya c m and indudhara swamy y m (1972), ‘Some physical properties of paddy and rice and their interrelation’, J Sci Food Agric, 23, 171−186. bhattacharya k r, indudhara swamy y m and sowbhagya c m (1979), ‘Varietal difference in equilibrium moisture content of rice and effect of kernel chalkiness’, J Food Sci Technol, 16, 214−215. bhattacharya k r, ramesh b s and sowbhagya c m (1982), ‘Dimensional classification of rice for marketing’, J Agric Engg, 19 (4), 69−76. brown g g et al. (1950), Unit Operations, Bombay, Asia Publishing House, 213. candole b l, siebenmorgen t j, lee f n and cartwright r d (2000), ‘Effect of rice blast and sheath blight on physical properties of selected rice cultivars’, Cereal Chem, 77, 535−540. chattopadhyay p k and hamann d d (1994), ‘The rheological properties of rice grain’, J Food Process Engg, 17, 1−17. cooper n t w, siebenmorgen t j and counce p a (2008), ‘Effects of nighttime temperature during kernel development on rice physicochemical properties’, Cereal Chem, 85, 276−282. counce p a, bryant r j, bergman c j, bautista r c, wang y j, siebenmorgen t j, moldenhauer k a k and meullenet j f c (2005), ‘Rice milling quality, grain dimensions, and starch branching as affected by high night temperatures’, Cereal Chem, 82, 645−648. fan j, siebenmorgen t j, gartman t r and gardisser d r (1998), ‘Bulk density of long- and medium-grain rice varieties as affected by harvest and conditioned moisture contents’, Cereal Chem, 75, 254−258. fao (1972), ‘Report of the seventh session of the sub-group on rice grading and standardization’, CCP: RI 72/12, Food and Agricultural Organisation of the United Nations, Rome, p. 11. goi (1968), Report on the Classification of Rice, New Delhi, Ministry of Food, Agriculture, Community Development & Cooperation (Department of Food), Government of India. halverson j and zeleny l (1988), ‘Criteria of wheat quality’, in Pomeranz Y (Ed.) Wheat: Chemistry and technology, Vol. I, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 15−45. hlynka i and bushuk w (1959), ‘The weight per bushel’, Cereal Sci Today, 4, 239−240. husain a, agrawal k k and pandya a c (1968)’, ‘Physical properties of wheat and paddy’, Harvester (Kharagpur, India), Anniversary No, 66. ikehashi h and khush g s (1979), ‘Methodology of assessing appearance of the rice grain, including chalkiness and whiteness’, in Chemical Aspects of Rice Grain Quality, Los Baños, Laguna, Phillippines, International Rice Research Institute, 223−229. indudhara swamy y m and bhattacharya k r (1982), ‘Breakage of rice during milling. 4. Effect of kernel chalkiness’, J Food Sci Technol, 19, 125−126. irri (1975), Standard Evaluation System for Rice, International Rice Research Institute, Los Baños, Philippines, p. 64. juliano b o, cagampang g b, cruz l j and santiago r g (1964), ‘Some physicochemical properties of rice in south-east Asia’, Cereal Chem, 41, 275−286. kamath s, stephen j k c, suresh s, barai b k, sahoo a k, reddy k r and bhattacharya k r (2008), ‘Basmati rice: Its characteristics and identification’, J Sci Food Agric, 88, 1821−1831. kamst g f, vasseur j, bonazzi c and bimbenent j j (1999), ‘A new method for the measurement of the tensile strength of rice grains by using the diametral compression test’, J Food Engg, 40, 227−232. kim s s, lee s e, kim o w and kim d c (2000), ‘Physicochemical characteristics of chalky



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kernels and their effects on sensory quality of cooked rice’, Cereal Chem, 77, 376−379. kuhn t s (1962), The structure of Scientific Revolutions, Chicago, The University of Chicago Press. kunze o r and calderwood d l (2004), ‘Rough-rice drying – Moisture adsorption and desorption’, in Champagne E T (ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 223−268. kunze o r, lan y and wratten f t (2004), ‘Physical and mechanical properties of rice’, in Champagne E T (ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 191−221. lisle a j, martin m and fitzgerald m a (2000), ‘Chalky and translucent rice grains differ in starch composition and structure and cooking properties’, Cereal Chem, 77, 627−632. lu r and siebenmorgen t j (1995), ‘Correlation of head rice yield to selected physical and mechanical properties of rice kernels’, Trans Am Soc Agric Engr, 38, 889−894. mahadevappa m, bhashyam m k and desikachar h s r (1969), ‘The influence of harvesting date and traditional threshing practices on grain yield and milling quality of paddy’, J Food Sci Technol, 6, 263−266. murugesan g and bhattacharya k r (1991), ‘Basis for varietal difference in popping expansion of rice’, J Cereal Sci, 13, 71−83. murugesan g and bhattacharya k r (1994), ‘Interrelationship between some structural features of paddy and indices of technological quality of rice’, J Food Sci Technol, 31, 104−109. obetta s e and onwualu a p (1999), ‘Effect of different surfaces and moisture contents on angle of friction of foodgrains’, J Food Sci Technol, 36, 58−60. oki t, masuda m, nagai s, take’ichi m, kobayashi m, nishiba y, sugawara t, suda i and sato t (2004), ‘Radical-scavenging activity of red and black rice’, Abstract no. 475, presented in World Rice Research Conference, Tsukuba, Japan, 5-7 November. patindol j and wang y-j (2003), ‘Fine structures and physicochemical properties of starches from chalky and translucent rice kernels’, J Agric Food Chem, 51, 2777−2784. prasertan s, satake t and yoshizaki s (1994), ‘Breaking behaviour of long grain brown rice’, Asean Food J, 9, 36−41. perdon a, siebenmorgen t j and mauromoustakos a (2000), ‘Glassy state transition and rice drying: development of a brown rice state diagram’, Cereal Chem, 77, 708−713. raju g n and srinivas t (1991), ‘Effect of physical, physiological, and chemical factors on the expression of chalkiness in rice’, Cereal Chem, 68, 210−211. resurreccion a and fitzgerald m a (2007), ‘Chalk – a perennial problem of rice’, Cereal Foods World, 52, A6. sandhya rani m r and bhattacharya k r (1989), ‘Slurry viscosity as a possible indicator of rice quality’, J Texture Studies, 20, 139−149. shitanda d, nishiyama y and koide s (2002), ‘Compressive strength properties of rough rice considering variation of cantact area’, J Food Engg, 53, 53−58. sowbhagya c m, ramesh b s and bhattacharya k r (1984), ‘Improved indices for dimensional classification of rice’, J Food Sci Technol, 21, 15−19. srinivas t, singh v and bhashyam m k (1984), ‘Research: grain chalkiness in rice. Physicochemical studies on the genetic chalkiness in rice grain’, Rice J, 87 (1), 8, 9, 12−14, 17−19. srinivas t, bhashyam m k and raju g n (1985), ‘Anatomical and chemical peculiarities caused during the translocation of solute in the developing cereal grains’, Plant Physiol Biochem, 12, 77−85. stansel j w, halick j v and kramer h h (1961), ‘Influence of temperature on heading dates and grain characteristics of rice’, in Proceedings, Rice Technical Working Group. webb b d (1985), ‘Criteria of rice quality in the United States’, in Juliano B O (Ed.) Rice Chemistry and Technology, 2nd edn, St. Paul, MN, American Association of Cereal Chemists, 403−442. © Woodhead Publishing Limited, 2011


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wouters a and baerdemaeker j d (1988), ‘Effect of moisture content on mechanical properties of rice kernels under quasi-static compressive loading’, J Food Engg, 7, 83−111. wratten f t, poole j l bal s and ramarao v v (1969), ‘Physical and thermal properties of rough rice’, Trans ASAE, 12, 801−803.



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3 Milling quality of rice

Abstract: Rice is used as wholegrains. Hence any breakage of the grain during milling is undesirable. The primary reason why rice breaks during milling lies not in the milling process but in the defects in the grain that enters the mill. The main factor is cracks in rice. The rice grain is mechanically strong, but it is susceptible to moisture stress and develops fissures upon rapid hydration or dehydration either in the field or in the process of drying. There is a critical moisture content only below which cracking occurs. This is related to the glass transition temperature (Tg), below which the material is glassy, i.e. brittle, and above which it is rubbery, i.e. more malleable. Moisture acts as a plasticiser for starch and lowers the Tg, which is why wet grain does not crack. Cracking can be avoided during drying by hot tempering to resolve the moisture gradient before cooling. Milling machinery affects rice breakage only to the extent of the defective/damaged grains. Gentle milling protects some defective grains from failing, while harsh milling leads to failure of more or all of them. Key words: Rice milling quality, grain defects in rice, cracks in rice, crack resistance, glass transition temperature in rice, machine-grain defects-moisture interaction in rice milling.

‘[Come, I’ll teach you] how much paddy gives how much rice!’ Bengali proverb ‘Although milling methods are important, the condition of the rice on arrival at the mill has a decisive effect on the yield and quality of the milled product.’ Angladette (1963)



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3.1

Rice quality

Milling of rice

Proverbs are usually considered as self-evident truths. The same applies to the above Bengali proverb, which suggests that the yield of milled rice from paddy is an unalterable given. Having been raised as a small-town Bengali, the present author grew up with this proverb and at no time had any occasion to doubt its veracity. It is only after he joined the Central Food Technological Research Institute (CFTRI) at Mysore, India, in 1960 and joined the tiny rice group therein that an occasion soon arose that questioned its truth. It so happened that a Ford Foundation team visited India in 1963 or thereabouts to study the rice milling system in India which, according to them, was of vintage quality and needed urgent modernisation. What they suggested was that the milling yield of rice depended on the milling system used and that India, suffering from a serious shortage of rice then, could increase her rice availability substantially, even with her current output of paddy, simply by replacing her age-old rice-milling system with modern equipment and systems (Faulkner et al. 1963). Needless to say, this assertion, while it raised an important policy issue to the Indian authorities, came as a piece of astounding information to the present author because of his long-standing belief in the proverb above. It is possible that the proverb grew out of the experience of people during the time of the hand-pounding days when probably varieties and samples did not show a great deal of difference in their milling outputs. But it clearly lost its validity once the technology changed, and thereby hangs a lesson. Be that as it may, milling yield is an important factor in the technology of rice. Rice is unique among cereals in that it is overwhelmingly used not as a flour but as wholegrains. Structure is an important factor in the culinary tradition of humankind. Cereals, even when milled into flours are rarely consumed as such or as slurries and pastes but are invariably formed, with the help of water and heat, into various structures for consumption. The various kinds of pasta are elegant examples of how we first grind and then form structures. In the case of rice structure is inherent in the grain itself and that is how it is largely consumed. The entire paddy grain is not edible. It contains roughly one-fifth its weight as a woody, siliceous covering, called the husk or the hull, that is inedible and must be removed, a process that is called shelling (or dehusking or dehulling). The resulting grain is called brown rice (or dehusked or dehulled rice) which has a somewhat fibrous and fatty covering which prevents its easy cooking in boiling water. This layer, collectively called bran, also therefore needs to be removed at least partially by a process of attrition or abrasion, this process being called whitening or pearling or simply milling or polishing. The entire process of producing milled rice or white rice from paddy is also referred to by the generic name milling. How a sample of paddy responds to this process is an important quality attribute of rice, called the milling quality. The amount of brown rice that a sample or variety produces after shelling is called its brown rice yield or © Woodhead Publishing Limited, 2011


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outturn. The amount of finished milled or white rice produced, including broken grains, after whitening is called total yield or total milled rice yield or outturn. The amount of unbroken grains (often including more than threequarters length broken grains) produced is called head yield or head milled rice yield or outturn. These three attributes together constitute the milling quality of rice.

3.1.1 Milling yield of rice There are three principal reasons of variation in milling yield of rice. The first is the variety, specifically the husk (hull) content, of the paddy. An examination of the literature shows that the amount of husk in paddy varies from a low of 14.6% (Sadanandeswara Rao and Bhattacharya 1977) to a high of 26.0% (Juliano et al. 1964). Clearly there is a potential difference in milling yield of rice of over 11 percentage points due to this factor alone. The reason why such a huge potential source of difference in milling yield is not given a pride of position in the literature on rice milling is simply because the above values are more in the nature of exceptions, most varieties having a husk content in the range of 19−22%. A second factor, now probably not so relevant but was very much so years or decades ago, is the degree of milling (DM), i.e. the extent to which the bran is removed from the rice grain during its whitening. Obviously the more one mills the grain, the less is the final yield and vice versa. In well-developed market economies, rice is usually milled ‘fully’, viz. to a degree of milling of about 10% or more, but this is not generally the case in a subsistence or semi-subsistence economy market, the existence of which is not yet uncommon in parts of most rice-growing countries of Asia. Even in India, until several decades back, rice was required by law to be milled only up to 4% DM – the idea being to augment the availability of rice in the country – no matter if the law was widely obeyed or not! Clearly there is a potential difference in milling yield of up to several percentage points due to this factor. However, thirdly, from the practical standpoint and from the standpoint of technology, the most important factor in the milling yield of rice is its breakage, i.e. to what extent the rice grains break while they are being milled. This is a factor of profound importance in rice milling. For, first, any grain breakage reduces the aesthetic as well as the monetary value of the milled product – at any rate in developed economies – broken grains being disliked and sold at a much discounted price to wholegrains; and second, grain breakage during milling is invariably accompanied by the production of some fine broken pieces or flour which are apt to be lost during either an aspiration or sieving operation, thus reducing even the quantitative yield. While the first of these three factors – husk content – is a matter of genetic make-up and the second factor of DM is determined by custom or law, the third factor of grain breakage is the one which is of profound importance to



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the rice technologist as well as to the rice miller. As a matter of fact how to avoid or minimise the breaking of the grain and so maximise the production of head rice is the single most important aim of the rice miller.

3.1.2 Breakage of rice during milling That some rice grains break during its milling is well known to the layperson, the consumer and the miller alike. It is not a matter of great importance in subsistence or semi-subsistence economies. In these economies the miller spends some effort in retrieving whatever reasonable-sized broken pieces may have been lost either into the bran or along with the husk and is therefore not greatly concerned about the matter unless the breakage is really excessive. Nor is the consumer much concerned about cooking and eating a sample of rice containing a fair proportion of broken grains. Indeed this was more or less the situation in India at the time the present author joined the CFTRI rice group. Today of course the situation has changed not only in developed market economies but also largely in India. In most places wherever rice is not traded as a subsistence commodity, the amount of broken grains in the product is a matter of much importance and largely determines its value. In highly developed markets and in export business, of course, the amount of broken grains in the product is built into the grade and regulated by law. The breakage of rice during milling is therefore a matter of great concern to the miller. Why some rice grains break, however, is not well understood by the miller, the layperson or the consumer. Perception on this topic has evolved differently, and in parallel, among some scholars on the one hand and millers and consumers on the other. A section of rice researchers took specific interest in the subject and have been developing theories and concepts about it for a long time. To other researchers and millers, the reason has been quite unclear with a prevalence of various half-truths and myths. When the present author started his career in research in rice technology, the prevalent ideas even among rice scientists not otherwise engaged in research on rice milling were manifold. It used to be believed that rice grain breakage was a varietal characteristic, i.e. some varieties inherently broke more and some less; or that some varieties were ‘soft’ and hence broke while others were ‘hard’ and hence did not. Another prevalent idea was that breakage of rice during milling was entirely a function of the milling equipment and milling systems, as exemplified by the views of the Ford Foundation team which visited India just before the mid 1960s, as mentioned before. There were other nebulous or vaguely felt ideas. Indian millers who produced parboiled rice knew that its improper drying could lead to ‘cuts’ in the grain that might result in it to break. But the matter was only vaguely understood. In any case since broken grains in rice, unless excessive, were not considered to be a matter of great concern in most Asian markets, there was no particular reason to be too concerned about its cause. No doubt the situation has now changed. © Woodhead Publishing Limited, 2011


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Grain cracking or fissuring at or around harvest

3.2.1 Grain cracking at or around harvest One reason why a rice grain may break during milling has been attributed to cracks in it. This concept arose from the work of many researchers who noticed from as early as the 1920s that the rice crop in the field, whether in the stalk or after harvest, developed cracks or fissures in the grain under certain conditions and that these samples were apt to show higher breakage when milled. Srinivas (1975) and Li et al. (2007) have shown histological pictures of how the cracks in rice were real fractures and lay along the cell walls. It is interesting that under the historical conditions then prevailing studies on rice cracking were mostly carried out by European researchers working in rice-growing colonies of their mother countries in Asia and elsewhere. (Surprisingly there was no such work done by the British in India, the largest rice-producing colony of a European country, although agricultural, and to a smaller extent nutritional, aspects of rice received a good deal of attention in India. Nor, for that matter, so far as is known, was any such work done in China, the largest rice-producing country in the world. One possibility is that it is mostly the rice-exporting colonies that drew the requisite interest in the topic. On the other hand independent rice exporters such as Thailand without the requisite tradition of modern science naturally showed little interest in the matter.) Some of these early works are difficult to come by in original today. Fortunately these have been described in some excellent reviews (Rhind 1962, Angladette 1963, Johnston and Miller 1966, Kunze and Calderwood 2004) which have been freely quoted from in the following discussion. Possibly the first report on the above subject came from Copeland (1924) in the Philippines, who wrote that the rice grain developed ‘sun-cracks’ in the field from exposure to the sun whether in stalk or during its drying. Kondo and Okamura (1930) independently carried out a pioneering and detailed study on the subject in Japan. They concluded that it was moisture adsorption following drying that led to formation of cracks in the kernel. For instance, when two varieties were well dried in the room after harvest and then exposed to rain for two hours, there was a clear and rapid development of cracks in the grains. In another experiment, they left sheaves in the field for 22 days during which period it rained on 4 days. When the sheaves were put there before drying, the moisture content being 24%, there was no cracking. But when the sheaves were dried on racks, the grains in the outer part had moisture content of 17.5% and developed 14−24% cracks; Grains in the inner parts on the other hand had 19.5% moisture and only 1−7% cracks. The more the exposure and the more the drying, the greater was the extent of cracking. Further, the extent of cracking increased with increasing duration of rain and also with increasing interval after wetting. Cracks developed progressively after wetting, as shown with paddy which



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was spread out in a room soon after wetting. Kondo and Okamura (1930) also observed that exposure to dew in the night was a major cause of cracking. Grant (1935; cited in Rhind 1962) in Burma independently carried out similar work with results that dramatically brought out the effect of dew. Drying harvested grain in the shade gave the best results, while drying in the sun and especially exposure to dew in the night led to excessive cracking (Table 3.1). Another similar result came from the work of Coyaud (1950; cited in Angladette 1963) in Vietnam. He found that drying in shade after reaping just at maturity gave the best milling results whereas delayed harvest or drying in sun produced high breakage (Table 3.2). He also confirmed the effect of dew in increasing breakage. Samples protected from dew gave low breakage whereas the breakage increased dramatically if the grains were left exposed to dew. Another point he made was that if drying was done in the shade, the effect of remoistening even by rain was relatively small. Results similar to the above were obtained by Stahel (1935) in Surinam. He treated reaped paddy in three different ways: (a) swaths laid on the stubble, Table 3.1 Effect of different methods of drying paddy after harvest on amount of cracked grain (%) Drying period (days)

Dried in shade

Dried in sun, covered at night

Dried in sun, exposed to dew at night

2 4 6 8 10

4.4 4.0 3.7 3.9 4.0

17.3 28.5 32.0 50.3 60.6

72.0 88.7 96.2 96.4 99.3

Reproduced from Grant (1935; cited in Rhind 1862).

Table 3.2 Variety

Effect of reaping stage and drying method on breakage upon milling Drying method

Brokens in milled rice (%) Reaped at Reaped 15 days Reaped 25 days maturity after maturity after maturity

Khsesoth

In shade In sun Under corrugated iron sheet Under black paper Kongkhsach In shade In sun Under corrugated iron sheet Under black paper

11.4 58.7 10.2

58.6 74.0 57.2

78.1 79.7 78.0

21.5 3.7 38.7 3.5

59.3 20.0 23.3 18.8

78.0 39.3 45.0 40.5

10.4

18.0

38.7

Compiled from Coyaud (1950; cited in Angladette 1963).



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(b) swaths bound into sheaves and stooked, and (c) sheaves protected by inverted sheaves on top. The results (Table 3.3) were very illuminating. Head rice yield was the highest when the sheaves were protected and the least when the cut plants were left on the stubble. His data gave other crucial information. It was observed that rapid drying alone did not cause the cracks but it was the reabsorption of water from the humid air or from dew that damaged the grain. In an attempt to determine the point at which cracking began, he dried paddy to different moisture contents, soaked them in water for an hour and a half and again dried in the sun. He found that remoistening had no effect when the paddy had 15% moisture or more; but remoistening of paddy with less moisture resulted in reduced head rice. There were varietal differences in the point at which remoistening caused damage, which was tested in seven varieties. But the basic principle applied to all with the head rice yield showing a sudden fall at a particular point. Even the data in Table 3.3, which work was not done specifically to test this aspect, clearly show the above effect of a critical moisture content. In the protected sheaves the moisture content never went down to 15% and head rice was virtually unaffected. In the stooked group, whenever the moisture went down below 14%, there was a drop in head rice. And in the first group where the moisture content dropped below the critical point within one day, all showed high breakage. Ranganath et al. (1970) and Srinivas et al. (1978) in India confirmed that drying alone was not harmful but exposure to dew was; protecting the panicles or the plants from dew by a cover prevented grain cracking.

3.2.2 Optimum time of harvest Another concept that evolved gradually from the observations of numerous researchers throughout the rice-growing regions, was that it was not just the handling of the crop after harvest but even the time of harvest had an effect on the milling quality of the grain. Indeed this fact is evident even in the data Effect of drying method on the whole rice outturn after milling

Table 3.3 Drying time (days)

Spread on stubble

Sheaves stooked

Sheaves protected

Moisture (%)

Whole rice (%)

Moisture (%)

Whole rice (%)

Moisture (%)

Whole rice (%)

1 3 4 5 6 7 8

18.6 12.6 9.1 12.1 13.0 11.2 10.7

59.0 30.0 17.2 10.4 11.6 17.0 8.8

22.0 16.8 13.3 13.8 13.2 14.6 14.2

60.8 62.4 58.6 48.0 54.2 57.0 54.6

22.5 20.8 19.6 20.5 18.8 17.3 17.7

60.2 58.4 59.0 60.2 60.6 58.2 57.6

Reproduced from Stahel (1935).



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of Coyaud (1950; cited in angladette, 1963) (Table 3.2) mentioned above. This view emerged most specifically from the work of American researchers after the system of combine harvesting was introduced there. This new system already necessitated that the plants be harvested a little early when they still stood firm and erect in the field. They were then rather more moist too, so the matter called for some examination. The matter was studied by many workers in the USA, the results of which have been summarised by Johnston and Miller (1966). It was found that far from being deleterious, a little – but not too much – early harvest (followed by artificial drying, if properly done) was in fact beneficial for milling. A series of other workers throughout Cambodia, Vietnam, Madagascar, Surinam, Mali and Chad also did similar work which have been summarised by Angladette (1963).Other workers on the same subject, including especially later Indian works, have been listed in the review by Bhattacharya (1980). The basic principle observed was that if the harvest was too early, a substantial proportion of the grains remained immature, being still in the process of maturing, which were rather fragile and therefore got broken rather easily during milling. Besides, the total milled rice yield was also relatively low because of the same reason of substantial immaturity. On the other hand whenever the harvest was delayed beyond a point, the head rice yield again decreased even though the total milled rice yield was not necessarily too much affected. The reason of the reduction in head rice this time was that the grains underwent extensive cracking either due to drying in the sun or, more especially, due to cyclic drying in the day and wetting from dew or just high humidity in the night, or even from rain. Kunze and Calderwood (2004) have discussed in detail and shown how the change in temperature and relative humidity in the field between the day time and the night, even without rain, could result in a potential difference in equilibrium moisture content (EMC) of the grain of up to 10 percentage points or more between the high points of day and night. The mature grain went through this change everyday even without direct exposure to dew or rain and thus became susceptible to fissuring. While this principle of an optimum time of harvest has been repeatedly confirmed by many workers over several decades, it should be clearly understood that this optimum is not a fixed point applicable to all situations in a constant manner. Obviously the situation is highly dependent on the prevailing weather conditions (temperature and humidity and their variations, wind and its velocity) and also the varietal specificity. Therefore no definite time can be outlined as being invariably optimum for all conditions. Generally the optimum time has been identified by • • • •

average grain moisture content (usually in the range of 19−25%); days after heading or flowering (usually in the range of 30−35 days); grain colour (usually when the top grains are almost fully yellow and the bottom grains are half or more green); and the stage of maturity, viz. when the top grains are fully developed and the bottom grains are in the hard dough stage. © Woodhead Publishing Limited, 2011


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One set of classical data is shown in Fig. 3.1. The best practice therefore, as it has emerged, is to reap the crop at the optimum time, thresh immediately and then dry the grains preferably in the shade or carefully in the sun or else in the mechanical dryer under prescribed conditions.

3.2.3 Variation in grain moisture in the field One problem with deciding the optimum harvest time based on moisture content of the grain to be harvested is that this is largely a notional value. This aspect was apparently first examined in India at the CFTRI by Desikachar and his colleagues (Mahadevappa et al. 1969, Srinivas and Desikachar 1973). There is a few days’ spread for all the spikelets to flower in a panicle, which may further vary among panicles, tillers and plants. So there is always a few days’ difference in grain setting and maturing in a field. Accordingly the former authors reported that the grain moisture was not a constant figure but varied within a panicle, from panicle to panicle and also with the time of the day. One set of data is shown in Table 3.4 from the former work. It is clear that the grains on top of a panicle were the first to mature and also to dry, while those at the bottom were the last. This phenomenon has been recently noticed afresh in the universities at Texas and Arkansas in the USA. Chau and Kunze (1982) gathered data which showed how great the variation in grain moisture was in a field at any particular time (Fig. 3.2). Another similar set of such data is seen in the work of Bautista et al. (2000), where the individual kernel moisture content at harvest showed a dramatic spread (Fig. 3.3). Consequences of this wide variation in grain moisture in the field are serious and also manifold. The resulting presence of grains having widely different moisture contents in the harvested lot would not only seriously impact its drying and handling, it also has other deleterious consequences.

Head rice yield, %

70 60 50 40 30 0

Fig. 3.1

40

35

30 25 Moisture, %

20

15

10

Effect of time of harvest of paddy on head rice yield. Reproduced from Morse et al. (1967).



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Rice quality Table 3.4

Variation of grain moisture in different parts of the panicle

Variety

Mean grain moisture in field (%)

Grain moisture (%) in panicle Top

Middle

Bottom

S 701

21.7 18.4 16.6

20.6 18.8 17.3

23.3 20.4 19.5

27.4 24.5 22.9

T (N) 1

21.6 16.8

20.6 20.2 18.8

22.8 21.5 20.6

23.5 23.8 22.6

Reproduced, with permission, from Mahadevappa et al. (1969). 60 – x – x – x – x – x – x

55

Average moisture content, %

50

(wb) (wm) (wt) (Db) (Dm) (Df)

45

40

35

30

25

20

15

10 8/10

8/12

8/14

8/16 8/18 8/20 Harvesting day

8/22

8/24

8/26

Fig. 3.2 Average moisture content of paddy grains from the top (t), middle (m) and bottom (b) positions of the wettest (w) and driest (D) panicles during the indicated date of the harvesting season. Reproduced, with permission, from Chau and Kunze (1982).

Kunze and Calderwood (2004) have discussed how such moisture variation itself can lead to additional kernel cracking. For instance, if some of the grains in the lot had already got dried to a sufficiently low level, i.e., below © Woodhead Publishing Limited, 2011


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Number of kernels

Cypress, Stuttgart 1999 250 14.3% avg. HMC

200 150

22.7% avg. HMC

100 50 0 5

10

15

20 25 30 Moisture content, % wb

35

40

45

Fig. 3.3 Individual kernel moisture content distribution in a panicle of rice harvested at two different harvest moisture content (HMC) levels. Reprinted from Bautista et al. (2000) with permission from Elsevier. Table 3.5 Variation in the content of cracked grains in different parts of the panicle Average grain moisture Cracked grains (%) in panicle (%) Top Mid-1 Mid-2 Bottom 20.0 15.5 14.5

14 38 52

8 27 54

4 22 48

0 2 32

Reproduced, with permission, from Srinivas and Desikachar (1973).

a critical level if any, then these dry grains might readsorb moisture from grains which were moist and thereby fissure from such readsorption. Similarly well-dried grain may be present in a mixture of wet and dry grain in a truck or a cart. Or some dry grain may be present ahead of the drying front in a stationary mass of wet grain undergoing heated-air drying and so get exposed to warm humid air. Such dry grain would likely fissure from readsorption as a consequence. Secondly, a wide moisture difference among grains in a lot is also indicative of difference in the stage of maturity of the grains. In other words, such a moisture difference would imply that some a grains are already mature and dry and already subject to fissuring, while some are still immature and thus again susceptible to breakage during milling. Srinivas and Desikachar (1973) have presented such data (Table 3.5). These results show how the top grains in a panicle are already undergoing cracking while the bottom grains are still in the process of maturing. Clearly this is a serious problem for the miller and the technologist. Kunze and Calderwood (2004) have pointed out that this situation has yet other harmful potential. For the difference seen within a panicle is magnified when one considers the difference from panicle to panicle, tiller to tiller and plant to plant in a field. The resulting harvest is thus a mixture of grains with greatly variable moisture content and maturity stage. A certain extent of this nonuniformity is inherent in the current stage of rice production technology



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even under the best conditions. Clearly it is for the breeder and agronomist to understand the consequences and address this issue.

3.3

Drying of rice

Cracking or fissuring of rice can and does occur not only before and soon after harvest as described in the preceding sections; the same can occur upon its subsequent drying as well, whether the harvest is at an optimum stage or otherwise. Harvested grain will in any case contain moisture that is not safe enough for storage so the grain must be adequately dried. Angladette (1963) has extensively discussed optimum conditions for drying harvested rice with minimum damage under the traditional systems of production that existed before the advent of modern heated-air drying technology and as these exist even today in many rice-growing countries. The optimum conditions were derived on the basis of a series of works carried out by scientists in various countries including Cambodia, Vietnam, Madagascar, Surinam, Guinea, Mali and Chad. What emerges from this extensive work is that avoiding grain fissuring, and consequently its likely breakage during milling, requires the adoption of certain principled practices. Namely, paddy should be harvested at the optimum stage and not left on stalk for long after maturity and then dried in the shade wherever possible, be it after or before threshing. In any case drying in the sun is permissible only provided it is done carefully. Regarding drying before threshing Angladette concludes the best conditions as: • •

the paddy should be shocked up or stacked immediately after cutting and not left on the ground in bundles or sheaves for even a day; the shocks or stacks should be capped by an inverted bundle of paddy or by the straw so as to protect the ears from exposure to the sun.

Alternatively, the rice should be threshed immediately and then dried carefully by regular turning of the spread out grains. At the least it should be protected from exposure to the elements in the night, i.e., under cover.

3.3.1 Drying with heated air With the advent of combine harvesting, natural (including sun-) drying was no longer able to cope with the task, and artificial drying became a necessity. Combine harvesting resulted in the sudden arrival of a large volume of threshed and more than usually moist paddy at one go which would rapidly spoil if it was not dried immediately. Thus arose the necessity of fast drying, in other words drying by heated air. But heated-air drying of paddy was found to be hazardous, resulting in serious damage to its milling quality. A series of studies were therefore carried out on this subject particularly in the USA during the 1930s to 1950s. These studies have been described in detail © Woodhead Publishing Limited, 2011


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by Dachtler (1959) and summarised by Johnston and Miller (1966). Initial research was carried out on various on-farm and in-bin and similar systems of drying. Subsequently when more and more large quantities of paddy had to be handled, continuous drying with heated air was standardised. A series of research work was done for this purpose in various agricultural experiment stations of all the rice-growing states in the USA, and especially in the Western Regional Research Centre of the USDA at Albany, California. These studies have been well summarised by Kunze and Calderwood (2004). The major point of the system that emerged is that since drying with heated air is too fast, it became a serious source of grain damage with extensive cracking after drying. This was hypothesised to be caused by a steep moisture gradient being set up in the grain by the rapid drying. It was considered that moisture evaporated from the paddy grain only from its surface; so the moisture from the inner layers had first to diffuse to the surface and only then was able to evaporate. This process therefore set up a steep moisture gradient in the grain, the surface being excessively dry and the centre still wet, which set up a serious stress in the grain due to unequal contraction in different layers. When the unequal distribution of stress went beyond the capacity of the grain to bear, it was apt to release the stress by cracking. This process was unavoidable, so the solution was found in drying rice in stages or passes, called multi-pass drying. In other words, not more than a small amount (approximately two percentage points) of moisture was to be removed in one pass, after which the drying was to be stopped and the grain allowed to rest. This process of rest is called tempering. During tempering, evaporation of moisture from the surface ceases but its diffusion from the centre to the surface continues unabated. As a result, the moisture gradient in the grain eventually subsides and the moisture gets equalised in it after a sufficient period of tempering; then the drying could be resumed for the next pass. Wasserman et al. (1957) on the basis of extensive research suggested that rice could be dried even at a fast rate at a very high temperature without damage provided the number of passes was increased and the time of passage (in other words, the amount of moisture removed) in a pass was reduced. This system is being successfully followed throughout the world at this time.

3.4 Why the rice grain fissures Kunze (1979) (elaborated in Kunze and Calderwood 2004) repeatedly and passionately asserted that, contrary to what had been believed all along, fissuring in rice was caused not by desorption, i.e., by its drying, but by adsorption of moisture. Adsorption is a clear and straight forward cause of cracking in rice, as can be easily seen by putting some milled grains under water, when these would visibly develop transverse cracks within a short time, as was observed by Desikachar and Subrahmanyan (1961). The same is true of vapour-phase adsorption. Kunze and Hall (1965) had long © Woodhead Publishing Limited, 2011


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before shown that when paddy, brown or milled rice, equilibrated at a lower relative humidity (RH) were exposed to a higher RH, they developed cracks, confirmed by Lan and Kunze (1996) more recently. There is also no doubt that the major cause of rice cracking in the field or during its natural drying in the traditional systems may have been primarily due to readsorption of moisture by the grain that had already been dried possibly below a critical point, as discussed above. But none of these examples implies that rice does not crack by drying. There is no doubt that fissuring in rice occurs as a result of its drying also. Milled rice grains can be seen to visibly crack in a few minutes when exposed to the sun or even when kept under the fan. The whole science of heated-air rice drying is a testimony to the phenomenon of fissuring by drying. Some confusion in the matter may however have been caused by the interesting fact, noticed by several researchers, that fissuring of rice as a result of rapid drying occurs not during but after drying. Historically it is interesting that this paradoxical fact was noticed first in drying of parboiled paddy. This was seen independently by three groups of researchers. The first such report came from Craufurd (1963) in Sierra Leone, West Africa, who found that fast-dried parboiled paddy showed no cracks during or immediately after drying but cracks appeared with passage of time after the drying had ceased (Table 3.6). Sluyters (1963) too around the same time was puzzled to observe in Nigeria that breakage in sun-dried parboiled paddy was rather low if milled soon thereafter but increased upon its storage for up to three hours. Bhattacharya and Indudhara Swamy (1967) again made an independent observation of this fact in their detailed study of the drying of parboiled paddy, to be presented in more detail below. They observed that when parboiled paddy was dried with hot air very rapidly down to storage moisture level and immediately milled hot, it yielded nearly 100% head rice, confirming it had not been damaged. But when this milled head rice was put in a polyethylene bag and kept aside, the grains visibly started to crack Table 3.6 Time of appearance of cracks in fastdried parboiled paddy Time after drying (hours)a

Cracks (%)

0.25 0.50 1.00 1.00b 2.75 3.00 4.50 5.00

0 0 8 29 44 49 61 62

Reproduced, with permission, from Craufurd (1963) John Wiley and Sons. a Paddy dried at 43 °C to 11.4% moisture. b Possibly a printing mistake for 1.5. © Woodhead Publishing Limited, 2011


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within a few minutes and became severely cracked within an hour or so. Kunze and Calderwood (2004) have reported that this fact of post-drying fissuring was also seen in the drying of field paddy by Ban (1971) in Japan after sun drying and by Kunze (1979) himself in hot-air dried field paddy. Kunze and Calderwood (2004) have also reproduced photographs showing progressive cracking of rice grains following drying. This paradox has now been explained which will be discussed below. There have been other confusions. For instance, one of the doubts that arose early was whether cracking was caused by changing temperature, especially heating. Henderson (1954) in particular claimed that temperature change was primarily responsible for cracking of rice. But this view could not be sustained. Many studies have shown that the temperature change or heating per se has little if any effect on grain breakage in rice. As an example, Bhattacharya and Indudhara Swamy (1967), as mentioned, found that parboiled paddy after fast drying and at a high temperature showed no breakage whatsoever when it was milled hot immediately after drying – although the milled grains cracked heavily subsequently. Besides, if hot-air drying was immediately followed by hot tempering, this heating and subsequent quick cooling of the paddy did not cause cracking at all – actually prevented it (these aspects are discussed further below). Similarly Kunze and Hall (1967) and Matthews et al. (1971) observed that heating or cooling of the paddy did not cause cracking or breakage as long as its moisture content remained unchanged. Archer and Siebenmorgen (1995) also found that grain temperature did not seem to have significant effect on the head rice outturn upon milling. Kunze and Chaudhary (1972) suggested from measurement of the tensile strength of rice that fissures arose from unresolved compressive and tensile stresses generated in different layers of the kernel because of the moisture gradient formed during moisture change. In other words we can conclude that the rice is a tough grain that is not easily susceptible to damage by mechanical or thermal stress but is very susceptible to moisture stress.

3.4.1 The phenomenon of critical moisture content and a few other puzzles One crucial factor seems to be that there is a critical moisture content, only below which the rice grain becomes susceptible to cracking. This was observed for the first time by Stahel (1935) as clearly seen in his data quoted in Table 3.3 and discussed there. This observation was confirmed by other workers especially in relation to parboiled rice. Craufurd (1963) first noticed that no cracks were formed in parboiled paddy even upon fast drying until its moisture content had dropped to 15% or below (Table 3.7). He found the same during wetting; parboiled paddy did not seem to crack when wetted if its moisture content was already more than 15%; further, the extent of cracking was greater, the lower the moisture below 15% (Table 3.7). He theorised that starch was plastic above 15% moisture and hence the rice



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Table 3.7 Effect of initial moisture content of paddy on development of cracks due to (a) fast drying and (b) moistening with water (a) Fast drying Raw

(b) Moistening Parboiled

Moisture (%)

Cracks (%)

Moisture (%)

Cracks (%)

18.0 16.3 15.6 14.1 13.1 12.0 11.2

6 6 6 19 30 48 50

17.0 15.5 14.8 13.2 12.2 11.5

0 0 0 30 43 65

Initial moisture Cracks (%) (%) Initial After moistening 10.9 11.2 12.5 13.1 14.2 14.8 15.7 18.4

8 9 10 13 14 11 10 10

60 43 28 16 15 10 12 8

Adapted, with permission, from Craufurd (1963) John Wiley and Sons.

could absorb the stress of differential expansion and contraction in different layers in this state; but starch become rigid below this moisture and hence the grain was inflexible when dry and so released the unabsorbed stress by cracking. As we shall see, this was remarkable anticipation. Siebenmorgen et al. (1998a) studied the critical moisture content for rice cracking by adjusting their moisture contents to 10−17% and then soaking in water at 10−40 °C. The moisture content at which head rice yield began to decline due to moisture adsorption ranged from 12.5 to 14.9% depending on the variety, harvest moisture and initial storage conditions. In another work Siebenmorgen et al. (1998b) found that milled rice at a higher moisture level suffered more cracking when exposed to a low RH (i.e. desorption) compared to lower moisture rice cracking when exposed to high RH (i.e. adsorption). The phenomenon of the critical moisture content was also independently observed by Bhattacharya and Indudhara Swamy (1967) during their work on drying of parboiled paddy, which is discussed in some detail here, for it threw up several puzzling facts simultaneously. First, they noted that when parboiled paddy was dried continuously either in the sun or with heated air, it did not become susceptible to fissuring damage until its moisture content dropped to 15−16%, after which the potential damage rose steeply (Fig. 3.4). This was true for all cases of fast drying whether the drying was in the sun or done with air at a temperature of 40−80 °C. Second, tempering the paddy at that point for a few hours eliminated the entire potential damage and it could be again dried to ~13% moisture without any harm (Fig. 3.5). Third, as already mentioned above, the cracks appeared in the paddy (when dried down to 10−14% moisture in one go) not during drying but only after the drying had ceased and over a period of time (Fig. 3.6). These three factors have been observed by other workers subsequently. But what is most interesting is



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60

50

Breakage, %

40

30

20

10

40

30

25 20 Moisture, %

15

10

Fig. 3.4 Effect of drying on milling quality of parboiled paddy. Paddy dried with air at 82 °C in an LSU dryer to various moisture contents then finish-dried in the shade and milled. Reproduced, with permission, from Bhattacharya et al. (1971).

that, the potential damage could be completely eliminated by hot tempering. In other words, even parboiled paddy dried continuously down to 10−12% moisture with air heated to 40−80 °C, which would otherwise result in heavy grain breakage, showed no milling breakage if it was hot tempered at the drying temperature for a few hours immediately after drying before cooling and milling (Fig. 3.6). The authors concluded that a steep moisture gradient and cooling were both necessary for the rice to crack, neither condition alone being sufficient. Thus when the grain was cooled or ambient-tempered immediately after heated-air drying, both conditions prevailed and the grain cracked. If on the other hand the hot, dried paddy was first hot-tempered before cooling, then only one condition prevailed (cooling), so the rice did not crack. Similarly if the rice was shade-dried, again only one condition prevailed (moisture gradient), if at all, and there was no cracking. The last of the four observations, viz. that hot-tempering after drying could prevent rice cracking, repeatedly confirmed in the work, was a puzzling one with no previous reference anywhere and remained unexplained – but unfortunately still unnoticed – until recently. Now, over three decades later, the matter is explained by the phenomenon of the glass transition temperature applied to the process of rice drying by workers at the University of Arkansas in the USA, which is discussed later.



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Rice quality 30

Temper 0 h

Breakage, %

20 2h

4h 8h

10

48 h

0 16

15

14 13 Moisture, %

12

11

Fig. 3.5 Effect of drying with tempering on milling breakage of parboiled paddy. Drying temperature, 60 °C; initial drying to 15.5% moisture (䊊). Tempered at room temperature (27–30 °C) for different periods shown and dried for 5 (X), 10 (D), and 15 (䊉) min each. Reproduced, with permission, from Bhattacharya and Indudhara Swamy (1967). 70

Tempered 0 h

60

Breakage, %

50 1/2 h

40

30

20

1h

10

3 h and 24 h 1

2 3 4 Time of storage, h

5

24

Fig. 3.6 Effect of hot tempering (50 °C) on milling breakage of parboiled paddy. Wet parboiled paddy continuously dried with 60 °C air down to 12.4% moisture; then immediately hot tempered for different time intervals shown. The tempered paddy was cooled and then held at RT for various times (abscissa) and milled. Reproduced, with permission, from Bhattacharya and Indudhara Swamy (1967). © Woodhead Publishing Limited, 2011


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Before proceeding further, another puzzling fact that remains to be explained can be mentioned. Today the fact has been repeatedly confirmed even in commercial scale that parboiled paddy (≥35% moisture) can be dried with heated air straight down to ~16% moisture in one continuous pass without any potential damage (Bhattacharya and Ali 1970, Bhattacharya et al. 1971). If this phenomenon is related to the concept of the critical moisture content and of the glass transition temperature, discussed below, one wonders why the same does not apply to the drying of field paddy (raw paddy after harvest). It is now common practice throughout the world that field paddy, although having a moisture content of 20−25% at the most, has to be commercially dried in several passes with not more than about two percentage points of moisture being removed in one pass. This is despite the fact that the phenomena of the critical moisture content and glass transition temperature, if true, should apply equally well to raw (i.e., field or nonparboiled) as to parboiled paddy. It would appear to this reviewer that this difference arises from the differential moisture contents in the paddy grains before drying. Parboiled paddy as produced has 35−40% moisture before drying and the moisture content is obviously more or less uniform in the mass. This is not the case in field paddy as already discussed. If we go by the data of Figs 3.2 and 3.3, there is a huge variation of moisture content within a harvested mass. Clearly field paddy is not a uniform lot and various grains in it reach the critical moisture content at various times during the process of drying. It seems to the present author that this is the reason why field paddy needs several passes for drying even up to the stage of 15−16% average moisture content, for this value is only an average value in this case and does not reflect the moisture of individual grains.

3.4.2 The phenomenon of crack resistance A phenomenon of considerable importance in the area of milling quality of rice is that of crack resistance originally brought to light by the work of the CFTRI group (Srinivas et al. 1977). These workers harvested 20 varieties at different stages and examined the cracks in the grains with a paddy crack detector and also determined their shelling breakage after drying in the shade. There was very wide varietal difference in the propensity to cracking among the varieties (Table 3.8). The authors (Srinivas et al. 1977, 1978) then developed a standard laboratory test to test the crack susceptibility or crack resistance of individual varieties: paddy is harvested at 20% moisture or more, shade dried, soaked in water at 30 °C for three hours and tested for cracks. A modified test was developed later (Srinivas et al. 1981). Varieties not only showed a difference in crack resistance but also a difference in the critical moisture content below which alone they developed cracks. Among four varieties, the critical moisture content varied between 14.2 and 18.3%. This was the moisture content at which the samples just started cracking (1% of the grains cracked). However, all the four samples showed 100%



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Rice quality Table 3.8 Variety

Halubbulu MR 297 MR 44 MR 298 IET 2254 MR 36 IET 2501 GMR 2 Sona MR 62 MR 301 Madhu Jaya Satya IET 2246 IR 20 IET 2295 Suhasini Surya MR 272

Varietal difference in rice cracking at harvest Cracked grains (%) at harvest moisture (%) 26

22

18

16

0.0 3.0 3.5 – 2.0 – 5.0 5.5 6.5 4.0 – – 11.0 – 12.5 15.0 10.0 – – –

1.0 7.5 7.5 13.0 3.5 15.5 21.0 13.0 15.0 23.0 28.0 22.0 24.0 30.0 36.0 35.0 44.5 – – 29.0

5.5 12.5 20.0 18.0 16.5 28.0 31.0 30.0 32.0 35.0 40.0 45.0 41.0 52.0 52.0 55.0 58.0 58.0 63.0 73.0

7.0 18.0 23.0 24.0 30.0 36.5 37.5 38.0 41.0 41.0 44.0 50.8 53.0 59.0 64.0 67.0 68.0 70.0 72.0 89.0

Reproduced, with permission, from Srinivas et al. (1977).

cracks in the range of 9.9−10.7% moisture. The authors also noticed the interesting fact that in an early-harvested lot, undamaged mature yellow grains showed greater propensity to cracking than sound and mature green grains collected from the same harvested lot. This latter observation remains to be explained. Subsequently International Rice Research Institute (IRRI) scientists (Juliano and Perez 1993, Juliano et al. 1993) took up this work and confirmed the concept. Among several varieties studied using the soaking-stress test, the critical moisture content varied from < 10% to as high as 16% moisture. Varieties showing a high critical moisture content (15 and 16%) were considered crack susceptible whereas those which had a low critical value (10−12%) were considered crack resistant. This phenomenon is obviously of paramount significance. As is evident in the discussion presented earlier, cracking or fissuring of rice is a matter of considerable importance for it promotes breakage of rice during milling. When varietal variation in crack resistance exists, it is open to breeders to use this property and breed varieties with greater resistance along with other desirable characteristics. Such an effort will automatically help in reducing the breakage of rice during milling. Bhashyam et al. (1984) and Raju et al. (1995) studied the relationship of various morphological and physicochemical factors to crack resistance.



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3.4.3 The concept of glass transition temperature and its relevance to rice grain fissuring As can be seen above, several puzzling facts emerged about the fissuring of rice grain during the research on the phenomenon. These facts emerged mainly in relation to the work on drying of paddy, especially that of parboiled paddy (Bhattacharya and Indudhara Swamy 1967), but also in relation to fissuring by adsorption. These puzzling observations were: •

• •

the rice (especially seen in parboiled rice) did not develop the potential cracking until its moisture content had dropped to ~16% or less (Table 3.7, Fig. 3.4); the same happened during adsorption, which caused fissures only if the grain moisture content before adsorption was less than a critical value, more or less same as above (Table 3.7); the rice did not crack during drying but only some time after drying (Fig. 3.6); and hot tempering of the paddy at or above the drying-air temperature for a sufficient length of time completely eliminated the cracking (Fig. 3.6).

All these three paradoxes now seem to be explainable by the concept of glass transition temperature (Tg) recently applied to rice drying by researchers at the University of Arkansas in the USA. A crystalline material, including a crystalline polymer, melts at a defined temperature when being heated. A semicrystalline polymer behaves differently (Zelenzak and Hoseney 1987). Starch is a semicrystalline polymer of glucose. It is composed of two components. One is the branched amylopectin molecule, which forms a crystalline domain in the parallel oriented branches region and an amorphous region at the branching points. The other component is linear amylose which is apparently amorphous. Thus the starch granule has a continuous amorphous phase interspersed with crystalline regions. At a low temperature, the molecular motion of the polymer chains is ‘frozen’ in a random orientation. So the polymer molecule is immobile and is said to be in a ‘glassy’ state. Upon heating, molecular motion is initiated and the molecules slide past one another. At this point the polymer becomes rather flexible and viscous and is said to be in a ‘rubbery’ state. The temperature at which this physical change occurs is the Tg. The polymer is said to be in a glassy state below and in a rubbery state above this temperature. Starch, being a semicrystalline polymer, shows this transition at a temperature (Tg) somewhat lower than the crystal melting temperature or the gelatinisation temperature (GT) (Slade and Levin 1995). As water acts as a plasticiser in starch, the latter’s Tg is heavily dependent on its moisture content, being quite low at high moisture values (around room temperature) and rising substantially (≥ 50 °C) as the moisture content of the starch falls below 10−14% (Perdon et al. 2000). The physical and mechanical properties of the starch change between the two states. The values of toughness, modulus of elasticity, tensile and compressive strengths, and hardness are higher in the glassy state; while those of specific heat, specific volume and coefficients of expansion are much lower.



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To examine the effect of this transformation, Perdon et al. (2000) used various thermo-analytical techniques to determine the Tg of rice and derived curves of Tg versus moisture at different moisture contents. From this they derived a state diagram of brown rice as the harvested rice with high moisture moved from the glassy state to rubbery when it was subjected to heated-air drying and then again back to glassy state when it was cooled following drying (Fig. 3.7). During drying the moisture content of the surface of the grain decreases while that in the centre lags behind. As a result, Tg of the surface layer increases over that of the centre, causing a gradient of Tg within the grain just like the moisture gradient that develops. In other words, depending on the temperature and the moisture state, the surface layer may become glassy while the centre remains in the rubbery state. That is to say, not only the grain as a whole, even the different layers of the grain may be characterised by different material properties as the grain dries. During tempering after a pass of drying or after completion of drying, moisture continues to diffuse from the centre to the surface and the moisture gradient gradually subsides. If the tempering temperature is below Tg, the grain again passes through a glass transition and attains a glassy state as the grain cools. As a result, the differential stress within the grain resulting from the temperature and moisture gradients, if sufficiently high, along with the change of state could very well cause the grain to fissure. The above has been shown diagrammatically by Cnossen and Siebenmorgen (2000) (Fig. 3.8). When the tempering temperature is sufficiently low, i.e. lower than the Tg, the surface, mid-point and the centre of the grain, originally at different 70 Rubbery region 60

Tg Drying

Temperature, °C

50 40

Heating drying

Cooling

30 20 10 Glassy region 0 0

5

10 15 20 Moisture content, % wb

25

30

Fig. 3.7 Brown rice state diagram and a hypothetical drying process for a rice kernel. Reproduced, with permission, from Perdon et al. (2000).



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83

70 65

Surface

Mid-point Centre

Temperature, °C

60 55 A

50 Glass transition line

45 40 35

A: Tempering temperature > Tg B: Tempering temperature < Tg

30 25

B

20 8

10

12

14 16 Moisture content, (% wb)

18

20

22

Fig. 3.8 Hypothetical response of the various sections of a brown rice kernel durng tempering for two tempering scenarios. Reproduced, with permission, from Cnossen and Siebenmorgen (2000).

moistures because of the moisture gradient created during drying, cross the Tg line at different moistures and attain a final position shown as point B in the diagram. Clearly the situation is such as may cause fissuring in the grain. This might explain why the rice grain has been found to fissure not during but after the drying has ceased. On the other hand, if the tempering is carried out at a temperature higher than the Tg, the equilibrium situation after the tempering would correspond to the position A in Fig. 3.8. Since position is above the Tg, the grain, being in the rubbery state, is not damaged by the gradient; nor would cooling the grain now, after hot-tempering, when the moisture gradient has already subsided, cause any damage. This would explain why hot tempering at or above the drying-air temperature did not result in any milling breakage observed by Bhattacharya and Indudhara Swamy (1967). Cnossen and Siebenmorgen (2000) similarly tested hightemperature tempering following drying of field paddy, and found that whenever the tempering temperature was equal to or above the Tg, even as high an amount of moisture removal as 6.5 percentage points in a single pass did not result in any reduction of the head rice yield. A series of such experiments were successfully done by these and other (Zhang et al. 2003) researchers with similar results. It is thus proved that if tempering can be done at a sufficiently high temperature, paddy can be dried in a single pass. Even though the feasibility of such high-temperature tempering in practice is a moot question, the theoretical clarification of this situation is one of the major developments in rice drying in recent times. The same property may also explain the phenomenon of rice cracking by adsorption and that of a critical moisture content. At a sufficiently high moisture content, where the Tg may be at or around the ambient temperature,



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the entire action of wetting and its subsequent natural drying may occur in the rubbery state or thereabouts, hence causing no damage to the grain. On the other hand, when the grain moisture content is rather low (at or below the ‘critical’ value) and the prevailing temperature is such that adsorption happens at or below the Tg, the grain may be in or may cross into the glassy state when the developing moisture gradient caused by hydration would surely cause the rice to fissure. This could explain the phenomenon of critical moisture content and that of rice cracking by wetting. This might also explain the phenomenon of crack resistance if we assume that the chemical composition or morphological or other grain property may have an influence in altering the Tg.

3.5

Miscellaneous factors that affect milling quality of rice

3.5.1 Chalky grains There is some controversy whether chalky grains are more susceptible to breakage during milling. Lay millers, even scientists, widely believe that chalky grains are ‘soft’ and break heavily during milling. However this is not necessarily supported by experimental data. Bhattacharya (1969) did a careful study of various factors that contributed to rice breakage, discussed further below, but could not find any evidence suggesting that chalky grains per se, not otherwise defective, broke any more than translucent grains. Matthews et al. (1970) also found similar results. However Indudhara Swamy and Bhattacharya (1982b) made a very interesting observation that chalky grains were more susceptible to cracking under adverse ambient conditions than translucent grains. They analysed several samples of paddy, separated them into chalky and translucent grains and examined them for cracks. It was found that the proportion of cracked grains was much more in chalky than in nonchalky grains (Table 3.9). Thus one can conclude that chalky grains per se probably do not break any more than translucent grains. However chalky grains are more susceptible to cracking under adverse atmospheric or handling conditions and thus may indirectly contribute more to breakage in a sample during its milling.

3.5.2 Moisture content at the time of milling It has already been discussed that addition or removal of moisture to or from the rice grain has a profound effect on its mechanical properties by virtue of their effect in causing fissures in the grain. However, in addition to the above, the moisture content itself also has some effect on milling. Pominski et al. (1961) noted that the laboratory milling yield (using McGill equipment) of rice (raw) was affected by the moisture content of paddy at the time of milling (Fig. 3.9). The head and total yields of rice increased as the moisture content of the sample decreased from over 14% down to 10%. © Woodhead Publishing Limited, 2011


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Milling quality of rice Table 3.9

85

Chalky kernels crack easily

Variety

Chalkiness score of grains having cracks numbering

Basmati 370 S705 Intan Jenugudu S749 Mahsuri Pankaj Ch2 Bala C435 Mean

0

1

>1

2.0 0.3 1.6 2.1 1.4 2.5 2.2 3.0 2.3 1.6 1.9

5.3 2.3 7.0 2.4 3.1 2.8 3.6 4.0 2.9 3.8 3.7

8.0 4.7 7.0 – 1.0 2.5 4.0 4.0 3.6 3.8 4.3

Reproduced, with permission, from Indudhara Swamy and Bhattacharya (1982b) 75 Total 70

Yield, %

65

60

55 Head 50 45 10

Fig. 3.9

11

12 13 Moisture, %

14

15

Effect of moisture content of paddy at the time of milling on yields of rice. Reproduced from Pominski et al. (1961).

They also quoted similar results of two other workers. The reduction in grain breakage was attributed to the better mechanical strength of the drier kernel, while the increased total yield was ascribed to a lower removal of bran from the harder kernel. Bhattacharya and Indudhara Swamy (1967) made similar observations with parboiled paddy. Milling breakage increased, even without any cracks or other damage, as the moisture content rose above 15%. Based on the above results, Wasserman and Ferrel (1963) devised a



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process wherein the paddy was first dried to 11.5% moisture and milled; the rice was then exposed to humid air to gradually raise its moisture content to 12–13% to restore the weight.

3.5.3 Environmental conditions Autrey et al. (1955) carried out a milling study with a pilot rice mill. They found that a low relative humidity in the mill room caused increased breakage of rice (Table 3.10). The best results were obtained when the mill room humidity was identical with the equilibrium RH of the rice entering the room, i.e., around 70% RH (the rice usually having 13−14% moisture). Many millers have also observed that breakage generally increased during dry summer days when the temperature was high and the humidity low. It appears to the present author that these results may be partly related to the propensity of the rice grain to lose moisture under the above conditions and thus possibly develop actual or latent cracks. The above authors also noted that cooling the grains in a closed container between passage from one unit to another, or humidifying the air in units where a large volume of air entered, also reduced the breakage. Stermer (1968) confirmed the susceptibility of the rice grain, which otherwise shows good mechanical strength, to temperature and humidity change. When rice grains equilibrated to a particular relative humidity were exposed to either much higher or much lower RH, the grains showed cracks in either case. Similar results were found by Noomhorm and Yubai (1991) in Thailand and Siebenmorgen et al. (1998b), Lloyd and Siebenmorgen Table 3.10 rice

Effect of mill room relative humidity (RH) on outturn of head

Variety

Lot no. RH (%)

Head rice yield Total rice yield (%) (%)

Zenith

2

32 45 58 60

37.4 39.4 41.0 41.2

68.0

Rexark

1

38 62 50 55 60 66

48.7 51.0 50.5 50.7 51.7 53.0

70.5

48 65 75

40.4 44.0 44.2

69.0

2

Blue bonnet 1

70.0

Reprinted, with permission, from Autrey et al. (1955). Copyright 1955 American Chemical Society.



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(1999) and Chen et al. (1999) in Arkansas, USA. It was found that both temperature and humidity change during milling affected head rice yield. The data of Lloyd and Siebenmorgen (1999) (Fig. 3.10) are very illuminating and show how delicate the rice is in relation to moisture change.

3.5.4 DM It is generally known that rice breakage increases with increasing DM, which is not surprising. However the results seem to vary from worker to worker and also from circumstances to circumstances. Actually this may be partly related to the shelling and whitening equipment used. The older equipment were less sophisticated and thus probably resulted in relatively heavy breakage at the early stages. Modern equipment is gentle and the breakage increases gradually as the milling proceeds. Benett et al. (1993) confirmed that head rice yield was inversely related to the DM. Moreover, the lowering of head rice yield with increasing DM was exaggerated by increasing moisture content. Another well-known fact is that, for a given DM, milling with greater pressure in one or two stages (‘breaks’) results in more grain breakage than gentle progressive milling in more stages.

3.5.5 Grain hardness The layperson often thinks that breakage of rice is related to an elusive category called ‘hardness’ of the grain. Often this is a case of inverted logic: If a rice breaks, it must be soft; so we conclude that soft rice breaks and hard rice does not! But the present reviewer is not aware of any experimental 100 90

Milled rice breakage, %

80 70

R2 = 0.8878 Root MSE = 7.22

60 50 40 30 20 10 0 0

10

20

30

40 50 60 Relative humidity, %

70

80

90

100

Fig. 3.10 Rice damage upon exposure to altered RH. Milled rice at 12.5% moisture exposed for 20 min to various RH values and subjected to a breakage test. Reproduced, with permission, from Lloyd and Siebenmorgen (1999).



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data supporting this view, nor does he feel this can be a significant factor. Measurement of grain hardness shows that chalky grains are softer than translucent grains (see Table 2.7 in Chapter 2). Yet, as mentioned above, chalkiness per se has not been found to make rice grains significantly more susceptible to breakage than otherwise.

3.5.6 Grain thickness For some time past a good deal of interest has been generated originally in the Southern Regional Research Centre (SRRC) at New Orleans and later in the University of Arkansas, USA, regarding a possible relationship of grain thickness to breakage of rice during milling. SRRC scientists (Matthews and Spadaro 1976, Wadsworth et al. 1982) noticed that when paddy was classified into different thickness fractions (using 1.60 to 1.98 mm slot sieves), there seemed to be a clear positive relationship between head rice yield and grain thickness (Table 3.11). The data showed that there was no appreciable difference in the head yield among the three thicker fractions, but fractions thinner than these showed increasingly greater breakage. The researchers argued from these data that there was a strong case for separating the thinner kernels and processing them separately, for then the thicker grains would produce much less breakage. Further the thinner grains could be used to produce a unique new product, for these contained much higher protein. Thus if these fractions were processed separately as a brown rice flour it could be usefully utilised in infant feeding. Handling of the thinner kernels separately could also help in drying and handling, for these thinner grains had 6−10 percentage points greater moisture content than the main fractions. Scientists at SRRC also studied and reported that the thinner kernels had distinct chemical, physical and physicochemical properties compared with the bulk of the paddy (Wadsworth and Matthews 1986, Wadsworth 1987, Wadsworth and Hayes 1991). Sun and Siebenmorgen (1993) also made a similar study with comparable results. Jindal and Siebenmorgen (1994) found that the thinner kernels had higher EMC. Table 3.11

Milling yield of different thickness fractions of paddy

Mean thickness Proportion in Brown rice (mm) paddy yield (%) (%) 1.91 1.85 1.77 1.67 1.45 1.29

17.7 22.5 45.4 9.3 2.0 2.9

81.2 81.1 79.9 75.7 63.5 55.9

Milled rice yield (%)

Breakage in milled rice (%)

74.0 73.5 71.7 62.0 33.4 23.9

19.9 18.2 23.3 61.1 89.2 95.4

Adapted, with permission, from Wadsworth and Hayes (1991) John Wiley and Sons.



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It would appear to the present author that the above results broadly reflect the poor milling quality of immature rice grains. The results of the relative proportions of the various fractions and their milling output are such that the last two or three fractions’ fragility may not be quite unexpected. The proposal to separate these fractions and process them separately also seems to be a reasonable one so that the main bulk of the rice is not mixed up with patently lower quality materials. The present author also feels that this problem may probably be more accentuated in the USA and other similar situations because of combine harvesting. Preliminary examination in the author’s laboratory at the Rice Research and Development Centre (RRDC) at Mysore, did not show such appreciable quantities of immature fractions, although a small immature fraction and its milling behaviour were quite similar. The real problem may be that there is no clear definition of immaturity. Indeed there may be different degrees of immaturity, and all of these may be more or less fragile and so liable to fail during milling. Deriving a clear definition or an index of immature rice grain will be helpful. Indeed fragility itself may be a good index. There could be ultimately two solutions for this problem. One is for breeders and agronomists to reduce the problem of variable maturity (cf. variable moisture problem discussed earlier) to the unavoidable minimum. The other is, as the researchers suggested, to separate and process the thinner kernels separately. What is truly interesting in the above work is the suggestion of the use of the thinner fractions as brown rice flour for infant food formulations. Perez et al. (1973) had suggested that endosperm starch was derived mainly from photosynthesis after flowering and hence it increased progressively during maturity. In contrast, grain protein came mainly from translocation of accumulated plant nitrogen at flowering. This may explain why the thin fractions were richer in protein.

3.5.7 Grain topography Raju and Srinivas (1991) showed that the variation in lemma–palea (i.e., husk) interlocking morphology created variation in the depth of grooves (and height of ridges) on the body of the brown rice. Deep grooves retained bran longer and hence required longer/more milling, making such samples liable to more breakage.

3.5.8 Protein content Leesawatwong et al. (2005) made an interesting observation that some varieties had greater storage protein accumulation in the lateral regions, especially upon higher nitrogen fertilisation. They observed that head rice yield seemed to be positively related to the abundance of storage protein in the lateral region of the endosperm in all cultivars. They hypothesised that



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a higher density of storage protein in the lateral region of the endosperm provided resilience to the grain during milling.

3.6 Fundamental cause of rice breakage: interrelationship and synergy between different factors The visit of the Ford Foundation team to India mentioned at the beginning of this chapter was followed shortly thereafter by a comparative rice mill evaluation study. Performance of seven modern commercial-scale rice mills, newly imported and set up on the Foundation’s advice in seven major riceproducing districts of India, was compared with that of prevalent commercial mills working in the respective region (GOI 1971). The work was carried out during 1966–68 by a joint team (of which the present author was a member) drawn mainly from the CFTRI, with participation of a few members from the Government of India and the Ford Foundation. While the study provided many valuable information and did bring out certain differences between the modern and the traditional mills in terms of milling outturn and grain breakage, it did little to throw light on the fundamental reasons of rice grain failure. Rather it indirectly reinforced the notion that the path to improved rice milling and reduced breakage lay in improved equipment alone. Yet, as the relevant dates would show, the science of rice breakage was then just in the stage of picking up momentum and the emerging ideas were rather in conflict with the equipment-centric notion. Another contradiction was the experience with parboiled rice. The latter, a common form of rice in India (Bhattacharya 2004), was well known to break much less than ‘raw’ (i.e., nonparboiled) rice during milling, indeed confirmed in the evaluation study as well (GOI 1971). The prevalent idea was that parboiled rice was ‘harder’ than raw rice and for this reason parboiling ‘improved’ the milling quality of rice. Yet in practice the extent of ‘improvement’ varied from sample to sample and time to time. Thus the theories of the importance of cracks in rice (Rhind 1962, Angladette 1963), the centrality of mill equipment (Faulkner et al. 1963, GOI 1971) and the role of parboiling in improving the milling quality of rice presumably by improved grain hardness did not quite gel together and left many gaps calling for clarification. A comprehensive study was therefore taken up by the present author to clarify these doubts (Bhattacharya 1969). He first determined the proportion of cracked, immature and insect-damaged grains in several paddy samples, identified by viewing in transmitted light after dehusking by the hand. Their relation to breakage of these samples after milling with laboratory McGill equipment (a sheller using one rubber roller and one ribbed-metal roller for dehusking and a metal roller friction-type whitener) was then examined. The same or similar samples were then parboiled in the laboratory, shade dried, milled and studied. It was noted with surprise that the amount of the



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above-mentioned defective grains in the hand-dehusked brown rice and the amount of broken rice obtained after milling corresponded remarkably well with each other, suggesting that it was precisely these grains which largely broke upon milling. When the samples were parboiled and milled after shade drying, there was virtually no breakage (see Chapter 8) and examination under transmitted light showed no trace of cracks and immaturity, etc. in the lot. The tentative conclusion was that it is the above defective grains which were the principal, if not only, source of breakage during milling and that parboiling healed all the pre-existing defects and therefore eliminated, not just ‘improved’, milling breakage. It is from these data that the concept evolved, that the rice grain per se was mechanically strong and did not break during milling simply because it was exposed to the stress of milling; the breakage arose mainly from pre-existing defects in the grain. In other words the cause of rice breakage during milling primarily lay in the grain itself and not in the act of milling; the latter only made the latent breakage manifest. In as much as there were different kinds of milling equipment (rubber-roll sheller, under-runner sheller, impact sheller; abrasive emery mill, friction mill of metal rollers), and in as much as rice grains had variable dimensions (primarily length and slenderness) as well as variable moisture contents at the time of milling, the interrelationship and synergy among these various factors were then studied in a series of papers by Indudhara Swamy and Bhattacharya (1979a, 1979b, 1982a, 1982b, 1984). The different types of cracked and immature grains present in the rice were first examined by careful observation in transmitted light (Fig. 3.11). Subsequent studies showed that grains with multiple transverse cracks (MTC) as well as those with longitudinal and irregular cracks (LC) along with patently immature grains could be considered as highly defective, for these grains largely disappeared from head rice (i.e., broke) upon milling under even relatively mild conditions. Grains having a single transverse crack (STC) did not always break or broke only partly except under harsh milling conditions. Accordingly the following classification was adopted: STC = grains with a single transverse crack MTC = grains with multiple transverse cracks LC = grains with longitudinal and/or irregular cracks ID = grains which are clearly damaged (or bored) by insect HD = highly defective grains = MTC + LC TD = total defective grains = HD + STC + ID. How the type and proportion of these various defective grains changed during progressive soaking and heated-air drying of paddy and also during delayed harvest were also investigated. It was found that soaking of paddy in water led only to transverse cracks, mainly STC but also some MTC grains later (Table 3.12). Heated-air drying, on the other hand, led initially to STC which changed quickly to MTC and upon severe drying to LC grains (Table 3.12).



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a

b

a

c

b

d

c

e

d

f

e

g

f

Fig. 3.11 Types of cracked and immature grains in brown rice. Top row: (a) single transverse crack (STC), (b) two transverse and (c) three transverse cracks (MTC), (d) longitudinal crack (LC) with STC, (e) LC with 2 TC, (f) LC with 3 TC, (g) badly cracked grains. Bottom row: (a) sound and mature, (b) slightly smaller, but otherwise sound and mature, (c), (d), (e) various degrees of immaturity, and (f) mature, but chalky-centre brown rice grains. Reproduced, with permission, from Indudhara Swamy and Bhattacharya (1982a). Table 3.12

Cracks formed on soaking and drying of paddy Time of soaking (hr)

Drying Final Cracked grainsa (%) temp. (°C) moisture (%) STC MTC

Soaking

Untreated 0.5 3.0

N/A RT RT

12.5 -

5.7 25.5 31.2

2.0 4.2 4.9

0.0 0.0 0.0

Drying

Untreated N/A N/A

N/A 60 70

24.2 13.5 9.2

4.1 24.3 4.1

0.0 12.6 36.6

0.0 1.1 44.1

Expt.

LC

Reproduced, with permission, from Indudhara Swamy and Bhattacharya (1982a). a STC = single transverse crack; MTC = multiple transverse cracks; LC = longitudinal or irregular cracks. N/A = Not applicable; – = Not available.

In the standing crop there were initially too many immature grains which rapidly declined as the crop ripened (Table 3.13). A few STC grains were present even before complete ripening of the field, which rapidly increased as days went by and the crop dried, then became transformed into MTC and, in very late stages, to LC grains (Table 3.13). The rationale of progressively



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Milling quality of rice Table 3.13

93

Defective grains in paddy harvested at various stages

Paddy moisture (%)

Cracked kernels (%) STC

MTC

LC

25.3 23.5 17.9 15.1 14.8

3.4 8.2 24.9 29.3 25.2

0.0 2.2 11.4 26.3 22.6

0.0 0.0 1.3 7.3 18.6

Immature grains (%) 21.0 13.0 9.7 5.6 4.8

Reproduced, with permission, from Indudhara Swamy and Bhattacharya (1982a).

increased breakage of rice seen upon milling of paddy undergoing progressive drying and wetting, including in the field, thus appeared explained. Following this, various samples of paddy having diverse sizes and shapes as well as various extents of different kinds of defective grains were shelled and milled with different laboratory equipment (laboratory Satake rubber roll sheller THU 35B, McGill sample sheller (a rather harsh sheller), centrifugal impact sheller; McGill no. 2 friction miller, Satake abrasion mill (Satake Testing Mill, TM 05), Olmia laboratory emery cone). A large number of results were collected, from which a sample set of data is presented for illustration in Table 3.14 (data on left). The following ideas emerged: •

•

•

•

•

Rice breakage was pre-eminently related to the content of defective kernels in the sample being milled. Whatever the grain type and quality and whatever the milling condition, the total grain breakage almost never exceeded the total count of defective grains (TD). Sound grains rarely if ever broke, barring in exceptionally long and slender grains where a certain amount of it could break under harsh milling conditions, especially if the grain moisture was on the high side. HD grains broke rather easily, rather fully, except in round varieties or when using gentle equipment, sometime even at the shelling stage if it was harsh. Not only sound grains but even grains with STC generally escaped breakage, especially under gentle and mild milling conditions, except in very long and slender grains, or after harsh milling systems, where STC might partly break or rarely fully in extreme cases. STC grains broke, if at all, more during shelling than during whitening, so that whatever STC grains that escaped during gentle shelling, would not generally break further, except in long and slender grains or upon harsh whiteneing, or with high moisture. The initial advantage of gentle shelling (e.g., using rubber-roll sheller) was progressively nullified during whitening, depending on the type of whitener used; on the other hand after harsh shelling (centrifugal impact sheller, laboratory McGill sample sheller, excessive shelling in



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29.4

Slender

46.2

67.1

TD

McGill-McGill 62, 0 100, 4 100, 23 100, 46 87, 0 100, 38 100, 64 100, 100, 2

R.Roll-McGill 18, 0 90, 0 100, 5 100, 44 28, 0 73, 0 95, 0 100, 55

R.Roll-Emery 0 0 0 0 0 0 0 0

18, 52, 73, 83, 28, 45, 54, 64,

McGill-McGill 26.6 44.2 48.6 54.2 25.7 35.7 40.2 47.2

R.Roll-McGill 7.8 38.8 44.5 53.6 8.2 21.4 27.7 38.7

R.Roll-Emery 7.8 22.5 31.4 35.7 8.2 13.2 15.9 18.7

0 L M H 0 L M H

Notional proportion of defective grains brokend (%)

Reproduced, with permission, from Indudhara Swamy and Bhattacharya (1984). a Satake laboratory rubber-roll and McGill sample sheller; Satake laboratory emery and McGill metal-roll miller (whitener). b Mean of 2 varieties and 2 harvest stages in each grain type. Full shelling. c 0 = brown rice; L, M, H = low, medium, high milling respectively. d First figure shows % of highly defective grains; second figure shows % of STC grains; and third figure shows % of sound grains broken.

43.2

Round

HD

Breakage (%)

Effect of shellera, whitenera, grain type and degree of milling on rice breakage (expressed as proportion of defective grains)

Grain typeb Defective kernels DMc (%)

Table 3.14


• •

•

95

one pass), which caused rather heavy breakage including of some STC grains, there might be only minor further breakage during whitening. A friction-type whitener (e.g., McGill laboratory mill) was harsher on grain compared to an abrasion mill (Satake emery). In round varieties, even some HD grains escaped breakage; but HD grains broke more or often entirely as the grain type became more long and slender. A relatively high grain moisture (>14%) promoted grain breakage, including even of some sound grain in favourable circumstances.

These data thus confirmed the earlier hypothesis that the rice grain per se was quite tough and did not break merely because it was being milled. The ultimate cause of rice breakage lay in the kernel, and it was almost always only the defective grains that broke. What precise proportion of these defective grains broke, however, was a function of the grain size and shape, the DM, the grain moisture, and certainly the milling equipment and system. It will be noticed that the statement of Angladette (1963) quoted at the start of this chapter expresses more or less the same conclusions. It is remarkable that he could arrive at these conclusions as early as 1963 even before the large amount of evidence required to substantiate them were far in the future. Incidentally the McGill laboratory sample sheller and the McGill laboratory miller were found to be the hardest among the laboratory equipment with respect to causing breakage in rice. However, as mentioned in the first bullet point above, this did not mean that the pair broke even sound grains. What broke was more or less equal to the amount of TD grains. In fact one of the virtues of this pair was that it could act as a very effective ‘TD grains counter’. Another point that emerged from this work was as follows. It can be assumed from the above work that, broadly speaking, the HD grains have the ‘first charge’ on grain breakage during milling, followed by the STC grains. Taking this fact into account, the breakage value can be notionally expressed first as the percentage of HD grains and any residual breakage as that of STC grains. This is shown in the right half of Table 3.14. This way of expression would enable one to compare the efficiency of different equipments even when samples having different amounts and types of defective grains were being milled.

3.7

References

angladette a (1963), Rice Drying: Principles and techniques, FAO Informal Work Bull, Agric Eng Branch, No 23. archer t r and siebenmorgen t j (1995), ‘Milling quality as affected by brown rice temperature’, Cereal Chem, 72, 304−307. autrey h, grigorieff w w, altschul a m and hogan j t (1955), ‘Effects of milling conditions on breakage of rice grains’, J Agric Food Chem, 3, 593−599. © Woodhead Publishing Limited, 2011


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ban t (1971), ‘Rice cracking in high rate drying’, Japan Agric Res Q, 6, 113−116. bautista r c, siebenmorgen t j and cnossen a g (2000), ‘Characteristics of rice individual kernel moisture content and size distribution at harvest and during drying’, in Proceedings of International Drying Symposium (IDS2000), Amsterdam, Elsevier Science, paper 325. benett k e, siebenmorgen t j and mauromoustakos a (1993), ‘Effects of McGill No. 2 miller settings on surface fat concentration of head rice’, Cereal Chem, 70, 734−739. bhashyam m k, raju g n, srinivas t and naidu b s (1984), ‘Physico-chemical studies in relation to cracking properties in rice using isogenic lines’ J Food Sci Technol, 21, 272−277. bhattacharya k r (1969), ‘Breakage of rice during milling, and effect of parboiling’, Cereal Chem, 46, 478−485. bhattacharya k r (1980), ‘Breakage of rice during milling: a review’, Trop Sci, 22, 255−276. bhattacharya k r (2004), ‘Parboiling of rice’, in: Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 329−404. bhattacharya k r and ali s z (1970), ‘Improvement in commercial sun drying of parboiled paddy for better milling quality, Rice J, 73(9), 3, 4, 9, 12–15. bhattacharya k r and indudhara swamy y m (1967), ‘Conditions of drying parboiled paddy for optimum milling quality’, Cereal Chem, 44, 592−600. bhattacharya k r, ali s z and indudhara swamy y m (1971), ‘Commercial drying of parboiled paddy with LSU dryers’, J Food Sci Technol, 8, 57−63. chau n n and kunze o r (1982), ‘Moisture content variation among harvested rice grains’, Transactions ASAE (Am Soc Agric Eng), 25, 1037−1040. chen h, siebenmorgen t j and du l (1999), ‘Quality characteristics of medium-grain rice milled in a three-break commercial milling system’, Cereal Chem, 76, 473−475. cnossen a g and siebenmorgen t j (2000), ‘The glass transition temperature concept in rice drying and tempering: effect on milling quality’, Transactions ASAE (Am Soc Agric Eng), 43, 1661−1667. copeland e b (1924), Rice, London, MacMillan. coyaud y (1950), Le Riz. Etude botanique, genetique, physiologique, agrologique et technologique appliqué a l¢Indochine, Archives de l¢Office Indochinois du Riz. (Saigon) No. 50, 312. craufurd r q (1963), ‘Sorption and desorption in raw and parboiled paddy’, J Sci Food Agric, 14, 744−750. dachtler w c (Ed) (1959), Research on Conditioning and Storage of Rough and Milled Rice: A Review Through 1958, United States Department of Agriculture, Agric Res Serv, ARS 20−27. desikachar h s r and subrahmanyan v (1961), ‘The formation of cracks in rice during wetting and its effect on the cooking characteristics of the cereal’, Cereal Chem, 38, 356−364. faulkner m d, reed g w and brown d d (1963), Increasing Milling Outturns of Rice from Paddy in India, Report to the Government of India, The Ford Foundation, New Delhi. goi (1971), Modern versus Traditional Rice Mills – A performance study, New Delhi, Ministry of Agriculture, Department of Food, Government of India. grant j w (1935), Annual Report of the Rice Research Officer Burma, 85. henderson s m (1954), ‘The causes and characteristics of rice checking’, Rice J, 57 (5), 16−18. indudhara swamy y m and bhattacharya k r (1979a), ‘Breakage of rice during milling – effect of kernel defects and grain dimension’, J Food Process Eng, 3, 29−42. indudhara swamy y m and bhattacharya k r (1979b), ‘Breakage of rice during milling – comparison among different shellers’, J Food Process Eng, 3, 43−47.



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indudhara swamy y m and bhattacharya k r (1982a), ‘Breakage of rice during milling. Types of cracked and immature grains’, J Food Sci Technol, 19, 106−110. indudhara swamy y m and bhattacharya k r (1982b), ‘Breakage of rice during milling Effect of kernel chalkiness’, J Food Sci Technol, 19, 125−126. indudhara swamy y m and bhattacharya k r (1984), ‘Breakage of rice during milling. Effect of sheller, pearler and grain types’, J Food Sci Technol, 21, 8−12. jindal v k and siebenmorgen t j (1994), ‘Effects of rice kernel thickness on head rice yield reduction due to moisture adsorption’, Transactions ASAE (Am Soc Agric Eng), 37, 487−490. johnston t h and miller m d (1966), ‘Culture’, in Rice in the United States: Varieties and Production, United States Department of Agriculture, Agric Res Serv, Handbook No. 289, 74−110. juliano b o and perez c m (1993), ‘Critical moisture content for fissures in rough rice’, Cereal Chem 70, 613−615. juliano b o, cagampang g b, cruz l j and santiago r g (1964), ‘Some physicochemical properties of rice in southeast Asia’, Cereal Chem, 41, 275−286. juliano b o, perez c m and cuevas-perez f (1993), ‘Screening for stable high head rice yields in rough rice’, Cereal Chem, 70, 650−655 kondo m and okamura t (1930), ‘Der durch die feuchtigkeits zunahme verursachte querris (Doware) des reiskorns’, Ber Ohara Inst Landwirtsch Forsch, 4, 429−446. kunze o r (1979), ‘Fissuring of the rice grain after heated air drying’, Trans ASAE (Am Soc Agric Eng), 22, 1197−1207. kunze o r and calderwood d l (2004), ‘Rough-rice drying – moisture adsorption and desorption’, in Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 223−268. kunze o r and chaudhary m s u (1972), ‘Moisture adsorption related to the tensile strength of rice’, Cereal Chem, 49, 684−696. kunze o r and hall c w (1965), ‘Relative humidity changes that cause brown rice to crack’, Trans ASAE (Am Soc Agric Eng), 8, 396−399, 405. kunze o r and hall c w (1967), ‘Moisture adsorption characteristics of brown rice’, Trans ASAE (Am Soc Agric Eng), 10, 448−450, 453. lan y and kunze o r (1996), ‘Relative humidity effects on the development of fissures in rice’, Cereal Chem, 73, 222−224. leesawatwong m, jamjod s, kuo j, dell b and rerkasem b (2005), ‘Nitrogen fertilizer increases seed protein and milling quality of rice’, Cereal Chem, 82, 588−593. li d, wang l j, wang d c, chen x d and mao z h (2007), ‘Microstructure analysis of rice kernel’, International J Food Prop, 10, 85−91. lloyd b j and siebenmorgen t j (1999), ‘Environmental conditions causing milled rice kernel breakage in medium-grain varieties’, Cereal Chem, 76, 426−427. mahadevappa m, bhashyam m k and desikachar h s r (1969), ‘The influence of harvesting date and traditional threshing practices on grain yield and milling quality of paddy’, J Food Sci Technol, 6, 263−266. matthews j and spadaro j j (1976), ‘Breakage of long-grain rice in relation to kernel thickness’, Cereal Chem, 53, 13−19. matthews j, abadie t j, deobald h j and freeman c c (1970), ‘Relation between head rice yields and defective kernels in rough rice’, Rice J, 73 (10), 6−12. matthews j, veal d m and deobald h j (1971), ‘Comparative head rice yields from commercial and laboratory milling equipment’, Rice J, 74 (4), 5−9. morse m d, lindt j h, oelke e a, brandon m d and curley r g (1967), ‘The effect of grain moisture at time of harvest on yield and milling quality of rice’, Rice J, 70 (11), 16−20. noomhorm a and yubai c (1991), ‘Effect of tropical environmental conditions on rice kernel breakage during milling’, J Sci Food Agric, 55, 521−528. perdon a, siebenmorgen t j and mauromoustakos a (2000), ‘Glassy state transition and rice drying: Development of a brown rice state diagram’, Cereal Chem, 77, 708−713. © Woodhead Publishing Limited, 2011


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4 Degree of milling (DM) of rice and its effect

Abstract: Paddy or rough rice, when dehusked, yields brown rice, which has an outer layer collectively called bran. The extent to which bran is removed during milling (or whitening) is called the degree of milling (DM) of rice. The DM affects rice quality in many ways. As the bran layer has a different composition, the DM affects the chemical composition of rice. Fat gets smeared on the grain surface during partial milling, hence DM affects flow and packing properties of rice as well as storage stability. Many micronutrients are concentrated in the bran layer, hence DM of rice affects its nutritive value. DM affects cooking too as the bran layer offers some resistance to cooking. Key words: degree of milling (DM) of rice, DM affects colour, appearance, composition, flow and packing, nutritive value.

‘The average chemical composition has generally been the basis for studying milled rice ... However, the rice kernel is not a homogeneous mixture of carbohydrates, proteins, and minor constituents ... Consequently, data on its average composition provide only an inaccurate knowledge of its chemical nature and properties.’ Barber (1972)

4.1

Milling paddy grain and how much to mill

The rice grain as it is received from the farm after harvesting and threshing is the spikelet, the paddy grain, also referred to as rough rice (Fig. 4.1(a)).



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Degree of milling (DM) of rice and its effect

(a)

(b)

(c)

101

(d)

Fig. 4.1 Photomicrograph of (a) paddy, (b) paddy with lemma (left) and palea (right) detached, (c) brown rice and (d) milled rice (IR 20 variety). Photo: courtesy RRDC.

Unlike the case of wheat and other cereal grains, where threshing directly removes the caryopsis from the glumes, in the case of paddy, threshing only detaches the spikelets from the plant along with the glumes. The two glumes are the lemma and the palea (Fig. 4.1(b)) which completely enclose the inner grain or caryopsis, which is also better known as the brown rice probably because of its general brownish colour (Fig. 4.1(c)). The glumes, or the husk or hulls, of the spikelet (paddy) are woody and siliceous and are not edible. Therefore it is essential to remove the husk before the grain can be rendered suitable for human consumption. This process of removing the husk is a part of the general process of rice milling and is referred to as dehusking, dehulling or more commonly as shelling. What is obtained after shelling during the process of milling is the actual fruit, the caryopsis, commonly known as the brown rice or dehusked/dehulled rice. Brown rice has several layers of outer covering which are botanically and compositionally quite different from the main edible portion inside, viz. the endosperm. These layers consist of the pericarp (itself composed of the epicarp, the cross cells and the cuticle), the testa (seed coat), the nucellus and the aleurone layer – in addition to the embryo attached to one end of the endosperm which is also similarly covered (Fig. 4.2). The aleuorone layer is firmly attached to the inner endosperm, which in turn has a somewhat distinct outermost layer, called the subaleurone layer. All these layers, as they are gradually eroded and separated as a powder during the process of rice milling, including the germ, are collectively called bran of commerce. Being botanically distinct, and having specific biological functions, the composition of the bran is quite different from that of the inner endosperm. The bran layers consist mainly of fibre, hemicelluloses, cellulose, minerals and phytate, but very little or no starch and little protein.



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102

Rice quality Awn

Lemma Palea Pericarp Seedcoat Nucellus Aleurone layer Subaleurone layer Endosperm

Scutellum Plumule Radicle Epiblast

Embryo

Sterile lemmae Rachilla

Fig. 4.2

Line drawing of a longitudinal section of a rice spikelet. Courtesy: K. S. Muthamma Milan.

Having thus a skin-like covering, brown rice, although edible per se, is resistant to cooking. Desikachar et al. (1965) showed that a certain amount of scratching or milling of the intact bran was essential to enable the brown rice to hydrate adequately in boiling water, i.e., to be cooked. In other words at least a partial removal of the bran layer is more or less mandatory to make the rice edible in practice. No doubt brown rice can be rendered edible after somewhat prolonged cooking, but the product has an ungainly appearance from the grain bursting and opening up. While there is indeed a tiny niche section of health-conscious, ‘naturalist’ population for whom brown rice is the preferred form of rice, the cooking defect and ungainly appearance of the cooked product are not acceptable in general. So some amount of bran removal along with that of the husk, already referred to above as the process of shelling, is a normal part of the rice milling process. The process of removing the bran from brown rice is called whitening, pearling or even milling or polishing, which yields the final product, the milled rice or white rice (Fig. 4.1(d)). There has been a long controversy about the extent to which the brown rice needs to be whitened. As in all cereal grains, the botanical and physiological requirements of the seed are such that the micronutrients important for humans, such as vitamins and minerals, are largely present in the rice grain in its outer layers, i.e., within the biologically active bran layers, including the germ. Extensive milling or whitening of the brown rice therefore produces a grain



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103

which is substantially devoid of these essential micronutrients, leading in the past to well-known deficiency diseases, especially beriberi. Nutritionists, particularly ‘naturalists’, have therefore long held, often passionately, that rice should be milled only minimally, viz. to remove say only 2−4% by weight of the brown rice at the most. The protagonists of undermilling offer additional arguments. They argue that excessive milling wastes available food, including nutrition. This argument per se is partly flawed. Few things in life are really wasted. If something is not used in one form, it usually becomes available for use in another form, especially in the social context in which the system is practised. For example, while in no way arguing for excessive milling, it must be understood that if vitamins and minerals located in the outer layers of rice are removed during milling and thus are not available for immediate human consumption, they become available to poultry or domestic animals or even for industrial extraction. The same thing is true of oil. If the oil becomes unavailable to humans because of excessive milling, it becomes available either to the domestic animals or for industrial exploitation. This controversy has been going on for almost a century, but ‘naturalists’ are probably fighting a losing battle. The reasons are partly technological and partly sociological. Initially, in ancient days, rice was milled largely manually using only rudimentary implements, the process being generically referred to as hand-pounding. Hand-pounding was prevalent in substantial pockets of rice-producing countries of Asia even as late as the time of World War II. While rice can certainly be milled to a very high degree even by hand-pounding if one wants to, this is hard work, laborious and time consuming. Hand-pounding therefore generally yielded only lightly milled rice as a matter of course, generally referred to as undermilled rice. Nor was there any objection to the system or the product. For one thing, hand-pounding by itself was a local or even a household process, yielding a familiar product. Besides, it could produce only a small amount of rice in a day, so there was not much storage, and consequent storage deterioration, of such rice. For another, the rice was automatically considered tasty and acceptable too, generally found as ‘sweet’ by the consumer – even literally so because of the sugars in the bran layer! However introduction of machine milling, first in small scale, e.g., by the huller, gradually going larger and larger in scale, changed all that. First, when machines produced large quantities of milled rice at a time and place, storage either with the miller or the trader or with the consumer became a necessity. Deterioration during storage therefore now became an important consideration. This fact gradually created a definite objection to undermilled rice. Undermilled rice contains a substantial quantity of fat and also lipase on its surface apart from other micronutrients and loose particles. It is therefore not suitable for medium or long-term storage, undergoing rancidity and other deteriorations, including insect attack. Technology created other problems and challenges. Hand-pounding or small-scale milling yielded small amounts of



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by-products (husk and bran) which were useful and welcome for immediate local use (domestic fuel, poultry or animal feed). But when milling became a large-scale process, the large amount of husk and bran produced could not only be not utilised locally or immediately but, on the other hand, provided opportunities for a by-product industry. Once large-scale milling and wide marketing of rice were established, therefore, production of undermilled rice gradually became more or less nonviable, except for niche markets. Sociopsychological factors also operated simultaneously, encouraging people to produce and consume more and more ‘refined’, ‘modern’, ‘white’ and ‘attractive’ products.

4.1.1 The degree of milling (DM) of rice Rice can be and is milled to different extents. That is, theoretically and practically, the bran layer can be removed to varying extents up to and including the aleurone layer or even a part of the subaleurone. This is referred to as the degree of milling (DM) of rice. Clearly there is a need to define and test for the DM of rice. Efforts for such definition have been going on for a long time. As a matter of fact numerous methods for estimating the degree of milling of rice have been proposed by a large number of workers, of whom Kik (1951) and Desikachar (1955a) can be said to have been pioneers. There is an interesting history about how the need for estimating the degree of milling of rice first arose. As we will discuss in detail in Chapter 11, the introduction of machine milling of rice during the turn of the nineteenth century suddenly raised the spectre of beriberi throughout the rice countries of Asia. The alarmed international food and nutrition authorities eventually realised that overmilling of rice by machines led to its vitamin B1 (thiamin) content being drastically lowered, which is what caused the deficiency disease of beriberi. Adoption of laws to restrict the milling of rice to a low or medium level such that the resulting rice still had sufficient amount of B1 was strongly suggested. But the suggestion came to nothing when it was soon realised that enforcement of such a law required a clear definition and a method of estimation of the degree of milling, which was then, alas, not available. So the national scientific agencies were strongly urged to develop methods for the purpose, which is what led to studies such as those of Kik (1951) in the University of Arkansas in the USA and Desikachar (1955a, 1955b, 1956) in the Central Food Technological Research Institute (CFTRI) at Mysore in India. A series of methods were thus devleoped over a long period of time. All the early methods were reviewed in detail by Hogan and Deobald (1965). Other reviews, including of more recent works, appeared later (Barber and Benedito de Barber 1979, Wadsworth 1994). The fundamental principle of all the methods of testing the DM is very simple – viz. to determine how much of the grain or any of its constituents has



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been lost (i.e., removed) during milling. Theoretically, the primary method is based on the extent of weight loss. If the brown rice grain has been milled such that say 5.0% of its weight has been removed during milling (as ‘bran’), then the product, i.e., the milled rice thus obtained, has a DM of 5.0%. However, loss of some other constituent – such as the bran layer visible either by the naked eye or as made visible by some staining, extractable bran pigment, or some chemical (such as phosphorus, thiamin, fat, surface fat) – can also be similarly made the basis of a method to estimate the DM. The reverse too is possible. Thus as the rice is milled its colour decreases, in other words its whiteness increases. So the increase in whiteness can be made an index of the DM. A very large number of methods have thus been proposed with varying degrees of success. All these will be discussed in Chapter 13. The limits and limitations of these methods will also be pointed out there. All these methods evolved over a long period of time. Initially no method was considered acceptable. (As a matter of fact, as we will discuss in Chapter 13, the situation cannot be said to have changed greatly even today despite such a steady series of works over a long period of time.) Meanwhile, initially, as international trade in rice was developing, there was need to define and certify the DM of the material entering the trade. So the agencies fell back to defining the DM by verbal descriptions. For example the Food and Agricultural Organisation (FAO 1972) defined various degrees of milling of rice as: ‘husked rice: paddy from which the husk only has been removed; also known as “brown rice”, “cargo rice”, “hulled rice”, “loonzain rice”, and “sbramato rice”; undermilled rice: paddy from which the husk, a part of the germ and all or part of the outer bran layers, but not the inner bran layers, have been removed; reasonably well milled rice (medium milled rice): paddy from which the husk, the germ (part of the germ in the case of round rice), the outer bran layers and the greater part of the inner bran layers have been removed, but parts of the lengthwise streaks of the bran layers may still be present on not more than 30 percent of the kernels; well milled rice: paddy from which the husk, the germ (part of the germ in the case of round rice), the outer bran layers and the greater part of the inner bran layers have been removed, but parts of the lengthwise streaks of the bran layers may still be present on not more than 10 percent of the kernels; and extra well milled rice: paddy from which the husk, the germ (part of the germ in the case of round rice) and the bran layers have been completely removed.’ The United States Department of Agriculture (USDA) too prescribed similar definitions (USDA 2005) for testing of market samples by experienced grain inspectors.



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No doubt such definitions of DM are not at all suitable for routine practical use especially in laboratory studies and researches, regardless of their continued use by trained inspectors in national and international grain trade. So, as already mentioned, a very large number of procedures have been developed over the years, to be reviewed in Chapter 13. Here we will now discuss how the DM affects the quality of the product.

4.2

Effect of degree of milling (DM) on rice quality

It was mentioned at the beginning of this chapter that the structure and composition of the outer layers of the rice caryopsis are very different from those of the inner endosperm. This is not just a question of a few layers, like a skin, but a continuous gradation from the outer periphery to fairly deep inside. Properties of the rice (which are what ultimately decide its quality), therefore change significantly as the rice is milled. Brown rice is a very different material from even undermilled rice and that from fully milled rice in terms of appearance, composition, functionality and nutrition.

4.2.1 Chemical composition The pattern of changing chemical composition within the rice grain from the periphery inwards does not stop with the pericarp or the testa or even the aleurone but continues deep inside. In fact, as far as the brown rice is concerned, one cannot really blandly prescribe where its ‘outer’ stops and the ‘inner’ starts. Although not entirely unknown, the full dramatic impact of this compositional kaleidoscope was for the first time revealed by Barber (1972) in his chapter in the first edition of the American Association of Cereal Chemists’ monograph on Rice Chemistry and Technology, appropriately illustrated with a thematic diagram which is reproduced in Fig. 4.3. Careful studies of the progressive change in composition with successive milling of brown rice have been carried out by numerous workers, but more notably by three groups of researchers: viz. V. Subrahmanyan and his colleagues in the Indian Institute of Science (IISc), Bangalore, India; J. T. Hogan and his colleagues in Southern Regional Research Centre (SRRC), USDA, New Orleans, Lousiana; and S. Barber, E. Primo and their colleagues in the Instituto de Agroquimica y Tecnologia de Alimentos (IATA), Valencia, Spain. The work of the IISc group (Subrahmanyan et al. 1938) was more or less the first detailed work of the change in chemical composition with progressive milling of rice. Their main emphasis was on the loss of nutrients, especially protein, fat and phosphorus, during milling. The SRRC group in New Orleans devised a special abrasive device to gently mill successive layers not only from the outer bran layer but also from the endosperm. With this technique they studied the progressive change in © Woodhead Publishing Limited, 2011


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107

100 Starch 80

%

60

40 Fibre Ash Fat

20

Protein 20 Outer layer

40

60

80

100 Centre

Proportion of the kernel

Fig. 4.3

Distribution of major milled rice constituents within the brown rice grain. Reproduced, with permission, from Barber (1972).

various rice constituents, especially protein, as layer after layer of the rice grain was abraded away (Hogan et al. 1964, 1968, Normand et al. 1966). Some of these revealing data are shown in Table 4.1. These authors were struck by the high-protein layer in rice just below the layer where normal milling generally ceased. From this observation they ended up by producing a high-protein rice flour, largely comprising the outermost layer of the conventionally milled rice, which they thought would be useful for formulation of weaning and infant foods. The protein content of this flour went up to as much as 20% or more under specified conditions. Significantly the protein and other nutrient constituents of the residual kernel was only slightly lower than that of the original milled rice, so the overmilled rice obtained after producing the high-protein flour could be still used in the same way as normal milled rice. The main focus of the third group at IATA, Valencia, was to study how the diverse components changed in successive layers in rice and their effect on the functionality of the grain. Barber (1972) has summarised how this group painstakingly followed the trajectory of change of the numerous and diverse chemical constituents and how each of these invariably showed a sharp drop a little inside the outermost periphery of the conventionally milled grain. Apart from thus presenting a novel vision of the rice grain consisting of rapidly changing composition as the DM changed, their major conclusion was that a thin layer of the milled-rice grain, constituting about 5% by weight in the outermost layer of the milled kernel, was a layer that was dramatically different from the inner kernel both in respect of composition



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2.00 11.90 2.03 12.08

2.17 12.91

2.36 14.04

2.54 15.11

2.77 16.48

3.00 17.85

3.25 19.34

3.48 20.71

3.67 21.84

3.74 22.25

3.66 21.78

1.33 7.91

0.86 5.12

Whole Residual kernel kernel

Reproduced from Normand et al. (1966). a 1 lb = 0.45 kg.

60.27 63.28 70.59 73.24 76.67 80.03 81.35 82.14 85.49 88.17 89.83 90.10 90.68 94.29 25.57 26.49 29.11 31.02 29.45 31.26 34.01 35.88 32.07 30.44 35.08 32.26 31.24 0.17 0.22 0.11 0.22 0.22 0.22 0.78 1.06 1.29 2.40 4.26 0.00 5.77 0.23 4.065 3.237 2.368 1.353 0.872 0.540 0.390 0.352 0.213 0.172 0.098 0.368 0.065 0.402 0.327 0.343 0.295 0.237 0.162 0.163 0.106 0.092 0.074 0.065 0.053 0.099 0.000 3.00 3.00 3.10 4.90 6.97 9.30 2.94 48.09 36.67 25.08 16.87 12.62 5.73 1.236 0.835 0.666 0.379 0.263 0.166 0.160 0.115 0.096 0.090 0.088 0.085 0.140 0.028 0.463 0.301 0.157 0.080 0.036 0.016 0.001 0.000 0.000 0.000 0.000 0.000 0.023 0.000

0.98 18.87

1.68 17.89

1.49 16.21

1.37 14.72

1.28 13.35

1.27 12.07

1.38 10.8

12

11

10

9

8

7

6 1.43 9.42

5 1.65 7.99

4 1.94 6.34

3

2.33 4.40

2

2.07

1

Successive fraction removed

Composition of successively removed fractions of conventionally milled rice (dry weight basis)

% of rice in fraction Cumulative % removed % N in fraction % protein in fraction % starch in fraction % amylose in starch % lipids in fraction Thiamin (mg/lb)a Riboflavin (mg/lb)a Niacin (mg/lb)a Phosphorus (%) Calcium (%)

Component

Table 4.1


109

and in respect of reactivity. A glimpse of this picture can be visualised from the data in Table 4.2 from Barber (1972). The concentration of various constituents including fat, certain proteins, amino acids, different sugars, sulphur compounds and so on in this thin layer could be up to as much as five times or more of those within the central kernel and it was a layer with Table 4.2 rice

Chemical composition of outer layer, nucleus and entire kernel of milled

Constituent

Unita

Starchd Amylosed Reducing sugars Nonreducing sugars Total sugars Fibre

% % g maltose/100 g rice g sucrose/100 g rice % %

61.86 16.12 0.50 2.42 2.92 1.47

Total N Nonprotein N Protein N Albumin Globulin Prolamin Glutelin Insoluble fraction Free amino N

g g g g g g g g g

2.53 0.04 2.49 1.75 1.12 0.72 7.93 3.28 25.11

Alpha-amylase Beta-amylase Protease

mg/100 g SKB units/g rice mg maltose/g rice

Total lipidse Free fatty acids Neutral fats Phospholipids

% % % %

4.44 1.34 2.53 0.57

0.45 0.15 0.26 0.04

0.66 0.21 0.38 0.07

Thiamind Riboflavind Niacind Pyridoxinef

mg/100 g mg/100 g mg/100 g mg/100 g

0.797 0.075 9.270 1.185

0.047 0.019 0.885 0.080

0.081 0.022 1.264 0.128

Ash Calciumd Ironf Phosphorusd

% % % %

6.10 0.359 0.028 1.022

0.45 0.007

0.72 0.023 0.001 0.140

N/100 g rice N/100 g rice N/100 g rice (N ¥ 5.95)/100 g (N ¥ 5.95)/100 g (N ¥ 5.95)/100 g (N ¥ 5.95)/100 g (N ¥ 5.95)/100 g (N ¥ 5.95)/100 g

Outer layerb

rice rice rice rice rice rice

1.0 223.8 6.0

Nucleus

Entire kernelc

92.00 29.85 0.07 0.11 0.18 0.22

90.68 29.46 0.12 0.26 0.38 0.28

1.27 0.018 1.25 0.29 0.60 0.22 5.05 1.48 2.55 0.07 31.2 0.6

– 0.099

1.39 0.019 1.37 0.30 0.67 0.25 5.25 1.69 3.40 0.1 44.9 0.9

Reproduced, with permission, from Barber (1972). a Dry basis. b 5% by weight of entire kernel, unless otherwise specified. c Approximately 10% milling, unless otherwise specified. d Commercially milled rice; outer layer 4.4%. e Chloroform:methanol (2:1) extractable lipids. f Commercially milled rice; outer layer 4.27%. – = Not available.



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very different properties and reactivity than the remaining part of the grain or even the grain as a whole.

4.2.2 Physical properties The brown rice grain contains 2−4% of lipid material. Most of this lipid is located in lipid bodies in the aleurone layer. Other tissues including the endosperm contain negligible quantities of lipid. So like other constituents within the bran, the fat content of the rice also decreases as the rice is milled (Fig. 4.4). Something more interesting also happens. With most of the fat being located in fat globules within the aleurone layer, close to the kernel surface, the lipid bodies are gradually disrupted as the brown rice grain is milled and the oil spreads on the surface of the rice grain. Thus brown rice has no fat on its surface, the entire quantity of fat being located as discrete globules in an inner layer close to the surface. It is only upon milling that the fat globules are disrupted and the fat spreads on the surface. Therefore the amount of fat freely present on the surface of the rice grain initially increases until about 4% degree of milling (by weight) and then decreases as the surface layers are removed (Fig. 4.4) (Bhattacharya et al. 1972). Interestingly, it will be noted that practically the entire amount of the grain lipid is present on the grain surface after about 4−5% DM.

2.5

2.0

Fat, %

t–PB 1.5 t–R

s–PB 1.0 s–R

0.5

2

4 6 Degree of milling, %

8

10

Fig. 4.4 Changes in the amount of fat in the total grain (t) and on the grain surface (s) in raw (R) and parboiled (PB) rice with progressive milling with a McGill miller. Adapted, with permission, from Bhattacharya et al. (1972) John Wiley and Sons.



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One can note in passing that this fact about the first rising and then falling level of the amount of fat on the surface of rice grain with progressive milling of rice, although described about four decades back, seems to have escaped general notice. This may be how the amount of surface fat on the rice kernel has been made into the basis of one method to determine the DM of rice which is widely, almost exclusively, being used by researchers in the USA. Obviously the unstated assumption is that the amount of surface fat in milled rice decreases with progressive milling – which is wrong. The matter is discussed in detail in Chapter 13 (Section 13.6.1). The anomaly of this method is clear from Fig. 4.4, which shows that a value of 0.5% surface fat, for instance, would correspond to, roughly, both 0.9% and 7.6% DM! Regardless of the fact that the former level of undermilling is unlikely to be encountered in practice, the theoretical possibility and hence the anomaly are clear enough. This smearing of fat on the grain surface has a profound effect on the physical properties of rice, especially its frictional, storage and flow properties (Fig. 4.5). The grain density marginally increases with progressive milling because of the reduction in fat. But the bulk density first decreases and then increases as the rice is milled because of the change in frictional property (angle of repose, AR). The smearing of fat on the grain surface increases its grain-to-grain friction (AR) at the beginning which finally decreases again as the fat is removed. As a result, the porosity changes in parallel, affecting the bulk density which first decreases and then increases steeply. As will be discussed in Chapter 8, the same changes occur to a much larger extent in 40

AR 0.78

1.45 d

p

46

1.44

0.77 d p

1.43

Bulk density, g/ml

47

Density, g/ml

Porosity (%)

38

36

Angle of repose, °

48

0.76

dB

34 45

1.42 0

2

4 6 Degree of polish, %

8

0.75

Fig. 4.5 Relationship of various physical properties to degree of milling of raw rice (BS variety). d, density; dB, bulk density; AR, angle of repose; p, porosity. Reproduced, with permission, from Bhattacharya et al. (1972).



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parboiled rice. In effect therefore both the brown rice (unmilled rice) and fully milled rice store and flow well, but intermediate milled rice is poor to store as well as to flow. This is one of the critical effects of the DM on rice quality and perhaps one of the reasons why partially milled rice is not viable in world trade.

4.2.3 Nutritive value As is quite evident from what has been discussed, the DM has profound effect on the nutritive value of rice. The major vitamins (B vitamins, vitamin E) as well as important minerals (calcium, iron …) progressively decrease in concentration as the rice is milled (see Tables 4.1 and 4.2). Antioxidants and other micronutrients also similarly decrease. To that extent, the rice becomes nutritionally poorer as milling progresses. On the other hand, phytic acid too decreases with milling so that its chelating action on minerals decreases, which may have some positive effect on dietary availability of minerals. These aspects will be discussed in greater detail in Chapter 11.

4.2.4 Storage stability The DM has a profound effect on the storage stability of rice. The lack of free-flowing nature of undermilled rice has been mentioned. Undermilled rice not only has a thin layer of fat on its surface but also residual lipase. It therefore easily develops free fatty acids, i.e., lipolytic rancidity, during storage (Hunter et al. 1951, Houston et al. 1952). Fat autoxidation (oxidative rancidity) can also occur either by oxidation from air (Narayana Rao et al. 1954) or through the action of lipoxygenase or both, although antioxidants naturally present in the bran tend to protect the fat from oxidation (Sowbhagya and Bhattacharya 1976). In brief, undermilled rice is poor for storage; brown rice if totally unscratched is not bad for storage (Houston et al. 1952) and fully milled rice stores the best (partly because it is so poor in nutrients!). Piggott et al. (1991) investigated the effects of undermilling and subsequent storage on eating and cooking qualities of rice using both chemical and sensory evaluation methods. Undermilled rice showed greater FFA and carbonyl development as well as deterioration in sensory properties during storage as compared to fully milled rice. Similar conclusions had been made earlier by Tsugita et al. (1980). The poor storage stability of undermilled rice may be one of the major reasons why, once rice became a widely marketed and packaged commodity for trade, undermilling was no longer a viable proposition.

4.2.5 Cooking quality As already mentioned, Desikachar et al. (1965) showed that brown rice did not cook well at all. It would not absorb water easily during boiling, but



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eventually the bran layer would crack open and the grain would burst after which hydration would proceed. But the ungainly appearance of cooked brown rice is not acceptable to any other than the specially motivated. Hydratability improves substantially after a little scratching of the bran (about 2–3% degree of milling by weight) and continues to improve with progressive milling (Fig. 4.6). Roberts (1979) made a similar observation. Cooking of undermilled rice therefore is not much of a problem, even though it may take a little longer and there might be objection to the somewhat coloured and ‘dull’ (less shiny) look to one who is not used to it, although some may actually prefer it (Roberts 1979). In a recent work in the Rice Research and Development Centre (RRDC, unpublished) it was found that (a) not only brown rice but even undermilled rice (3−5% DM) showed a fair amount of grain bursting (fine splitting) when cooked, (b) some curling of the cooked grains also occurred, and (c) the results were independent of whether the samples were milled in a metal or an emery whitener. On the other hand there may be a small but significant difference in the cooked rice texture obtained from undermilled, normal and overmilled rice. When rice is overmilled, especially when the subaleurone layer or part of the endosperm is abraded, the resulting cooked rice becomes slightly stickier and softer than normally milled rice after cooking (Andrews et al. 1993). In fact, this patent suggests that this is one way by which a desired cooked

3.5

Bangara Sanna 6% 4% 2%

Swelling ratio by weight

3.0

1% 0.5% Brown rice

2.5

2.0

1.5

1.0

0.5

5

10

15 20 25 Time, min

30

35

40

Fig. 4.6 Swelling ratio (ratio of weight of cooked rice to uncooked rice) of rice (BS variety) of different degrees of milling (indicated) when cooked with water at 96 °C. Reproduced, with permission, from Desikachar et al. (1965).



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rice texture can be imparted to a given sample of rice if it happens to be too hard and integral after cooking, and conversely a sample can be made to cook a little more hard and free-flowing by milling it somewhat less. Perdon et al. (2001) showed that Rapid Visco Analyser (RVA) peak viscosity of rice increased with increasing DM. The significance of this result is not clear. One possibility is that it may be related to the presence of alpha-amylase in fresh rice. Indeed inactivation of amylase by the authors increased the peak viscosity of the samples. In any case, as will be discussed later in Chapters 7 and 13, peak viscosity as such does not give much of a useful information about the functionality of a sample of rice. What is important is primarily breakdown and secondarily total setback at a fixed peak viscosity value. Park et al. (2001) studied the pasting and sensory properties of rice milled to different degrees. Peak viscosity, breakdown and setback viscosities increased with increasing milling. Again, as will be explained in Chapter 13, it is hard to comment on the implications of these data. Hardness and chewiness of cooked rice decreased while adhesiveness increased. DM had a significant effect on the sensory and physicochemical characteristics of milled rice and cooked rice. Yanase and Ohtsubo (1985) also had made similar observations. To conclude, clearly the properties of the rice grain change, to a lesser or to a greater extent, as its bran layers are progressively removed, i.e., as its DM is progressively increased. In other words the quality of the rice changes with its DM. Some if not all of these changes, especially those in the chemical composition, the nutritive value and the flow and storage properties of the grain, have a substantial effect on the rice quality. In that sense, along with varietal difference in rice quality (Chapters 2, 3, 7, 9, 10, 11), progressive quality change caused by ageing (Chapter 5) and differences in rice properties introduced by processing (parboiling) (Chapter 8), the degree of milling introduces a fourth dimension of quality variability in rice.

4.3

References

andrews r d, locke d, mann j a and stroike j e (1993), ‘Milling process for controlling rice cooking characteristics’, US Patent, 5,208,063, 4 May 1993. barber s (1972), ‘Milled rice and changes during aging’, in Houston D F (Ed.) Rice Chemistry and Technology, 1st edn, St. Paul, MN, American Association of Cereal Chemists, 215−263. barber s and benedito de barber c (1979), ‘Outlook for rice milling quality evaluation systems’, in Chemical aspects of rice grain quality, Los Baños, Laguna, Philippines, International Rice Research Institute, 209−221. bhattacharya k r, sowbhagya c m and indudhara swamy y m (1972), ‘Some physical properties of paddy and rice and their interrelations’, J Sci Food Agric, 23, 171−186. desikachar h s r (1955a), ‘Determination of the degree of polishing in rice, I. Some methods for comparison of the degree of milling’, Cereal Chem, 32, 71−77.



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desikachar h s r (1955b), ‘Determination of the degree of polishing in rice, II. Determination of thiamine and phosphorus for process control’, Cereal Chem, 32, 78−80. desikachar h s r (1956), ‘Determination of the degree of polishing in rice. IV. Percentage loss of phosphorus as an index of the degree of milling’, Cereal Chem, 33, 320−323. desikachar h s r, raghavendra rao s n and ananthachar t k (1965), ‘Effect of degree of milling on water absorption of rice during cooking’, J Food Sci Technol, 2, 110−112. fao (1972), ‘Degree of milling of rice, The standard method adopted by the food agency, government of Japan’, Sub-group on Rice Grading and Standardisation, Intergovernmental Group on Rice, Rome, Food and Agricultural Organisation, Document CCP: RI/CS C.R.S. 1, May 10−12. hogan j t and deobald h j (1965), ‘A review: measurement of the degree of milling of rice’, Rice J, 68 (10), 10, 12−13. hogan j t, normand f l and deobald h j (1964), ‘Method for removal of successive surface layers from brown and milled rice’, Rice J, 67 (4), 27−34. hogan j t deobald h j, normand f l, mottern h h, lynn l and hunnell j w (1968), ‘Production of high-protein rice flour’, Rice J, 71 (11), 5−6, 8−9, 32. houston d f, mccomb e a and kester e b (1952), ‘Effect of bran damage on development of free fatty acids during storage of brown rice’, Rice J, 55 (2), 17−18, 27−28. hunter i r, houston d f and kester e b (1951), ‘Development of free fatty acids during storage of brown (husked) rice,’ Cereal Chem, 28, 232−239. kik m c (1951), ‘Determining the degree of milling by photoelectric means’, Rice J, 54 (13), 18−22. narayana rao m, viswanatha t, mathur p b, swaminathan m and subrahmanyan v (1954), ‘Effect of storage on the chemical composition of husked, undermilled and milled rice’, J Sci Food Agric, 5, 405−409. normand f l, soignet d m, hogan j t and deobald h j (1966), ‘Content of certain nutrients and amino acids pattern in high-protein rice flour’, Rice J, 69 (9), 13−18. park j k, kim s s and kim k o (2001), ‘Effect of milling ratio on sensory properties of cooked rice and on physicochemical properties of milled and cooked rice’, Cereal Chem, 78, 151−156. perdon a a, siebenmorgen t j, mauromoustakos a, griffin v k and johnson e r (2001), ‘Degree of milling effects on rice pasting properties’, Cereal Chem, 78, 205−209. piggott j r, morrison w r and clyne j (1991), ‘Changes in lipids and sensory attributes on storage of rice milled to different degrees’, Int J Food Sci Technol, 26, 615−628. roberts r l (1979), ‘Composition and taste evaluation of rice milled to different degrees’, J Food Sci, 44, 127−129. sowbhagya c m and bhattacharya k r (1976), ‘Lipid autoxidation in rice’, J Food Sci, 41, 1018−1023. subrahmanyan v, sreenivasan a and das gupta h p (1938), ‘Studies on quality of rice. I. Effect of milling on the chemical composition and commercial qualities of raw and parboiled rice’, Indian J Agric Sci, 8, 459−486. tsugita t, kurata t and kato h (1980), ‘Volatile components after cooking rice milled to different degrees’, Agric Boil Chem, 44, 835−840. usda (2005), United States Standards for Rice, United States Department of Agriculture, Federal Grain Inspection Service, Washington, DC. wadsworth j i (1994), ‘Degree of milling’, in Marshall W E and Wadsworth J I (Eds.), Rice Science and Technology, Marcel Dekker, New York, 139−176. yanase g and ohtsubo k (1985), ‘Relation between rice milling methods and palatability of cooked rice. I. Relation between the quality and physico-chemical properties of milled rice and textural parameters of cooked rice’, Rep Nat Food Res Ins, 46, 148−161.



6/8/11 8:59:02 AM

5 Ageing of rice

Abstract: The age of rice after harvest strongly affects its eating quality. Consumers in south Asia dislike new rice but those in northeast Asia dislike old rice. The water uptake and loss of solids during cooking, viscogram breakdown and rice stickiness progressively decrease during ageing of rice, while volume expansion, viscogram setback and cooked-rice hardness and fluffiness increase. In other words, the grain substance becomes progressively organised and reinforced as rice ages. Cold storage retards ageing and heating promotes it. Ageing is unique in rice: no other grain shows such behaviour. Many theories have been proposed to explain rice ageing – including disulphide or carbonyl or fatty acid reinforcement of starch or protein bodies or cell walls – but none has been clinched yet. Rice ageing remains the last frontier of rice research. Key words: ageing of rice, effect on cooking and texture, free fatty acids, carbonyl compounds, strength of starch granules, last frontier of rice research.

‘It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), ... And so these men of Indostan Dispute loud and long, . . . Though each was partly in the right, And all were in the wrong!’ John Godfrey Saxe (1873)



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117

Introduction

Rice is unique among cereals in more ways than one. Not the least of these is the response of rice to storage. Cereal grains are seasonal crops, produced once or at best twice a year. But as staples, they are required for food throughout the year. Obviously foodgrains, including rice, need to be stored for long periods. What is unique in rice is that its cooking and eating quality is found to undergo a profound change during its storage after harvest. Rice milled from paddy soon after harvest cooks rather soft, sticky and lumpy. These properties change after the grain has been stored for several months. When cooked, the rice is now fluffy and nonsticky. This transformation is a phenomenon that is unique with rice and is not known to occur with any other cereal grain. The reaction of rice-eaters to the above phenomenon of storage change in rice varies with their culture, habit and geography – and the type of rice that grows in the region. The rice country of the world can be depicted as a sort of a crescent stretching from south Asia at one end to northeast Asia at the other (Fig. 5.1). What is interesting is that people of south Asia at one end of this crescent have a hearty dislike for rice that has been freshly harvested

Each dot represents 10 000 ha

Fig. 5.1 Map of the rice country. Over 90% of world’s rice is grown and consumed here. It can be depicted as a crescent stretching from south Asia at one end to northeast Asia at the other. The original map is reproduced from IRRI (2007). The crescent sign has been superimposed on the map by the author.



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(henceforth called ‘new’ rice) and look forward to its ageing. They even strongly believe that new rice does not cook well, is not digested well and gives rise to digestive problems! People of Japan and Korea at the other end of the crescent, on the other hand, have a equally hearty dislike of aged rice (henceforth called ‘old’ rice) – for it is said to yield an undesirable hard and dry texture and a ‘stale’ flavour – and would do everything to prevent the ageing of their rice. The fact that the indica rice that is natively grown in south Asia is by nature comparatively free, fluffy, nonsticky and firm when cooked, while the native japonica rice of northeast Asia is quite the reverse (these aspects will be discussed in detail in Chapter 7), may obviously have much to do with these preferences. The influence of geography continues. The intrinsic character of rice in southeast Asia lying at the belly of the crescent is intermediate in character between these two extremes (discussed in Chapter 7); so also appears to be the reaction of the people therein to the fact of rice ageing. Ageing of rice as a phenomenon is no doubt recognised and accepted there, but it does not look as if people there are overly concerned about it. One may add here that the people of west Asia (so-called middle east), Saudi Arabia in particular, who have taken to eating of rice in a big way during the last several decades, too seem to have much dislike of new rice and want their rice to be old. This fact is evidenced by the careful marking of the claimed date of harvest and the date of milling of the sample in all packages of basmati rice sold there – supposedly testifying to the fact of how well the rice has been aged before it is sold to the consumer! As far as we can surmise, rice consumers in Europe and North America too on the whole seem to prefer old to new rice, though they are clearly not so psychologically sensitive about it. In line with the Indian’s obsession with new and old rice, scientific attention to this phenomenon was also first drawn, as best as we can see, in this region. Attention of the then fledgling scientific institutions in Bangalore in India was drawn to this phenomenon in the 1930s. Since then numerous workers have been repeatedly drawn to this fascinating phenomenon and been devoting time in efforts to understand why rice changed in its cooking quality during the period of its storage. A mass of data has thereby been collected, but it is difficult to say that as a result today the enigma is close to being solved. In fact it seems that the phenomenon of ageing of rice and the reasons thereof are the last frontier of rice research. Great progress has been made in understanding rice and its properties during the last five to six decades. But despite strenuous efforts, the phenomenon of rice ageing has still defied a clear understanding. If someone says that rice researchers cannot yet confidently claim to have crossed the stage of ‘the blind men and the elephant’, quoted at the beginning of this chapter, (s)he may not be exaggerating too much. The present author recalls that he was the advisor for a student writing a Master’s level dissertation on this very topic over three decades ago (Mohan Reddy 1977). If one were to compare that review with any current one, one



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would find a great accretion of data now compared with then but not a great deal of difference in understanding. The present author also recalls that as an aspiring biochemist he used to be thrilled in the 1950s and 1960s reading the yearly reviews on carbohydrate metabolism in the Annual Reviews of Biochemistry. But the mass of contradictory data therein used to shed as much confusion as light: until, one day, a review came along from Krebs (1943) and, like a magic show, all the pieces seemed to suddenly fall in place. Let us all hope rice ageing’s Krebs will arrive soon.

5.2 Consumers’ perception of changes in rice behaviour during storage The main reason why cereal grains have occupied such an important place in human civilisation is the fact that grains are in equilibrium with the ambient atmosphere. In other words, cereal grains are low-moisture foods and hence can be stored for long periods. Food security is synonymous with foodgrains. This is not to say that grains are immune to deterioration during storage. Such deterioration may involve physical disappearance (consumption by pests), weight reduction (due to loss of moisture or caused by respiration) and quality deterioration on several counts. Deterioration of cereals during storage and its prevention is a subject of huge scientific and economic interest on its own right. There are many treatises written on the subject and this subject will not be discussed further here. While dealing with storage of rice, however, a matter of equal concern is the profound changes in its cooking and eating quality that rice undergoes during its storage after harvest. Because of the civilisational importance of this subject – taste and acceptability of food being of profound importance to human culture – this phenomenon of ageing of rice has attracted a good deal of scientific attention since the 1930s. Early researchers verified and vividly described these perceptional changes as a preliminary to undertaking research on these subjects.

5.2.1 Colour and appearance Colour is one characteristic of milled rice that the consumer normally is only aware of when it is unusual. The colour of rice changes marginally during storage, old milled rice being a faint shade darker (brownish) than new rice. Grains of old milled rice also have a characteristic faint shade of opacity compared with the faintly more shining and translucent grains of new milled rice. Actually these are two characteristics by which experienced people can fairly easily identify if the rice is aged, but generally this change is faint except under conditions when there is perceptible deterioration in



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the quality of rice. This aspect has been reviewed by Barber (1972), who showed that the colour change was more or less negligible under good and hygienic conditions of storage. However, a small shift towards yellow or brownish colour might become perceptible especially when the rice moisture and the ambient temperature were somewhat on the high side. Slight browning was also facilitated if the rice was somewhat undermilled. Pelshenke and Hampel (1967) examined the effect of storing milled rice in air, oxygen, nitrogen and carbon dioxide at 2, 20 and 35 °C; they found the slight yellowing effect at 35 °C was independent of the storage atmosphere. Kim et al. (1988) made similar observations. In brief, change in colour and appearance of rice when stored could be considered as marginal and very characteristic under normal or good conditions of storage, and in any case was inconsequential compared with the profound changes that occurred in the culinary properties of the rice.

5.2.2 Odour Change in flavour, especially odour, was more or less similar to that of colour. Barber (1972) again discussed this aspect and showed that significant change in odour occurred only under somewhat unfavourable conditions of storage, viz. a relatively high temperature and high grain moisture content. The Japanese consumer, however, is extremely sensitive to flavour changes and many Japanese workers, especially Yasumatsu et al. (1966), invariably reported that rice stored especially under somewhat unfavourable conditions of temperature and moisture developed a ‘stale flavour’ typical of stored rice. Pelshenke and Hampel (1967) also made similar observations.

5.2.3 Cooking and eating quality While the above changes are subtle and may be considered negligible under normal storage conditions, profound changes are perceived to occur in the culinary and eating qualities of rice during its storage. The major changes are: the cooked new rice in its bulk does not swell within the cooking pot as much as one might expect; the cooked rice feels perceptibly sticky, lumpy and moist; and the excess cooking water, whenever rice is cooked by boiling in excess water, appears thick and viscous. These properties change dramatically after the rice has been stored for several months. The mass now looks fluffy and swells in the vessel during cooking, the cooked grains are rather dry on the surface and do not cling together, and the excess cooking water is thin and free-flowing. The Indian consumer has also traditionally persuaded him/herself that cooked new rice is rather hard to digest and is apt to cause digestive problems (Sahasrabudhe and Kibe 1935, Sreenivasan 1939). Indian rice scientists often rationalised that the lumpy character of cooked new rice with a slimy watery film surrounding it probably provided a barrier to the permeability of digestive enzymes (Sanjiva Rao 1938), which was what reportedly caused digestive problems whenever new rice was eaten. © Woodhead Publishing Limited, 2011


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5.3 Changes in physicochemical properties of rice during ageing as measured in the laboratory While the above are the changes in the visual, odorific and culinary properties of rice as perceived by the consumer, scientists have tried to make careful measurements of related properties in the laboratory. Scientists in India were probably the first, but they were soon followed by others. These include Subbaramaiah and Sanjiva Rao (1937) and Sanjiva Rao (1938, 1948) in the Central College, Bangalore, Sreenivasan (1938, 1939) in the Indian Institute of Science (IISc), Bangalore, and Desikachar (1956) and Desikachar and Subrahmanyan (1959, 1960) at the Central Food Technological Research Institute (CFTRI), Mysore in India; Hogan (1963) and Kester et al. (1963) at the Southern (SRRC) and Western (WRRC) Regional Research Centres of the United States Department of Agriculture (USDA) at New Orleans, Louisiana and Albany, California, respectively, in the USA; Yasumatsu and colleagues (Yasumatsu and Moritaka 1964, Yasumatsu et al. 1964, 1965, 1966) and Tani et al. (1964) in the Takeda Company at Osaka and the National Food Research Institute (NFRI) at Tsukuba respectively in Japan; Pelshenke and Hampel (1967) at the Federal Research Centre for Cereal and Potato Processing (BFAGKT) at Detmold, Germany; and Primo and his group of Spanish scientists, whose sustained work at the Instituto de Agroquimica y Tecnologia de Alimentos (IATA), Valencia, during the 1960s and 1970s has been summarised by Barber (1972) in his seminal chapter in the first edition of the rice monograph brought out by the American Association of Cereal Chemists (AACC).

5.3.1 Grain hardness and milling quality Textbooks on rice and reviews on rice ageing routinely mention that the grain hardness as well as milling quality of rice improves during ageing: hardness increases and milling breakage decreases, i.e., the head rice yield (HRY) increases after storage. Many new papers on rice ageing also routinely make these statements quoting each other. Careful search indicates that all these statements seem to originate from one old report. The report by Villareal et al. (1976) mentioned, quoting a work by Juliano shortly before, that there was a great increase in HRY (from 15% to 64% in one variety and 28% to 60% in another variety) after ageing for six months. Simultaneously grain hardness too increased (after a standard grinding test, the amount of flour retained on an 80-mesh sieve increased from an average of 40% to 45%). There was no change in the hardness of defatted and waxy milled rice. This report remains to be confirmed. The observed increase in HRY is of such magnitude as to seem most unlikely. Two relatively recent studies from the University of Arkansas provided some more evidence. Daniels et al. (1998) observed an increase in HRY of approximately 10 percentage points after storage of rice for 3−8 weeks. This



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value too seems too high to be obtainable in actual milling practice. This was followed by another study by Pearce et al. (2001) where interestingly the findings were contradictory. Here the HRY again initially increased but then later it curiously decreased, which is difficult to understand. The situation is similar with respect to grain hardness. Clearly, despite being widely reported, these conclusions need to be confirmed. The present author on the basis of his long experience, including decades of work in the CFTRI and in the Rice Research and Development Centre (RRDC), Mysore, doubts whether there is any significant change in these properties after ageing, although he admittedly did not do any direct experiment on the matter. It may be mentioned that Shu et al. (2000) did not find any change in milling quality upon ageing.

5.3.2 Water uptake, hydration and swelling As rice is invariably cooked in water before it is put up for consumption, hydration, i.e., water absorption by rice during cooking, was what was first measured by early scientists. Sanjiva Rao (1938) devised careful systems to measure the water absorption of rice during cooking (‘swelling number’, ‘water uptake’). He reported that the water uptake of new rice was rather low and increased upon ageing. Sreenivasan (1938, 1939) made similar observations with respect to actual volume expansion of the grains during cooking as measured by water displacement. Desikachar (1956), Desikachar and Subrahmanyan (1959, 1960), Hogan (1963), Pelshenke and Hampel (1967), Barber (1972) and a host of later and more recent workers also observed that the water uptake of rice increased after storage. A set of classical data is shown in Table 5.1. Villareal et al. (1976) studied the changes in rice properties during ageing. They stored four varieties of rice (waxy and low- , intermediate- and highamylose rice, differing in protein contents as well) for six months, both as paddy and as milled rice and also as isolated starch and as surface-defatted rice, at 2 and 29 °C. This was a well-conceived experiment designed to bring out the possible roles of starch, protein and fat. The limitation was that the samples were tested for their various properties only at the beginning and again after six months of storage. The present author can vouchsafe that the rice grain is too elusive an entity to reveal its secrets in only one encounter! Other details of this work will be discussed under appropriate paragraphs. For the present we note that the authors observed a clear increase in the water uptake of rice after ageing at 29 °C regardless of the form in which the rice was stored and also regardless of the amylose content, but the increase was negligible or nil after storage at 2 °C. Desikachar and Subrahmanyan (1959), apart from water uptake or bulk swelling, also measured the actual physical expansion of the individual rice grain during cooking. They noted that the rice grain expanded much more in length and very little in its width upon cooking. More importantly,



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+5 +25 +35

+5 +25 +35

+5 +25 +35

13.0

14.3

15.7

Reproduced, with permission, from Barber (1972). – = not applicable.

Temperature (°C)

Moisture content (%)

262 262 262

258 258 258

256 256 256

Feb

270 271 288

272 269 286

269 277 283

Apr

291 304 302

284 302 309

284 317 307

May

287 304 288

273 – 311

291 291 294

Oct

Water absorption (g water/100 g rice)

6.1 6.1 6.1

6.3 6.3 6.3

6.4 6.4 6.4

Feb

5.8 5.8 3.4

6.3 5.7 4.0

6.6 6.4 5.0

Apr

6.3 5.7 2.9

6.4 5.7 3.4

6.7 5.8 4.9

May

Solids leached (g/100 g rice)

Changes in water uptake and total solids in residual cooking liquids during airtight storage of milled rice

Storage conditions

Table 5.1

6.4 5.2 2.6

6.2 5.3 3.1

6.6 5.3 4.6

Oct

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the stored grain elongated much more than the newly harvested grain upon cooking. For example, after 15 min of cooking new rice elongated 1.41 times its original average grain length whereas the same rice when old elongated 1.64 times. They also observed that the new rice tended to burst (split) open relatively more during cooking than the old rice. The same authors (1960) also verified in a quantitative volumetric experiment that old rice swelled much more in volume after cooking, and thereby appeared more free and fluffy, than new rice. It should be noted that this reference to swelling is only to bulk swelling and not necessarily to actual expansion of the individual grains as could be measured by liquid displacement. It is a cliché known to every Indian who runs a household that old rice ‘increases’ (i.e., swells) more than new rice after cooking; so one can feed more people with a given amount of rice (uncooked) if it is old than if it is new! Similar results of increased water uptake during cooking after storage have been noted by very many other workers up to the present, too numerous to list here. However, there is a limitation in most of these works. A majority were conducted for too brief a period of storage – often a few weeks, generally up to a few months, and rarely at best a year from the time of harvest – to be able to catch the long-term trend. Indudhara Swamy et al. (1978) carried out an elaborate work running up to a little less than four years. They stored seven varieties of rice, comprising japonica, tall indica and dwarf indica groups, both as paddy and milled rice, in dark and exposed to light, at 1−3 °C (cold), room temperature (RT) and 37−40 °C (hot) and studied their properties at six time intervals over the entire period (plus the beginning). Hydration capacity was measured not only at boiling temperature but also at 80 °C and at the ambient temperature. The results clearly showed that while the rate or extent of hydration indeed increased upon storage up to 6−10 months, there was a steady decline in hydration in subsequent months without any indication of its reaching an equilibrium state (Fig. 5.2). This was true of all classes of rice, stored both as paddy and milled rice, and at all the temperatures of hydration tested. The change was negligible or very slow in cold and increased with increasing temperature of storage. Clearly the long-term trend was a progressive reduction in hydration upon ageing, although there was indeed an initial increase at the beginning. As subsequent and other related evidence suggests, to be discussed later, the initial increase was apparently a result of progressive decline in residual amylase activity in the grain, which declined and ultimately disappeared with time. The reflection of this long-term decline in hydration ability was seen in other properties as well, for example in paste viscosity, as will be discussed below. Most workers who studied the ageing phenomenon after a brief storage or at only one or two time intervals − even long after the above work was published, until today − invariably missed this and other related long-term trends. Bolling et al. (1978) also carried out a similar work of storage for several years and they too found a decline in swelling number upon storage. However, one should note that they measured the swelling number of their rice at



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1.55

RT W96 (g/g)

1.45

C

H

1.35

1.20 0

12 24 36 Time of storage, months

Fig. 5.2 Change in water uptake upon cooking during storage of rice. Mean of seven varieties, two states (paddy, milled rice), two storage states (dark, light). Stored at 1−3 °C (C), room temperature (RT) and 37–40 °C (H). Reproduced, with permission, from Indudhara Swamy et al. (1978) John Wiley and Sons.

77 °C. Because of the influence of the gelatinisation temperature (GT) of the rice upon its hydration at this temperature, as will be discussed in detail in Chapter 6, hydration at 77 °C did not necessarily parallel the water uptake at boiling temperature. Such data are often difficult to interprete because of the influence of two contrary forces. Indeed, Lee et al. (1993) also studied water uptake at 77 °C for up to five years and they found the water uptake to increase with time. Unnikrishnan and Bhattacharya (1995) again studied the effect of ageing on various properties of several raw as well as parboiled rice stored for nearly four years; they confirmed the long-term trend of decline in water uptake after an initial rise. One or two stray reports mentioned that old rice also differed from new rice in the time required to cook. Sreenivasan (1939) mentioned that old rice took a little longer to cook than new rice; Pushpamma and Reddy (1979) mentioned that old rice needed at least 3−4 minutes more to cook than new rice. No other researcher has claimed such difference. On the face of it this difference looks peculiar. As will be discussed in detail in Chapter 6, Bhattacharya and Sowbhagya (1971) showed, confirmed by others, that all rices when cooked in boiling water up to the grain centre attained a moisture content of approximately 74% (wet basis). If that is the case, and if water uptake of old rice, as generally found by other researchers, is more than that of new rice, than old rice if at all should cook faster than new rice. In any case, the present reviewer feels that there is not much of a difference between new and old rice as far as cooking time is concerned.



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5.3.3 Loss of rice solids during cooking The other visible associated effect of cooking, viz. loss of soluble as well as insoluble rice solids into the excess cooking water, has also been studied by most workers. All scientists without exception have noted that the loss of solids during cooking was high (up to 6−10%) in milled rice obtained from freshly harvested paddy but the loss of solids steadily declined as the rice aged, even for relatively short periods (Table 5.1). Here again the reduction in loss of solids was often absent or negligible when the rice was stored in the cold (1−5 °C) and was hastened by high-temperature storage. The iodine-blue colour of the excess cooking water, determined only by a few researchers, has however been often found to increase as the rice aged (Tsugita et al. 1983, Shibuya and Iwasaki 1984, Lee et al. 1993), the meaning of which remains to be understood. One should remember that ‘cooking’ of rice is often a laboratory-specific procedure, with varying time, temperature, scale and protocol, in view of which straight comparisons are often not feasible. Another property noted by a few researchers is the acidity of the excess cooking water. Tsugita et al. (1983) and Tamaki et al. (1993) noted that the pH of the excess cooking water decreased slightly upon storage; these two studies were limited to only 60 and 90 days of storage, respectively. This acidity is probably related to the development of fat acidity, to be discussed below.

5.3.4 Volume expansion Along with water uptake and loss of solids, the expansion in volume of the rice upon cooking was noted by a few scientists (Sreenivasan 1939, Desikachar and Subrahmanyan 1960, Indudhara Swamy et al. 1978, Tsugita et al. 1983, Lee et al. 1993). This property, as we can understand now, is largely a reflection of the decrease in the stickiness of cooked rice. New rice as we shall see is very sticky when cooked and the resulting grain-to-grain adhesion prevents the cooked grains from detaching themselves free and thereby expand in bulk. Upon storage, as the adhesion decreases, the cooked grains are now enabled to expand relatively more freely. Another property which facilitates the expansion in volume is the increased elongation of rice when cooked after storage. As we discussed in Chapter 2, the porosity of a grain mass increases with increased grain slenderness (see Fig. 2.2), so when ageing increases grain elongation after cooking as mentioned above (Desikachar and Subrahmanyan 1959), this fact too would promote increased volume expansion. Volume expansion of the rice when cooked under standard conditions in a graduated tube, such as by the Desikachar and Subrahmanyan (1959) procedure, is thus a good reflection of the overall change in cooking/eating properties of rice during ageing (Fig. 5.3). Indeed Indudhara Swamy et al. (1978) used this property as an indirect estimate of the progressive decline of the stickiness of cooked grain after ageing in their study mentioned above (Fig. 5.4).



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1

2

127

3

5 di 4

di

d

ti

t

j

j

50

40

ti 30

3

j C

RT

H

20

2 12

24

36

12 24 36 Time of storage, month

12

24

36

Increase in volume, %

Increase in volume, ml

Fig. 5.3 Photograph of rice cooked in graduated tubes illustrating poor volume expansion of freshly harvested rice (tube no. 1) and improved expansion after its ageing (no. 2) or ‘curing’ treatment (no. 3). Photo: courtesy K. R. Bhattacharya.

48

Fig. 5.4 Change in volume expansion of 4 g rice cooked with 8 ml water in a graduated tube during storage of the rice at 1−3° C (C), room temperature (RT) and 37−40 °C (H). j, Japonica; ti, tall Indica; di, dwarf Indica. Mean of different varieties and different storage conditions. Reproduced, with permission, from Indudhara Swamy et al. (1978) John Wiley and Sons.

5.3.5 Pasting properties The other property which has been most extensively studied and by virtually all researchers is the change in the pasting property of rice flour upon ageing,



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determined using the Brabender amylograph (or viscograph) in the pre-1990s and more often the Rapid Visco Analyser (RVA) from the 1990s onwards. In fact this has been probably the most widely studied property of rice in connection with its ageing. No doubt, the pasting profile determined under a controlled regimen of heating and cooling, as in the viscograph or RVA procedure, is an extremely sensitive index of the intrinsic properties of all starchy materials including rice. Thus it should ideally provide an excellent reflection of any change that may be occurring in the grain during ageing. Almost all researchers recorded that the paste viscosity in general and the peak viscosity in particular – determined at a constant flour/paste concentration (usually 10%), as is customary – increased upon storage of rice (Hogan 1963, Kester et al. 1963, Yasumatsu and Moritaka 1964, Yasumatsu et al. 1965, 1966, Pelshenke and Hampel 1967, Barber 1972, Villareal et al. 1976, Perez and Juliano 1981, Shibuya and Iwasaki 1982, Shin et al. 1985, Perdon et al. 1997). A set of random data from Shin et al. (1985) is shown in Fig. 5.5. This increase was considered by all a sufficiently satisfactory outcome of the efforts, confirming a significant intrinsic change in rice during ageing. Two caveats are due here. First, contrary results too have been recorded, i.e., several workers noted a decline in peak viscosity upon storage (Kim

25

95

95

Temperature, °C 50 25

(a)

95

95

50

(b)

1000 800 12 mo

Brabender viscosity

600 400 0 mo

200 0 (c)

(d)

1000 800 600 400 200 0

40

80

120

0 Time, min

40

80

120

160

Fig. 5.5 Amylograms of undefatted (a), ether-defatted (b) and methanol-defatted (c) brown rice flour, and brown rice starch (d) during storage at 35 °C. ––– Stored 0 month, ------- 12 months. Reproduced, with permission, from Shin et al. (1985).



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et al. 1988, Matsue et al. 1991, Noomhorm et al. 1997, Tulyathan and Leeharatanaluk 2007). And this drop too was considered an equally satisfactory finding testifying to the fact of ageing change! Second, a rise (or drop) in viscosity no doubt proves some change during storage, but this fact of change was proved by changes in other properties, including water uptake, anyway. One would wish the pasting profile to provide more insight into the actual nature of the ageing. There are a couple of conceptual and procedural issues here which would explain why so much effort yielded so little, even contradictory, information. The present author and his group in their detailed work mentioned above (Indudhara Swamy et al. 1978) also studied the pasting properties of their seven rice varieties (stored as paddy and milled rice at three temperatures) over a period of nearly four years. Two conclusions emerged from this exhaustic work. One, the paste viscosity, especially peak viscosity, determined as per the prevailing paradigm at a constant flour concentration (10%), first rose for several months and then slowly but steadily declined until the end of the studies. In other words the paste viscosity was just like, perhaps a reflection of, the hydration property shown in Fig. 5.2. As the hydration power initially increased and later on steadily decreased, so did the paste viscosity. Clearly the perception of the very large number of researchers that the paste viscosity increased during ageing of rice was wrong. It no doubt increased at the beginning but the long-range trend was actually a decline. Most workers who studied the property after only a brief storage or at only one or two time intervals missed the overall trend. Why some workers saw a decline in peak viscosity upon storage may also be explained, for probably their one experiment came at the declining stage. Before proceeding further, it can be mentioned that the reason of this initial increase and then later decline may be related to the results of separate work of several researchers. Kester et al. (1963) observed that the changes in amylase enzyme and in peak viscosity occurred in parallel during maturation of rice in the field. That is, as the grain amylase decreased, the peak viscosity increased. Many workers noticed a gradual decline in amylase activity during rice storage (Sreenivasan 1939, Hogan 1963, Yasumatsu et al. 1965). Shibuya et al. (1983) and Shin et al. (1985) noted that when they added mercuric chloride to the slurry during the determination of their pasting curve, to inactivate the amylase, the peak viscosity increased. Although Ohno et al. (2007a) did not find a similar effect when they added CuSO4 to the mix, the overall trend of data suggests that the suppressed peak viscosity observed in freshly harvested rice could be a result of amylase activity in such rice; the amylase activity progressively declined as the rice was stored, as a result of which the peak viscosity increased. After several months the amylase activity virtually disappeared and from this period onwards the peak viscosity steadily declined over time, showing clearly that the long-term trend was a decline in viscosity. In other words, the initial increase seen in the first few months



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was only a disturbance in the long-term trend caused by an external agent, viz. amylase. One can also surmise that the initial increase followed by a steady decrease in the water uptake seen earlier (Fig. 5.2) could also have been caused by the same amylase in the fresh rice and its steady decline over a few months. Having settled this issue that ageing of rice was accompanied by a longterm trend of a steady decline in its paste viscosity, the question arises as to what it signified. Here comes the second limitation of the large amount of pasting work carried out by so many workers. The problem of viscograph technique arises here. Indudhara Swamy et al. (1978) in their work further noted that, while the paste viscosity of rice at a fixed paste concentration initially increased and later on decreased, the huge number of viscograms ran by them failed to provide any additional useful information. The breakdown and setback in particular, which are considered characteristic features of the pasting profile, gave confusing and even contradictory trends of change over the storage time. The authors became conscious from these results about a deficiency in the technique of viscography itself. A detailed examination suggested that the determination of the pasting profile at a fixed flour concentration as well as arithmetical calculation of the breakdown and setback, as per the prevailing paradigm, were the culprits. To be useful, the authors concluded, the profiles had to be determined at a fixed peak viscosity value, when the breakdown and setback immediately became meaningful. A new method of pasting studies was proposed by the group on this basis (Bhattacharya and Sowbhagya 1978, 1979). This matter will be explained in detail in Chapter 13. The veracity of the comments above has been proved by the work carried out later by Sowbhagya and Bhattacharya (2001) on ageing using the new technique. These workers studied 15 rice varieties of varying amylose contents and varying quality characteristics over a period of over four years studying their pasting profiles at three to five flour concentrations each at each time interval of storage (a total of some 500 viscograms). This mass of data revealed a world of information (for definitions, see Table 5.2). The trend of change in breakdown (actually relative breakdown [BDr] alone is shown here) as also of other paste characteristics at a fixed peak viscosity (P) value are shown in Figs 5.6 and 5.7. What is striking is the progressive decrease in breakdown with time of storage (Fig. 5.6). Simultaneously the setback (SB) or total setback (SBt), the time (or temperature) at which the viscosity attained the peak (tp), the lowest P value at which the paste first registered a breakdown (Pmin (BD)), and the P value at which it equalled the cold-paste viscosity (P = C) – all determined, to remind, at a fixed P value, not a fixed flour concentration – steadily increased over the storage period (Fig. 5.7). Clearly the changes were such as to suggest as if the starch granules were becoming progressively more resilient and reinforced as the rice aged. It may be recalled that a precisely similar increase in the strength



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131

Definition of terms

= Peak viscosity, i.e., the maximum viscosity reached during the initial ‘heating’ period (increasing temperature, 43.3 min if in the Brabender) plus the ‘cooking’ period (constant highest temperature, usually 20 min at 95 °C if in the Brabender). Where the viscosity does not show a peak but continues to rise until the end of cooking, the H value is taken as the P value also (H = P) H = Hot-paste viscosity, i.e., the final viscosity reached at the end of cooking at 95 °C (or the highest set temperature) C = Cold-paste viscosity, i.e, the viscosity value attained as the paste is cooled to 50 °C (or any other lowest cooling temperature set) tp = Time of heating needed to reach the P, which is an index of the temperature at which P is reached; tp £ 43.3 min in the Brabender means the P is reached within the ‘heating’ time up to 95 °C; tp > 43.3 min in the Brabender means P is reached not during ‘heating’ but during 95° ‘cooking’; tp = 63.3 min in the Brabender means the sample shows no P, so that H value is taken as P BD = Breakdown = P – H BDr = Relative breakdown = BD/SBt = (P – H)100/(C – H) % SB = Setback = C – P SBt = Total setback = C – H C/H = Cold-paste: hot-paste viscosity ratio PC intersection point = Zero–SB point = viscosity value at which P = C Pmin (BD) = The minimum P value at which the paste registers a breakdown during heating or cooking, i.e., the minimum P value at which there is a fall in viscosity during ‘heating’ or ‘cooking’ P

From Bhattacharya and Sowbhagya (1979).

and resilience of rice starch granules were observed with increasing amylose content among rice varieties (Radhika Reddy et al. 1993, 1994, Sandhya Rani and Bhattacharya 1995a, 1995b). This aspect will be discussed in detail in Chapter 7. These pasting data thus provided a clear clue regarding the ageing process. That is, apparently the starch granules gradually became more reinforced and internally strengthened such that they did not hydrate, swell or break down as easily as before and this is what caused the changes during ageing. As we shall see later, there is some evidence against any change in the starch granule per se. If that is true, then the conclusion might be that some external changes occurred (cross-linking, for instance, by carbonyl bridges) which might indirectly reinforce the granules. Incidentally the P value of the samples as well as the final cold-paste viscosity value (C) at a fixed flour concentration (10%) were also determined. The change in the P value (of 10% paste) with time (Fig. 5.7(b)) was exactly as reported in the earlier work of Indudhara Swamy et al. (1978) and again paralleled the initial increase and later progressive decrease in water uptake seen in the earlier work as well as here (Fig. 5.2). The final C value at a fixed concentration increased steadily, indirectly reflecting the increased hardness of the cooked rice as the rice aged. Another interesting result that came from this pasting work was about the index C/H, i.e., the cold-paste : hot-paste ratio, in other words, how many



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400

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Fig. 5.6 Progressive fall in viscogram relative breakdown (BDr) during storage of different quality-type rices (types I, II, III, IV, VII and VIII rice, decreasing in total and water-insoluble amylose contents in the same order). The BDr values reported are means of 1−4 varieties for each quality type and are for the peak viscosity value of 1000 Brabender units (BU). Reprinted from Sowbhagya and Bhattacharya (2001) with permission from Elsevier. (a)

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Fig. 5.7 Progressive change in viscogram parameters during storage of rice. (a) Setback (SB) and total setback (SBt) at P = 1000 BU, (b) peak viscosity (P) at 10% paste concentration, (c) PC intersection (zero setback) point and minimum P value where a breakdown is first registered (Pmin (BD)), and (d) time of heating required to reach P (tP) at 10% concentration. Reprinted from Sowbhagya and Bhattacharya (2001) with permission from Elsevier.



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times the viscosity of the hot paste rose during its cooling. Unlike the other indexes above, this index did not change at all during the storage. These data and their implications will be discussed a little later below. Thus these data not only shed valuable light on the real changes in pasting property as also on the process of rice ageing, they also showed how much valuable information could be extracted from pasting studies if one shed dead paradigms and adopted proper techniques.

5.3.6 Cooked-rice texture Early researchers had no instrumental help to measure the texture of cooked rice, but they noted their subjective impression that cooked grains of new rice appeared soft and sticky while those of old rice felt hard and nonsticky. Indudhara Swamy et al. (1978), as mentioned above, tested the stickiness of cooked rice indirectly by its volume expansion (Fig. 5.4). By that time texture-measuring equipments had become widely available and practically all workers from around 1980s onwards measured the hardness as well as stickiness of cooked rice using various items of equipment (Bolling et al. 1978, Perez and Juliano 1981, Shibuya and Iwasaki 1984, Matsue et al. 1991, Tamaki et al. 1993, Shu et al. 2000, Sodhi et al. 2003, Ohno et al. 2007a, 2007b, Tulyathan and Leeharatanaluk 2007). Despite differences in the meaning and/or method of cooking from laboratory to laboratory, all workers invariably found that cooked old rice was harder and less sticky than cooked new rice. What is perhaps more important to the Japanese, the S : H ratio (stickiness : hardness) therefore invariably fell in old rice compared with that in new rice. A random set of data is shown in Fig. 5.8. This difference in texture is the quintessential property difference that the consumer encounters when encountering ageing of rice. As a result, one of the standard indexes one considers when examining the effectiveness of any intervention (converting new rice to old or old rice to new – see below) is to test the hardness and stickiness of cooked rice before and after treatment. Interestingly one may note that here again the effect of rice ageing and that of increasing amylose content simulate each other.

5.3.7 Other properties Changes in a few other parameters of rice have also been examined, although not so extensively. GT Changes in GT if any during storage of rice have been examined by some researchers, mainly as a part of their study of the pasting properties. A majority failed to notice any significant change, although a few researchers observed a marginal to small gradual increase in GT with storage (Bolling et al. 1978), some found a marginal decrease (Qiu et al. 1998) and some



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1.4

1.2

Texture value

1.0

0.8

0.6

0.4

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6

Fig. 5.8 Change in cooked-rice hardness (–––) and stickiness (-----) of several varieties of rice with their storage. Reproduced, with permission, from Tamaki et al. (1993).

definitely observed absence of any change (Noomhorm et al. 1997). Shu et al. (2000) noted that there was no change in alkali score, i.e., an inverse estimate of the GT, during storage; but Lee et al. (1993) noticed a distinct increase in the alkali score, i.e., a fall in GT, after storage. Some indirect evidence about the GT was also available. Several researchers studied the water uptake of rice at 70−80 °C (Bolling et al. 1978, Indudhara Swamy et al. 1978, Lee et al. 1993) and some of them did notice a gradual change with storage therein. While part of this change could be due to a change in hydration ability, it could also be partly due to a change in GT. Yong et al. (1995) as part of their differential scanning calorimetric (DSC) studies of rice during storage noticed an increase in GT. A marginal increase in GT by the DSC was also observed by Zhou et al. (2003a) but Sodhi et al. (2003) observed the opposite. It can be concluded from these contradictory data that no significant change of the GT probably occurs in rice during its storage.



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Gel consistency (GC) This property in rice during its storage has been studied by only a few researchers. Villareal et al. (1976) and Perez and Juliano (1981) noted after storing rice for six months that the GC hardened after storage; but the change occurred equally after storing rice at 29 and at 2 °C. Lee et al. (1993) observed a similar decrease in gel length after storage. Unnikrishnan and Bhattacharya (1995) also observed a progressive decrease in gel length during storage both in raw and parboiled rice. One might say that these observations on the whole tallied with the hardening of the cooked rice upon storage. However, repeated work in the RRDC (unpublished) in recent years over several seasons did not find a consistent trend of change in gel length during long-term storage of several varieties of Basmati rice. DSC Only a few workers have examined the DSC pattern of either rice flour or of its isolated starch after or during storage of rice. Yong et al. (1995) found a decrease in crystallinity of starch (by X-ray studies) after 16 weeks’ storage of rice both at 5 and 30 °C. GT, ‘melting temperature’ and ‘melting enthalpy’ of starch (by DSC) were reported to have increased whereas the gelainisation enthalpy decreased. Storage at 30 °C showed greater effect than at 5 °C. Sodhi et al. (2003) noticed a decrease in GT and enthalpy after one and two years of storage. Zhou et al. (2003a) noticed a small change in the peak temperature and of the peak breadth after storage, which tended to be restored after protease treatment. Obviously information available in the subject is fragmentary and contradictory. More systematic work seems to be called for. Miscellaneous properties of rice A series of papers have been published from the University of Arkansas in somewhat related subjects (Daniels et al. 1998, Fan and Marks 1999, Fan et al. 1999, Perdon et al. 1999, Meullenet et al. 1999, 2000, Pearce et al. 2001). The main focus of these papers was not so much the process of rice ageing per se, but mainly to examine the effect of various handling and processing treatments (harvest moisture; pre-drying waiting time; time, temperature and protocol of drying; time, temperature and ambient conditions of storage) on the functional and sensory properties of rice. No new or novel information accrued from these studies on the subject of rice ageing, hence these papers are not discussed further.

5.4 Theories of rice ageing: relation to individual constituents The results summarised in the previous section give an account of the changes that have been seen to occur in the physicochemical and functional properties © Woodhead Publishing Limited, 2011


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of rice during its ageing. Now it is necessary to consider what factors have so far been thought of or put forward to account for these changes. The first point to consider is the changes observed, if any, in the chemical constituents of the rice grain. If we disregard the husk and the bran of the grain for the time being, for the consumer is concerned mainly with the milled product, then the major constituents of rice are: starch, other minor carbohydrates, lipids and protein.

5.4.1 a-Amylase Sreenivasan (1939), being a biochemist, soon noted that freshly harvested rice had a relatively high concentration of a-amylase, the activity of which gradually declined and ultimately tapered off during storage. He immediately concluded that the observed ageing changes were related to this amylase activity. New rice, he felt, cooked soft and sticky and lost more solids precisely because it had more a-amylase, while old rice behaved otherwise because it had less and finally no amylose. Sanjiva Rao (1938), being a physical chemist, had no interest in enzyme activity but was aware of changes in silicic acid gel as a result of treatment with heat and moisture. On this basis he proposed that starch, which he considered a colloidal entity, to undergo a sol–gel transformation over a period of time during storage which is what, he felt, caused the ageing process. Desikachar and Subrahmanyan (1960), being biochemists by training, came back to the question of amylase. However, a thorough examination led them to reject the theory. They observed that amylase in rice was destroyed within a few minutes after the beginning of cooking but the loss of grain solids continued nevertheless; that cooking new rice in the presence of mercuric chloride did not improve its cooking; and that cooking old rice with added amylase did not make it cook like new rice. On the other hand treatment of new rice with formalin or with steam made it cook as if it was old. They were thus led to believe that the ageing in rice had to do more with some change in starch than in amylase enzyme.

5.4.2 Sugars Sugars among the carbohydrates are a minor constituent of the milled rice grain. Significant changes in these sugars have been observed during storage of rice by a number of workers (Sreenivasan 1939, Kester et al. 1956, Houston et al. 1957, Tani et al. 1964, Barber 1972, Cao et al. 2004). The major change observed was decrease in nonreducing sugars and increase in reducing sugars. Sometimes decrease in total sugars was also observed. Changes were generally found to be more at high grain moistures and less at low moisture contents. Barber (1972) and his colleagues also showed that the changes occurred at a far greater rate or extent in an ultra-thin outermost layer of the milled grain than in the nucleus. Much of these changes may be



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autonomous, probably a result of low levels of enzyme activity and part of it probably a result of microbial action. However, no one has so far shown any link of these changes with the process of rice ageing. Considering the overall functional effects on the grain, these changes in sugars appear to be more an incidental phenomenon rather than overtly relevant to the process of rice ageing.

5.4.3 Starch Starch constitutes about 90% of the dry matter of a milled rice grain and any change in this constituent is bound to have a profound influence on its behaviour. A number of investigations have found no significant quantitative change in the content of either starch as a whole or of amylose or amylopectin during rice storage (with the sole exception of Sodhi et al. 2003, who reported a decrease in amylose content, which remains to be verified). However, there have been many tantalising reports of many qualitative changes in starch. First is the profound change in the pasting properties of rice upon storage and the inference therefrom about increasing resilience of the starch granule that was discussed above. As against this, a number of workers reported that isolated rice starch behaved quite differently (Barber 1972, Shibuya et al. 1977a, Shibuaya and Iwasaki 1984, Shin et al. 1985, Rajendra Kumar and Ali 1991, Teo et al. 2000, Zhou et al. 2003b). They noticed no change in the pasting properties of starch isolated from rice after storage even when rice flour consistently showed substantial changes. The results of Shin et al. (1985) are shown in Fig. 5.5(d)). Even though Villareal et al. (1976) did notice a distinct change in the paste viscosity of isolated rice starch just like that in the rice flour, none of the later workers confirmed this finding. If indeed isolated starch did not show any storage change, this result may be considered a prima facie evidence that rice starch per se had little role in the grain’s ageing. However, as explained above, the granules in intact rice may have been affected by external interactions, producing the same result as if the granule itself had changed. Second, other starch properties, viz. GT and DSC profile, have been mentioned above. Not much work has been done on them so far, but not much significant change has been found in them either. Yet a number of early workers noticed some minor changes in other properties of rice starch after storage. For example, Sahasrabudhe and Kibe (1935) noticed that isolated starch from new rice was not easily digested either by acid or by amylase enzyme as compared with starch from old rice. But Tulyathan and Leeharatanaluk (2007) did not find any difference in the in vitro digestibility in starch of new and old rice. Desikachar (1956) and Desikachar and Subrahmanyan (1960) found that the properties of starch from fresh rice were somewhat different from those from old rice: the intrinsic viscosity and solubility in perchloric acid of starch and/or amylose from new rice were somewhat higher and their iodine-binding capacity somewhat lower than of those from old rice. Spanish



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workers, as reported by Barber (1972), also found similar differences with respect to limiting viscosity and iodine-binding capacity. Rajendra Kumar and Ali (1991) found the starch from new rice had a higher number-average molecular weight, swelling power and solubility and amylose-solubility than these from stored rice. Yong et al. (1995) found swelling power, solubility, blue value and amylose-solubility to decrease in starch as rice aged. Thus there is some ambiguity about any change in starch. Third, another starch property which has been apparently found to change during ageing by the present author and his colleagues (Indudhara Swamy et al. 1978, Unnikrishan and Bhattacharya 1995) is the hot-water solubility of the amylose content of the rice. It was observed that the hot-water solubility of the amylose seemed to progressively decrease during storage of the grain. Rrajendra Kumar and Ali (1991), Yong et al. (1995) and Zhou et al. (2003a) too found a lowering in the solubility of amylose either in isolated starch or in flour after rice was stored. This finding has many ramifications. Ali and Bhattacharya (1972), in their pioneering study of the fundamental character of parboiled rice, observed that the solubility of the amylose content of rice decreased after parboiling and the decrease was proportional to the severity of the parboiling treatment. Also, the cooked rice simultaneously got increasingly hardened. This observation was confirmed by Unnikrishnan and Bhattacharya 1987) with 13 varieties of rice (very low to very high amylose content) parboiled under two conditions each. Lastly Unnikrishnan and Bhattacharya (1995) not only confirmed a progressive drop in amylose solubility in raw (i.e., nonparboiled) rice during ageing, but also observed exactly similar drop in amylose solubility even in parboiled rice during its ageing. If these findings are correct and are considered in the background of another set of works from the same laboratory, they assume a newer significance. Bhattacharya et al. (1972, 1978, 1982) (see detailed discussion in Chapter 7) established that the hotwater solubility of the amylose content of rice differed among rice classes and that the calculated hot-water insoluble amylose (total amylose minus soluble amylose) correlated very well with the experimentally determined texture of cooked rice. Now here we seem to have three parallel sets of relationships: •

•

the hot-water solubility of the analytically determined amylose content of rice differed among rice varieties and the calculated content of hotwater-insoluble amylose showed a highly significant correlation with the texture of the rice after cooking and a host of other rice properties (breakdown, setback, storage modulus and relaxation time of rice-flour paste); the solubility of rice amylose decreased upon parboiling of the rice and in proportion to the severity of parboiling; and here too the cookedrice texture and other rice properties changed correspondingly and proportionately; and



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amylose solubility again seemed to decrease in both raw (i.e., nonparboiled) and parboiled rice as they aged, and their texture after cooking and paste and other properties again changed similarly.

Unnikrishnan and Bhattacharya (1987, 1995) were struck by this parallelism. On this basis they examined the relationship between the calculated amount of insoluble amylose and cooked-rice hardness in different varieties of rice, both raw and parboiled, and after different periods of storage in their two studies and found all the respective data to fall in a fairly close unified relationship each (Fig. 5.9). The amount of insoluble amylose of a sample seems to be actively related to the latter’s texture irrespective of the variety, its parboiling and ageing before or after parboiling. It might be imprudent to extend this argument further. One should be cautious about concluding that the insoluble amylose content may have a central role to play in rice behaviour regardless of the variety, its age or its processing. At the same time such a signal cannot be totally ignored either. If insoluble amylose is not a cause of rice behaviour, is it an effect caused by some master change? Fourth, the above-mentioned study on pasting properties by Sowbhagya and Bhattacharya (2001) revealed another enigma. Earlier studies from the same group (Bhattacharya and Sowbhagya 1978, 1979) had shown that the cold-paste : hot-paste (C:H) ratio at a given peak viscosity value was a characteristic property of each starch (see details in Chapter 13 Tests for rice quality). Rice varieties of different quality types also had shown differing and characteristic values for this ratio. The above study (Sowbhagya and Bhattacharya 2001) confirmed those values for different varieties (the value of the ratio varied from 1.23 for waxy to 2.02 for high-amylose rice). But what came as a total surprise was that these C:H values remained practically constant over four years of storage time; this, one should remember, when every other paste index changed systematically over time (Figs 5.6 and 5.7)! If the C:H ratio is considered as a characteristic property of a starch, then this constancy of C:H value would imply that the rice starch basically did not change during the entire storage period. This conclusion may seem to go against the grain of the other interpretations mentioned above, but this is the type of surprises that the process of rice ageing constantly throws up.

5.4.4 Lipids Comparatively speaking, fats or lipids are among the more reactive entities of foods. Possible changes in the lipid content of rice and their roles if any in the ageing process of the cereal have therefore been extensively examined. This has happened more especially in Japan where the consumer is extremely sensitive about flavour changes and lipid deterioration is strongly associated with changes in flavour of foods. The total fat content of milled rice has not been found to change significantly during storage. However, its composition could change, viz. free fatty acids © Woodhead Publishing Limited, 2011


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(a)

Firmness, %

70

Raw : MPB : SPB : Combined :

r r r r

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60 MPB 0 months SPB MPB 4 SPB MPB 10 SPB MPB 20 SPB MPB 40 SPB , + Raw 0, 40

40

20 10

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Fig. 5.9 Effect of rice variety (a) and variety as well as ageing time (b) on the relation between water-insoluble amylose content of raw and MPB (mild) and SPB (severe) parboiled rice and their viscoelastograph firmness after cooking. (a) 13 varieties of different quality types (I-VII, differing in amylose content). Reproduced, with permission, from Unnikrishnan and Bhattacharya (1987). (b) 5 varieties (types I, II, III, V, VII) stored for 0, 5, 10, 20 and 40 months. Reproduced, with permission, from Unnikrishnan and Bhattacharya (1995).

(FFA) could increase and neutral fat (NF) and phospholipids (PL) decrease (Narayan Rao et al. 1954, Houston et al. 1957). Barber (1972), reviewing earlier work, showed that lipid deterioration was common in foods. Such deterioration led to changes in flavour, off-odours and acidity. Oxidative



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changes led to formation of peroxides, aldehydes, ketones, acids and rancidity. Little or no change occurred in total fat content, but the lipid composition changed. FFA increased, while NF and PL decreased somewhat. The pattern of this change was similar throughout the grain but quantitatively speaking this change was very high in an ultra-thin outer layer of the milled rice. Fatty acid composition of the different fractions remained more or less unchanged, but there was a small change in the proportion of individual acids, especially oleic and linoleic acid. But on the whole fatty acids released from the fractions were more or less in the same proportion as they were combined in the fraction. Many authors observed changes in various constituents of lipids, i.e., in FFA, NF and PL during storage of rice, which changes became more pronounced at higher temperatures and grain moisture contents (Yasumatsu and Moritaka 1964, Lee et al. 1965). Yasumatsu and Moritaka (1964) observed progressive increase in FFA in rice during storage. Yasumatsu et al. (1964) proposed on this basis that the fatty acid complexed with the helical rice amylose, which interfered with the gelation and swelling, which is what led to increased peak viscosity. Yasumatsu et al. (1966) further proposed that the fat in rice underwent autoxidation leading to formation of carbonyl compounds which is what led to the stale flavour of old rice. Many other researchers confirmed the development of FFA in rice during storage (Bolling et al. 1978, Kim et al. 1988, Matsue et al. 1991, Lee et al. 1993, Sodhi et al. 2003, Lam and Proctor 2003). Tamaki et al. (1993) in a long-term storage experiment (six years) found increase in fat-by-hydrolysis and fat acidity and lowering of the pH of the cooking liquid. Increase in fatby-hydrolysis suggested that it might be a factor in the hardening of stored rice after cooking. Fujita et al. (2005) identified six carbonyl compounds and eight sulphur compounds in the vapour space and found a clear relation of this with old rice smell. The proportion of the carbonyl compounds increased with age. Contrary observations have also been made. Villareal et al. (1976) and Perez and Juliano (1981) in their work mentioned earlier contested a big role of lipids in the process of rice ageing. They observed that development of FFA was low in paddy and the highest in waxy rice. Still paddy aged nearly as well as milled rice and waxy rice hardly aged. Carbonyl compounds were low in paddy and very low in defatted milled rice. But both aged nearly as well as ordinary rice. Peak viscosity of waxy rice increased like nonwaxy rice, yet it had no amylose to complex with FFA. The role of carbonyl compounds in stale flavour was clear but any other role seemed doubtful. Surface defatting with petroleum ether had little effect on texture change but reduced change in gel consistency and amylogram. Shibuya et al. (1977b) also did not feel that FFA could increase viscosity. Tsugita et al. (1983) stored brown rice at 4 and 40 °C for 60 days. The latter sample simulated the flavour and texture of old rice (higher water uptake, expanded volume, blue value and solids loss in cooking liquid and decrease



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in its pH, increase in cooked-rice hardness and decrease in stickiness). The authors detected many carbonyl compounds by gas chromatography analysis but thought that this chiefly affected flavour rather than texture. Shin et al. (1985) stored one variety of brown rice for 12 months at 35 °C. The untreated flour and ether-defatted flour showed increase in peak viscosity and total setback; methanol-defatted flour and isolated starch amylograms, which already showed high viscosity, did not change (Fig. 5.5). FFA changed in undefatted flour only, not in ether-defatted flour, hence any role of FFA would tend to be discounted. Mercuric chloride raised the peak viscosity of undefatted and ether-defatted flour (fresh before storage), showing the effect of amylase activity. They agreed with the view of Bhattacharya and Sowbhagya (1978, 1979) that varietal difference in peak viscosity was caused by extraneous factors, such as amylase activity, rather than being an intrinsic property. Total setback increased in undefatted and ether-defatted flour, but was unaffected by mercuric chloride; no increase was observed in isolated starch or methanol-extracted flour. Therefore increase in total setback could be due to change in structural component such as bound lipid. Yasumatsu et al. (1964) also had observed that methanol extraction eliminated increase in peak viscosity caused by storage. Shibuya et al. (1977a) stated that increase in FFA hardly increased peak viscosity and setback of rice-flour pastes. On the other hand Zhou et al. (2003b) observed that removing free lipids led to little changes in RVA pattern of rice paste, but removing bound lipids led to a drastic change. However the change was the same in both 4° and 37° C stored rice, raising doubts whether lipids were involved in the ageing process. On the whole there is no doubt that a good deal of micro-level changes took place in the lipid component of rice during storage. There is also little doubt that such changes affect sensory perceptions especially with respect to acidity, odours and flavours. However, to what extent these changes were overtly related to the gross changes observed during the ageing process of rice seems a moot question.

5.4.5 Cell walls: nonstarch polysaccharides – structure-maintaining entities The other minor carbohydrate in rice is the nonstarch polysaccharides (NSP) which constitute the cell walls. Desikachar and Subrahmanyan (1959), during their studies mentioned earlier, made one interesting observation. When they examined the transverse sections of cooked rice under the microscope, they observed that the cell walls were rather disorganised in cooked new rice but were more orderly and organised in cooked old rice. This matter has been recently re-examined in the RRDC (unpublished) after a lapse of half a century. Indeed the above observation has been confirmed. There is no doubt that the cell walls in transverse sections of cooked rice appeared orderly in old rice and somewhat disorderly in new rice. When the rices



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were overcooked, the cell walls became more disorganised, and when new rice was steamed (a method of accelerated ageing, discussed below), the cell walls became more orderly (Fig. 5.10). It is true that this observation alone cannot provide a sufficient explanation of the ageing process, for it merely transfers the onus of explanation from the rice grain to the cell walls. Nonetheless it does raise an interesting question.

(a)

(c)

(b)

(d)

Fig. 5.10 Photomicrographs of transverse sections of variously stored rice after cooking. Cell walls are highly disorganised in rice stored at 5 °C (b) and overcooked rice (e, f); relatively disorganised in 3-months-old rice (a) and well organised in wellstored (c, d) and steamed (g) rice. Courtesy, RRDC, Mysore.



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(e)

(f)

(g)

Fig. 5.10

Continued.

This aspect had been extensively studied by Shibuya and his coworkers at NFRI from the chemical angle. After they were somewhat disillusioned about the theory of changes in fat being behind the rice ageing process, they shifted their attention to cell walls. Shibuya and Iwasaki (1982) stored one japonica brown rice at 4 and 23 °C for 10 months. The pasting profile of the 23 °C stored sample changed as usual after storage. However when the same sample was treated with a cellulase and a pectinase preparation, the © Woodhead Publishing Limited, 2011


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difference disappeared. From this they concluded that it was a change in the ‘structure-maintaining components’ of rice that brought about the change in rice property upon ageing. The same authors (Shibuya and Iwasaki 1984) repeated a similar experiment, wherein the cellulase- and endoxylanase-treated rice was tested for its texture after cooking. Texturometer analysis showed that the enzyme treatment led to a decrease in hardness (H) and an increase in adhesiveness (A). As a result, the H/A ratio decreased, which the Japanese consider a very important index of rice acceptability. Similarly stored milled rice treated with the enzymes showed increased water uptake, solids loss and iodine blue value during cooking and decreased amylogram viscosity as in new rice. Zhou et al. (2003b) recently confirmed that cellulase treatment changed the RVA profile of stored rice towards new rice (Fig. 5.11) (but hemicellulase treatment did not). In view of the above results, Shibuya (1984) studied the phenolic acids in rice cell walls. He detected the presence of ferulic acids, p-coumaric acid and diferulic acid in the alkaline extract of rice endosperm cell walls. He also observed phenolic-carbohydrate esters in enzymatic digests of cell walls, including ferulic acid esters of arabinoxylan fragments and fractions containing diferulic acids, suggesting diferulic bridges cross-linking the matrix polysaccharides in cell walls. A model for cell walls including diferulic bridges is shown in Fig. 5.12. Tsugita et al. (1983) detected phenolic acids in larger amounts in flours from rice stored at 40 than at 4 °C. It is difficult to say what all these findings signify. They may not be considered as conclusive evidence to suggest that a change in the ‘structuremaintaining components’ of rice (a euphemism for cell walls) is the origin of its ageing. However, they undoubtedly suggest that the cell wall materials, especially formation of diferulic and similar reinforcement bridges in the cell walls and thereabouts, may have some role to play in the process.

5.4.6 Protein In recent times there has been a spurt in interest in a possible role of protein in the process of rice ageing. The voices of scientists suggesting that protein plays a substantial role in the varietal difference in texture of cooked rice have been becoming louder of late (discussed in Chapter 7). Perhaps following from this interest, more and more researchers have been recently suggesting that the proteins play a big role in rice ageing as well. Actually the possible involvement of protein in the process of rice ageing is an old theory. Many researchers noticed that the solubility of salt-soluble and acetic acid-soluble protein progressively decreased during rice storage, and so did nitrogen rendered soluble by digestion with pancreatin (Narayan Rao et al. 1954, Desikachar and Subrahmanyan 1960, Barber 1972, Bolling et al. 1978). Saio and Kubo (1963) studied the denaturation of protein during storage in connection with change in phytic acid. The results indicated that the functional groups in rice glutelin, the absorbance at 240 nm in alkali



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Fig. 5.11 Effect of cellulase treatment on pasting properties of two varieties of rice stored at 37 °C for 16 months. Reprinted from Zhou et al. (2003b) with permission from Elsevier.

solution and the content of phosphorus in glutelin-phytic acid precipitate increased during storage, showing possible denaturation of glutelin. Villareal et al. (1976) did not agree with the suggestion of the Spanish scientists that carbonyl cross-linking of proteins was the major factor in rice ageing. They did confirm a decrease in salt-soluble protein, but the



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Protein–polysaccharide linkage?

Diferulic bridges

Cellulose microfibrils

Heteroxylans

Isodityrosine bridge

Structural proteins

Fig. 5.12 Model for maize bran cell walls showing diferulic bridges and other cross connections. Reproduced, with permission, from Saulnier and Thibault (1999) John Wiley and Sons.

reduction was as high in defatted milled rice as in untreated milled rice inspite of the obvious difference in the contents of carbonyl compounds. Paddy also showed a similar decrease despite its lower carbonyl content. Hence the authors felt that protein denaturation might be a continuation of the decrease in salt-soluble protein observed in the developing rice grain (Palmiano et al. 1968). Chrastil and collaborators at the SRRC presented a series of papers in the 1990s suggesting that the protein played a key role in the process of rice ageing. All their early results were summarised in a chapter (Chrastil 1994a); this was followed by a few other papers especially by Chrastil and Zarins (1994). The conclusions of this group can be summarised as follows. Most ageing changes were due to changes in protein, starch and protein– starch interaction. There were small but definite changes in the molecular weight of starch during storage. On the other hand the molecular weight of oryzenin, the major component of rice protein, also of its subunits and disulphide bridges, increased appreciably. These changes were brought about by specific activities of enzymes which remained substantially active in the rice grain even during long storage. Starch synthetase enzymes were also © Woodhead Publishing Limited, 2011


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active. Chrastil (1994b) explained that during cooking, protein and starch bodies were partially destroyed, which enabled interaction between them by reversible adsorption. This is what caused stickiness of cooked rice. After cooking, grain surfaces were covered by small particles, made mostly of oryzenin and starch, which were responsible for stickiness. Greater binding of the two caused increased stickiness and vice versa. The equilibrium binding constant (Keq) and equilibrium binding ratio of oryzenin to starch (n:m) decreased appreciably during storage. One might find it difficult to appreciate the above hypothesis. The conclusion about starch-protein binding was arrived at by observing a shift in absorbance at 285 nm when starch and protein solutions were mixed in the spectrophotometer cell. It is not known whether there was any chemical reaction or it was only a case of light scattering caused by formation of some micro-particles. In any case, the hypothesis hardly throws any light on the change in hardness of the cooked rice. Chrastil’s observation on disulphide bridges within the oryzenin has, however, been supported by other workers, discussed below. Qiu et al. (1998) observed that oryzenin from stored brown rice had lower sulphydryl content, lower hydrophilic activity and higher molecular weight when the rice had been stored at 38 °C than at 8 °C or before storing. Teo et al. (2000) noted that the DSC of flour from fresh rice gave a weak peak at 47−66 °C, which they felt could be attributed to denaturation of oryzenin. This transition was found to shift to higher temperatures with increasing storage time and temperature and to broaden and finally disappear. It appeared therefore that conformational changes in protein occurred during ageing, intermolecular disulphide cross-links were formed which rendered the protein less soluble and less binding. The authors therefore felt that the modification of protein rather than starch was responsible for the rheological changes in rice during ageing. Zhou et al. (2003a, 2003b) noticed that treatment of stored rice flour with b-mercaptoethanol changed the RVA profile. But the profile remained similar in rice stored at 4 and 37 °C. Moreover the RVA shape was not restored to that of unstored rice. On the other hand treatment with protease (which removed 85% of the protein) showed similar change in the RVA profile and also restored the original shape completely (Fig. 5.13). The amount of propanol-extractable protein fraction, i.e., prolamine, decreased. In the starch-granule-associated-protein (SGAP) also there was a suggestion of its lowering (i.e., when extracted with a solvent containing dithioerythritol, DTT). Ohno and Ohisa (2005) studied the effect of reducing and oxidising agents on the hardness and stickiness of cooked rice after storage (only for two months) of rice. They surmised that a very thin outer layer of the milled rice was the portion which really underwent change during storage. Thus when milled rice was further polished to remove another 7% grain matter by weight, the S/H (stickiness : hardness) ratio of the cooked rice, which had decreased somewhat upon storage, now seemed to be restored to the



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Fig. 5.13 Effect of protease treatment on the viscograms of rice flour after storage at (a) 4 °C and (b) 37 °C for 16 months. Reproduced, with permission, from Zhou et al. (2003a) John Wiley and Sons.

level of the new rice. Extraction of protein from rice and examination by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) showed that the protein in aged rice had more of higher molecular weight (HMW) bands than in new rice. Similarly treatment with oxidising agents increased the HMW bands while treatment with reducing agents lowered



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them. However, one must note that most of these changes mentioned were marginal or small. The authors continued similar studies (Ohno et al. 2007a, 2007b). This time rice was stored at 30 °C for five months. It was observed that the S/H ratio which decreased upon ageing was more or less restored by treating with a reducing agent (Na2SO3). HMW bands in extracted protein were again found to have increased after ageing (Fig. 5.14) and/or by addition of oxidising agents, while they were reduced by reducing agents (Na2SO3, cysteine or DTT). The authors felt ageing of rice involved formation of higher molecular weight protein subunits by oxidation of sulphydril groups to disulphide bridges. Cleaving of these disulphide bonds by reducing agents increased extractable solids. A gelatinised paste formed a layer around the grain surface increasing its stickiness. One should note, however, that neither addition of reducing agent nor extra milling of the outermost layer completely restored aged rice to the parameters of new rice although there was a shift in that direction. Tulyathan and Leeharatanaluk (2007) confirmed that the content of HMW bands increased in aged rice compared with fresh rice.

5.4.7 Conclusions Let us now try to sum up the various informations above and see where they lead us to. First, let us again summarise the various changes that the

kDa

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14.4 1

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Fig. 5.14 (a) SDS-PAGE analysis of proteins extracted from external layer of milled rice samples. Lane 1, new rice; lane 2, new rice stored at 30 °C, 5 months; lane 3, stored at 30 °C, 5 months in vacuum pack. (b) Relative intensity of the 5 protein bands in (a). Reproduced, with permission, from Ohno et al. (2007b).



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rice grain undergoes during ageing. Any theory that one may propose about what causes rice to age must satisfy all these conditions. The ageing process of rice is characterised by a progressive • • • •

•

• • •

decrease (barring an initial broad spike caused probably by decline of a-amylase, an external factor) in the hydration ability of the rice; decrease in solids loss during cooking; decrease (barring an initial disturbance) in paste viscosity at a given flour concentration; increase in time of heating required to reach the peak viscosity (tp), i.e., a decrease in either the ease of pasting or the ease of granule swelling or an increase in granule stability; decrease in paste breakdown; and increase in setback, P–C intersection point and Pmin (BD) value, i.e., increasing paste stability, in other words increasing strength and resilience of the swollen starch granule; increase in the final cold-paste viscosity (at a fixed flour concentration); increase in hardness and decrease in stickiness of cooked rice; and decrease in solubility of the grain substance (amylose and protein).

Putting these observations together a picture that emerges is that the rice grain or rice substance, or perhaps the starch granule itself, as an entity becomes progressively more compact, organised, reinforced and strengthened as it ages. It does not hydrate as much or as easily as before. The granules do not swell as easily as before, nor do they disintegrate as easily during continued heating of the paste. The rice constituents (amylose, protein) are not rendered soluble as easily, and the cooked grain remains progressively more firm and intact with progressive reduction of solids loss. So one has to primarily account for why the rice substance or the starch granule becomes progressively more strengthened and reinforced as it ages. Moritaka and Yasumatsu (1972), with remarkable prescience, had proposed a scheme (Fig. 5.15) some four decades back that has been widely quoted – alas, sometimes with an incorrect or no acknowledgement! The perceived effect of the main thrust of the scheme – the progressive cross-linking and reinforcement – is undoubtedly in line with the perceived changes described above. However, it is also clear that there is no definite evidence for some of the reactions suggested or in any case some are debatable. Some more facts that contradict some aspect or other of all of the proposed theories are that ageing: •

•

proceeds more or less equally regardless of whether the rice is in the form of paddy (less FFA or oxygen) or milled rice, whether it is stored in the dark (less autoxidation) or in the light, and whether it is stored in air or in vacuum or in an atmosphere of other gases (nitrogen, carbon dioxide); proceeds equally well, including showing similar protein denaturation,



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Cooking

Sensory effect

Increase of free fatty acid

Appearance

Oxidation (air)

Lipid Oxidation (air)

Increase of hydroperoxide and carbonyl compounds

Increase of volatile carbonyl compounds

Aroma

Protein

Oxidation (air)

SH S–S Interaction with protein

Decrease of volatile sulphur compounds Formation of helix with amylose

Starch

Fig. 5.15

• •

•

Increase of strength of micelle binding

Inhibits swelling of starch granule

Texture

Mechanism of ageing of rice grain as proposed by Moritaka and Yasumatsu (1972). Reproduced with permission.

in stored surface-defatted rice despite little development of FFA or carbonyl compounds; is probably slightly facilitated by increase in amylose content of the rice; is not a random happening but proceeds in a precisely ordered fashion, whereby the initial difference in behaviour among different amylose classes, say in paste breakdown, is maintained throughout the ageing process (see Fig. 5.6); and happens as much in parboiled rice (no enzyme, no FFA, easier autoxidation) as in raw rice; in other words it is not related to any enzyme activity (including lipase, amylase), nor on the development of FFA, and is apparently not overly promoted by relatively more rapid development of carbonyls as it happens in parboiled rice.

Considering all these factors, we can see that all the proposed theories discussed earlier, including that of Moritaka and Yasumatsu (1972), partly satisfy the conditions but none seems to do so fully. This is true, for example, of the case of fat. Formation of FFA and carbonyl compounds, the latter in particular, should be a good explanation in the sense that the carbonyls would reinforce and strengthen the grain just as artificial exposure of rice grain to formalin does. But there are many snags, as already discussed. Similarly, changes in cell walls and possible cross-linking by diferulic acid, etc, may fit the data well in many respects. Much more clinching evidence, however, is needed in support of such a theory to be considered acceptable. © Woodhead Publishing Limited, 2011


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Changes in protein again provide a tantalising theory. Disulphide bonding and increase in molecular weight of monomer units and total protein have no doubt been proved. But how do they affect the whole grain? Protein (less than 10% of starch) largely resides as minute protein bodies surrounding the starch granules, mostly in the subaleurone layer (Shih 2004). So the question would be where and when do they form disulphide bridges and how does that affect the total structure? The case of starch is the most ambiguous. On the one hand every physical and functional change is suggestive of a change in starch. This is especially true of the change in pasting properties, cooked-rice texture, amylose solubility and others. Yet the reported absence of change in the pasting behaviour of isolated starch or in cold-paste hot-paste (C:H) viscosity ratio and other inconsistencies cited create doubt about a role of starch per se. External reactions affecting starch behaviour (recall how formalin causes the cooking-eating behaviour of rice to change) cannot of course be ruled out.

5.5

Some final rice paradoxes

Before leaving the subject it is necessary to point out three other enigmas in relation to ageing of rice for which we have no answer yet.

5.5.1 Three processes, similar effect First, there is a striking parallelism among the effects of three circumstances in rice, viz. (a) varietal difference in amylose content, (b) effect of ageing and (c) effect of parboiling. (a) It is well known (discussed in Chapter 7) that the texture of cooked rice becomes progressively more hard and less sticky as the amylose content (strictly speaking amylose-equivalent content, AE, mainly its insoluble portion) of the grain increases. (b) Coming to ageing, now we find that a similar effect operates during ageing. The overall effect of ageing – in the sense of increase in cooked-rice hardness and in paste stability, i.e., in starch granule resilience – is exactly the same as would have happened had the AE content, or more correctly the proportion of extra long chains in amylopectin, of the rice been increased. In fact the present writer recalls that a japonica rice, stored for several years in his laboratory in the CFTRI, had cooked exactly like indica rice! Yamamoto and Shirakawa (1999) recently subjected old stored japonica rice to ‘annealing’ to get rid of its ‘indicalike cooking behaviour’ (see below). (c) Considering parboiling, it has been shown (discussed in Chapter 8) that parboiling again has more or less the same effect on the texture of cooked rice, as if the AE content of the rice has been increased. So here we have a situation where varietal difference with reference to AE content, the ageing of the cereal and the effect of parboiling © Woodhead Publishing Limited, 2011


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and its severity, all have more or less a similar effect. One wonders if this similarity is purely accidental.

5.5.2 Temperature can substitute for time Second, a curious fact, discussed below, is that ageing of rice – at any rate the effect thereof – can be simulated by heat treatment. The temperature can even be as high as 100 °C (steam). The end product, for all one can see, is similar. Then is ageing of rice a natural, a biological (enzymic) or a purely physical/chemical process?

5.5.3 Is ageing unique in rice? Third, there is another extraordinary piece of information that we have reserved for the end. While considering the ageing changes in rice, after one has spent some years on it, a troubling question remains: why such changes do not seem to be reported in other cereal grains? Chemically and structurally all cereal grains are basically similar. So surely if the ageing of rice is a normal biological and/or physicochemical process, one should expect that similar changes should occur in other grains as well. But it is noteworthy that no strong popular perception about ageing changes seems to exist in respect of other grains (wheat, maize, barley, sorghum . . .). Surprisingly, a rough general search of the literature also did not reveal any discussion of a similar phenomenon in the case of other grains. The only reference found to some comparable changes are rather tangential and relate to the following. One, the case of ‘maturation’ of wheat flour is well known (Mailhot and Patton 1988), but it is noteworthy that maturation refers to flour and not to the wheat grain. Irrespective of how long after harvest the wheat is milled (i.e., ‘new’ or ‘old’ wheat), the flour needs to be matured. On the other hand, there does not seem to be any reference to maturation or any other change in the unmilled wheat grain during storage. The same seems to be true of maize, barley, sorghum, etc., except that there is decrease in viability during storage, resulting in poorer malting of barley if attempted. Two, certain legumes or pulses (pea, beans, green gram . . .) are known to undergo some changes during certain conditions of storage which render these pulses hard to cook. But this hard-to-cook (HTC) phenomenon has been generally ascribed to unfavourable storage conditions (moisture content, temperature, humidity) and is said to involve hardening of the middle lamella due to reactions of pectin, divalent ions (Ca++, Mg++), protein and phytin (Reyes-Moreno and Paredes-López 1993). Third is the case of fruits and vegetables. Fruits after plucking undoubtedly undergo softening and ultimately spoilage. This is clearly a case of biochemical and enzymic changes, a sort of continuation of the ‘ripening’ process, in no way similar to those in rice ageing. The case of vegetables may be marginally different. Vegetables do seem to undergo some changes after harvest. We



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are all familiar with the fact that vegetables if stored seem to harden, i.e., become tougher (fibrous) to cook as well as to eat. It is generally considered that this is primarily due to dehydration. There does not seem to be any clear literature indicating whether there are some changes in the content or in the structure of fibre and/or in the cell-wall material or nonstarch polysaccharides. In any case, one can, if at all, surely notice some faint parallels here between the toughening of vegetables and the cell-wall changes theory of rice ageing. Even though possibly far-fetched, this parallelism can be an area worth further exploration. Fourth is the well-known case of potato. The starch and the sugars in the tuber are well known to be in a dynamic state and/or enzymatically interconvertible depending on the storage conditions and other circumstances. So either the processes are very different or there seems to be no ageing of other grains at all. To examine this question further, a preliminary study of the matter was carried out during the last two or three seasons at the RRDC with wheat, maize and sorghum (RRDC, unpublished). The properties studied were room-temperature hydration (water absorption index, equilibrium moisture content after water-soaking), grain hardness, texture of cooked milled (pearled) grain or grits (determined using the TA-HDi Texture Analyser), pasting profile (Brabender viscogram) and hot-water soluble amylose. The surprise is that none of the grains showed any significant change in the above properties during or after storage at room temperature for upto one year! This is extraordinary. These results would imply that ageing is a unique property of the rice grain. Ageing of rice must be caused either by biological (enzymic or induced by microorganisms) or by physicochemical processes (oxidation, free radical, ionic, etc.). If other cereal grains are essentially similar to rice in composition and structure, then why should such biological/ physicochemical changes occur in rice but not in other cereal grains is dumbfounding. In any case one must now add one more condition that any rice-ageing theory must satisfy. Any theory that one proposes to explain the ageing process of rice must also account for why this process does not operate in other cereal grains. One feels humbled. We still look like being in the same state as the six blind men quoted at the beginning of the chapter. Or, we can recall the classical Sanskrit saying quoted in Chapter 1: ‘even gods do not know, what to speak of mere mortals’!

5.6

Can the ageing process be hastened or retarded?

South Asians on the one hand and northeast Asians on the other always felt a vital concern in the process of rice ageing which people from other regions find it hard to appreciate. This concern ultimately led to the idea of human intervention in the natural process – aimed at hastening it in the former region and in retarding or even reversing it in the latter. © Woodhead Publishing Limited, 2011


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5.6.1 Artificial/accelerated ageing Scientists in Bangalore and Mysore in India while researching in the area of rice ageing gradually came to the point where they felt inclined to ask the question: Why not artificially age the new rice? Experience with field practices, where harvested paddy in stalk was often stored in a stack, leading to a certain amount of heating and discoloration of the grain (‘stack burning’) provided some clue. It was found that such practice while lowering the market quality of the rice (discoloration), nonetheless rendered the new rice somewhat like aged. Following from such considerations, Desikachar and Subrahmanyan (1957) succeeded in developing a process wherein freshly harvested paddy was steamed followed by its drying. Although there were some technological problems, viz. cracking and resulting milling breakage, the rice was found to cook just like old rice. The process was called ‘curing’. It gradually became popular in regions around Mysore (the state of Karnataka) where the product was sold as ‘steamed rice’. Consumers happily took to the rice despite its apparent shortcoming in the form of broken grains and a certain amount of opacity (chalky-like grains), because the rice cooked well. Some efforts at refining the system have been done (Desikachar et al. 1969, Gujral and Kumar 2003). Lately the process has been spreading. Some steamed rice is now a regular product even in north India. Desikachar and Subrahmanyan (1957) even devised a domestic rice cooker to help the consumer overcome the new-rice problem. In this cooker, rice is taken in a perforated tray which is initially raised and hung above the level of water boiling in the main pot. The tray is lowered into the water after the rice is adequately steamed, which is then cooked as usual. The rice cooks just like old rice. Following from the above, on further consideration, Bhattacharya et al. (1964) in the same laboratory devised a process to cure milled rice itself. It was found that treatment of rice with an appropriate combination of temperature and humidity in a humidity oven rendered the rice to cook like old rice (Fig. 5.3). The authors then tested a crude mini-pilot system in which new milled rice was heated under agitation in a closed rotating drum and then slowly cooled in another closed container, and found the rice was well cured. Normand et al. (1964) around the same time tested a similar process at the SRRC at New Orleans, LA. An interesting question arises, whether the internal changes that cause the rice to age naturally and the process of heat treatment (‘curing’) that renders the fresh rice to cook like old rice are similar. Are the intrinsic processes that cause the change in either case similar? That would eliminate any biological (or enzymic) causation, for curing has been tried even at very high temperatures as also by steaming and found to cause the same culinary changes. It has in fact been found that the boundary between ageing and curing could get blurred if one treated the rice over a range of temperatures. Examination of literature data, reviewed above, the experience of Barber (1972) where new rice was subjected to somewhat higher temperatures and so-called cured



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rice was obtained, the experience of research mentioned above where rice was stored not only at a very low temperature but also at such high temperatures as 50 °C, as well as experience at the RRDC (unpublished) of heating rice at still higher temperatures make one wonder whether it is a continuum. Ageing has been repeatedly found to be strongly related (positively) to temperature and the same relation seems to continue at not only ‘normal’ high temperatures but even at ‘abnormal’ high temperatures. One must also recall that Normand et al. (1964), Barber (1972) as well as Gujral and Kumar (2003) found the treated rice not only to cook like old rice but also behave like old rice in other properties including viscography. Considering the above thoughts, the RRDC recently prepared an Arrheniuslike plot (Fig. 5.16) of the ageing process, where the approximate equivalent ageing/curing time of new rice at different temperatures were plotted against the reciprocal absolute treatment (i.e., storage) temperature. The surprising finding was that it yielded a straight line up to 100 °C (actually two slightly different lines). The implication is that rice ageing is not quite a biological or ‘natural’ process, for it can be simulated by heat treatment, even at 100 °C. However, there were actually two slightly different straight lines, one below 40 °C and one above (Fig. 5.16). The calculated activation energy was 47.2 kJ h/mol at temperatures below the break point (~ 40 °C) and 135.1 kJ h/ mol above. The slight spurt in the rate of ageing at around 40 °C was repeatedly confirmed. One possibility is that this rate change could possibly be ascribed to the glass transition temperature. It could be that the rate of the processes that cause ageing was slow in the glassy state and was fast at the rubbery state. Alternatively it might suggest that we are looking at a ‘natural’, 5.00 4.50

log Curing time, hr

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2.8

2.9

3.0 3.1 3.2 3.3 3.4 3.5 Inverse curing temperature, K–1 ¥ 1000

3.6

3.7

Fig. 5.16 Arrhenius-like plot of inverse ageing/curing temperature against log of time (RRDC unpublished).



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biological process below 40 °C and an artificial, purely physical/chemical process beyond it. While on this subject, it may be worthwhile mentioning a parallel development. A new sub-branch of starch science – heat-moisture treatment of starch – has cropped up lately after the initial observation of Sair (1964). A large number of papers have been intermittently appearing on the subject, including one from Takahashi et al. (2005). It appears to the present author that this new sub-branch may have some light to throw on the subject of rice curing/ageing.

5.6.2 Retarding/reversing ageing While Indians intuitively thirst for accelerated ageing, the Japanese are instinctively inclined to retard it if they can. Scientists, being human, and also consumers, have actually tried it. A few approaches have been considered. One, the rice is stored in the cold. This would be obvious not only from the experience of numerous researchers detailed above but also from the plot in Fig. 5.16. In any case the Japanese researchers have repeatedly shown (Tani et al. 1964, Shibuya and Iwasaki 1982) and prescribed that rice be stored at low temperatures (lower than 15 °C) to maintain its ‘freshness’. As a matter of fact storing brown rice at slightly below 15 °C is said to be a relatively common industrial practice in Japan which not only retards its ageing but also enables the long-range storage of rice without the use of pesticides and also allows the grain to be stored at normal or even slightly higher than normal moisture contents (16−17%). Another approach follows from various research mentioned above, where treating of old rice with hemicellulases and other related enzymes and/or proteases were seen to restore the cooking and pasting properties of it to that of new rice. Based on these principles, Saito et al. (1964; cited in Shibuya and Iwasaki 1984) and Arai et al. (1993) tested the possibility of rendering old rice to cook like new rice by treating it with appropriate enzymes before cooking. One would not be surprised if some such method is commercialised some day in future for, alas, rice to the Japanese is fresh only transiently, with a long season of nostalgia to follow! A variant of the second approach above has also been thought of. Ohno et al. (2007a, 2007b), on the basis of their research already discussed, suggest that the cooking quality of old rice can be at least partially mitigated either by overmilling the rice or by cooking it in the presence of Na2SO3. A third approach tried, apparently successfully, is to subject the old rice grains to pressurisation (Watanabe et al. 1991). It is claimed that the process partially degrades the cell walls and changes the cooking character. Finally a variant of heat-moisture treatment, viz. annealing, has been claimed to reverse the ‘indica-like cooking behaviour’ of japonica rice stored for a very long time (Yamamoto and Shirakawa 1999).



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5.7

159

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fujita a, kawakami k, mikami t, masumoto k and nomura m (2005), ‘Difference in aroma compounds and quality variation in milled rice from 1996 to 2001’, J Oleo Sci, 54, 311−316. gujral h s and kumar v (2003), ‘Effect of accelerated aging on the physicochemical and textural properties of brown and milled rice’, J Food Engg, 59, 117−121. hogan j t (1963), ‘Rice research at Southern laboratory’, Rice J, 66 (7), Annual Issue, 38−41. houston d f, straka r p, hunter i r, roberts r l and kester e b (1957), ‘Changes in rough rice of different moisture content during storage at controlled temperatures’, Cereal Chem, 34, 444−456. indudhara swamy y m, sowbhagya c m and bhattacharya k r (1978), ‘Changes in the physicochemical properties of rice with aging’, J Sci Food Agric, 29, 627−639. irri (2007), Annual Report, Los Baños, Laguna, Philippines, International Rice Research Institute. kester e b, houston d f, ferrel r e, hunter i r and finfrock d c (1956), ‘Storage behaviour of rice in experimental bins’, Rice J, 59 (9), 24−27. kester e b, lukens h c, ferrel r e, mohammad a and finfrock d c (1963), ‘Influence of maturity on properties of Western rices’, Cereal Chem, 40, 323−336. kim b s, park n h, jo k s, kang t s and shin d h (1988), ‘Comparison of quality stability of rice and rice flour during storage’, Korean J Food Sci Technol, 20, 498−503. krebs h a (1943), ‘The intermediary stages in the biological oxidation of carbohydrate’, Adv Enzymol, 3, 191−252. lam h s and proctor a (2003), ‘Lipid hydrolysis and oxidation on the surface of milled rice’, J Amer Oil Chemists Soc, 80, 563−567. lee i k, kim k h and choi h c (1993), ‘Changes in physicochemical properties of rice grain during long-term storage’, Korean J Crop Sci, 38, 524−530. lee t, wu w and williams v r (1965), ‘The effect of storage time on the compositional patterns of rice fatty acids’, Cereal Chem, 42, 498−505. mailhot w c and patton j c (1988), ‘Criteria of flour quality’, in Pomeranz Y (Ed.), Wheat Chemistry and Technology, Vol II, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 69−90. matsue y, mizuta k and yoshida t (1991), ‘Varietal difference in palatability of stored rice’, Jap J Crop Sci, 60, 537−542. meullenet j f, marks b p, griffin v k and daniels m j (1999), ‘Effects of rough rice drying and storage conditions on sensory profiles of cooked rice’, Cereal Chem, 76, 483−486. meullenet j f, marks b p, hankins j a, griffin v k and daniels m j (2000), ‘Sensory quality of cooked long-grain rice as affected by rough rice moisture content, storage temperature, and storage duration’, Cereal Chem, 77, 259−263. mohan reddy i (1977), ‘Aging of rice’, Dissertation submitted in partial fulfilment for the award of the M.Sc. degree in Food Technology, University of Mysore. moritaka s and yasumatsu k (1972), ‘The effect of sulfhydryl groups on deterioration of milled rice during storage’, J Jap Soc Food Nutr, 25 (2), 59−62. narayana rao m, viswanatha t, mathur p b, swaminathan m and subrahmanyan v (1954), ‘Effect of storage on the chemical composition of husked, undermilled and milled rice’, J Sci Food Agric, 5, 405− 409. noomhorm a, kongseree n and apintanapong n (1997), ‘Effect of ageing on the quality of glutinous crackers’, Cereal Chem, 74, 12−15. normand f l, hogan j t and deobald h j (1964), ‘Improvement of culinary quality of freshly harvested rice by heat treatment’, Rice J, 67 (13) (Dec), 7−11. ohno t and ohisa n (2005), ‘Studies on textural and chemical changes in aged rice grains’, Food Sci Technol Res, 11, 385−389. ohno t, tomatsu m, toeda k and ohisa n (2007a), ‘Gelatinization properties of aged rice and improvement of rice texture by external layer removal’, Food Sci Technol Res, 13, 301−304. © Woodhead Publishing Limited, 2011


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6 Cooking quality of rice

Abstract: Conventional water uptake (g water absorbed per g rice in a given time) when rice is cooked directly in boiling water is related only to the grain surface area per its unit weight. Hence small and slender grains cook faster than big, round grains. When just cooked to the grain centre, all rice absorbs 2.5 times its weight of water regardless of its composition and character. In contrast, hydration of rice at ambient temperature is inversely affected by its amylose content and gelatinisation temperature (GT) and is promoted by grain chalkiness. Hydration at 70–80 °C is inversely related to GT. Presoaking before cooking reduces cooking time, promotes grain elongation and affects grain bursting and breakage. Cooking procedure affects cooked-rice texture and other parameters. Hence standardisation of laboratory cooking is desirable. Key words: hydration of rice, rice-water ratio, cooking time, grain elongation, grain bursting.

6.1

Introduction

Rice is well recognised as the most important cereal in terms of human food. More people of the Earth subsist on rice as their staple than on any other cereal grain. A small amount of this rice is made into various food products, which we will discuss separately, but the main form in which rice is used is as table rice. Millions of people in the world use rice in this form as the main staple. Many use it as the bulk of their food intake. Table rice by and large is nothing but plain cooked rice, i.e., rice cooked in water. This situation is a direct consequence of the unique form in which



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rice is handled and used – viz. in wholegrain form. In this form, rice is necessarily cooked in water until it becomes soft and is only then fit for being served. In the last 50 years or so, various innovative means of cooking rice have come into vogue especially in the industrialised countries and of late among small segments of people in developing countries – such as in a pressure cooker, in a double boiler or a rice cooker, or by microwave. But in terms of the total quantity of rice use, these practices are minuscule. The overwhelming form of cooking rice worldwide is simply to put rice in water in a pot and boil the mixture over an open fire (or other source of heat) until the grain becomes soft. In most cases excess water is taken, and the residual cooking water, after the rice becomes soft as perceived by pressing a few grains between the fingers, is drained (either discarded or used separately). Alternatively, an exact amount of water, learnt from experience, is taken and the mixture is boiled over a fire till the water is fully absorbed. Many processes occur and the rice grain undergoes many changes during its cooking, some desirable, some neutral and some undesirable. These may be summarised as follows: ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑ ∑

the primary process is of course hydration, the grain absorbing up to over twice its weight of water; chemically, the native starch granules in the grain, which form some 90% of the dry grain solids, get gelatinised; the uncooked rice which is hard and ungelatinised becomes soft and fit for eating as well as for easy digestion; the grain expands; the expansion occurs along all three radii but usually happens more in length than in other directions; the grain volume, both true volume and bulk volume, increases; the uncooked grain that was translucent or semitranslucent becomes opaque; some of the grains may break into two or more pieces; varying amounts of solids, both dissolved solids and suspended particles, are leached out from the grain during cooking and come into the excess cooking water, if any; some or all grains may open up longitudinally either along the ventral or dorsal edge or even along the lateral sides to varying extents or depths (bursting); some grains may get curled (attain ‘C’ shape); and the edges of some or all of the cooked grains may become more or less frayed.

A few of these changes, especially hydration and gelatinisation, are of course desirable but some, as we can easily see above, are not. Overall, all these changes together may be considered to constitute the cooking quality of rice. Before proceeding to discuss the above processes and changes, it is important to remember that the nature and extent of all the above events



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depend strongly on three factors. One is the variety, for the course and extent of the changes often differ from variety to variety, and that is precisely what we wish to mainly examine here. Second is the age of the rice after harvest, as discussed in Chapter 5. Finally, the changes also vary with the method of cooking – viz. whether the rice is soaked in ambient water and for how long before it is cooked, whether it is cooked in an open vessel over an open fire (or some other source of heat) or in a closed cooker, in excess water or in a limited amount of water and, especially, if the latter, how much, and so on. These aspects will be discussed as we go along in the following discussion of the cooking events.

6.2 Absorption of water by rice during cooking at or near the boiling temperature Hydration is obviously the most important event or process that occurs during cooking of rice. We might even say, that is the raison d’être of rice cooking. Scientists studying rice would therefore have been naturally interested in this phenomenon very early and many questions arise in relation to it. Some of these questions are: ∑ ∑ ∑ ∑

How much water does a sample of rice absorb when it has been properly cooked? Do different rices absorb the same amount of water or does the amount of water vary with the rice variety/grain type/quality type? Do they all get cooked in the same time interval? Do they all absorb water at the same rate?

One cannot say hydration of rice during cooking has been generally studied with such clear questions in mind. The approach was at least initially much more restricted. Indian scientists were probably the first to study hydration of rice during cooking. B. Sanjiva Rao (1938, 1948) in the Central College and A. Sreenivasan (1938, 1939) in the Indian Institute of Science (IISc), both in Bangalore in southern India, pioneered these studies. They measured the amount of water that a given amount of rice absorbed in a given time interval (say, 20 or 30 min) while being heated with water under certain standard conditions. Clearly the parameter as measured was more restricted in intent than the above questions would imply. This parameter has since then been variously called ‘water uptake’, ‘swelling number’, ‘water absorption ratio’ (g water absorbed/g or 100 g rice in a definite time), ‘swelling ratio’ (g cooked rice/g or 100 g rice, i.e., water uptake + 1.0) or ‘volume expansion ratio’ (expansion in volume, as measured by displacement of water or kerosene).



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6.2.1 The water-uptake cult The above and other workers observed early that ‘water uptake’ or ‘swelling number’, as defined, showed considerable variation among varieties. It also seemed to change with storage of rice after harvest (Sreenivasan 1939). There were other extraneous sources of variation, including those caused by imprecise milling equipment and unstandard milling conditions in those early days. With such variations superimposed over intrinsic variability, a wide range of values of water uptake began to be reported, both then and later. Moreover Sanjiva Rao et al. (1952) concluded shortly afterwards that ‘good varieties’ had both a high content of amylose starch and a high water uptake. The reason of this fortuitous association will be explained later. But these and similar reports also led to propounding of impromptu theories about how the amylose and amylopectin starch differed in their propensity to hydrate and so on. Water uptake, as defined above, thus slowly assumed the status of a paradigm of rice quality which influenced the thinking of subsequent researchers for over a generation. The parameter was felt to somehow reflect the inherent fundamental characteristics of rice varieties and began to be routinely tested in all rice quality researches or in description of new breeding lines regardless of what the value indicated. This practice is, if somewhat weakened, still being fairly widely followed even today especially among breeders. Also meanings began to be read in the value not warranted by the circumstances. The parameter as defined (g water absorbed/g of rice in a given time, say 20 min, as arbitrarily chosen) clearly meant that it was a reflection of only the hydration rate of the sample. But this aspect was usually missed and the meaning of water uptake, considered out of its definitional context, generally got semantically distorted to reflect the inherent hydratability of the sample. That is, the parameter was often implicitly thought to be a reflection of not how fast the sample got hydrated but of how much it had the capacity to hydrate. Thus a higher water uptake value was often taken to mean that the rice inherently absorbed more water − i.e., without reference to time – and hence became softer and perhaps tastier when cooked, and vice versa. These perceptions were helped by the fact that the earlier-mentioned ‘correlation’ between amylose and water uptake was repeatedly confirmed in later literature (e.g., see El-Saied et al. 1979). This inadvertent association then tended to suggest that high-amylose rice absorbed, or needed to absorb, more water to cook properly and vice versa. Such belief may have been partly at the back of the intuitive feeling observed among virtually all Western rice researchers (for whom, be it noted, first, rice is not a staple and, second, high-amylose rice is uncommon) that water : rice ratio for cooking for experimental purposes should be progressively increased as the amylose of the sample increased. These studies were continued later in a somewhat more sophisticated manner by Italian workers. Ranghino (1966) determined the ‘gelatinisation time’, i.e., the cooking time of rice by noting the time the opaque core disappeared when pressed between two glass slides, just as Desikachar and



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Subrahmanyan (1961) had done earlier. But while the latter workers took the value just as such, i.e., its cooking time, the Italians thought it to reflect certain fundamental properties of the samples. Again Refai and Ahmad (1958) and De Rege (1963, 1966) studied the volume expansion of rice when cooked at 80 °C (‘Refai index’) and considered it as another fundamental index of rice quality. Studies on water uptake have been continued elsewhere as well. Juliano et al. (1965) at the International Rice Research Institute (IRRI), Manila, cooked rice in an automatic electric cooker. They arrived at the conclusion that water uptake by rice during cooking was inversely related to the protein content as well as the gelatinisation temperature (GT). As a result, highprotein or high-GT rice was thought to need a longer cooking time and more amount of water to cook satisfactorily. These conclusions may be open to review, for rice was cooked in these studies in an automatic electric cooker with rice and water in a predetermined ratio (200 g : 280 ml) being taken in the inner pot, and a predetermined amount of water (40 ml) being put in the outer pot. ‘Cooking time’, i.e., the time for which the electrical switch remained on, under these circumstances, would, one feels, be determined more by the amount of water taken in the outer and inner pots than by any quality of the rice inside. Similarly when the amount of water taken along with rice was in a predetermined ratio, which was entirely absorbed, meaning of ‘water uptake’ under these conditions may not be clear, nor why it should vary at all. Indeed the reported values of cooking time and water uptake among the varieties had extremely small coefficients of variation. The authors also mentioned that amylose, though not a dominant factor in cooking, did influence the cooking time positively if calculated independent of protein (partial correlation). Juliano (1970, 1972) also repeatedly mentioned that the water absorption and volume expansion by rice up on cooking were positively related to the amylose content, and that for optimum texture, more water was needed to cook high-amylose than low-amylose rice (see also Perez and Juliano 1979, Del Mundo 1979). Assuming that the latter conclusion was true, even though ‘optimum texture’ was not defined, it would not imply that high-amylose rice absorbed more water than low-amylose rice in a given time, other things being equal. Juliano et al. (1969) again reported, this time after cooking rice (of unspecified size and shape) in excess water in a tube, that the time needed for cooking was longer by at least one minute for high-GT rice than that for low-GT rice. Yet GT was mentioned as being unrelated to water uptake or volume expansion ratio.

6.2.2 Study of hydration per se While the water uptake in the restricted sense mentioned above was routinely measured and reported by practically all rice researchers until the 1970s or perhaps even 1980s, and which many, especially rice breeders, perform



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even today, studies on the hydration phenomenon per se of rice have been limited. Parthasarathi and Nath (1953) cooked rice for different time periods – a key factor in understanding the phenomenon of hydration rather than comparing a single index figure. They suggested that when 2.0 g of any rice had absorbed 4.5 g of water (which means an water uptake value of 2.25), the rice was optimally cooked (no hard centre when pressed between the fingers). This was a remarkable finding for the time which attracted little attention. As we shall see below, this result was not far off the mark. Batcher et al. (1956, 1957) and Halick and Keneaster (1956) determined the water uptake of a number of American rice varieties as part of a US project to define and characterise rice quality (discussed in the next chapter). They made the perceptive observation that the water uptake after cooking rice in excess boiling water for a predetermined time was more in long- than in short- and medium-grain American rice. But the value was practically constant within a grain type, regardless of differences in their behaviour (i.e., differences in their amylose content and GT, as it later transpired) (Table 6.1). In contrast, Hogan and Planck (1958) and Halick and Kelly (1959) noted that water uptake at 70–80 °C was in the reverse order, being more in short and medium grains than in long grains. Also, the 70–80 °C value, unlike the 100 °C value, differed among varieties within a grain type corresponding to the other intrinsic properties of the samples (viz. their amylose content and GT). Halick and Kelly (1959) commented that while the 70–80°C value reflected the compositional characteristic, ‘it is suggested that at high temperatures water uptake may become a function of size and shape of the endosperm, and varietal difference in composition are then masked.’ Unfortunately the import of these findings, and especially the above statement, was lost in the general feeling of wellness that the cult-like figure of ‘water uptake’ provided. A thorough investigation of the phenomenon of water absorption by rice during cooking was carried out by the present author in the 1960s (Bhattacharya and Sowbhagya 1971). This work has an interesting history. The first research work that the present author was entrusted with after joining the fledgling rice group in the Central Food Technological Research Institute (CFTRI) at Mysore, India, in 1960 was to study the characteristics and the varietal differences in 20 varieties of rice. The first thing he did was to dutifully determine, as per the prevailing paradigm, the water uptake of the varieties as defined above, viz. the amount of water absorbed by the samples after cooking in a boiling-water bath for 20 min (W ¢20) (bath temperature 98 °C at Mysore, cooking temperature ~ 96 °C). The symbol W¢20 indicates that the value was the apparent water uptake [(weight of cooked rice/weight of uncooked rice) – 1.0] uncorrected for the solids loss during cooking. The corrected value (W20) would be roughly 5% more. It was observed that the value of W¢20 indeed differed widely among the 20 varieties. But a glance at the grain size of the varieties (which luckily differed appreciably) clearly



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Table 6.1

Some classical USDA data on rice properties

Grain type

Variety

Water Alkali Amylose Gelatinisation Blue uptakea spreading contentc temperatured Valuee (% (g/g) scoreb (%, db) (°C) transmittance)

Long

Bluebonnet Bluebonnet 50 Century Patna 231e Fortuna Impr Bluebonnet Rexoro Sunbonnet Texas Patna Toroe TP 49

3.32 3.24 3.38 3.08 3.24 3.39 3.29 3.35 3.39 3.22

2.9 4.5 2.0 3.9 4.7 4.7 4.5 4.9 6.0 5.2

– 21.5 12.9 – 23.2 25.0 22.7 23.4 14.0 22.7

– 72.0 79.5 70.5 72.5 73.5 72.5 70.5 67.5 73.5

– 35 88 34 31 26 35 21 75 25

2.84 3.04 3.02 2.98 3.12 3.23

6.0 6.1 2.4 5.9 6.0 5.7

15.2 – – 14.0 13.5 14.9

64.5 67.5 75.5 66.5 67.5 65.0

3.09 2.95

6.2 6.2

14.3 –

67.5 66.0

67 77 86 70 65 54 64

Medium Blue Rose Calrose Early Prolifice Magnolia Nato Zenith Short

Caloro Colusa 1600

48

Compiled from various sources, particularly those listed in (a) – (d) below. The majority of samples were common in the different studies, making properties and values are comparable. a Batcher et al. (1957). b Little et al. (1958). c Williams et al. (1958). d Halick and Kelly (1959). e Atypical varieties.

Adapted with permission from (Batcher et al. 1957), (Little et al. 1958), Williams et al. (1958), Halick and Kelly (1959), Bhattacharya and Subba Rao (1966a, b), Subba Rao and Bhattacharya (1966) and Unnikrishnan and Bhattacharya (1987a). Copyright 1957, 1958, 1959, 1966 and 1987 American Chemical Society.

suggested to the author that the W ¢ values apparently varied inversely with the grain size. Measurements indeed showed that the W ¢20 value was inversely related to the grain thickness of the variety (r = –0.832***, n = 20) (Fig. 6.1, left). The above finding raised a question about the premise of the ‘water uptake’ concept itself. The author argued that since rice was never cooked (in excess water) in Indian homes for a given time but only for a given end point (disappearance of a hard core), cooking rice for a predetermined time – as may appear mandatory in ‘scientific studies’ in a laboratory – was fallacious. It strongly appeared, on the contrary, that water uptake should be compared after cooking each variety for its own ‘optimal cooking time’, i.e., the time when the hard centre disappeared. Desikachar and Subrahmanyan (1961) had offered a precise test for the latter, viz. the time when the opaque central core just disappeared when the grain being cooked was pressed between two glass slides. When this ‘optimum cooking time’ (ot) of these © Woodhead Publishing Limited, 2011


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20 varieties was determined, the values again differed widely among the varieties but, interestingly, were directly proportional to the grain thickness (r = 0.877***) (Fig. 6.1, right). As a result, W ¢20 was inversely related to the optimum cooking time (r = –0.843***). It further appeared from these data that water uptake of rice might in fact be a constant if different varieties were cooked for their respective optimum times. Indeed, on determination, the values of optimum-time water uptake (W ¢ot) of all the samples fell within a narrow range with a mean value of 2.35 (g/g), with a small coefficient of variation (CV) of 5.02%. All cooked rice at this stage attained a moisture content of approximately 73−74% (wet basis). This meant that all rice varieties, regardless of their other attributes, absorbed a more or less constant ratio of roughly 2.5 times their weight of water (W value, corrected for solids loss) when cooked in water at or near the boiling temperature upto the grain centre. Clearly the ‘water uptake’ parameter, as defined (i.e., the value at a given time), was only an expression of the rate of hydration of the sample, and was not by any means indicative of its hydration ability, nor of its affinity for water, as were generally intuitively felt. In fact the authors experimentally verified that water absorption by rice continued unabated with increasing time of cooking even beyond its ‘optimal time’. While ruminating over these results (unpublished then), the author came across a paper by Husain et al. (1968) where surface area of rice grain was measured for certain properties, and felt a new window had opened. It was immediately speculated that perhaps the surface area per unit weight of the rice was the determining factor. That would be entirely logical and would explain why the value varied inversely with the grain size, especially grain thickness. It is well known that a sphere (length = breadth = thickness) has the least surface area for a given weight (Brown et al. 1950). It is similarly known that the surface area per unit weight of a particle increases as its size (particle weight) decreases and/or as its shape deviates from the spherical

20

32

20 min

2.4 2.2

W¢

(g/g)

1

2.0

30

5 3 2

15

1.8

10

18

19

15 20

1.6 1.5

28 19

4 1112 14 6 8 9 13 7 17

10 13 14 89 12 6 11 7 5

2 1 3 1.8 1.9 1.6 1.7 Average kernel thickness (mm)

16 17 18

26 24

Cooking time (min)

2.6

22

16

1.6

1.7

1.8

1.9

2.0

Fig. 6.1 Relation between grain thickness of 20 rice varieties and their water uptake after cooking at 96 °C for 20 min (left) and their optimum cooking time (right). Reproduced, with permission, from Bhattacharya and Sowbhagya (1971). © Woodhead Publishing Limited, 2011


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(i.e., as the ratios of its three diameters increase). A new series of 45 rice samples was therefore again tested in 1968−69 jointly with his coauthor along with calculation of the surface area per unit weight (S) of the varieties by using the approximate formula of Husain et al. (1968): S

3 4

L (B 2

T 2 )/2 ¥ 10/w (cm 2 /g)

where L, B, T represent the mean grain length, breadth and thickness, respectively, in mm and w the mean grain weight in mg (the full derivation is shown later). Indeed W20 turned out to be highly correlated with S, even though the value of S was somewhat approximate (r = 0.747***; n = 45). This explained why small, slender and thinner grains had a greater water uptake value (at a given time) than big and round ones. In agreement, broken rice was found to absorb water much faster than wholegrains. Cracked and chalky grains too hydrated faster, the latter presumably because it visibly cracked very quickly when placed in water. The authors further noted that the W20 values of their samples were not related to the amylose (17−29% on dry basis), protein (6.0−11.5%) contents or the alkali score (0−8: an inverse index of the GT) of the samples. However, the W/S ratio (mean value of 1.31 mg of water absorbed per 1 mm2 of rice surface area when it was cooked at 96 °C for 20 min) had a small range (CV, 8.2%) and showed a small correlation with the protein content (r = –0.421**) and the GT (r = –0.353*; n = 45). The authors concluded that water absorption by rice during cooking in excess boiling (or near-boiling) water for a given time was a function predominantly of the physical size and shape of the grain (and cracks and chalkiness); but it was also marginally affected (inversely) by its protein content and GT but was independent of its amylose content. The results of the American workers mentioned above can be understood in the light of the above data. Because of the lower GT of the medium and short varieties than the long-grain ones (Table 6.1), the former types naturally absorbed more water at 70−80 °C than the latter. The 70−80 °C value also brought out the atypical varieties within each grain type on account of the differences in their GT. But the 100 °C value, being far above even the highest GT, was dependent mainly on the surface area and largely independent of GT or amylose. Hence it was more in the long grains (longer and more slender) than in the other two types. It also failed to differentiate the atypical varieties within a type which had different GT and/or amylose values (Century Patna 231, Toro, Early Prolific) (Table 6.1). These results also explained why Sanjiva Rao et al. (1952) thought ‘good’ rice varieties had both a high amylose content and a high water uptake or why many researchers found that water uptake was positively related to amylose content. After all indica rice generally contains a comparatively high amylose content and is comparatively slender-grained (high surface area per unit weight, so high water uptake). And Indians generally prefer



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rice with small and slender grains (high W) and hard texture (high amylose). Japonica rice on the other hand is usually rounder-grained (less surface area) with a lower amount of amylose. Juliano et al. (1981) studied 10 samples of rice by cooking in excess boiling water for their individual optimal cooking times (i.e., until the opaque core disappeared). They reported that the moisture content of all cooked samples, regardless of large differences in their cooking time, as well as in amylose, protein and GT, was relatively constant around 75% moisture (wet basis). This finding confirmed one of the observations of Bhattacharya and Sowbhagya (1971) mentioned earlier. It should automatically imply that the real water uptake of rice, in the sense of water affinity when fully cooked, was a constant. A paradigm, however, is not easily dislodged (Kuhn 1962). Juliano and Perez (1983) concluded from the grain-dimension data of the above 10 varieties that their cooking times were positively related to their GT (r = + 0.75*), but were not related to their surface area (r = − 0.49NS). The authors concluded that hydration during cooking was inversely related to the GT and was independent of the grain surface area. This is a difficult conclusion to reconcile, to say the least. One can say only it is hazardous to conclude on this point on the basis of observed data from 10 samples when a much more extensive study gave an unambiguous and opposite conclusion earlier. It is a bit unfortunate that this point has still to be laboured. There is enough experimental evidence in the literature to show the validity of the importance of surface area in determining cooking rate and time. The US study has been mentioned earlier. In this, Batcher et al. (1957) observed in 66 samples (26 varieties) that the water uptake after cooking rice in excess boiling water for 20 min was (Table 6.1): ∑ ∑

more in long- than in short- and medium-grain American rice, although the former was known to have a higher GT than the latter two; and reasonably constant within a grain type, regardless of difference in GT among the varieties, such as Century Patna 231 (very high GT) and Toro (low GT) among the long grains and Early Prolific (very high GT) among the medium grains.

Evidently GT had little to do with the water uptake when rice was cooked in excess boiling water for a specified time. These results became easy to understand if it was remembered that American long-grain rice was invariably long (>6.6 mm) and slender (length/width, >3), while American short- and medium-grain rice was shorter and roundish, and also heavier (Adair et al. 1973). This size and shape difference meant that the former had a larger surface area per unit weight than the latter. The data of Batcher et al. (1956) were more forthright (Fig. 6.2). Here the curves for long-grain rice (higher GT) were throughout above those of medium- and short-grain rice (lower GT). Clearly it is the size and shape (i.e., surface area) and not the GT that decided the issue. © Woodhead Publishing Limited, 2011


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Water uptake ratio

Long-grain Century patna Rexoro Texas patna Medium-grain

40

35

Calrose Zenith Short-grain

30

California pearl Caloro Colusa

25

0

4

8

12 16 Cooking time (min)

20

24

20 28

Fig. 6.2 Water uptake of long- (3), medium- (2) and short-grain (3) US rice. Reproduced from Batcher et al. (1956).

Again Suzuki and Murayama (1967) in their study of early-season and late-season rice in Japan observed that the late-season (with lower GT) rice absorbed less water after cooking in excess hot water for a constant time compared with the same varieties grown by early-season cultivation (higher GT). These results were in direct contradiction to the supposed negative influence of GT on water uptake. The contradiction resolved when it was noted that the late-season samples were bigger and heavier, and thus had lower surface area per unit weight, than the early-season crop. Our finding that broken rice derived from the same sample absorbed water much faster than the wholegrains also discounted any influence of GT but obviously pointed to the influence of grain surface area. If GT was the deciding influence and not surface area, there was no reason why broken grains should have had higher W values than wholegrains. Again, in our later study of 177 samples (Bhattacharya et al. 1982) we had determined the apparent water uptake (uncorrected for solids loss) for 15 min (W¢15) and the approximate grain length and breadth, enabling calculation of the apparent grain surface area per g (S¢) from the formula S¢ = 2LB10/w (cm2/g) (unpublished). Statistical examination of these data showed that the W¢15 values were strongly related to the S¢ values (r = +0.593***, n = 134). Interestingly, in later literature, one finds repeated mention that perhaps the grain thickness and ‘coarseness’ did after all affect water uptake adversely (e.g., Juliano and Perez 1983, Juliano 1985). Indeed this relation was amply demonstrated earlier (Fig. 6.1) and is in fact what triggered the whole investigation. However, the best mathematical expression for coarseness (big size) or greater thickness (more roundness) is the surface areas per unit weight. One can conclude that water uptake of rice (in a given time) is without



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any doubt related primarily to the grain surface area, but perhaps marginally, inversely, to the protein and the GT.

6.2.3 Some caveats The actual circumstances of cooking may modify the relationship to some extent. The grain surface area per unit weight would no doubt always influence the rate of hydration: that is the greatest influence at boiling temperature. So at the boiling temperature, as we have seen, the water uptake is determined mostly by the surface area alone. But this is not how rice is generally cooked in an electrical cooker or at homes. Here, rice is put in ambient water and the mixture is then gradually heated, so the rice is exposed for the initial several minutes to a temperature far below boiling. As will be discussed below, room temperature hydration of rice is inversely related to both amylose and GT, so these influences will operate at lower temperatures. Hence in this situation low-amylose and low-GT rice would already have absorbed a greater amount of moisture by the time the water comes to a boil than high-amylose or high-GT rice and vice versa. Further, a low-GT rice would start rapid hydration, as the temperature passed through the 60−80 °C range, earlier than a high-GT rice would do. Thus whenever rice is cooked by putting it in ambient water and then heating, which is by far the more common practice, there should be little doubt that lower-amylose and lower-GT rice would show a somewhat higher water uptake (i.e., in the conventional sense of in a given time) than the opposite kind of rice, other things being equal, and vice versa. These facts may partly explain the persistent belief that high amylose or high GT lowers the water uptake of rice or increases its cooking time. The above caveat, however, does not in any way detract from the three general propositions that: ∑ ∑ ∑

the surface area per unit weight of rice, i.e., its grain size and shape, always has an influence on its rate of hydration; the grain size and shape of rice are the primary factors in its rate of hydration during cooking at the boiling (or near-boiling) temperature; and all rice, when fully cooked in boiling temperature up to the grain centre, absorbs about 2.5 times its weight of water (i.e., has a nearly constant water uptake of about 2.5 at that time), and attains a moisture content of approximately 74% (wet basis).

It clearly follows from the above that the water uptake value in the conventional sense does not normally provide any useful information that cannot be obtained or inferred from other attributes (for instance, size and shape). However, this does not apply to specific situations. Especially when rice is subjected to some process or treatment, such as ageing, heat treatment or hydrothermal treatment, water uptake data may provide useful information, for the grain size and shape remain unchanged by such treatment, and so any change in water uptake can be attributed to the process or treatment.



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6.2.4 Other observations Sinelli et al. (2006) used Fourier transform near-infrared (FT-NIR) spectroscopy and an electronic nose to evaluate the optimum cooking time of rice. Barton et al. (2002) studied the progress of and changes in rice constituents (interaction of water with starch and protein) during cooking using near-infrared, midinfrared and Raman spectra. Rashmi and Urooj (2003) suggested that cooking conditions (pressure cooking, boiling, steaming and straining) had an effect on the formation of rapidly digestible starch, slowly digestible starch and resistant starch in rice. Chang et al. (1996) studied 12 varieties in Taiwan. They found that diffusion was the dominant process controlling water uptake at low temperatures while gelatinisation was predominant at high temperatures. Correlations between physicochemical properties and reaction parameters at high temperature were rarely found. Takeuchi et al. (1997a, 1997b) studied the change in moisture distribution in rice grain during boiling using nuclear magnetic resonance (NMR) imaging. The cell wall was considered to offer little resistance to migration of water. Lee and Osman (1991) suggested that the rate of water diffusion during cooking was related to the cell arrangement and was inversely proportional to the protein content; the protein seemed to act as a barrier to hydration and swelling. Choi et al. (1999) studied 19 japonica and tongil-type rices for their water absorption at room temperature as well as under boiling conditions.

6.2.5 Kinetics of hydration and energy relations This aspect has been studied by many workers. Parthasarathi and Nath (1953) seem to have been the first to examine this aspect. They cooked rice in water between 80 and 100 °C and found that the time required for absorbing definite amounts of water (1.5, 2.5, 3.5, 4.5 g/g each) varied exponentially with the temperature. From a plot of the logarithm of the above times against the reciprocal of the absolute temperature, the energy relations were calculated. It was found that the activation energy was approximately 13 kcal per mole between 80 and 100 °C, which remained constant for various lengths of cooking. A temperature of 80 °C was considered to be more or less the lowest limit for proper cooking of rice. Suzuki et al. (1976, 1977) made a detailed study of the kinetics of rice cooking between 75 and 150 °C. They measured the softening of cooked rice using a parallel plate plastometer to estimate the degree as well as the end point of cooking. The rice was cooked after preliminary soaking in ambient water for 30 min. The cooking rate followed the equation of a first order chemical reaction, the proportional constant being designated the cooking rate constant. The Arrhenius plot of the cooking rate constant against the reciprocal of absolute temperature (Fig. 6.3) gave two straight lines intersecting at approximately 102 °C (stated as 110 °C in the paper). The activation energy of cooking was approximately 19 kcal and 9 kcal below and above this temperature, respectively. The rate of hydration was governed



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1.0

0.5

110 °C 102 °C 100 °C

0.1

0.05

0.02 2.3

2.4

2.5

2.6

2.7

2.8

2.9

Fig. 6.3 Arrhenius plot of the cooking rate constant of rice. Reproduced, with permission, from Suzuki et al. (1976) John Wiley and Sons.

by simple diffusion up to 50 °C, by reaction rate with starch in the range of 80–100 °C, and by diffusion of water through the gelatinised starch shell above 100 °C. Similar studies were reported by Cheigh et al. (1978) and Cho et al. (1980). The heat of hydration of rice was studied by Miyakawa and Shiotsubo (1977) and Kanemitsu and Miyagawa (1974). Interestingly the latter workers while studying the heat of hydration of whole and broken rice found that the value was approximately proportional to the total surface area. As the above studies were made with japonica rice, Juliano and Perez (1986) studied the hydration phenomenon of tropical rice using a technique similar to that of Suzuki et al. (1976) between 80 and 120 °C. They used different proportions of rice:water for the four different varieties as being ‘optimum’ for each. The samples were cooked between 80 and 120 °C for different times, using the Instron to measure the softening and the end of cooking. They arrived more or less at similar conclusions as Suzuki et al. (1976), noting that the Arrhenius plot showed a break at 90 °C, the slope being higher (higher activation energy) below 90 °C. The respective activation energies varied between 76 and 121 kJ/mole below 90 °C and 32 and 57 kJ/ mole above 90 °C for the four varieties. Others who made similar studies are Lin (1993) and Chang et al. (1996). The pattern of results was similar in all these studies although the derived activation energy values differed somewhat with each. Lakshmi et al. (2007) measured the amount of energy consumed during cooking of rice by microwave.



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6.3

Hydration at lower temperatures

6.3.1 Hydration at 70−80 °C A few studies have been made to see how rice grains hydrated at 70−80 °C. American scientists (Hogan and Planck 1958, Halick and Kelly 1959) studied this aspect as part of the historic cooperative US study on rice quality already noted above. They noted that in contrast to boiling temperature, at which long-grain US rice hydrated faster than medium- and short-grain US rice, the pattern of hydration was the reverse at 70−80 °C. At this temperature it was the medium- and short-grain rice that hydrated faster because the latter varieties had lower GT than the former. So the medium- and short-grain samples started substantial gelatinisation at 70−80 °C and therefore underwent rapid hydration as compared with the long-grain rice. It is for the same reason that hydration at 70−80 °C could differentiate atypical varieties – viz. Century Patna 231 (high GT) and Toro (low GT) among long-grain and Early Prolific (high GT) among medium-grain rice – among the grain types. Cooking at boiling temperature could not differentiate atypical varieties. The reason was that the cooking temperature here was much higher than the GT of either group and therefore the rate of hydration was decided more by the physical factor of surface area than by the chemical property. Even today, in the quality testing protocol of new breeding materials in the USA, water uptake and solids loss when rice is cooked at 77 °C is included as a routine test (Webb 1985). The test instantly picks out the low-GT lines. Italian scientists (Refai and Ahmed 1958, 1961, Borasio et al. 1964) in their study of rice quality included a study of cooking rice at 80 °C. They found that the volume expansion of the rice when it was cooked at 80 °C (called ‘Refai index’) was a good indicator of Italian rice quality. Here again the reason was simple. Italian rice, mostly japonica, had a lower GT and therefore hydrated and swelled more at 80 °C than foreign imported indica-type rice. Bhattacharya and Sowbhagya (1971) too studied hydration of rice at 70−80 °C. As expected, they too found that the extent of hydration of the rice was strongly inversely affected by the GT of the variety. In fact this value could be used to index the GT. However, they argued that since water absorption per se was also strongly affected by the grain surface area, it would be best for a correct estimation of GT to express the 80 °C value as a percent of that at 100 °C. Thereby the effect of surface area would be cancelled. Bhattacharya et al. (1972, 1982) confirmed that the above ratio (W80 °C/W100 °C) was very highly correlated to the GT (r = –0.791***, n = 144) as well as to the alkali digestion score (r = 0.924***, n = 40). It may be mentioned here that in the starch-iodine blue value (SIBV) test devised by early US scientists (described in the next chapter), the rice flour was extracted with water at 77 °C. Thus the amount of amylose extracted in the test was affected by both the solubility of the amylose and the GT of the variety. This double effect had the advantage of magnifying the differences among some of the limited number of US varieties (discussed in the next © Woodhead Publishing Limited, 2011


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chapter), but it had the unfortunate effect of partly masking the differences when considering world rice as a whole. This is because grain type and chemical properties (amylose content, GT) were rigorously controlled by US breeders (Adair et al. 1973) while these properties varied at random in rice outside the USA.

6.3.2 Hydration at room temperature Tani et al. (1969) were the first to show that the rate of hydration of rice in ambient water differed among varieties. Japanese rice hydrated more than long-grain Thai or Burmese rice; also new rice absorbed more water than old rice (Fig. 6.4). Similar data were independently obtained by Indudhara Swamy et al. (1971) and Kongseree and Juliano (1972). The latter workers and Antonio and Juliano (1973) concluded that the water content of steeped brown rice was inversely proportional to the amylose content, but was not related to the GT. But Bhattacharya et al. (1972, 1978, 1979, 1982) showed that the equilibrium moisture content attained by rice when soaked in ambient water (EMC-S) was an inverse function of the amylose content as well as the GT, and a positive function of kernel chalkiness. In fact the EMC-S (y) of a sample of rice could be calculated from its regression equation with amylose (x1), alkali score (x2: scores 0−8, an inverse index of the GT), and chalkiness score (x3: 0% chalky area = score 0, £20% = 1, >20% = 2) by the formula

30

New rice

Absorption ratio, %

Japan

Old rice

20

Burma 10 Thailand

0

0

40

80

120

Time, min

Fig. 6.4 Absorption of ambient water by milled rice. Reproduced, with permission, from Tani et al. (1969).



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Rice quality y = 34.84 – 0.28x1 + 0.26x2 + 0.73x3 (R2 = 0.925, n = 165)

Thus the mean EMC-S value varied among varieties from a low of about 27.5% (wet basis) in non-chalky, intermediate-GT, high-amylose rice (the typical indica rice of south Asia) to about 32% for the typical japonica rice (low amylose, low GT, low chalky) to a high of about 36% in low-GT waxy rice (Bhattacharya et al. 1982).

6.4

Loss of solids during cooking

Another important factor to be considered in cooking of rice is the amount of solids lost into the excess cooking water. There are two kinds of such solids: dissolved and undisolved (i.e., suspended particles). This parameter used to be routinely measured in the early days of rice grain research but no longer receives much attention. Specifically a good deal of data are available in the works of Batcher et al. (1956, 1957), Chikubu et al. (1960), Borasio et al. (1964), Hampel (1965, 1967) and Juliano et al. (1969) among others. However, no clear trend is visible in any of these results. Studies in the author’s laboratories in CFTRI and in the Rice Research and Development Centre (RRDC unpublished) also did not reveal any trend. Theoretically, the amount of solids lost during cooking of rice can have potential relation to several factors, viz. ∑ ∑ ∑

water uptake 䊊 at a fixed time of cooking (possibly positive relation) 䊊 at complete cooking surface area per unit weight of the sample grains (positive) chemical composition of rice, especially 䊊 amylose content (inverse relation?) 䊊 gelatinisation temperature (inverse relation?) 䊊 protein content

So far no relation seems to have been found with any of these variables. Only one factor that has been conclusively shown is the effect of the age of the rice after harvest. Loss of solids is definitely high soon after harvest and gradually decreases with storage time. This aspect was discussed in the previous chapter. As far as varietal difference is concerned, no clear clue as to the reason why a fair amount of sample-to-sample difference in the parameter is often seen to exist has been found so far. Cooking time should certainly be a factor, for the amount of solids lost should undoubtedly increase with increasing time of cooking, albeit at a slow rate after some time. This creates an additional difficulty. For what is measured at a fixed time of cooking may not mean anything with respect to a variety. As discussed earlier, for true comparison, samples ought to be compared at the respective optimum times of cooking, but these values also



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did not show any relationship. One would intuitively feel that perhaps the amount of solid loss would be inversely proportional to the amylose content, but this is not necessarily true as the large amount of data of Batcher et al. (1957, 1958) and others show. Nor does it necessarily increase or decrease with the GT or the surface area of the variety per unit weight. To be fair, there has been no clear systematic work to investigate this parameter. Considering its possible economic importance, such an investigation is worth doing. The matter should be of some importance for industries wherever rice is cooked in excess water, which is later drained, such as in making quick-cooking rice and canned rice. At the same time from the practical standpoint it is good to remember that whenever rice is cooked in a fixed amount of water that is allowed to be fully absorbed, the question of solids lost during cooking remains only of academic interest.

6.5

Effect of presoaking in ambient water on cooking

In many homes rice is initially presoaked in ambient water for some time before being cooked by heating. Even the usual practice where rice is put in ambient water before starting to heat also entails partial presoaking. This is a fairly important aspect, for presoaking affects the cooking parameters substantially. The effect of presoaking was studied in detail for the first time by Desikachar and Subrahmanyan (1961). They observed that the rice grain developed transverse cracks upon soaking. The cracks formed within several minutes in raw milled rice but required up to 1−2 h in the case of parboiled rice. Both raw and parboiled milled rice hydrated faster and cooked earlier after such presoaking than when the rice was cooked directly in hot water. Observations suggested that the cracks probably provided additional openings for entry of water and hence hastened the hydration. Also grains elongated considerably more when cooked after presoaking compared with direct cooking. Apparently, it is the transverse cracks again that helped in the greater longitudinal expansion or elongation. Sowbhagya and Ali (1991) confirmed the above findings with detailed data. As a matter of fact these transverse cracks have been shown by microscopic and histological techniques to be responsible for the characteristic ‘rings’ that are seen to develop in basmati group of rice upon cooking (Kamath et al. 2008). Desikachar and Subrahmanyan (1959) had earlier provided detailed information regarding the swelling along different axes of new and old rice during cooking. Another observation has come from Japan. Horigane et al. (1999, 2000) made an interesting observation that cooked rice when observed by NMR microimaging showed internal hollows within it. It has to be noted that although they took care to exclude all cracked or fissured grains from their experimental lot, they presoaked rice in ambient water for one hour before starting to heat for cooking. Clearly the hollows would have developed from © Woodhead Publishing Limited, 2011


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the transverse cracks that developed during presoaking, as discussed above. Indeed, the authors felt that the hollows originated from the fissures whose ends became sealed by gelatinised starch, thus leading to the development of the hollows.

6.6

Other changes/events occurring during cooking

As listed at the beginning of this chapter, many other minor to major changes occur during cooking of rice. These changes or events have been incidentally noticed by different scientists during their work on rice, but have rarely been investigated on their own right. Only recently some notice has been taken of these phenomena at the RRDC (unpublished). Although the work at the RRDC has mostly, though not entirely, concerned itself with basmati group of rice, there are reasons to believe that many of these observations would apply to rice in general. Some of the important observations are summarised below. Wherever other sources exist, these have been cited; otherwise the remaining observations are to be understood as unpublished observations from the RRDC. Presoaking of rice in ambient water before heating has a profound influence on the course of various events/changes that occur during cooking. It particularly reduces cooking time and enhances grain elongation as already mentioned. There are also other effects. One development that occurs during cooking is that some grains split or burst open to varying extent during the process. The bursting may be minor, even negligible, or deep and wide, leading to the grain opening almost like a book. There are several significant points about this grain bursting during cooking. First, there is a strong variety to variety difference in bursting. But, so far as has been examined, admittedly limited, no relationship of it with other grain quality parameters has been established. Second, the age of the sample after harvest plays a major role. Regardless of whether the variety is more or less prone to bursting during cooking, the prevalence of bursting is invariably more in the early days after harvest and gradually decreases as the grain ages. Third, presoaking makes a profound difference. The number of grains that burst during cooking is very high, almost 100%, if the rice is cooked directly in hot water, but it decreases drastically, or even disappears, if it is cooked after presoaking. Fourth, yet, even though the percentage of grains that bursts is high when the rice is directly cooked in hot water, the degree or depth of bursting is usually very small, may be even negligible. On the other hand, when cooked after presoaking, the percentage of grains that burst is drastically reduced; but the few grains that do burst do so to a very high degree. Fifth, there is difference in the site of the splitting or bursting. Upon direct cooking in hot water, the fine bursting that occurs, does so along both the ventral and dorsal edges. But after presoaking, the few grains that burst wide open almost invariably do so only on the ventral © Woodhead Publishing Limited, 2011


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side. Finally, there is some indication that grain bursting probably leads to some lowering of the instrumental measure of hardness of cooked rice. Another event that may occur during cooking is that some grains break into smaller pieces. Presoaking has little or no influence on the breakage. Some samples break more, some less. Apparently excessive milling and too low grain moisture promote grain breakage during cooking. The type of equipment used for milling of the rice has a small influence on the incidence of cooking defects. Generally abrasive milling (as typified by the laboratory emery mill, the Satake Testing Mill TM 05) tends to cause a slightly greater incidence of cooking defects (bursting and breakage) than friction-type whiteners (as typified by the laboratory McGill mill). The degree of milling has some influence on the events that occur during cooking. Undermilling affects cooking in many ways, generally negatively. It leads to substantial reduction in both grain elongation and ring formation as well as to increase in grain bursting. It also causes two additional defects. One is grain curling (‘C’ type grains) caused by residual bran on the dorsal edge. The other is, water absorption may be reduced. This last aspect was studied in detail by Desikachar et al. (1965). They demonstrated that brown rice did not cook well at all. It was resistant to hydration, which led ultimately to its sudden bursting open into an ungainly shape. Even some minor scratching of the bran facilitated hydration. A minimum of some 2% degree of milling led to near normal cooking (see Fig. 4.6). Cheigh et al. (1978) too reported that undermilling prolonged cooking time. They additionally observed that neither presoaking nor the degree of milling had much effect on the energy relations during cooking. Grain chalkiness had a curious effect on the events that occurred during cooking. If directly cooked in hot water, chalky grains interestingly burst less but broke more than translucent grains. When cooked after presoaking, both defects were enhanced.

6.7

Laboratory cooking of rice for various tests

Although this subject may more rightfully belong to the chapter on quality testing, it is convenient to discuss it here in view of the proximity of the subject. Rice has to be cooked in the laboratory for various measurements including grain elongation and other changes but especially for sensory testing and instrumental texture measurement. The method of cooking of rice should be crucial for such measurements. As mentioned above, and as we shall see further below, the method of cooking may have a profound influence on the various attributes of cooked rice. Clearly a logically determined and universally accepted method of cooking should be followed if results of different laboratories are to be compared. Yet, in actual practice, there seems to be little unanimity among laboratories about either the rice : water ratio used for cooking or about the actual procedure of cooking. No © Woodhead Publishing Limited, 2011


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doubt Batcher et al. (1963) stated that differences in cooking method of rice between laboratories did not seem to affect the sensory scores appreciably. One wonders. This might perhaps be somewhat true of sensory evaluation, if at all. But it is certainly not so in the case of instrumental texture measurements. In the latter case, the hardness or firmness of the cooked rice is bound to be directly affected by the rice : water ratio used, as can be seen from the data of Juliano and Perez (1983) (Fig. 6.5). After all water acts as a plasticiser in starch pastes. Therefore more or less water is bound to change the paste viscosity, in this case the texture of the cooked rice. The method of cooking of rice is thus crucial to the whole science of quality testing of rice. To take an example, Rousset et al. (1995) included ‘white core rate’ in their sensory texture profile analysis procedure for cooked rice. This could be an indication that the cooking method (water ratio) adopted by them probably did not cook all the grains up to the grain centre. If this was indeed the case, the assessment results could be considered flawed. To take another example, Fig. 6.5 shows that the hardness of cooked rice is heavily dependent on its moisture content (shown as water : rice ratio in the figure). So any uninformed adjustment of the water ratio is fraught with serious hazard. Yet there is absolutely no unanimity among scientists about this ratio. One finds different investigators using different water : rice ratios and

Instron cooked-rice hardness, kg

40 RD4 (1.7% amylose) IR841–67–1 (15.1% amylose) C4 – 63G (24.4% amylose) IR42 (27.9% amylose) Ratio used for rice cooker method (IRRI) Ratio for 75% water in cooked rice

30

20

10

0

0

1 2 3 Water: rice ratio (uncorrected for steam loss)

4

Fig. 6.5 Effect of water : rice ratio (uncorrected for steam loss) on cooked-rice Instron hardness of four milled rices representing four amylose types. Reproduced, with permission, from Juliano and Perez (1983) John Wiley and Sons.



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even the same investigator using different ratios for different amylose-class rices for cooking. In fact the latter practice is widespread. The method of cooking also varies. Different laboratories generally adopt different practices originating arbitrarily in their past. The origin of the practice of arbitrarily adjusting the water ratio to the amylose content is not clear and can only be speculated upon. ∑

∑

∑

∑

It may have started from the original Indian (Sanjiva Rao) flawed paradigm − that water uptake of rice was proportional to its amylose content and to ‘good cooking’ − which profoundly influenced the next generation of researchers. It may have originated from the repeated observation of indica rice (generally higher amylose) usually having a greater water uptake than japonica rice (lower amylose). These observations perhaps gradually got transformed in one’s mind into a positive association between amylose and water absorption. The actual reason was, of course, indica grain is usually comparatively long and slender (greater surface area), while japonica rice is generally fatter and more round (lower surface area). It may have originated from investigators outside south Asia, who are not used to high-amylose rice, finding the latter too hard. Therefore they may have felt it appropriate to add more water for its cooking to make it ‘acceptable’. This practice then probably gradually got transformed into a generalised rule of adjusting the water to the amylose. It may have originated from generalised statements on this subject that abound in reviews and chapters.

As an example of the last point, Juliano (1979) states: ‘The cooking and eating characteristics of milled rice are determined mainly by its amylose/ amylopectin ratio. Water absorption and volume expansion during cooking are directly affected by amylose content.’ The first sentence is obviously true. The second is also true, but only in some contexts, not in others. For example, as we have seen earlier under the section on hydration during cooking, amylose of course does not affect water absorption during cooking in boiling water. At low temperatures amylose does affect hydration somewhat. This act however may more correctly be called ‘soaking’ rather than ‘cooking’. As for volume expansion, amylose again does affect the bulk volume, but not true volume of cooked rice. The former effect is indirect, through its effect on cooked-rice stickiness. Sticky cooked rice is prevented from swelling freely because of mutual grain adhesion. Nonsticky rice is not so prevented and hence swells more freely. This swelling creates a good deal of empty space, i.e., porosity, hence a lower bulk density. But true volume (excluding inter-granular empty space), i.e., true density, is the same, related only to the amount of water absorbed. As more water is absorbed, so the true volume is increased. But the perceptible positive relationship of the increase in bulk volume of cooked rice to the amylose content often creates a false impression. The clearly visible greater swelling looks like proof that volume increase,



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and hence by inference water absorption, is positively related to amylose content. When statements to such effect are found repeatedly in the literature, and are considered out of context, a climate is created of the need for the water : rice ratio to be increased as the amylose is increased. It is not that no attempt has been made to address the issue of water ratio. Del Mundo (1979) did a good deal of work to find out the optimal amount of water to be used for cooking different types of rice. Unfortunately she tried to adjust the cooking to ‘similar doneness’, which was defined by no more than ‘neither too dry nor too soft’. Whether this is a sufficiently objective criterion and, equally important, will be understood similarly by others, is a moot point. Batcher et al. (1963) collected a considerable amount of information about the methods of cooking followed in different countries, but the information was not such as could be easily translated into the required water ratio. Juliano et al. (1981, 1982) and Juliano and Perez (1983) paid a good deal of attention to this issue, which was discussed further in detail by Juliano (1985). He mentions that they adjusted the water : rice ratio for high-amylose rice to obtain an ‘acceptable soft texture (less than 10 kg of Instron hardness)’. Even if it is assumed to be a fair judgement, the value is clearly arbitrary. Again he suggests that the water : rice ratio used for cooking should be: 0.9–1.1 for waxy rice, 1.2–1.4 for low-amylose rice, 1.5–1.6 for intermediate-amylose rice, and 1.7–2.0 for high-amylose rice. This is clearly a perception, not a finding based on measurable criteria. In the study of the effect of different water : rice ratios on the hardness values of cooked rice (Fig. 6.5), he showed that these above-mentioned ratios gave almost identical range of hardness values among the varieties as would have been the case had an identical ratio of 2.65 (final rice moisture, 75%) been chosen for all rices. Assuming that is so, nonetheless the ratios are clearly arbitrary. And in any case if the results are indeed similar, then one finds it difficult to understand why one should substitute a straightforward constant water ratio by an arbitrarily perceived one. What is more, even these ratios are not constant. In Juliano and Perez (1983), IRRI was reported to have been using the ratios of 1.3, 1.7, 1.9 and 2.1, respectively. In a survey, Juliano (1982) found the water ratios being used by different investigators were: 0.8–1.3 (waxy), 1.1–1.7 (low-) and 1.5–2.5 (intermediate- and highamylose rice). Arbitrariness is obvious. And when the texture is heavily dependent on the moisture content (Fig. 6.5), the reported results would remain in question. A light-hearted instance of the hazard of using arbitrary ratios can be shown from the same Fig. 6.5: if one were to choose a water : rice ratio of approximately 1.6 for waxy rice, 2.1 for low-amylose rice, 2.3 for intermediate-amylose rice and 2.7 for high-amylose rice, all the samples would give an identical Instron hardness reading. The investigator would then proclaim that cooked-rice hardness was independent of amylose! One arbitrary decision cannot be claimed to be superior to another.



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The present author feels that the cooking method should be decided on the following considerations.

6.7.1 Water : rice ratio ∑ As discussed before, all rices, regardless of type or composition, absorbed an identical amount, viz. approximately 2.5 times its weight, of water when cooked in excess boiling water until the central opaque core just disappeared. This is a fact and not a matter of debate. On this consideration, one logical approach would be to cook all samples with precisely 2.5 times its weight of water. The present author and his colleagues in the CFTRI school adopted this approach. ∑ Japanese scientists (Okadome et al. 1999a, 1999b) recently used a constant water : rice ratio of 1.6. While this ratio too is arbitrary, it has the merit of being the same for all varieties (in the logic that all rices had the same water uptake at complete cooking). So the comparative position remained unchanged. In any case, there should be a strong case for using an identical water ratio for different varieties in all experiments in addition to any other differential pattern of ratios that the investigators choose to use. ∑ At the same time cooking with adjusted water : rice ratios would be appropriate if there were data to demonstrate that such was the actual practice in homes of different population/ethnic groups. It would also be justified if there were data to show that the amount of water was actually so adjusted in homes as to yield a defined texture. One example of justifiable variation of the ratio may be cited. Waxy rice is a staple in northern Thailand and parts of Laos. It is observed that rice was not cooked in boiling water there. Rice was actually soaked in ambient water overnight (the equilibrium water content would then be around 40%, including the adhering water) and then the soaked rice was steamed on a perforated basket or cloth mesh. Clearly here the final water uptake value would lie somewhere in the range of 0.5−1, and cooking waxy rice with such a ratio for its testing could be justified on this count. A detailed international survey of this kind would be of great help. Despite the heroic efforts made by Batcher et al. (1963), the information they collected could not be expected to be much more than fragmentary. The survey should include the exact practice and method of cooking as well as the final moisture content of the cooked rice. Its scope should especially include practices among Asians who eat rice as a staple every day. Finally it should be directed to practices in homes and catering establishments rather than in laboratories (whose practitioners carry a heavy baggage of literature, tradition and prevailing paradigm (Kuhn 1962)!). Until that time use of different water : rice ratios for cooking rice would continue to be considered an arbitrary choice.



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6.7.2 Cooking procedure This is another variable on which there is no unanimity. Cooking by open boiling in a beaker, in an automatic cooker, and indirectly by steam were all being adopted. Probably microwave cooking would be adopted soon. Juliano and Sakurai (1985) discussed how the cooking procedure might affect palatability of cooked rice. If a precise water : rice ratio was used, the cooking would preferably have to be performed indirectly by steam, only then the entire amount of water would be absorbed by the rice. In an automatic cooker some loss of steam would occur, which would have to be suitably compensated. A second point was the uniformity of absorption of water by all the rice grains in the sample. This could be ensured in one of two ways. One was to cook the rice in a shallow layer. This is what was done by the CFTRI group (Bhattacharya et al. 1978, Deshpande and Bhattacharya 1982) in the following manner: a given quantity of rice (2−10 g) along with exactly 2.5 times its weight of water was taken in flat shallow steel dishes. The dishes were covered and carefully arranged perfectly horizontally in a special cooking stand, which was then lowered into an autoclave and steamed at atmospheric pressure for 45 min. The other way to do it, in the case of a water ratio of 2.5, would be to cook in excess water for the exact cooking time of each sample. The cooking time was found out in a preliminary experiment by Juliano et al. (1981) using the Desikachar test of pressing between two glass slides till the opaque core disappeared. Sowbhagya and Ali (1991) found out the time, especially for parboiled rice by finding the time required for the rice to attain 73−74% moisture. Both these methods could well be subject to some random error as discussed by Juliano (1985). On the other hand many researchers cook rice in a deeper layer either in a cup or in a cooker in steam while using a given water ratio. In such a case there would be a clear gradient of moisture content from the top (least) to the bottom (most) rice layer. This error is generally attempted to be rectified either by discarding the top and the bottom layers and using only the middle layer for the test or by collecting the rice in a plastic bag and allowing it to equilibrate for some time. Whether either is sufficient correction is a moot point.

6.7.3 Presoaking Presoaking of rice in ambient water before the beginning of cooking has a profound influence on the properties of the cooked rice. Similarly putting rice in excess of ambient water and then heating is equivalent to partial presoaking and thus exerts a partial influence of presoaking. Results of measurement of any property of rice cooked with and without presoaking would therefore not be comparable.



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References

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juliano b o, oñate l u and del mundo a m (1965), ‘Relation of starch composition, protein content and gelatinization temperature to cooking and eating qualities of milled rice’, Food Technol, 19, 1006-1011. juliano b o, nazareno m b and ramos n b (1969), ‘Properties of waxy and isogenic nonwaxy rices differing in starch gelatinization temperature’, J Agric Food Chem, 17, 1364-1369. juliano b o, perez c m, barber s, blakeney a b, iwasaki t, shibuya n, keneaster k k, chung s, laignelet b, launay b, del mundo a m, suzuki h, shiki j, tsuji s, tokoyama j, tatsumi k and webb b d (1981), ‘International cooperative comparison of instrument methods for cooked rice texture’, J Texture Stud, 12, 17-38. juliano b o, balkeney a b, butta i, castillo d t, choudhury n, iwasaki t, shibuya n, kongseree n, lapis e t, murty v v s, paule c m, perez c m and webb b d (1982), ‘International cooperative testing of the alkali digestibility values for milled rice’, Starch/Stärke, 34, 21-26. kamath s, stephen j k c, suresh s, barai b k, sahoo a k, radhika reddy k and bhattacharya k r (2008), ‘Basmati rice: Its characteristics and identification’, J Sci Food Agric, 88, 1821-1831. kanemitsu t and miyagawa k (1974), ‘Calorimetric studies on swelling of rice. 1. Swelling of rice grain and rice powder’, Cereal Chem, 51, 336-344. kongseree n and juliano b o (1972), ‘Physicochemical properties of rice grain and starch from lines differing in amylose content and gelatinization temperature’, J Agric Food Chem, 20, 714-718. kuhn t s (1962), The Structure of Scientific Revolutions, Chicago, The University of Chicago Press. lakshmi s, chakkaravarthi a, subramanian r and vasudeva s (2007), ‘Energy consumption in microwave cooking of rice and its comparison with other domestic appliances’, J Food Engg, 78, 715-722. lee y e and osman e m (1991), ‘Physicochemical factors affecting cooking and eating qualities of rice and the ultrastructural changes of rice during cooking’, J Korean Soc Food Sci Nutr, 20, 637-645. little r r, hilder g b and dawson e h (1958), ‘Differential effect of dilute alkali on 25 varieties of milled white rice’, Cereal Chem, 35, 111−126. lin s h (1993), ‘Water uptake and gelatinization of white rice’, Lebensm. Wiss. Technol, 26, 276-278. miyakawa k and shiotsubo f (1977), ‘Emission of heat of water absorption during swelling of rice’ (in Japanese), J Soc Brew Japan, 72, 405−409. okadome h, toyoshima h and ohtsubo k (1999a), ‘Multiple measurements of physical properties of individual cooked rice grains with a single apparatus’, Cereal Chem, 76, 855−860. okadome h, kurihara m, kusuda o, toyoshima h, shimotsubo k, matsuda t and ohtsubo k (1999b), ‘Changes in physical properties of cooked rice grains cultivated with different nitrogenous fertilizers’, in Proceedings of the US–Japan Coop Prog in Nat Res (UJNR), 28th Annual Meeting, Tsukuba, Ibaraki, 7–12 Nov., 43−48. parthasarathi n v v and nath n (1953), ‘Cooking and energy requirements’, J Sci Indus Res, 12B, 224−226. perez c m and juliano b o (1979), ‘Indicators of eating quality for non-waxy rices’, Food Chem, 4, 185−195. ranghino f (1966), ‘Evaluation of the resistance of rice to cooking as a function of the time of gelatinization of the grains’ (in Italian), Riso, 15, 117−127. rashmi s and urooj a (2003), ‘Effect of processing on nutritionally important starch fractions in rice varieties’, Int J Food Sci Nutr, 54, 27−36. refai f y and ahmad j a (1958), ‘Development of a rapid method for determining the cooking quality of rice’ (in German), Getreide Mehl, 8, 77−78. refai f y and ahmad j a (1961), ‘The cooking behaviour of long and short-grained rice in



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relation to the amylograph characteristic’ (in German), Getreide Mehl, 11, 21−24. rousset s, pons b and pilandon c (1995), ‘Sensory texture profile, grain physico-chemical characteristics and instrumental measurements of cooked rice’, J Texture Stud, 26, 119−135. sanjiva rao b (1938), ‘Investigation on rice’, Current Sci Bangalore, 6, 446−447. sanjiva rao b (1948), ‘Rice: physico-chemical aspects of its curing to secure vitamin conservation and improvement in storage and cooking quality’, in: Proc 35th Indian Sci Congress Part II: Presidential addresses, 259−270. sanjiva rao b, vasudeva murthy a r and subrahmanya r s (1952), ‘The amylose and the amylopectin contents of rice and their influence on the cooking quality of the cereal’, Proc Indian Acad Sci, 36B, 70−80. sinelli n, benedetti s, bottega g, riva m and buratti s (2006), ‘Evaluation of the optimal cooking time of rice by using FT-NIR spectroscopy and an electronic nose’, J Food Sci, 44, 137−143. sowbhagya c m and ali s z (1991), ‘Effect of pre-soaking on cooking time and texture of raw and parboiled rice’, J Food Sci Technol, 28, 76−80. sreenivasan a (1938), ‘Investigations on rice’, Current Sci, Bangalore, 6, 615−616. sreenivasan a (1939), ‘Studies on quality in rice. IV. Storage changes in rice after harvest’, Indian J Agric Sci, 9, 208−222. suzuki h and murayama n (1967), ‘Effect of temperature on the rice grains and rice starches’, IRC Newsletter (Spl. Issue), 82−92. suzuki k, kubota k, omichi m and hosaka h (1976), ‘Kinetic studies on cooking of rice’, J Food Sci, 41, 1180−1183. suzuki k, aki m, kubota k and hosaka h (1977), ‘Studies on the cooking rate equations of rice’, J Food Sci, 42, 1545−1548. takeuchi s, fukuoka m, gomi y, maeda m and watanabe h (1997a), ‘An application of magnetic resonance imaging to the real time measurement of the change of moisture profile in a rice grain during boiling’, J Food Engg, 33, 181−192. takeuchi s, maeda m, gomi y, fukuoka m and watanabe h (1997b), ‘The change of moisture distribution in a rice grain during boiling as observed by NMR imaging’, J Food Engg, 33, 281−297. tani t, chikubu s and horiuchi h (1969), ‘Physicochemical quality of rice’, J Jap Soc Starch Sci, 17, 139−153. webb b d (1985), ‘Criteria of rice quality in the United States’, in Juliano B O (Ed.), Rice Chemistry and Technology, 2nd edn, St. Paul, MN, American Association of Cereal Chemists, 403−442. williams v r, wu w t, tsai h y and bates h g (1958), ‘Varietal differences in amylose content of rice starch’, J Agric Food Chem, 6, 47−48.



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7 Eating quality of rice

Abstract: The thousands of rice varieties of the world show much variation in their eating quality. Initial research had repeatedly suggested that the amylose starch was the single largest determinant of its texture. Later research has now shown that what was being measured as amylose or its water-insoluble part was in fact largely the long branches of amylopectin starch. Rheological studies suggest that these long branches by their inter- and intramolecular interaction affected the strength of the starch granules. The resilience/fragility of the starch granules caused thus by the relative abundance/scantiness of these amylopectin chains is what determined the texture of cooked rice. The protein content probably played some subsidiary role. Key words: rice texture, cooked-rice hardness, stickiness, rheology of rice, pasting properties, amylose, amylopectin branch profile.

7.1

Introduction

Rice is primarily grown and consumed in south, east and southeast Asia, what we have called the ‘rice country’ of Asia. But despite this concentration, it is the most important staple food of humankind (Maclean et al. 2002). Indeed, rice is very adaptable and is one of the most versatile agricultural crops on Earth. Rice is grown in all continents (except Antarctica) and in more than 100 countries, between 40°S and 53°N latitudes, from sea level to 3000 m in altitude, and from dry land to under 1-2 m of water. Having been thus adapted to such diverse agroclimatic conditions, rice comes in thousands of



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cultivars which differ greatly in their attributes, including cooking, eating and product-making qualities. No other food grain has so much varietal diversity. Rice palatable to one cultural group or suitable for one product is, as often as not, not so for another. A striking example of this variation is the systematic change in rice quality within the heartland of rice in Asia. There is an interesting trend of textural difference in the native rice from the east to the south of Asia (Fig. 7.1). Rice in the north and east of Asia (japonica) is largely soft and sticky after cooking, whereas that in south Asia (indica) is generally dry and fluffy. Rice of southeast Asia comes in between (Juliano and Pascual 1980). But this is only an overview, an example of the gross variation in rice quality. Individual cultivars actually differ much more from each other. This extraordinary diversity in the quality attributes among rice cultivars, particularly in their cooking-sensory attributes (henceforth called eating quality), naturally raises the question: what causes this diversity? What chemistry is behind it? Scientists have been trying to grapple with these questions for nearly a century. It may not be an exaggeration to say that this

Fig. 7.1 Home of world’s rice. More than 90% of world’s rice is grown and consumed in the shaded area. The arrow shows the direction of gradation in the sensory quality of cooked rice from soft and sticky to firm and fluffy.



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Eating quality of rice

195

topic has been one of the most intensively researched fields in the realm of cereal chemistry. The eating quality of rice can be largely defined as the sensory perception of the person who eats the rice. It is mostly made up from: aroma, flavour, tenderness (or hardness), cohesiveness (or stickiness), whiteness and gloss of the cooked rice (Juliano et al. 1965, Del Mundo 1979). Recently other perceptions such as wetness, dryness, stickiness to lip, stickiness to teeth, etc. have been included in sensory analysis at the Southern Regional Research Centre (SRRC) of the US Department of Agriculture (USDA) at New Orleans, LA (Champagne et al. 2009). The discussion in the following will focus chiefly on the texture of cooked rice, i.e., its hardness (firmness, tenderness) and stickiness. Research on the subject of the physicochemical basis of the eating quality of rice can be divided into three phases. These are: the initiation (pre-1950s), the period of data accumulation (1950s to mid-1980s), and exploration at the molecular level (post–mid-1980s).

7.2 The initiation: the water-uptake paradigm The subject was first explored close to a century ago. The work of Warth and Darabsett (1914), who studied the progressive digestion of rice grains by increasing concentrations of dilute alkali, was probably the first attempt to understand the varietal difference in rice quality. However, since rice is cooked by boiling it in water before consumption, water uptake by rice during cooking (g water absorbed per g rice in a definite time, say 20 min) was the issue that was mainly examined during this period. This subject has been discussed in detail in Chapter 6, and hence need not be gone into again here. We need only to note here, for the sake of continuity, that this work was initiated and mostly pursued in India, Sanjiva Rao et al. (1952) finally concluding that ‘good’ rice varieties had a high water uptake and also a high amylose content. As explained earlier, this assertion originated from a fortuitous association. For the popular Indian rice, indica, contained a relatively high amount of amylose starch (see below). It was also generally fine-grained (Bhattacharya et al. 1980), thus having also a relatively high surface area per unit weight and so a high water uptake. However, in the then absence of this understanding, the above fortuitous association between amylose and water uptake looked like a novel relation. It thus almost set up a paradigm of rice quality which lasted for decades. Rice researchers began compulsorily examining the water uptake of their samples. Most studies of rice quality carried out after the 1940s until not too many years back invariably contained a report on its water uptake, no matter what those data revealed. Many reports, especially from breeders, still do. The futility and hazards



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of these studies have been explained in that chapter. Needless to say these studies shed little light on the varietal difference in rice quality, but they had the merit of bringing this issue to the stage of world’s science.

7.3 The period of data accumulation: the amylose paradigm In a remarkable coincidence of history, systematic research on the physicochemical basis of rice quality was initiated at about the same time in several laboratories of the world shortly after World War II. Perhaps the post-war food shortage in Japan, Germany, Italy and some newly independent third-world countries played a part. Perhaps the vigorous resumption of world trade contributed. Perhaps the great exposure of tens of thousands of soldiers to alien lands and alien cultures prepared the ground, and a bizarre event in the USA acted as a trigger (see below). Be that as it may, research on rice quality was initiated in the USA, Europe (Italy, W. Germany, Spain, France), Japan, the Philippines and India in the 1950s and 1960s. The work in the USA and Europe could be said to be micro-studies, limited both in time and range, triggered by some specific circumstances. But the last three studies (in Japan, the Philippines and India) were macro-studies, covering between them the entire cross-section of world’s rice. The Japanese work – apart from an initial involvement with the problem of post-war food shortage – dealt by and large with low-amylose, temperate, japonica rice of east and north-east Asia. The International Rice Research Institute (IRRI) work covered the entire cross-section of world’s rice, yet concentrated on tropical south-east Asian, i.e., intermediate-amylose rice. Finally the Indian work, again wishing to cover the entire cross-section of world’s rice, was forced for logistical reasons to concentrate on the tropical south Asian, i.e., mainly high-amylose, rice. Thus these three strands of research complemented each other and helped form a holistic view. A later flowering of research in Korea (Lee et al. 1984, 1989, Hong et al. 1988, Rho and Ahn 1989, Kim and Oh 1992, Choi and Son 1993, Kang et al. 1994, 1995a, 1995b, 1995c, Jang et al. 1996, Kim et al. 1997) has added to the japonica, and more importantly japonica ¥ indica (= tongil type), strands of this global effort.

7.3.1 American work The USA may be said to be the pioneer in this research field and the way it came about has an interesting history. In keeping with their socio-economic background, rice to American growers and processors is more an object of trade than food. So rice in the USA has been all along carefully bred, selected and categorised into three standardised grain types - long, medium and



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197

short – each with precise dimensional and cooking-processing characteristics (Adair et al.. 1973). The inadvertent release in the early 1950s of Century Patna 231, which had the dimensions of a typical American long-grain rice but a totally atypical cooking-processing characteristic, came as a shocking revelation and a near disaster for the rice industry (Mackill and McKenzie 2003). This event forcefully brought to attention the urgent need to understand the basis of rice quality and to devise simple tests for it. What followed was a remarkably well-organised, well-coordinated, inter-disciplinary, interlaboratory research effort, coordinated by the USDA, which within a span of less than five years clarified the essentials of rice quality that remain substantially valid even today. Beachell and Halick (1956, 1957) recorded its first steps and Beachell and Stansel (1963) its outcome. The effort started off with a parameter called the starch-iodine blue value (SIBV). While studying the quality of parboiled rice and its products, Roberts et al. (1954) had earlier observed that a warm-water extract of its flour gave a blue colour with iodine, the intensity of which gave a good index of the degree of parboiling of a sample. In trying out this test in the newly set up Rice Quality Laboratory at Beaumont, Texas – set up precisely as part of this coordinated project – Halick and Keneaster (1956) now observed that the SIBV could differentiate raw (i.e., nonparboiled) rice varieties as well. The long-grain varieties produced a deeper blue colour than the medium and short varieties (Table 7.1). What is more, the index also easily picked out the atypical varieties, such as Century Patna 231, Toro and Early Prolific. In fact the index correlated very well with the known processing quality of rice. The results in Table 7.2 illustrate the optimism generated by the index. The Human Nutrition Research Division of the USDA at Washington, DC developed techniques for controlled cooking of rice and its sensory evaluation as background reference benchmarks (Batcher et al. 1956). In addition Little et al. (1958), in the same laboratory, following the pioneering work of Warth and Darabsett (1914) nearly half a century earlier in India, developed an alkali digestion test. Grains of different varieties were found to be digested to different extents by a given dilute solution of alkali which thus provided a useful index of varietal difference. This index, too, easily picked out the atypical varieties (Table 7.1). Williams et al. (1958) in Louisiana State University at Baton Rouge, following the preliminary work of Sanjiva Rao et al. (1952) in India, estimated the amylose starch content in rice by its iodine blue colour reaction and found that it strikingly differed among rice types as well as picked out the atypical varieties (Table 7.1). Halick and Kelly (1959) studied the pasting behaviour of rice using the Brabender viscograph (variously called amylograph or visco/amylograph) and reported on the gelatinisation temperature (GT), peak viscosity, breakdown and setback of the samples. They observed distinct differences especially in GT between rice types and also of the atypical varieties within each type



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– 35 88 34 31 26 35 21 75 25 67 77 86 70 65 54 64 48 – 72.0 79.5 70.5 72.5 73.5 72.5 70.5 67.5 73.5 64.5 67.5 75.5 66.5 67.5 65.0 67.5 66.0

– 21.5 12.9 – 23.2 25.0 22.7 23.4 14.0 22.7 15.2 – – 14.0 13.5 14.9 14.3 –

2.9 4.5 2.0 3.9 4.7 4.7 4.5 4.9 6.0 5.2 6.0 6.1 2.4 5.9 6.0 5.7 6.2 6.2

Water uptakea (g/g) 3.32 3.24 3.38 3.08 3.24 3.39 3.29 3.35 3.39 3.22 2.84 3.04 3.02 2.98 3.12 3.23 3.09 2.95

Variety

Bluebonnet Bluebonnet 50 Century Patna 231e Fortuna Impr Bluebonnet Rexoro Sunbonnet Texas Patna Toroe TP 49 Blue Rose Calrose Early Prolifice Magnolia Nato Zenith Caloro Colusa 1600

Grain type

Long

Compiled from different sources as indicated in footnotes a–d. Although each study was different (as shown in footnotes a–d), the majority of the samples were common in all the four studies. Hence the properties and values are comparable. a Batcher et al. (1957). b Little et al. (1958). c Williams et al. (1958). Used with permission. d Halick and Kelly (1959). e Atypical varieties.

Short

Medium

Blue Valued (% transmittance)

Gelatinisation temperatured (°C)

Amylose contentc (%, db)

Alkali spreading scoreb

Some classical USDA data on rice properties

Table 7.1

Eating quality of rice Table 7.2

199

Quality evaluation of US rice varieties

Variety

Blue value (% transmission)

Parboilingcanning test

Cooking soaking testa

Nira Texas Patna Improved Bluebonnet TP 49 Rexoro Bluebonnet Bluebonnet 50 Sunbonnet Rexark Century Patna 231

16 21 24 25 26 29 31 32 59 82

VG G G G G F F F VP VP

VG VG VG VG VG G G G VP VP

Reproduced, with permission, from Beachell and Halick (1957). VG = very good; G = good; F = fair; P = poor; VP = very poor. a Rice is cooked, then soaked overnight in water. Poor varieties show grain bursting, curling and fraying of the edges.

(Table 7.1). Also the amylose content seemed to be positively correlated to the setback. The indices (amylose content, SIBV, alkali value) along with the protein content were also found to be closely related to the parboil-canning stability of rice (Webb 1985). These few indices together proved very successful for monitoring new lines in breeding programmes not only then (Beachell and Stansel 1963) but also all over the world until today. The amylose content, SIBV, alkali digestion and Brabender viscogram became standard tools for research on rice quality in every laboratory around the world. Paradoxically, with this successful conclusion, interest in further basic research in rice quality seemed to temporarily wane for two to three decades in the USA.

7.3.2 European work Italian scientists, following the water-uptake tradition, concentrated on cooking studies (Refai and Ahmed 1958, 1961, De Rege 1963, 1966, Ranghino 1966). The volume expansion of rice when cooked at 80 °C (‘Refai index’) was thought to be a good indicator of Italian rice quality. This can be understood with hindsight, for the popular local Italian rices, being japonica, had low GT and therefore expanded well at 80 °C. The locally unpopular imported indica rices, usually of moderate to high GT, did not. Ranghino (1966), like Desikachar and Subrahmanyan (1961) earlier, determined the ‘gelatinisation time’, i.e., the cooking time of rice, by noting the time the opaque core disappeared during cooking when the grain was pressed between two glass slides. Possible correlation of Refai and Ranghino indices with amylose,



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SIBV and alkali digestion were later examined (Borasio et al. 1964, Baldi and Ranghino 1972, Fossati and Fantone 1982), understandably without much success. Hampel (1958) and Pelshenke and Hampel (1958, 1960) in the Federal Research Center for Cereal and Potato Processing in Detmold, Germany, first developed a ‘rice swelling coefficient’ test which was somewhat akin to the SIBV. They also developed an ‘oryzogram’ test as a measure of the softness of cooked rice to assess the optimum time and water needed for cooking. Most important, Hampel (1961) developed perhaps the first successful instrumental method for assessing the texture of cooked rice using the Haake Consistometer. Even though dependent on the limited number and range of rice varieties of unknown source and age available in the German market, his studies using the above techniques as well as the new American quality criteria re-emphasised the essentials of rice quality testing (Hampel 1965, 1966, 1967, 1968). Spanish scientists at the Instituto de Agroquimica y Tecnologia de Alimentos (IATA) at Valencia directed their attention mainly to the effect of ageing, discussed in Chapter 5. But they also studied the possible role of protein in rice texture, discussed later in this chapter. French scientists at the Institut National de la Recherche Agronomique (INRA), Montpellier, studied mainly the role of amylose in the texture of parboiled rice using the Chopin-INRA Viscoelastograph (Feillet and Alary 1975, Alary et al. 1977).

7.3.3 Japanese work The Japanese have been among the oldest contributors to rice quality research and their work has continued off and on throughout the last over half a century. In fact their studies preceded the US project but initially went largely unnoticed, being limited in rice varietal range and linguistic reach. The Japanese were pioneers in devising new instruments and new techniques. Fukuba (1954) and Fukuba and Yamamoto (1954) used the Brabender viscograph for the first time for study of rice, although this work went largely unnoticed, until Halick and Kelly (1959) revived it. Chikubu et al. (1957, 1958) used an oscillating rheometer to study the dynamic viscoelasticity of a cold rice-starch paste. Later Chikubu et al. (1965a) devised another instrument, the parallel plate plastometer, to study the viscoelasticity of cooked rice grains. Again Kurasawa et al. (1962) devised a table balance system to measure the adhesiveness (stickiness) of cooked rice. The same workers also developed a sensory method to index the palatability of cooked rice, called the gross palatability index (GPI). The later Japanese achievements in devising and using rice texture instruments (the Texturometer, the Tensipresser) are well known. Suzuki and Taketomi (1956) devised alkaliviscogram as an alternative system to study rice pasting pattern. Here rice-flour slurry was pasted not by heating but by increasing concentrations of alkali. © Woodhead Publishing Limited, 2011


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The Cereal Properties Laboratory in the National Food Research Institute (NFRI) in Tsukuba, the main centre of research on rice quality in Japan, has an uninterrupted history of rice chemistry research from the 1950s. In an effort to grapple with the immediate problem of post-war food shortage, the NFRI scientists (Chikubu et al. 1957, 1958, 1960, 1965a, Horiuchi 1967, Horiuchi and Tani 1966) first compared the relative properties of the local rice with those of the domestically disliked imported rices. These included water uptake at 70-80 and 100 °C, solids loss during cooking, volume expansion, blue value and pH of the excess cooking water, alkali digestion, amylose content, SIBV, pasting properties, alkaliviscogram, the bound protein content of rice starch, as well as the cold starch-paste rigidity and cooked-rice viscoelasticity. Chikubu (1967) and Tani et al. (1969a) have summarised these studies. They found a distinct difference in all the properties among rices from different countries, as seen in the clear gradation of the properties from the top to the bottom of Fig. 7.2. There was a clear difference between indica and japonica rices: the relative water uptake at 70-80 and 100 °C of the two were quite the reverse (cf. the American work) and indica rices cooked firm and fluffy, while japonica rices were soft and sticky. Amylose content was correlated to starch-paste rigidity, Brabender cold-paste viscosity (positively) and breakdown (negatively). The starch-bound protein was proportional to the amylose content (r = 0.91***, n = 20), an observation which was confirmed later by other scientists (Suzuki and Juliano 1975, Sano 1984, Villareal and Juliano 1986, 1989) who showed that the residual starch-bound protein was in fact a starch synthase, the waxy gene product. The GT as revealed by Brabender viscograph was closely correlated to the gelatinisation normality of alkali in alkali viscogram. Horiuchi (1967) carefully studied the parameters of Brabender viscogram using his own and past literature data of both rice starch and flour. He showed that the peak viscosity was positively correlated with the breakdown; the higher the peak viscosity, the more the viscosity dropped at the end of heating. The blue value, an index of the amylose content, was highly significantly correlated (negatively) with the ratio of breakdown to consistency (called ‘relative breakdown’ by Bhattacharya and Sowbhagya, 1978, 1979, later) (Fig. 7.3). This important work of Horiuchi unfortunately went largely unnoticed. It partly anticipated the more detailed work of the latter Indian workers. In the interim, Japanese scientists were confronted with a new problem. Rice cropping had been extended after the war to nontraditional areas and nontraditional seasons in the north in an effort to tackle the food shortage, but this was found to result in rice of poor quality. Much effort was devoted to understand this phenomenon (Horiuchi et al. 1965, Chikubu et al. 1965b, Chikubu 1967, Suzuki and Murayama 1967). Subsequently, NFRI scientists made a novel study of the factors related to the palatability of Japanese rice for the Japanese consumer. They grew



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202

Rice quality Thailand Burma (Ngasein) Burma (Meedone) Italy Spain Egypt USA (California) China mainland Taiwan (1st crop) Taiwan (2nd crop) Japan Water uptake ratio

2.0

Expanded volume

3.0 30

Iodine blue value

40 ml 0.1

Total solid

0.2

0.3

0.4 0.3 0.4 0.5 0.6

pH

5

6

7

Thailand Burma (Ngasein) Burma (Meedone) Italy China mainland Japan Spain Egypt Taiwan Glutinous Protein content Gelatinisation KOH Amylose content Max. viscosity

0

0.4%

0.1

0.3 N

0

10

20 300

Rigidity 100

30 500

700

900 BU

1000 dyn/cm2

Fig. 7.2 Cooking qualities of milled rice (top) and some starch properties (bottom) of various rices imported into Japan. Reprinted, with permission, from Tani et al. (1969a).

several varieties of rice in several locations in Japan. All these large number of samples were tested for their palatability by experienced sensory panels. Simultaneously many kinds of physicochemical properties of these samples were determined in the laboratory. The multiple correlation between the two sets of data was examined. This study was carried out several times over



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Eating quality of rice 900

203

0.40

0.35 Blue valve

Peak viscosity (BU)

700

500

0.30 300

100

0

200 400 Breakdown (BU)

0.25 0

0.5 1.0 Breakdown/consistency

Fig. 7.3 Relation of peak viscosity to breakdown (a) and of starch-iodine blue value (absorbance) to ‘relative breakdown’ (b) in rice flour viscograms. Solid and dotted lines in the left figure show regression line and 95% confidence ranges, respectively. Reprinted, with permission, from Horiuchi (1967).

a period of some two decades (Chikubu 1967, Tani et al. 1969a, 1969b, Endo et al. 1976, Chikubu et al. 1983, 1985). In the last of these studies, the authors concluded that the palatability of rice for the Japanese could be estimated by measuring five physicochemical properties: protein content, maximum viscosity, minimum viscosity, breakdown and SIBV of residual cooking liquid. Yanase and Ohtsubo (1985) mentioned that the degree of milling and the proportion of broken and cracked kernels also affected the palatability index. H. Kurasawa’s laboratory at the Niigata University was another active centre of research during the 1960s and early 1970s. The results, broadly similar to those in NFRI, were presented in a series of papers, summarised in Kurasawa et al. (1969, 1972, 1973). They evaluated their results, as mentioned before, against the table balance stickiness score and the sensory GPI score. Japanese workers also devoted considerable attention to the instrumental measurement of palatability on the basis of texturometer readings of hardness (positive peak, H) and stickiness (negative peak, –H). For example Okabe (1979) used the General Foods Texturometer to construct a texturogram (Fig. 7.4), wherein zones of various levels of acceptability could be designated on the basis of the values of H and the ratio of – H/H. Okadome et al. (1999a, 1999b) later used the Tensipresser to measure the texture of cooked rice.



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204

Rice quality 0.05

5.0

0.1

E5

D5

0.13 C5

0.15

B5

A5

4.6 E4

D4

C4

B4

0.2

A4

4.2 E3

4.0 3.8

D3

E2

B3

C3

D2

C2

A3

0.22 AA3

A2

B2

AA2

3.4 D1

Hardness, (H)

E1

C1

B1

AA1

A1

3.0

0.3

2.0

1.0

0.1

0.2

0.3

0.4 0.5 0.6 Stickiness (–H)

0.7

0.8

0.9

Fig. 7.4 Texturogram for cooked rice showing zones of acceptability as a function of hardness and stickiness. A – excellent; B – good; C – slight poor, but acceptable; D – poor; E – unacceptable. Reprinted, with permission, from Okabe (1979) John Wiley and Sons.

7.3.4 IRRI work The IRRI was established at Los Baños, Philippines, in 1961. The Chemistry Department of IRRI under the leadership of Bienvenido O. Juliano took up an intensive study of the subject of rice quality from the very beginning. One can say this school took over from early 1960s where the USDA effort had left it in the late 1950s. It soon became the strongest centre of rice quality research in the world. As already mentioned, the IRRI work for the first time looked into the rice of tropical Asia, the rice bowl of the world. Juliano et al. (1964a) found in 16 varieties that the SIBV value was a good reflection of the amylose content and that the latter correlated well with the amylograph setback. The alkali digestion value was an excellent inverse estimate of the Brabender GT. In a parallel work, Juliano et al. (1964b) studied 55 varieties of south-east Asian rice. They concluded that the



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205

amylose was the most important determinant of rice quality, for its content correlated very well with the preferred varieties in the different southeast Asian countries. The authors confirmed that the amylose content was unrelated to the Brabender peak viscosity but that it was well correlated both with the breakdown (negatively) and the setback (positively). The setback became positive after 23% amylose. The protein content was not related to anything except that it seemed to be negatively correlated with the peak viscosity. The relative contribution of amylose, protein and the GT to sensory characteristics of cooked rice was studied next (Juliano et al. 1965) in a carefully designed experiment, using the method of Oñate and Del Mundo (1963) for sensory scoring. Several carefully selected pairs of varieties differing in one property each was used. Amylose was found to correlate excellently with tenderness, stickiness, colour and gloss (all negative) and aroma (positive) of cooked rice (Table 7.3). Protein was not a significant contributor except that it played some role in colour and flavour and apparently also in other properties when interaction with amylose was minimised (partial correlation). The GT had no effect, its reported influence in many works being due to its general association with amylose. These correlations were confirmed later in another similar study (Juliano et al. 1972), except that protein seemed to have still less effect in this work. Later, using the Instron for an instrumental measure of texture (hardness, stickiness), Perez and Juliano (1979) confirmed the key role of amylose. The same result was obtained later in an international cooperative study with 10 varieties using various instrumental texture methods (Juliano et al. 1981). So it appeared that the sensory quality of cooked rice was determined mainly, if not entirely, by its amylose starch. However, conflicting results came from IRRI as well as other laboratories. For instance, IRRI scientists repeatedly found that waxy rice often showed substantial varietal difference in texture after cooking or in product quality (Kongseree 1979, Perez et al. 1979, Merca and Juliano 1981, Juliano and Villareal 1987). These differences Table 7.3 Correlation between eating-quality scores and composition of 23 samples of milled nonwaxy rice Quality criterion

Amylose (r)

Protein (r)

Gelatinisation temperature (r)

Aroma Flavour Tenderness Cohesiveness Colour Gloss

+0.608** +0.104 –0.610** –0.640** –0.712** –0.581**

–0.324 –0.718** –0.382 –0.310 +0.704** –0.343

–0.166 –0.037 –0.070 +0.106 +0.037 +0.021

Reproduced, with permission, from Juliano et al. (1965). © Institute of Food Technologists.



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could not obviously be explained on the basis of amylose content. Differences were found in nonwaxy rices of similar amylose as well. It thus appeared that amylose, though undoubtedly important, was not sufficient; additional factors were involved. A stream of research papers came out from the IRRI lab during the mid 1960s to the mid 1980s) in an effort to identify these elusive ‘other factors’. This effort included a series of studies on isolated rice starches (Reyes et al. 1965, Lugay and Juliano 1965, Vidal and Juliano 1967, Juliano et al. 1969, Antonio and Juliano 1974, Juliano and Perdon 1975), which provided a wealth of information about the molecular weight, mean chain length, intrinsic viscosity, iodine binding capacity, the degree of branching, etc of waxy and nonwaxy rice starches. Cagampang et al. (1973) meanwhile devised a gel consistency (GC) test, wherein a 4.4% alkaline rice-flour gel was allowed to flow in a horizontal tube and the length of its flow was noted. This test, the authors felt, supplemented the amylose test very well. Rice varieties having a hard GC (less flow) generally cooked hard and fluffy and vice versa. The test seemed especially useful for high-amylose varieties. The researchers also felt that it was an index of certain properties of the amylopectin and hence was a natural complement to the amylose test (Perez 1979). Nevertheless the extraordinary diversity in tropical rice created many deviations. Data in Table 7.4 are just a sample example of how the parameters and indices overlapped. After a series of studies (Perez and Juliano 1979, Perez et al. 1979, Juliano and Pascual 1980, Juliano et al. 1981, Merca and Juliano 1981, Juliano and Perez 1983, Juliano and Villareal 1987) the general conclusion was that one or more of the three indices, viz. the GC,

Table 7.4 Hardness and stickiness of cooked high-amylose milled rices differing in gel consistency Property

Gel consistency

Number Cooked rice hardness (kg) Range Mean Cooked rice stickiness (g cm) Range Mean Alkali spreading value Range Mean

Hard (27–40 mm)

Medium (41–60 mm)

Soft (61–100 mm)

8

6

6

7.2–9.2 8.2

6.8–7.9 7.5

6.5–7.6 6.9

28–50 34

39–54 46

44–60 53

5.0–7.0 6.8

3.5–7.0 4.6

3.0–5.0 4.2

Reproduced, with permission, from Perez (1979).



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207

the GT and the Brabender setback and consistency, together with amylose content, could account for the properties of rice. Nonetheless the results were not consistent. One parameter fitted the rice behaviour in one case, another parameter fitted another variety in another.

7.3.5 Central Food Technology Research Institute (CFTRI) work Work in India at the Central Food Technological Research Institute (CFTRI) at Mysore also has an interesting history. The present author and his colleagues at CFTRI initially studied the mandatory water uptake of rice as per the prevailing paradigm (Chapter 6). It so happened that the semidwarf, highyielding rice development programme started in full swing in India at that time (second half of the 1060s), and the rice lab at CFTRI was requested by the All India Coordinated Rice Improvement Project (AICRIP) of the Indian Council of Agricultural Research (ICAR) to evaluate the promising new lines. Bhattacharya et al. (1972) noted with surprise that the new lines being tested, as well as most of the traditional Indian varieties studied, all had more or less identical high amylose contents. Of course later work (Bhattacharya and Sowbhagya 1980) showed that there was nothing surprising about it, for rice of India (and of south Asia in general, the other major tropical homeland of rice), including the high-yielding lines being developed in India, then or later, was predominantly high-amylose rice (over 26% amylose on dry basis). Yet paradoxically the authors noted with surprise that the SIBV, which should be proportional to amylose, differed sharply among the samples. Juliano et al. (1968) too had just observed that SIBV, while in general indeed proportional to amylose, fell off sharply in some varieties with very high amylose content tested (Fig. 7.5). They concluded that when the amylose content increased beyond a point, the SIBV decreased due to in situ retrogradation. But Bhattacharya et al. (1972) found sharp differences in SIBV precisely in high-amylose rice. So it did not appear as if the SIBV decreased when the amylose content increased; it appeared there was a varietal difference in SIBV. The CFTRI workers drew a key conclusion at this point. They concluded that the SIBV was nothing but an index of hot-water soluble amylose. Hence the difference in it signified a difference in the hot-water solubility of amylose among the varieties. Calculations then showed that the original Taiwanese high-amylose, low-GT dwarfing-gene donors (Dee-Geo-Neo-Gen, Taichung Native l) and most of their initial progenies (IR 8, IR 22, Jaya, as well as a majority of the new lines being then tested in India) all had an amylose solubility of roughly 35-40%. About a third of the traditional Indian rices gave an amylose solubility value of about 45-50%; and the bulk of the remaining traditional Indian rices gave a solubility of about 55-60% (Bhattacharya et



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Starch iodine blue absorbance (590 nm)

0.6

0.5

0.4

0.3

0.2

0.1

0

Fig. 7.5

0

10 20 Milled rice amylose, % dry weight

30

Starch-iodine blue colour of rice samples having different amylose contents. Reprinted, with permission, from Juliano et al. (1968).

al. 1972). Later studies showed that intermediate- and low-amylose rices tested by the CFTRI workers (the number of these varieties available to them was small) also by and large gave an amylose solubility of 55-60%. In other words, this striking difference in amylose solubility existed mainly in high-amylose rice. In this sense the fact that the CFTRI school had a good access to high-amylose rice may have been of some advantage. Researchers elsewhere rarely encountered high-amylose rice and so missed many of their peculiar properties, including the above variable amylose solubility and its consequence. It was then but a small step for this group to be led to the observation that the hot-water-insoluble amylose (‘insoluble amylose’ for short, i.e., the difference between the total and the soluble amylose) correlated excellently with the pasting and textural properties of rice in general and of high-amylose rice in particular (Table 7.5) (Manohar Kumar et al. 1976, Bhattacharya et al. 1978, 1982, Deshpande and Bhattacharya 1982, Sowbhagya et al. 1987). It is interesting to note that Kongseree and Juliano (1972), Juliano and Perdon (1975) and Maniñgat and Juliano (1978) also later observed that varieties showing low solubility of amylose generally showed very high setback and low breakdown, hard gel consistency and faster retrogradation. However, these authors did not pursue this aspect further. One possibility is that solubility of amylose has not generally been found to vary significantly, so far as tested, among rices other than high-amylose class. So laboratories outside south



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Eating quality of rice Table 7.5

209

Role of insoluble amylose in texture of cooked rice

Amylose class

High Intermediate Low

No. of varieties

Total amylose Insoluble (% db) amylose (% db)

5 11 8 4 4

28.7 28.7 28.8 23.8 20.6

18.4 13.0 11.9 9.0 8.2

Cooked rice Stickiness Hardness score (105 cP)a 1.6 2.5 3.2 4.6 6.4

151 88 48 19 12

Reproduced, with permission, from Bhattacharya et al. (1978) John Wiley and Sons. a As measured by Haake Consistometer.

Asia have not had the occasion to take much interest in it. However it may be significant that Kang et al. (1994, 1995b, 1995c) recently observed low solubility of amylose (i.e., higher insoluble amylose) even in low-amylose rice, and it correlated with relatively hard and fluffy texture after cooking. Whether amylose solubility does differ in some varieties of other classes of rice needs to be tested in a large number of samples. The CFTRI group also devised a new system of viscography which gave an excellent correlation with rice texture. This was based on the finding, as attested to by the work of Mazurs et al. (1957) and Horiuchi (1967) earlier, that the conventional viscogram parameters of breakdown and setback varied with the paste viscosity, which varied randomly among the varieties, thus preventing a true comparison. The proper way to compare the properties was therefore to run the test at a fixed peak viscosity rather than at a fixed concentration, when the varietal differences came out in bold relief (Bhattacharya and Sowbhagya 1978, 1979). The excellent differentiation obtained among rice classes based on a new parameter called ‘relative breakdown’ (BDr = breakdown/total setback) by this technique is shown in Fig. 7.6. The rationale of this technique will be discussed in detail in Chapter 13. Bhattacharya and Sowbhagya (1972, 1980) also re-examined the alkali digestion test, and revealed another interesting rice property. They noted that the rice varieties differed not only in the extent of digestion in alkali (the ‘alkali digestion score’, an inverse estimate of the GT) but also in the type or pattern of digestion. Five reaction types were encountered which correlated well with the amylose-based quality type classification of rice (Table 7.6). Priestly (1976) found that this alkali reaction type correlated well with the apparent solubility of cooked rice when the latter was dispersed by sonication and extracted with water. This fact was confirmed by Maniñgat and Juliano (1978). Thus this alkali reaction type test provided a kind of rapid spot test of rice quality. Unfortunately in an international cooperative



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210

Rice quality 200 VI

IV V

100 VIII

VII

Relative breakdown, %

60 40

20

III

V VI

VII 10

III

II I

6 4

2 200

IV

400

600 1000 Peak viscosity, BU

2000

Fig. 7.6 ‘Relative breakdown’ (breakdown/total setback) of rice of eight quality types (I -VIII: see Table 7.7) at various peak viscosities. Reprinted, with permission, from Bhattacharya and Sowbhagya (1979) John Wiley and Sons. © Institute of Food Technologists.

test of the alkali test (Juliano et al. 1982), some of the cooperators did not feel confident about this type-classification, after which this test was not widely used. Another fact investigated was the blue value of the excess cooking water (CW-BV), which many (especially in Japan) started using, having been encouraged by the success of the SIBV test. Bhattacharya et al. (1972) showed that this value gave no useful information in most cases. The reason was that the CW-BV varied, first, with the amylose content; second, with the latter’s solubility (which varied among rice types); and third, with the water uptake (which varied with the kernel size and shape). However, this negative assessment probably does not apply to Japanese rice. Japanese rice has broadly similar sizes and shapes and, being of low-amylose type, generally has similar amylose solubility. Hence its CW-BV may be a fairly good and simple index of its amylose. This may be the reason why Japanese scientists have repeatedly found the CW-BV as a useful index. The CFTRI group concluded that the three criteria, viz. the amylose content, the insoluble amylose and the viscograph ‘relative breakdown’ gave a clear and unambiguous picture of cooked-rice texture, whether determined sensorily or instrumentally (Sowbhagya et al. 1987).



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Eating quality of rice Table 7.6 of rice

Interrelation between alkali reaction type and other quality characteristics

Rice quality typea

I II, III IV, V, VI VII VIII

211

Amylose (% db)

Alkali reaction type

Total

Insoluble

>26 >26 22–26 15–22 15 ≤15 – – –

B, Mixed B A, B1 Mixed C C D

Adapted, with permission, from Bhattacharya and Sowbhagya (1972, 1980) John Wiley and Sons. a See Table 7.7.

After a study of 177 samples representing a cross-section of world’s rice, the CFTRI school classified rice into eight distinct quality types based mainly on these three criteria (Table 7.7). A few features of this classification are worth mentioning. The first three types were all high-amylose rice; but they sharply differed in all their behaviours including cooked-rice texture, and this was precisely correlated to differences in their insoluble amylose (see also Table 7.5). The intermediate-amylose rices too were classified into three groups, although they had similar total and insoluble amylose. But these rices differed sharply in their viscogram criteria (Fig. 7.6) and of course in sensory and instrumental texture. The case of aromatic rices (type IV) is especially instructive. Even though collected from different parts of India, which is a huge country, the 25 or so scented rices of the country tested all showed virtually identical properties with a distinct BDr pattern. Hence these scented rices were unhesitatingly assigned a separate type despite having similar total and insoluble amylose as in types V and VI. It is significant that Glaszmann (1987), in his later classification of rice based on isozyme polymorphism, found the traditional aromatic rices of south and west Asia different from usual indica rice (his Group I) and hence assigned them a distinct class (his Group V). Similarly the bulu rices (CFTRI’s type VI) had a pasting breakdown curve quite similar to that of low-amylose japonica rice (type VII) (Fig. 7.6). According to Glaszmann, again, bulu rice (tropical japonica) belonged to the same group as temperate japonica (Group VI). The CFTRI school also confirmed the general usefulness of the GC test, but they found that it often gave rather inconsistent response. As a result, overall the insoluble amylose and the paste BDr gave a far better correlation with texture than the GC (Sowbhagya et al. 1987).

7.3.6 Conclusions Summarising, a very intensive amount of work was thus conducted in various laboratories of the world to explain the physicochemical determinants of



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Table 7.7

Quality classification of rice

Rice quality

Amylose (% db)

BDrb (%) Cooked rice

Type

Designationa

Total

Insoluble

I

HA: Hard

> 26

> 15

0–5

II III IV V VI VII VIII

HA: Intermediate HA: Soft IA: Aromatic IA: Normal IA: Bulu LA WX

> 26 > 26 22–26 22–26 22–26 15–22 140 °C), cooking resistance would be extreme. Derycke et al. (2005a) re-examined the matter with two varieties of brown rice (12% and 24% amylose) after steaming the soaked brown rice at 112 and 121 °C. They obtained essentially identical results as above, except that they also employed a new technique, viz., temperature-resolved wide-angle X-ray scattering (TR-WAXS). This system was used to study the thermal-cum-X-ray properties of the respective flours. Three different moisture contents (66%, 40% and 25%) were used. TR-WAXS showed a clear A-type diffraction pattern melting at definite temperatures. The crystallinity index (CI) at 66% and 40% moistures started decreasing at approximately 65–70 °C and disappeared at roughly 90 and 105 °C, respectively. At 25% moisture content, the CI decreased very gradually and disappeared only at 140 °C or so. These combined thermal and X-ray data clearly confirmed the earlier-mentioned principle that the thermal property of starch was heavily dependent on the moisture content. DSC thermograms similarly showed endotherms M1 at 65% moisture content and M1 and M2 at 40% moisture. No endotherm was shown by the 25% moisture sample. Again L-Am II endotherm was not shown by the 66% moisture sample at all; but it was shown at 40% and 25% moisture at ~ 100–130 °C. By static X-ray the parboiled samples showed weak A + B + V patterns. So all these data confirmed the earlier conclusions. Manful et al. (2008) again examined the effect of certain soaking and



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Rice quality

steaming conditions on the X-ray and DSC patterns of the product. The results were as expected. The A X-ray pattern gradually decreased but V pattern was hardly discernible. The same group (Himmelsbach et al. 2008) later studied product properties using Rapid Visco Analyses (RVA), DSC, Fourier transform (FT)-Raman spectroscopy and solid-state 13C charge polarised magic angle spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy. Unfortunately the soaking conditions were such that the samples would have been very incompletely parboiled. Besides, the information that the heavy-artillery techniques provided were not very different from what could have been obtained by simple techniques like EMC-S, water uptake and alkali test. Thermal breakdown of starch Many reports in the past suggested that there were some quantitative changes in starch, but these remained unconfirmed (see Bhattacharya and Ali 1985 for early references). The total amylose content of starch was verified to remain unchanged after processing (Raghavendra Rao and Juliano 1970, Ali and Bhattacharya 1972a). However one transformation of starch, in addition to gelatinisation and reassociation, has been confirmed, viz. thermal breakdown. Upon gel permeation chromatographic (GPC) fractionation, starch of parboiled rice showed a decrease in the proportion of high-molecular weight (MW) major fraction I and an increase in that of the low-MW, minor fraction II (Mahanta and Bhattacharya 1989). The difference increased with increasing time and pressure of steaming (Table 8.3). In accord with this quantitative change, the lmax of the iodine complex also changed simultaneously. However, Table 8.3

Fractionation of parboiled rice flour on sepharose Cl-2B column Parboiling condition

Paddy moisture (% wb)

Raw rice 30 22 17 12

Carbohydrate in GPCa Fr I (% of total)

Steam Pressure (kg/ cm2, gauge)

Time (min)

0 1 3 1 3 1 3 3

10 10 20 10 20 10 20 20

69.5 68.5 53.8 33.0 47.4 16.1 55.8 20.9 38.4

lmax of iodine complex (nm) Fr I

Fr II

571 571 569 581 577 588 564 581 587

634 638 647 622 644 620 649 621 621

Reproduced, with permission, from Mahanta and Bhattacharya (1989) John Wiley and Sons. a Gel permeation chromatography. Fr = fraction.



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Effect of parboiling on rice quality

269

very little is known about what effect this starch degradation has on the properties of the resultant product at this time. One possible effect on eating quality of the rice is discussed later.

8.2.4 Changes in the disposition of fat Fat is an interesting constituent of rice. The total amount of fat in brown rice is about 3%. But the location of the fat in the grain (concentrated close to the surface) and the method of milling of the cereal as a wholegrain are such that a large proportion of the fat comes into the bran when the rice is milled. As a result, although relatively poor in fat, the rice grain (strictly speaking its milling by-product, rice bran) becomes a reasonably good source of oil. Rice is thus not only a staple cereal but also in a figurative sense a kind of oil seed. Subrahmanyan et al. (1938) in their classical work on rice composition observed that milled parboiled rice contained less fat than milled raw rice after equivalent degrees of milling. From this they, with remarkable prescience, postulated that as the water solubles moved into the grain during parboiling, the oil moved out. Regardless of the validity of the former hypothesis (water solubles moving in), the latter hypothesis has been confirmed by many workers (Bhattacharya et al. 1972, Padua and Juliano 1974, Benedito de Barber et al. 1977, Sondi et al. 1980). Bhattacharya et al. (1972) estimated the fat content both on the grain surface and also in the total grain during progressive milling of both raw and brown rice. The results (Fig. 8.12) showed that the fat, obviously lying slightly under the surface of brown rice, quickly appeared on the grain surface as the brown rice was scratched (i.e., milled). The amount of fat on the grain surface thus increased during the milling up to about 4% degree of milling and then progressively decreased with further milling. The peak layer, as seen in Fig. 8.12, lay slightly to the outer side in parboiled rice compared with that in raw rice. The same result was obtained by Sondi et al. (1980) when they analysed the oil content in successive layers of bran during milling (Fig. 8.13). The results clearly showed (a) that the oil content of parboiled bran was more than that of raw bran and (b) the peak layer moved a little towards the grain surface in parboiled rice compared with raw rice. The reasons of the above changes could be inferred from the earlier work of Mahadevappa and Desikachar (1968). They observed that most of the oil in raw rice was present as distinct globules just below the grain surface in the aleurone layer. These globules got disrupted and converted into a band upon parboiling (Fig. 8.14). We can assume that the outer movement of oil (Figs 8.12 and 8.13) after parboiling was related to this phenomenon. The oil released by the disruption due to parboiling apparently moved into the soft bran tissue but could not penetrate the harder endosperm.



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Rice quality

2.5

2.0

Fat, %

t–PB 1.5

t–R

s–PB 1.0 s–R

0.5

2


8

10

Fig. 8.12 Changes in the amount of fat in the total grain (t) and on the grain surface (s) in raw (R) and parboiled (PB) rice with progressive milling with a McGill miller. Reproduced, with permission, from Bhattacharya et al. (1972). © Society of Chemical Industry. Permission is granted by John Wiley & Sons on behalf of the SCI.

Many researchers claimed that the outward movement of oil could be modulated by adjusting the soaking and steaming conditions (Chakravarty and Ghose 1966, Vasan et al. 1971, Padua and Juliano 1974). However, Sondi et al. (1980) could not confirm this contention. They found that the grain oil content or its fraction in the bran remained virtually constant irrespective of the processing condition as seen in the data of Fig. 8.13. However, varietal differences in the extent of oil have been reported (Mukherjee and Bhattacharjee 1978), suggesting differences either in content of oil or its location in the grain cross-section. These changes in fat have some profound effects not only on the oil content of bran but on certain physical properties of the rice as well (Fig. 8.2). These aspects will be discussed later when discussing the properties of the resultant parboiled rice.

8.2.5 Changes in protein The protein bodies in the rice grain have been reported to be ruptured during the steaming process. It has been well documented that the solubility of rice protein was reduced after parboiling and the extent of reduction was proportional to the severity of the process (Raghavendra Rao and Juliano



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Parboiled

35

20 Parboiled

25

10

Oil in successive layers, %

30 Raw

Oil in bran, %

271

Raw

15

2


8

Fig. 8.13 Oil content of raw and parboiled rice bran after various degrees of milling (DM). Lower curves, oil content in composite bran of indicated DM; upper curves, calculated mean oil content in successive bran layers. Different symbols in the parboiled-rice curve represent different conditions of soaking (room temperature to 80 °C) and steaming (0.0−1.4 kg/cm2 gauge, 10−60 min). Milling was with the McGill miller. Adapted, with permission, from Sondi et al. (1980); © Elsevier Science, used by permission.

Fig. 8.14 Transverse sections of raw (left) and parboiled (right) brown rice, showing distinct bodies containing oil in the aleurone layers of the former and their disruption in the latter. Reproduced, with permission, from Mahadevappa and Desikachar (1968).

1970, Dimopoulos and Muller 1972). The data of the latter authors are shown in Table 8.4. However, little is known about how this change affected the overall rice quality. As far as is known, the total protein of the brown or milled rice or its amino acid composition did not seem to be affected by parboiling (see Bhattacharya and Ali 1985). On the other hand there seems



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Table 8.4

Effect of processing conditions on solubility of proteins in parboiled rice

Soaking timea (h) Untreated 3.50 5.58 7.00

Steaming pressureb (kg/cm2)

Total protein extracted by detergent solutionc

0.35 0.70 1.05 0.70 0.70

73.4 31.1 25.9 20.1 24.5 22.2

Reproduced, with permission, from Dimopoulos and Muller (1972). a At 64° C. b Steamed for 5 min at gauge pressure indicated. c Sodium alkyl benzene sulphonate, 3% solution.

to be some effect on the nutritional value of protein as a result of parboiling. This aspect will be discussed later. Derycke et al. (2005b) presented evidence to suggest that protein plays a substantial role in the cooking and eating qualities of rice (see Chapter 7). The disulphide bonds in the protein were proposed as the source of the effect. In that connection they suggested that this effect was true in parboiled rice as well. They showed that the hydrolysis of the protein or reduction of its disulphide bonds led to changed properties, including increase in RVA viscosity and reduction in cooked-rice hardness in both raw and parboiled rice. However, whether the suggested effect, if any, was merely carried over from raw to parboiled rice or was an additional effect introduced during parboiling was not clear.

8.2.6 Other changes As shown in Fig. 8.2, parboiling, especially the steaming (or other ways of heating) step had other effects on the grain constituents. One aspect was the effect of heat per se. The heating conditions were such that most of the enzymes in the grain were largely inactivated during parboiling (Barber et al. 1983). It also tended to destroy the antioxidants. The resulting retardation in production of free fatty acids (FFA) on the one hand, and promotion of oxidative rancidity on the other obviously affected the quality of both the milled rice and bran. Similarly the heating step also brought about the Maillard reaction and the consequent grain discoloration. In addition changes occured in the distribution or disposition of certain small molecules such as vitamins, sugars, amino acids and minerals. One should note that these latter were not real changes in the molecules but only in their location or disposition. The effects of all these changes will be described later when discussing the properties of parboiled rice.



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8.3

273

Properties of parboiled rice

The properties of parboiled rice, which are a consequence of the various changes in grain constituents elaborated above (Fig. 8.2), are now considered. The precise link of these properties with the respective constituent changes will also be pointed out.

8.3.1

Grain characteristics

Size, shape and appearance It has been observed that the size and shape of the milled parboiled rice grain were slightly different from those of the raw milled rice. The former was a little shorter and a little fatter than the latter (see Bhattacharya and Ali 1985). The exact cause of this change is not known but should be related to some sort of realignment of the plastic kernel substance within the husk at the moment when it was cooked during parboiling. Sowbhagya et al. (1993) observed that the above change was true of only conventional steam-parboiled rice. On the other hand, dry-heat- and pressure-parboiled rice grains were somewhat longer and thinner. In addition, rice had natural ridges on the face of brown rice. These ridges apparently arose during grain development from the pressure exerted by the lemma and the palea. These ridges were seen to get largely evened out in the conventional process but got accentuated in the other two. This accentuation of the ridges apparently arose from the quick drying and consequent freezing of the grain surface topography during these processes. This sequence of events is proved by the somewhat amusing observation that if the dry-heat parboiled rice was moistened and held for a while, the grain rearranged itself and the pronounced ridges disappeared! Other differences in the appearance between raw and parboiled rice grains are well known. Raw rice grains were, relatively speaking, somewhat opaque and white. Parboiled rice grains on the other hand, again relatively speaking, somewhat glassy and translucent. Any chalky area pre-existing in the raw rice grains also completely disappeared after parboiling. One can assume that the gelatinised starch granules, as well as the disrupted protein bodies, probably merged and adhered to each other, the resulting compactness reducing light scattering at the granule boundaries (Fig. 8.15). Another difference was grain hardness, the parboiled rice grains being harder than raw rice grains (Raghavendra Rao and Juliano 1970, Kimura et al. 1976, Pillaiyar and Mohandoss 1981). This hardness too was apparently a result of the above-mentioned compactness. Grain colour The next important difference is grain colour. Milled raw rice grains are



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Rice quality

B

A 10 mm

Fig. 8.15 Photomicrograph of an incompletely parboiled rice endosperm in phasecontrast illumination, showing a clear difference between the gelatinised outer layer (A) and the ungelatinised inner portion (i.e., ‘white belly’ area) (B). Reprinted, with permission, from Raghavendra Rao and Juliano (1970). Copyright 1970 American Chemical Society.

relatively white. In contrast parboiled rice grains have a shade of amber, which could be very pale to dark depending on the processing conditions. The fact that the discoloration was inhibited by bisulphite (Charlton 1923, Houston et al. 1956, Mecham et al. 1961, Jayanarayanan 1964) showed that the discoloration was caused by nonenzymatic browning of the Maillard reaction. The effect followed from the enhanced level of reducing sugars and amino acids in the soaked grain coupled with the heat treatment during steaming (Fig. 8.2). The intensity of the discoloration was related to the degree of total heat treatment during soaking and steaming (see Bhattacharya and Ali 1985 and Bhattacharya 2004 for references). Both the time and temperature of soaking and the time and pressure of steaming influenced the effect (Fig. 8.16 and Table 8.5). The enhanced discoloration effect of soaking at high temperatures (70, 80 °C) was either due to absorption of some husk colour from the soak water once the husk had opened or due to the increase in the cumulative heat treatment or both. Similarly, as can be seen from Fig. 8.8, and also from Table 8.5, high pressure and prolonged time of steaming produced the highest discoloration. A combination of low grain moisture and high steam pressure (equivalent to so-called pressure-parboiled rice) probably produced the worst sample in terms of discoloration. The pH of the soaking water was also reported to have some effect (Jayanarayanan 1964). This last aspect needs further clarification. For one thing, in the above work the colour was measured in brown rice and the results might or might not apply



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275

(a)

Colour difference, DE

80° 16 70° 12

60° 50° RT

8

4 40

80 120 160 200 Time of soaking, % of optimum

240

(b)

Colour difference, DE

25

20

15

10

60 48

303 .9 (134 )

202 . (12 6 Pre 1) s s (Tem ur per e, kPa atu re, °C)

101 .3 (100 )

12

36 n 24 e, mi m i T

0

Fig. 8.16 (a) Effect of temperature and time of soaking of paddy on colour of resultant milled parboiled rice (open-steamed for 10 min in each case). RT = room temperature. Reprinted, with permission, from Bhattacharya and Subba Rao (1966b). Copyright 1966 American Chemical Society. (b) Response surface of colour difference of parboiled rice against different pressures and times of steaming. Reprinted from S. Bhattacharya (1996) with permission from Elsevier.

to milled rice. Secondly, the bran pigment was an acid–base indicator and turned rather strongly yellow-brown in alkaline pH and a pale yellow in acid pH (Bhattacharya and Subba Rao 1966b). So whether the observed effect of pH on colour was due to colour production or to colour change was a moot



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Table 8.5

Effect of pressure and time of steaming on colour of milled parboiled rice

Steaminga

Colour difference (DE)

Pressure (kg/cm2, gauge)

Time (min)

0

2 5 10 20 40 60 10 10 10

0.35 0.70 1.05

8.8 9.2 10.0 12.1 11.4 12.1 12.8 11.9 16.6

Reproduced, with permission, from Bhattacharya and Subba Rao (1966b); © American Chemical Society. a Paddy soaked at 70 °C for 2.5 h, then steamed at indicated pressure.

question. At the same time some starch hydrolysis might occur in acid pH which might intensify the Maillard reaction and the pH might itself have an effect on Maillard reaction (Ajandouz and Puigserver 1999). The characteristic of the colour was studied by Bhattacharya and Subba Rao (1966b). They were surprised to note that the ‘dominant wave length’ of the colour remained practically unchanged at ~ 578 nm (yellow-brown) in both raw and parboiled rice. However, there was a progressive increase in the ‘excitation purity’ and decrease in ‘luminance’ (Y) with increasing heat treatment during soaking and steaming. What this means is that the colour undoubtedly appears deeper and darker to the eye with progressive discoloration; yet the ‘hue’ remains largely unchanged. Ali and Bhattacharya (1982) also noticed that even the pronounced discoloration perceived by the eye of pressure-parboiled rice mainly resulted from its darkening (lowering of Y) with little or no change of the hue. In her study, S. Bhattacharya (1996) (Fig. 8.16(b)) observed that the Hunter L value (lightness = luminance, Y) mainly decreased, more with time than with pressure of steaming, but the Hunter b (yellowness), chroma, was affected by both. All the changes followed zero order kinetics. Lamberts et al. (2006, 2008) recently examined the discoloration during parboiling of brown rice. Colour as usual increased with severity of parboiling. But redness increased more than yellowness, red rice became black. Levels of sugars changed. Sucrose decreased partly by leaching into soak water and partly by enzymic conversion (cf. Ali and Bhattacharya 1980a). Levels of glucose and fructose varied accordingly to the conditions. There was some isomerisation (glucose to fructose), some starch conversion and some loss by Maillard reaction. Content of certain indicators of Maillard reaction correlated with the colour.



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8.3.2 Milling quality The greatly improved milling quality, viz. reduction in grain breakage during milling, of parboiled rice was one of the factors next to its vitamin content which endeared parboiled rice to millers and producers in Western countries. Numerous researchers who were thus led to examine this aspect repeatedly found that parboiling led to considerable ‘reduction’ in grain breakage. It was generally vaguely thought that this ‘reduction’ arose from the improved ‘hardness’ of parboiled rice. The matter was examined by Bhattacharya (1969). He observed that properly made parboiled paddy, if air-dried in shade, always gave virtually zero milling breakage regardless of the breakage of the sample before parboiling. Even paddy deliberately damaged to give nearly 100% breakage, as well as separated immature grains that shattered almost completely during milling, yielded nearly 100% whölegrains after parboiling (Table 8.6). This applied to damaged parboiled paddy as well! Examination in transmitted light showed no trace of cracks or chalkiness or other grain defects in the parboiled grains. It was therefore concluded that cooking and swelling of the starchy endosperm during the process completely healed all the preexisting defects in the rice grain. Consequently rice grain breakage was not just reduced but virtually eliminated by parboiling. However, one should remember that parboiled paddy, like raw paddy, too could crack if improperly dried and could break during milling as a consequence (see Figs 3.4–3.6). Clearly not only proper and complete parboiling (no residual white belly from insufficient soaking) but also proper drying of the paddy was essential to get the benefit of parboiling so far as head-rice yield was concerned. Improper processing, especially drying, may therefore explain why different workers got different extents of ‘improvement’ after parboiling. Grain hardening after

Table 8.6 paddya Paddy

Raw Parboiled

Effect of parboiling on milling quality of damaged raw and parboiled

Treatment

Nil Oven-driedb Wettedb Nil Oven-dried Wetted

Breakage (%) on milling As is

After parboiling

35.9 100.0 71.0 0.6 99.0 10.5

0.7 0.9 0.8 – 0.4c 0.7c

Reproduced, with permission, from Bhattacharya (1969). a After parboiling, paddy was air-dried and milled in a McGill miller to 8–10% degree of milling. b After over-drying or wetting, samples were exposed to ambient air to equalise moisture. c Indicates reparboiling. – = Not available.



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parboiling was widely thought to be responsible for the ‘improvement’. But no experimental data were available to answer how far it really contributed to the better milling quality of the parboiled rice. The present author’s guess is that it played only a minor part, if at all. Milling of parboiled rice differed from that of raw rice in other minor ways. Shelling of parboiled paddy was easier than raw paddy as the husk was slightly opened after steaming during parboiling. On the other hand, being harder, parboiled rice needed greater energy and time for whitening. Further, as the parboiled bran was oily (see below), it tended to be somewhat flaky and thus tended to clog the mill screen during milling. Not only this. Even the milled rice might appear somewhat oily and sticky, especially if undermilled in metal pearlers (Halim and Bhattacharya 1978), discussed below. Millers tend to overcome these problems by adding a little husk or chalk to the brown rice during whitening or maintain a higher pressure in the whitening machine (Gariboldi 1984).

8.3.3 Flow and packing properties The change in the disposition of the fat discussed earlier (Section 8.2.4) had a profound effect on certain properties of the milled rice. A peculiar effect of the location of oil globules fairly close to the grain surface in rice (Fig. 8.14) is that fat quickly appeared on the grain surface during milling (Fig. 8.12). The surface fat initially increased and then decreased as the milling proceeded, being very low in both unmilled and fully milled rice. As the oil globules were disrupted and converted into a band (Fig. 8.14) during parboiling, and the oil was somewhat pushed outwards (Fig. 8.13), oil on the grain surface appeared earlier in parboiled rice during milling and reached a higher peak (Fig. 8.12). This oil disposition had a strong effect on the physical properties of milled parboiled rice, including its appearance. The milled rice might appear perceptibly sticky and somewhat dirty (attached some bran particles), especially if processed in metal pearlers. Halim and Bhattacharya (1978) showed by experimental measurement of its actual flow through an orifice that undermilled parboiled rice was stickier and somewhat ‘viscous’ in comparison with the corresponding raw rice or even fully milled parboiled rice. This drawback was not confined to perception alone. Bhattacharya et al. (1972) in careful measurement determined the physical properties of rice, viz. density (D), bulk density (BD), porosity (P) and angle of repose (AR). These properties of both raw and parboiled rice were determined over the entire range of degree of milling from 0 to 8% (Fig. 8.17). The density increased slightly as the milling progressed, obviously due to the loss of the lighter fat, in both raw and parboiled rice. Interestingly the friction (angle of repose) and, as a consequence, porosity initially increased and then decreased.



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Effect of parboiling on rice quality Raw

48

Parboiled

1.46

0.79 AR

42

48

P

1.45

0.77

D D

AR

1.44

0.75

P 46

1.43

Bulk density, g/ml

50

Density, g/ml

Porosity, %

BD

40

36

0.73

BD

Angle of repose, degree

52

279

32 44

1.42

0

2

4

6 8 2 Degree of milling, %

4

6

8

0.71

Fig. 8.17 Physical properties of raw and parboiled rice as affected by degree of milling. D = density, BD = bulk density, P = porosity, AR = angle of repose. Milling with the McGill miller. Reproduced, with permission, from Bhattacharya et al. (1972) John Wiley and Sons. © Society of Chemical Industry (SCI).

The bulk density followed in tandem, in reverse, first decreasing and then increasing in both raw and parboiled rice. This was obviously a consequence of the initial increase and subsequent decrease in the amount of fat on the surface of the milled grain (Fig. 8.12). These properties, viz. AR, P and BD, determined the flow (related to AR) and packing and storage (related to P and BD) properties of the grain. What was striking was that this adverse effect of the degree of milling on these physical properties was much greater on parboiled than on raw rice (Fig. 8.17). This was obviously related to the greater amount of surface fat in the former (Fig. 8.12). Clearly the change in disposition of the grain fat as a result of parboiling, tiny in amount though it might be, caused a substantial effect on the appearance, flow and packing properties of parboiled rice. The effect was especially marked if the rice was undermilled and more so if it was processed in metal pearlers. One should note, however, that this difference was virtually eliminated once the rice was well milled.

8.3.4 Cooking and eating qualities After having been drawn to parboiled rice from the wonder of its therapeutic and preventive properties against beriberi and, later, milling quality, researchers then discovered that parboiled rice cooked and tasted somewhat differently from raw rice. This finding would have undoubtedly dampened



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the international health authorities’ enthusiasm for parboiled rice to some extent. But the apparent drawback was gradually taken in its stride. Many workers in repeated work showed that parboiled rice needed a longer time to cook than raw rice. At the same time the cooked grain retained better shape and was firmer, the rice was fluffier and fewer sticky, and it lost fewer solids into the cooking water (see Bhattacharya and Ali 1985 and Bhattacharya 2004 for the many references). The rationale of this change has already been discussed above while examining the effect of parboiling on starch. The change was a clear consequence of the initial gelatinisation followed by relevant reassociation of starch (primarily to lipid–amylose complex I and/or II) occurring during the process. While the above is the general picture, the effect was obviously modulated according to the parboiling type and circumstances. Each type of processing condition had its own effect on the specific property of low-temperature hydration, solubility and viscosity on the one hand and that of high-temperature hydration (i.e., cooking, in other words, water uptake), viscosity and solids loss, etc. and cooked-rice texture on the other. As a result the property profile of the end product varied qualitatively from type to type and quantitatively from mild to severe degree of each process. For example, as has been explained earlier in Section 8.2.3, dry-heat parboiling and very low-moisture pressure-parboiling, especially the former, led to the unique property of the product having a very high EMC-S. The high EMC-S arose from the fact that the low moisture of the grain at the time of processing prevented amylopectin retrogradation. One effect of this situation was that such dry-heat parboiled rice appeared partially cooked even when simply soaked in cold or warm water. Low-moisture pressure-parboiled rice also showed similar effect, although to a lesser extent. Pressure-parboiled rice on the other hand would invariably show very low high-temperature hydration, i.e., high cooking resistance, because of the formation of lipid– amylose complex II and possibly amylose retrogradation under very adverse conditions. The texture of the cooked rice too would vary accordingly. In fact the texture largely mirrored the cooking resistance, a highly cookingresistant rice showing a very hard cooked-rice texture and vice versa. The data in Fig. 8.8 give an idea of the wide tapestry of different combinations of properties that one can obtain based on a combination of the different processing conditions. Clearly parboiled rice is not just one product but a cluster of a large number of products. Adding the varietal difference and the difference originating from rice age to the above indicates what a mix of rice property combinations one could potentially obtain. An incidental peculiarity of the cooking property of parboiled rice is that it elongated less but expanded more in width during cooking in comparison



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with that of raw rice (see Bhattacharya 2004 for references). For this reason cooked parboiled rice appeared rather shorter and more stocky than cooked raw rice (Fig. 8.18). It is not known why this change occurs. It may be remembered that parboiled grains (uncooked) are already a little shorter and thicker. Clearly this difference is magnified after cooking. Effect of starch breakdown What remains is to consider the effect of thermal breakdown of starch during processing (see page 268). Mahanta and Bhattacharya (1989) obtained clear evidence of thermal breakdown of starch under high steam pressures (Table 8.3), but the effect it had on rice property was not known. There was one hint, however. As discussed above, increasing steam pressure during parboiling progressively reduced the water uptake (increased the cooking resistance) as well as of the parboiling-canning solids loss of the resulting product. However, there had been some exceptions. Unnikrishnan and Bhattacharya (1987b) confirmed the initial trend of decreasing parboil-canning solids loss with increasing steam pressure, but they observed that this trend was arrested and then reversed at very high pressures. This was confirmed in Mahanta’s (1988) work who used an intermediate-amylose variety (Intan) and found the effect quite perceptible. She also observed a clear reversal of the trend of not only canning solids loss but also of high-temperature water uptake (i.e., ease of cooking) and cooked-rice firmness after treatment at high steam pressures (2–3 kg/cm2) (Fig. 8.19). That means, while water uptake and canning solids loss initially decreased with increasing temperature of

R

Fig. 8.18

M-PB

S-PB

Raw (R), mildly (M) and severely (S) parboiled (PB) rice cooked with 2.5 times its weight of water. Variety: BT.



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60

12

12

50

30

3.2

17

30 F, %

W96, g/g db

3.4

40 22

3.0 30

22 17 0

1

2

3

0

1

(a)

2

3

(b) 32

22

PCSL, % db

28

24

17

12

20

30

16 0

1

2 (c) Steam pressure, kg/cm2

3

Fig. 8.19 Effect of steam pressure during parboiling (steamed for 20 min each at the gauge pressure indicated) on 96 °C water uptake (a), cooked-rice viscoelastograph firmness (b) and parboiled-canning solids loss (c). The numerals along the curves indicate the initial paddy moisture. Symbols enclosed in circles indicate white-bellied samples. F = firmness (viscoelastograph), PCSL = parboil-canning solids loss. Source: Mahanta (1988).

steaming, as expected, this decrease seemed to get halted and then even reversed at still higher steaming temperature, especially in lower-amylose rice. There was a hint of similar reversal in the data of Biliaderis et al. (1993) as well. Mahanta (1988) speculated that this peculiar reversal might be related to the thermal breakdown of starch at high steam pressures observed by her. To put it in another way, starch breakdown might help increase water uptake and solids loss during cooking, i.e., tend to reverse the normal effect of parboiling in resisting cooking and hardening of the cooked rice. If these



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283

inferences were correct, thermal breakdown of starch may play a role in the cooking behaviour of parboiled rice especially in low-amylose varieties after high-pressure steaming. There might be a glimmer of a positive signal here: there may be here a mechanism to ultimately reverse one disadvantage of parboiling, viz. its effect in resisting cooking.

8.3.5

Nutritive value

B vitamins and other trace constituents The legendary superior nutritive value of milled parboiled over milled raw rice was the reason why the world outside south Asia initially took interest in parboiled rice. Numerous workers repeatedly found that milled parboiled rice contained more thiamin as well as nicotinic acid than milled raw rice (see Bhattacharya and Ali 1985 and Bhattacharya 2004 for references). Initially little was understood as to why this happened. It became apparent that the total change was quite complex. The total amount of thiamin in brown rice actually decreased after parboiling. There was an initial loss by leaching during soaking. This loss was normally negligible but could be moderate if the soaking was done under conditions where the husk split open. Then a part of the vitamin was thermally destroyed during steaming which was again negligible under normal conditions but could be substantial if done under pressure (Table 8.7). Similar loss could occur in nicotinic acid as well but does not seem to have been measured so thoroughly. To put the matter in perspective, therefore, the greater vitamin content in milled parboiled rice arose simply because a vastly lower proportion of the residual vitamin was removed (along with the bran) during its milling compared with that of raw paddy (Table 8.7). This is clearly shown in the classical data of Aykroyd et al. (1940) (Fig. 8.20), repeatedly confirmed by others later. The milled grain probably retained similarly larger amounts of other B vitamins (Kik 1955), but apparently not of riboflavin (Bolling and El Baya 1975, Ocker et al. 1976). Before discussing the mechanism of the reduced milling loss of vitamins, it may be appropriate to point out a similar situation with respect to sugars, amino acids and minerals. Williams and Bevenue (1953) showed more than half a century ago that milled parboiled rice contained more sugars than milled raw rice. As it transpired (Ali and Bhattacharya 1980a), the final sugar content was again a dynamic end product. Considerable enzymatic conversion of nonreducing to reducing sugars and additional production of sugars (Fig. 8.3) occurred; a substantial part of the sugars was leached out into the soak water; and there was a small drop during heating due to the Maillard reaction. However, here again, the main factor was a greatly reduced loss of sugars during milling as compared with that in raw rice (Fig. 8.21).



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Table 8.7

Loss of thiamin during soaking, steaming, and milling of paddy

Soaking Temperature (°C)

Not soaked Room temperature 50 60 70

80

Percent lossb of thiamin

Steaming Timea Pressure (min) (kg/cm2, gauge)

a b c a b c a b c a b

0 0 0 0 0 0 0 0 0 0 0 0

c a b c

0.35 0.70 1.40 0 0 0 0

Time (min)

10 10 10 10 10 10 10 10 10 10 10 2 5 10 40 60 10 10 10 10 10 10 10

During parboiling Soaking

0 0 1 0 1 3 0 6 16 3 6

37 13 17 31

During milling

Steaming Before After steaming steaming 10 – – – 10 – 8 – – – – 3 3 4 14 19 11 15 28 – – 2 –

90 93 93 93 85 80 76 82 69 68 60 46

31 33 34 32

64 35 34 32 30 25 23 23 19 24 23 26 25 24 – 25 – 25 29 27 29 30 30

Adapted, with permission, from Subba Rao and Bhattacharya (1966), © American Chemical Society. a Soaking time: b = time required for optimal soaking at the indicated temperature; a and c = undersoaking (50-75% of optimum time) and oversoaking (125-150% of optimum time), respectively. b Loss expressed as percent of B1 present in original paddy (for soaking), soaked paddy (for steaming), and soaked and steamed paddy (for milling). – = Not available.

The striking similarity between Figs 8.20 and 8.21 is obvious. As the extent of enzymatic production and leaching of the sugars varied according to the processing conditions, the final sugar content in the milled rice could vary widely (see Bhattacharya and Ali 1985 for some data). Lamberts et al. (2006, 2008) recently reported on the sugars of parboiled rice (see page 276). Enzymatic production of amino acids (Fig. 8.3) and their reduced loss during milling also occurred similarly. Here again the final content varied widely according to the conditions. The contents of minerals in parboiled rice have been estimated by different authors. These data were collated by Bhattacharya and Ali (1985). The data showed that there was little change in any as a result of parboiling



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Effect of parboiling on rice quality 5 Parboiled

4

Parboiled 3

3

2

2 Raw Raw

1

0

1

Nicotinic acid, mg/100g

5

4 Thiamin, mg/g

285

0 0

5

10

15

20 0 5 Millings removed, %

10

15

20

Fig. 8.20 Effect of milling on content of thiamin (left) and nicotinic acid (right) in raw and parboiled rice. Reprints, by permission, from Aykroyd et al. (1940) copyright 1940 of the IJMR. 1.0 Raw Parboiled

Sugar, %

0.8

0.6 Suc 0.4

RS

0.2

RS 0

2


8

10

Fig. 8.21 Changes in reducing sugars (RS) and sucrose (Suc) contents in raw and parboiled brown rice during progressive milling with a McGill miller. Reproduced, with permission, from Ali and Bhattacharya (1980a) John Wiley and Sons.

process per se (in brown rice before milling), but the content of many (ash, phosphorus, calcium, iron, manganese, molybdenum and chromium) was definitely greater in milled parboiled as compared to milled raw rice. In other words, the milling loss of these constituents was prevented after



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parboiling just like those of B vitamins and sugars. On the other hand, the content of other minerals (magnesium, zinc, copper) after milling were not affected by parboiling. The latter phenomenon was reminiscent of the case of riboflavin. Yang and Cho (1995) re-examined the effect of parboiling and milling on chemical composition; unfortunately their detailed data are difficult to obtain. Mechanism of reduced loss of constituents during milling Two theories have been prevalent to account for the reduced loss of watersoluble components (B vitamins, sugars, amino acids, many minerals) during milling of brown rice after parboiling. The case of thiamin has received the widest attention (see Bhattacharya and Ali 1985 for references). One can assume that the remaining cases were similar. The earliest proposed and the prevalent theory was that the water-soluble constituents diffused into the endosperm during soaking for parboiling. The fact that certain water-soluble constituents were found to leach out into the soak water during soaking (Ramalingam and Anthoni Raj 1996), including that of thiamin under conditions where the husk split open during soaking (Table 8.7), raised some doubts about this theory. There were other doubts. Subba Rao and Bhattacharya (1966) reviewed the earlier data and were not convinced by them. They also made a very important observation that the retention of thiamin content in rice after its milling was always low in all samples that were merely soaked (not steamed), no matter how long the paddy was soaked or oversoaked. If inward migration of B1 during soaking was the reason of the reduced milling loss of parboiled rice, then milling after soaking alone, especially prolonged soaking, should also have shown an identical high retention of thiamin in the milled product. But this happened only marginally, if at all (Table 8.7). On the other hand, the milling retention of B1 jumped sharply and promptly after the same soaked paddy had been steamed for a few minutes. In other words the retention jumped at the point the starch got gelatinised. Similarly milling retention of B1 was raised even after mere soaking (no steaming) if the soaking was done at high temperatures (≥ 70 °C) favouring gelatinisation (Table 8.7). The case was identical with respect to milling loss of sugars and amino acids (Ali and Bhattacharya 1980a). In other words, it appeared that gelatinisation was the proximate cause of the greater milling retention of the molecules. The conclusion drawn was that the reduction of milling loss of the small water-soluble molecules was not caused primarily by their inward migration during soaking but mainly by their immobilisation due to adhesion to the gelatinised starchy endosperm during steaming. Padua and Juliano (1974) and Benedito de Barber et al. (1977) contested this hypothesis. They showed that successive three millings of 10% by weight of the brown rice in the former work (Table 8.8), and 5% by weight



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Table 8.8 Distribution of thiamin in successive milling fractions of raw and parboiled rice Milling fractiona Raw rice

Parboiled riceb

Weight Thiamin Weight fraction Thiamin fraction (percent of Amount Percent (percent of Amount Percentc brown rice) (mg/g) of totalc brown rice) (mg/g) of total First Second Third Residual rice

11 9 10 70

26.9 6.0 0.60 0.05

82 15 2 1

12 9 7 72

4.6 4.4 3.8 2.0

21 15 10 54

Reproduced, with permission, from Padua and Juliano (1974) John Wiley and Sons © Society of Chemical Industry (SCI). a Milling with Satake Laboratory emery mill. b Soaked at 60 °C for 6 h, steamed at 121 °C for 10 min. c Thiamin content of brown rice: raw, 3.58 mg/g; parboiled, 3.10 mg/g.

of brown rice in the latter, virtually completely removed thiamin from raw rice but only about half of the vitamin from parboiled rice. These data, they concluded, were a clear proof of inward migration. We are thus left hanging in mid-stream. If the second set of data strongly suggests inward migration, the earlier set strongly suggests that starch gelatinisation had a hand in the phenomenon. What it would then imply is that the molecules migrated precisely at the point of gelatinisation. That would be something difficult to comprehend. Further carefully planned research is needed. Another point that needs to be explained in whatever hypothesis one may propose is: why was milling loss of water-soluble components after parboiling strongly prevented in some cases (B1, niacin, sugars), moderately prevented in others (most minerals), and virtually not at all in some (riboflavin, magnesium, zinc and copper)? Other nutritional aspects Milled parboiled rice did not just contain more B vitamins. The loss of the residual vitamins during storage of rice or during its washing preliminary to cooking were also reduced by parboiling (see Bhattacharya and Ali 1985). It also contained minerals, especially calcium, phosphorus and iron in greater amounts. All these added to its nutritive value. Protein content remained on the whole unchanged in parboiled rice, as mentioned before. Kik (1955, 1965) mentioned that the nutritive value of the protein was improved. Eggum (1979) noted that the digestibility of the protein was reduced but the biological value was increased. As a result the total utilisable protein remained unchanged. On the other hand, according to Benedito de Barber et al. (1977), parboiling resulted in a small (lysine)



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to considerable (tryptophan and methionine) drop in available amino acids. Lamberts et al. (2008) found epsilon-amino group of protein-bound lysine was affected more than free lysine. Tetens et al. (1997) suggested that parboiled rice contained more resistant starch. Probably the complexed polymorphs of starch were responsible.

8.3.6 Storage quality Apart from flow and packing properties, storage quality of rice is a function of deterioration of its fat (rancidity) and susceptibility to insect and microbial attack. Parboiling affected these properties of rice too. Under normal circumstances, all enzymes in rice were by and large inactivated during parboiling by the heat treatment (Barber et al. 1983). Especially important in terms of storage quality was the effect on lipase. Since the lipase was virtually destroyed during steaming, development of FFA was stopped. In other words, parboiled rice hardly underwent any hydrolytic rancidity. On the other hand, the antioxidants in rice too were largely destroyed during the heat treatment (Fig. 8.2). Remember that the oil globules were also disrupted and then converted into a band and pushed somewhat outward during the process (Fig. 8.14). As a result of this, as was found by Houston et al. (1954) and Sowbhagya and Bhattacharya (1976), milled parboiled rice was very much more susceptible to lipid autoxidation, i.e., oxidative rancidity, than milled raw rice (Fig. 8.22). Low grain moisture, exposure

60° Dark: Open stored

Peroxide, m Eq/g; Carbonyl, mM/g fat

1200

Parboiled

, ,

Peroxide Carbonyl

600

400

200 Raw

20

60 Storage time, days

100

Fig. 8.22 Development of peroxide and carbonyl compounds in raw and parboiled milled rice during open storage at 60 °C in the dark. Reproduced, with permission, from Sowbhagya and Bhattacharya (1976) John Wiley and Sons; © Institute of Food Technologists. © Woodhead Publishing Limited, 2011


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to light, increasing degree of milling and high temperature (also grinding) were found to accelerate the oxidation. Kato et al. (1983) studied changes in carbonyl compounds, head-space volatiles and FFA after parboiling. One should note, however, the rancidity did not appear to pose much problem under normal circumstances. Storage in woven bags was especially helpful, for the darkness retarded oxidation and the natural aeration helped to dissipate any odour formed. It is a matter of some interest to note that milled parboiled rice was less susceptible to insect infestation than milled raw rice. In a study, the number of two species of moths and six species of beetles found at the end of storing for four months in a warehouse followed by two months’ storage in the laboratory was counted. It was about two-thirds in parboiled rice of that in raw rice (Vinacke et al. 1950). In laboratory studies the results were more dramatic. Progeny development of several common species of insects in the laboratory was virtually nil in parboiled rice whereas raw rice was highly susceptible (McGaughey 1974, Sing 1980). The conclusion is clear that parboiled rice was easier to store than raw rice in terms of insect infestation. It has been said that the smooth surface and the hard kernel prevented the insects from getting a foothold on the grain surface and boring through the kernel. However, parboiled paddy, by virtue of its husk being slightly open, could be more susceptible for infestation than raw paddy. But then, in practice, no one usually stores parboiled paddy. Normally in commercial practice parboiling and drying are soon followed by its milling and disposal. Unnikrishnan and Bhattacharya (1995) made the interesting observation that milled parboiled rice aged virtually identically as milled raw rice. This phenomenon was significant in terms of our understanding of the phenomenon of rice ageing, as discussed in Chapter 5. It virtually dispelled the notion that either lipase (destroyed during parboiling) or carbonyls (produced in far greater amounts in parboiled than in raw rice) played a major role in the ageing process.

8.3.7 Bran quality Although strictly not a part of grain quality, any role of parboiling in affecting the value of the bran cannot be ignored. In actual fact parboiling did affect the bran quality, some advantageous, some not so. As the lipase enzyme was virtually inactivated during steaming, the nuisance of FFA development in bran was almost completely eliminated (Fig. 8.23). This is a great advantage. Another advantage is that the oil content in parboiled rice bran was more than that in raw rice. This was an effect of parboiling pushing the oil in the grain outwards (Fig. 8.13), so a greater amount of bran oil of better stability was obtained as a result of



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80 Untreated

FFA in oil, %

70

30

Soaked 70 °C, 3 h

20

10 Parboiled

20 40 Storage time, days

Fig. 8.23 Development of free fatty acids (FFA) in bran from raw, soaked and parboiled rice (soaked at 70 °C for 3 h, open-steamed for 10 min). Reproduced, with permission, from Viraktamath and Desikachar (1971).

parboiling (Bolling and El-Baya 1975, Shaheen et al. 1975, Kumaravel et al. 1980). Interestingly any amount of FFA already developed in the original rice (bran) was also claimed to be lowered by parboiling (Anthoni Raj and Singaravadivel 1982). As against this, one may note that parboiled rice bran oil was somewhat discoloured (colour fixing) compared with raw rice bran. Therefore it needed additional bleaching. Extraction of oil from parboiled bran was also said to be somewhat more difficult.

8.3.8 Advantages and disadvantages of parboiling Summarising, one can say that the following are the advantages and disadvantages of parboiling. The advantages are: (1) shelling of the paddy is easier, (2) grain breakage during milling of the rice is dramatically reduced (in fact, parboiling can salvage rain- or drying-damaged paddy), (3) milled rice contains greater amounts of B vitamins and minerals, (4) loss of nutrients during washing is reduced, (5) grains remain integral and do not mash after cooking, and the loss of solids in cooking water is reduced, (6) insect infestation and loss of nutrients during storage of rice are reduced, (7) the bran contains more oil, which is relatively stable to FFA development, and (8) parboiled rice is suitable for making three rice products (canned, puffed, and flaked rice) for which raw rice is not suitable (see below). The disadvantages are: (1) brown rice needs more time and energy to



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whiten, (2) the mill screen tends to get choked during its whitening, (3) rice undermilled in metal pearlers looks oily and unattractive, (4) the flow and packing properties of undermilled rice are poor, (5) the rice needs more time and hence more energy to cook, (6) its harder texture after cooking is not liked by many consumers, and (7) it is more prone to oxidative rancidity.

8.4

Effect of rice variety on properties of parboiled rice

It is a measure of the great versatility of rice and the wide diversity of its quality that rice varieties do not only themselves vary in their property profile (Chapter 7), they also in turn influence the property of parboiled rice. An early signal of this effect was obtained by American and French workers in connection with their effort to produce canned rice. The inevitable retorting step for canning required that the rice grains inside be strong and resilient enough not to be damaged by the high-temperature treatment. This not only required the use of parboiled rice rather than raw rice, but scientists also noted that there was a difference between different rices. Webb et al. (1968) in the USA observed that the parboiled-canning solids loss varied, especially, inversely with the amylose content. Similarly French workers (Feillet and Alary 1975, Alary et al. 1977) noted that the firmness of canned rice was proportional to the amylose content. The above works, and the gradual clarification of the varietal difference in rice eating quality (Chapter 7), led Unnikrishnan and Bhattacharya (1987a) to a deeper study of this subject. They used 13 rice varieties of varying quality types, including two waxy rices. What they noted with surprise was that the effect of processing was virtually identical in all varieties: identical not in final product character, but in the extent of change. In other words, all varieties advanced to a measurable extent in a particular direction as a result of processing, so that the original varietal difference was carried over nearly unchanged even after processing. Thus all varieties after parboiling showed the usual effect of contrary low- and high-temperature hydration, solubility, solids loss, slurry viscosity, pasting behaviour, etc., as also increased cooking resistance and cooked rice hardness. But the extent of the changes in the properties compared with the corresponding raw rice properties remained largely constant in all varieties (Table 8.9), so the original gradation in the properties among the varieties remained more or less unchanged. The texture of the cooked rice also advanced similarly. Arai et al. (1975) and Kimura (1983) showed that stickiness and tenderness of cooked rice no doubt decreased after parboiling, yet japonica varieties continued to remain stickier and tenderer than indica varieties even after processing. Unnikrishnan and Bhattacharya (1987a, 1987b) found in their 11 nonwaxy rices that the texture values changed in all the samples in a similar way,



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Table 8.9 Variety (code no.b) 11 12 21 22 31 32 41 42 51 71 72 81 82

Change in physicochemical properties of rice as a result of parboilinga W96c (ratio)

SA96c (ratio)

Gel mobilityc (ratio)

Firmnessd (%)

M/R

S/R

M/R

S/R

M/R

S/R

R

M

S

0.93 0.91 0.92 0.90 0.91 0.90 0.93 0.91 0.92 0.90 0.91 0.92 0.86

0.69 0.68 0.70 0.69 0.72 0.72 0.72 0.71 0.70 0.72 0.72 – –

0.89 0.90 0.89 0.89 0.89 0.90 0.88 0.90 0.88 0.87 0.86 – –

0.68 0.72 0.71 0.75 0.70 0.73 0.71 0.69 0.69 0.69 0.66 – –

1.51 1.57 1.39 1.32 1.35 1.30 1.32 1.41 1.39 1.63 1.61 1.20 1.18

2.03 2.00 1.86 1.81 1.62 1.60 1.57 1.75 1.76 1.84 1.95 – –

54.2 62.0 49.4 45.5 38.3 36.6 29.1 27.7 24.0 22.2 20.3 14.1 14.0

60.1 65.3 55.4 52.6 45.8 44.2 39.2 42.5 36.5 28.3 24.6 14.8 15.2

65.0 70.0 60.2 59.4 54.9 54.2 50.4 52.7 47.1 37.2 35.6 – –

Compiled from Unnikrishnan and Bhattacharya (1987a). Used with permission. a R = raw (unprocessed), M = MPB (mildly parboiled), and S = SPB (severely parboiled) rice, W96 = water uptake (W) when cooked at 96 °C for 1 h (g/g), SA96 = amylose of rice flour dissolved in 96 °C water (% db). b The first digit of the code identifies the quality type of the rice. The subsequent digit stands for the serial no. within a type. Thus, 21 indicates that the variety belongs to quality type II, 72 = type VII and so on. The rice quality types are characterised by the amounts of total and insoluble amylose contents, which are highest in type I and least in type VIII (waxy) (see Chapter 7). c The results are expressed as a ratio of the values of parboiled rice (M or S) to that of raw rice. d Chopin-INRA viscoelastograph firmness value of cooked rice. – = Not available.

thereby maintaining the original intervarietal gradation more or less intact (Table 8.9). Biswas and Juliano (1988) studied 12 varieties having different amylose–GT combinations after steam-parboiling at 100–131 °C. Paddy of low-GT varieties imbibed a greater amount of moisture after soaking. (This would happen only if one did not adjust the soaking temperature and/or time downward according to the GT, which must be done. If not so adjusted, soaking would end up in splitting of the husk, and the process would not be operatable in practice.) These higher moisture-imbibed samples thus obtained showed a greater degree of gelatinisation upon steaming, suggesting that not only amylose but GT too influenced product quality. However, this is at best true only in theory, not in practice as explained. Otherwise amylose content was the major factor in the product qualities. We thus arrive at a situation that varietal difference in parboiled rice properties, produced under identical conditions of processing, mirrored that of the starting raw rice. This is great both for the processor and the investigator for one can predict the quality of the product from that of the feedstock. Also, one can choose the variety for processing depending on



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what end product one desires. For example, one can select a high-amylose rice of type I or II to produce hard-textured parboiled rice. Similarly one can use low- or intermediate-amylose varieties to produce parboiled rice that could be fairly acceptable to consumers of high-amylose raw rice or to revive the pressure-parboiling process (Unnikrishnan and Bhattacharya 1987a, 1989).

8.5

Products from parboiled rice

Parboiled rice yields at least three products for which raw rice is not at all suitable. These are puffed rice, flaked rice and canned rice. This subject will be taken up in Chapter 9 and hence is not further discussed here.

8.6

References

ajandouz e h and puigserver a (1999), ‘Nonenzymatic browning reaction of essential amino acids: effect of pH on caramelization and Maillard reaction kinetics’, J Agric Food Chem, 47, 1786–1793. alary r, laignelet b and feillet p (1977), ‘Effect of amylose content on some characteristics of parboiled rice’, J Agric Food Chem, 25, 261–264. ali s z and bhattacharya k r (1972a), ‘Hydration and amylose-solubility behaviour of parboiled rice’, Lebensm Wiss Technol, 5, 207–212. ali s z and bhattacharya k r (1972b), ‘An alkali reaction test for parboiled rice’, Lebensm Wiss Technol, 5, 216–218. ali s z and bhattacharya k r (1976a), ‘Comparative properties of beaten rice and parboiled rice’, Lebensm Wiss Technol, 9, 11–13. ali s z and bhattacharya k r (1976b), ‘Starch retrogradation and starch damage in parboiled rice and flaked rice’, Stärke, 28, 233–240. ali s z and bhattacharya k r (1980a), ‘Changes in sugars and amino acids during parboiling of rice’, J Food Biochem, 4, 169–179. ali s z and bhattacharya k r (1980b), ‘Pasting behaviour of parboiled rice’, J Texture Stud, 11, 239–245. ali s z and bhattacharya k r (1982), ‘Studies on pressure parboiling of rice’, J Food Sci Technol, 19, 236–242. anthoni raj s and singaravadivel k (1980), ‘Influence of soaking and steaming on the loss of simpler constituents in paddy’, J Food Sci Technol, 17, 141–143. anthoni raj s and singaravadivel k (1982), ‘Reduction in free fatty acids due to parboiling of paddy’, J Food Sci, 47, 692–693. anthoni raj s, singaravadivel k and subramaniyan v (1996), ‘Excretion of chlorides during soaking of paddy’, J Food Sci Technol, 33, 57–59. arai k, raghavendra rao s n and desikachar h s r (1975), ‘Studies on the effect of parboiling on japonica and indica rice’ (in japanese), Jap J Trop Agric, 19, 7–14. aykroyd w r, krishnan b g, passmore r and sundararajan a r (1940), The Rice Problem in India, Indian Med Res Memoirs, No. 32, 84. barber s, benedito de barber c and novo m (1983), ‘Inactivation of rice bran enzymes during parboiling’, in McLoughlin J V and McKenna M B (Eds) Research in Food



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Science and Nutrition, Vol. I, The production, preservation and processing of food, Dublin, Boole Press, 85–86. benedito de barber c, martinez j and barber s (1977), ‘Effects of parboiling processes on the chemical composition and nutritional characteristics of rice bran’, in Barber S and Tortosa E (Eds) Rice By-products Utilization, Vol. IV, Valencia, Spain, Inst Agroquimica Technol Alimentos, 121–130. bhattacharya k r (1969), ‘Breakage of rice during milling, and effect of parboiling’, Cereal Chem, 46, 478–485. bhattacharya k r (2004), ‘Parboiling of rice’, in Champagne E T (Ed) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 329–404. bhattacharya k r and ali s z (1976), ‘A sedimentation test for pregelatinized rice products’, Lebensm Wiss Technol, 9, 36–37. bhattacharya k r and ali s z (1985), ‘Changes in rice during parboiling, and properties of parboiled rice’, in Pomeranz Y (Ed.) Advances in Cereal Science and Technology, Vol. VII, St. Paul, MN, American Association of Cereal Chemists, 105–167. bhattacharya, k r and subba rao p v (1966a), ‘Processing conditions and milling yields in parboiling of rice’, J Agric Food Chem, 14, 473–475. bhattacharya k r and subba rao p v (1966b), ‘Effect of processing conditions on quality of parboiled rice’, J Agric Food Chem, 14, 476–479. bhattacharya k r, sowbhagya c m and indudhara swamy y m (1972), ‘Some physical properties of paddy and rice and their interrelations’, J Sci Food Agric, 23, 171–186. bhattacharya s (1996), ‘Kinetics on colour changes in rice due to parboiling’, J Food Eng, 29, 99–106. biliaderis c g (1992), ‘Structures and phase transitions of starch in food systems’, Food Technol, 46, 98–109, 145. biliaderis c g and galloway g (1989), ‘Crystallization behavior of amylose-V complexes: structure–property relationships’, Carbohydr Res, 189, 31–48. biliaderis c g, page c m, maurice t j and juliano b o (1986), ‘Thermal characterization of rice starches: a polymeric approach to phase transitions of granular starch’, J Agric Food Chem, 34, 6–14. biliaderis c g, tonogai j r, perez c m and juliano b o (1993), ‘Thermophysical properties of milled rice starch as influenced by variety and parboiling method’, Cereal Chem, 70, 512–516. biswas s k and juliano b o (1988), ‘Laboratory parboiling procedures and properties of parboiled rice from varieties differing in starch properties’, Cereal Chem, 65, 417–423. bolling h and el-baya a w (1975), ‘Einfluss von Parboiling auf die physikalischen und chemischen Eigenschaften des Reises (Effect of parboiling on the physical and chemical properties of rice)’, Getreide Mehl, 29, 230–233. chakravarty h b and ghose t k (1966), ‘Studies on the hydration of Indian paddy. IIIA: Efficiency of elevated temperature on composition of oil in bran’, Souvenir, Seminar on Modern Technology of Rice Milling, East India Rice Mills Assn, Calcutta, pp. 21–25. charbonnier r (1975), ‘Structure physique de l’amidon du riz natif, étuvé, précuit, appertisé, retrogradé (Physical structure of native, parboiled, precooked, canned, and retrograded rice starch)’, Bull Inf Rizicul France, 160, 14–21. charlton j (1923), ‘The prevention of nuisances caused by the parboiling of paddy’, Bulletin, 146, Agric Res Inst, Pusa, Calcutta. chinnaswamy r and bhattacharya k r (1986), ‘Pressure-parboiled rice: a new base for making expanded rice’, J Food Sci Technol, 23, 14–19. derycke v, vandeputte g e, vermeylen r, de man w, goderis b, koch m h j and delcour j a (2005a), ‘Starch gelatinization and amylose-lipid interactions during



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rice parboiling investigated by temperature resolved wide angle X-ray scattering and differential scanning calorimetry’, J Cereal Sci, 42, 334–343. derycke v, veraverbeke g e, vandeputte g e, de man w, hoseney r c and delcour j a (2005b), ‘Impact of proteins on pasting and cooking properties of nonparboiled and parboiled rice’, Cereal Chem, 82, 468–474. dimopoulos j s and muller h g (1972), ‘Effect of processing conditions on protein extraction and composition and on some other physicochemical characteristics of parboiled rice’, Cereal Chem, 49, 54–62. eggum b o (1979), ‘The nutritional value of rice in comparison with other cereals’, in Chemical Aspects of Rice Grain Quality. Los Baños, Laguna, Philippines, International Rice Research Institute, 91–111. feillet p and alary r (1975), ‘Influence des caracteristiques variétales, de l’étuvage et de l’usinage sur l’aptitude des riz a l’appertisation (Influence of varietal characteristics, of parboiling, and of milling on the suitability of rice for canning)’, Bull Inf Rizicul France, 58, 15–18. gariboldi f (1984), Rice Parboiling, FAO Agric Services Bulletin 56, Rome, FAO. halim a and bhattacharya k r (1978), ‘Effect of degree and pressure of milling on the stickiness of milled parboiled rice’, J Food Qual, 1, 349–358. himmelsbach d s, manful j t and coker r d (2008), ‘Changes in rice with variable temperature parboiling: thermal and spectroscopic assessment’, Cereal Chem, 85, 384–390. houston d f, hunter i r, mccomb e a and kester e b (1954), ‘Deteriorative changes in the oil fraction of stored parboiled rice’, J Agric Food Chem, 2, 1185–1190. houston d f, hunter i r and kester e b (1956), ‘Storage changes in parboiled rice’, J Agric Food Chem, 4, 964–968. indudhara swamy y m, ali s z and bhattacharya k r (1971), ‘Hydration of raw and parboiled rice and paddy at room temperature’, J Food Sci Technol, 8, 20–22. jayanarayanan e k (1964), ‘Der Einfluss der verarbeitungsbedingungen auf das Braunwerden von “parboiled” Reis (Effect of operating conditions on the browning of parboiled rice)’, Nahrung, 8, 129–137. kato h, ohta t, tsugita t and hosaka y (1983), ‘Effect of parboiling on texture and flavor components of cooked rice’, J Agric Food Chem, 31, 818–823. kik m c (1955), ‘Influence of processing on nutritive value of milled rice’, J Agric Food Chem, 3, 600–603. kik m c (1965), ‘Nutritional improvement of rice diets and effect of rice on nutritive value of other foodstuffs’, Ark Agric Exp Stn Bull, 698. kimura t (1983), ‘Properties of parboiled rice produced from Japanese paddy’, Agric Mecha Asia Africa and Latin Amer, 14, 31–33. kimura t, matsuda j, ikeuchi y and yoshida t (1976), ‘Basic studies on parboiled rice. Part III. Effect of processing conditions on the rate of gelatinization of parboiled rice’ (in Japanese), J Jap Soc Agric Mach, 38, 379–383. kumaravel s, singaravadivel k, vasan b s and anthoni raj s (1980), ‘Storage of parboiled bran’, J Oil Technol Assoc India, 12, 49–51. lamberts l, de bie e, derycke v, veraverbeke w s, de man w and delcour j a (2006), ‘Effect of processing conditions on color change of brown and milled parboiled rice’, Cereal Chem, 83, 80–85. lamberts l, rombouts i, brijs k, gebruers k and delcour j a (2008), ‘Impact of parboiling conditions on Maillard precursors and indicators in long-grain rice cultivars’, Food Chem, 110, 916–922. mahadevappa m and desikachar h s r (1968), ‘Some observations on histology of raw and parboiled rice’, J Food Sci Technol, 5, 72–73. mahanta c l (1988), ‘Studies on nature of parboiled rice’, unpublished Ph.D. Thesis, University of Mysore, Mysore, India.



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mahanta c l and bhattacharya k r (1989), ‘Thermal degradation of starch in parboiled rice’, Starch/Stärke, 41, 91–94. mahanta c l, ali s z, bhattacharya k r and mukherjee p s (1989), ‘Nature of starch crystallinity in parboiled rice’, Starch/Stärke, 41, 171–176. manful j t, grimm c c, gayin j and coker r d (2008), ‘Effect of variable parboiling on crystallinity of rice samples’, Cereal Chem, 85, 92–95. mcgaughey w h (1974), ‘Insect development in milled rice: effects of variety, degree of milling, parboiling and broken kernels’, J Stored Prod Res, 10, 81–82. mecham d k, kester e d and pence j w (1961), ‘Parboiling characteristics of California medium-grain rice’, Food Technol, 15, 475–479. mestres c, colonna p and buleon a (1988), ‘Characteristics of starch networks within rice flour noodles and mungbean starch vermicelli’, J Food Sci, 53, 1809–1812. mukherjee r k and bhattacharjee m (1978), ‘Distribution of oil in the bran layers of slender, medium and short grain varieties of rice, and effect of parboiling’, J Am Oil Chem Soc, 55, 463–464. ocker h d, bolling h and el-baya a w (1976), ‘Effect of parboiling on some vitamins and minerals of rice: thiamine, riboflavin, calcium, magnesium, manganese and phosphorus’, Riso, 25, 79–82. ong m h and blanshard m v (1994), ‘The significance of the amorphous–crystalline transition in the parboiling process of rice and its relation to the formation of the amylose–lipid complex and the recrystallisation (retrogradation) of starch’, Food Sci Technol Today, 8, 217–226. ong m h and blanshard m v (1995), ‘The significance of starch polymorphism in commercially produced parboiled rice’, Starch/Stärke, 47, 7–13. padua a b and juliano b o (1974), ‘Effect of parboiling on thiamine, protein and fat of rice’, J Sci Food Agric, 25, 697–701. pillaiyar p (1984a), ‘Applicability of the rapid gel test for indicating the texture of commercial parboiled rices’, Cereal Chem, 61, 255–256. pillaiyar p (1984b), ‘A rapid test to indicate the texture of parboiled rices without cooking’, J Texture Studies, 15, 263–273. pillaiyar p (1985), ‘A gel test to parboiled rice using dimethyl sulphoxide’, J Food Sci Technol, 22, 1–3. pillaiyar p and mohandoss r (1981), ‘Hardness and colour in parboiled rices produced at low and high temperatures’, J Food Sci Technol, 18, 7–9. priestley r j (1976), ‘Studies on parboiled rice. I. Comparison of the characteristics of raw and parboiled rice’, Food Chem, 1, 5–14. raghavendra rao s n and juliano b o (1970), ‘Effect of parboiling on some physicochemical properties of rice’, J Agric Food Chem, 18, 289–294. ramalingam n and anthoni raj s (1996), ‘Studies on the soak water characteristics in various paddy parboiling methods’, Bioresource Technol, 55, 259–261. schoch t j (1964), ‘Swelling power and solubility of granular starches’, in Whistler R L (Ed.) Methods in Carbohydrate Chemistry, Vol. IV. New York, Academic Press, pp. 106–108. shaheen a b, el-dash a a and el-shirbeeny a e (1975), ‘Effect of parboiling of rice on the rate of lipid hydrolysis and deterioration of rice bran’, Cereal Chem, 52, 1–8. sing k (1980), ‘Influence of milled rice on insect infestation. I. Oviposition and development of post-harvest pests in different types of milled rice’, Angew Entomol, 90, 1–9 (Food Sci Technol Abstr 13: 6M562, 1981). singaravadivel k and anthoni raj s (1979), ‘Leaching of phenolic compounds during soaking of paddy’, J Food Sci Technol, 16, 77–78. sondi a b, reddy i m and bhattacharya k r (1980), ‘Effect of processing conditions on the oil content of parboiled-rice bran’, Food Chem, 5, 277–282. sowbhagya c m and bhattacharya k r (1976), ‘Lipid autoxidation in rice’, J Food Sci, 41, 1018–1023.



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sowbhagya c m, ali s z and ramesh b s (1993), ‘Effect of parboiling on grain dimensions of rice’, J Food Sci Technol, 30, 459–461. subba rao p v and bhattacharya k r (1966), ‘Effect of parboiling on thiamine content of rice’, J Agric Food Chem, 14, 479–482. subrahmanyan v, sreenivasan a and das gupta h p (1938), ‘Studies on quality of rice. I. Effect of milling on the chemical composition and commercial qualities of raw and parboiled rice’, Indian J Agric Sci, 8, 459–486. tetens i, biswas s k, glitso l v, kabir k a, thilsted s h and choudhury n h (1997), ‘Physico-chemical characteristics as indicators of starch availability from milled rice’, J Cereal Sci, 26, 355–361. unnikrishnan k r and bhattacharya k r (1981), ‘Swelling and solubility behaviour of parboiled rice flour’, J Food Technol, 16, 403–408. unnikrishnan k r and bhattacharya k r (1983), ‘Cold-slurry viscosity of processed rice flour’, J Texture Stud, 14, 21–30. unnikrishnan k r and bhattacharya k r (1987a), ‘Influence of varietal difference on properties of parboiled rice’, Cereal Chem, 64, 315–321. unnikrishnan k r and bhattacharya k r (1987b), ‘Properties of pressure-parboiled rice as affected by variety’, Cereal Chem, 64, 321–323. unnikrishnan k r and bhattacharya k r (1988), ‘Application of gel consistency test to parboiled rice’, J Food Sci Technol, 25, 129–132. unnikrishnan k r and bhattacharya k r (1989), ‘Reviving the pressure-parboiling process by use of low-amylose varieties of paddy’, Indian Food Ind, 8, 25–28. unnikrishnan k r and bhattacharya k r (1995), ‘Changes in properties of parboiled rice during ageing’, J Food Sci Technol, 32, 17–21. vasan b s, iengar n g c, subramanyam t v, chandrasekaran r and subrahmanyan v (1971), ‘Effect of processing on the movement of oil in the rice kernel and its relation to the oil content of paddy and the yield of oil from rice bran’, Interregional Seminar on the Industrial Processing of Rice, Food and Agriculture Organization, ECAFE, and Government of India, Madras. vinacke w r, hartzler e and tanada y (1950), ‘Processed rice in Hawaii’, Tech Bull, 10, Honolulu Univ Hawaii Agric Exp Stn. viraktamath c s and desikachar h s r (1971), ‘Inactivation of lipase in rice bran in Indian rice mills’, J Food Sci Technol, 8, 70–74. webb b d, bollich c n, adair c r and johnston t h (1968), ‘Characteristics of rice varieties in the U.S. Department of Agriculture Collection’, Crop Sci, 8, 361–365. williams k t and bevenue a (1953), ‘A note on the sugars in rice’, Cereal Chem, 30, 267–269. xavier i j and anthoni raj s (1996), ‘Enzyme changes in rough rice during parboiling’, J Food Biochem, 19, 387–389. yang m o and cho e j (1995), ‘The effect of milling on the nutrients of raw and parboiled rices’, J Korean Soc Food Sci, 11, 51–57.



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9 Product-making quality of rice

Abstract: Many products are made from rice. The patterns of the products vary among zones. In northeast and southeast Asia, cooked or steamed cakes predominate. Puffed, popped or flaked wholegrain products predominate in south Asia. In Western societies, rice products are mainly used for its hypoallergenic property. Rice flour is made into baby foods by drum-drying or into baked products, pet foods and drinks. Rice grits are used in brewing. Cooked cakes are made mainly from low-amylose or waxy rice. But noodles require high-amylose rice. The fermented, leavened cake idli is best made from high-amylose rice. Popped and puffed rice have specific graincharacteristic requirements. Baby foods are best made from low-amylose rice. Key words: zone-wise variation in rice products, cakes, flakes, puffed and popped rice, baby foods, baked foods.

9.1

Introduction

Rice is consumed mainly as cooked table rice. In south, southeast and east Asia, where over 90% of the world’s rice is grown as well as consumed, people have been eating cooked rice twice a day as their main meals for thousands of years. However, rice has also been used here, of necessity, as subsidiary food for breakfast and snacks. In most parts of this region, little else of other cereals is grown. Rice therefore has had to be adapted to provide for the requirements of breakfast and snack foods as well. A great many types of rice snack and breakfast foods are made, but



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299

there are interesting regional differences. The majority of these products in southeast and east Asia are made from wet-ground rice, with or without other adjuncts, made into a variety of cakes, pancakes, fries, crackers, puddings and allied foods. A further form of consumption of rice in these regions is in the form of noodles, again made from wet-ground rice; but that is generally for the main meals. Puffed and flaked products are also made, but to a limited extent. In south Asia, the situation has been somewhat different. While cakes and fries have been made here too, four products dominate the scene. Three of these are flaked rice, puffed rice and popped rice, which are made not from flours but from wholegrain rice as the starting material. The fourth is the fermented and steamed rice cake, idli, made from flour (wet-ground) in this instance. This is a traditional native item of south India, now becoming popular throughout the region. Rice does not have gluten and therefore cannot be leavened by itself. Idli is an attempt to leaven rice by adding a legume and fermenting. Breakfast and snack foods from rice have lately been introduced into wheat-consuming regions of the world. The objectives have been three-fold. One was to add variety to people’s diets. The second was for health reasons: wheat has been known to cause allergy. As rice is largely nonallergenic, baby food and snacks and bread made from rice fill the gap in such cases. The third motive was the need to utilise broken rice. The USA is unique in growing a fair amount of rice (about 1.5% of the world production), in being a major exporter of rice, and in being a highly industrialised country. This combination inevitably led to the availability of a substantial amount of broken rice – much of which in nonexporting and less industrialised countries would generally not be separated at all. An outlet other than table rice had to be found for this broken rice. Production of rice snacks and products arose as a consequence, and inevitably straight away evolved into a modern industry. Many of these products are made starting with rice flour as a raw material. Rice flour too has therefore evolved as an item of industrial production and commerce in these regions, which is then converted into various products in downstream industries. Some other products are made starting with granular milled rice. A list of the more important rice products and a scheme for their classification are presented in Fig. 9.1. There are no statistics showing how much of rice is used as breakfast and snack foods in the world. It is generally believed in India (Narayanswami 1956, Ghose et al. 1960) that some 10% of the production of rice is used for the four main products, viz. flaked, puffed and popped rice and idli. There is no way of knowing how much rice is used for adjunct foods in other regions of Asia, but it can quite easily be similar, or even more, if one takes noodles also into consideration. A substantial amount of rice is also used in the USA for making various products. Some interesting statistics



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Flour

Cooked QuickWettable rice cooking ground substitutes rice batter/paste

Centralised cooking

Cooked products

Baby food

Cooked granular products

Baked products

Canned rice Retort rice

Noodle

Bread

Frozen rice

Cakes

Cake

Fermented cakes

Cracker

Fries

Cookie Muffin

Flaked rice Popped rice

Miscellaneous products

Diverse snacks

Puffed rice

Pet foods

Rice flakes

Brewing

Shredded rice

Germinated brown rice

Pinipig, Emping Hurum, Bhaja chawal

Crackers Puddings

Fig. 9.1

Rice products and their classification.

are available here. Production of milled rice in USA was approximately 6.1 million tonnes in the year 2000 (Coats 2003), about 42% of which or about 2.56 million tonnes was exported. According to Wilkinson and Champagne (2004), use of milled rice for processed foods in the marketing year 19992000 was approximately 1.2 million tonnes. In other words, around the turn of the millennium about 2.3 million tonnes of rice was used as table rice and about 1.2 million tonnes as rice products in the USA. One should note that about 62% of the 1.2 million tonnes was used for brewing and for pet foods, leaving about 0.5 million tonnes for making human food products. Descriptions of the various products, their methods of preparation and their properties have been extensively covered in the standard textbooks on rice (Houston 1972, Juliano 1985, 2003, Luh 1991a, Champagne 2004) and in a detailed review (Juliano and Hicks 1996). The following discussion is based largely on the above presentations. Only recent or crucial references are cited here. The above textbooks may be consulted for additional process details and references. The emphasis here is not on the methods of production but on the quality characteristics of the products themselves and of the rice required for their production.



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9.2 Table rice 9.2.1 Rice cooked at home Cooked, wholegrain table rice is the form in which rice is overwhelmingly used. Cooking for this purpose is almost universally done at home. Rice is cooked in water in quantities as required for the family meal and served.

9.2.2 Table rice from outside the home However, a very small quantity of table rice is made commercially outside the home. A part of this comes from food service meal systems. A small quantity is provided by centralised cooking systems. Distributed cooked rice In modern technological societies, where a large number of people work in large organisations employing large groups of people, or numerous students in schools and colleges, distribution of cooked food for lunch becomes feasible. Among rice-eating societies, such a situation is true of Japan. The Japanese are overwhelmingly rice eaters and Japan is a modern technological society, which combination is ideally suited to the development of a centralised cooked-rice distribution system. Basically for such a system the primary arrangement has to be a centralised cooking system combined with an efficient, fast and hygienic distribution system. Juliano and Sakurai (1985) described the continuous cooking-distribution system being operated in Japan. The system was developed based on research to optimise not only the mechanical machine aspects but also the individual steps of cooking per se. Once the cooking was complete, arrangements were required to put the rice in appropriate containers and for their movement through the public transport system and their distribution to appropriate destinations. The entire process had not only to be technologically and economically efficient but also had to produce a product which was hygienic and of desirable taste. Clearly there is nothing special about the suitability of the raw material rice for such a product. Whatever variety and type of rice the target consumers are accustomed to or have the liking for would be used in the process. The product quality mainly depends on the efficiency of the process developed for the cooking as well as for the distribution. Canned rice If we move away from centralised production and distribution of food for a large number of people eating together in institutions under a single roof, cooked rice can also be produced and distributed for individuals or families eating at homes. This happens in the form of small quantities of rice cooked and put in cans or pouches. Canned rice was one of the earliest rice products introduced into the



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market. The first attempt was made nearly a century back. Willison (1926) was the first to file a patent for canned rice which was followed by several other patents (see Burns 1972, Burns and Gerdes 1985 for early references). Raw rice was necessarily used in those initial attempts, for it was the only form in which rice was then available in the USA. Around World War II or thereabouts, news about parboiled rice and its extraordinary qualities (see Chapter 8) reached the Western rice industry circles. Several patents were taken out on modern methods of parboiling and a parboiling industry soon grew up in the USA. It came as a boon to the rice products industries. Efforts to make canned rice from parboiled rice showed immediate promise (Roberts et al. 1952, Ferrel and Kester 1959). Cooked parboiled rice being firmer in texture and not much frayed during cooking (see Chapter 8), the cooked rice stood up to the punishment of retorting much better than raw rice. Meanwhile Ghosh and Sarkar (1959) determined the effects of various salts (sodium chloride, sodium sulphate, calcium chloride, magnesium salts) on the rate of water absorption and the amount of solids loss during cooking of rice. This information was helpful in selecting appropriate conditions. There were several other step-by-step improvements and innovations that the industry went through and fairly satisfactory products were being marketed. The potential products for canned rice are rice in soups, salads, rice dishes, desserts, baby foods. However, the inherent difficulties in canning, deshaping of rice and availability of newer products in rice in pouches prevented accelerated development of this industry. The major problem in preparing canned rice is two-fold. One is mashing of the product or splitting of the grain and fraying of its edges (Fig. 9.2). The second is loss of solids during cooking and also into the canning liquid which is a direct loss to the industry in addition to causing unfavourable consumer response. Any movement of particulate matter in a mashed rice

Fig. 9.2 Canning stability of: left, untreated control parboiled rice; right, epichlorohydrin-treated. Reprinted, with permission, from Rutledge and Islam (1973). Copyright 1973 American Chemical Society. © Woodhead Publishing Limited, 2011


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product into the canning liquid or soup is liable to make the liquid turbid and so somewhat unacceptable. Efforts made to overcome the above deficiency were four-fold. First were fine adjustments of manufacturing conditions and production systems, including washing, precooking, rate of temperature increases, addition of salts, etc. These efforts helped, but only a little. The second was the introduction of parboiled rice. This step immediately upgraded the product quality quite a few notches because of the inherently superior integrity of cooked parboiled rice (Fig. 9.3). This is not to say that production of canned rice became very successful. Only the deficiencies were reduced. Meanwhile a well-coordinated, well-planned, inter-laboratory collaborative study of the eating quality of rice was initiated in the USA in the 1950s in the wake of the release of Century Patna 231 (see Chapter 7). This coordinated research revealed the factors involved in cooking and eating quality of rice, showing especially the crucial importance of the rice amylose content. This aspect of varietal difference then became a third source of innovation in canned-rice production, for it became clear that US short- and medium-grain rice (low amylose) were not at all suitable for canning. Research on this aspect was carried out both in the USA and in France. Webb et al. (1968) screened four thousand rice varieties in the United States

1

2

3

4

5

6

Fig. 9.3 Photographs of cooked rice (high amylose, BT variety). 1, 2, raw rice; 3, 4, mild parboiled; 5, 6, severe parboiled; 1, 3, 5, normal cooking; 2, 4, 6, pressure cooking.



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Department of Agriculture’s (USDA) world collection. Parboil-canning stability, expressed as loss of solids when rice was heated with excess water under pressure in a standardised test (Webb and Adair 1970), correlated very well with the amylose content, starch-iodine blue value, alkali digestion score, kernel width and protein content in 2322 nonwaxy varieties. The USDA derived a formula on this basis to determine the suitability of a variety for canning. Needless to say the amylose content was the most important criterion. Feillet and Alary (1975) and Alary et al. (1977) similarly tested 49 French varieties in France. Their conclusion was that the firmness of the canned rice was mainly proportional to the amylose content. A fourth stage was an attempt to strengthen the rice grains by chemical treatment. Rutledge and his colleagues (Rutledge et al. 1972, Rutledge and Islam 1976, Islam et al. 1974) tried cross-linking of starch in rice with either phosphorus oxychloride or epichlorohydrin to improve the structural integrity of the rice. The cross-linking no doubt improved the grain integrity to a dream level (Fig. 9.2) but it had its problems. Besides, US Food and Drug Administration (FDA) approval has been pending. Apart from these, efforts have also been made for innovation in parboiling technology. Thus Unnikrishnan and Bhattacharya (1987) showed that ‘pressureparboiling’ (see Chapter 8) under certain conditions gave a product which would stand upto the punishment of retorting much better than conventional system of parboiling. Retort rice (rice in pouches) The other form of distribution of cooked rice is in the form of rice in retort pouches. After the development of flexible heat-sealable pouches, the system of canning not only of cooked rice but also of other food material is being gradually replaced by production in pouches. The principle is largely the same as in canning. Rice is either cooked and put in pouches or rice and water are taken in pouches. The pouch is then sealed and retorted under appropriate conditions. Use of retort rice has been increasing in recent times, for these are extremely convenient for use of single persons or even by small families for dinner. All one does is put the pouch in hot water for a few minutes, or alternatively, puncture and microwave it for one minute and the material is ready for serving. The system is very versatile. For not only one can produce plain rice in this system but it is also suitable for production of a complete meal by mixing with appropriate sauces, spices, vegetables or meats or a combination thereof. The product quality here depends as much on the development of the optimal production process as on the variety or source of the rice. However, one must bear in mind that here again, as in the case of canning, a higheramylose rice is more likely to yield a better product, with lesser grain distortion or fraying than a lower-amylose variety. Similarly an appropriate level of parboiling is essential to make the rice tolerate the stress of retorting. © Woodhead Publishing Limited, 2011


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Frozen rice Another alternative method of distribution of cooked rice ready for consumption as table rice or in mixed cooked form (e.g., pilaf) is as frozen rice. Basically in this system rice is cooked and then frozen. It is then preserved and distributed in a frozen condition. Whether rice is distributed in individual pouches, or is centrally made in cooking centres and then delivered to chain restaurants or eating outlets, the frozen rice is heated in microwave ovens and then served to customers at homes, eating centres or in dining cars and trains. Rice alone or rice with various combinations of sauces, vegetables or meat can be made in this way. Luh (1991b) has provided references as well as details of the process. Here again the quality of the product is basically an outcome of the process more than of the variety of rice used. However, retrogradation of starch is induced by freezing, which aspect has to be kept in mind. This aspect has been studied by Roseman and Deobald (1959).

9.2.3 Quick-cooking rice This is a product where cooking at home is not completely avoided, merely reduced. Milled rice requires anything between 15 and 30 min of boiling in water to cook, depending on the size and shape of the variety. Parboiled rice needs at least one and a half times of that duration of boiling to cook. Processes were developed starting around the middle of the last century to enable rice to be cooked in much less time, sometimes as little as 5 min. A large number of patents have been taken out on these processes from the time when the General Foods Corporation in the USA for the first time took out a patent on this subject (Ozai-Durrani 1948) and newer patents continue to be taken out. The basic principle of these processes lies in precooking the rice, followed by drying it in such a way as to leave a large number of cracks or to make the product somewhat porous so that the product hydrates and cooks quickly. Alternatively the grain is micro-damaged by suitable dryheat or freezing techniques. The basic processes consist of a few well known systems, indicated below. Roberts (1972) and Luh (1991c) have given more detailed descriptions of these processes. One system is the soak–boil–steam–dry method. In this method milled rice is soaked, partially or fully cooked, then steamed and again cooked and so on until it attains a moisture content of ~70%. The cooked rice is then carefully dried so as to maintain a fissured structure. Alternatively the partially cooked rice is bumped or rolled, i.e., lightly flattened, and dried. The increased surface area enables the product to cook faster. In another approach, the bumped grain above can be lightly toasted to generate partial puffing which further helps in quick hydration. The same can also be done without bumping. The grain is gelatinised or cooked and then dried. The dense, glassy cooked grains are lightly puffed at an appropriate temperature to yield a porous product which hydrates quickly. © Woodhead Publishing Limited, 2011


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In addition, milled rice itself can be adjusted to a suitable moisture and be lightly gun-puffed. The resulting porous material hydrates quickly and becomes a quick-cooking rice. Processes using suitable chemicals, such as phosphate, during cooking are also used to develop products that can cook relatively faster. The structure of uncooked milled rice can also be lightly damaged by suitable treatment to enable it to absorb water quickly. Thus milled white (or brown) rice may be heated in air to develop fissures in the structure. These fissures enable the rice to absorb water more rapidly and thus cook quickly. Alternatively raw or cooked rice can be frozen and then freeze dried. The resulting fissured structure enables the grain to cook quickly. Recently there has been an entirely new development. Uncle Ben’s company has developed an altered process of milling parboiled rice such that it yields quick-cooking rice without further cook-processing (Lin and Jacops 2002). In this process freshly parboiled rice is milled by a special process when still somewhat moist, such that the grain is claimed to develop internal stresses resulting in micro-cracks within the internal structure that are not visible outside. These cracks enable the rice to cook quickly. This is a major innovation, for a special merit of this process is that rice treated in this manner looks like normal milled parboiled rice. The appearance does not give an indication that it has been processed to produce quick-cooking rice. Products from all other processes described above, on the other hand, are sufficiently altered in their appearance to make it evident that these are processed products (Fig. 9.4).

1

2

3

4

5

6

Fig. 9.4 Photographs of: 1, parboiled rice (commercial); 2, 3, 4, 5, quick-cooking rice (commercial: different brands); 6, quick-cooking rice of basmati (RRDC). Samples 1–5 were all purchased from US supermarket. Courtesy, RRDC, Mysore.



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The quality of quick-cooking rice is virtually independent of the raw material rice to be processed. Any rice can be processed in these ways. Whatever rice is prevalent and in use in the particular community in question can be processed thus. The product quality therefore depends mostly on the process being used far more than the variety being processed or its chemical or physical attributes. However, one should remember that, being precooked and somewhat porous, these products are liable to turn rancid faster than regular rice. And the quality characteristics of the starting rice will always remain.

9.3

Rice flour and products thereof

9.3.1 Rice flour Strictly speaking, rice flour is not a product in the sense of something that can be heated or boiled or cooked by the final consumer and eaten. However, rice flour is an important commercial commodity today which is fairly widely traded and used for preparation of various products, especially in industrialised countries (see Fig. 9.1). Rice flour is regularly made at homes and catering establishments in Asian countries too; but that is not as a separate product, but as an intermediate material – mostly as a batter or a paste. It is used here as part of an integrated process of making various types of traditional home products such as cakes, mochi, puto, idli, suman, noodles, etc. Most often grinding, usually wet grinding, of rice is the first step in the preparation of such home-made foods. Many products are now being made in industrial scale using rice flour as the starting material. This is true not only of Japan, a rice-eating country, where such rice products were being traditionally made and used at homes for a long time, but also of Europe and the USA for manufacture of relatively newer products. It is now required for production of baby foods, extruded breakfast cereals and snack foods, as also pet foods. Besides, it is used in baking of breads, pan breads, waffles, pizza, muffins, biscuits and cookies wherever wheat cannot be used. It is also used as dusting flour for separating dough pieces, for pan release and to impart crispness. Rice flour as an independent commodity is now being manufactured and traded for such uses, including for export especially after the latest WTO trade agreement on agricultural commodities. It is usually manufactured from broken rice for these purposes, while whole and broken grains are both used for preparation of home batter for making different products. Some 15% of rice grains are broken during modern milling processes and these are the grains that are most commonly used in the manufacture of rice flour in modern commercial practice. Second head rice (pieces of rice approximately half the length of a full grain) is most widely used but a part of the brewer’s



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rice (less than half grain size) may also be used. Special care for cleaning has to be exercised in the latter case. One important reason for such use of rice flour is its nonallergenic nature. There is a small proportion of the population who are allergic to wheat gluten. Rice is widely used in baking for such wheat-intolerant people, i.e., those suffering from coeliac disorder. Its nonallergenic property also makes rice as one of the first cereals to be used in infant feeding. Besides, one strong point in favour of the use of rice flour lies in its versatility. First, rice varieties differ widely in their inherent qualities (see Chapter 7), which enables different use values to be imparted to the flour. Second, the flour can be prepared either by dry or wet grinding, which again imparts different characteristics into it. Third, there are different types of mills (grinder) which impart different properties to the product, including difference in particle size and starch damage. Fourth, one can use either raw rice (regular rice) or parboiled or otherwise pregelatinised rice, producing flours with different properties. Fifth, while flour is normally made from milled (polished) rice, one can also prepare flour from unmilled brown rice which imparts a different flavour as well as texture to the product made from the flour. Sixth, flour made from fresh and aged rice would obviously have different product property profiles (see Chapter 5). Clearly rice flour has a wide versatility in terms of use for diverse products. In addition rice starch granules are among the smallest in size (3–10 mm), which impart special properties to it, including its use as fat replacement. In general one can say that traditional use of rice flour by and large corresponds to the prevailing varieties in use as table rice. However, special products may call for special varieties. For instance, certain products can be made only from waxy rice. Waxy rice flour has excellent thickening property for sauces, gravies, puddings and oriental snack foods. Its special advantage lies in the fact that waxy rice starch is the least susceptible to retrogradation among all waxy starches. Its paste can undergo several freeze–thaw cycles without liquid separation (syneresis). This property led to the use of waxy rice in many manufactured products. However, certain products, for example noodles, are best made from high-amylose rice which provides a better texture and cook-stability to the product. But high-amylose rice may be unsuitable for other products, for example, pancake mixes and expanded rice. Similarly certain high-amylose rice, e.g., IR 8, provides a harsh, dry and crumbly texture to bread. Rice flour is prepared by wet, semi-wet and dry grinding methods (Fig. 9.5). By and large wet grinding is used for making traditional products, more often in the form of batters and pastes as part of the integrated process for making the products (see Fig. 9.1). Flour per se is less often made by wet or semi-wet processes. Wet grinding has the advantage that it reduces starch damage. The presence of water provides cooling as well as lubrication



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Product-making quality of rice Brown rice Milled rice Broken rice

Soaking

Draining

Wet milling

Semi-dry milling

Drying and grinding Rice flour

Cleaning

Dry milling

Rice flour

309

Noodles Cakes Crackers Rice paper Egg roll wrapper Baked products (breads, muffins, crackers, cake, puddings) Puffed and extrusion-cooked (baby foods, breakfast cereals, snacks)

Air classification or enzyme digestion

Fig. 9.5

High-protein rice flour

Instant milk Gruel Pudding

Manufacture of rice flours and their applications. Reprinted, with permission, from Yeh (2004).

during grinding so that the starch granules largely escape being damaged. However, the disadvantage is that the material has to be dried involving extra expenditure and there is the problem of disposal of the waste water. The cost is minimised in semi-wet grinding where excess water is removed before grinding and the final ground material is dried either in natural air or by heated air. Wet or semi-wet ground flour is used for making various cakes, crackers, noodles, traditional puto, idli, etc. Dry-ground flour has more starch damage but can be used for baked products, baby foods and extruded products. Dry-ground product has a composition identical to that of the rawmaterial rice. Wet or semi-wet ground flour on the other hand has a lower content of protein and other constituents (lipid, ash, sugar) (Table 9.1). In dry milling, different grinding equipment produce different particle sizes (Fig. 9.6). Turbo and hammer mills in particular cause considerable rise in temperature, resulting in more starch damage. Wet-milled flour is especially good for bread and other baking, though it is not often used because of the higher expense. One way of ameliorating the disadvantage of dry-milled flour is by its intensive mixing with water before use. The intensive mixing improves the texture of bread and the texture and volume of white layer cakes (Bean et al. 1983, Bean 1986). A roller grinder is best for baking. A flour having a particle size such that 50% passes through a 100-mesh screen is best for baking. Finer flour has more starch damage. Certain products require precooked or pregelatinised starch. Flour made from parboiled rice or quick-cooking rice may be suitable for such purposes.



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Table 9.1

Chemical compositions of rice flours from various milling processes

Variety

Milling

Protein (%)

Lipid (%)

Ash (%)

Reducing sugar (%)

TNu 70

Dry Semidry Wet Dry Semidry Wet

8.02 7.55 6.67 7.91 7.56 5.70

0.41 0.03 0.03 0.27 0.08 0.03

0.45 0.15 0.17 0.57 0.20 0.22

0.90 0.12 0.15 0.74 0.16 0.15

TCS 10

Reprinted from Lu and Lii (1989). Through sieve number 200

100

140 120 100

70

50

Cumulative percent of sample

80 Pinmill 2X Pinmill 1X Hammer mill

60

Turbo mill Roller (quad. jr) Blade (wiley)

40

Burr (bauer) Burr (coffee) 20

0

0

Fig. 9.6

50

100 150 200 Particle diameter (micron)

250

300

Cumulative particle size distribution by weight for rice flours ground by various mills. Reprinted from Nishita and Bean (1982).

Alternatively the starch cake or the dough is cooked by steaming before drying for such purposes. Brown rice flour imparts some special properties to the products. It imparts a more chewy texture to baked products which is liked in some markets. However, one difficulty of brown rice flour is that it is likely to show development of free fatty acids due to the presence of the enzyme lipase. One way out of this difficulty is to add stabilised ground bran to milled-rice flour.



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High-protein rice flour is another product which has application in infant foods. It can be prepared either by gradual abrasion of the outer layers of the rice grain which contain higher levels of protein (Kennedy et al. 1974 ), or by air classification of normal rice flour (Houston et al. 1964). Alternatively it can be made by partial digestion of the starch in regular rice flour by enzymes (Hansen et al. 1981). Finally rice flour is often used for baking for many purposes. One objective is to make up any shortage of wheat flour. Secondly, rice bread and other baked rice products are especially used for infants, for the nonallergenic property of rice, and for patients who cannot use wheat products (coeliac disorder patients). However, bread cannot be made from rice flour alone. As rice does not contain gluten, rice alone cannot provide leavening, hence incorporation of some gums and mucillages, such as hydroxypropyl methylcellulose, is essential.

9.3.2 Cooked/semicooked products from rice flour A variety of cooked or semicooked rice products are traditionally made throughout the rice countries of Asia, more especially in southeast and east Asia, for use as supplementary snacks or breakfast. Noodles are exceptions, for they are used as substitutes for rice in the main meal. All these products are more usually prepared from wet or semi-wet ground flour (batter or paste) produced as part of the process itself and then partly or fully cooked (see Fig. 9.1). Noodles Noodles are basically structured products made from flour obtained by initially pulverising whole (structured) grains. Rice is one cereal which is eaten after cooking that is already in granular form. The granular structure of rice was probably broken before the rice was cooked in order to produce the rice again in other granular forms, so imparting variety and newness to the diet. Besides, this way one could also use broken grains that are invariably obtained during milling of rice. Noodles, pasta and macaroni are well-known structured food products made from wheat flour in the West, especially in Italy. In the same manner rice can be ground and formed into noodles. This is a very popular food throughout southeast and east Asia. It is interesting that while noodles are so popular in the regions just mentioned, they are not so popular in the other important rice-growing region, viz. south Asia. The basic process of preparing noodles consists of wet milling of rice followed by forming a dough and steaming or otherwise cooking it partially in water. This is then kneaded and then extruded in a simple press either by hand at home or mechanically in the cottage industry. The extruded noodles are then cooked in boiling water or by steam followed by drying (Fig. 9.7).



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Fig. 9.7

Photograph of noodle.

There are any number of variations in this process both for small-scale and for large-scale preparation at home or in industry. Noodles can no doubt be made also from dry-milled flour but that is not the most appropriate. Dry milling causes partial surface gelatinisation, which renders the very fine particles somewhat soft and sticky. Besides the particles are not as uniform in size as after wet grinding. So some of the particles are somewhat coarse which makes the products somewhat rough. Rice quality plays an important role in noodle quality. High-amylose rice varieties make much better noodles compared to lower-amylose varieties. They provide a better ordered structure, appropriate strength and lower density (higher swelling) and also white colour. For instance, high-amylose, lowgelatinisation temperature (GT) rice such as Taichung Native 1 is considered to give the best structure. It is interesting that in Japan too, where indica rice is rarely used for table rice, noodles are usually prepared from indica rice. Bhattacharya et al. (1999) and Yoenyongbuddhagal and Noomhorm (2002) confirmed that a high-amylose content and hard gel were correlated with better quality of noodles. The method of milling (grinding), particle size and degree of starch damage were also correlated with quality. Particle size 15

0-5

HA: Intermediate HA: Soft IA: Aromatic IA: Normal IA: Bulu LA WX

> 26 > 26 22-26 22-26 22-26 15-22 7.4 > 5.6 8.3 8.8 7.2 7.0 7.4 8.8

6.2 6.3 5.6 8.3

– – – – 4.8 4.5 4.6 7.7

Reproduced, with permission, from Shams-ud-Din and Bhattacharya (1978) John Wiley and Sons. a DM as per loss of weight of brown rice upon milling. b BS variety rice was milled by different laboratory millers to different degrees of milling (by weight). The resulting rice samples were tested for visual DM response by two methods of residual-bran visualisation.

The above would imply that the degree of milling of rice, as might be defined by weight loss of the brown rice kernel on the one hand, and as analytically or visually determined by product quality (colour, whiteness, staining, fat or phosphorus content or whatever) on the other, may not necessarily be identical. A given weight loss may correspond, even in a single lot of rice, to slightly different product qualities as defined by visual appearance or chemical reaction, depending on the mill type used. This situation produces a dilemma. While grain weight loss might appear to be a natural basis of reckoning the degree of milling, as a consumer or as a practitioner it might be as (or even more) logical to think of a colour change (or change in whiteness) or a change in some chemical (say B1 or surface-fat content) to be a more appropriate index of the degree of milling. But then this principle has an inherent handicap. Weight or material loss is something we can see, perceive and measure. It is a natural standard scale by itself. For a constituent chemical or colour, what would be the reference scale, especially when the amount of the constituent or colour may vary among varieties? The way out of this dilemma might be to prepare weight-loss versus colour- and chemical-change curves for different mills as in Fig. 13.21 (top). Then taking a metal-roller whitener such as the McGill curve as a standard – for milling in this mill has been shown to remain more confined to the ‘true bran’ region – one can determine the standard weight loss by comparing the curves for equivalent colour or chemical change. For instance we can see from Table 13.5 that 12.6% DM produced by the Minghetti mill is equivalent to 8.4% DM from the McGill in terms of alcoholic alkali bran-staining test. So the Minghetti sample, 12.6% DM as obtained, can be designated nominally as 8.4% DM (assuming that the McGill mill is universally accepted as the standard).



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Another incidental result of these findings is the light they throw on the efficiency of whitening equipments. Clearly the emery mills need to remove more kernel matter to yield rice with a given degree of whiteness or some other constituent than the metal whiteners. Therefore, other things being equal, it may be more profitable to use metal-roller whiteners particularly for high DM and thereby obtain slightly higher mill outturns.

13.7

Hydration and cooking quality of rice

The predominant way rice is consumed is as table rice. For this, rice is cooked in boiling water to soften it and gelatinise the starch. So cooking of rice involves both hydration and gelatinisation. Hydration of rice as well as its cooking process concern important quality features of rice.

13.7.1 Equilibrium moisture content attained by rice upon soaking in water at ambient temperature (EMC-S) It is true that rice grains in practice are rarely hydrated in water at ambient temperature. If at all, this is done only preliminary to making some product from rice (flour, cakes, etc.). Predominantly rice is cooked only at boiling temperature. Nonetheless, hydration at room temperature is an important property of rice and throws light on a number of other parameters that affect rice quality. For instance, rice varieties differ in the equilibrium moisture content they attain when they are soaked in water under ambient conditions (EMC-S). This difference arises from physical as well as chemical characteristics of rice: EMC-S is affected negatively by amylose content and gelatinisation temperature (GT) and positively by kernel chalkiness (Indudhara Swamy et al. 1971). Thus the EMC-S of milled rice goes up from a low of about 27.0% for high-amylose, intermediate-GT, vitreous-kernel varieties to about 36% for low-GT waxy rice (Table 13.6). This reflects varietal difference related to both physical (chalkiness) and chemical (amylose, GT) factors. But EMC-S is affected still more by processing, especially hydrothermal processing. For example parboiling and flaking can produce rice with EMC-S values anywhere in the range of 40% to75% (wb, which translates on dry basis to 67–300%) (Table 13.6). Clearly EMCS-S is a very useful tool to study rice quality. The determination of EMC-S is simple. A small quantity (5–15 g) of rice is washed roughly with water and is covered with enough distilled water. After overnight soaking (for milled rice; soaking should be continued for three days in the case of paddy), the grain is strained, surface-dried, and tested for its moisture content by the abridged oven method (see page 442). Instead of the EMC-S, the ambient-temperature hydration property can be determined in terms of the water absorption index (WAI). This index also provides more or less an identical information although the numerical © Woodhead Publishing Limited, 2011


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Analysis of rice quality

475

Table 13.6 Approximate values of equilibrium moisture content attained by some rice and rice products when soaked in water at ambient temperature (EMC-S) Rice type Process

Type

Amylose

GT

Chalkiness

Approximate EMC-S (% wb)

Raw milled rice

– – – – – –

High High Intermediate Low Low Waxy

Intermediate Low Intermediate Low Low Low

Nil Medium Nil Nil High –

27 29 29 31 33 36

Steam-parboiled rice

Mild Medium Severe

High High High

Intermediate Intermediate Intermediate

40 50 56

Dry-heat parboiled Medium High rice Moisture treateda

Intermediate

– – – –

Intermediate Intermediate Intermediate

– – –

60 70 >75

Flaked rice

Thick Medium Thin

High High High

65 55

a Dry-heat parboiled rice moistened (~30% moisture) and tempered to promote amylopectin retrogradation, then dried.

values obviously differ. Estimating WAI also enables another index to be determined at the same time, the water solubility index (WSI). For estimating these indices, the rice is ground to about 30- to 40-mesh flour. An accurately weighed quantity (2–5 g) is taken in a tared centrifuge tube and covered by 50 ml distilled water. The mixture is allowed to remain undisturbed for one hour. After that, it is centrifuged and the weight of the residue is determined after decanting the supernatant. The difference in weight gives the amount of water absorbed which when divided by the weight of the rice flour, gives the WAI (Anderson 1982). The supernatant is then evaporated and weighed. The amount of dissolved solid is expressed as a percent of the original rice and expressed as WSI. Properly carried out, the WAI (and WSI) value gives similar information as the EMC-S. It is widely used in starch research, obviously because the material is already in powder form. Flours and powders are more suitably tested by WAI test, but grains and material which are in the form of particles of substantial size are more amenable to EMC-S study.

13.7.2 Cooking of rice Rice has to be cooked in boiling water before consumption. The cooking behaviour can thus be considered as a notable property of rice. In addition, many properties of rice such as texture and sensory qualities can be tested



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only after cooking. Cooking and its consequences are thus very important quality-testing steps for rice. The method of cooking rice itself varies from person to person. Some cook rice by open-pan boiling in excess water with heating from below. The excess cooking water is later on discarded or used separately. Some cook rice in a similar way but with an exact amount of water (predetermined by a rule of thumb) that is allowed to be fully absorbed. Some cook in a doubleboiler in steam and some cook in a pressure cooker. Nowadays cooking is also done in an electrical rice cooker with a predetermined amount of rice and water. Not only the method of cooking (i.e., heating), even the rice : water ratio may vary from culture to culture – although unfortunately there are no precise or even broad data about these aspects. The laboratory methods for cooking of rice also vary accordingly from laboratory to laboratory. No universally agreed uniform method of cooking exists. Each laboratory follows a system as evolved traditionally by its practitioners. This variation exists not only in the procedure of different steps but also in the rice : water ratio. The variation is quite disturbing and creates a good deal of confusion with regard to comparison of properties of cooked rice observed in different laboratories. Some common cooking parameters and their basic procedures are described below first before taking up the procedure of cooking per se. Water uptake by rice during cooking Water uptake by rice is defined as the amount or weight of water absorbed by a given amount of rice (g/g) in an arbitrarily fixed time period (say 20 min) in boiling water. It is true that this parameter does not provide much useful information with respect to varietal difference in rice, as discussed extensively in Chapter 6. However, it has some value in interpreting the effect of various processes, such as ageing, parboiling, etc. This parameter can be determined by putting a small quantity of rice (1–2 g) directly into hot water inside a tube that is already kept immersed in a boiling water bath. The rice is strained after a predetermined time (15–30 min), surface dried and weighed. The increase in weight gives the amount of water absorbed. This amount when expressed in terms of unit weight of rice gives the apparent water uptake. It is called apparent water uptake (W¢) because it does not take into consideration the amount of solids lost. The true water uptake can be determined by estimating the grain moisture before and after cooking: W =

Mc Mo 1 + Mo

in which Mc and Mo are the moisture contents (dry basis, g/g) of the cooked and original rice, respectively.



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Loss of solids during cooking The amount of solids (dissolved as well as undissolved) that leaches out from the rice grain during cooking and comes into the cooking water is another property of rice. It does show some varietal difference, although no clear relationship of this index with any other varietal characteristic in rice has so far been found (see Chapter 6). This property is perceptively affected by ageing (Fig. 13.22) and by any processing such as parboiling. The property is thus of some value in studying ageing and processing of rice. The parameter is simply determined by collecting the excess cooking water, drying and weighing. The amount of solids lost is expressed in terms of the original weight of the rice taken for cooking. In theory, determination of the optical density of the excess cooking water (appropriately made up in volume) should provide a good index of the solids lost. But the reproducibility is poor. Some have tried to add iodine to the liquid to measure the colour (with or without centrifugation), but that indicates more the dissolved solids, if at all. Webb and Adair (1970) and Webb (1979) described a method for determining solids loss during parboil-canning of rice. Apart from direct weighing, the solids loss (s) during cooking can also be calculated from the true (W) and apparent (W¢) water uptakes (g/g on dry basis) by the formula (Indudhara Swamy et al. 1978): s = W W ¢ ¥ 100% 1+W

Fig. 13.22 Excess cooking water of rice showing high (left tube) and low (right) loss of solids in fresh and aged rice, respectively. Photo: courtesy, T. Dhanalakshmi.



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Cooking time Cooking time can be determined by the Desikachar and Subrahmanyan (1961) method. A small quantity of rice is put directly into water already preheated in a test tube by keeping it immersed in a vigorously boiling water bath. A few grains are removed at intervals and pressed between two glass slides. The time at which the opaque central core just disappears is the cooking time. In the literature, this test is often referred to as the Ranghino (1966) test. Elongation ratio and ‘rings’ A relatively high kernel elongation during cooking is considered an important desirable characteristic of the basmati group of rices. The test commonly used to measure this trait is a modification by Juliano and Perez (1984) of the original method by Azeez and Shafi (1966) The principle is to presoak the uncooked rice in water, then cook the soaked rice for a set time by putting it directly into hot water. The ratio of the average length of the cooked grain to that of the uncooked grain is the elongation ratio. Any ‘rings’ in the cooked rice are also observed simultaneously (Fig. 13.23). This is a characteristic property of the basmati group (Kamath et al. 2008). A quantitative measure is not essential, but the number of grains having rings and the number of rings per grains are broadly noted. The extent of ring formation is expressed in terms of a hedonic scale (very high to nil). Grain defects in cooked rice Some proportion of rice grains undergoing cooking may show various kinds of undesirable traits. These traits or defects include breaking of the grain, curling, bursting, etc. (Fig. 13.24). These defects are not generally well revealed when a small quantity of rice (2–10 g) is cooked in excess water,

HBC 19

Sharbati

Fig. 13.23 Photograph showing good ‘rings’ in rice after cooking in HBC 19 (left) and poor ‘rings’ in Sharbati (right). Reproduced, with permission, from Kamath et al. (2008).



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A

B

C

D

479

Fig. 13.24 Grains showing defects developed during cooking of rice. A, normal grains; B, burst grains; C, curled grains; D, broken grains. Source: RRDC (unpublished).

say, in a tube, as is done for some tests as above. It is best for this purpose to presoak at least 50–100 g rice and then cook it either in a beaker in excess water or in an electric cooker. A portion of the cooked rice is now put under water in a plate, when the defective grains can be separated and counted. Volume expansion of rice during cooking Rice expands upon cooking. Rice grains if cooked right up to the grain centre in boiling water absorb about 2.5 times their weight of water (Bhattacharya and Sowbhagya 1971). Each grain thus expands in volume but this expansion of the grain per se is not a useful index for measurement, for that is more or less a constant. If all varieties absorb 2.5 times of their weight of water at full cooking, expansion in true volume also would be identical. But increase in bulk volume need not be, in fact would not be, the same. Variation in the increase in bulk volume happens primarily due to variation in the stickiness of the cooked rice. Grains of rice that is sticky would remain clinging to each other and so would not increase so much in bulk volume. On the other hand, a variety that remains flaky and nonsticky after cooking would expand much more even after absorbing an identical amount of water. The amount of



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expansion in cooked-rice volume is thus a reasonably good index especially of the stickiness of cooked rice. For this reason, the volume increase upon cooking becomes an excellent index especially when studying the ageing of rice. Freshly harvested rice is very sticky when cooked and shows a very low cooked-rice volume. The volume increases after ageing and becomes a good index of the extent of ageing. A volume expansion test can be used for this purpose (Desikachar 1956b). The test is very simple. Weighed 20 g of rice is cooked with 50 ml water in steam in a graduated boiling tube and the final volume is noted (Fig. 13.25). Soon after harvest, different varieties of rice thus cooked would normally show a volume of approximately 65–70 ml. After ageing for six months or more, the volume would be close to 90 ml for high-amylose rice, 85–87 ml for intermediate-amylose and 80–85 ml for low-amylose varieties. Knowing the variety, the cooked volume thus gives a fair idea about the extent of ageing of rice. A varietal difference in cooked-rice volume, primarily based on the amylose class, also exists but this difference is relatively small. Also it can be recognised only if the rice samples are of the same age. The rice age has a greater effect on the cooked-rice volume then the varietal difference.

1

2

3

Fig. 13.25 Photograph of rice cooked in graduated tubes illustrating poor volume expansion of freshly harvested rice (tube. 1) and improved expansion after its ageing (no. 2) or ‘curing’ treatment (no. 3). Photo: courtesy, K. R. Bhattacharya.



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Cooking of rice in the laboratory Rice has to be cooked in the laboratory for various tests. Apart from testing for elongation ratio, various cooked-rice defects and so on, cooking is necessary to test rice for its texture and sensory response. Unfortunately cooking can be considered as one of the weak areas of rice research. Researchers do not follow an agreed standard method of cooking. There is actually no standard method of cooking at all. In fact there is not even much of a discussion on rice cooking methods. As a result the methods adopted vary from laboratory to laboratory. Each laboratory follows a custom as set by its tradition. This is unfortunate for the method of cooking may have a strong influence on the results of any tests. A few facts that may be relevant in this connection are as follows: ∑

∑ ∑

∑

∑

If rice is cooked by putting grains directly in boiling water, then any rice, irrespective of its amylose, GT or any property, would absorb approximately 2.5 times its weight of water by the time the grain centre just gets cooked (i.e., its opaque centre just disappears). This is a fact and not a matter of debate. Unfortunately this fact does not seem to be yet widely recognised. In any case, following from this, there would be a strong logic for cooking all rices with exactly 2.5 times their weight of water (excluding vapour losses, if any). If 2.5 was considered too high for any reason, there would be good logic on the basis of the above equality to decide that all varieties should be cooked with at least a constant rice : water ratio, e.g., 1:2 or 1:1.5. Rice can also be cooked in excess water over a heater or hot plate such that any hard or opaque centre just disappeared. This method would automatically take care of the rice : water ratio (which under this condition would automatically be 1:2.5). Presoaking is an important variable in rice cooking. Presoaking strongly affects cooking time as well as the characteristics of the cooked rice in bulk and as grains. In domestic cooking, partial presoaking is the norm, for one usually washes rice and puts it in water taken in a vessel and only then starts heating. This way of cooking automatically involves a partial presoaking. Therefore it may not be a bad idea to adopt some presoaking of rice for a definite time, say 15 or 30 min, before the mixture is heated. Washing of the rice may be optional, but whether to presoak or not should be a matter of deliberate policy. Either should be acceptable if deliberately chosen and indicated. The rice : water ratio should also be a matter of deliberate policy. It should be decided on the basis of some experimentally verifiable logic rather than on the basis of some vague notion, heresay or tradition.

As regards the precise procedure of cooking, there are different choices: ∑

For measurement of texture, the general practice is to cook a small quantity of rice (say 5.0 g) taken in a covered aluminium cup or glass evaporation dish with a deliberately chosen rice : water ratio (1 : 2 in © Woodhead Publishing Limited, 2011


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∑

Rice quality the author’s laboratory). Again it is one’s choice whether to presoak (generally 30 min) or not. One should remember that whatever way this is done, the procedure would affect the results. Therefore the precise conditions and steps should be clearly spelled out. The cup or the dish in above case would be cooked in a loosely covered autoclave or pressure cooker in steam for 20 min. (If the rice is parboiled then it would be soaked for 1 h instead of 30 min and steamed for 30 min instead of 20 min.) The rice should be gently stirred with a spatula or glass rod once immediately after taking out from the autoclave and again after 1 h of cooling. It is then immediately tested. The precise system of heating for the cooking is another variable. One way is to cook in excess water over a hot plate. Another is to take rice and water in a tightly covered cup and cook in steam. Finally cooking can be done in a modern electric cooker. In the last case there would be loss of steam during cooking and that must be compensated for in the rice : water ratio taken.

13.8 Alkali digestion score Alkali score refers to the visually observed extent of digestion of rice kernels when they are immersed in a specified concentration of alkali. About 90% of the dry weight of rice is starch, and starch is gelatinised by alkali. So the alkali digestion score gives an indirect estimate of the GT of the rice starch. It is true that the GT of rice has not so far been found to greatly influence the eating quality of rice (see Chapter 7). Nonetheless it is an extremely important property of rice in the sense that it affects the processing of rice for making any product. If the process involves cooking of the rice, then the GT is of prime importance for it would determine at what temperature the cooking should be done. Similarly for brewing, if broken rice (brewer’s rice) is used as an adjunct, it is always better to have the grits from low-GT rather than from high-GT rice. At any rate all the grits should have a uniform GT, rather than different grits originating in different GT stocks. Heating water costs money, and the lower the GT the lower would be the temperature of heating required to cook the rice. The GT also affects the EMC-S and other ways of hydration. It also affects the eating quality and other properties in the case of waxy rice. It is an important identificatory property too. Another important use of this test is to identify varietal segregation and adulteration (as shown by abnormal grain to grain variation in score). Warth and Darabsett (1914) were the pioneers to study the alkali digestion of rice, but it was the work of Little et al. (1958) that set this property as a standard test. They immersed six grains of rice in a rectangular plastic box in 10 ml of 1.7% KOH. The extent of kernel digestion after 20 h (Fig.



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13.26) was scored with the help of a score card (Table 13.7). The test has been used extensively over the years by various workers. Bhattacharya and Sowbhagya (1972b) made an in-depth study of this property by examining the progressive digestion of rice kernels over incremental concentrations of KOH. They observed that the kernels of any variety went through increasing digestion starting from zero effect to complete dispersion with an increment of 0.7–0.8% KOH. This was true of all rice varieties, although the respective KOH concentration range differed and the pattern of digestion too differed from variety to variety. Thus there was a

Untreated (Spreading 1) (Clearing 1)

Spreading 2 Clearing 1

Spreading 2, 3 Clearing 1, 2

Spreading 3, 4, 5 Clearing 2

Spreading 4 Clearing 2, 3



Spreading 7 Clearing 7

Fig. 13.26 Alkali digestion scores of Little et al. (1958). Pictorial chart reproduced from Simpson et al. (1965).



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Rice quality Table 13.7 Alkali digestion of rice: score card Score

Description

1 2 3 4 5 6 7

Kernel not affected Kernel swollen Kernel swollen, collar incomplete or narrow Kernel swollen, collar complete and wide Kernel split or segmented, collar complete and wide Kernel dispersed, merging with collar All kernels completely dispersed and intermingled

Adapted from Little et al. (1958).

Fig. 13.27 Alkali digestion test of rice. Photo showing the successive stages through which grains of three rice varieties of different types go through when treated by increasing concentrations of alkali. Scores at bottom. Reproduced, with permission, from Bhattacharya and Sowbhagya (1972b).

natural eight-stage progression in kernel digestion in any variety, leading to a natural 8-point scoring. Besides, they found that the 1.7% KOH of Little et al. (1958) pushed most varieties towards the higher end of the digestion scale and so tended to fail to bring out fine distinctions among varieties. On this basis, Bhattacharya and Sowbhagya (1972b) selected 1.4% KOH as the test concentration and devised an 8-point score card (Fig. 13.27). Bhattacharya (1979a) further simplified the procedure by carrying out the test in a Petri dish, the amount of alkali required and the optimum number of rice grains to be used, depending on the size of the petri dish, also were specified. The score card too was finalised with slight modification later (Table 13.8) as part of an international collaborative study (Juliano et al. 1982). For the test, the 1.7% KOH and the score card of Little et al. (1958) may be sufficient for deciding in what GT class (low, intermediate, high) an unknown variety fell (Juliano 1985). But the Bhattacharya and Sowbhagya (1972b) test (1.4% KOH) along with their score card (Table 13.8) was useful for



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Table 13.8 Alkali digestion scores of rice Score Kernel consistency

Collar

Total diameter across (mm)

0

Wholly chalky

Absolutely none

–

1

Wholly chalky

Trace hairy

≤5

2

Nearly wholly chalky

Mostly on one side; slight white painty deposit in inner ring, outer ring cottony

7 (±2)

3

Substantially chalky

More on one side; wide white painty 10.5 (±2) deposit in inner ring, outer ring cottony; radially streaked

4

Fairly chalky

All around; some white painty deposit in inner ring, outer ring cottony; radially streaked

14 (±3)

5

Slightly chalky

All around; very little white painty deposit in inner ring, outer ring cottony; radially streaked

18 (±3)

6

Cottony, or cottony + gel

All around; no white painty deposit; 18 (±3) semi-transparent, faintly streaked

7

Compact gel (one or more pieces)

All around; practically transparent; smooth (i.e., not streaked)

Narrow (i.e., nearly transparent)

8

Loose gel

All around; practically transparent; smooth (i.e. not streaked)

Narrow (i.e., nearly transparent)

Adapted, with permission, from Bhattacharya and Sowbhagya (1972b) and submitted during the international cooperative test (Juliano et al. 1982).

fine distinction between varieties. For this reason it also enabled the precise GT (y) of the variety to be calculated from the score (x) (Bhattacharya et al. 1982): y = 74.54 – 1.40x (n = 157 nonwaxy rice) (r = –0.848***) or = 74.80 – 1.57x (including waxy rice, n = 165) (r = –806***). This test along with the revised score card, enabling fine distinction, was useful for varietal identification as well. Kamath et al. (2008) used this as one of the tests to distinguish and identify among over 30 lines and varieties of basmati group of rices. The test is very simple. Ten grains of rice are put in a 60 mm Petri dish and 20 ml of 1.4% KOH (precisely diluted from a stock after standard acid titration) are poured. The grains are gently rearranged at equal distance from each other and left overnight. Next day the pattern and the degree of digestion, including especially the total diameter of the digested kernels, are examined and scored as per the score card.



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13.9

Gel mobility test

This is a test devised by Cagampang et al. (1973) and discussed by Perez (1979). A dilute dispersion of the rice flour in alkali (4.4% db) is allowed to flow in a tube and the distance travelled is noted. The test has some predictive value about the texture of the rice after its cooking. The reason would be clear if the principle is understood. Starch can be gelatinised either by heating with water or by treating with an alkali solution. For instance, the pasting properties of rice flour (or starch) can be studied with a viscograph (the Brabender or the Rapid Visco Analyser, RVA), where the flour and water slurry is passed through a regimen of controlled heating and cooling. Alternatively, very similar information can be obtained by alkali-viscography, wherein rice flour is treated with increasing concentrations of alkali (Suzuki and Taketomi 1956, Suzuki 1979a). Both processes throw light on the gelatinising and pasting behaviours of the material. In the same manner the gel mobility property is a kind of a reflection of the expected texture of the cooked rice. Cooked rice can be considered as more or less equivalent to a roughly 26% rice-flour paste (cooked rice has approximately 73–74% moisture, wb). The gel mobility test involves the preparation of a 4.4% (db) dispersion of rice flour in dilute alkali (heated to make the gel transparent). The extent of horizontal flow within the tube is an approximate inverse index of the viscosity (high viscosity = short flow, low viscosity = long flow) (Fig. 13.28). The idea is that knowing this viscosity should give an approximate idea of the expected viscosity, in this case texture, of the cooked rice. This is the basic principle of the test. Cagampang et al. (1973) called it a gel consistency test, which is what it is in terms of the

Fig. 13.28 Gel mobility test of rice, showing hard (low mobility: top) intermediate and soft (high mobility: bottom) gels. Source: RRDC (unpublished).



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property studied. However, as conducted (high consistency equals short gelflow reading, low consistency equals long gel-flow reading), the values read become semantically contradictory and therefore a little confusing. That is why it may be better called the gel mobility test. The principle is theoretically sound. Yet, unfortunately, in the author’s and many researchers’ experience, the correlation between the gel mobility and the cooked-rice texture is not that good. One possible reason of the less than expected good correlation lies in the non-Newtonian character of the system. Viscosity increases linearly with concentration only in the case of Newtonian liquids. In non-Newtonian systems, the viscosity increase with increasing concentration is not linear. As a result, the ratio of viscosity at a higher to a lower concentration may not be the same from sample to sample. As starch paste is a non-Newtonian system, the ratio of apparent viscosity between a 4.4% paste (gel mobility test) and cooked rice (equivalent to approximately 26% paste) need not be the same in all varieties. This may explain the somewhat low predictability of the gel mobility test about the expected texture of cooked rice. The test is extremely simple. Accurately weighed 100 mg of 100-mesh rice flour is first wetted with a small amount of alcohol and then dispersed in 2 ml of dilute potassium hydroxide solution by boiling. The dispersion is cooled in ice and then the tube is laid horizontally on a table. The flow of the gel after 1 h is noted. Some precautions are necessary about this test. The main hazard is nonuniform dispersion of the flour at the point of heat gelatinisation, leading to lump formation. In that case the partly dispersed gel would be too thin and would run freely. Some of the precautions have been discussed by Perez (1979). She pointed out that the flour should be fine indeed, 100-mesh (preferably ground in an Wig-L-Bug amalgamator). Degree of milling should be uniform, for more or less lipid (from bran) can cause problems. According to the present author, further problems may occur in mixing. Mixing the flour and alkali by a vortex mixer is not enough, in fact it gives a false sense of safety. Vigorous shaking of the tube by the hand at the point of immersing it in the boiling bath, continuing for 30 s, is essential. Once uniform non-lump dispersion is ensured, the rest is trouble free.

13.10

Estimation of amylose content

Amylose content is the most important quality indicator of the eating and processing qualities of rice. No doubt, from what we now know, it is no longer the molecule of amylose, the linear polymer of anhydroglucopyranose, alone that is measured by this analysis. What is actually measured by the colorimetric or amperometric or potentiometric reaction with iodine is only the amylose-equivalent (AE). This is somewhat like free fatty acids (FFA)



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being expressed in terms of oleic acid, regardless of whether oleic acid is actually present or not. It can be partly amylose, but a part, probably the largest part, is actually the long branches of amylopectin. This fact is not relevant for the purpose of quality analysis of rice. What matters is that the amylose or AE that one measures by this iodine reaction correlates very well with the eating quality (texture) of rice. So the analysis of this amylose or AE is still very relevant. In the following we will designate the measured entity as amylose or AE interchangeably as convenient. It would be good to recognise one fact about this test at the very outset. This is one of the most important tests of rice quality and it was one of the earliest such tests developed in the literature. Yet there is unfortunately a lot of ambiguity about the results. Historically, there has been a large laboratoryto-laboratory and time-to-time difference or variation in the results of the amylose analysis. This is because several underlying sources of errors are not widely understood. This unfortunately happens quite often in breeding stations, where in fact the result may be of the greatest importance. The amylose method for rice was originally developed by Williams et al. (1958) by a suitable modification of the original method prescribed by McCready and Hassid (1943). However, this Williams method was not easy for routine adoption. Hence it was modified by Sowbhagya and Bhattacharya (1971, 1979), Juliano (1971), Bolling and El Baya (1975), Perez and Juliano (1978) and Juliano et al. (1981a). The problems in the original Williams et al. method, which are what caused the wide variations in the results, as well as steps to solve these problems suggested by the various revisions, are the following. The matter was recently raised by Bhattacharya (2009).

13.10.1 Method of dispersing the flour in alkali Two points need to be remembered. First, the rice flour should be at least of 60-mesh size as actually obtained by passing through a standard 60mesh sieve. Second, the flour should be either soaked in alkali overnight at room temperature and then heated for a few minutes (soaking overnight is not enough, heating after soaking is a must) or else heated immediately in a boiling-water bath, while swirling the flask by the hand, for a sufficient length of time (at least 10 min).

13.10.2 Whether to defat the sample and how This is a major source of error. In many laboratories the question of defatting is simply ignored, which is absolutely wrong. Fat content in milled rice flour is very low (0.5% or less), yet this small amount of lipid without doubt interferes with the amylose assay. Of course that does not mean that defatting is a must. One can live without defatting, but one must know its consequences. Alternatively, it is not difficult to defat the sample. It is true that a proper amylose analysis requires defatting, but then the



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error can be approximately accounted for by an appropriate factor. Many researchers have examined the factor by which the result of an undefatted flour has to be multiplied to get the true results. Such factors for nonwaxy rice reported are 1.16 (Sowbhagya and Bhattacharya 1971), 1.25 (Bolling and El Baya 1975), 1.29 (RRDC unpublished). The factor for waxy rice is 1.06. Juliano (1979) suggested a constant correction factor (to be added to the result) of 2.0% amylose. This procedure may be more or less satisfactory for an approximate work, but this method cannot be relied on for an accurate amylose assay. After all the presence of more or less bran, or more or less lipid may render the above factor only approximate. It is therefore best for accurate results, and not difficult, to remove the fat interference by physically removing the fat. Defatting can be done in two ways. One is to defat the flour, but mistakes may happen here if an important fact is ignored: a part of the fat is chemically bound to the starch granule and it cannot be removed by nonpolar fat solvents such as petroleum ether. To remove the fat completely, therefore, the flour needs to be refluxed with a highly polar solvent – 85% methanol or watersaturated butanol – for at least eight hours (preferably more). Several samples of rice flour (about 1 g each) can be put in small filter paper packets in one big ampule and defatted together. The samples are to be air-exposed for a day or two after defatting. An alternative and simple method is the one devised by Sowbhagya and Bhattacharya (1979). Here the undefatted flour is first dispersed in alkali, then the fat is removed in the liquid phase by shaking an aliquot with organic solvents. The process is very simple and is being routinely used in the RRDC. Two manual liquid-phase extractions with a suitable solvent is enough to remove the fat completely. Three extractions can completely remove the fat even from brown rice flour.

13.10.3 Adjustment of the extract pH This is another simple issue, often not well appreciated. The pH of the extract in the Williams et al. (1958) method was around 10. Actually this pH was not suitable because the blue colour is unstable at this pH. Ideally a pH of 8.0–8.5 is best, which can be attained very easily by a simple procedure prescribed by Sowbhagya and Bhattacharya (1971). A drop or two of phenolphthalein solution is put directly into the dispersion as an internal indicator. First 0.1N and then 0.01N hydrochloric acid solutions are put dropwise until the pink colour just disappears. At this point adding a drop of very dilute alkali (~ 0.01N) would bring the colour back to a faint pink. That would mark that the solution was in the ideal pH. The faint pink colour did not interfere with the colour reading. But one precaution was needed at this point. Care was needed that the water to be used for diluting the solution at this stage was neutral. Distilled water made in the laboratory by the usual distillation system attached to the municipal water



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line often contained dissolved carbon dioxide and hence had a pH of about 5. Therefore once the pH of the extract had been adjusted to about 8.0–8.5 as above, the subsequent dilution had to be done only with boiled (which would expel the dissolved CO2) and cooled distilled water so that the pH would not change. Alternatively, as suggested by Juliano et al. (1981a), the neutralisation of the alkali can be done by adding dilute acetic acid in slight excess. No doubt this will not bring the pH to the ideal level but to about 4.0–4.5 which is not ideal for reading the blue iodine colour. As a matter of fact this will lower the amylose value by about 0.5 percentage points. However once this error is recognised and accepted by all, it can be easily ignored for the benefit of the ease of work. Another advantage of this procedure is that one need not worry about the pH of the dilutant distilled water.

13.10.4 Source of standard amylose The problem of the amylose estimation has been compounded by the fact that the amylose standard itself is not often reliable. In earlier times people used to isolate their own standard amylose from rice. Later potato amylose in pure form became available and there was no problem. Of late, however, pure and reliable amylose has become difficult to procure. This again is not an insurmountable problem. The first thing is to realise that the standard one buys may or may not be pure. It is not difficult to test its purity: its colour can be tested against that of a sample of pure amylose already existing in the laboratory, or a small sample borrowed from another laboratory, or a sample of known rice, or even a rice of known amylose content or as reported in the literature, and so on. Having done this, one can get either a given ‘bad’ standard or a few samples of rice analysed by a source of reliable standard and then use these samples as standards. In other words, the main thing is not to trust the ‘standard’ blindly – the ‘meter syndrome’. While talking of standard amylose, one more point needs to be remembered. When rice is estimated for amylose by its reaction with iodine, it should not be forgotten that rice has some 0–30% ‘amylose’ (or AE) and approximately 60–90% amylopectin (on dry matter basis). That amylopectin also contributes a small amount of colour reading. Waxy rice by the above method always gives a reported amylose content of 2–4%, which is generally considered to be contributed not by any amylose but only by the background amylopectin. Juliano et al. (1981a) suggested one ingenious way of correcting this error. They proposed that a standard curve should be prepared not with pure amylose but with mixtures of amylose and amylopectin varying in proportion from 0:90 to 20:70, 40:50, 50:40, 70:20 and 90:0 mg. (Note that the total starch content in the standard solution is 90 and not 100 mg, for rice has approximately 90% starch on dry basis.) One can use a sample of waxy rice as the source of amylopectin.



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491

There is no doubt that this method will produce results which should be considered as more correct. At the same time it is also necessary to remember that this procedure will lower the reported amylose values by 2–4 percentage points from the results that have been reported in the literature for several decades. Whether it is thus desirable to get results which do not tally at all with the past for the sake of getting a ‘truer’ amylose value is a moot point, since what we are measuring is not necessarily amylose but amylose-equivalent. It is nothing more than an index. In the author’s opinion it is better to sacrifice this small correction to agree with historical results rather than to get results which might create confusion. It is the considered opinion of the author that if one remembers the above sources of error and the approaches to their correction, one can always get highly accurate results with ease. Recently Fitzgerald et al. (2009) began an international collaborative work on the estimation of amylose in rice. In the first part, 17 varieties were tested for amylose in different laboratories by their own methods. The results showed predictably wide laboratory-to-laboratory and procedure-to-procedure variation in the amylose values. Suggestions for improvement are to follow in the next phase of the project.

13.11

Hot-water-insoluble amylose content

This is another extremely useful test of rice quality It has been explained in Chapter 7 that a part of the ‘amylose’ content of rice can be extracted from rice flour with hot water. The other part cannot be so extracted and remains in the rice. The proportion of these two components varies from variety to variety as per the quality type of rice. Results so far indicate that this variation in the solubility of ‘amylose’ or AE happens more in high-AE rice and apparently little in intermediate and low-AE rice. In fact, based on this difference, high-AE rice has been classified into three distinct quality types. These are type I rice, which has very high insoluble AE (> 15.0% db); type II with intermediate (12.5–15.0%); and type III with low (≤12.5%) insoluble amylose. The cooked-rice texture of high-amylose rice has been found to correlate exceedingly well with this insoluble amylose content. As such, estimation of this hot-water insoluble amylose or insoluble amylose or insoluble AE in short, is an indispensable test for high-AE rice. No other property or test differentiates high-AE rice as well as this one property. A legitimate question may arise as to what chemical, molecule or component this insoluble AE represents, and in truth this is not known. But this fact is immaterial. It is a similar situation as in the case of ‘amylose’; there also we do not really know what that entity represents. What we know for certain is that both the AE and the insoluble AE correlate excellently with the texture of cooked rice and that is enough justification to test them. To test this parameter is very simple and largely follows the method of ‘amylose’ tests. The main difference is that for estimating the amylose, one © Woodhead Publishing Limited, 2011


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Rice quality

has to disperse the rice flour completely in dilute alkali. To estimate the insoluble AE, the rice flour is only extracted with hot water. The rest of the test is essentially similar to the amylose test.

13.12

Pasting characteristics

This is another extremely useful quality test for rice (or any other starchy material). If properly conducted, it can provide information of outstanding value. Unfortunately the way the test is traditionally conducted means the full potential benefit of it is not being obtained. This point would be clear from the discussion below. A misconception needs to be put out of the way first. The test can be conducted with different equipments. The Brabender viscograph (variously called amylograph, visco/amylograph) (Barbender GmbH, Duisburg, Germany) was being routinely used for the purpose until 1980s or early 1990s. Since then the RVA (Newport Scientific, New South Wales, Australia) has largely replaced the Brabender. The RVA requires only a small fraction of the time or of the starch (or flour) for the test. In that sense the RVA is a great improvement over the Brabender viscograph, but otherwise there is no basic difference between the two. Often people have the impression that the RVA is of a different genre, but this is not true. The principles and the outcome are very similar. Recently Brabender too has come out with a micro version (Micro Viscoanalyser, MVA), which again is basically similar. The following discussion will relate to the use of the Brabender viscograph, because of the author’s long personal experience of it, but it should be understood to apply equally well to the RVA and to the MVA. To understand the deficiencies of the current method of viscography and how to rectify them, one has to understand the basic principles of viscography. The basic use-values of starch depend on its three properties. First, when starch is heated with water it swells, forms a paste and provides a body. So how easily and to what extent it swells is the first issue. The second is how stable that body or paste or the viscosity is. The third is how much the paste congeals when it is cooled. The viscograph is designed to study precisely these three properties of starch. In this instrument, the starch (or flour) is mixed with water and the slurry and the resulting paste are passed through a regulated process of heating and cooling. The slurry is first heated at a regulated rate under continuous mixing until the paste reaches a predetermined temperature slightly below the boiling point, usually 95 °C. Then the paste is maintained at that temperature for a specified time (20–30 min) to see how stable the paste is. Finally the paste is cooled at a regulated rate to 50 °C. A characteristic viscosity curve is thus produced as illustrated in Fig. 13.29. The curve starts rising once the starch begins to gelatinise at around 70 °C, rises to a maximum value



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Analysis of rice quality 20

Viscosity, BU

1000

Time, min 60

40

493

80

Gelatinisation temperature = GT Breakdown = BD = P – H Setback = SB = C – P Total setback = SBt = C – H Relative breakdown = BD/SBt = (P – H)/(C – H)

C

P 600 H

200 GT 60

70

80

90 95

95 90 Temperature, °C

80

70

60

50

Fig. 13.29 A viscogram of rice, showing peak (P), hot-paste (H) and cold-paste (C) viscosity. Calculation of other indices (BD, SB, SBt and BDr are also shown. Courtesy, S. Z. Ali.

(called the peak, P) at around 90 °C, then decreases as the paste continues to be heated to reach a trough (called the hot-paste viscosity, H), then rises again as the paste is cooled to reach the final cold-paste viscosity (C). Various criteria are now read from the viscogram to reveal the properties of the starch. These are, first, the peak viscosity attained during the heating (P). Then the stability of the paste is studied from its ‘breakdown’ (BD), i.e., the amount of drop in viscosity during continued heating BD = P – H Finally the property of paste congealing is studied from two measures of the ‘setback’, i.e., the quantum of thickening or rise in viscosity of the paste during cooling. These are the traditional setback (SB) and the total setback (SBt) (also called the consistency) SB = C – P SBt = C – H The former (SB) measures the difference between the C and P, which is a bit bizarre, for the rise in viscosity occurs not from the point P but from the point H. That is why SBt is the real measure of the setback. In any case calculating these by arithmetical difference compounds the error (discussed below). These parameters (P, BD, SB or SBt) can provide useful information about the property of the starch. But unfortunately this is hampered, if not nullified,



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by the way the test is routinely conducted. This would be understood from the following discussion. ∑

∑

∑ ∑

The peak viscosity (P) is no doubt, potentially, an important index. Unfortunately it is observed that the peak viscosity attained by a starch or flour at a given slurry concentration varies widely from botanical species to species and also, to some extent, from variety to variety. But there is no clear theory or documented reason why the peak viscosity differs. So this value, always dutifully recorded and reported, does not really provide much useful information. At the same time, the varying P affects all other parts of the viscogram and partly masks the informatory power of the other indices, as explained next. A property of the pasting curve is that the entire viscogram is heavily dependent on the slurry concentration, for the relation between concentration and viscosity is not linear but exponential. This is illustrated by the series of viscograms obtained with a sample of rice flour at different concentrations, depicted in Fig. 13.30. As a result, P increases with the increasing slurry concentration, not linearly but exponentially. Unfortunately, it then affects every other part of the curve, that too again almost exponentially. The observed breakdown or BD, which should have been the most important quality indicator of the starch, is thus heavily influenced by P (i.e., the concentration) regardless of the starch characteristic. The higher P, the greater the breakdown, and more than proportionately, and vice versa. This can be easily seen from Fig. 13.30. One can see 20

40

Viscosity, 100 BU

30

Time, min 60

80 Ptb 10

78 g 20

70 g 60 g 56 g

10 50 g 44 g 40 g 60

70

80

90 95

95 90 Temperature, °C

80

70

60

50

Fig. 13.30 Viscograms of Ptb 10 rice at different slurry concentrations (g wb flour per 500 g total). Courtesy, S. Z. Ali.



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∑

∑

495

that the higher the curve rises, the greater is the following drop, that too, more than proportionately. This is shown more clearly when the interpolated P and BD data from Fig. 13.30 are drawn against each other (Fig. 13.31). Clearly paste breakdown varies more or less as the square of the peak viscosity. As a result, if P was not controlled, one would not know whether a variation in the BD arose because of the characteristic of the given starch or simply because of the variation in the P. Thereby the interpretative power of the BD is largely nullified. The same argument applies to the setback (SB or SBt). The SB or SBt depends heavily on the H value, i.e., the BD. The more the earlier drop in viscosity (P – H), the less the later setback (Fig. 13.30). That is, again the SB is dependent on P and again quite unrelated to the property of the material under test. There is another computational problem. The relationship among the various criteria in a viscogram is exponential or geometrical, not linear (see Fig. 13.30). Yet by tradition the BD and SB values are read by arithmetical difference, which, frankly, is meaningless. Had the BD and SB values been expressed as ratios (BD = (P – H)/P; SB = C/P; SBt = C/H) and not as arithmetical differences, at least some meaning could have been derived even from the existing data. But the faulty practice persists even decades after its fallacy was pointed out. The power of an established paradigm in science is well known (Kuhn 1962).

To sum up these arguments, the slurry concentration obviously has a profound influence on the entire viscogram, including all the indices. Yet this concentration is decided totally arbitrarily by some traditional practice. For example rice-flour viscogram is traditionally run at a constant 10% 40

Breakdown, BU

30

20

10

0 0

Fig. 13.31

10 20 Peak viscosity, 100 BU

30

Rice-flour paste ‘breakdown’ is strongly dependent on the ‘peak viscosity’. Curve drawn from viscograms in Fig. 13.30.



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Rice quality

concentration or thereabouts for which there is no logical argument. Since the P itself varies from variety to variety for unknown reasons and as P has an exponential influence on the other criteria, namely BD and SB, very little valid conclusion can be drawn from the results thus obtained from an arbitrarily fixed concentration. For valid comparison, samples (i.e., viscograms) should be compared at a fixed peak viscosity value (rather than at a fixed slurry concentration) so that the effect of variable P on the BD and SB would be eliminated and the latter parameters would reflect the starch characteristics alone. As an example, consider the following data of Honsch (1956) for 20-, 40- and 60-fluidity acid-modified wheat starches. At the mandatory equal slurry concentration, the 20-fluidity starch gave the highest breakdown and the 60-fludity starch the least (Table 13.9). This result was an absolute artefact that made no sense. It arose because with the equal starch concentration, the peak viscosity itself declined with increasing fluidity and thus dragged the BD down. But the picture completely changed, and became meaningful, if the concentrations of the samples were readjusted such that all three starches gave an equal peak viscosity. For example at the P value of 600 BU for all, the 60-fludiy starch gave the highest breakdown and the 20-fludity starch the smallest, a result that clearly made sense. The literature is replete with such fallacies where viscograms of flours and starches have been compared at a fixed concentration, ignoring the concurrent change in the P that led to artefacts being produced. The long history of efforts to study rice ageing with an inappropriate viscogram procedure has been pointed out in Chapter 5. One should note that these above fallacies were not present in the viscograph procedure when it was first proposed. While Anker and Geddes (1944) had first used this test, its potential was studied in detail by Mazurs et al. (1957). Although not explicitly stated, these authors were clearly conscious of these above arguments, for they prescribed that each starch be viscographed at a number of slurry concentrations. Then, they suggested, the resulting curve points, namely P, H, C, etc. values, be plotted in a semilog graph against Table 13.9 Viscogram indices of 20-, 40- and 60-fluidity acid-modified starcha Slurry concentration

Starch fluidity

P (BU)

BD (BU)

SB (BU)

10.7% for all

20 F 40 F 60 F

620 430 290

260 220 150

130 150 160

Different for each

20 F 40 F 60 F

600 600 600

260 320 390

150 80 (–) 100

Reproduced, with permission, from Bhattacharya and Sowbhagya (1978) John Wiley and Sons. a Values interpolated from data in Figs 2 and 3 of Honsch (1956). Used with permission.



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497

the concentration, as shown in Fig. 13.32(a) for potato and (b) corn starch. They suggested that the resulting curve pattern be compared among the samples. For instance, the curves in Fig. 13.32 clearly show that potato has 1000 15 800

600

10

B

A

E 400 5

C

D

Viscosity, Poises

(a)

Viscosity, BU

D

200

2

3 4 5 6 Starch concentration, g/100 ml

7

8

1000

15 800

600

A

D

400

B

10

C

Viscosity, Poises

(b)

Viscosity, BU

E

5 200

4

5 6 7 8 Starch concentration, g/100 ml

9

Fig. 13.32 Semilog graph of viscogram indices against concentration of potato (a) and corn (b) starch. A, peak; B, 95 °C beginning; C, 95 °C end; D, 50 °C (cooling cycle beginning); E (50 °C cooling cycle end). Breakdown = A – C; setback = D – A; total setback = D – C. Reproduced from Mazurs et al. (1957).



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a very high breakdown (C/A ratio in the figure) and correspondingly a very low setback (D/C ratio in the figure), while corn has a very low BD and a high SB. Unfortunately, it is tedious to run several viscograms for each sample. So researchers apparently thought it prudent to sacrifice a bit of clarity but derive as much information as possible from an abbreviated procedure. This is probably how they intuitively selected a convenient concentration (apparently such that the curve largely remained within the width of the recording chart) and run a single curve for the test. Halick and Kelly (1959), for instance, standardised the technique for rice with 10% slurry, and this set up a paradigm, although the grave fallacies introduced thereby were not noticed at the time. Bhattacharya and Sowbhagya (1978, 1979), examined the above arguments and concluded that it was necessary to go back to the original concept of Mazurs. However the graphical comparison was too tedious and had to be replaced by some numerical indices. So they suggested that viscogram parameters be compared, not at a fixed concentration at all, but at a fixed P value. This paradigm shift might appear bizarre initially, but in reality it should cause no surprise, for unrelated parameters could only be compared with a common denominator. This is precisely the reason why we use concepts such as per cent or per capita while comparing unrelated absolute magnitudes or changes therein. In addition, the breakdown and setback parameters, being nonlinear, were better expressed as ratios [H/P, or better (P–H)/P, C/P and C/H] rather than arithmetic differences. The authors therefore proposed that ideally any sample under test should be viscographed at a number of slurry concentrations, à la Mazurs. The resulting P, H and C values should then be plotted against slurry concentration in a log–log graph (better than the semilog used by Mazurs), yielding a set of three characteristic curves as shown in Fig. 13.33. BD, SB, etc. values could then be read at any fixed P value from this graph, which provided excellent criteria for comparison between samples. The authors noted with surprise that the remarkable pattern of the curves had yet more stories to tell. The three viscosity indices gave a slightly curved line each rising with concentration and together gave a remarkably characteristic pattern common to all the samples, be they high- or low-amylose or waxy (Fig. 13.34). The P line rose with the steepest slope. The C line lay initially above the P line (positive setback), but it had a lower slope and so eventually crossed the P line (positive and then negative setback) at a certain viscosity value (SB = 0). The H line was initially identical with the P line (zero breakdown) but subsequently became the lowest line (positive breakdown). This H line ran throughout more or less parallel to the C line (i.e., being in log–log coordinate, the ratio C/H was nearly constant throughout, as mentioned earlier). It is immediately evident that for each sample there was a minimum P value (Pmin (BD)) below which the breakdown was zero (P = H) and above which it progressively increased not only in absolute magnitude but also



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499

4000 P 2000 P

Viscosity, BU

C

H

1000

P

C 600 H C

400

H

200 PTB 10 100

7

8

10

12

Phoudum

ASM 44

6 8 10 5 6 Slurry concentration, % db

8

10

Fig. 13.33 Log–log pattern of peak (P), hot-paste (H) and cold-paste (C) viscosities against concentration in viscograms of three rice varieties. Varieties: high- (left) and low- (centre) amylose and waxy (right). Reproduced, with permission, from Bhattacharya and Sowbhagya (1978) John Wiley and Sons.

Viscosity, 100 BU

Group I

Group II

Group III

Group Group V IV

Group VI

Group VII

Group VIII

40

40

20

20

10

10

6

6

4

4

2 Jaya 1

7

9 11 13 7

Prosadbhog 9 11 8

T 141

Basmati 370

Usi

Benong Changlei 130

10 12 14 7 8 10 8 10 12 6 Slurry concentration, % db

8 10 7

9

5

Purple puttu 7

2 1

9

Fig. 13.34 Log–log P–H–C patterns of eight rice varieties representing eight rice quality types (I to VIII). Reproduced, with permission, from Bhattacharya and Sowbhagya (1979) John Wiley and Sons.

as a proportion of the peak viscosity. Similarly, with increasing slurry concentration, the setback first rose, then declined, eventually becoming zero at a particular point (the P–C intersection point), and then became negative.



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500

Rice quality

As a proportion of the peak viscosity, it continually decreased with increasing P, eventually becoming zero and then negative. It is remarkable that the above features were true for all starches, including low-amylose and waxy rice varieties. The general understanding was that these varieties, particularly waxy rice, were characterised by a negative setback, as they indeed showed with the usual 8–10% slurry. However, with a very dilute slurry (i.e., below a certain P value) even these rices gave a positive SB (see the curves on the right in Figs. 13.33 and 13.34). In other words, no rice or starch inherently gave a low or a high BD, or a negative or a positive SB as such, as generally assumed. Each variety gave a zero BD below a certain P value (Pmin (BD)) and a rising BD above it, as well as a positive SB below another critical P value (P–C intersection point) and a negative SB above it. Clearly, it was quite meaningless to compare the BD or SB of two samples without reference to the corresponding peak viscosity values. The above observations, with starches and rice flour alike, can perhaps be explained as follows. At a very low starch concentration, the granules have plenty of room to swell and to move about freely and hence are not sheared between the pins of the rotating viscograph bowl. Therefore, they continue to swell till the end of heating; i.e., the breakdown is zero. At a high slurry concentration, on the other hand, the swollen granules occupy most of the volume and are, therefore, severely sheared between the pins. As a result they tend to disintegrate, i.e., the sample shows a breakdown. This would explain why the BD increases at an ever-increasing rate with rising P (i.e., the concentration) in any given sample. However, the actual extent of granule disintegration (i.e., BD) at any P value would surely depend on the internal organisation and resilience of the granule and hence would also vary from starch to starch and from cultivar to cultivar, as indeed observed. This aspect was extensively explored by application of viscography, viscometry and rheometry in rice-quality studies in Chapter 7. It is the business of the viscography test to detect this difference in granule organisation by the BD criterion. But, to do that, maintaining a constant P value is a must. During cooling, the viscosity would increase – first simply due to cooling, and second due to starch retrogradation. The quantum of this increase in viscosity must obviously be visualised not as an addition (C minus H), as is erroneously thought, but as a ratio (C/H), i.e., how many times. The first factor (effect of cooling) being constant, the extent of increase in viscosity would vary to the extent the second factor (retrogradation) varied. This would explain why the ratio C/H tended to remain relatively constant in a sample over a range of slurry concentrations and also within a species. No doubt, it may vary slightly with varying amylose content of the starch, hence the slightly decreasing value of the ratio with decreasing amylose in rice (Table 13.10) . On the other hand, retrogradation is known to vary greatly in starch from species to species; and this may account for the large difference in the value of C/H among different starches (Table 13.10).



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501

Table 13.10 Viscogram indices of various starches and rice flours P at 10% slurry (BU)

Slurry concentrationc (% db)

H/Pc

C/Pc

C/Hc, d

BDr

Starcha Potato Waxy sorghum Cationic corn Tapioca Corn Wheat

4500 2700 2200 2800 1400 500

2.1 5.0 2.7 5.2 7.0 10.0

0.20 0.28 0.22 0.26 0.56 0.65

0.28 0.44 0.45 0.56 1.60 2.80

1.36 1.57 2.05 2.12 2.86 4.31

11.1 4.5 3.4 2.5 0.42 0.16

Rice flourb Asm 44 Phoudum Ptb 10

870 1310 880

7.8 7.6 8.9

0.75 0.79 0.97

0.94 1.38 1.92

1.25 1.75 1.98

1.32 0.36 0.03

Sample

Reproduced, with permission, from Bhattacharya and Sowbhagya (1978) John Wiley and Sons. a Values as calculated approximately from Figs 2–8 of Mazurs et al. (1957). b Values calculated from Fig. 13.33. c At a P value of 500 BU. d The value of this ratio remained relatively constant at other P values also.

The usefulness of the above approach can be illustrated even with the data of Mazurs et al. (1957). It can be seen from Table 13.10 that the different starches gave widely different peak viscosities at any given slurry concentration (10% db shown). This was precisely why no meaningful comparison of breakdown and setback, etc., could be made between them at an arbitrary or constant concentration. However, at a fixed peak viscosity, say 500 BU illustrated in Table 13.10, the perceived visual distinctions between the different prototypes could be given a quantitative shape with the help of a few simple ratios. The breakdown and setback ratios H/P, or better (P–H)/P, and C/P brought out the high breakdown and low setback of the first four starches as compared to the last two. The total setback ratio C/H was evidently another distinguishing characteristic of each starch type. Moreover, as mentioned before, its value for each starch remained relatively constant over the entire peak viscosity range (see the parallel C and H lines in log–log coordinates in Figs 13.33 and 13.34). Next, the parameter relative breakdown (BDr) BD r = BD = BD SBt BD + SB clearly distinguished the different starches most effectively. (As the P was constant, arithmetic calculation too would be good enough here.) The importance of BDr probably lay in the following. It is true that the breakdown was the principal relevant property of starch. The subsequent rise in viscosity,



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Rice quality

or the setback (be it SB or SBt), was largely an obverse reflection of the BD. The more the BD, the lower the H value from where the viscosity rise on cooling would start and so the SBt would tend to be lower, and vice versa. So putting the breakdown as a ratio of BD/SBt (i.e., BDr) would magnify the difference among varieties. It would also take account of whatever difference in setback there was. It is of interest that Horiuchi (1967) had already observed that this parameter correlated excellently (inversely) with the starch-iodine blue value, a good index of amylose in Japanese rice. That the above indices could effectively distinguish between rice varieties as well is shown by the results calculated for the three rice flour viscograms of Fig. 13.33 as shown in Table 13.10. Having established the general validity of this approach, the technique was applied to a study of rice quality using a very large number of samples of rice (illustrated in Fig. 13.34 with eight rice varieties, one from each quality type). The remarkable uniformity of the pattern among the highest to the lowest amylose class of rices, and the gradation of the index values, are testimony to the soundness of the concept. Its success can be further judged from the fact that the BDr criterion over a range of P values showed a distinct pattern each among the eight quality types of rice (Fig. 13.35).

200 IV

VI 100

VIII

V

VII

Relative breakdown, %

60 40

III

V

20

VI VII 10

III II

6

I

IV

4

2 200

400

600 1000 Peak viscosity, BU

2000

Fig. 13.35 ‘Relative breakdown’ (breakdown/total setback) of rice of eight quality types (I–VIII: see Table 7.7) at various peak viscosities. Reprinted, with permission, from Bhattacharya and Sowbhagya (1979). © Institute of Food Technologists.



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The multi-concentration procedure mentioned above is thus the ideal. With the RVA and MVA being now available, it is not such a daunting task either. Its powerful value lies not only on the BDr (at a fixed P) criterion above but also on other insights, such as the Pmin (BD), the P–C intersection point (Figs. 13.33 and 13.34) and the pattern of BDr over a range of P values (see Fig. 13.35). The remarkable value of these indices was shown in the study of rice ageing in Chapter 5. However, it goes without saying that such a multi-concentration procedure cannot be adopted for routine testing with the Brabender. It may be somewhat irksome even with the RVA or MVA. The present author along with his colleagues (Sandhya Rani and Bhattacharya 1995, Sowbhagya and Bhattacharya 2001, RRDC unpublished) have now devised a simplified twoconcentration viscographic procedure which is less demanding but retains the key benefits: ∑

∑

One viscogram is run with a 10% slurry. This would provide two useful items of information. (i) One is the P value at 10% slurry (P10%). Although the meaning of this parameter is not known at this time, it is envisaged that statistical examination of a large number of these values accumulated over years with different varieties may reveal some interesting new relation. (ii) The other item of information is the C10% (the cold-paste viscosity of a 10% paste). Many researchers have hinted that this value correlates well with cooked rice texture (Lorenz and Saunders 1978, Okadome et al. 1998a). This is quite logical. All cooked rice can be considered as equivalent to a 26.5% paste (rice cooked up to the centre has a moisture content of about 73–74%, as explained above). Hence its hardness (equivalent to apparent viscosity in the present context) may well correlate somewhat with the viscosity of a 10% paste. Recent data at RRDC (unpublished) showed the r value between the two as 0.722 (n = 120). The C10 value may then act as a surrogate of the cooked-rice hardness. The above P10% value is then used to readjust the slurry concentration such that it would yield a P value as close to 500 BU as possible. A second viscogram is now run with this readjusted concentration. (i) The resulting P, H and C values are used to calculate the BDr [(P–H) ¥ 100/(C–H)%]. This BDr has a strong correlation with the amylose, insoluble amylose and sensory and instrumental texture of cooked rice, as already discussed in Chapter 7. (ii) The parameter C/H, a measure of the retrogradation tendency of the sample under test, is also calculated. This ratio too, once a large number of its values have been accumulated, may reveal some interesting property of starches in future.

To conclude, viscography is an extremely useful quality test of rice (or any starch or starchy food), but its fullest benefit can be obtained only if one follows the two-run procedure above. This would yield the four following important indices:



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∑ ∑ ∑

Rice quality First is the most informative relative breakdown (BDr) value at a given peak viscosity (say 500 BU). This (or at any event at least the BD at a fixed P value) is the key parameter that the viscogram provides. After all the main difference between starches is the relative resilience-fragility of the starch granule. This is amply demonstrated in the extensive data and discussion on the rheological difference among rice varieties in Chapter 7. The viscogram breakdown is an excellent reflection of that granule resilience, and therein lies its real value. P10% (the peak viscosity of a 10% paste); although its meaning is not known at this time, some day it may reveal something useful and interesting. C10% (the final cold-paste viscosity of a 10% paste); this is a surrogate for the texture of cooked rice. The retrogradation index (at 500 BU), C/H. This index is the true measure of the setback (not the C–P value, which the prevailing paradigm wrongly prescribes).

13.13

Gelatinisation temperature (GT)

It is known that on an average starch forms about 90% of the dry weight of rice (Juliano 1985). There should be little wonder then that starch properties should strongly affect all behavioural properties of rice, especially in the wet or cooked condition. Indeed as we have seen in Chapters 6 to 9, the hydration, cooking, eating and product-making properties of rice are heavily influenced by the properties and composition of its starch. The three main properties of starch that may be relevant in this context are the granular size and shape, the GT and the molecular structure of starch. The last property, viz. molecular structure of starch, has been shown in Chapter 7 to be the main determinant of the pasting properties of rice and the texture of cooked rice. It also influenced the grain’s processing and product behaviour. As far as can be surmised, the size and shape of the granule are as yet not known to appreciably affect the properties either of the uncooked or of the cooked rice or paste behaviour. An intriguing question arises about the GT. The GT is an extremely important property of starch and should be undoubtedly expected to have an effect on the properties of rice. However, to the best of our knowledge, the starch GT has not yet been shown to have any significant effect on the basic behaviour of nonwaxy rice – viz. its paste properties, product quality and texture of cooked rice (see Chapter 7). It has some effect, though, in the case of waxy rice. The GT, on the other hand, does have a strong influence on the hydration and cooking of rice at lower temperatures. No doubt rice-starch GT has hardly any effect on the cooking time and rate of water uptake of the rice whenever rice is cooked by putting it directly into boiling or near-boiling water (see Chapter 6). However, it is also known that GT does influence the © Woodhead Publishing Limited, 2011


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room-temperature hydration of rice as well as the cooking time wherever rice is presoaked before putting in boiling water. Besides, in industries where the rice grain is soaked, hydrated or cooked – always aimed to be achieved at the lowest possible temperature to save energy – the GT would play a strong role on the economics by virtue of the hydration being inversely related to the GT. The same is the case where rice grits are used as brewing adjuncts. It is an interesting fact of starch property, as has now become apparent from studies of starch chemists during the last two or three decades, that most properties of starch – pasting, viscosity, granule resilience/fragility, paste strength, rigidity, elasticity – are by and large all determined by the branch-structure profile of the amylopectin molecule. Even the GT of starch is no exception. As has been shown in very recent times (see Chapter 7), the proportion of the very short to the short branch chains in the amylopectin molecule is what determines the GT of the starch (see Chapter 7: Figs 7.19 and 7.20). Starch is a semicrystalline polymer and gelatinisation represents the melting of its crystallites. The temperature at which this melting occurs is characteristic of each starch. Fundamentally, therefore, GT is defined and determined by two changes. One is the disappearance of birefringence of the starch granule, i.e., the loss of the Maltese cross of the granule when it is viewed under polarised light. The other is the melting of the starch crystallite as it is being heated in presence of sufficient water in a DSC. The classically correct method of determining the GT therefore is either by determining the loss of birefringence (birefringence end point temperature, BEPT) or by studying the crystallite melting pattern and position during studying the heat flow in a starch slurry in the DSC. A number of other changes accompany starch gelatinisation. The GT can therefore be identified and measured by these changes as well. These include the sharp changes in optical, hydration, swelling, viscosity and microscopic properties of a starch slurry that occur either during heating or during treatment with increasing concentrations of alkali. These latter changes are simple to measure and are normally routinely employed in the laboratory to estimate the GT of rice. These methods depend on studying either the change in optical properties or viscosity of a starch slurry or hydrated starch. This change, again can be brought about either by heating or by increasing concentrations of alkali. Several methods can be devised on this basis, some actually in use and some potential. This range of potential scope of the approaches was reviewed by Bhattacharya (1979b) in the 1978 IRRI conference on rice quality. Some of the more commonly used and some important potential methods are as follows.

13.13.1 Photometric method This method was devised at the IRRI by Ignacio and Juliano (1968) and Suzuki and Juliano (1975). The principle of the method is as follows. A



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slurry of rice flour dispersed in water is heated at a slow but steady rate. The transmittance of the slurry is simultaneously observed in a photoelectric system. The temperature at which the transmittance suddenly changes (increases) is taken as the GT. The value has been found to be in good agreement with the GT as determined by more classical physical chemistry methods. In actual practice a few rice grains are triturated with water and the dispersion is put in a specially midified optical tube of a photoelectric colorimeter. The slurry is kept in agitation with a magnetic stirrer and is simultaneously heated at a steady rate. The light transmittance is kept under observation and any sudden change is noted.

13.13.2 Alkali photometry Similar to the above is a method devised by IRRI (1978) to screen breeding material into high- , intermediate- and low-GT groups. The principle is very simple. The brown rice is powdered and the brown rice flour is soaked in 1.6% KOH for 24 h at 30 °C. Next day the transmittance of the alkaline slurry is measured with a probe colorimeter. The percent transmittance (T) gives a rough idea of the GT as follows High-GT group Intermediate-GT group Low-GT group

< 10% T 21–50% T > 60% T

13.13.3 Water-uptake ratio method It has been shown that the water absorption by the rice grain during cooking, when the rice grains are cooked by placing them directly into boiling water, is hardly affected by any chemical property, including the GT. The absorption is determined almost entirely by the surface area per unit weight of the rice (see Chapter 6). This is because the boiling temperature is so much higher than even the highest range of GT (~ 80 °C) that the latter hardly exercises any influence. However, if rice is cooked (or hydrated) by putting it in water at a temperature around 70–80 °C, then the hydration is bound to be strongly influenced by the GT. This fact was observed by many workers (Chapter 6). This principle has been suggested as a method by Bhattacharya et al. (1972b, 1982). While considering the hydration of the rice at such temperatures, the authors at the same time recognised that the effect of the surface area on the hydration would not disappear. Therefore the authors prescribed that the best way to estimate the GT was to determine the water uptake at 80 °C and divide the value by the water uptake at boiling or near-boiling temperature for the same time period (W¢80 °C /W¢96 °C). The effect of the surface area would thereby get cancelled and the property would be almost entirely determined by the GT. This ratio was in fact found to be highly significantly correlated with the alkali score (r = 0.924, n = 40) and therefore inversely related to © Woodhead Publishing Limited, 2011


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the GT. It was later shown in a larger study (Bhattacharya et al. 1982) that GT was strongly related to the above ratio and could be determined from the above water-uptake ratio (WR) by the following equation GT = 77.89 – 0.28 WR (excluding japonica varieties, n = 133) (r = – 0.854***) Or = 77.03 – 0.24 WR (including japonica varieties, n = 144) (r = 0.791***)

13.13.4 Viscographic method The pasting curve gives a rough idea of the GT of the sample. The temperature at which the viscogram curve starts rising from the baseline can be called the pasting temperature, which is related to the GT of the sample. However, this rising point is spread over a fairly long temperature range when a viscogram is run with the usual 10% slurry. That makes it is therefore very difficult to read the transition temperature from this graph. Halick et al. (1960) devised a method by which this difficulty was avoided. After running the viscogram for its normal information, they suggested that a second viscogram be run with a 20% slurry. In view of the high concentration, the viscogram now showed a very sharp rise from the baseline. This sharp transition point gave a good indication of the GT of the sample.

13.13.5 Alkali digestion test method This is the well-known test of rice quality (see Section 13.8). As shown there, digestion of the rice kernels in a dilute solution of alkali is obviously related to the GT of the sample. Samples with low GT would naturally show a relatively high degree of digestion, while varieties having a high GT would necessarily show a relatively low degree of digestion. The degree of kernel digestion, i.e., the alkali digestion score, is thus an excellent inverse index of the GT. Especially the alkali score (AS) read after digestion with 1.4% KOH by the Bhattacharya and Sowbhagya (1972b) score card (Table 13.8) can be converted into the GT with a fair degree of accuracy by using the equations on page 485 (Bhattacharya et al. 1982).

13.14 Tests for eating quality of rice Rice is overwhelmingly used as cooked table rice. The quality of the cooked rice as perceived by the consumer during eating is therefore a very important attribute of rice quality. It can be broadly called the eating quality of rice. The eating quality of rice has been thought to consist mainly of hardness (or



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firmness)/softness (or tenderness), stickiness (earlier literature often referred to this perception as cohesiveness), flavour, aroma, colour and gloss of the cooked rice (Del Mundo 1979). Of these, cooked-rice hardness (firmness)/ softness and stickiness/fluffiness are probably the most important basic attributes of cooked rice with which the consumer is concerned. These two attributes can be subsumed in the broad term texture. We can thus say that texture after cooking is probably the most important property of rice so far as its eating quality is concerned. Accordingly the largest degree of attention and research consideration in measuring the eating quality of rice has been bestowed on its texture after cooking. Rice texture has been measured both by sensory and instrumental methods. Sensory attributes are no doubt the ultimate standard which all other modes of assessment aim to match. At the same time it is easy to appreciate that sensory methods require a much higher level of planning, organisation, time, effort and cost to execute. Sensory methods are therefore used more as standards to calibrate other methods than for routine analysis. Sensory examination of cooked rice by the researcher or by rudimentarily trained or untrained panels would have been performed in a variety of ways and over a relatively long time in the past. In contrast instrumental methods have a shorter history, their development probably coinciding with the general beginning of more intensified research on rice that started from around the middle of the last century.

13.14.1 Instrumental measurement of texture of cooked rice Various approaches and instruments have been used to measure the texture of cooked rice, but the broad principles of all the methods and approaches can be said to be more or less similar. These measurements on cooked wholegrain rice are rarely if at all made along strict scientific criteria of stress, strain, viscosity, elasticity, storage modulus, stress relaxation and so on. Practically all the methods are based on empirical procedures involving application of a force and studying the response of the cooked rice to this force. Broadly speaking, in one class of methods, a probe or a plunger is made to apply a given amount of force on to a sample of cooked rice and the amount of deformation or the resistance to the deformation is recorded. A measure of hardness or firmness or softness is calculated from this measurement. Alternatively a force may be applied by a plunger on a rice grain and the amount of spring-back of the grain when the force is removed may be observed as a measure of its latent elasticity. In a third approach a force is applied by a plunger on the cooked rice which is made to extrude either through holes at the bottom or back-extrude through the top of the plunger. Either the force recorded or the distance the plunger travelled or the amount of extrudant that was produced under a given force or in a given amount of time is recorded. An empirical measure of the hardness, firmness or softness is obtained from these measurements.



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The degree of stickiness or tackiness of the cooked-rice grains is measured similarly. Broadly speaking a plunger is made to apply a force on grains of cooked rice. After a suitable time the plunger is made to withdraw and the degree of resistance offered by the adhesion between the cooked rice and the plunger is measured. This value then gives an empirical measure of the stickiness. In another approach the cooked-rice grains may be screened through a suitable sieve. The amount of cooked mass either retained by or passing through the sieve would provide a measure of the degree to which the grains tend to form lumps by clinging to each other, i.e., their stickiness. In another approach, the number and size of clusters that the grains may make in a mass on their own by clinging to each other may be measured. It would thus automatically give an idea of the stickiness. An historical overview of the techniques employed is provided in the account below. Kurasawa et al. (1962, 1969) employed a table balance to measure the stickiness of cooked rice. They measured the weight required to lift the pan of the balance from its contact with cooked rice to which it had been pressed earlier. Manohar Kumar et al. (1976) also made a similar attempt, but found the reproducibility rather poor. Japanese workers at the National Food Research Institute (NFRI) at Tsukuba used a parallel plate plastometer to study the vicoelasticity of cooked rice. The cooked rice was pressed between two parallel plates, and viscosity and elasticity were calculated from the observed deformation (Chikubu et al. 1965, Chikubu 1967, Endo et al. 1976). This technique was more than empirical and was possibly the first and only attempt to study the classical rheological parameters of the cooked rice in terms of viscosity and elasticity. Nonglutinous rice had greater viscosity and elasticity than glutinous rice, and indica rice more than japonica rice. Arai et al. (1981) proposed a sixelement rheological model for cooked rice from the above data. Sugiyama et al. (1990) and Yoshii et al. (1993) further refined this technique. Hampel (1961) at the Federal Research Centre for Cereal and Potato Processing at Detmold, Germany, used the Haake consistometer to determine the ‘consistency’, i.e., the hardness, of cooked rice with much success. A plunger was pressed into the rice taken in a cylinder under a given weight, when the rice back-extruded through the plunger, and the time needed to penetrate a given distance (or the distance travelled in a given time) was measured. He found the consistency was well related to the amylose content. Manohar Kumar et al. (1976), Bhattacharya et al. (1978) and Deshpande and Bhattacharya (1982) at the Central Food Technological Research Institute (CFTRI), Mysore, also used the same system, and found the measured consistency (hardness) of cooked rice to be very well correlated to the total and water-insoluble amylose contents, pasting indices and other parameters. Cooked-rice hardness and stickiness were very highly inversely correlated. The effects of rice-to-water ratio and of storage of cooked rice were also studied.



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Other equipment meanwhile became available. Chikubu et al. (1971) employed the General Foods Texturometer and suggested that the index hardness : adhesiveness gave the best indication of rice texture. Okabe (1979) studied the texture of Japanese rice with the above instrument and demonstrated the measurement of hardness, cohesiveness, adhesiveness, gumminess, etc. from the data. He constructed a texturogram from these readings and showed that a ratio of stickiness : hardness of 0.15–0.20 under standard conditions was the most acceptable to Japanese consumers (see Fig. 7.4). Yoshikawa et al. (1974) and Endo et al. (1980b) devised an improved apparatus. The instrument has been employed extensively by other workers (Tanaka 1975, Endo et al. 1976, 1980a, Ebata and Hirasawa 1982, Ebata et al. 1982, Suzuki et al. 1983a, 1983b). Suzuki (1979b) reviewed the method. Scientists at the IRRI, Los Baños (Perez and Juliano 1979, 1993, Perez et al. 1979, Merca and Juliano 1981, Juliano and Perez 1983), used the Instron. The maximum force generated during extrusion of cooked rice through the bottom of a modified Ottawa texture measuring system (OTMS) cell was used as a measure of the hardness. The force needed to detach a plate after pressing a mass of cooked rice was measured as an index of its stickiness. Blakeney (1979) reviewed the use of Instron with the above and other cells. Mossman et al. (1983) used the same method for measuring stickiness of rice and Fellers et al. (1983) studied the effect of various heat treatments of rice on the reduction of its stickiness. Blakeney and Allen (1983) observed that increasing initial moisture content of rice led to decreasing hardness after cooking, while increasing protein content had a slight increasing effect. Scientists at the Institut National de la Recherche Agronomique (INRA) at Montpellier, France devised a new instrument, called Chopin-INRA Viscoelastograph, for studying ‘firmness’ and ‘elastic recovery’ of cooked rice. Three cooked grains were pressed between two parallel plates under a given weight for a specified time after which the weight was withdrawn. The initial deformation and the final recovery of the grains were noted (Feillet et al. 1977, Laignelet and Alary 1978, Laignelet and Feillet 1979). The values correlated well with the sensory attributes of rice as well as with its amylose content. Sowbhagya et al. (1987), Unnikrishnan and Bhattacharya (1987) and Sandhya Rani and Bhattacharya (1995) in CFTRI used the same instrument with a large number of samples and found that both the parameters were very well correlated with sensory as well as physicochemical parameters of rice. This instrument could also make some rudimentary measurements of classical rheological property in the sense that it could measure the progressive deformation under a weight and the later relaxation on withdrawing the weight. Tsuji (1981, 1982, 1988) in Japan devised a new instrument, Tensipresser, and a new method. Adhesiveness and hardness of cooked rice were measured as usual from two bites from a plunger. The ratio of the adhesiveness/ hardness value at 50% deformation to that at 90% deformation of the rice grain provided the best index of the texture of rice. More recently scientists in NFRI (Ohtsubo et al. 1998, Okadome et al. 1998b, 1999) revised the



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technique for this instrument, using a combination of a low (25%), high (90%) and a continuous progressive compression system, with much success for precise measurement of various textural attributes of cooked rice. This technique enabled them to show that protein played a role in determining the surface hardness of cooked rice. The use of TA.XT2 Texture Analyser (Stable Micro Systems, UK) has recently become very popular. It is being used now in most rice laboratories of the world, particularly in the USA (Champagne et al. 1998, 1999, Lyon et al. 2000). Meullenet and his collaborators at the University of Arkansas, Fayetteville (Meullenet et al. 1998, 2000, Meullenet and Gross 1999, Sitakalin and Meullenet 2000), have used this instrument in compression mode as well as with an extrusion cell. It has been used both in simple maximum load mode, and also with more complex spectral stress–strain analysis mode. Very good correlation of these data with sensory profile data have been reported. This instrument (improved version, TA.HDi) is being used in the RRDC. Scientists at CFTRI (Manohar Kumar et al. 1976, Bhattacharya et al. 1978, Deshpande and Bhattacharya 1982) used a sieve test to measure cooked-rice stickiness. The result agreed very well with sensory stickiness scores and were strongly inversely related to instrumental and sensory hardness scores. However, careful cleaning and drying of the sieve after each test were crucial, which rendered the procedure tedious. Lee and Peleg (1988) used a surface tensiometer to measure the attractive force between cooked rice grains. One cooked grain was pressed against another and then pulled apart, the force being measured. They observed distinct differences between sticky and flaky varieties. Chrastil (1990) used a novel technique to measure stickiness of cooked rice. He determined the number of various clusters (2, 3, 4… grains clinging together) in a sample of cooked rice, and plotted a frequency curve of the clusters. Stickiness was the mode (hmax) of this frequency curve. Demont and Burns (1968), Lisch and Launay (1975) and Alary et al. (1977) used a Kramer cell with Kramer shear press for measuring the texture of parboiled rice. Santoprete (1978) measured the resistance of rice grains to squashing under a weight increasing at a constant rate. Pillaiyar and Mohandoss (1981) used a pressing device to press cooked rice between two glass plates and measured the area. An international cooperative test of various instrumental methods of measuring the texture of cooked rice was done (Juliano et al. 1981b), which found instrumental methods more sensitive than sensory methods.

13.14.2 Sensory evaluation of cooked rice In any experimental or marketing study of rice, understanding the sensory response of the consumer to the cooked rice has always been crucial. In olden days when there was no instrumental methods of studying rice texture, the response of some colleagues, including the experimenter, would have been the method of choice for testing the eating quality of rice. Later on,



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after instrumental methods became available, the need for eating and testing undoubtedly decreased. Nonetheless, one must remember that it is the sensory attributes of rice which are the ultimate standards which the instrumental methods or any other indirect criteria (amylose, pasting, etc.) must always aim to match. Scientific methodology of applying sensory methods to test the eating quality of rice probably started with the broad intensification of research on rice from after the time of World War II. Batcher et al. (1956, 1963) in the Human Nutrition Research Division of the USDA were probably among the first to develop techniques for sensory evaluation of cooked rice. They got the colour, cohesiveness (stickiness), off-flavour and taste of cooked rice tested by sensory panels, for which they developed appropriate scoring systems. According to Juliano (1985), cooked-rice characteristics frequently assessed were aroma, flavour or taste, tenderness or hardness, cohesiveness or stickiness, appearance and whiteness or colour. The number of points in the judging scale varied between 2 and 11. The ranking scores of Larmond (1977) were most commonly used in consumer tests and the samples were presented in a randomised manner. Consumers were asked to rank the samples and provide reasons thereof. For panel testing, general score cards showing the highest to the least degree of perception of the quality was used. Others who have studied and employed sensory techniques to study eating quality of rice are Oñate et al. (1964), Oñate and Del Mundo (1966a, 1966b), Del Mundo (1979) and Juliano et al. (1965, 1972) at the University of Philippines and IRRI. Recently sensory panel testing of cooked rice has been taken to a higher level of sophistication by Elaine Champagne and her team at the Southern Regional Research Centre of the USDA at New Orleans, LA, working in collaboration with researchers at other USDA laboratories dealing with rice in southern USA. They developed techniques to test for 13 attributes of flavour and 14 attributes of texture of cooked rice by smelling, mouthfeel and biting and masticating (Champagne et al. 1997, 1998, Lyon et al. 1999, 2000). They also used these techniques to study the effects of various factors on rice eating quality, such as rice drying, milling, genotypic and phenotypic variation, storage, effect of presoaking, etc. (Champagne et al. 1997, 2004a, 2004b, Lyon et al. 1999). Others who have worked on similar lines are Park et al. (2001) and Rousset et al. (1995). Very similar work, but perhaps a little smaller in scope, was done by Jean-Francois C. Meullenet and his colleagues at the University of Arkansas, Fayetteville (Meullenet et al. 1998, 1999, 2000).

13.15 Testing for aroma of scented rice varieties As discussed in Chapter 10, there are a number of scented or aromatic rice varieties throughout the world. Two groups of them, basmati and jasmine, © Woodhead Publishing Limited, 2011


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have become commercially very important. International trade in these two rices have grown spectacularly during the last two decades or so. Testing for the aroma of the above two groups and of scented rice in general is of much commercial importance. The authenticity and purity of these rices in international trade were accepted more or less on trust until about 10–20 years ago. But in view of many surrogate lines and of mixing and possible adulteration, testing for the purity of the varieties has become important. One aspect of this purity testing is to test for the intensity of their characteristic aroma. Testing for aroma is also essential in all programmes of breeding of these aromatic rices. There was no special method of testing for aroma of scented rices in the past. Breeders, traders and buyers had to cook the rice and smell the result. Then it was found that if one bit the raw grains in the mouth, one could know if the rice had aroma. Attempts were made from the 1950s to see whether a test could be developed that would detect the presence and the degree of aroma in these varieties. Some researchers warmed the leaves of the scented rice plant in a glass vial. It was observed that the characteristic scent would evolve thereby, which could be assessed by inhaling (Nagaraju et al. 1975, Tripathi and Rao 1978), but the test was not precise enough. Sood and Siddiq (1978) then found a novel method by which the aromatic rice grains could be made to give out the characteristic smell that could be detected by the nose. The method consisted of adding dilute alkali to the grains or the plant parts in a Petri dish and inhaling the evolved aroma. This method was quite successful and has been used for decades especially by the breeders. Vasudeva Singh et al. (1986) examined the matter and found that the method of Sood and Siddiq (1978), sound in principle, could be further standardised. They found that the amount of grain, the concentration of the alkali solution and the volume of the alkali were all important. For instance, too deep a layer of alkali prevented the odour from evolving freely. After several trials, the best conditions for the test were specified (see Appendix). Applying the test blindfolded to a large number of breeding lines showed a high level of correct identification. The authors also successfully applied the test to brown rice and to the leaves of the plant by slightly modifying it. Meanwhile a large number of scientists have been trying to characterise the aroma of scented rice varieties by gas chromatographic analysis. This aspect has been reviewed in detail in Chapter 10. Buttery et al. (1983) concluded after a series of studies that 2-acetyl-1-pyrroline (2-AP) was the principal characteristic component of the volatile compounds evolved from scented rices. They also concluded that measuring 2- AP by gas chromatographic analysis of the volatile material in the rice could give a conclusive as well as a quantitative estimate of the aroma of the rice. The 2-AP test is now recognised worldwide as an authentic standard test of aromatic rices. No doubt because of the cost and the sophisticated



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technique involved, this test has yet to be widely used. It is being used only in a very few selected reference laboratories. Until its general adoption, therefore, the alkali test still remains the workhorse of the scented-rice testing methodology. The above discussion refers specifically to the characteristic aroma of the scented rices. But apart from these, every rice develops some specific aroma or other after cooking. This is an important issue in the sensory quality of rice and has been reviewed by Champagne (2008).

13.16 Various constituents There are a number of trace to minor to substantial chemical constituents of rice that may play some role in rice quality. These constituents are protein, nonstarch polysaccharides, lipids, sugars, free amino acids, sulphydryl and disulphide compounds as well as several enzymes. Protein in rice is very important. Apart from possibly influencing rice quality in general, protein of rice in itself is of paramount importance. After all rice even today forms the bulk of the diet of a vast number of rather impoverished populations. The protein content of rice therefore plays a crucial role in terms of their nutrition, for it may form the lion’s share of the protein that they have access to in their diet. Most if not all the other components mentioned above, including the various enzymes, do play some role or other in terms of rice behaviour. The free sugars may play a role in the development of colour in any processed rice product, apart from any other functional role. The same may be true of free amino acids. Sulphydryl and disulphide components have been repeatedly suggested as playing a strong role in the ageing of rice, in the quality of rice products and in the flavour of rice. Enzymes may have an effect in storage and processing. Nonstarch polysaccharides are important in rice structure. Even though very small in quantity, the various components of lipid, including free fatty acids, are well known to play a strong role in the flavour, odour, storage and product quality of rice. All of these components are standard components of foods in general. Testing for these components are well described in detail in several monographs and handbooks dealing with food analysis and is therefore not further discussed here.

13.17 Tests for parboiled rice As has been discussed in Chapter 8, about one-fifth of the world’s rice is parboiled. The region of south Asia is the home of parboiled rice, where probably some 60–70% of the rice produced is converted into the parboiled



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form. Most of this production in south Asia is for local consumption. This production therefore, as it happens, is not necessarily subjected to a great deal of quality control. Nonetheless international trade in parboiled basmati group of rices has picked up in recent years, where quality becomes an important factor. Apart from that, a substantial amount of rice is parboiled in the USA and a small amount in other places (Thailand, Surinam, etc.) meant for international trade. These are obviously under a reasonably strong quality control regimen. Although basically the same cereal, parboiled rice in many ways has properties that are quite different from those of raw (i.e., nonparboiled) rice. The same system of quality testing discussed so far in this chapter may or may not therefore be applicable or sufficient in full to parboiled rice. At the same time a substantial amount of parboiled rice is traded internationally and systems are needed to ensure its quality. Even the very large quantities of parboiled rice produced in south Asia and also to some extent in Brazil, even though not under a good deal of pressure of strict quality assurance, at least require tests to understand their nature and properties. From all these standpoints, a system of tests for understanding the quality of parboiled rice is essential. Parboiled rice in reality is an umbrella term since parboiled rices have at least three different strands of property profiles because of three different methods for their production. The attributes also vary accordingly and the tests would differ. First, the product may have to be tested to find out the temperature at which the paddy grain had been soaked in water (which may vary from the ambient to about 70 °C). The second objective would be to know what type of parboiling by which the material was produced. There are basically three types of parboiling. These are, first, the regular steam-parboiling, in which the paddy is fully soaked in water to saturation and then steamed to cook the starch. The steaming may vary from open steaming (i.e., at atmospheric pressure) to being under an elevated pressure (up to 2–3 kg/cm, gauge). One can thus see that although the type is identical (steam-parboiling), the degree of parboiling could vary greatly according to the time and pressure of steaming. The second type is pressure-parboiling. In this process, the paddy is not fully soaked but merely wetted or lightly soaked and is then steamed under a high pressure. This process produces rather more discoloured and more hard-cooking rice, but here again the degree of parboiling may vary depending on the pressure and time of steaming. Finally, the third process of parboiling is dry-heat parboiling. In this case paddy is soaked fully, but then instead of steaming, is subjected to conduction heating usually with hot sand. Clearly, therefore, there are three types of parboiling but under each type there may be different degrees of parboiling. So all the three aspects: temperature of soaking, type of parboiling and degree of parboiling need to be tested.



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13.17.1 The tests and their basis The tests basically depend on the fact that parboiled rice is a precooked product. In other words, the starch in this product is in a state of being gelatinised and also partially retrograded. In addition, varying amounts of other polymorphs of starch may be present, depending on the parboiling type and degree (see Chapter 8). Therefore the testing methods would have to mainly determine the extent of these various starch polymorphs, or to put it in the proper perspective, their effect. Coming to the testing methods, the possible approaches are summarised in Table 13.11. The first obvious property to test is the room-temperature hydration ability, i.e, the EMC-S. This property would easily distinguish between raw and parboiled rice, the latter showing a much higher EMC-S value (see Table 13.6). Moreover as the EMC-S value increases with increasing degree of gelatinisation, the actual test value should also give a good idea about the degree of parboiling. The EMC-S value would further rise in dry-heat parboiled rice (because the simultaneous cooking and drying Table 13.11 Tests for parboiled rice Property

Principle of test

EMC-S

Rice is soaked in ambient water overnight. The Ali and Table 13.6 equilibrium moisture content attained by it goes Bhattacharya up from about 27% (wb) for raw rice to 50% or (1972a) more for severely parboiled rice.

Slurry viscosity

Rice flour is made into a slurry with water (20%, db). Viscosity increases with increasing degree of parboiling.

Unnikrishnan Fig. 13.36 and Bhattacharya (1983)

Sediment volume

A 10% slurry of rice flour in 0.05N HCl is allowed to stand in a measuring cylinder. Sediment volume increases with increasing degree of parboiling.

Bhattacharya Fig. 13.37 and Ali (1976)

Alkaline 100 mg of rice flour is shaken with 4 ml of gel volume 1.25% KOH and allowed to stand for 30 min. The gel volume increases with increasing severity of parboiling.

Reference

Pillaiyar (1984)

Illustrated by

Fig. 13.38

Rice kernels are put in 0.5-1.0% KOH. Extent Ali and Alkali Fig. 13.39 degradation of kernel degradation increases with increasing Bhattacharya of kernel degree of parboiling. (1972b) Affected kernels also show characteristic patterns of chalky mass or gel mass and/or kernel splitting depending on parboiling type. Alkaline A few grains are heated with 1 ml 5% KOH. kernel gel Pressure-parboiled rice forms a lump of gel. Others are dispersed without forming a translucent gel.

Mahanta and Fig. 13.40 Bhattacharya (1995) Pillaiyar et al. (1997)

–



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of the paddy grain by conduction heating retards amylopectin retrogradation) (Table 13.6). The same effect would happen partly in pressure-parboiled rice. The test would thus be able to partially distinguish between different types of parboiling as well. Two related properties are viscosity and sediment volume of a slurry of rice powder in water. Because of being gelatinised, parboiled rice powder, when slurried in water, develops a perceptible viscosity which raw rice slurry hardly does (Fig. 13.36). For the same reason again, such a dispersion of parboiled-rice powder in water produces a higher sediment volume than a raw rice powder slurry (Fig. 13.37). In the same manner, the rice powder can be dispersed in dilute alkali or in dimethyl sulphoxide (DMSO), when they would produce a distinctly different volumes of gel, depending the state of the starch (Fig. 13.38). In both the above cases (slurry viscosity, sediment or gel volume), the property would get intensified with increasing degree of parboiling. The result would thus be able not only to distinguish parboiled rice but also indicate the extent and nature of parboiling. D-PB FL

104

Viscosity, cP

VS-PB DR-PB 103

S-PB

M-PB Raw 102

101 100

101 Shear rate, s–1

102

Fig. 13.36 Apparent viscosity of raw and processed-rice flour slurries in water (20%) read in a coaxial cylinder viscometer. PB = parboiled rice; M, S, VS, D, DR = mild, severe, very severe, dry-heat, D retrograded (D moistened and tempered), respectively; FL = flake rice. Reproduced, with permission, from Unnikrishnan and Bhattacharya (1983) John Wiley and Sons.



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Fig. 13.37 Sediment volume of various rice products (~10% aqueous rice-flour slurries allowed to stand). 1, raw rice (5.5 ml); 2, mild parboiled rice (6.2); 3, parboiled rice (6.7); 4, severely parboiled rice (8.1); 5, dry-heat parboiled rice (12.8); 6, flaked rice – thick (16.2); 7, flaked rice – thin (19.5); 8, unmodified tapioca starch (3.6); 9, pregelatinised tapioca starch (22.5 ml). Reproduced, with permission, from Bhattacharya and Ali (1976).

Fig. 13.38 Alkaline gel volume test for parboiled rice. 100 mg rice flour shaken with 4 ml 1.25% KOH and stood 30 min. Samples are (from left): raw rice, light, moderate and severe parboiled rice. Source: RRDC (unpublished).

Another closely related property is the digestion of rice kernel in dilute alkali. Being pregelatinised, the parboiled rice kernels would be amenable to digestion by dilute alkali much more easily than raw rice kernels. As a result, parboiled kernel is attacked or digested by such dilute alkali as would not attack the raw rice kernels at all. Moreover the degree of digestion



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would increase with increasing degree of parboiling (Fig. 13.39). The extent of kernel digestion in very dilute alkali would thus provide a very simple test of parboiled rice and also an approximate measure of the degree of parboiling. The alkali test also has the ability to distinguish between different types of parboiling. In steam-parboiled rice produced under ambient or low steam pressure, even though the kernel is attacked and degraded by the alkali, the kernel central mass remains chalky in appearance. Pressure-parboiled rice, on the other hand, is not only attacked by alkali but also shows a completely translucent kernel mass. However if the grain is not fully parboiled up to the kernel centre but has a white belly, then the alkali-dispersed mass would also have a chalky centre. Pressure-parboiled rice can thus be distinguished by the test. Dry-heat parboiled rice shows a characteristic kernel splitting in this alkali test (Fig. 13.40). This test thus can distinguish both the type and the extent of parboiling. Pressure-parboiled rice can be distinguished by another alkaline gel test similar to the alkali-kernel test above. If a few grains of rice taken in a test tube are heated with 1 ml of 5% KOH in a boiling water bath for 4 min, the pressure-parboiled grains are converted into a lumpy gel, while others remain chalky (Pillaiyar et al. 1997). We can thus see that several elegant tests, although not entirely quantitative, can give a highly sensitive qualitative and an approximately quantitative test of both the type and the degree of parboiling of rice. As for the temperature of soaking, this can be approximately determined by a heat-discoloration test (see Chapter 8). Potential enzyme action during soaking of the paddy (the first step in parboiling) produces more or less quantity of sugars in the rice grain depending on the temperature of soaking. Maximum sugars are produced at around 60 °C soaking, the least at ambient temperature and practically nil if the paddy is presteamed before soaking (‘double-boiling process’: see Chapter 8). Accordingly, if the milled parboiled rice is heated in a closed tube at 90 °C for overnight, the grains develop discoloration proportional to the sugars produced (Table 13.12). This test thus can provide a fair idea of the condition under which the paddy had been soaked for parboiling.

Fig. 13.39 Degradation of raw (R) and very mild (V), mild (M), and severe (S) steam-parboiled rice grains in very dilute (≤1.0%) aqueous KOH. Reprinted from Ali and Bhattacharya (1972b) with permission from Elsevier.



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Rice quality Raw

30-0-10

30-0-60

30-1-10

12-3-10

12-3-20

17-1-10

17-3-20

Raw

200°-2-20

250°-1-20

275°-0.75-20

30-0-10

200°-4-16

250°-2-16

275°-1.5-16

Fig. 13.40 Degradation of various parboiled rice kernels in 0.9% KOH after 24 h. Top: top line, normal steam-parboiled rice; bottom line, pressure-parboiled rice. Sample code: first number = nominal grain moisture (%), second number = steaming pressure (kg/cm2 gauge), third number = steaming time (min). All pressure-steamed kernels become translucent mass, except those with white belly. Bottom: dry-heatparboiled rice kernels are chalky mass and longitudinally split. Code: first number = roasting temp (°C), second number = time (min), and third number = final grain moisture (%). Reproduced, with permission, from Mahanta and Bhattacharya (1995).

The degree of starch gelatinisation in the rice can be fairly quantitatively determined by a test originally suggested by Birch and Priestley (1973) and adapted by Mahanta (1988: see Mahanta and Bhattacharya 2010). In this method, the amount of amylose extracted from the rice flour by 0.2 N and 0.5 N KOH is measured. The ratio of the two values gives a measure of the degree of starch gelatinisation, unhampered by starch retrogradation or lipid–amylose complexation. When the degree of starch gelatinisation is very high, the ratio approaches 100%. But the ratio falls off as the gelatinisation decreases and is least in untreated raw rice (40%).



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Table 13.12 Effect of the temperature of soaking of paddy on the discoloration of resulting milled parboiled rice upon reheatinga Soaking temperature (°C)

RTc 50 60 70 80

Approximate colourb of parboiled rice Before reheating

After reheatinga

± ± ± ½+ +

+ 3+ 5+ 3+ 1½+

Data of K. R. Bhattacharya, unpublished. a Milled rice placed in a closed tube and heated in an oven at 90 °C for 18 h. b The number of plus signs shows the approximate relative colour. ± = faint. c Room temperature (23-27 °C).

13.18

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irri (1978) Annual Report for 1977, International Rice Research Institute, Los Baños, Philippines (cited in IRRI 1979). irri (1979), Chemical Aspects of Rice Grain Quality, Los Baños, Laguna, Philippines, International Rice Research Institute. johnson r m (1965), ‘Light reflectance meter measures degree of milling and parboiling of parboiled rice’, Cereal Chem, 42, 167−174. juliano b o (1971), ‘A simplified assay for milled-rice amylose’, Cereal Sci Today, 16, 334–336, 338, 360. juliano b o (1979), ‘Amylose analysis in rice – a review’ in Chemical Aspects of Rice Grain Quality, Los Baños, Laguna, Philippines, International Rice Research Institute, 251–260. juliano b o (1985), ‘Criteria and tests for rice grain qualities’, in Juliano B O (Ed.) Rice Chemistry and Technology, 2nd edn, St. Paul, MN, American Association of Cereal Chemists, 443–524. juliano b o and perez c m (1983), ‘Major factors affecting cooked milled rice hardness and cooking time’, J Texture Stud, 14, 235–243. juliano b o and perez c m (1984), ‘Results of a collaborative test on the measurement of grain elongation of milled rice during cooking’, J Cereal Sci, 2, 281–292. juliano b o, oñate l u and del mundo a m (1965), ‘Relation of starch composition, protein content and gelatinization temperature to cooking and eating qualities of milled rice’, Food Technol, 19¸1006–1011. juliano b o, oñate l u and del mundo a m (1972), ‘Amylose and protein contents of milled rice as eating quality factors’, Philippine Agriculturist, 56, 44–47. juliano b o, perez c m, blakeney a b, castillo d t, kongseree n, lagnelet b, lapis e t, murty v v s, paule c m and webb b d (1981a), ‘International cooperative testing on the amylose content of milled rice, Starch/Stärke, 33, 157–162. juliano b o, perez c m, barber s, blakeney a b, iwasaki t, shibuya n, keneaster k k, chung s, laignelet b, launay b, del mundo a m, suzuki h, shiki j, tsuji s, tokoyama j, tatsumi k and webb b d (1981b), ‘International cooperative comparison of instrument methods for cooked rice texture’, J Texture Stud, 12, 17–38. juliano b o, balkeney a b, butta i, castillo d t, choudhury n, iwasaki t, shibuya n, kongseree n, lapis e t, murty v v s, paule c m, perez c m and webb b d (1982), ‘International cooperative testing of the alkali digestibility values for milled rice’, Starch/Stärke, 34, 21–26. kamath s, stephen j k c, suresh s, barai b k, sahoo a k, radhika reddy k and bhattacharya k r (2008), ‘Basmati rice: Its characteristics and identification’, J Sci Food Agric, 88, 1821–1831. kik m c (1951), ‘Determining the degree of milling by photoelectric means’, Rice J, 54 (13), 18−22. kuhn t s (1962), The Structure of Scientific Revolutions, Chicago, The University of Chicago Press. kunze o r and calderwood d l (2004), ‘Rough-rice drying – Moisture adsorption and desorption’, in Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 223–268. kunze o r, lan y and wratten f t (2004), ‘Physical and mechanical properties of rice’, in Champagne E T (Ed.) Rice Chemistry and Technology, 3rd edn, St. Paul, MN, American Association of Cereal Chemists, 191–221. kurasawa h, igaue i and hayakawa t (1962), ‘Study on the eating-quality (especially stickiness) of white-milled non-waxy rice. (Part III) A starch-iodine blue test of rice powder as a quality indication of the rice’ (in Japanese), J Jap Soc Food Nutr, 15, 84–87. kurasawa h, kanauti y, yamamoto i, hayakawa t and igaue i (1969), ‘Some physicochemical properties of nonwaxy paddy rice starch in Niigata prefecture. Part I. Relation of properties of starch to eating and cooking qualities of milled rice’, Agric Biol Chem, 33, 798–806. © Woodhead Publishing Limited, 2011


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Appendix: some selected rice quality test procedures

Procedures for some of the rice quality analytical methods are presented in this appendix. The presentation is by no means intended to be exhaustive: only the routine, commonly used test methods are presented. Most of these analyses are daily staples, at any rate fairly routinely required tests used in any rice laboratory. Here again the intention is not to present all procedures for the different tests. By and large the tests that the author is familiar with, or those which are regularly used in the author’s laboratory, are presented. This appendix should be read in conjunction with Chapter 13. The background to the tests, their underlying theoretical basis, as well as the limits of their applicability and limitations are all discussed in detail there. Therefore the relevant section in Chapter 13 should be consulted before undertaking a test as described below.

A.1

Physical properties

A.1.1 Grain length and breadth Materials 1. Sample of rice (paddy, brown rice, milled rice) to be tested, appropriately cleaned. 2. Sample divider, quartering arrangement. 3. Steel centimetre rulers – 2 Nos. 4. Tweezers/forceps


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Procedure 1. Divide the sample appropriately, using sample divider and or quartering, as appropriate, to end up with a quantity close to approximately 1 g. Note: Selecting 50 grains randomly from a larger sample is theoretically feasible, but it is found in practice that one acquires a subconscious bias to select well-formed grains despite being vigilant. Therefore it is better to divide the sample to as close to 50 grains as possible and select 50 grains therefrom. 2. Arrange the two steel rulers perpendicularly (see Fig. 13.9). 3. Pick up grains one by one and arrange ten grains end to end, barely touching each other, along the edge of the horizontal ruler (see Fig. 13.9). Select the grains randomly but reject immature, insect- or fungusinfected, tip-broken and otherwise damaged or defective grains; but do not reject sound but smaller grains. 4. Read the length at least up to half a millimetre. Divide by 10 to get the grain length. 5. Repeat five times in all and take the average of five means as the grain length. Alternatively add up all the five readings and divide by 50. 6. Proceed identically for the grain breadth (width) (see Fig. 13.9).

A.1.2 Grain thickness Materials 1 Dial gauge 2 Rest as above Procedure 1 Proceed as in A.1.1, except to measure 10 grains one by one in the dial gauge. 2 Take the average and report thickness up to second place of decimal of a millimetre.

A.1.3 Grain weight Materials 1 The rice sample (paddy, brown rice, milled rice) to be tested, appropriately cleaned. 2 Sample dividing arrangement. 3 Chemical balance (weighing up to fourth place of decimal of a gram). Procedure 1 Divide the sample, using suitable dividing techniques, to a quantity equivalent to approximately 300 grains. 2 Spread the grains on a tray or table and reject obviously immature, tip-broken and other defective grains (but do not reject sound but small grains). © Woodhead Publishing Limited, 2011


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Appendix: some selected rice quality test procedures 3 4 5

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Weigh the entire quantity up to the third place of decimal of a gram. Remove carefully on to a tray or table top and count without missing any. Divide the weight by the number of grains to obtain the mean grain weight in milligram upto the first place of decimal (the numerical value is the same if expressed in g/1000 grains). Repeat at least two times and calculate the average of the means. Note: If an electronic grain counter is available, the procedure becomes simpler and a larger number of grains (say up to 1000 grains each time) can be tested.

A.1.4 Density Materials 1 Sample of the grains to be tested, appropriately cleaned Note: Expose the grains suitably in the room (or in an air-conditioned room) or adjust its moisture suitably so that its moisture content is known and adjusted approximately at 13.0 ± 0.5%. 2 Chemical balance. 3 Density flask (see Fig. 13.11). Wash thoroughly and dry in an oven. 4 Solvent (kerosene, toluene, benzene, isopropyl alcohol). Procedure 1 Divide the sample appropriately to obtain approximately 5-8 g. 2 Spread the sample on a tray, examine and discard obviously defective grains such as immature, infected and discoloured grains. 3 Weigh 5-8 g of grains accurately, correct up to at least the second place of decimal of a gram. 4 Pour solvent into the density flask up to a mark towards the bottom of the stem. 5 Allow to drain for a few minutes. Hang the flask vertically from the top with two fingers, adjust the meniscus to the eye level and read the solvent level without parallex upto the second place of decimal of a millilitre. Avoid standing near a window or door, if the outside is too warm or too cold, or if there is a breeze, or near a hot stove or flame or an ice box. 6 Pour the rice through the funnel into the flask. Swirl gently once or twice. Wait for a few minutes for the bubbles to escape and the solvent to drain. Note: In the case of paddy, bubbles will not cease, so a compromise has to be made. Wait until there is heavy escape of bubbles and then proceed to measure the liquid level. 7 Read the liquid level as before.



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Calculation Divide the weight of the sample taken by the change in the liquid volume and report density at least up to the third place of decimal (g/ml).

A.1.5 Bulk density Materials 1 Sample to be tested, appropriately cleaned and divided. 2 Test weight apparatus (see Fig. 13.12). 3 Top-pan balance Procedure 1 Divide appropriately and take approximately 0.5–1.0 kg of grain. The sample should have been previously cleaned to remove chaff and impurities and well mixed. Notes: (i) If the material is paddy, and if it has awns, one should know whether one wants to measure the bulk density with or without awns. It should be a deliberate decision and reported accordingly. The value without awn should be mandatory, the other being optional. (ii) If paddy has awn, in the absence of a deawner, awn can be removed by taking a few handfuls of paddy in a towel and rubbing for a few minutes. (iii) Again bulk density can be determined including or excluding the immature grains. The decision should be deliberate and reported accordingly. An unspecified value would be understood to be including immature grains if any. 2 Arrange the test cup below the funnel. Pour sufficient quantity of grain into the funnel with the stopper closed. Open the stopper allowing grain to fall on to the cup placed over a tray, excess grain overflowing the cup. Remove the funnel. Level the grain in the cup with zigzag strokes of the blunt ruler. 3 Weigh the content. 4 Repeat two or three times and take the average value. 5 Report the bulk density as g/l. Note: (i) If only a small quantity of a sample is available, it can still be measured. Take a suitable beaker and get it cut at the top to remove the lip. Determine its volume by filling with water and weighing. This beaker can be used to determine the bulk density of a smaller quantity of grain. Check its accuracy by measuring a suitable sample both in it and in the standard cup, and apply a correction factor to the small-cup value if needed. Porosity can be calculated from the density and bulk density values by the formula shown on page 457.



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A.1.6 Angle of repose Materials 1 Sample of grain, appropriately cleaned and divided. 2 Angle of repose set (see Fig. 13.13). Procedure 1 Place the angle of repose set on a perfectly horizontal table or bench. Place it at the edge, so that excess grain can fall into a tray placed on a stool below. 2 Pour grain into the Perspex box preferably using the test weight pouring funnel to overflow the box. In any case the height from which the grain is poured on to the box should be the same in each case. 3 Level the grain in the box by zigzag strokes of a blunt ruler. 4 Flick open the front wall of the box allowing excess grain to fall. 5 Read the angle on the etched sides of the box. The top bounding line of the grain may or may not be a straight line. It may be slightly convex. In such a case, read an imaginary tangent at the middle portion. 6 Repeat the determination at least three times. 7 Report value as degree angle to the horizontal.

A.1.7 Grain hardness Materials 1 Sample of grain, appropriately cleaned and divided Note: The grain moisture should be adjusted to around 13%. 2 Kiya hardness tester (see Fig. 13.14). 3 Tweezers/forceps Procedure 1 Rotate the apparatus wheel and raise the pressing ram. 2 Place a grain with a tweezer on the platform under the ram in the centre and place it flat. 3 Lower the ram by rotating the wheel at a steady rate until the typical sound of the grain cracking is heard. Note: Do not stop when the grain touches the ram but continue to rotate until the rice cracks. 4 Read the hardness value from the dial. 5 Repeat with approximately 30 grains and take the average. 6 Report hardness as kg/grain. Note: Chalky grains are perceptively softer than translucent vitreous grains. Therefore, even if otherwise identical, the hardness of a sample with more chalky grains will be less than that of a sample with less chalky grains. For this reason, it is better to reject all chalky grains from brown or milled rice while measuring its hardness. The value should be reported as that of translucent kernels. Such a recourse is not possible



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Rice quality in the case of paddy, for which reason the sample of paddy should be thoroughly mixed with extra care.

A.1.8 Tightness of lemma–palea interlocking of paddy Materials 1 Paddy sample suitably cleaned and divided. 2 Rubber-roll dehusker: Satake Testing Husker (THU 35B). 3 Triangular trays. 4 Feeler gauges. 5 Dial gauge. Procedure 1 Determine the average thickness of the paddy grain using the dial gauge (Method A.1.2). 2 Set the clearance or distance between the two rubber rolls of the dehusker at five-eighths of the average thickness of paddy, using a feeler gauge of that value. 3 Select randomly 100 sound grains of the paddy. 4 Start the dehusker and pour the grains at a slow and steady rate from the top. 5 Count the paddy remaining undehusked. 6 Repeat at least three times. 7 Report mean value as percentage grain dehusked.

A.2

Milling of rice, and degree of milling (DM)

A.2.1 Milling quality There will be occasions when the milling quality of a sample would have to be determined. This estimation would generally include the determination of the ∑ ∑ ∑ ∑

yield yield yield yield

of of of of

total brown rice; head brown rice; total milled rice; and head milled rice.

This need would more particularly arise in rice mills where large quantities of paddy was being purchased. An assessment of the milling quality of a sample intended for purchase would be essential for a healthy economics of the company. The procedure described in Section A.2.2 below, with minor variations as appropriate for the work in hand, can be easily adopted for the purpose of estimating the milling quality of the sample of paddy. It is therefore not separately described here to avoid duplication. © Woodhead Publishing Limited, 2011


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A.2.2 Preparing standard DM samples Materials 1. Sample of paddy (or brown rice), appropriately cleaned and divided. 2. Satake Testing Husker (THU 35B). 3. McGill miller No. 2 or No. 3. 4. Satake Testing Mill (TM 05C). 5. Standard sieve – 18 mesh. Note: Use a standard, certified sieve. In absence of such, a nonstandard sieve can be used provided an appropriate one has been tested against a standard one and found reasonably equivalent (ignore its mesh-size label). 6 Satake Grader (TRG 05B), or other equivalent, with appropriate indented cylinder; or a sizing device with appropriate indented plates. 7 Triangular trays. Procedure 1 Dehusk the paddy suitably using the Satake dehusker in sufficient quantity (see below). The dehusker (sheller) should be so set as to dehusk roughly 85% of the paddy in one pass. 2 Remove paddy from the brown rice, first using the grader or sizing device and later with the hand with or without the help of a winnower (or a suitable riddle in a dockage tester). Reshell or discard the paddy so separated. Remove broken brown rice grains too with the help of the grader or sizer. Collect sufficient amount of whole brown rice, enough to prepare several samples having different degrees of milling. Mix well. Note: The presence of some unshelled paddy in the product of laboratory dehusking creates a tricky situation (see discussion in Section 13.4). Complete (100%) shelling in a pass is not possible without causing excessive grain breakage. The presence of residual paddy can be ignored if the objective is only to produce milled white rice for some study. But if the milling quality is to be tested (including brown-rice yield) or if reference samples of known DM are to be prepared, separating the unshelled paddy is a must. However there is little equipment which can separate paddy from brown rice in a go. Size separation using indented plates/cylinder, riddles (riddle 000 with the Carter dockage tester), and finally the hand is to be employed. The unshelled paddy so separated can be discarded if the objective is only to prepare standard DM samples. But if the objective is to test the milling quality, then the separated paddy must be separately shelled and the brown rice added to the main stock. Mix well. 3 Take a weighed amount of brown rice for each milling (~ 100 g for McGill 2, ~ 750 g for McGill 3, ~ 200 g for the Satake). 4 Prepare samples having different degrees of milling, using one of the millers (McGill No. 2, No. 3 or Satake). Appropriate weight and time should be used with the McGill to get the degree of milling desired. The weight should not be so high that the desired extent of milling is achieved in no time, nor so low that the milling cannot be achieved in © Woodhead Publishing Limited, 2011


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6

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8

9 10

Rice quality a reasonable time. For the Satake, choose the proper rpm and the time of milling. Collect the entire quantity of bran material carefully, using a brush to brush down from every nook of the milling chamber. Screen through the 18-mesh sieve. Screen the milled rice too as needed. Weigh the bran. Divide by the amount of brown rice taken for milling and calculate the degree of milling by weight as a percentage. Prepare each sample in replicate if necessary. Prepare several samples with different DM in the range of 0-12%. Note: It will be hard to produce DM beyond 8-9% with the McGill. But the Satake can produce up to 12% DM or even more. Collect the rice immediately after milling and put inside double polythene bags to prevent loss of moisture and cracking. Preferably keep these bags under cover to allow slow cooling. Pass each sample through the grader to remove all brokens and collect head rice. Label appropriately and store.

A.2.3 Estimating the DM of rice: methylene blue staining method Materials 1 Petri dish (about 9 cm dia). 2 Standard volumetric flasks. 3 Forceps. 4 Magnifying glass. Reagents 1 Approximately 0.5N hydrochloric acid (~95 ml of distilled water + 5 ml conc. HCl). 2 Methylene blue solution – 0.05% (50 mg in 100 ml distilled water). Procedure 1 Take about 5 g representative sample of rice in a Petri dish. 2 Add about 15 ml methylene blue solution and allow to stand for 2 min. Decant. 3 Add about 15 ml 0.5N HCl, swirl 6-8 times. Decant. Repeat once more. 4 Add about 20 ml distilled water, swirl 6-8 times. Decant. Repeat two more times. 5 Add about 20 ml of distilled water and allow to stand for 15 min. Decant and replace with fresh water and observe. Observation 1 An approximate idea of the degree of milling can be had from the distribution of deep blue bran layers/streaks against a very light blue endosperm (see Table A.1). © Woodhead Publishing Limited, 2011


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Table A.1 Estimating the approximate degree of milling (DM) of rice by methylene blue staining of bran Approx. DM Approx. % distribution of grains (McGill) (%) > ¼ of grain One full length surface dark blue dark blue streak

Small dark blue streaks (1 or 2)

Without streaks

3 5 6 7 8

40 60 13 3 0

0 20 85 97 100

40 4 0 0 0

20 16 2 0 0

2. Apart from indicating the approximate DM, the main value of the test lies in confirming the completion of the milling process. The presence of even a tiny fractional blue streak anywhere on the endosperm would indicate that a small portion of the bran is still remaining, while no such streak would mean that the bran removal has been completed.

A.2.4 Estimating the DM of rice: residual bran pigment method Materials 1 Rice sample to be tested. Separate and discard broken grains. 2 Reference brown rice sample. It should be the same stock from which the test rice sample originated. The brown rice should have been prepared using a rubber roll sheller so as to have no scratches. 3 Grind both test and reference samples to 30-40 mesh powder. 4 Glass-stoppered conical flask – 100 ml, Pyrex. 5 Centrifuge and centrifuge tubes. 6 Spectrophotometer (or Spectronic-20 colorimeter). Reagents 1 2% aqueous KOH solution. 2 n-Propanol (or isopropanol). 3 Solvent – mix the KOH solution and propanol in the proportion of 1:2 (v/v). Make fresh for each test. Method 1 Make an approximate visual appraisal of the DM of the test sample. Weigh the requisite amount of the sample, based on its approximate DM as shown in Table A.2, into a conical flask. 2 Pour the solvent gently into the flask through its side. The amount of solvent should be 1.0 ml for every 0.1 g of the powder (e.g., 2.0 ml solvent for 0.2 g sample, 5.0 ml for 0.5 g sample, etc.). Stopper and leave the flask undisturbed in dark overnight.



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Rice quality Table A.2 Amount of sample to be taken for the residual bran pigment test

3 4 5 6 7

Approximate DM (by wt) (%)

Sample wt (g)

0–1 1–2 2–3 3–4 4–5 5–6 Over 6

0.2 0.3 0.4 0.5 0.6 0.8 1.0

Add the balance of 10.0 ml solvent next morning (e.g., 8.0 ml for 0.2 g powder, 5.0 ml for 0.5 g powder, etc.) into the respective flask. Shake the solution vigorously for 10–15 min. Transfer solution to a 10–20 ml centrifuge tube covered with an aluminium foil cap and centrifuge at low speed for a few minutes. Decant and read absorbance of the extract in a spectrophotometer at 400 nm against the solvent blank as soon as possible. Read similarly for the reference brown rice simultaneously.

Calculation 1 Calculate the absorbance for each sample per gram of rice per 10 ml solvent. For example – if 0.2 g sample in 10 ml gives an absorbance reading of 0.60, then its absorbance/g/10 ml is 3.00. Or if 0.5 g of sample (in 10 ml) gives an absorbance of 0.70, then its absorbance/g/10 ml is 1.40. 2 Express all above absorbance results of all test samples as a percentage of that for the corresponding brown rice. 3 A standard curve should be prepared by using reference samples of known DM (see Fig. A.1). 4 Read the % absorbance value of the test sample against the curve to indicate its DM. Comments 1 If the corresponding paddy or brown rice from which the test sample originated is available, then the method can give quantitative results. It has been shown that when the amount of bran pigment lost during milling is expressed as a percent of the pigment in the original brown rice, different varieties give a reasonably uniform curve (Fig. A.1) (Bhattacharya and Sowbhagya 1972). Therefore by referring against the standard curve (or a chart mode therefrom), a quantitative estimate of the degree of milling of the sample can be obtained. 2 In case the corresponding brown rice is not available, the exact DM of sample cannot be estimated. This is because the actual content of bran © Woodhead Publishing Limited, 2011


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Absorbance, % of brown rice

100

80

60

40

20

2

4 6 8 Degree of polish, (%)

10

Fig. A.1 Fall in relative absorbance of pigment extract of rice with progressive milling. Four varieties. Reproduced, with permission, from Bhattacharya and Sowbhagya (1972) John Wiley and Sons.

3

4

5

pigment varies from variety to variety, so that the absorbance value per se would not correspondent to a definite degree of milling. However, even in the absence of the corresponding brown rice, the approximate conclusion of the milling process can be estimated. The absorbance value for all varieties comes down to more or less a constant low level in the range of 0.4–0.5 (per g/10 ml) at a DM of 8–9% (by the McGill). Therefore the test value obtained can indicate whether the sample is well milled or not. The colour is inherently somewhat unstable, but is partly stabilised in presence of rice powder. That is the reason why a relatively constant amount of the pigment and a constant ratio of rice powder to solvent is maintained during the overnight holding, the balance of the solvent being added just before centrifugation. As the colour is not linear, a filter colorimeter is not suitable.*

*As explained in Chapter 13 (page 468), DM of rice is widely, and apparently exclusively, determined in the rice laboratories in the USA by estimating the amount of fat on the surface of the milled rice grain. Because of this widespread use, a brief outline of the method was initially planned to be included in the Appendix. However, a cursory study of the procedures presented in the large number of publications emanating from US laboratories on this subject (see references on page 468) showed a fair degree of interlaboratory as well as intralaboratory difference in the procedures. In the absence of familiarity with the methods of the present author, therefore, it was decided not to venture into the effort. The reader is therefore referred to the literature cited.



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Estimation of moisture

A.3.1 Simplified oven method: heating at 105 °C for 24 h Materials 1 Moisture cups (made of steel or aluminium, slightly tapered (widening) towards the top, with fitting lids). The dishes should have matching numbers etched on them both on the lid and on the cup. 2 Tongs. 3 Desiccator with active fused calcium chloride or silica gel or other dehydrating agent. 4 Chemical balance. 5 Electrical oven. Procedure 1 Tare the moisture cups by heating in the oven, cooling in the desiccator and weighing. 2 Weigh accurately approximately 2-3 g of the well-mixed rice into a tared moisture cup. 3 Switch on the oven and adjust to a steady 105 °C. There should be small, appropriate openings in the oven at the bottom and top for a gentle convection of air (or built in air-circulation system within the oven). 4 Place moisture cups with rice in the oven in a central position exposed to the convection current and where temperature has been tested previously to be steady at 105 °C. 5 Place the lids below the cups. 6 Close the door of the oven and wait till the temperature regains 105 °C. Count time from now and heat for about 24 h. Note: The door must not be opened during this drying for 24 h. 7 Open the door of the oven and quickly replace the lids of the cups with the tong. 8 Take out the cups one by one and put in the desiccator, allowing the heated air in the desiccator to go out. Note: Not more than 4-6 cups should be placed in one desiccator. 9 Allow the desiccator to stand as long as it takes for the cups to cool (say about 30 min). 10 Weigh the cups quickly, but steadily, correct at least up to the third place of decimal of a gram. Calculation Determine moisture loss from the difference in weight. Divide by the weight of rice taken and express as a percentage. Correction factor A correction factor of 1.3% moisture db for paddy, 0.6% moisture db for milled or brown rice, and © Woodhead Publishing Limited, 2011


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0.3% moisture db for rice flour has to be added to the percent weight loss determined above to account for the empirical difference from the value obtained by the standard vacuum oven method with ground material (i.e., notionally equal to the moisture retained by the whole-grain material under the conditions stated). Note that the correction factor stated above is expressed on dry basis and its value on wet basis will go on decreasing as the moisture content of the sample increases (see Table A.2). Add the wb correction factor to the amount of % weight loss determined.

A.3.2 Simplified oven method: heating at 130 °C for 2 h This method is exactly the same as Method A.3.1, except that the heating in oven is done at 130 °C for 2 h. Therefore substitute the words 130 °C and 2 h for 105 °C and 24 h, respectively, in the procedure A.3.1 above to get the procedure for A.3.2. The correction factor will change. Add the appropriate correction factor shown in Table A.3 to the percentage weight loss to get the moisture content of the material.

A.3.3 By moisture meter: calibration of the meter Materials 1 Sample of grain (paddy, rice). 2 Available moisture meter. 3 Arrangements for moisture estimation by the simplified oven method. Table A.3 method

Correction factor for moisture estimation by simplified hot-air oven

Sample

Paddy

Brown/milled rice

Moisture range (% wb)

10 15 20 25 30 35 40 10 15 20 25 30 35 40

Correction factor to be added to per cent weight loss At 105 °C, 24 h

At 130 °C, 2 h

1.2 1.1 1.0 1.0 0.9 0.8 0.8 0.5 0.5 0.5 0.4 0.4 0.4 0.4

2.2 2.1 1.8 1.6 1.3 1.0 0.8 1.5 1.3 1.1 0.8 0.6 0.4 0.4



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Procedure 1 Collect appropriate quantities of several samples of the grain each having a different moisture content in the range of, say, 10-30% (wb). One way of obtaining such samples is to collect samples at different stages as wet paddy is undergoing drying in a dryer plant. Alternatively, wet paddy may be dried in a laboratory dryer working at a low temperature. Several samples can be taken at intervals having different moisture contents in the range above. 2 Alternatively divide a sample of the dry grain into various portions. Approximately calculate the amount of distilled water to be added to different subsamples so as to yield samples with the target moisture levels. Add the water and mix well. 3 Dry a small portion in an oven under gentle conditions to obtain lowermoisture samples (8-13%). 4 Put all samples in wide-mouth closed bottles full to the brim or in double polyethylene bags and keep aside for a day for the moisture to equilibrate. 5 Determine the moisture of all the above samples in the moisture meter, as per the procedure prescribed by the manufacturer, at least in duplicate, preferably in triplicate. 6 Determine the true moisture of the samples by the simplified oven method described above in duplicate. 7 Draw a calibration curve of the moisture as indicated in the moisture meter against the true moisture content of the samples (see Fig. 13.6). 8 Prepare a calibration chart from the above calibration curve (see Table 13.1). 9 Add the appropriate corrections from the above chart to the moisture readings as determined in the moisture meter. Note: 1. Calibrate separately for each material to be tested (paddy, milled rice, raw, parboiled). 2. Repeat calibration after a few months, at any rate at the beginning of each season.

A.3.4 By moisture meter: grain undergoing drying The above correction is not enough for grain undergoing drying (see discussion in section 13.3.2). A further correction is necessary here. Procedure 1 Collect several representative samples of the grain undergoing drying in a dryer plant over a period of time during its flow. 2 Put each sample in a double polyethylene bag immediately and keep aside for a few minutes to cool. 3 Estimate moisture content of each sample using the moisture meter immediately (applying the correction factor as determined above).



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Appendix: some selected rice quality test procedures 4 5 6

7

8

A.4

545

Keep the polyethylene bag aside under cover overnight. Again determine the moisture content of the sample in the moisture meter next day. The difference between the two readings above gives the additional correction factor to be applied to determine the moisture content of grain undergoing drying by using a moisture meter. Determine the above correction factor at different grain moisture ranges. This additional correction factor may be constant at different grain moistures or may itself change with changing grain moisture. Determine the trend and use appropriately. Optionally, check the accuracy of the above procedure by also testing the moisture contents of the samples by the oven method. Note: The procedures at A.3.3 and A.3.4 above can be combined to obtain both the calibrations simultaneously.

Hydration and cooking parameters

A.4.1 Equilibrium moisture content attained by rice upon soaking in water at ambient temperature (EMC-S) Materials 1 Beaker – 100 ml; Petri dish. 2 Filter paper sheets. 3 Distilled water. 4 Tea strainer, spatula. Procedure 1 Take a suitable quantity (5-15 g) of rice in a 100 ml beaker. 2 Wash roughly once or twice with water. 3 Cover with excess distilled water, cover beaker with a Petri dish and leave overnight. Note: Leave overnight for milled rice or brown rice. Stand for at least 60 h with changes of water in case of paddy. 4 Strain through the tea strainer and dry bottom of the strainer by dabbing against pieces of filter paper. 5 Transfer the rice on double-fold filter paper, spread a little with a spatula, cover with another double-fold filter paper. Press well to remove the adhering water. 6 Collect sample in a tared moisture dish and estimate moisture by the simplified oven method. Calculation Express as percentage moisture (wet basis) after applying appropriate correction factor.



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A.4.2 Water uptake by rice during cooking Materials 1 Test tubes or boiling tubes (o.d. 2.5-3.0 cm). 2 Metal stand for above tubes. 3 Boiling water bath. 4 Thin glass rod or stiff wire; spatula or glass rod; funnel. 5 Stop watch. 6 Top-pan balance. 7 Filter papers sheets. 8 Evaporating dish (tared). 9 Tea strainer. Procedure 1 Put about 20 ml distilled water in each tube. Place tubes in the stand and put the entire stand in the water bath. The water level in the bath should be slightly above the level of water in the tubes. 2 Place the thin glass rod or a stiff wire into one of the tubes (to keep it hot). 3 Let the tubes be in the bath for at least half an hour for the temperature inside to equilibrate (the temperature of the water inside the tubes would be approximately 2 °C less than the bath temperature). 4 Accurately weigh 2.00 g of a sample rice, pour into one of the tubes and start the stop watch. 5 Put the glass rod or wire inside the tube and gently swirl once to remove any air bubble and put the rod back in its tube. 6 Cook rice thus for the desired time (15-30 min). 7 Strain cooked rice through the tea strainer and collect the excess cooking water in an evaporating dish (if the loss of solids also has to be determined. Also, in the latter case wash the rice on the strainer once or twice and collect the washings into the excess cooking water). 8 Transfer rice on to a double-fold filter paper. 9 Spread with a glass rod or a spatula, cover with another double filter paper, press gently to remove adhering moisture. Repeat if necessary in another filter paper. 10 Weigh cooked rice up to the second place of decimal. 11 Alternatively determine the moisture content of the cooked rice by the oven method. In such a case determine the moisture content of the uncooked rice also similarly. Calculation The apparent water uptake is calculated from the amount of water absorbed, i.e., gain in weight: W¢ =

mc

mo (g/g) mo



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where W ¢ is the apparent water uptake, mc is the weight of cooked rice (g) and mo is the weight of original uncooked rice (g). This is called apparent water uptake (W¢) because it does not take into consideration the amount of solids lost. The true water uptake can be determined by estimating the grain moisture before and after cooking: W =

Mc Mo (g/g) 1 + Mo

in which Mc and Mo are the moisture contents (dry basis, g/g) of the cooked and original rice, respectively.

A.4.3 Loss of solids during cooking This test is combined with the previous one. Procedure 1 Take the evaporating dish containing the excess cooking water and washings as above and evaporate on a water bath. 2 Wipe bottom and dry in an oven at 105 °C for 1-2 h. 3 Weigh. Calculation 1 Express the amount of solids thus weighed as a percentage of the amount of rice taken for cooking. 2 Apart from direct weighing, the solids loss (s) during cooking can also be calculated from the true (W) and apparent (W¢) water uptake (g/g on dry basis) by the formula: s=

W W¢ ¥ 100% 1+W

Note: Instead of evaporating the entire quantity, one can also make up volume to a suitable value and evaporate a portion after thorough mixing.

A.4.4 Cooking time Materials 1 As in A.4.2 above. 2 Glass slides, glass spatula. Procedure 1 Put about 20 ml distilled water in each tube. Place tubes in the stand and put the entire stand in the water bath. The water level in the bath should be slightly above the level of water in the tubes.



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548 2 3

4 5

6 7

Rice quality Place the thin glass rod or a stiff wire and the glass spatula into one of the tubes (to keep them hot). Let the tubes be in the bath for at least half an hour for the temperature inside to equilibrate (the temperature of the water inside the tubes would be approximately 2 °C less than the bath temperature). Put 2–5 g of rice in one of the tubes and start stop watch. Withdraw a few grains with the glass spatula at intervals. Place between two glass slides and press. If the rice upon pressing shows an opaque central core, the rice is still not fully cooked. The time at which for the first time a few grains show no opaque central core, is the cooking time. Repeat in another tube and determine time in two or three replicates and take the mean time in min.

A.4.5 Elongation ratio and ‘rings’ Materials 1 Beaker/conical flask (100 ml, 500 ml). 2 Hot plate or Bunsen burner with heating arrangement. 3 Stop watch. 4 Tea strainer. 5 Black plastic dish. 6 Spatula/spoon/fork. Procedure 1 Take about 5 g milled rice in a 100 ml beaker. Wash once or twice with distilled water. 2 Add about 20 ml water and soak rice for 30 min. 3 Allow distilled water to boil in a 500 ml beaker or conical flask. 4 Add about 60 ml boiling distilled water into the beaker (with rice) and transfer it immediately on to the hot plate or the burner. 5 Start stop watch. 6 Boil gently for 10 min. Note: Alternatively test the cooking time (method A.4.4 above) in a portion (to be done previously) and cook for that time. 7. Strain through the tea strainer over the sink and transfer rice on to a black plastic dish containing some water. 8. Collect five randomly selected grains and measure their length (test A.1.1). 9 Repeat five times. 10 Determine the length of the uncooked grains. Calculation The elongation ratio is calculated as



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Appendix: some selected rice quality test procedures ER =

Lc

549

Lo Lo

where Lo and Lc represent mean grain length of the original uncooked grain and that after cooking, respectively. Note: Any ‘rings’ in the cooked rice are also observed simultaneously (Kamath et al. 2008).

A.4.6 Volume expansion Materials 1 Graduated boiling tubes (3.0–3.5 cm o. d.). 2 Circular metal stand for above. 3 Autoclave (or high-dome pressure cooker). Procedure 1 Take 20.0 g rice in one tube. 2 Add slowly 50 ml distilled water. 3 Stir gently with a thin glass rod or a stiff wire to expel air bubbles. Tap gently to level the rice. 4 Arrange other tubes of other samples similarly in the stand. Plug tubes with cotton. 5 Meanwhile keep ready the autoclave (or pressure cooker) with water at the bottom being heated and boiling. 6 Put stand containing the tubes into autoclave, put autoclave lid loosely on. 7 Steam for 45 min. 8 Remove stand and note level of cooked rice in each tube. Note: Arrange all the tubes together and quickly. There should be as little time lapse between adding water to rice and start of heating. The idea is to avoid presoaking as far as possible, and in any case avoid significant differences in presoaking times between samples. Observation This test is especially suitable for studying ageing of rice. See discussion on page 479 and see Fig. 13.25.

A.5 Amylose content Note: It is now known that what is measured by the methods to be described is not necessarily amylose starch, i.e., the linear or near-linear polymers of a-d-anhydroglucose, alone but also, or mainly, long anhydroglucose chains of amylopectin. Therefore the entity should not strictly be called ‘amylose’ but is henceforth better called ‘amylose-equivalent’ (AE) or ‘apparent © Woodhead Publishing Limited, 2011


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amylose (AC). However, because of the long usage tradition, it is also often interchangeably called ‘amylose’ here.

A.5.1 Total amylose (total AE) Materials 1 Test rice sample. It should be preferably well milled. Grind to pass a 60-mesh sieve. 2 Potato amylose (standard) Note: Expose rice flours and standard amylose in the room (preferably in an air-conditioned room) for a day or two. Moisture contents in all samples (and standard) will thereby be equalised and hence can be ignored. So the results will be automatically expressed on dry basis. 3 Boiling water bath. 4 Volumetric flasks – 100 ml. 5 Pipettes; glass-stoppered conical flasks – 100 ml. 6 Graduated, glass-stoppered measuring cylinder – 50 ml. 7 Fine grinder – Udy cyclone sample mill or equivalent. Reagents 1 Sodium hydroxide, 1N and 0.1N (approx). 2 Hydrochloric acid, 1N and 0.1N (approx). 3 Iodine solution, 0.2% in 2% aqueous KI – dissolve 20 g KI and 2 g iodine in water and dilute to 1. l Store in an amber bottle. 4 Distilled ethyl alcohol. 5 Distilled water. 6 Acetic acid, 1N (approx). 7 Petroleum ether, boiling range 60-80 °C. 8 Carbon tetrachloride. 9 Standard amylose – weigh accurately about 100 mg standard potato amylose into a stoppered conical flask. Add 1 ml of distilled alcohol to wet the powder. Then add 10 ml of 1 N NaOH with gentle mixing. Heat on a boiling water bath for a few minutes and cool. Add about 50 ml distilled water and about three-quarters the calculated amount of HCl required to neutralise the solution. Make up volume to 100 ml. Store in the refrigerator. It can be used for at least a week (usually a month). Procedure 1 Weigh accurately about 100 mg finely ground rice powder into a 100 ml stoppered conical flask. Add 1 ml distilled alcohol to wet the powder, then add 10 ml of 1N NaOH gently by the side. Leave overnight, and heat the next day in a boiling water bath for 2-3 min and cool. Alternatively, instead of overnight soaking, heat the flask directly for about 10 min in a boiling water bath and cool. Make up volume to 100 ml. 2 Take about 20 ml of the alkaline rice dispersion in a 50 ml graduated,



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3

4

5 6 7

551

glass-stoppered measuring cylinder. Add 7 ml of petroleum ether and shake intermittently for 10 min. Allow to stand for 10-15 min. Suck off the ether layer with a water suction. Repeat the extraction with 7 ml of CCl4 and allow to stand for 10-15 min. Pipette 5 ml of the aqueous layer from the top (leaving the CCl4 layer undisturbed below) into a 100 ml volumetric flask. Note: Amylose content can be determined even in undermilled and brown rice by the above method. However, the following slight alterations are necessary: (a) use 10 ml (not 7 ml) solvent for each extraction; (b) for brown rice, use two extractions with petroleum ether and one extraction with CCl4. Add about 50 ml distilled water, followed by 1 ml acetic acid and 2 ml iodine. Make up volume to 100 ml with distilled water. This will give a pH of about 4.5. Take 1 ml of standard amylose solution and treat in the same way as the rice dispersion has been treated. Make up 2 ml iodine solution to 100 ml with distilled water, which serves as a blank. Read the colour in a spectrophotometer or spectro-colorimeter (e.g., Spectronic-20 or Spekol) at 630 nm after a definite time (say, 20 or 30 min) against the blank. Note: The colour can also be read in a filter colorimeter (e.g., KlettSummerson). But the range of linearity of the readings in filter colorimeters is rather low and should be tested in advance.

Calculation Amylose content R a 1(ml) = ¥ ¥ ¥ 100 (% dry r basis) A r 5 (ml) = R ¥ a ¥ 20 A r where R = reading of rice flour dispersion, A = reading of standard amylose solution, a = amount of standard amylose weighed (mg), r = amount of rice powder weighed (mg).

A.5.2 Hot-water insoluble amylose Materials As in total amylose estimation (method A.5.1). Procedure 1 Weigh accurately about 100 mg rice powder in a 100 ml conical flask. Add 1 ml distilled alcohol, then about 50 ml distilled water. Cover the conical flask with a bulb stopper. Heat for 20 min in a vigorously boiling water bath with occasional shaking. Cool in water to room temperature © Woodhead Publishing Limited, 2011


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2

3 4 5 6 7

Rice quality and make up to 100 ml. Filter through Whatman No. 4 filter paper, rejecting the first portion (about 20 ml). Defat the extract as in the total amylose method above except that petroleum ether is used for both the extractions. Suck off the solvent layer. Note: CCl4 cannot be used, as the dispersed starch precipitates at the CCl4–water interface in the neutral pH medium here. Pipette 5 ml of the extract for colour development into a 100 ml volumetric flask. Add about 50 ml distilled water and mix. Add 0.5 ml 1N acetic acid. Add 2 ml of iodine solution. Make up volume to 100 ml and mix. Prepare a standard as in method A.5.1. Prepare an iodine blank. Read colour against iodine blank after a fixed time as in total amylose method.

Calculation Calculate amylose content as in amylose determination. This value gives the amount of dissolved or water-soluble amylose content. Subtracting this value from the total amylose content gives the water-insoluble or nonextractable amylose content: insoluble amylose = total amylose (%) – soluble amylose (%) (% dry basis)

A.6 Alkali digestion score Materials 1 Milled rice – sound whole grains, fully or partially milled. 2 Petri dish – 7 cm dia, Corning or Pyrex, with smooth, level inner surface. 3 Black plastic sheet or cloth. 4 Spatula. 5 Magnifying glass. 6 Transparent plastic measuring ruler. 7 Stock KOH solution, 10–12%. Dilute a portion exactly 10 times and titrate with standard phthalate, using phenolphthalein as indicator, to determine its exact concentration. Store carefully stoppered. 8 KOH, 1.4% (and other concentrations as needed) – made by calculated dilutions from the stock. 9 Standard acid potassium phthalate, 0.4N. Heat the acid salt in an oven at 120 °C for 2 h and cool in desiccator for 15–20 min. Weigh exactly 20.42 g, dissolve in boiled and cooled distilled water and make up to 250 ml. 10 Phenolphthalein, 0.1% in 95% alcohol. © Woodhead Publishing Limited, 2011


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Procedure 1 Place six rice grains in a Petri dish which is kept on a black cloth or plastic sheet. 2 Add 20 ml 1.4% KOH. Arrange the grains symmetrically with a spatula. 3 Cover the Petri dish with the lid and leave it undisturbed overnight (18–20 h). 4 Next day observe the grains carefully and determine the extent of degradation as per the score card in Table A.4.

A.7

Gelatinisation temperature (GT)

A.7.1 Photometric method Materials 1 Mortar and pestle. 2 Modified Spectronic-20 colorimeter. The modification consists of the following. The 18 mm tube adapter of the instrument is electrically heated at an appropriate rate (Fig. A.2). Also the suspension in the measuring tube is kept agitated with a Teflon-coated stirring rod by means of a magnetic stirrer placed below the colorimeter. Procedure 1 Triturate a few rice grains with water. 2 Put a 0.5% suspension of the ground rice into the colorimeter tube. 3 Start the magnetic stirrer to keep the suspension in agitation. 4 Read the transmission at 525 nm. 5 Start the heating at an appropriate rate, adjusted by a suitable rheostat system. 6 Keep on checking the transmittance. Observation The temperature corresponding to an increase in transmittance of 5% T over the original value is considered as the GT of the sample.

A.7.2 Alkali photometric method This method is suitable for screening of breeding materials and classifying into three GT groups (high, intermediate and low). Materials 1 Probe colorimeter with an 8 mm light path. 2 1.6% KOH. 3 Defatted brown rice flour.



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Table A.4 Alkali digestion scores of rice Score

Kernel consistency

0 1 2

Wholly chalky Wholly chalky Nearly wholly chalky

3 4 5 6 7 8

Collar

Absolutely none Trace hairy Mostly on one side; slight white painty deposit in inner ring, outer ring cottony More on one side; wide white Substantially chalky painty deposit in inner ring, outer ring cottony; radially streaked All around; some white painty Fairly chalky deposit in inner ring, outer ring cottony; radially streaked All around; very little white Slightly chalky painty deposit in inner ring, outer ring cottony; radially streaked Cottony, or cottony + gel All around; no white painty deposit; semi-transparent, faintly streaked Compact gel (one or more All around; practically transparent; smooth (i.e., not pieces) streaked) All around; practically Loose gel transparent; smooth (i.e., not streaked)

Total diameter across (mm) – ≤5 7 (±2) 10.5 (±2) 14 (±3) 18 (±3) 18 (±3) Narrow (i.e. nearly transparent) Narrow, (i.e., nearly transparent)

Explanations and precautions: ∑ ∑ ∑

∑ ∑

Measure diameter by placing a transparent ruler across the kernel, up to the visible collar edge. While scoring, give importance to diameter, collar, and kernel, in that order, for scores 0-5. For scores 6-8 give equal importance to all three. The ± ranges shown for the diameter are partly to account for differences in grain size. Generally, add 1-2 mm to the reading for small grains, and subtract 1-2 mm from the reading for large grains. Thus, a reading of 13 mm should be scored 4 for a very small grain but 3 for a very large grain. The kernel may have 1 or 2 minute cracks for scores 0 and 1. It may be cracked/opened/segmented/ corroded/hairy for scores 3-5 (in increasing degree). These features concern the degradation type and should be ignored for scoring. Waxy rice hardly shows any collar for any score (type D). Their scoring is, therefore, only approximate.

For Petri dishes of the stated or any other size, calculate the volume of alkali to be used (v) as follows: v

p r 2 · 00.55 ª 3 r 2 3 d 2 ml 2 8

where r = radius of the Petri dish in cm and d = diameter in cm Calculate the number of rice grains (n) to be placed in a pair of Petri dishes as: n

p r2 1 6



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Thermometer

Cover

To variable transformer

Nichrome wire

Magnetic stirring bar

Fig. A.2 Schematic diagram of arrangement for determining gelatinisation temperature by photometry. Sketch: Courtesy B. O. Juliano (see Bhattacharya 1979).

Procedure 1 Soak defatted brown rice flour (50 mg) in 10 ml of 1.6% KOH for 24 h at 30 °C. 2 Measure percentage transmittance of the slurry at 620 nm with the probe colorimeter. Observation The percentage transmittance gives a rough idea of the GT as follows: High-GT group Intermediate-GT group Low-GT group

< 10% T 21-50% T > 60% T

A.7.3 Alkali digestion test method The test is the same as described in method A.6 above. As has been explained in Section 13.8, the alkali score, particularly in the method described



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above (Bhattacharya and Sowbhagya 1972), can be used not only for broad classification of varieties into high-, intermediate- and low-GT groups. The score can also be used for determining (a) the GT of the variety and (b) for varietal identification. However, for such purposes the score has to be carefully determined as per the score card (Table A.4). Some of the points to be noted are explained in Notes under Table A.4. Calculate the GT from the alkali score (AS) from the formulae: GT = 74.54 – 1.40 AS (n = 157 nonwaxy rice) (r = -0.848***) or = 74.80 – 1.57 AS (including waxy rice, n = 165) (r = -806***)

A.8

Gel mobility test

Materials 1 Milled rice. Grind in a Wig-L-Bug amalgamator (or others) to pass a 100-mesh sieve 2 Test tubes, size 15 (o. d.) ¥ 150 mm, Pyrex or Corning. 3 Wig-L-Bug amalgamator (Crescent Dental Mgf. Co., USA) or other suitable grinder. 4 Cyclo mixer (Vortex mixer). Reagents 1 Ethanol, 95% containing 0.1% (ª 100 mg in 100 ml) thymol blue. 2 Potassium hydroxide 0.2N (exactly 0.2N by titration with standard potassium hydrogen phthalate). Method 1 Accurately weigh 100 mg of rice powder into a test tube. 2 Wet with 0.2 ml 95% ethanol containing 0.1% thymol blue. 3 Shake well to wet the powder thoroughly. 4 Add 2.0 ml of 0.2 N potassium hydroxide while shaking the tube. 5 Disperse the mixture using a cyclo mixer. 6 Cover the tube with a bulb stopper and place in a vigorously boiling water bath to reflux for 8 min. 7 Remove and cool the tube at room temperature for 5 min and then further in an ice-water bath for 30 min. 8 Finally lay the tubes horizontally over a graph paper graduated in millimetres. Observation Measure the length of the gel from the bottom of the tube to the gel front after 60 min. Conduct each test in duplicate.



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Appendix: some selected rice quality test procedures Classification Hard consistency Medium consistency Soft consistency

– -

557

26-40 mm 41-60 mm >60 mm

Note: A few precautions are mandatory for this test. The main objective is to prevent lump formation when dispersing the rice flour in alkali. If a lump is formed, the remaining gel will be too thin and will run freely. Such a tube has to be cancelled. The following precautions will ensure proper gel formation: ∑ ∑ ∑ ∑

A.9

The rice should be well milled, so the fat content is low and uniform. The flour must be at least 100-mesh. Vortexing after adding alkali is not enough. Shake vigorously by the hand when the tube is immersed in the boiling water bath and continue shaking for several seconds. Heating in the bath should be so adjusted that the liquid should come up to about 7.5 cm in the tube.

Pasting behaviour: Brabender viscography

The following method applies specifically to the Brabender visocgraph which has been around for some three-quarters of a century. Recently the RVA (Newport) and the MVA (Brabender) have become more widely used. Such equipment requires much less flour/starch and much less time to complete a run. Hence the obvious advantage. However, essentially the principles and the processes are the same and the essentials of the procedures described will apply equally well to the two latter items of equipment. Materials 1 Grind the rice to be tested to pass at least a 60-mesh screen. 2 Distilled water. 3 Beaker, glass rod/spatula. 4 Brabender viscograph (Model E). Procedure 1 Weigh 50 g rice flour into a 500 ml beaker. 2 Add 250 ml water and mix well with a glass rod or a spatula. 3 Transfer to the bowl of the viscograph. Note: (i) Mixing is preferably done in a mixer grinder, but it is not essential. Mixing in a beaker is equally satisfactory, but avoid lumps. (ii) It is preferable to consider the flour weight always on dry basis or on 14% moisture basis to facilitate comparison. 4 Add the balance of 450 ml of water (450 - 250 = 200) in one or two



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558

5

6 7 8

9

Rice quality instalments to wash the beaker and transfer to the bowl. If flour is weighed on dry or 14% moisture basis, calculate the amount water to include the flour moisture. Set the programme of the equipment to heat from 30 to 95 °C (or any other suitable temperature in the range of 92-96 °C) at the rate of 1.5 °C/ min, then hold the temperature at the same value (95 °C as stated) for 20 min, and finally cool at the rate of 3 °C/min to 50 °C. Note: Cooling too has been classically done at 1.5 °C/min, but the model E of the equipment has facilities to set other rates; cooling at the rate of 3 °C/min gives essentially same results but saves time. Run the equipment as per the above programme. Stop when the programme is finished and wash. Note the peak viscosity (P) value from the viscogram obtained. From the P value thus obtained, make an estimate of the slurry concentration required to obtain a P value of 500 BU (or any other constant value, such as 600 or 750 BU, etc.). Run another viscogram in exactly the same programme with the revised concentration estimated above.

Observations Note the following indexes from the two viscograms above: 1 2

3

The peak viscosity (P) value at 10% slurry concentration. The cold-paste viscosity (C) value at 10% paste concentration. Note: C is the cold-paste viscosity, i.e., the viscosity after cooling to 50 °C. This value has some relationship to the texture of cooked rice and is useful in that sense. The relative breakdown at P ª 500 BU, i.e., BD r =

4

(P – H ) ¥ 100% ( – )

where H = the hot-paste viscosity, i.e., the viscosity reached after holding at 95 °C for 20 min. The total setback index, C/H, which is an index of how many times the viscosity rises during cooling. Note: A fair amount of useful information from viscography can be obtained from a single viscogram also. For this the viscogram can be run from previous experience at a concentration which would give a P value as close to 500 BU as possible. Then the same indexes as above can be noted. Or, even the breakdown per se can give a fair amount of useful information if it is calculated not as P-H but as (1 – H/P) ¥ 100%



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559

A.10 Instrumental measurement of cooked-rice texture using the TA.HDi Texture Analyser As explained in Section 13.14.1), a large number of different types of equipment are being used to measure the texture (especially hardness and stickiness) of cooked rice. Essentially there is no great difference in results among these instruments. The TA.HDi is being widely used these days and the measurement using this equipment is described here. Even here one can adopt different programmes for measurement, but these all essentially give the more or less same information. One particular programme which has been found useful as well as convenient in the RRDC is described below. Materials 1 Glass evaporation dish. 2 Petri dishes. 3 Autoclave or high-dome pressure cooker. 4 Filter papers. 5 Distilled water. 6 Spatula or glass rod. 7 Forceps. 8 TA.HDi Texture Analyser along with its operating accessories. Procedure A 1 Weigh 5.0 g wholegrain rice into an evaporation dish. 2 Add 10.0 ml water, cover with a Petri plate and allow to soak for 30 min. 3 Put inside an already boiling autoclave/pressure cooker, put the latter’s lid on loosely and steam for 20 min. 4 After 20 min take out the cup and stir the rice gently. Cover with a Petri plate with filter paper lining to absorb the condensate. Leave aside in an air-conditioned room for 1 h. 5 After 1 h, stir once more and immediately take for texture measurement. 6 Set up the TA.HDi equipment with #P25 (25 mm dia) cylindrical aluminium probe. 7 Calculate the number of uncooked grains that add up to approximately 50 mg (for instance, if the grain weight is 16 mg, then three grains will make ~50 mg; if the grain weight is 22 mg, then approximately two and half grains may make about 50 mg). 8 Transfer the number of cooked grains equal to the number calculated above on to the equipment platform below the probe with the help of forceps and place the grains flat on the platform. 9 Bring the probe manually close to the cooked rice (with a gap of 2-5 mm). 10 Set up the TA.HDi Texture Analyser in a programme such that the probe



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will move at a speed of 0.1 mm/s, compress the cooked grain to 50% of its thickness, return back immediately, wait 10 s, again compress the grain, this time to 90% of its thickness and finally immediately return back at the same speed. Note: Set up the programme such that when the instrument recognises a force of 5 g, that position is taken as zero. This is to make sure that the probe just about touches all three grains. 11 Switch on the programme. The following indexes are read from the curve: (a) the peak force at 50% compression which is considered as hardness and (b) the peak negative force at 90% compression, which is considered as stickiness or adhesiveness. 12 Repeat the test at least ten times and take the mean of the values (rejecting any one or two value which may be too much outside the range). Procedure B Steps 1–5 as in procedure A above. 6 Set up the TA.HDi with the #P/100 (100 mm dia) compression platen. 7 Nil 8 Transfer 1 g of cooked rice on to the platform and arrange in a circle in a single-grain layer under the probe. Level flat as far as possible without pressing. 9 Bring the probe manually close to the cooked rice (with a gap of 2-5 mm). 10 Set up the TA.HDi Texture Analyser in a programme such that the probe will move at a speed of 0.1 mm/s, compress the cooked grain to 50% of its thickness, return back immediately, wait 10 s, again compress the grain to 50% of its thickness and finally immediately return back at the same speed. Note: Set up the zero position of the probe as when the instrument senses a force of 25 g. This is needed to bring the probe to touch the large number of grains. 11 Switch on the programme. The following indexes are read from the curve: (a) the peak force at 50% compression which is considered as hardness, (b) the peak negative force after the first bite (or second bite), which is considered as stickiness or adhesiveness, and (c) the ratio of the area of the second bite to that of the first bite, expressed as a percent, which is called cohesiveness. Note: This is where the advantage of taking 1 g rice for the test lies. With ~ 3 grains (procedure A), the negative force generated after 50% compression is too small to provide a reasonable reading of adhesiveness. So the second bite has to be made to provide 90% compression. Thereby a good reading for adhesiveness is no doubt obtained, but a reading of cohesiveness is lost. Besides, one wonders whether the adhesiveness of the smashed grain (90% compression) is a correct representation of the adhesiveness of the grain surface. By taking 1 g rice in Procedure B, a



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561

fair reading for adhesiveness is obtained even at 50% compression, which is welcome in itself. In addition, thereby a reading for cohesiveness is obtained as a bonus. However, weighing 1 g cooked rice and arranging it properly on the platform does test one’s patience! So it is a question of choice. 12 Repeat the test at least ten times and take the mean of the values (rejecting any one or two value which may be too much outside the range).

A.11 Aroma in scented rice Materials 1 Injection vials with bark cork. 2 Pipettes. 3 KOH solution: 0.1N, 0.25N. Procedure 1 Weigh 200 mg of wholegrain milled rice in an injection vial. 2 Pipette 0.5 ml of 0.1N KOH into the vial. Close with a bark cork (not rubber cork). Note: If using brown rice, use 0.25N KOH instead of 0.1N. 3 Remove cork after 15 min and smell. 4 In case of doubt, again cork and leave aside for 15 min, then smell. 5 Score 1-5 (or 1-3) in a hedonic scale from nil to strong basmati (or jasmine) aroma. 6 Test by at least three persons and take mean score. Note: The test can also be conducted with leaves of the rice plant. For this 2 g of leaves cut into small pieces are taken in a 5 cm dia Petri dish and covered with 10-15 ml 0.1N KOH, then covered with the lid. Smell after 15-20 min.

A.12 Tests for parboiled rice The tests are described in Section 13.17.1 and summarised in Table 13.11. EMC-S and kernel digestion in alkali test procedures are the same as described in test A.4.1 and test A.6 above. These two are the simplest possible tests for parboiled rice, but are very informative. The other tests, viz. slurry viscosity, sediment volume and alkali gel volume are such that these can be easily carried out based on the information already provided in Table 13.11. Hence these are not further described here. The temperature at which the paddy had been soaked for preparing the parboiled rice under examination can be tested by the oven heating method mentioned in Section 13.17.1 and presented in Table 13.12.



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A.13

References

bhattacharya k r (1979), ‘Gelatinisation temperature of rice starch and its determination’, in Chemical Aspects of Rice Grain Quality, Los Baños, Laguna, Philippines, International Rice Research Institute, 231–249. bhattacharya k r and sowbhagya c m (1972), ‘A colorimetric bran pigment method for determining the degree of milling of rice’, J Sci Food Agric, 23, 161-169. kamath s, stephen j k c, suresh s, barai b k, sahoo a k, radhika reddy k and bhattacharya k r (2008), ‘Basmati rice: Its characteristics and identification’, J Sci Food Agric, 88, 1821–1831.



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Index

a-amylase, 136 abrasion mill, 447–8 2-acetyl-1-pyrroline (2-AP), 347–8 AE see amylose-equivalent ageing, 116–58 acceleration and retardation, 155–8 Arrhenius-like plot of inverse ageing/curing temperature, 157 artificial/accelerated ageing, 156–8 retarding/reversing ageing, 158 changes in other properties, 133–5 DSC, 135 gel consistency, 135 gelatinisation temperature, 133–4 miscellaneous properties, 135 changes in physicochemical properties, 121–35 change in cooked-rice hardness and stickiness, 134 cooked-rice texture, 133 grain hardness and milling quality, 121–2 loss of rice solids during cooking, 126 consumers’ perception of changes in rice behaviour during storage, 119–20 colour and appearance, 119–20 cooking and eating quality, 120 odour, 120 final rice paradoxes, 153–5 ageing unique in rice, 154–5 temperature can substitute for time, 154

three processes, similar effect, 153–4 map of rice country, 117 mechanism, 152 pasting properties, 127–33 amylograms of different brown rice flour and brown rice starch, 128 definition of terms, 131 progressive change in viscogram parameters during storage of rice, 132 progressive fall in viscogram relative breakdown, 132 relation to individual constituents, 135–53 a-amylase, 136 cellulase treatment on pasting properties, 146 lipids, 139–42 maize bran cell walls model, 147 nonstarch polysaccharides, 142–5 protease treatment on viscograms of rice flour after storage, 149 protein, 145–50 SDS-PAGE analysis of proteins from external layer of milled rice samples, 150 starch, 137–9, 140 sugars, 136–7 transverse sections of variously stored rice after cooking, 143–4 water-insoluble amylose content


WH-Rice quality-Index.indd 563

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564

Index

of raw and mild and severe parboiled rice, 140 rice, 20 volume expansion, 126–7 change in cooking/eating properties of rice during ageing, 127 decline of stickiness of cooked grain after ageing, 127 water uptake, hydration and swelling, 122–5 water uptake and total solids in residual cooking liquids, 123 water uptake upon cooking during storage of rice, 125 Agtron, 466 alkali digestion score, 209, 236, 482–5 digestion scores of rice, 485 digestion test of rice, 484 extent of kernel digestion after 20 h, 483 score card, 484 alkali digestion test, 209–10 All India Coordinated Rice Improvement Project (AICRIP), 207 American basmati, 357 amylopectin, 213–24 amylose, 221–3 isomylase-debranched rice starch, 222 cluster structure of L-type and S-type, 234 ferment in the area of starch structure, 213–14, 215 GPC patterns of normal and amylomaize starches, 215 gel-permeation chromatography patterns debranched amylopectin of legume starches, 216 normal and amylomaize starches, 215 rice starch characteristics, 220 rice starch elution pattern on Sepharose 2B column, 217 soluble/insoluble amylose conundrum, 223–4 structure, 214, 216–21 gel-permeation HPLC pattern, 219 insoluble amylose content, 218

long-B chains before and after b-amylosis, 221 relationship with onset of gelatinisation temperature, 235 rice starch elution pattern on Sepharose 2B column, 217 amylose, 57, 196–213, 221–3 American research, 196–9 US rice varieties quality evaluation, 199 USDA data on rice properties, 198 CFTRI research, 207–11 alkali reaction type and other rice quality characteristics, 211 rice relative breakdown, 210 role of insoluble amylose in texture of cooked rice, 209 starch-iodine blue colour of rice samples having different amylose contents, 208 European research, 199–200 IRRI research, 204–7 cooked high-amylose milled rice hardness and stickiness, 206 milled nonwaxy rice eating-quality scores and composition, 205 Japanese research, 200–4 cooking qualities of milled rice and starch properties, 202 peak viscosity to breakdown and SIBV to relative breakdown, 203 zones of acceptability as a function of hardness and stickiness, 204 rice quality classification, 212 amylose content, 201, 205, 210, 236, 487–91 defatting, 488–9 extract pH adjustment, 489–90 flour dispersement in alkali, 488 hot-water-insoluble, 491–2 standard source, 490–1 amylose-equivalent, 216, 220, 236, 426 angle of repose, 45, 278, 279, 457–8 Perspex box, 457–8 2-AP test, 513–14 apparent amylose content, 216, 219, 236



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Index Appendix, 531–61 alkali digestion score, 552–3 amylose content, 549–52 aroma in scented rice, 561 estimation of moisture, 542–5 gel mobility test, 556–7 gelatinisation temperature, 553–6 hydration and cooking parameters, 545–9 instrumental measurement of cooked-rice texture using TA.HDi Texture Analyser, 559–61 milling of rice and degree of milling, 536–41 pasting behaviour, 557–8 physical properties, 531–6 tests for parboiled rice, 561 Arborio, 371 aromatic rices, 339–48 aroma compounds, 347–8 distribution, 342–7 cultivars belonging to different rice groups, 344 milled rices sizes and shapes, 345 nano-sized scented rices of Bangladesh and India, 345 scented rice cultivars characteristics grown in different countries, 346 group V rice physicochemical characteristics, 341–2, 343 quality classification, 342 rice flour pasting behaviour, 343 taxonomy, 340–1 group V rices centre of diversity and dispersal routes, 341 Automatic Patent Minghetti, 471 bag trieurs, 433–4 baking, 316 Basmati breeding, 349–53 approved Indian basmati varieties, 352 high-yielding derivatives developed in India, 350 varieties developed and released in Pakistan, 353

565

international trade, 354–5, 356 export from India and Pakistan, 356 physicochemical characteristics and derivatives, 355, 357–60 additional derivatives released recently in India, 358 good ‘rings’ in rice after cooking in HBC, 360 milled grains varieties, 359 milled rice grains sketch showing different distal-end shapes, 359 production, 353–4 area, production and yield in India, 354 vs Jasmine milled rice, 362 beriberi, 380–1 bhaja chawl, 330 Boerner sample divider, 434–5 illustration, 434 Boutique rice, 369 Brabender amylograph, 128 Brabender moisture analyser, 443 Brabender viscograph, 200, 492 bran, 62, 100, 101, 381, 404 breeding end-use quality, 425–8 eating quality, 426–7 product-making quality, 427–8 rice total and hot-water amylase contents relationship and expansion ratio, 428 paddy grain physical and morphological properties that affect the final product quality, 416–23 grain hardness, 422–3 grain size and shape, 417–21 husk content, 416–17 Lemma–palea interlocking, 423 surface ridges, 421–2 plant characteristics for optimum harvest, 413–16 grain moisture variation in panicle different parts, 414 time effect of harvest of paddy on head rice yield, 413 variation in the content of cracked



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566

Index

grains in panicle different parts, 415 rice for desirable quality, 410–29 rice grain susceptibility to cracking, 423–5 critical moisture content concept, 424–5 varietal difference in rice cracking at harvest, 424 rice quality, 411–13 brewer’s rice, 331 brown rice, 101, 105 yield, 62 bulk density, 41, 278, 279, 456 test weight apparatus, 456 carbohydrate, 402 cargo rice, 105 Carter, 437 Central Food Technological Research Institute, 207–11, 327, 414 Century Patna 231, 197, 303 cereals, 8 CFTRI see Central Food Technological Research Institute chalky grains, 53–7, 84 dependence of white belly chalkiness on grain breadth in rice, 55 effect of grain chalkiness, 56–7 relation of chalkiness to growing temperature, 56 slender milled rice kernels with no white belly, 55 white belly, 54–5 chapatti, 318 Chopin-INRA Viscoelastograph, 510 cleaning, 437 winnowing tray, 437 coefficients of friction, 45–6 coloured rice, 369–70 brown rice, 369 cone polisher, 379 contrariness, 1, 25 conventional parboiling, 251 cooking behaviour, 475–82 cooking time, 478 elongation ratio and ‘rings,’ 478 good and poor ‘rings’ in Sharbati, 478

grain defects, 478–9 defects developed during cooking, 479 loss of solids, 477 ageing, 477 rice cooking in laboratory, 481–2 volume expansion, 479–80 poor volume expansion, 480 water uptake by rice, 476 cooking procedure, 188 cooking quality, 20 hydration at lower temperatures, 178–80 ambient water absorption by milled rice, 179 70–80°C, 178–9 room temperature, 179–80 laboratory cooking for various tests, 183–8 cooking procedure, 188 effect of water : rice ratio on cooked-rice, 184 presoaking, 188 water : rice ratio, 187 milled and starch properties of rice imported into Japan, 202 rice, 164–88 effect of presoaking in ambient water, 181–2 loss of solids during cooking, 180–1 other changes/events during cooking, 182–3 water absorption by rice at/near boiling temperature, 166–77 caveats, 175 hydration, 168–75 kinetics of hydration and energy relations, 176–7 other observations, 176 water-uptake, 167–8 cooking time, 168 crack resistance, 79–80 cracking rice grain, 423–5 critical moisture content concept, 424–5 varietal difference in rice cracking at harvest, 424



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Index critical moisture content, 424–5 curing, 156 degree of milling, 20, 63, 87, 100–15, 461–74 compatibility of approaches, 471–4 residual bran test result, 473 weight, bran pigment and surface fat, 472 effect on rice quality, 106–14 changes in amount of fat, 110 chemical composition, 106–10 chemical composition of outer layer, nucleus and entire kernel of milled rice, 109 composition of removed fractions of conventionally milled rice, 108 cooking quality, 112–14 major milled rice constituents distribution within the brown rice grain, 107 nutritive value, 112 physical properties, 110–12 storage stability, 112 swelling ratio, 113 milling paddy grain and how much to mill, 100–6 degree of milling of rice, 104–6 longitudinal section of rice spikelet, 102 paddy, paddy with lemma and palea detached, brown rice and milled rice, 101 relationship to various physical properties, 111 theoretical basis of estimation, 462–71 changes in grain fat content, 470 illustration of rice milled to different degrees, 468 loss of bran constituents, 464–5 loss of grain weight, 463–4 optical methods, 465–6 rice grain constituents, 464 rice stained with alcoholic alkali, 468 rice stained with methylene blue, 467

567

staining methods, 467 surface fat content, 467–70 visualisation of residual bran, 466–7 dehulling see shelling dehusking see shelling density, 40–6, 278, 279 density and bulk density, 40–4 dependence of bulk density of paddy on its harvest moisture content, 43 porosity, 44–5 porosity dependence on grain shape, 42 and related properties, 454–8 angle of repose, 457–8 bulk density, 456 density flask, 455 porosity, 457 density in a mass see bulk density Desikachar test, 188 dietary fibre beneficial effects, 397–9 differential scanning calorimetry (DSC), 319 discoloration, 254 dry-heat parboiling, 251 drying, 437–8 Early Prolific, 197 eating quality, 20–1 amylopectin paradigm, 213–24 amylose, 221–3 ferment in the area of starch structure, 213–14 rice amylopectin structure, 214, 216–21 soluble/insoluble amylose conundrum, 223–4 amylose paradigm, 196–213 American research, 196–9 CFTRI research, 207–11 European research, 199–200 IRRI research, 204–7 Japanese research, 200–4 rice quality classification, 212 other influencing factors, 229–34 amylograms of rice flour, 231 amylopectin and onset of gelatinisation temperature, 235



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568

Index

cluster structure of L-type and S-type amylopectin, 234 gelatinisation temperature, 232–4 protein content and Tensipresser hardness of cooked rice, 233 role of protein, 229–32 rice, 193–238 water-uptake paradigm, 195–6 worldwide production, 194 rice-flour paste rheology, 224–8 change of apparent viscosity, 225 paste breakdown vs total AN and insoluble AE, 226 starch granules in 12% pastes, 227 storage modulus and relaxation time, 228 testing for rice quality, 234–8 new synthesis, 236–8 Engelberg Company, 379 Engelberg huller, 379 environmental conditions, 86–7 epichlorohydrin, 304 equilibrium moisture content, 50, 474–5 rice and rice products soaked in water at ambient temperature, 475 equilibrium relative humidity (ERH), 50 evolved basmati, 351 Export Act (1963), 351 Export (Quality control and inspection) Act, 351 extra well milled rice, 105 extrusion process, 331 fermented cakes, 313, 315 idli and dosai, 315 friction, 45–6 friction mill, 447–8 gas chromatography mass spectrometry (GCMS), 347 gel consistency, 135, 206 gel mobility, 236 gel mobility test, 486–7 hard, intermediate and soft mobility, 486 gelatinisation, 256 gelatinisation temperature, 125, 133–4, 168, 232–4, 265, 292, 341–2, 504–7

alkali digestion test method, 507 alkali photometry, 506 photometric method, 505–6 viscographic method, 507 water-uptake ratio method, 506–7 gelatinisation time, 167–8, 199 General Foods Corporation, 305 General Foods Texturometer, 203, 510 genetically modified (GM) rice, 406 germinated brown rice, 331 glutinous rice, 365–9 Southeast Asia map showing origin centre, 367 glycaemic index, 402 grain appearance, 32–40 colour, 39–40 dimensional classification of rice, 34–9 characteristics of US rice, 37 classification as per Ramiah Committee, 38 regression curve of normalised grain weight, 40 size and shape classification, 36 paddy, brown rice and milled rice proportionality, 35 size and shape, 32–4 grain hardness, 52, 87–8 grain moisture moisture adjustment, 438–9 moisture estimation, 440–6 methods involving evaporation of moisture, 441–3 use of moisture metres, 443–6 grain thickness, 88–9 grain topography, 89 granule-bound starch protein (GBSP), 231–2 granule-bound starch synthase (GBSS), 232 grinding, 439–40 gross palatability index (GPI), 200 group V rices, 340–3 centre of diversity and dispersal routes, 341 distribution, 342–7 cultivars belonging to different rice groups, 344 physicochemical characteristics, 341–2



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Index quality classification, 342 rice flour pasting behaviour, 343 gun-puffing, 325–6 Haake consistometer, 509 haemagglutinin, 404 hand pounding, 103 hard-to-cook (HTC) phenomenon, 154 harvest moisture content (HMC), 43 head milled rice yield, 63 head rice yield, 51, 63, 67, 121–2 high-temperature, short-time, 322 Hom Mali rice see Jasmine HTST see high-temperature, short-time hull, 62 hulled rice, 105 Human Nutrition Research Division of the USDA, 197 husk, 62 husk opening, 255 husked rice, 105 hydratability, 167, 259 hydration, 166, 168–75 grain thickness of rice varieties and their water uptake, 171 kinetics, 176–7 rice cooking rate constant, 177 lower temperatures, 178–80 ambient water absorption by milled rice, 179–80 70–80°C, 178–9 room temperature, 179–80 USDA data on rice properties, 170 water uptake of long-, medium-, and short grain US rice, 174 hydroxypropyl methylcellulose, 317 Indian Council of Agricultural Research (ICAR), 207 Indica, 24, 185, 194 Industrial Revolution in Europe, 379 infrared moisture analyser, 443 insoluble amylose, 210, 216, 218 Instron, 510 insulin, 400 International Rice Research Institute (IRRI), 196, 204–7, 368, 411 International Year of Rice (2004), 370 iron, 406

569

IRRI see International Rice Research Institute Japanese sake, 331 Japonica, 24, 185, 194 Jasmine, 361–4 rice export from Thailand, 363 vs Bamati milled rice, 362 Javanica, 24 Kett Digital Whiteness Metre for Rice, 466 Khao Dawk Mali 105 (KDML 105), 361 Kiya hardness tester, 52 Kojiki, 7 Kola Joha, 40 Koshihikari, 371–2 Kramer cell, 511 Kramer shear press, 511 lignans, 400 lipids, 139–42, 405 loonzain rice, 105 low-moisture parboiling, 251 machine milling, 103 Madras Presidency, 384 McGill equipment, 90, 93, 95 McGill miller no. 1, 471 McGill miller no. 2, 447–8 McGill miller no. 3, 447–8 McGill sample sheller, 446–7 medium milled rice, 105 milling, 62, 183, 379, 439 effect on other vitamin contents in rice, 389 effect on raw and parboiled rice, 279 effect on vitamin contents in rice, 389 parboiled rice, 277–8 milling quality, 19, 61–95, 446–9 drying of rice, 72–3 fundamental cause of rice breakage, 90–5 cracks formed on soaking and drying of paddy, 92 defective grains in paddy harvested at various stages, 93 sheller, whitener, grain type and



6/8/11 8:57:09 AM

570

Index

degree of milling on rice breakage, 94 types of cracked and immature grains in brown rice, 92 grading cylinder, 450 grain cracking or fissuring at or around harvest, 65–72 average moisture content of paddy grains, 70 content variation of cracked grains, 71 different methods of drying paddy after harvest on amount of cracked grain, 66 drying method on whole rice outturn after milling, 67 grain cracking at or around harvest, 65–7 individual kernel moisture content distribution, 71 optimum time of harvest, 67–9 paddy harvest time effect on head rice yield, 69 reaping stage and drying method on breakage upon milling, 66 variation in grain moisture in the field, 69–72 variation of grain moisture in different parts of the panicle, 70 indented plate, 449 milling of rice, 62–4 milling yield of rice, 63–4 rice breakage during milling, 64 miscellaneous affecting factors, 84–90 chalky grains, 84 chalky kernels crack easily, 85 degree of milling, 87 environmental conditions, 86–7 grain hardness, 87–8 grain thickness, 88–9 grain topography, 89 mill room relative humidity effect on outturn of head rice, 86 milling yield of different thickness fractions of paddy, 88 moisture content at time of milling, 84–6 paddy moisture content at the time of milling on yields of rice, 85 protein content, 89–90

rice damage upon exposure to altered relative humidity, 87 rice grain fissures, 73–84 brown rice state diagram and hypothetical drying process for rice kernel, 82 drying with tempering on milling breakage of parboiled paddy, 78 effect of drying on milling quality of parboiled paddy, 77 glass transition temperature concept, 81–4 hot tempering on milling breakage of parboiled paddy, 78 hypothetical response of various sections of brown rice kernel, 83 initial moisture content of paddy on cracks development, 76 phenomenon of crack resistance, 79–80 phenomenon of critical moisture content, 75–9 time of cracks appearance in fastdried parboiled paddy, 74 varietal difference in rice cracking at harvest, 80 moisture content, 75–9 effect on angle of friction of milled rice, 49 effect on rice, 46–9 swelling of paddy during soaking at various temperatures, 47 at time of milling, 84–6 moisture metres, 443–6 calibration chart, 445 calibration curve, 444 monotheism, 7 mud civilisation, 7 multi-pass drying, 73 National Academy of Sciences (USA), 382 National Food Research Institute (NFRI), 201 net protein utilisation (NPU), 404 ‘new’ rice, 117–18 Njavara, 40 nonstarch polysaccharides, 142–5



6/8/11 8:57:09 AM

Index noodles, 311–12 illustration, 312 normalised grain weight, 39 Northern Circars, 384 nutritional quality, 22 biotechnological approach to upgrade the rice nutritive value, 405–7 diet overall defects of poor-rice eaters, 392–4 cheap Madrassi diet, 392 growth of children on poor rice diet with and without supplementation, 394 nutrients in the cheap Madrassi diet, 393 young rats growth in poor rice diet with and without supplement, 393 effect of milling on other vitamin contents in rice, 389 on vitamin contents in rice, 389 nutrition perspective of poor riceeater, 383–94 rice diet other deficiencies, 388–90 thiamin question, 384–8 nutritive value, 378–83 evolution of the focus of studies, 381–3 technology changes and its nutrition, 379–81 nutritive value perspective of individual rice constituents, 401–5 anti-nutritional factors, 404 glycaemic index, 402 other constituents, 403–4 other factors, 405 protein quality and quantity, 404–5 resistant starch, 403 rice, 377–407 washing effect and cooking of rice, 390–2 effect of cooking in excess water on percentage loss of vitamins, 391 vitamins percentage loss after washing, 391

571

wholegrains, 394–401 untold benefits, 397–401 vs refined grains, 394–7 oatmeal, 396 ‘old’ rice, 118 optimal cooking time, 170–1 Oryza sativa, 340 oryzacystatin, 404 oryzogram test, 200 paddy, 100 paddy grain grain hardness, 422–3 chalkiness effect on the brown rice hardness, 422 grain size and shape, 417–21 chalky kernels crack easily, 421 milled rice kernels of BR 2655, 420 porosity dependence on grain shape in paddy and milled rice, 419 water uptake relation during rice cooking, 418 white belly chalkiness dependence on grain breadth in rice, 421 physical and morphological properties that affect the final product quality, 416–23 husk content, 416–17 Lemma–palea interlocking, 423 surface ridges, 421–2 grain of brown rice, 422 palatability, 203 pandan leaf, 348 parallel plate plastometer, 509 parboiled rice, 247–51 bran quality, 289–90 free fatty acid development, 290 changes in rice grain and its constituents, 252–72 effect on product quality, 253 other changes, 272 cooking and eating qualities, 279–83 raw and mildly and severely parboiled rice, 281 starch breakdown effect, 281–3 steam pressure, 282



6/8/11 8:57:09 AM

572

Index

effect of rice variety on properties, 291–3 change in physicochemical properties, 292 fat disposition, 269–70, 271 changes in total grain and on grain surface, 270 distinct bodies containing oil, 271 oil content of raw and parboiled rice bran, 271 flow and packing properties, 278–9 effect of milling, 279 grain characteristics, 273–6 incompletely parboiled rice endosperm, 274 size, shape and appearance, 273 grain colour, 273–7 effect of pressure and time of steaming, 276 effect of temperature and time of soaking, 275 milling quality, 277–8 damage raw and parboiled paddy, 277 nutritive value, 283–8 B vitamins and other trace constituents, 283–6 changes in reducing sugars and sucrose contents, 285 loss of thiamin, 284 milling on content of thiamin and nicotinic acid, 285 other nutritional aspects, 287–8 reduced loss of constituents during milling, 286–7 thiamin distribution in successive milling, 287 parboiling advantages and disadvantages, 290–1 products, 293 properties, 273–91 protein, 270–2 processing effects on solubility, 272 soaking, 252–5 effect of temperature, 255 enzymatic activity, 252–5 husk opening, 255 possible migration of small

molecules, 255 reducing sugars, sucrose, and free amino acids, 254 South Asia, 248 starch transformation, 257–69 amylose–monomyristin complexes, 264 amylose–monostearin complexes, 266 apparent viscosity, 260 DCS thermal curves at different moisture contents, 263 digestion in aqueous KOH, 261 fractionation of rice flour on sepharose C1-2B column, 268 hydration of raw and mildly parboiled rice at room temperature, 259 hydration of raw and parboiled rice at different temperatures, 258 paddy moisture, steam pressure and steaming time, 262 status of starch, 259–68 thermal breakdown, 268–9 unusual properties of parboiled rice, 257–9 steaming, 255–7 soaking and steaming effect, 256 storage quality, 288–9 peroxide and carbonyl development, 288 tests, 514–21 alkaline gel volume test, 518 degradation of raw, very mild, mild and severe rice grains, 519 degradation of various rice kernels in 0.9% KOH after 24 h, 520 effect of temperature of soaking of paddy on discoloration, 521 possible testing methods, 516 sediment volume, 518 viscosity of raw and processed rice flour slurries, 517 types, 250–1 vs raw rice, 249–50 parboiling, 21 advantages and disadvantages, 290–1 changes in rice grain and its constituents, 252–72



6/8/11 8:57:09 AM

Index effect on product quality, 253 fat disposition, 269–70, 271 other changes, 272 protein, 270–2 during soaking, 252–5 starch transformation, 257–69 during steaming, 255–7 effect on rice quality, 247–93 changes in rice grain and its constituents, 252–72 parboiled rice, 247–51 products from parboiled rice, 293 properties of parboiled rice, 273–91 rice variety on parboiled rice properties, 291–3 see also parboiled rice parkhi see bag trieurs pasting characteristics, 492–504 log–log graph, 499 log–log P-H-C patterns of eight rice varieties, 499 Ptb 10 rice at different slurry concentration, 494 relative breakdown of rice, 502 rice-flour paste ‘breakdown,’ 495 rice viscogram, 493 semilog graph, 497 starch and rice flours viscogram indices, 501 viscogram indices of 20-, 40- and 60-fluidity acid-modified starch, 496 pearling see whitening phosphate, 306 phosphorus oxychloride, 304 physical properties, 19 physicochemical properties, 20–1 phytic acid, 396, 398–9, 404, 405 polishing see whitening porosity, 44–5, 278, 279, 457 presoaking, 181–2, 188 pressure-parboiling, 251, 267 product-making quality, 21 rice, 298–31 breakfast cereals and snacks made from wholegrain rice, 319–30 flour and products, 307–19 other rice products, 330–1

573

table rice, 301–7 protein, 145–50, 229–32 protein content, 89–90 quartering, 435 Ramiah Committee, 37–8 Rapid Visco Analyser (RVA), 114, 128 Ratiospect, 465 raw rice, 249–50 reasonably well milled rice see medium milled rice Refai index, 168, 178, 199 relative breakdown, 209, 210, 236 resistant starch, 403 rice, 402 some glycaemic index values of rice and rice products, 403 rice, 1–25 ageing, 116–58 acceleration and retardation, 155–8 changes in physicochemical properties as measured in laboratory, 121–35 consumers’ perception of changes in rice behaviour during storage, 119–20 final rice paradoxes, 153–5 relation to individual constituents, 135–53 aromatic rices, 339–48 aroma compounds, 347–8 distribution, 342–7 group V rice physicochemical characteristics, 341–2, 343 taxonomy, 340–1 baked products from rice flour, 316–19 rice bread, 317–18 rice cakes and crackers, 318 unleavened bread, 318–19 Basmati, 348–60 breeding, 349–53 international trade, 354–5, 356 physicochemical characteristics and derivatives, 355, 357–60 production, 353–4 breakfast cereals and snacks made from wholegrain rice, 319–30



6/8/11 8:57:09 AM

574

Index

common Indian rice products, 319 flaked rice, 326–9 popped rice, 319–21 puffed rice, 321–6 shredded rice, 329–30 breeding for desirable quality, 410–29 end-use quality, 425–8 paddy grain physical and morphological properties that affect the final product quality, 416–23 plant characteristics for optimum harvest, 413–16 rice grain susceptibility to cracking, 423–5 classification as per Ramiah Committee, 38 cooked/semicooked products from rice flour, 311–15 fermented cakes, 313, 315 noodles, 311–12 rice cakes, 312–13, 314 cooking quality, 164–88 effect of presoaking in ambient water, 181–2 hydration at lower temperatures, 178–80 laboratory cooking for various tests, 183–8 loss of solids during cooking, 180–1 other changes/events during cooking, 182–3 water absorption at or near boiling temperature, 166–77 cracks in rice, 460 brown rice, 460 degree of milling, 100–15 effect on rice quality, 106–14 milling paddy grain and how much to mill, 100–6 eating quality, 193–238 amylopectin paradigm, 213–24 amylose paradigm, 196–213 influencing factors, 229–34 rice-flour paste rheology, 224–8 testing for rice quality, 234–8 water-uptake paradigm, 195–6 worldwide production, 194

flour and products, 307–19 baby foods and infant rice cereals, 315–16 chemical compositions, 310 cumulative particle size distribution, 310 manufacture and their applications, 309 grain chalkiness, 459–60 chalky grains, 459 grain hardness, 458–9 Kiya hardness tester, 458 inherent variability, 22–5 cannot be pinned down to any single rule, 24–5 map of rice country, 23 subspecies classification, 24 three zones and three broad categories, 22–3 Jasmine, 361–4 export from Thailand, 363 vs Bamati milled rice, 362 milling quality, 61–95 drying, 72–3 fundamental cause of rice breakage, 90–5 milling of rice, 62–72 miscellaneous factors that affect rice milling quality, 84–90 rice grain fissures, 73–84 miscellaneous, 369–72 Arborio, 371 Boutique rice, 369 coloured rice, 369–70 Koshihikari, 371–2 soft rice, 370–1 wine rice, 371 miscellaneous properties, 50–3 effect of chalkiness on hardness of brown rice, 52 hygroscopic properties, 50–1 mechanical properties, 51–2 thermal properties, 52–3 nutritional quality, 377–407 biotechnological approach to upgrade the rice nutritive value, 405–7 nutrition perspective of poor rice-eater, 383–94



6/8/11 8:57:09 AM

Index nutritive value, 378–83 nutritive value perspective of individual rice constituents, 401–5 wholegrains untold benefits, 397–401 wholegrains vs refined grains, 394–7 other, 364–72 waxy or glutinous rice, 365–9 other rice products, 330–1 alcoholic drinks, 331 germinated brown rice, 331 miscellaneous products, 330–1 pet foods, 331 paradoxes, 8–12 rice and poverty seem to be related, 9–10 rice feeds half the world, 8–9 rice is a typical Asian staple, 11 rice yield is proportional to latitude, 10–11 waking up of ‘mud civilisation,’ 11–12 yield of paddy in different latitudes, 10 physical properties, 26–57, 450–61 chalky grains, 53–7 characteristics of US rice, 37 density and friction, 40–6 density and related properties, 454–8 dial gauge, 452 effect of moisture content, 46–9 grain appearance, 32–40 grain dimensions, 450–3 grain length and breadth measurement, 451 grain weight, 453–4 HBC-19 brown rice, 453 other properties, 458–61 paddy, brown rice and milled rice proportionality, 35 range, 29–31 size and shape classification, 36 product-making quality, 298–331 products and their classification, 300 properties USDA data, 170, 198

575

rice data, 12–18 arable area, paddy area and population of major countries of the world, 14–15 arable area and population by continents, 17 arable area and population of traditional European countries, 16 paddy production, arable area and population of rice countries of Asia, 13 role in history, 2–8 beginning of agriculture, 2 concentration and spread of rice cultivation, 3–4 frail but economically mighty grass, 2–3 not simply food or economics, 5–8 rice as food and as a source of employment, 5 rice countries of Asia, 3 speciality, 337–72 summary of various properties of paddy and rice, 31 table rice, 301–7 from outside the home, 301–5 quick-cooking rice, 305–7 rice cooked at home, 301 tabulated physical properties, 30 testing for aroma of scented varieties, 512–14 tests for eating quality, 507–12 instrumental measurement of texture, 508–11 sensory evaluation, 511–12 see also specific specialty rice rice cakes, 312–13, 314 illustration, 314 rice country, 3, 22 rice flour, 307–15 baked products, 316–19 rice bread, 317–18 rice cakes and crackers, 318 unleavened bread, 318–19 cooked/semicooked products, 311–15 fermented cakes, 313, 315 noodles, 311–12 rice cakes, 312–13, 314



6/8/11 8:57:09 AM

576

Index

high-protein rice flour, 311 rice quality, 18–22 ageing of rice, 20 analysis, 431–521 alkali digestion score, 482–5 amylose content, 487–91 degree of milling, 461–74 gel mobility test, 486–7 gelatinisation temperature, 504–7 hot-water-insoluble amylose content, 491–2 hydration and cooking quality, 474–82 milling quality, 446–9 moisture estimation, 440–6 pasting characteristics, 492–504 physical properties, 450–61 sample preparation, 433–40 test for aroma of scented varieties, 512–14 test for eating quality, 507–12 various constituents, 514 classification, 212 cooking quality, 20 degree of milling, 20 effect of parboiling, 21, 247–93 changes in rice grain and its constituents, 252–72 parboiled rice, 247–51 products from parboiled rice, 293 properties of parboiled rice, 273–91 rice variety on parboiled rice properties, 291–3 milling quality, 19 nutritional quality, 22 physical properties, 19 physicochemical properties and eating quality of rice, 20–1 product-making quality, 21 specialty rice, 21–2 testing, 234–8 new synthesis, 236–8 Rice Research and Development Centre (RRDC), 357 rice starch characteristics, 220 properties, 202 rice swelling coefficient test, 200

RiceTec Inc., 357 roti, 318 rough rice see paddy RVA, 492 sampling, 433–6 Boerner sample divider, 434–5 probes and bag trieur, 433 rice being quartered, 436 scoops used in quartering, 435 sand civilisation, 7 sandahguri, 330 Sataka Milling Metre, 466 Satake emery, 95 Satake Grain Testing Mill, 471 Satake test rice, 449 grading cylinder, 450 Satake testing husker, 446–7, 460 Satake Testing Mill, 447–8 Satake Testing Mill TM 05, 183 SatakeTesting Pearler, 471 sbramato rice, 105 scanning electron microscopic (SEM), 324 Seeds Act, 351, 352, 353 selenium, 400 shelling, 62 Shukla Committee, 38 SIBV see starch-iodine blue value simplified oven method, 442–3 soak–boil–steam–dry methods, 305 soaking, 252–5 soft rice, 370–1 specialty rice, 21–2, 337–72 aromatic rices, 339–48 Basmati, 348–60 Jasmine, 361–4 other, 364–72 Speed Act, 351 ‘sphericity,’ 45 standard method, 441 starch, 81, 137–9, 140 starch breakdown, 281–3 starch-iodine blue value, 178, 197, 207 starch transformation parboiled rice, 257–69 steaming, 255–7 stickiness, 185 stone civilisation, 7



6/8/11 8:57:09 AM

Index sugars, 136–7 surface tensiometer, 511 swelling number, 166, 167 swelling ratio, 166 table rice, 164–5, 301–7 canned rice, 301–4 canning stability, 302 cooked rice, 303 from outside the home, 301–5 distributed cooked rice, 301 frozen rice, 305 retort rice (rice in pouches), 304 quick-cooking rice, 305–7 parboiled rice, 306 rice cooked at home, 301 Taichung Native 1, 312 TA.XT2 Texture Analyser, 511 tempering, 73 Tensipresser, 200, 203, 510–11 Texturometer, 200 The Art of Rice, 6 The Nutritional Improvement of White Rice, 382 The Rice Problem in India, 382 thermal conductivity, 53 thiamin, 380–1 question, 384–8 milling effect on thiamin and nicotinic acid in raw and parboiled rice, 387 recent prevalence of beriberi in the Orient, 385 South India map showing beriberi area, 386 tocotrienols, 400 Toro, 197 total milled rice yield, 63 total yield, 63 traditional basmati, 351 undermilled rice, 103, 105, 112 United States Department of Agriculture, 303–4, 355 US Food and Drug Administration (FDA), 304 vitamin A, 406 vitamin B1 see thiamin

577

vitamin E, 400 volume expansion ratio, 166 water : rice ratio, 187 water absorption index, 474–5 water absorption ratio, 166 water solubility index, 475 water uptake, 166, 167–8, 195–6 long-, medium-, and short grain US rice, 174 parameter, 171 waxy rice, 187, 365–9 Southeast Asia map showing origin centre of glutinous rice, 367 waxy rice flour, 308 well milled rice, 105 Western diseases, 397 wet grinding method, 308 wheat gluten, 308 white belly, 54–5, 419 white centre, 419 white core, 54 white core rate, 184 white rice, 247–8 whitening, 62 wholegrain rice breakfast cereals and snacks, 319–30 common Indian rice products, 319 shredded rice, 329–30 flaked rice, 326–9 edge-runner, 328 gram roaster, 328 improved process for making flaked rice, 329 traditional process, 327 popped rice, 319–21 lemma–palea interlocking in paddy, 321 process, 320 puffed rice, 321–6 milled parboiled rice grain transverse section, 325 parboiled rice expansion interdependence during HTST puffing, 324 parboiled rice relationship between total and insoluble amylose



6/8/11 8:57:09 AM

578

Index

contents and expansion ratio, 323 process, 322 wholegrains, 394–401 untold benefits, 397–401 dietary fibre beneficial effects, 397–9

not dietary fibre alone, 399–401 vs refined grains, 394–7 not in rice alone, 395 wholegrains negative side, 396–7 wine rice, 371 World War II, 395, 396



6/8/11 8:57:09 AM

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