Thermal Technologies in Food Processing

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Thermal technologies in food processing

Related titles from Woodhead’s food science, technology and nutrition list: Extrusion: Technologies and applications (ISBN: 1 85573 559 8) Extrusion is a major technology for a wide range of food products. The first part of this book introduces this technology, examining extruders and their use in thermal transitions of raw materials into functional forms for the manufacture of particular foodstuffs. The second part of the book looks at the application of extrusion in specific product groups. It will be invaluable reference for improving efficiency and product quality in this area of the food industry. Frying (ISBN: 1 85573 556 3) Edited by one of the world’s leading authorities in the field, this collection covers the key issues in fried product quality. Part I covers general issues such as the market for fried foods, regulation and health issues. Part II covers issues on raw materials such as composition of frying oils and factors affecting the quality of oils. Part III concentrates on processing issues such as the frying process and packaging, all of which affect the flavour, texture and colour of fried foods. Food processing technology: Principles and practice (ISBN: 1 85573 533 4) The first edition Food processing technology was quickly adopted as the standard text by many food science and technology courses. The publication of this completely revised new edition is set to confirm the position of this textbook as the best singlevolume introduction to food manufacturing technologies available. New chapters include computer control of processing, novel ‘minimal’ technologies including processing using high pressures or pulsed electric fields, ohmic heating and an extended chapter on modified atmosphere packaging. Details of these books and a complete list of Woodhead’s food science, technology and nutrition titles can be obtained by: • visiting our web site at www.woodhead-publishing.com • contacting Customer services (email: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 30; address: Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB1 6AH, England)

If you would like to receive information on forthcoming titles in this area, please send your address details to: Francis Dodds (address, tel. and fax as above; e-mail: [email protected]). Please confirm which subject areas you are interested in.

Thermal technologies in food processing Edited by Philip Richardson

Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB1 6AH, England www.woodhead-publishing.com Published in North and South America by CRC Press LLC, 2000 Corporate Blvd, NW Boca Raton FL 33431, USA First published 2001, Woodhead Publishing Limited and CRC Press LLC ß 2001, Woodhead Publishing Limited The authors have asserted their moral rights. 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 authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, 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 the publishers. The consent of Woodhead Publishing Limited and CRC Press LLC 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 or CRC Press LLC 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 Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing Limited ISBN 1 85573 558 X CRC Press ISBN 0-8493-1216-7 CRC Press order number: WP1216 Cover design by The ColourStudio Project managed by Macfarlane Production Services, Markyate, Hertfordshire ([email protected]) Typeset by MHL Typesetting Limited, Coventry, Warwickshire Printed by TJ International, Padstow, Cornwall, England

Contents

List of contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of notation 1

ix xiii

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . P.S. Richardson, Campden & Chorleywood Food Research Association, Chipping Campden

1

Part I Conventional technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

2

3

Retort technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N.S. May, Campden & Chorleywood Food Research Association, Chipping Campden 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The basic retort cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Selection of container . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Selection of a retort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 The influence of heating medium on retort performance . . . . . 2.6 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Sources of further information and advice . . . . . . . . . . . . . . . . . . . 2.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous heat processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S.P. Emond, Campden & Chorleywood Food Research Association, Chipping Campden 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Indirect heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Direct heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 7 7 11 11 13 25 27 27 29 29 30 39

vi

Contents 3.4 3.5 3.6 3.7

Holding section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sources of further information and advice . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42 44 45 48

Part II Measurement and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49

4

5

6

7

Pressure and temperature measurement in food process control P.G. Berrie, Endress+Hauser Process Solutions AG, Reinach 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Temperature measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 General instrument design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51

Validation of heat processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G.S. Tucker, Campden & Chorleywood Food Research Association, Chipping Campden 5.1 Introduction: the need for better measurement and control . . 5.2 Validation methods: objectives and principles . . . . . . . . . . . . . . . 5.3 Temperature distribution testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Heat penetration testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Microbiological spore methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Biochemical time and temperature integrators . . . . . . . . . . . . . . . 5.7 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Sources of further information and advice . . . . . . . . . . . . . . . . . . . 5.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Modelling and simulation of thermal processes . . . . . . . . . . . . . . . . B.M. Nicolaı¨, P. Verboven and N. Scheerlinck, Katholieke Universiteit, Leuven 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Modelling of conduction heat transfer: the Fourier equation . 6.3 The Navier–Stokes equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Numerical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

91

Modelling particular thermal technologies . . . . . . . . . . . . . . . . . . . . . . S. Bakalis, P.W. Cox and P.J. Fryer, University of Birmingham 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Processing of packed and solid foods . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Continuous heating and cooling processes . . . . . . . . . . . . . . . . . . . 7.4 Heat generation methods: ohmic and microwave heating . . . . 7.5 Developments in the field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51 52 57 68 73

75 76 79 81 83 85 87 88 89

91 92 93 97 104 108 109 109 113 113 116 122 127 131 133

Contents 8

vii

Thermal processing and food quality: analysis and control A. Arnoldi, University of Milan 8.1 Introduction: the importance of thermal processing . . . . . . . . . . 8.2 The importance of the Maillard reaction . . . . . . . . . . . . . . . . . . . . . 8.3 Thermal processing and food safety . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Thermal processing and nutritional quality . . . . . . . . . . . . . . . . . . . 8.5 Thermal processing, food flavour and colour . . . . . . . . . . . . . . . . 8.6 Maillard reaction and lipid oxidation . . . . . . . . . . . . . . . . . . . . . . . . 8.7 Controlling factors in the Maillard reaction . . . . . . . . . . . . . . . . . . 8.8 Methods of measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.9 Application to the processing of particular foods . . . . . . . . . . . . 8.10 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.11 Sources of further information and advice . . . . . . . . . . . . . . . . . . . 8.12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

138 139 142 143 145 148 149 150 151 153 154 154

Part III New thermal technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

161

9

Radio frequency heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.T. Rowley, EA Technology Ltd, Chester 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Basic principles of RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Application to food processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Advantages and disadvantages of RF heating . . . . . . . . . . . . . . . . 9.5 RF heating technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Case studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.7 Future trends in RF heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.8 Sources of further information and advice . . . . . . . . . . . . . . . . . . . 9.9 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

138

163 163 163 166 167 169 173 175 176 177 177

10 Microwave processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Regier and H. Schubert, University of Karlsruhe 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Physical principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Microwave applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Modelling and verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

178

11 Infrared heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Skjo¨ldebrand, ABB Automation Systems (formerly Swedish Institute of Food Research (SIK)), Tumba 11.1 Introduction; principle and uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Theories and infrared properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

208

178 180 194 199 203 204

208 210 215 217

viii

Contents

11.5 11.6 11.7

Applications: case studies and modelling . . . . . . . . . . . . . . . . . . . . Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

222 225 227

12 Instant and high-heat infusion J. Andersen, APV Systems, Silkeborg 12.1 Instant infusion: an introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Instant infusion in detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Advantages and disadvantages of instant infusion . . . . . . . . . . . 12.4 High-heat infusion: an introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5 The problem of heat resistant spores (HRS) . . . . . . . . . . . . . . . . . 12.6 High-heat infusion in detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7 Advantages and disadvantages of high-heat infusion . . . . . . . . 12.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

229

13 Ohmic heating R. Ruan, X. Ye, P. Chen and C.J. Doona, University of Minnesota and I. Taub, US Army Natick Soldier Center 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Ohmic heating process and equipment . . . . . . . . . . . . . . . . . . . . . . . 13.3 Monitoring and modeling of ohmic heating . . . . . . . . . . . . . . . . . 13.4 Major challenges and needs for future research and development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

241

14 Combined high pressure thermal treatment of foods L. Ludikhuyze, A. Van Loey, Indrawati and M. Hendrickx, Katholieke Universiteit, Leuven 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Effect of high pressure on micro-organisms . . . . . . . . . . . . . . . . . 14.3 Effect of high pressure on food quality related enzymes . . . . 14.4 Effect of high pressure on food structure and texture . . . . . . . . 14.5 Effect of high pressure on sensorial and nutritional properties of food . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.6 The use of integrated kinetic information in process design and optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.7 High pressure processing technology and products . . . . . . . . . . 14.8 Conclusive remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.9 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.10 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

266

Index

229 230 232 234 234 235 238 239

241 245 247 260 264

266 267 268 271 273 275 276 278 278 278 285

Contributors

Chapter 1

Chapter 3

Professor Philip Richardson Head – Process and Product Development Department Campden & Chorleywood Food RA Chipping Campden Glos GL55 6LD England

Mrs Sue Emond Campden & Chorleywood Food RA Chipping Campden Glos GL55 6LD England

Tel: +44 (0)1386 842036 Fax: +44 (0)1386 842100 E-mail: [email protected]


Chapter 4 Chapter 2 Mr Nick May Department of Process and Product Development Campden & Chorleywood Food RA Chipping Campden Glos GL55 6LD England Tel: +44 (0)1386 842031 Fax: +44 (0)1386 842100 E-mail: [email protected]

Dr Peter Berrie Endress+Process Solutions AG Christoph-Merian-Ring 23 CH 4153 Reinach BL1 Switzerland Tel: +41 (0) 61 715 7340 Fax: +41 (0) 61 715 7301 E-mail: [email protected]

x

Contributors

Chapter 5

Chapter 8

Mr Gary Tucker Department of Process and Product Development Campden & Chorleywood Food RA Chipping Campden Glos GL55 6LD England

Professor Anna Arnoldi DISMA University of Milan Via Celoria 2 IT-20133 Milan Italy


Tel: +39 02 2362721 Fax: +39 02 70633062 E-mail: [email protected]

Chapter 9 Chapter 6 Professor Bart Nicolaı¨ Department of Agro-Engineering and Economics Katholieke Universiteit Leuven de Croylaan 42 B-3001 Heverlee Belgium Tel: +32 16 322375 Fax: +32 16 322955 E-mail: [email protected]

Chapter 7 Dr P. W. Cox, Dr S. Bakalis and Professor P. J. Fryer Food Processing Group Centre for Formulation Engineering School of Chemical Engineering University of Birmingham Edgbaston Birmingham B15 2TT England Tel: +44 (0)121 414 5310 E-mail: [email protected]

Dr Andrew T. Rowley EA Technology Limited Capenhurst Chester CH1 6ES England Tel: +44 (0)151 347 2392 Fax: +44 (0)151 347 2560 E-mail: [email protected]

Chapter 10 Mr Marc Regier and Professor Helmar Schubert Institut fu¨r Lebensmittelverfahrenstechnik Universita¨t Karlsruhe Kaiserstr. 12, Geb 30.44 D-76128 Karlsruhe Germany Fax: +49 (0)721/694320 E-mail: [email protected]

Contributors xi

Chapter 11 Dr Christina Skjo¨ldebrand ABB SE 147 80 Tumba Sweden Tel: +46 08 530 66704 Fax: +46 08 530 66110 Business e-mail: [email protected] Home e-mail: [email protected]

Chapter 12 Dr Joergen Andersen APV Systems Pasteursvej 8600 Silkeborg Denmark E-mail: joergen.andersen@invensys

Chapter 13 Dr Roger Ruan, Mr Xiaofei Ye, Dr Paul L. Chen and Mr C.J. Doona Department of Bioscience and Agricultural Engineering University of Minnesota 1390 Eckles Avenue St Paul MN 55108 USA

Tel: +1 612 625 1710 Fax: +1 612 624 3005 E-mail: [email protected] Dr Irwin A. Taub Sustainability Directorate US Army Natick Soldier Center Natick MA 01560 USA

Chapter 14 Professor Dr Ir M. Hendrickx Department of Food and Microbial Technology Katholieke Universiteit Leuven Kardinaal Mecierlaan 92 BE-3001 Leuven (Heverlee) Belgium Tel: +32 16 321 585 Fax: +32 16 321997 E-mail: [email protected]

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Notation

a A A a, b Ap Bi hd= ! B B C C

surface area surface area coefficient matrix constants particle surface area Biot number magnetic flux density flux density capacity matrix concentration of bacteria in liquid food

C1, C2 C c c, c0 ca cP ! D D D Dref Dp De d ! E E

constants turbulence constant heat capacity (Chapter 6) velocity of light, in vacuum (Chapter 10) specific moisture capacity of vapour phase specific heat capacity (constant pressure) electric flux density flux density decimal reduction time reference decimal reduction time field penetration depth diffusion coefficient diameter electric field electric field strength

m2 m2 kg s

1

m2 VSm 2 J ºC 1 number of bacteria m J kg

1

ºC

J kg 1 K VSm 2 VSm 2 min min m m2 s 1 m Vm

1

3

1

1

xiv

Notation

F ^ F f f fi Fi Fo G SR=V g g, h H H ! H H h h

1

h hevap hi Ii i ! j K K k k k ! k kt L M M, Ml m, n, o, p. q, r m N n ni n? ! P p Q

F-value: integrated lethality (Chapter 7) volumetric average thermal load vector frequency volumetric body force Cartesian component of particle force Fourier number shape factor of the container acceleration due to gravity (Chapter 7) constant (Chapter 10) static enthalpy (Chapter 6) specific enthalpy (Chapter 7) magnetic field (Chapter 10) magnetic field (Chapter 7) surface heat transfer coefficient (Chapter 6) convective heat transfer coefficient (Chapter 7) Planck’s constant evaporation heat density enthalpy of phase i mass sink or source density of phase i imaginary unit electric current density turbulent kinetic energy stiffness matrix electrical conductivity thermal conductivity (Chapter 7) Boltzmann’s constant (Chapter 10) wave vector reaction rate constant at temperature T characteristic length molecular weight (Chapter 6) moisture content, liquid moisture content (Chapter 10) exponents (Chapter 6) constant (Chapter 10) shape function constant concentration of ion i outward normal to surface polarisation pressure power dissipated per unit volume (Chapter 7)

min sm 3 W GHz Nm 3 N ms

2

J kg Jm

3

Am Wm Wm

1

1 2 2

ºC K

m2 s 2 W ºC 1 Sm 1 W m 1 ºC s 1 m kg mol

Pa Wm

3

1

1 1

1

Notation Q Q Q(R) Q Qem qR R R r r r 300 mg/100 g. Abnormal values indicate fraud, for example the addition of reconstituted milk powder. Great interest has been dedicated to the nutritional value of dry milk powder, whose most important use is for the preparation of infant formulas. The main processes used include preconcentration using film evaporating systems to 40– 50% solids and spray drying. An alternative method is drum drying where the liquid is applied in a thin layer to a heated cylinder and after some time removed by a knife. With the latter method, however, the thermal exposure is much higher. 8.9.2 Baked products Wheat flour contains only very small amounts of free sugars and the fermentation process is very important to generate glucose and maltose that are indispensable MR precursors. This can be improved by the addition of flour from malted grains very rich in -amylase. Bread baking time and temperatures vary very much as a function of the bread dimensions and constituents, in general small products (about 45 g) may be heated for 18–20 min at 240–250ºC, while big products (1000 g) may be heated at 240–220ºC for 55–65 min. Naturally, especially in the case of large products, the heat transfer occurs slowly and there is a temperature gradient from the outer to the inner part of the dough. Water evaporates efficiently only in the crust, while the inner part remains softer and colder. The extent of the MR and its consequences are not homogeneous: the differences in colour, flavour and texture of the crumb and the crust are probably part of the hedonistic pleasure of bread consuming. In crumb the most important flavours are autoxidation products of linoleic acid, methional and diacetyl, while 2-methyl-3-ethylpyrazine, 2-acetylpyrazine, 2acetyl-1-pyrroline and 5-methyl-(5H)cyclopenta(b)pyrazine and furaneol are responsible for the caramel, malty and roasty notes of crust. Free proline is the precursor of many volatile compounds that have been characterised in detail

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(Tressl et al. 1985). The pleasant colour of crust is due in part to the MR and in part to caramelisation of sugars. 8.9.3 Coffee During roasting, coffee beans undergo many pyrolytic reactions, which lead to the substances responsible for its particular colour and flavour. The knowledge of green coffee composition is still incomplete and during roasting other reactions take place besides the MR, so that the composition of roasted coffee, especially the non-volatile part, is still far from being clear. The process can be divided into two phases: drying and roasting. An important parameter is heat transfer which is correlated with time and temperature of roasting. The intensity of colour is correlated with the final roasting temperature. The taste changes from slightly sweet (green coffee) to bitter. Most of the constituents are transformed, with some exceptions such as caffeine. Free amino acids and sucrose, which is around 8.0% in arabica green coffee and 4.0% in robusta green coffee, disappear completely, but also polysaccharides are transformed, together with other compounds not directly involved in the MR like chlorogenic acid. Any effect that changes the concentration of the precursors in green beans before roasting influences the quality of the roasted material. At the end the condensation and caramelisation products are about 25%. More than 700 volatile compounds have been identified in coffee aroma: important classes deriving from the Maillard reaction are pyrans, pyrazines, pyridines, pyrones, pyrroles, thiazoles and thiophenes (Illy and Viani 1995).

8.10

Future trends

Extrusion cooking is a high-temperature/short-time technique was introduced in 1930s in the cereal industries. Nowadays this process is used extensively in the food industry for the production of expanded snacks, breakfast cereals, textured soy protein, and other foods. Due to the nature of this technique, involving high temperatures, pressures and shear forces, the food matrix is subjected to chemical changes (gelatinisation of starch molecules, cross-linking of proteins) and the production of flavours can be induced by the Maillard reaction (Riha et al. 1996). Another recent technique for food processing is microwave heating. As experienced also during domestic cooking, the aroma profile of foods cooked in this way is a very critical point. The lack of volatiles is due to the speed of heating and surface moisture and temperature; actually many aroma compounds are steam volatile and during microwave heating they evaporate rapidly; moreover the surface aw, that generally is around 1.0, is very unfavourable for the Maillard reaction. In order to enhance the flavour profile of microwaved foods several approaches have been attempted, e.g. addition of commercial

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flavourings or Maillard precursors, use of microwave absorbing susceptor sheets, combination of heating treatment (van Eijk 1992).

8.11

Sources of further information and advice

The reader will find a useful discussion of the consequences of thermal treatment of the most relevant foods in Belitz and Grosch (1999). Every 3–4 years the researchers meet for international symposia, whose proceedings are a very useful source of information (Waller and Feather 1983; Fujimaki et al. 1986; Finot et al. 1990; Parliment et al. 1994; O’Brien et al. 1998).

8.12

References

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Part III New thermal technologies


9 Radio frequency heating A. T. Rowley, EA Technology Ltd, Chester

9.1

Introduction

Radio frequency (RF) or high frequency (HF) heating, is more correctly described as RF dielectric heating, and is a well established thermal processing technology which has found applications in many areas. The most notable of these are found in the plastics (welding1), textiles (drying2), paper and board (drying3), wood (gluing4 and drying4) and food industries. RF heating has much in common with microwave and ohmic heating – all three are electroheat technologies in which heat is generated volumetrically throughout a product rather than having to rely on the slow conduction of heat through its surface. Following an overview of the fundamentals of RF heating, this chapter will outline why the use of this technology in the food processing industry can lead to clear advantages over other thermal techniques. In particular, the technology will be compared to microwave and ohmic heating. The limitations of the technology will also be discussed. A major section of this chapter describes the technology associated with RF heating and how it is generally used for food processing. Two case studies, RF-assisted baking and meat defrosting, will be presented which emphasise the importance of RF heating to the food processing industry. Finally, the chapter concludes by considering the future direction of the technology, and the implications for the thermal processing of food.

9.2

Basic principles of RF heating

In very simple terms, radio frequency heating of foods arises from the direct conversion of electrical energy to heat within the volume of the food itself. This

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electrical energy is provided by a high frequency electric field applied between the plates of a capacitor (the RF applicator). In electrical terms, foods are dielectrics (materials which increase the charge stored in a capacitor) which possess a significant dielectric loss factor (an indication of the amount of energy which will be dissipated). The term dielectric heating can equally be applied to radio frequency and microwave systems – in both cases the heating is due to the fact that energy is absorbed by a lossy dielectric when it is placed in a high frequency electric field. In foods, at radio frequencies, this loss principally arises from the electrical conductivity of the food, and the heating mechanism is simply resistance heating (i.e. similar to ohmic heating). Although microwave heating also relies on a dielectric loss to provide the heat, the principal loss mechanism in food products at microwave frequencies is different (resonant dipolar rotation5). The radio and microwave frequency bands occupy adjacent sections of the electromagnetic spectrum, with microwaves having higher frequencies than radio waves. The actual dividing point between the two frequency bands is imprecisely defined, with, for example, some applications at around 900 MHz being referred to as RF (cellular telephones), and some as microwaves (dielectric heating). However, the technology used to generate and transmit the high frequency electric fields can be used to distinguish them. RF systems are generally based on high power electrical valves (to produce the RF power), transmission lines (to carry the RF energy), and applicators in the form of capacitors; whereas microwave systems use magnetrons (to generate the microwaves), waveguides (to transport the microwaves) and cavities (in which the microwaves are applied). There is a relatively small number of internationally agreed and recognised frequency bands which can be used for RF and microwave heating. These are known as the Industrial, Scientific and Medical (ISM) bands, and are defined in Table 9.1. Electromagnetic compatibility (EMC) regulations set very low limits for any emissions outside of these bands, and, in most countries, compliance is a legal obligation. Consequently, virtually all RF process heating equipment will operate at one of the three allowed ISM frequencies. It is worthwhile noting that the wavelength at radio frequencies is substantially greater than at microwave frequencies – 11 m at 27.12 MHz compared with only 12 cm at 2450 MHz. It is this difference which leads to a number of significant advantages of RF over microwaves, particularly for industrial food processing applications. Table 9.1

The ISM bands available for dielectric heating

Heating technology

Frequency

Radio frequency

13.56 MHz 0.05% ( 0.00678 MHz) 27.12 MHz 0.6% ( 0.16272 MHz) 40.68 MHz 0.05% ( 0.02034 MHz) 900 MHz (depending on country) 2450 MHz 50 MHz

Microwave

Radio frequency heating

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Fig. 9.1 The effect of a dielectric on a capacitor.

A calculation of the actual amount of energy (or power) absorbed by a dielectric body is essential to a full understanding of radio frequency (or microwave) heating. An expression for the power dissipated in a dielectric can be derived following directly from the premise that, in essence, all applicators used for RF dielectric heating are some form of capacitor. When a dielectric material such as food (with a dielectric constant, "r0 , and a dielectric loss factor, "r00 ) is placed in this capacitor, it will be affected in two ways (see Fig. 9.1). First, the new effective capacitance, C0 , will be greater than the original capacitance (Co) by a factor er’ (by definition, "r0 is always greater than one), and secondly, a finite resistance, R (proportional to 1/Co"r00 ), will appear across the capacitor. The increase in capacitance arises from a change in the distribution of electric charge within the RF applicator, and the presence of the resistance gives the possibility of heat generation within the dielectric. Assuming that the power, P, dissipated in this resistance to be equal to V2/R, then it can be shown that the power dissipation per unit volume or power density, Pv, is given by:6 Pv 2f "0 "00r E2

9:1

where f is the frequency of the applied field (RF), "0 is a constant (the permittivity of free space) and E is the electric field strength in the dielectric. Although derived in another way, the same equation is used to describe microwave dielectric heating.5 Inspection of equation [9.1] reveals that the power density is proportional to the frequency of the applied field and the dielectric loss factor, and is proportional to the square of the local electric field. This equation is crucial in determining how a dielectric will absorb energy when it is placed in a high frequency electric field. For a given system, the frequency is fixed and f and "0 are both constants. The dielectric loss factor "00r can, in principle, be measured. The only unknown left in equation [9.1] is the electric field, E. To evaluate this, the effect of the dielectric material itself on the applied electric field (due to the RF voltage across the RF applicator) must be considered. For materials, such as food products, where the dielectric loss arises principally from the electrical conductivity, then the loss factor, "00r , is given by "00r =2f "0 , and equation [9.1] can be further reduced to Pv E2

9:2

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which is the same equation as that used to describe ohmic heating. However, unlike ohmic heating, in RF heating this electric field can be achieved without the need for the electrodes to be in contact with the product. This is a consequence of the much higher frequency used.

9.3

Application to food processing

Radio frequency heating has been used in the food processing industry for many decades. In particular, RF post-baking of biscuits and cereals, and RF drying of foods are well established applications. More recently, RF thawing equipment has demonstrated substantial benefits over conventional techniques and over comparable microwave tempering systems. Furthermore, as the public concern over food safety issues continues to grow, and as the demand for convenience foods increases, RF pasteurisation and sterilisation processes are becoming more important. 9.3.1 Baking The post-baking of biscuits7 is one of the most accepted and widely used applications of RF heating in the food processing industry. The addition of a relatively small RF unit to the end of a conventional baking line results in a substantial increase in product throughput, together with improvements in product quality. Similarly, the same process has also been applied to cereal, pastry and bread products. More recently, the RF system has been incorporated directly into the hot air oven, allowing RF-assisted baking of a wide range of products to be carried out in a very compact unit. 9.3.2 Drying The principal role of RF heating in baking is the removal of moisture, particularly at the end of the process when conventional heating is inefficient. RF drying is intrinsically self-levelling,6 with more energy being dissipated in wetter regions than in drier ones. This RF levelling leads to improvements in product quality and more consistent final products. As well as baking applications, RF drying applications in the food industry include the drying of: food ingredients (e.g. herbs, spices, vegetables); potato products (e.g. French fries), and a number of pasta products. 9.3.3 Defrosting A more recent, and rapidly growing, application of RF in the food processing industry is its use for the bulk defrosting of meats and fish. Conventionally, large blocks of meat are thawed slowly, often over a period of days. The volumetric nature of RF heating allows the thawing process to be accelerated, whilst still


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maintaining control of the temperature distribution within the food product. Typically, RF defrosting times of 1–2 hours are possible. 9.3.4 Pasteurisation/sterilisation With the increasing demand for convenience foods and ‘ready meals’, re-heating of food products is becoming more common, often carried out in a microwave oven, and it can no longer be assumed that all of the food is heated to a high enough temperature to kill bacteria. Consequently, there is a demand for more in-package pasteurisation. As a non-contact volumetric heating technique, RF is an ideal process for this application. Increased public concern and awareness of food safety issues are leading to more RF pasteurisation and sterilisation applications being investigated, and it is likely that a number of these will become industrial processes in the near future.

9.4

Advantages and disadvantages of RF heating

9.4.1 Advantages In common with microwave and ohmic heating, the volumetric nature of radio frequency heating gives rise to a number of significant advantages over more traditional, surface heating techniques. The most important of these to the food industry are: • Improved food quality. The main reason for using RF heating in food processing (rather than any other thermal technology) is improved food quality. First, the volumetric process leads to more uniform heating, removing the risk of overheating food surfaces while trying to heat the centre of products. Secondly, the selective nature of RF heating, with energy being dissipated according to the local loss factor, can produce very uniform products, even when there are relatively large variations in the unprocessed food. • Increased throughput. Conventional surface heating often has to heat foods relatively slowly to avoid the risk of overheating the surface. Moreover, once the surface of foods have dried out, they often form a good thermal barrier layer, making it even more difficult to heat the centres. By contrast, volumetric RF heating avoids these effects, allowing production lines to operate much faster. • Shorter process lines. As an alternative to increased throughput, food processing lines which include RF systems can be significantly shorter for a given throughput. • Improved energy efficiency. Since the RF energy is dissipated directly within the product being heated, processing lines using this technology can be very efficient, particularly when the increased throughput is also taken into account.

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• Improved control. Since the power dissipated within the food is due to the presence of an electric field (equation [9.1]), if this field is changed, or switched off, the heating of the food products responds almost instantaneously. In this way, RF heating can be controlled very precisely, again leading to improvements in food quality

In food processing, RF heating also has a number of advantages over the alternative volumetric technologies, namely microwave and ohmic heating. The main ones are: • Contactless heating. Although the heating mechanism is essentially the same as with ohmic heating, RF does not require the electrodes to be in contact with the food. This removes the constraint that the food product has to be pumpable, and allows RF heating to be applied to solid as well as liquid heating. • Increased power penetration. The longer wavelength at radio frequencies compared with microwave frequencies, and the dielectric properties of foods, mean that RF power will penetrate further into most products than microwave power. For example, the penetration depth (the distance for the power to fall to 1/e of its initial value) in unfrozen meat products is typically only a few millimetres at microwave frequencies, but tens of centimetres at RF frequencies. • Simpler construction. Large RF applicator systems are generally simpler to construct than microwave ones. In particular, the longer wavelength at radio frequencies allows relatively large entry and exit ports to be designed – 2 m wide ports are not untypical. Moreover, the geometries of RF applicator systems (see Section 9.5.3) naturally lend themselves to industrial food processing applications. • Improved moisture levelling. In food products, the variation of the dielectric loss factor with moisture content is generally greater at radio than at microwave frequencies. Consequently, the use of RF heating for baking and drying applications leads to improved moisture levelling and correspondingly higher quality final products.

9.4.2 Disadvantages When compared with conventional heating techniques, and, to some extent with ohmic heating, the main disadvantages of RF heating relate to equipment and operating costs. In comparison with microwave heating, the main limitation of RF heating arises from a lower power density. • Equipment and operating cost. For an equivalent power output, RF heating equipment is more expensive than conventional convection, radiation or steam heating systems. It is also more expensive than an equivalent ohmic heating system. However, in some applications, improvements in product quality and throughput often more than justify the initial capital investment.


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As an electroheat technology, the unit energy costs of a RF system will be higher than an equivalent conventional heating system. Nevertheless, when factors such as increased energy efficiency and increased throughput are taken into account, the total energy cost may be comparable to (or even less than) a conventional system. • Reduced power density. Equation [9.1] shows that the power density is directly proportional to the frequency. Given that the electric field is limited to avoid the occurrence of an electrical breakdown, then the power density will be much higher at microwave than radio frequencies. The main consequences of this are that RF systems are usually significantly larger than microwave heating systems of the same power rating, and that faster heating rates can often be achieved with a microwave system.

9.5

RF heating technologies

The available systems for producing and transferring RF power to dielectric heating applicators can be divided into two distinct groups; the more widespread conventional RF heating equipment,8 and the more recent 50 RF heating equipment.9 Although conventional RF equipment has been used successfully for many years, the ever tightening EMC regulations, and the need for improved process control, are leading to an increased use of RF heating systems based on 50 technology. 9.5.1 Conventional RF equipment In a conventional system,8 the RF applicator (i.e. the system which applies the high frequency field to the product) forms part of the secondary circuit of a transformer which has the output circuit of the RF generator as its primary circuit. Consequently, the RF applicator can be considered to be part of the RF generator circuit, and is often used to control the amount of RF power supplied by the generator. In many systems, a component in the applicator circuit (usually the RF applicator electrodes themselves) is adjusted to keep the power within set limits. Alternatively, the heating system is set up to deliver a certain amount of power into a standard load of known conditions, and then allowed to drift automatically up or down as the condition of the product changes. In virtually all conventional systems, the amount of RF power being delivered is only indicated by the DC current flowing through the high power valve (usually a triode) within the generator. A typical conventional RF heating system is shown schematically in Fig. 9.2. 9.5.2 50 RF equipment RF heating systems based on 50 equipment are significantly different, and are immediately recognisable by the fact that the RF generator is physically separated from the RF applicator by a high power coaxial cable (Fig. 9.3).

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Fig. 9.2 Conventional RF dielectric heating system.

The operation frequency of a 50 RF generator is controlled by a crystal oscillator and is essentially fixed at exactly 13.56 MHz or 27.12 MHz (40.68 MHz is seldom used). Once the frequency has been fixed, it is relatively straightforward to set the output impedance of the RF generator to a convenient value – 50 is chosen so that standard equipment such as high power cable and RF power meters can be used. For this generator to transfer power efficiently, it must be connected to a load which also has an impedance of 50 . Consequently, an impedance matching network has to be included in the system which transforms the impedance of the RF applicator to 50 . In effect, this matching network is a sophisticated tuning system, and the RF applicator plates themselves can be fixed at an optimum position.

Fig. 9.3

50 RF dielectric heating system.


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The main advantages of this technology over the conventional systems are: • Fixed operation frequency makes it easier to meet onerous international EMC regulations. • The use of 50 cable allows the RF generator to be placed at a convenient location away from the RF applicator. This is of particular importance to food processing applications, where the RF applicator will need to be cleaned regularly. • The RF applicator can be designed for optimum performance, and is not itself part of any tuning system. • The use of a matching network gives the possibility of an advanced process control system. The positions of components in the matching network give on-line information on the condition of the dielectric load (such as its average moisture content). This information can be used to control the RF power, the speed of conveyor or the temperature of air in applicator as appropriate.

9.5.3 RF applicators Whether conventional or 50 dielectric heating systems are used, the RF applicator has to be designed for the particular product being heated or dried. Although the size and shape of the applicator can vary enormously, they mostly fall into one of three main types – throughfield, fringefield or staggered throughfield. Whatever the type of applicator, RF food processing systems often benefit from the combination of RF with hot air convection heating.10 This hot air can either be introduced conventionally into the applicator enclosure, or directed onto the surface of the product through the electrodes themselves. This combination of volumetric and conventional surface heating optimises the cooking, baking and drying processes in such a way that relatively small amounts of RF energy can lead to large improvements in throughput and food quality, whilst minimising the size of the combination heating unit. Throughfield applicator Conceptually, a throughfield RF applicator is the simplest, and the most common, design, with the electric field originating from a high frequency voltage applied across the two electrodes which form a parallel plate capacitor (Fig. 9.4a). This type of system can be used for both batch and continuous processing applications, and is mainly used with relatively thick products, or blocks of material. For example, this electrode arrangement is found in RF meat defrosting systems. Fringefield applicator An alternative RF applicator arrangement, often used in drying applications, is known as the fringefield system. In this case, the product passes over a series of bars, rods or narrow plates which are alternately connected to either side of the

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Fig. 9.4

Alternative RF electrode configurations.


173

RF voltage supply (Fig. 9.4c). The major advantage of this configuration is that the product runs close to the electrode bars and there is only a small air gap between the RF applicator and the product. This ensures that there will be a virtually constant electric field in the material between the bars (an important requirement to maximise moisture levelling performance). It also reduces the electric field that has to be applied between the electrodes to generate a given power density within the product. The major disadvantage of this arrangement is that only relatively thin layers of product can be used, otherwise there will be an electric field variation throughout the product thickness. This electrode arrangement is found in some pasta drying and cereal baking applications. Staggered throughfield applicator For intermediate thickness products, a modified form of the throughfield applicator is often used. This is known as a staggered throughfield applicator (Fig. 9.4b). This arrangement reduces the overall capacitance of the applicator which, in turn, makes the overall system tuning easier. It also reduces slightly the voltage that has to be applied across the electrodes to produce a given RF power density within the product. This electrode configuration is commonly used in RF post-baking applications.

9.6

Case studies

9.6.1 RF-assisted biscuit baking Conventional process In a conventional baking oven, convective and/or radiative energy is applied simultaneously to the top and bottom surface of the biscuits. The biscuit drying proceeds from these surfaces towards the biscuit centre. Once the surface has dried the conventional heat starts to bake the surface. A typical industrial biscuit baking oven is about 60 m long. The main disadvantages arise from the difficulty associated with the use of conventional surface heating to remove the small amount of moisture in the centre of the biscuit at the end of the process. These are: • the biscuit baking line has to be very long, due to the disproportionately high amount of energy needed towards the end of the biscuit baking process • for the same reason, conventional biscuit baking is very energy intensive • the conventional process leads to only average or even poor quality biscuits.

RF process The traditional RF solution to these problems is to add a relatively small (3–4 m long) RF drying unit at the end of the conventional processing line. The intrinsic moisture levelling associated with RF heating allows the final thin layers of moisture at the centre of the biscuit to be rapidly and efficiently removed. Such a RF post-baking unit can increase throughput by up to 30%. More recently, the

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RF applicator systems have been incorporated directly into the conventional high temperature ovens allowing hot air and RF energy to be applied simultaneously. This RF-assisted biscuit baking system is comprised of a number of RF zones, with typically 40–50 kW of RF power per zone. The main advantages of the RF-assisted unit, over both conventional and RF post-baking systems, are: • the length of the biscuit oven can be substantially shorter • it is a very energy efficient process • biscuit quality is substantially improved: the biscuits are typically larger, have a less dense and more uniform structure, are more difficult to break and have a less ‘pasty’ taste.

9.6.2 Meat defrosting Conventional process Once removed from cold storage, large (typically 10–20 kg) blocks of meat have to be carefully stacked in a temperature controlled room where they slowly thaw out over a period of several days. Even at this slow thawing rate, there is often a large variation in temperature within individual blocks and between different blocks. The main disadvantages of the conventional process are: • the slow processing speed means that the supply of defrosted meat cannot respond to a rapidly changing demand • during conventional thawing, there is a large drip loss from the meat which can account for up to 12% of the product volume, and which reduces significantly the value of the product (meat is sold by weight), and also gives effluent handling problems • the conventional process is very labour intensive • the slow thawing process can lead to significant biological growth which reduces the product shelf-life, and may present a potential health risk • a large amount of floor space has to be allocated to the thawing process.

RF process The RF meat defroster is a continuous, conveyorised unit made up of three independent RF zones, each with a relatively simple throughfield electrode arrangement. A generator with a maximum output power of typically 30–40 kW is connected to each zone. This RF unit can continuously defrost around one tonne of meat per hour, with a defrost time of less than two hours. A typical unit would be around 20 m long with a conveyor belt width of about 2 m. The main advantages of the RF meat defroster are: • with a thawing time of less than 2 hours, the RF unit allows the supply of thawed meat to respond rapidly to any changes in demand • the drip loss is reduced to less than 1% – increasing the value of the final product and reducing any effluent handling problems


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• any biological growth is minimised, increasing the shelf-life of the meat products, and reducing any health hazard • the RF defrosting process is not labour intensive – up to 75% less than the conventional process • compared with the conventional thawing process, the RF unit is very compact.

9.7

Future trends in RF heating

The future direction of the use of RF heating in the food processing industry will be influenced by many factors. Developments in RF technology could significantly benefit both existing and emerging food applications. Similarly, changes in consumer food preferences could lead to new applications, but could also lead to some existing applications becoming redundant. Moreover, changes in the food (or radio) regulations could have a major impact on the future use of RF heating in the food industry. 9.7.1 Technology Although predicting the future direction of any technology is, at best, difficult, there are a number of general trends in RF heating technology which are likely to influence food processing applications. 50 RF equipment specifically developed for the dielectric heating market is now commercially available, and the technology has been proven for a range of food applications. The clear benefits of this technology over conventional equipment (see Section 9.5.2) will lead to its more widespread future use, both for new processing lines, and as replacements for conventional RF equipment in existing lines. At present, virtually all RF generators used for the thermal processing of foods use thermionic valves. However, future developments in transistor technology (particularly in MOSFETs), will lead to the upper power limit of solid state generator systems being increased from its present level of 5–10 kW. Ultimately, cost competitive solid state generators will be available in the range 20–50 kW typically used in most industrial food processing applications. Such RF generators will be compact, light and very controllable. The same developments in transistor technology could also lead to low power (i.e. 0.5–2 kW) RF generators becoming much cheaper. At present, microwave heating completely dominates dielectric heating in the commercial (and domestic) food processing sectors. Even though RF heating has a number of clear advantages over microwave heating (see Section 9.4.1), the cost of low power RF systems is prohibitively expensive compared to equivalent microwave systems. The availability of low cost RF power sources could lead to a major growth in the use of RF heating in the commercial food sectors.

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9.7.2 Applications The current and increasing demand for high quality food products will mean that RF post-baking and RF-assisted baking will continue to be important stages in the processing of biscuit, cereal and pastry products. Similarly, public concern over food hygiene issues will continue to require rapid and safe food thawing techniques, such as RF meat and fish defrosting systems. Increasing public awareness of general food safety issues, and the rising demand for convenience, pre-packaged foods will lead to a growth in the demand for RF (and microwave) pasteurisation and sterilisation techniques.

9.8 9.8.1

Sources of further information and advice Further reading

BLEANEY B I, BLEANEY B,

Electricity and Magnetism, Oxford, Oxford University Press, 1978. METAXAS A C, MEREDITH R J, Industrial Microwave Heating, IEE Power Engineering Series, London, Peter Peregrinus Ltd, 1983. METAXAS A C, Foundations of Electroheat – A Unified Approach, Chichester, John Wiley & Sons Ltd., 1996. HULLS P (Secretary, dielectric heating working group), Dielectric Heating for Industrial Processes, UIE, 1992. Journal of Microwave Power and Electromagnetic Energy, The International Microwave Power Institute (IMPI). 9.8.2 Organisations and other contacts EA Technology Capenhurst Chester CH1 6ES, UK Website: http:///www.eatechnology.com EA Technology provide advice and supply RF heating equipment to the food industry. They also have expertise in microwave and ohmic heating systems. British National Committee for Electroheat (BNCE) 30 Millbank London SW1P 4RD, UK Website: http://www.electricity.org.uk/services/bnce/ The International Union for Electroheat (UIE) Tour Atlantique Cedex 06 92080 Paris La Defense, France The International Microwave Power Institute (IMPI) 10210 Leatherleaf Court


177

Manassas, VA 20111, USA Website: http:/www.impi.org/ The Electric Power Research Institute (EPRI) Hillview Avenue, Palo Alto California 94304, USA Website: http:/www.epri.com/

9.9

Acknowledgements

The author would like to thank the Directors of EA Technology for permission to publish this work. In compiling the case studies, the author would also like to gratefully acknowledge the assistance of Petrie Technologies Ltd., Ackhurst Road, Chorley, Lancs PR7 1NH, UK.

9.10 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

References FEDERATION OF HF WELDERS,

High Frequency Welding Handbook, Ashburton, Federation of High Frequency Welders, 1995. JONES P L, Radio Frequency Drying in the Textile Industry, International Dyer, June 1980. JONES P L, Radio Frequency Processing in Europe, J Microwave Power and Electromagnetic Energy 1987 22(3) 143–53. POUND J, Radio Frequency Heating in the Timber Industry, London, E & FN Spon Ltd., 1973. METAXAS A C, MEREDITH R J, Industrial Microwave Heating, IEE Power Engineering Series, London, Peter Peregrinus Ltd, 1983. JONES P L, ROWLEY A T, Dielectric Dryers. In Baker C J, ed. Industrial Drying of Foods, London, Chapman and Hall, 1997. HOLLAND J M, Dielectric Post-baking in Biscuit Making, Baking Industries Journal, 1974 8(6). HULLS P, Dielectric Heating for Industrial Processes, UIE, 1992. MARCHAND C, Recent Developments in Industrial Radio Frequency Technology, UIE, Proc. High Frequency and Microwave Processing and Heating, Arnhem, 26–29 Sept. 1989, 4.4.1–4.4.10. JONES P L, ROWLEY A T Dielectric Drying, Special issue of Drying Technology, 1996, 14(5) 1063–98.

10 Microwave processing M. Regier and H. Schubert, University of Karlsruhe

10.1

Introduction

In this chapter an overview of microwave heating as one method of thermal food processing is presented. Due to the limited space, this overview cannot be complete; instead some important theoretical information and also examples of practical uses at home and in industry are shown. This chapter provides a starting point, and the interested reader is directed to the references, where more information about the special themes discussed in this chapter can be found. Additional to the references in the text the interested reader is also referred to two bibliographies that cover more or less all the published work on microwaves (Goldblith and Decareau 1973; Dehne 1999). 10.1.1 History of microwave heating The total sales number of microwave ovens in the United States stays at a constant level of approximately 10 million per year (Anon. 1998). The corresponding number in Europe is in the same range. These enormous sales point to the importance of microwave heating today. Nevertheless, it took some time for the development of this technique starting from the microwave source invention in 1921 by Hill (Knutson et al. 1987). The first continuous magnetron (see Section 10.2.4) was built by Randall and Boot who tried to produce a microwave source to power radar sets for the British military during World War II (Reynolds 1989). It was brought to the United States in order to use America’s production potential. Raytheon Co. was the company that received a contract to make copies of the magnetron, where the electrical engineer Spencer improved its manufacturability for large productions. He filed a patent in 1942 concerning

Microwave processing

179

the ameliorated magnetron that was issued nine years later. In 1945 Spencer occasionally observed the heating of various substances (a legend tells about his own body, popcorn and an exploding egg) by the microwave energy of the antenna horn (Reynolds 1989). In the same year, he applied for a patent (issued in 1950) called ‘method of treating foodstuffs’ describing for the first time a closed microwave oven. This technique was applied in Raytheon’s Radarange oven in 1946. With further development and falling prices (also due to the expiring of the basic patent) the domestic microwave oven market grew very fast, starting in the late 1960s, reaching a peak of 12 million ovens sold in the United States in 1988, later becoming constant near 10 million per year. The development of industrial dielectric heating applications started in the radio frequency range in the 1930s (Pu¨schner 1966). Due to the proportionality of the electromagnetic power loss to the used frequency (see equation [10.24]) the energy rate could be enhanced by increasing the frequency. The first patent describing an industrial conveyor belt microwave heating system was issued in 1952 (Spencer 1952). However, the first conveyor belt microwave application only started in 1962 due to the slow development of high power microwave generators. Its first major applications were the finish drying of potato chips, pre-cooking of poultry and bacon, tempering of frozen food and drying of pasta (Decareau 1985). 10.1.2 Today’s uses, advantages and disadvantages of microwave heating applications Today’s uses range from these well known applications over pasteurisation and sterilisation to combined processes like microwave vacuum drying (see also Section 10.3). The rather slow spread of food industrial microwave applications has a number of reasons: there is the conservatism of the food industry (Decareau 1985) and its relatively low research budget. Linked to this, there are difficulties in moderating the problems of microwave heating applications. One of the main problems is that, in order to get good results, they need a high input of engineering intelligence. Different from conventional heating systems, where satisfactory results can be achieved easily by intuition, good microwave application results often do need a lot of knowledge or experience to understand and moderate effects like uneven heating (e.g. edge heating or focused heating) (see Section 10.2.2), or the thermal runaway (see Section 10.2.3)). Another disadvantage of microwave heating as opposed to conventional heating is the need for electrical energy, which is its most expensive form. Nevertheless, microwave heating has a number of quantitative and qualitative advantages over conventional heating techniques that make its adoption a serious proposition. One main advantage is the place where the heat is generated, namely the product itself. Because of this, the effect of small heat conductivities or heat transfer coefficients do not play such an important role. Therefore, larger pieces can be heated in a shorter time and with a more even temperature distribution. These advantages often yield an increased production

180


rate and/or an improved product quality. Another advantage is the almost entire energy conversion from electromagnetic radiation into heat, where it is needed. Depending on the various applications, there could be further advantages like space savings or low noise levels.

10.2

Physical principles

10.2.1 MW-frequency range Microwaves are a kind of electromagnetic wave within a frequency band of 300 MHz to 300 GHz. By equation [10.1] the frequency f is linked by the velocity of light c to a corresponding wavelength . cf

10:1

The term microwaves is a little bit misleading since the vacuum wavelength of them is in the range between 1 m and 1 mm. Their name rather points to their wavelength within the matter, where their wavelength can be in the micrometer range. In practice, for microwave heating applications not all the microwave spectrum is used, in effect there are some discrete frequency bands, which have been set aside from telecommunication applications for industrial, scientific and medical (so called ISM) applications. The most important and most used ISM microwave frequency bands are 915 25 MHz and 2450 50 MHz, where a certain limited radiation level has to be tolerated by other applications (like communication devices). 10.2.2 Maxwell’s equations, wave equations and exemplary solutions Maxwell’s equations As already mentioned above, microwaves belong to electromagnetic waves, which can be basically described with Maxwell’s equations [10.2–10.5]: !

rD !

rE !

10:2 !

@B @t

rB 0 ! ! ! @D rH j @t

10:3 10:4 10:5

In order to include the interactions of matter with electromagnetic fields, the material equations, also called constitutive relations [10.6–10.8], have to be added, where the permittivity or dielectric constant " (interaction of nonconducting matter with an electric field), the conductivity and the permeability (interaction with a magnetic field) appear to model their behaviour (see also Section 10.2.3). The zero indexed values describe the behaviour of a vacuum, so that " and are relative values.

Microwave processing !

!

D "0 " E !

!

B o H !

!

j E

181 10:6 10:7 10:8

In the most general form all these material parameters, describing the properties of matter, can be complex tensors (with directional dependent behaviour). For practical use with food substances some simplifications are possible: the relative permeability can be set to 1, since food behaves non-magnetically, and the permittivity tensor can be reduced to a complex constant with real ("0 ) and imaginary part ("00 ), which may include the conductivity (see Section 10.2.3). Wave equations and exemplary solutions Starting from Maxwell’s equations, with the simplifications of no charge ( 0) ! and no current density ( j 0), there is an easy way to infer the wave equations for electric and magnetic fields (here only shown for the electric field case). Applying the curl-operator (r) on [10.3] yields [10.9]: !

! @B @ r B r r E r 10:9 @t @t Using the material equation for the magnetic field [10.7], supposing to be constant and introducing [10.5] into [10.9], this can be transformed to [10.10]: ! ! @ @D r r E 0 10:10 @t @t !

The last step is to utilise the material equation for the electric field [10.6], ! ! Maxwell’s equation [10.2] and the vector identity r r X rr X ! X to get the following well known wave equation [10.11]: !

@2E 10:11 E 0 "0 " 2 0 @t Similarly the corresponding wave equation for the magnetic component can be derived, yielding [10.12]: !

!

@2B 0 10:12 @t2 By comparing the wave equations [10.11] and [10.12] with the standard one, one can infer that in this case the wave velocity is defined by [10.13]: !

B

0 "0 "

1 c0 c p p " 0 "0 "

10:13

In order to illustrate the nature of solutions of [10.11] or [10.12], we consider the case where the electric field has only a component in the z-direction Ez (socalled linearly polarised) and depends only on the x coordinate (so-called plane wave). Additionally the material parameters should be frequency independent. Equation [10.11] then reduces to

182


@ 2 Ez 1 @ 2 E z 0 10:14 c2 @t2 @x2 Often used as solutions also for the more complex case [10.11], [10.12] are time harmonic functions: !

!

!!

!

! E0

!!

E E0 cosk x

!t;

sink x !t; E ! ! !! E < E0 exp i k x !t

10:15

!

Here k is the wave vector pointing to the direction of propagation with its absolute value defined by !2 10:16 c2 It should be added that the magnetic and the electric field are not independent from each other, since the wave equations cannot completely replace Maxwell’s equations. These lead to further conditions listed in Table 10.1. Both the dispersion (the dependence of c on !) and the absorption are included in this theory, the latter by a complex permittivity and with this a complex wave vector. Another case of absorption occurs when a current ! ! j E is allowed due to finite conductivity in [10.10]. Then [10.11] becomes: !2

k

!

!

@2E @2E 0 "0 " 2 0 10.11a E 0 @t @t Using time-harmonic functions for the electric field as above, [10.11a] reduces to: ! ! 10.11b E !2 0 "0 " i E 0 "0 ! !

This equation shows that a finite conductivity is equivalent to an imaginary term in the permittivity ". Coming back to an exemplary solution, the time harmonic plane wave in the case of an absorbing material, where the permittivity " has an imaginary part " "0 i"00 total, Table 10.1 Further conditions on electric and magnetic fields of an electromagnetic wave Transversality !

!

!

!

k E0 0 k B0 0

Correlation of electric and magnetic fields !

!

!

!

!

k E0 !B0 k B0

!

!0 "0 "E0

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183

Table 10.2 Boundary conditions for the electric and the magnetic fields, which have to be satisfied by the wave solutions at the boundaries Prerequisite

Boundary condition

No surface charge – No surface current – Ideally conducting wall (metallic) Ideally conducting wall (metallic)

Continuity Continuity Continuity Continuity Ejj 0 B? 0

"00total "00

of of of of

D? B? Hjj Ejj

"0 !

10:17

has to be a solution of [10.11c]. @ 2 Ez !2 0 "0 "0 i"00total Ez 0 10.11c @x2 A similar equation can be derived for the magnetic component of the plane wave, leading to a general solution with g, h, m and n constants to satisfy the boundary conditions (see Table 10.2): Ez g expfik xg h expf ik xg Hy m expfik xg n expf ik xg

10:18

One boundary condition, namely the continuity of E|| has to be emphasised, since it can explain the often observed effect of edge or corner overheating. Later it will be shown that the power dissipation in a sample volume is proportional to the squared electric field [10.24]. At edges (corners) the microwaves cannot only intrude from two (three) directions, but also at these volumes electric fields of two (three) polarisations find a parallel surface to intrude continuously, which means without amplitude decrease. Therefore the heat generation there will be very large. The solution [10.18] is an exponentially damped wave, with wave number k and damping constant , where the dependency on " can be derived by solving !2 0 "0 "0 leading to

i"00 ik2

r o "0 "0 k! 2

sr ! "002 1 02 1 "

r o "0 "0 ! 2

sr ! "002 1 02 1 "

and

10:19

10:20

10:21

184


The corresponding electric field penetration depth, the distance in which the electric field is reduced to 1/e is defined by e1 1=. An important consequence of the frequency dependency of is that microwaves of 915 MHz do have an approximately 2.5 times larger penetration depth than waves of 2450 MHz, when similar permittivities at both frequencies are assumed. With the assumption of the excitation and the propagation of a plane wave, that satisfies the boundary conditions, first estimations of the field configurations are possible. This yields, for example, the laws of the geometric optics, which are also valid for microwaves, when a typical object size is much larger than the wavelength. With this approach the particular centre heating of objects of cm-dimensions with convex surfaces (like eggs) can be easily understood, since at the convex surface the microwave ‘rays’ are refracted and focused to the centre. In order to calculate the temperature change within an object by microwave heating, it is important to determine the power density, starting from the electromagnetic field configuration. Since normal food substances are not significantly magnetically different from vacuum ( 1), in most cases the knowledge of the electric field is enough to calculate the heat production, by power dissipation. This power dissipation (per unit volume) pV is determined by ohmic losses which are calculable by 1 ! ! pV 1:51 0.33 0.31

direct penetration ability of infrared radiation makes it possible to increase the energy flux without burning the surface and thus reduce the necessary heating time that conventional heating methods require. This is especially true for thin products. In a special study a method was developed to determine optical properties of bread at different degree of baking (Skjo¨ldebrand et al. 1988). The results were that the transmission by the crust is less than the crumb. Even the thinnest dough sample did not transmit any radiation. Reflection curves for crust and dough are very similar while the reflection for the crumb is about 10–15% less. Table 11.2 shows calculated penetration depths for crust and crumb for radiators used in baking ovens. Measurements have been carried out for other foods and Table 11.3 shows some examples (Dagerskog and ¨ sterstro¨m 1979). O

11.3

Technologies

In IR heating, heat is transferred by radiation, the wavelength of which is determined by the temperature of the body – the higher the temperature, the shorter the wavelength. Present interest in industrial heating applications centres on short wave IR (wavelengths around 1 m) and intermediate IR (around 10 m), since these wavelengths make it possible to start up and reach working temperatures in seconds, while also offering rapid transfer of high amounts of energy and excellent process control. In some food materials, moreover, short wave IR demonstrates a penetration depth of up to 5 mm (see Section 11.1). The best known industrial applications (for non-food uses) are in the rapid drying of automobile paint and drying in the paper and pulp industry (see Section 11.1) For paper drying IR has superseded microwaves because it offers superior process control and economy. IR technology has long been under-estimated in the food field, despite its great potential. The main commercial applications of IR heating are drying low moisture foods (examples are drying of breadcrumbs, cocoa, flours, grains, malt, pasta products and tea). The technique is often used as one part of the whole process very often at the start to speed up the first increase of the surface

216


temperature. Such processes are frying, baking and drying. Radiant heating is used in baking or roasting ovens and is also used to shrink packaging film. In a recent search in the literature it was found that there is some use of the IR technique on drying of fish products (Wei-Renn-Lein and Wen-Rong-Fu 1997). A lot of research in recent years has been done in Taiwan, China and Japan (Afzal and Abe 1998; Wen-Rong-Fu and Wei-Renn-Lien 1998). They have studied drying of fish, drying of rice, rice parboiling and potato. The basic characteristics of infrared radiation are the high heat transfer capacity, heat penetration directly into the product, fast regulation response and good possibilities for process control. These qualities indicate that infrared radiation should be an ideal source of energy for heating purposes. As distinguished from microwave heating the penetration properties are such that suitable balance for surface and body heating can be reached which is necessary for optimal heating (see Section 11.1). Some empirical work in this field can be found in the literature by, for example, Ginzburg (1969). Suggestions have been made that radiant heating elements should be operated at temperatures between 1200 and 1800ºC as only wavelengths longer than 2 m are effective in developing colour. Successful results have been reported for several frying applications (Dagerskog 1978). Asselberg et al. (1960) used quartz tube heaters (1000 1300ºC) at 2.2 W/m2 for braising of beef stew. For the parboiling procedure, for a similar degree of heat treatment as compared with conventional technology, the infrared treatment required a shorter time (83%) with lower weight losses (50%). The flavour, colour and texture of the infrared braised meat were claimed to be superior. An industrial process for pre-cooking of bacon in a continuous infrared oven at Swift & Company has been investigated by Hlavacek (1968). Electric resistance heaters below the seamless stainless steel belt supplemented the 288 kW of infrared radiant heating from overhead quartz lamps. The frying time was 2–3 minutes and pre-cooked bacon was found to taste as good or better than freshly fried bacon. Several studies have been reported by Soviet investigators concerning the frying of meat with infrared radiation, but the only work published in English is that of Bolshakov et al. (1976) on the production of baked pork meat products. By analysing transmittance spectrographs of lean pork they showed that the maximum transmission of infrared radiation is for the wavelength region of 1.2 m. For wavelengths longer than 2.5 m the transmission was negligible. Consequently, it is necessary to use sources with the maximum radiation falling in the region of maximum transmission to achieve deep heating of pork. For heat treatment of the pork surface radiators in the region of maximum transmittance and reflectance (max > 2:3 m must be used. The authors therefore designed a two-stage frying process. In the first stage surface heat transfer was bought about by a radiant flux with max at 3.5–3.8 m. In the second stage the product was subjected to an infrared radiation flux with max at 1.04 m providing deep heating of the product. The result showed that the final moisture content and sensory quality of the product heated by the two-stage process were higher than those heated by conventional methods.

Infrared heating

217

The effect of radiation intensity (0.125; 0.250; 0.375 and 0.500 W/cm2 and slab thickness (2.5, 6.5 and 10.5 mm) on moisture diffusion coefficient of potato during far IR drying have been investigated by Afzal and Abe in 1998 in Japan. They found that the diffusivity increased with increasing radiation intensity and with slab thickness. In contrast activation energy for moisture desorption decreased with increasing slab thickness and resulted in higher drying rates for slabs of greater thickness. In Taiwan also far IR has been used for dehydration of fish. Over 90% of the far IR dried products had higher quality than currently marked sun dried product (Wei-Renn-Lein and Wen-Rong-Fu 1997).

11.4

Equipment

IR ovens or equipment of various sizes and constructions have been developed and tested in many countries. The main component – the radiator – may be of various types and shapes. Early tests with tube heaters revealed that successful radiant cooking required both the quality and quantity of energy used should be suitably balanced. As described in Section 11.2.1 the radiators may be divided into the following main groups: 1. 2.

Gas-heated radiators (long waves) Electrically heated radiators – tubular/flat metallic heaters (long waves) – ceramic heaters (long waves) – quartz tube heaters (medium- and short wave) – halogen heaters (ultra short waves)

Various reflector systems are also used (see Fig. 11.5) (Hallstro¨m et al. 1988): • individual metallic/gold reflectors • individual gilt twin quartz tube • flat metallic/ceramic cassette reflector.

Some of the high intensity radiators need water or compressed air cooling to avoid overheating. Table 11.4 shows the infrared emitter characteristics. IR equipment may be either of batch or continuous type. The radiator cassettes are positioned above the transport belt, which usually are wire mesh as indicated in Fig. 11.6. Some equipment also uses IR heating from below if the product allows this from a contamination point of view. The equipment shown in Fig. 11.7 has been used in reheating and frying in catering, utilises individual pans for the product to avoid fat dripping. In other equipment the IR system is combined with air convection to control the surrounding air temperature and humidity. On most equipment the degree of heating is controlled by thyristor systems. The simplest way is to use pure on/off systems where the number of radiators/tubes operating at any one time is controlled by switches. Advantages

218


Fig. 11.5

(a) Individual reflector (b) Individual gilt twin quartz tube (c) flat metallic ceramic cassette reflector.

Fig. 11.6

Fig. 11.7

Continuous process infrared oven.

Infrared oven in which heating is from above and below the product: this oven utilises individual pans for the product.

Table 11.4

Infrared emitter characteristics (Fellows 1988)

Type of emitter

Radiant heat

Convection heat

Heatingcooling time

(kW/m2)

Maximum process temperature (ºC)

(%)

(%)

(s)

2200 2300 2200

10 2 80

300 1600 600

75 98 80

25 2 20

1 1 1

5000 h – 5000 h

Medium wavelength Quartz tube

950

60

500

55

45

30

Years

Long wavelength Element Ceramic

800 700

40 40

500 400

50 50

50 50

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