Detecting Foreign Bodies in Food

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Detecting foreign bodies in food

Related titles from Woodhead’s food science, technology and nutrition list Analytical methods for food additives (ISBN 1 85573 722 1) This volume discusses methods of analysis for 30 major additives where methods are incomplete or deficient. In each case the authors review current techniques, their respective strengths and weaknesses, method procedures and which method to adopt. Each chapter includes detailed tables summarising particular methods, statistical parameters for measurement and performance characteristics. Detecting allergens in food (ISBN 1 85573 728 0) Allergens pose a serious risk to consumers, making effective detection methods a priority for the food industry. Bringing together key experts in the field, this important collection reviews the range of analytical techniques available and their use to detect specific allergens such as nuts, dairy and wheat products. Mycotoxins in food: detection and control (ISBN 1 85573 733 7) Mycotoxins are toxic compounds produced by fungi. They are produced in foods of plant origin and pose a significant contamination risk in cereal and other foods. With its distinguished editors and international team of contributors, Mycotoxins in food summarises the wealth of recent research on how to assess the risks from mycotoxins, detect particular mycotoxins and control them at differing stages in the supply chain. 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 (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 30; address: Woodhead Publishing Ltd, Abington Hall, Abington, Cambridge CB1 6AH, England)

Selected food science and technology titles are also available in electronic form. Visit our web site (www.woodhead-publishing.com) to find out more. 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.

Detecting foreign bodies in food Edited by M. Edwards

Cambridge England

Published by Woodhead Publishing Limited, Abington Hall, Abington Cambridge CB1 6AH, England www.woodhead-publishing.com Published in North America by CRC Press LLC, 2000 Corporate Blvd, NW Boca Raton FL 33431, USA First published 2004, Woodhead Publishing Ltd and CRC Press LLC © 2004, Woodhead Publishing Ltd 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 and CRC Press 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 or CRC Press 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 ISBN 1 85573 729 9 (book) 1 85573 839 2 (e-book) CRC Press ISBN 0-8493-2546-3 CRC Press order number: WP2546 The publisher’s policy is to use permanent paper from mills that operate a sustainable forestry policy, and which have been manufactured from pulp which is processed using acidfree and elementary chlorine-free practices. Furthermore, the publisher ensures that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by SNP Best-set Typesetter Ltd., Hong Kong Printed by TJ International, Padstow, Cornwall, England

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xi

Contributor contact details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii

Part I 1

2

Management issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Identifying potential sources of foreign bodies in the supply chain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. A. Marsh and R. E. Angold, RHM Technology, UK 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Sources of contamination in the food chain . . . . . . . . . . . . 1.3 The role of the manufacturer . . . . . . . . . . . . . . . . . . . . . . . 1.4 Concluding comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Sources of further information and advice . . . . . . . . . . . . . GMP, HACCP and the prevention of foreign bodies . . . . . . . . . R. R. Gaze and A. J. Campbell, Campden and Chorleywood Food Research Association, UK 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 The role of good manufacturing practice (GMP) and prerequisite programmes . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 The role of the hazard analysis and critical control point (HACCP) system . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Sources of further information . . . . . . . . . . . . . . . . . . . . . .

1

3 3 5 9 11 11 14

14 16 19 26 28

vi 3

Contents Managing incidents involving foreign bodies . . . . . . . . . . . . . . . . T. Hines, Leatherhead Food International, UK 3.1 Introduction: managing consumers, manufacturers and retailers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 The crisis management team . . . . . . . . . . . . . . . . . . . . . . . 3.3 The crisis management plan . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Managing internal and external communications . . . . . . . 3.5 Successful crisis management . . . . . . . . . . . . . . . . . . . . . . . 3.6 Categories of consumer complaints . . . . . . . . . . . . . . . . . . 3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Sources of further information and advice . . . . . . . . . . . . . 3.9 References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part II 4

5

6

29

29 31 34 37 40 41 42 43 43

Detection and identification . . . . . . . . . . . . . . . . . . . . . . . . .

45

Metal detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J. P. Craig, Thermo Electron Corporation, UK 4.1 Introduction and the history of metal detection . . . . . . . . 4.2 Types of detection systems . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 The balanced coil system . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Factors affecting the application of metal detection systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Operational and quality control procedures . . . . . . . . . . . 4.6 Future trends and conclusions . . . . . . . . . . . . . . . . . . . . . .

47

Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Apoussidis and I. Wells, Eriez Magnetics Europe, UK 5.1 Introduction: magnetic separators and the principles of magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Methods of producing magnetic fields: permanent magnets and electromagnets . . . . . . . . . . . . . . . . . . . . . . . 5.3 Safety and environmental issues . . . . . . . . . . . . . . . . . . . . 5.4 Types of magnetic separator used in the food industry . . . 5.5 Factors affecting the use of magnets in food processing . . 5.6 Examples of magnet use for particular foods . . . . . . . . . . 5.7 Advantages and disadvantages of using magnets . . . . . . . . 5.8 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Sources of further information . . . . . . . . . . . . . . . . . . . . . . Optical sorting systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. C. Bee and M. J. Honeywood, Sortex Ltd, UK 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 The principal components of optical sorting systems . . . . . 6.3 Inspection systems: selection of wavelength bands, filters and illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47 49 52 55 58 62 63

63 65 69 70 76 82 84 85 85 86 86 87 98

6.4 6.5 6.6 6.7 6.8 7

8

9

10

Contents

vii

The product feeding, ejection, cleaning and dust extraction systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The electronic processing systems in sorting machines . . . Strengths and weaknesses of colour sorting . . . . . . . . . . . . Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sources of further information . . . . . . . . . . . . . . . . . . . . . .

104 110 115 116 118

Applying optical systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G. Doménech-Asensi, Polytechnic University of Cartagena, Spain 7.1 Introduction: foreign bodies in fruits and vegetables . . . . . 7.2 Developing sorting systems for the removal of foreign bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Foreign body detection in the processing of olives and potatoes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Sources of further information and advice . . . . . . . . . . . . . 7.6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Microwave reflectance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. Benjamin, University of Bristol, UK 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Microwave imaging techniques . . . . . . . . . . . . . . . . . . . . . 8.3 Microwave inspection of food products . . . . . . . . . . . . . . . 8.4 Strengths and weaknesses of microwave sensors . . . . . . . . 8.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nuclear magnetic resonance imaging . . . . . . . . . . . . . . . . . . . . . . B. Hills, Institute of Food Research, UK 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Principles of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) . . . . . . . . . . . . . . 9.3 The use of NMR and MRI techniques in food processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Factors affecting the development of low-cost on-line MRI foreign body sensors . . . . . . . . . . . . . . . . . . . 9.5 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Sources of further information . . . . . . . . . . . . . . . . . . . . . . 9.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface penetrating radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . U-K. Barr, SIK and H. Merkel, Chalmers University of Technology, Sweden 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Principles of surface penetrating radar . . . . . . . . . . . . . . . 10.3 Detecting foreign bodies using microwaves . . . . . . . . . . . .

119 119 119 127 129 130 131 132 132 133 141 151 153 154 154 155 159 164 168 170 171 172

172 173 180

viii

11

12

13

14

Contents 10.4 Setting up radar systems in food processing . . . . . . . . . . . 10.5 Strengths and weaknesses of the radar method . . . . . . . . . 10.6 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7 Sources of further information and advice . . . . . . . . . . . . . 10.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

181 185 188 189 192

Electrical impedance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. Dowdeswell, Kaiku Ltd, UK 11.1 Introduction: measuring the electrical properties of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Capacitance, resistance and impedance-based systems . . . 11.3 Conclusion and future trends . . . . . . . . . . . . . . . . . . . . . . . 11.4 Sources of further information . . . . . . . . . . . . . . . . . . . . . . 11.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193

Ultrasound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O. A. Basir and B. Zhao, University of Waterloo and G. S. Mittal, University of Guelph, Canada 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Principles of ultrasound . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Types of ultrasonic transducer . . . . . . . . . . . . . . . . . . . . . . 12.4 Ultrasound signal processing to detect foreign bodies . . . . 12.5 The use of ultrasound techniques in food processing . . . . 12.6 Conclusions and future trends . . . . . . . . . . . . . . . . . . . . . . 12.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Using X-rays to detect foreign bodies . . . . . . . . . . . . . . . . . . . . . B. G. Batchelor, University of Cardiff, E. R. Davies, Royal Holloway, University of London and M. Graves, Spectral Fusion Technologies Ltd, UK 13.1 Introduction: principles of X-ray systems . . . . . . . . . . . . . . 13.2 Single axis X-ray systems . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3 Dual axis X-ray systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Dual energy X-ray imaging . . . . . . . . . . . . . . . . . . . . . . . . 13.5 Using X-ray systems in practice . . . . . . . . . . . . . . . . . . . . . 13.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.7 Sources of further information . . . . . . . . . . . . . . . . . . . . . . 13.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.9 Appendix: factors affecting system performance . . . . . . . . Separation systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R. O’Connell, Russell Finex Ltd, UK 14.1 Introduction: the need for separation systems . . . . . . . . . . 14.2 The location and design of separation systems . . . . . . . . . 14.3 Traditional types of separation equipment . . . . . . . . . . . . .

193 196 202 202 203 204

204 205 209 215 219 221 223 226

226 232 244 249 255 258 258 259 260 265 265 267 268

15

Contents

ix

14.4 Innovative types of separation equipment: sieves . . . . . . . 14.5 Innovative types of separation equipment: filters . . . . . . . 14.6 Future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.7 Sources of further information . . . . . . . . . . . . . . . . . . . . . . 14.8 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

269 276 280 280 281

Identifying foreign bodies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Edwards, Campden and Chorleywood Food Research Association, UK 15.1 Introduction: definition and sources of foreign bodies . . . . 15.2 Approaches to foreign body identification . . . . . . . . . . . . . 15.3 Foreign bodies of biological origin: identification and testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4 Foreign bodies of non-biological origin: identification and testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.5 Effects of food processing on foreign bodies and future trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.6 Sources of further information and advice . . . . . . . . . . . . . 15.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.8 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

282

282 284 287 290 294 296 296 296

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

Preface

Foreign bodies are the biggest single source of customer complaints for many food manufacturers, retailers and enforcement authorities. The accidental inclusion of unwanted items can sometimes occur in even the best-managed processes. Foreign bodies in foods are therefore quite rightly a matter of concern to all food manufacturers and retailers. Publicity given to contamination incidents of glass in food products, concerns about Salmonella in a range of foods, and more recently issues such as BSE and GM foods have left consumers very aware of the safety of what they eat. This is encouraged by increased coverage in the media of consumer rights, and, unfortunately, our increasingly litigious society. Therefore, any action that can be taken to lessen the incidence of foreign bodies in foods is of interest to food manufacturers, retailers and enforcement authorities. A foreign body may be defined as something that the consumer perceives as being alien to the food. The perception of the consumer is important, since not all foreign bodies are in fact alien to the food, though all have the potential to give rise to a consumer complaint. Hence foreign bodies can range from items that are demonstrably alien to the food, such as pieces of glass, metal or plastic; through items that are related to the food, such as fragments of bone in meat products; to part of the food itself, such as crystals of sugar or salt that are mistaken for glass. It follows from the definition given above that the range of possible foreign bodies is virtually limitless. One commonly made distinction is between intrinsic and extrinsic foreign bodies. Intrinsic foreign bodies are those that are related either to the raw materials used in the food product itself or to the packaging materials. Extrinsic foreign bodies are those that

xii

Preface

are not so related, and become incorporated in the product flow from an external source. The control of foreign bodies in food products has to be seen within the commercial environment within which it takes place. The criterion must therefore be to choose an approach to foreign body control in relation to the risks and costs involved. The choice of method will be influenced both by the size of the enterprise and the impact the cost will have on the commercial viability of the enterprise. For example, a low cost manual system may be the correct solution for a small operation, where a larger enterprise would perhaps install a machine. A low cost manual system may be the appropriate solution when the problem is short term. The choice of equipment will depend on the technical problem to be solved, the cost of the equipment, the particular foreign body hazard and the risk involved.Assessment of the risk will involve not only the legal position but also the publicity risk to the business of a foreign body incident. Approaches to the technical methods of combating foreign bodies on the food production line fall into two main categories: • •

Detection and removal systems. Separation systems.

Separation systems are mechanical methods such as sieving and flotation that aim to separate foreign bodies from the food as a result of basic physical differences. In many cases these methods are intrinsic to the production system itself. Possibly the most ancient is the process of winnowing to separate wheat from chaff, but more recent technologies have become much more complex. Detection and removal systems, in contrast, are systems designed specifically to detect the presence of a foreign body in the food and remove it as a consequence of having discovered it. The oldest of these methods is manual sorting, whilst the newest such methods use extremely sophisticated electronic technology. However, all rely on some physical difference between the food and the foreign body, be it colour, shape, density, electrical properties or another characteristic. Many of the more established techniques such as sieving or magnets use physical differences that are relatively obvious to the layman. Some of the more recent methods, such as colour sorters, have been devised to allow a machine to make judgements regarding a sample that may be obvious to the human eye, but to do so extremely quickly. These methods have relied upon microprocessors to process the information at speeds suitable for a modern production line. The most recent method to gain widespread acceptance in the food industry, the use of X-ray machines, is based upon a combination of a more subtle physical difference, that of density to X-rays, with the development of microprocessors to enable the system to work fast enough to be a realistic commercial proposition. This leaves some foreign bodies that are regarded as being ‘difficult’ to detect, generally because of the lack of a readily detectable physical dif-

Preface

xiii

ference between them and the food substrate from which they must be separated. Thus the challenge for developers of entirely new methods is to identify some point of difference in physical characteristics that is more extreme than the natural variation within either the food material or the target foreign body material, and then to find a method that will reliably detect this difference, with a low false positive rate, at a speed that will be acceptable on a commercial production line. The idea for this book grew out of a FoodLINK workshop organised by DEFRA in London in March 2002, to review the needs and current state of the art, identify gaps in capability, and assess new technologies for ‘difficult to detect’ materials. Food manufacturers and retailers and a wide range of researchers investigating a number of novel approaches to foreign body detection, as well as the improvement of existing methods of foreign body separation, detection and removal, attended the meeting. The types of foreign bodies occurring in foods depend to some extent on the food concerned. Stones and dirt, for example, may sometimes be harvested inadvertently with field crops, whilst fragments of bone are sometimes found in meat products. However, some overall patterns can be discerned, and two of the commonest types of foreign body, plastic fragments and insects, also present some of the greatest challenges to developers of detection equipment. Many of the unwanted items in food products are not foreign, but intrinsic – parts of the raw material that should have been removed during processing, such as bone, cartilage, shells and stalks. The application of good manufacturing practice (GMP) and hazard analysis and critical control point (HACCP) through the supply chain, monitored by supplier assurance and auditing, are the main ways in which a product is protected. Unfortunately, these can fail, so technology to separate, detect and remove any unwanted materials is required. For materials such as metal or stone, where large electro-magnetic or density differences exist between the food and the contaminant, technologies such as metal detection, magnets, density separation and more recently X-ray inspection systems are often quite adequate. However, plastics and materials of biological origin do not possess such clear differences from the food, and so alternative approaches are required. The requirements for new technologies are demanding. The methods must be non-invasive and they must match the speeds of raw material flow. Packing lines may be dealing with 103–104 items per minute, passing at several metres per second. Inspection systems for raw grains may have to deal with millions of units per minute. The equipment must also be able to demonstrate impressive reliability, particularly with regard to false reject rates. Methods which falsely reject good product are likely to be left unused by a factory manager under pressure to keep production rates high. Equipment must also be affordable. The level of acceptable cost will depend upon both the risk of causing real harm to consumers or to the brand reputation and on the value of the operation. However, as a very rough guide,

xiv

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equipment for detection and exclusion of foreign bodies which costs in excess of £50 000 is unlikely to find a significant market amongst food manufacturers. The first part of this book begins by considering the different types and sources of foreign bodies reported from foods, and goes on to discuss the role of quality management in preventing foreign body contamination. The sensitive subject of managing incidents involving foreign bodies is then dealt with, including approaches to handling press and publicity issues. The second part of the book deals with the various technologies for separating, detecting, removing and identifying foreign bodies in food processing systems. The removal of foreign bodies early in the production chain is often the preferred option, where less value has been added and the degree of heterogeneity is lower than that of most prepared foods. On the other hand, this strategy often involves the use of many different removal systems in order to cover each and every raw material. Detection applied at the end of the production line may be reduced to one or a few instruments, possibly justifying higher expenditure because more valuable material is being inspected. Moreover, such an approach may allow the detection and removal of foreign bodies that have been introduced during the production process. In practice, the strategy adopted is often a compromise between these approaches. Some of the detection and removal systems discussed in this section are well established in the food industry. Metal detection is probably the method that comes first to mind when considering detection and removal systems, and metal detectors have indeed been in use for very many years. Whilst the basic technology is well established, the performance of these detectors was considerably enhanced by the development of microprocessor technology. Magnets are also a well-established method of removing ferrous metals from a production line, and they can be used in a variety of different ways, depending on the type of food product or raw material involved. We next come to two chapters discussing optical sorting systems. The first of these deals with the principles of design and operation, and the possibilities for future developments. Optical sorting systems have been available for some time which are capable of inspecting and removing individual items of grain, fruit and vegetables presented on monolayers, and these are now capable of handling volumes up to around 10 tonnes per hour, equivalent to 104–105 items per second. A range of criteria for sorting based on surface characteristics can be used. The second chapter of the pair deals with some of the practical considerations of applying such systems to production lines. The next five chapters consider research on potential new technologies for the detection of foreign matter in foods, each dealing with different approaches towards exploiting differences in physical characteristics between foods and foreign bodies.The first of these discusses possible appli-

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cations of microwave reflectance, dealing with the principles behind microwave imaging, its application to food products, and the advantages and disadvantages of such an approach. Nuclear magnetic resonance imaging, well-known in medical work, is next considered in a similar way. We next return to the theme of microwaves, looking at the application of microwaves to surface penetrating radar. Measurements of the electrical properties of materials are next to be dealt with, in the chapter on electrical impedance. The final chapter in this group deals with ultrasound, a technique that has seen some limited application in food systems in the past. The next three chapters return to established methods of foreign body detection. The potential of X-rays for the detection of foreign bodies has been evident for many years to anyone who has had to pass through security at an international airport. However, as the authors of the next chapter explain, something which is plainly evident to the layman may not be easily detected by a machine. Applications of X-ray technology in the food industry were limited until advances in microprocessor technology resulted in computers capable of processing information at realistic speeds for on-line use in a food factory. Since that time, X-ray machines have become much more widely used, and developments in the technology have resulted in considerable reductions in cost. Separation systems are often regarded as part of the production process rather than as foreign body removal technologies as such, but they are in fact often responsible for removing the vast majority of foreign bodies that are present in the raw material. Although such systems are often seen as ‘low-tech’, the next chapter shows that there have been a number of innovative developments in the design of sieving systems that can have a significant influence upon their effectiveness. A key factor linking all of these aspects of foreign bodies together is the identity of the offending object. Until this is established, it is impossible to design equipment to detect and remove the foreign body, nor to develop appropriate quality management systems to control the risks. The final chapter in the book therefore deals with laboratory methods for examining foreign bodies in order to identify the likely source of the problem and thereby help to eliminate possible recurrences. This book describes the technology behind different foreign body detection systems in some detail, and it is to be hoped that this will help to stimulate new thinking on methods of detecting and removing foreign bodies. Those seeking a guide to the practical application of these techniques should consult CCFRA Guideline 5 (George R M (Ed.) (2004) Guidelines for the prevention and control of foreign bodies in food. CCFRA Guideline No. 5, 2nd ed. Campden and Chorleywood Food Research Association. Chipping Campden, UK.). What of the future? There will always be a need to operate systems to detect and remove foreign bodies from both raw materials and finished food products. Customer expectations are likely to grow still further, with

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greater demands for products completely free of any defects such as foreign matter. The established technologies for foreign body detection and removal are extremely effective, and will continue to be applied. Challenges still remain, particularly in the area of ‘hard to detect’ foreign bodies such as plastics and insects, which often lack any clear-cut differences to foodstuffs in their physical characteristics. The developing technologies discussed in this book have varying potential to answer some of these problems. However, there is still a need for some inspired thinking to find approaches that will address some of the more difficult foreign bodies and food matrices. It is hoped that this book will encourage those already working in this area, help to clarify the needs of the food industry and stimulate new ideas, either for importing technologies from other disciplines, or for the development of entirely novel approaches to foreign body detection. Dr Mike Edwards

Contributor contact details

(* = main point of contact)

Chapter 1 Mr R A Marsh* Kingfisher House 68 Pattison Lane Woolstone Milton Keynes MK15 0AY Tel: +44 (0) 1908 604432 E-mail: [email protected] [email protected] Dr R E Angold Pyxis CSB Ltd Tall Trees Park Lane Lane End, High Wycombe Bucks HP14 3NN UK Tel: 07702 429 455 Fax: 07092 383 978 E-mail: [email protected] Chapter 2 Mr R R Gaze* and Mr A J Campbell Campden and Chorleywood Food Research Association

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Chipping Campden Gloucestershire GL55 6LD UK E-mail: [email protected] Chapter 3 Mr T Hines mbe Leatherhead Food International Randalls Road Leatherhead Surrey KT22 7RY UK Tel: +44 (0)1372 376761 Fax: +44 (0)1372 386228 E-mail: [email protected] Chapter 4 Mr J Craig Thermo Electron Corporation Swift Park Rugby Warwickshire CV21 1DZ UK E-mail: [email protected] Chapter 5 E Apoussidis* and Dr I Wells Eriez Magnetics Europe Ltd Bedwas House Industrial Estate Bedwas, Caerphilly CF83 8YG United Kingdom Tel: +44 (0)29 2086 8501 Fax: +44 (0)29 2085 1314 E-mail: [email protected] Chapter 6 Dr S C Bee* and Dr M J Honeywood Sortex Ltd

Contributor contact details Pudding Mill Lane London E15 2PJ Tel: +44 (0)20 8522 5136 Fax: +44 (0)20 8519 3232 E-mail: [email protected] Web: www.sortex.com Chapter 7 Dr G Doménech-Asensi Polytechnic University of Cartagena Spain E-mail: [email protected] Chapter 8 Professor R Benjamin 13 Bellhouse Walk Bristol BS11 0UE UK E-mail: [email protected] Chapter 9 Dr B Hills Institute of Food Research Norwich Research Park Colney Norwich NR4 7UA UK Tel: +44(0)1603 255000 Fax: +44(0)1603 507723 E-mail: [email protected] Chapter 10 Dr U-K Barr* The Swedish Institute for Food and Biotechnology (SIK) PO Box 5401 SE-402 29 Göteborg Sweden

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Tel: +46 31 33 55 600 Fax: +46 31 83 37 82 E-mail: [email protected] Dr H Merkel Department of Microelectronics Microwave Electronics Laboratory Chalmers University of Technology SE-412 96 Sweden Tel: +46 31 772 18 48 Fax: +46 31 16 45 13 E-mail: [email protected] Chapter 11 Dr R Dowdeswell Kaiku Ltd Greenheys Centre Manchester Science Park Pencroft Way Manchester M15 6JJ Tel: +44 (0) 161 227 8900 Fax: +44 (0) 161 227 8902 E-mail: [email protected] Chapter 12 Dr O A Basir* Department of Systems Design Engineering University of Waterloo Waterloo Ontario Canada N2L 3G1 E-mail: [email protected] Dr B Zhao Department of Systems Design Engineering University of Waterloo Waterloo Ontario Canada N2L 3G1 Dr G S Mittal School of Engineering

Contributor contact details University of Guelph Guelph Ontario Canada N1G 2W1 Chapter 13 Professor B G Batchelor* School of Computer Science Cardiff University The Parade Cardiff CF24 3XF UK Tel: 029 2087 4390 Fax: 029 2087 4598 E-mail: [email protected] Professor E R Davies Department of Physics Royal Holloway, University of London Egham Hill Egham TW20 0EX UK Tel: 01784 443497 Fax: 01784 472794 E-mail: [email protected] Dr M Graves Spectral Fusion Technologies Ltd 45 Roman Way Coleshill Birmingham B46 1JT UK Tel: 01675 466111 Fax: 01675 467111 E-mail: [email protected] Chapter 14 Mr R O’Connell Russell Finex Ltd Russell House

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Browells Lane Feltham Middlesex TW13 7EW UK Tel: +44 (0)20 8818 2000 Fax: +44 (0)20 8818 2060 E-mail: [email protected] Chapter 15 Dr M Edwards Chemistry and Biochemistry Department Campden and Chorleywood Food Research Association Chipping Campden Gloucester GL55 6LD UK Tel: +44 (0)1386 842017 Fax: +44 (0)1386 842100 E-mail: [email protected]

Part I Management issues

1 Identifying potential sources of foreign bodies in the supply chain R. A. Marsh and R. E. Angold, RHM Technology, UK

1.1

Introduction

This short chapter can only provide a superficial introduction to a series of techniques that are dealt with in detail in the following chapters. We have taken the opportunity, therefore, to cite a few situations to put the subject in context. Food manufacturers and food retailers work continuously to eliminate foreign bodies in food. Foreign bodies are defined as objects that are visible to the naked eye, and are not intended to be present in the food. Included within this very broad definition are packaging materials, such as plastic, wood, ceramics and glass; by-products of food materials that have been inadequately removed such as bone in meat, leaves, stalks and other extraneous vegetable matter (known as EVM) in fruit and the introduction of materials inadvertently through the food chain, such as string and metal or insects. Outside this definition are contaminating microbiological agents that arise through poor hygiene, poor manufacturing practice and storage, such as yeasts, moulds and food-poisoning bacteria. Also excluded are food contaminants, which range from unwholesome chemicals, like toxins and carcinogens, to adulterants and food grade components not declared on the label. Clearly, the presence of foreign bodies in products leaving the retailer indicates a lack of control in the food process, for which the implications are severe. Consumers who find foreign bodies in the food that they have eaten or are about to eat need to be looked after sensitively, if they are ever to be persuaded to purchase the product again. (Who will return to the restaurant which serves a slug in the lettuce?)

4


Consumers have the choice of returning an unsatisfactory product either to the manufacturer, declared on the label, or to the retailer from whom they purchased it. Most choose the retailer if the foreign object is not too severe. There have been many cases where the Environmental Health Officer or the media have been the first contact, where the foreign object has been injurious to health, or newsworthy. The multiple retailers take the brunt of the complaints and have a well-honed system for involving their manufacturers and the supply chain, for which suppliers are charged. Systematic breakdown in control which leads to a pattern of foreign objects in a batch of products (e.g. glass in food) which could be injurious to health, creates a ‘crisis’ usually resulting in a product recall, with major implications for the security of the brand and a multimillion pound penalty for the recall, and subsequent reinstatement of uncontaminated product. The consumer attitude to foreign bodies in food has changed over the last two or three decades. Where, once, the customer would return to the retailer with a complaint, there is now a tendency to choose to ring up the local paper or to go to the local Trading Standards Officer. Local newspapers with column inches to fill will often want to sensationalise the story and the result may be coverage by the national press, resulting in an adverse impact on the brand. Many of these complaints, however, are the result of carelessness in the domestic environment and are outside the control of the manufacturer of the product. Nevertheless, the impact on the brand perception is no less and it is therefore important that these cases are investigated thoroughly and the circumstances of the complaint fully explained, whether it is the manufacturers’ fault or not. Some of these complaints are an attempt to secure substantial compensation. A consumer who believes that the company will wish to protect its good name may indicate to the supplier that a cash payment will prevent the ‘unfortunate publicity’. The vulnerability of major brands to deliberate foreign body contamination has led to several high profile malicious events that require careful handling with police help and consumers wishing to extort money will go to some lengths to make the complaint appear credible. A recent example is a used condom allegedly found in a canned product. The complainant claimed it was found inside the tin on opening. Subsequent investigation using microscopy and X-ray and infrared spectroscopy revealed that the condom had been boiled with the product but had not been retorted at the canning temperature. This fraudulent approach was clearly based on the assumption that there would be an investigation but the extortionist failed to realise that it is possible to distinguish between latex with spermicidal lubricant that had been heated to around 100 °C with the product in a saucepan and a similar combination that had been through the retorting process with the product. HACCP (Hazard Analysis and Critical Control Point) analysis is the control system used to identify risk universally in the food industry. Detect-

Identifying potential sources of foreign bodies in the supply chain

5

ing the source of foreign bodies is crucial to risk analysis and the subsequent elimination of future contamination.

1.2

Sources of contamination in the food chain

The very nature of our complex food industry, which provides, in a large retail store, the consumer the choice of over 30 000 different food products made from raw materials from all over the world, implies a process and distribution system with many different stages, each having potential risks for foreign body contamination. Figure 1.1 shows the main stages in the production of a complex food. Many raw materials are agricultural products, which are intimately in contact with potential contaminants and have to be actively separated. Stones, soil, chaff and insects will be in contact with cereal products: skin, bone and hair with animal products: stalks, leaves, and other vegetable materials with fruit. Each stage of transportation usually implies some degree of packing and contact with such items as wooden or cardboard boxes, plastic sacks and ties, paper labels. Most food manufacturers have systems in place to observe good manufacturing practice: for example, sieving or sorting raw materials, and metal detection or in some cases Xray inspection of the finished product to eliminate metal, bone or glass. However, post manufacture, the product passes through the retail or wholesale chain and is stored and prepared within the home or by restaurants and caterers before consumption. It should not be underestimated how many foreign bodies are introduced in these final stages, particularly in the home. Frequently, hard foreign body complaints turn out to be parts of teeth and fillings that have not originated with the food at all.

Harvesting

Prime producer

Transportation

Raw material processing (milling, abattoir)

Collation/ transportation

Food manufacturer Storage

Retail display

Retailer RDC or wholesaler/ caterer/ storage

Consumer purchase & transportation

Home storage

Fig. 1.1

Distribution

Food preparation

Collation, tertiary packaging, storage

Primary & secondary packaging

Food processing & assembly

Presentation & consumption

The main stages in the production of a complex food.

6


1.2.1 Inadvertent contamination Inadvertent contamination can occur within the production site. Any process using trays and racking is a source of fine metal shavings as trays are handled roughly in the haste to meet production volumes and deadlines. Careful design is important. Modifications to equipment to solve one problem may introduce another. For example, wear problems on aluminium couplings on tanker blow lines were addressed on one occasion by a site engineer who solved the wear problem by fabricating stainless steel couplings. These hard couplings lasted extremely well and during their lifetime displaced a remarkable number of aluminium chippings from the couplings at the delivery points, introducing metal contamination into the product as it was delivered. Fragments of glass in jars are a recurring problem. Where they are found at the bottom of the jar they are often associated with a malformed jar. Very occasionally, a fine web of glass is left in the neck during the production of the jar and this is not removed when the jars are inverted prior to filling. When the product is injected into the jar it enters with sufficient force to break the glass filament in the neck which ends up at the bottom of the jar. Some manufacturers who have chosen to use glass packaging have attempted to solve the problem of glass fragments within glass containers. With low viscosity fluids, (e.g. beer), the bottle can be spun around its long axis, centralising any foreign objects to the centre of the base. Light techniques focused at this position will detect small fragments. Alternatively, a specialised X-ray technique that subtracts the background of the jar can be used but these are expensive and have the problem of a high false reject rate.

1.2.2 Contamination in the distribution chain There are opportunities for contamination in the handling and distribution of products through the supply chain from production site to table. When it leaves the factory, the product is normally well-wrapped and protected but it will usually go from there to a distribution centre where it will be mixed with other products for assembly into loads for distribution to the retailer. It is at this point that problems can begin. Pallets are knocked against pillars and splinters form. Sharp fragments of blue-painted softwood will be pushed through product packaging as the pallet is then inserted into the load. Products in multiwall sacks are placed on the ground, pick up gravel on the base and are then stacked again, pressing the gravel into the sack or depositing it in the seam to fall in when the sack is opened. At the retail outlet, there is a new set of risks to the product. Setting aside deliberate and malicious contamination, accidents will occur. The carbonated drinks section contains, usually, both glass bottles and cans lined along the shelves. Occasionally, a glass bottle is dropped and smashes on the floor. Breaking glass flies a long way on its own: assisted by the pres-


7

sure of carbonation it can scatter over a wide area. Sticky fragments of glass will land on the rims of canned beverages. A fragment may be discovered in the mouth of the consumer of the can should the fragment land at the rim edge by the ring pull. If glass is found in the first mouthful drunk from a can, this is the most probable source. In the shopping bag, products are pressed together. Pins used to secure men’s shirts have been transferred, through the packaging, into packaged confectionery and baked products as the bags are pressed and jostled on the journey home.

1.2.3 Contamination in the home The home itself is a major source of contamination. The care taken by the manufacturer and distributor is not always matched by the final consumer. In the celebration cake industry, regular complaints of pins in cakes result from the use of pins to secure the paper band around the cake. Needles find their way into a product through packaging, eye end first, when a product is set down on a tablecloth into which a darning or sewing needle has been temporarily placed. The direction of entry and, if provided, the examination of the packaging for the entry hole reveals how this contamination had occurred. Stuffing mixes are frequently contaminated with flakes of borosilicate glass as the stuffing is stirred with a metal implement which beats against the rim of the bowl. Surprisingly, this seems to occur much less often with cake mixes. Another regular complaint faced by bread manufacturers is the sliver of glass knocked from the base of a conserve jar as the last scrapings are removed from the interior. As a knife is vigorously scraped around the base of the interior of the jar, a fragment of the jar may be knocked out and fall onto the bread or toast onto which the conserve is about to be spread. It will probably then be found in the complainant’s mouth, and as shown in Fig. 1.2, the fragment is a crescent shaped with extremely sharp edges and is very likely to inflict a cut inside the mouth. Unjustly, it is the bread manufacturer who will get the blame for this, and a very aggrieved customer will be communicating vehemently with the retailer or bakery. This customer needs to be treated with tact: the problem may be one of their own making but they have probably been hurt, are possibly frightened and are not going to like being told that the injury is the result of their own actions. What they are really hoping for is a large sum in compensation and possibly a grovelling apology. One might expect the conserve manufacturer to be blamed equally often for these glass fragments but in at least ninety-nine cases in a hundred, the consumer will assume that it is the baker’s fault. The baker also gets the blame for fruit stalks and the occasional thorn that have definitely come from the conserve and even wasps are regularly alleged to have been baked into bread when they have no traces of adherent starch. However, in these cases, the jam manufacturer will be blamed as often as the baker. The

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Fig. 1.2

A macrophotograph of a chip from the base of a jar found in association with marmalade on toast.

phosphatase test will usually demonstrate that the offending insect has neither been baked in bread nor immersed in hot jam. Fragments of glass found at the top of a jar usually originate from the rim and have been sheared off by the consumer’s attempts to release the cap with an implement, often a knife. A very emotive event is the finding of a pharmaceutical tablet or capsule in food. This is a surprisingly common occurrence. Not all such findings are, however, true pharmaceuticals. Sweeteners frequently bounce away from the cup at which they were aimed and can find their way onto a plate. Small and not easily seen, they may be found later in the mouth. Capsules or tablets dispensed to an elderly or infirm member of the household may be placed on a plate and picked up by food also placed on the plate. One complaint returned for analysis to RHM Technology’s laboratories concerned a pack of six teacakes: one was claimed to have been found to contain half a tablet and the investigation revealed that the other five all contained tablets that had been pushed into the product after unwrapping. The tablets were for the alleviation of the symptoms of Alzheimer’s syndrome. Most discoveries of ‘pharmaceuticals’ in food are a result of carelessness or confusion in the home. 1.2.4 Deliberate and malicious contamination In the last decade, there has been a worrying increase in the number of incidences of malicious contamination. This usually comes to light as a series (three or more) of similar types of complaint and triggers a ‘crisis response’ within retailers and/or manufacturers. Examples have been glass in jars of baby food, pins in cakes and mercury injected into oranges.


9

Often the worst forms of malicious contamination come as a threat for blackmail or extortion (such as anthrax spores or AIDS contaminated blood) and are usually accompanied with a letter or telephone call. There are many courses organised by security organisations on training and procedures to deal with this kind of threat. The police have special officers trained to help, and most manufacturers and retailers have built relationships with these specialists in anticipation of any potential problem. Occasionally these issues are ‘local’ and related to disgruntled ex-employees who want to wreak revenge on their former employers. It is usually only the more spectacular events, politically motivated by extremist pressure groups, that receive media attention.

1.3

The role of the manufacturer

1.3.1 Data on the extent of foreign body contamination Foreign object data are closely held by individual manufacturers and will be highly dependent on the product. The Rank Hovis McDougall group of companies manufactures a wide range of food products mostly, but not exclusively, destined for the UK market. It has over 50 production sites and manufactures ambient dry flour and starch-based products, ambient wet products such as jams and sauces, and frozen products. Annual collations of customer complaints in the non-bread, non-cake, branded products sector has shown that of the 12–15 000 complaints received each year, approximately one quarter relate to foreign matter. This approximates to fewer than ten complaints per million units manufactured. Analysis of the types of complaint is given in Table 1.1. It is not surprising that an extremely common complaint in one of the years studied relates to psocids (booklice). Most houses contain them and they are a very common coloniser of cereal products. Cereal-based Table 1.1 The analysis of the types of complaint Complaint Total

1995 3971

2001 2711

Psocids Infestation Fruit/vegetable matter Bone Plastic Glass Wood Metal Animal matter Stones

19 % 11 % 10 % 9% 8% 7% 5% 5% 3% 2%

9.2 % 9.2 % 7.8 % 6% 15.5 % 7.2 % 2.1 % 6.0 % – 7.8 %

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products stored in paper or cardboard are easily invaded and they will not normally be observed until there is a substantial population of them. Psocid numbers vary markedly from year to year and their numbers depend on the ambient temperature and humidity, which is the probable explanation for the difference in the number of complaints in the two different years for which records are presented. Plastic complaints are clearly on the increase. This is one material that is finding its way into product at the production site with increasing frequency. Ingredient wrappers are a major source: often packs are torn open rather than cut open and fragments become associated with the ingredient as it is mixed. Plastic materials are usually composed of carbon and oxygen and are of similar density to the product; they have no magnetic or conductive properties. These facts make the material very difficult to detect and good manufacturing practice is essential. Far too many fragments of plastic pen find their way into product.

1.3.2 Manufacturers’ responsibilities Manufacturers have to produce safe and wholesome food, fit for the purpose described on the label. They must comply with all current legislation, and, in the case of private label products, must conform to the guidelines and specifications defined by their retail customer. Manufacturers have the responsibility to design and operate processes which minimise the risk of inadvertent foreign body contamination, and to review the HACCP process to maintain continuous improvement. Features include (a) the elimination of all sources of glass from a production area, (b) the installation of metal detection equipment at the end of a production line, (c) installation of X-ray equipment at the end of a production line, for example to detect bone fragments in meat products, (d) the physical separation of packaging material from ingredients and processing operations, (e) covering glass light fittings in operation areas, (f) careful design of clothing, footwear, hair nets and beard snoods to eliminate contamination of product with hair, pens, pencils, buttons or any other object brought onto the production area. Typically, food processing operatives are required to remove jewellery, watches and other objects which could fall from their person during their work. Any plasters to cover cuts and abrasions are highly and distinctively coloured (typically blue), to be very visible should they enter the product inadvertently. The policy is one of prevention rather than cure, coupled with appropriate detection systems that will identify the risks from the hazard analysis programme, to detect objects that may have entered the food process with the raw materials. Handwashing and the scrubbing of wellington boots are obligatory for operatives entering production areas. This is primarily for reasons of hygiene but also serves to minimise objects introduced this way. Manufacturers also have the responsibility to handle and resolve customer complaints particularly those which involve the enforcement


11

agencies. They must also be able to refer foreign body complaints to an independent agency which can advise on the nature and likely source of the foreign body. Many large food manufacturers have specialist forensic experts as part of their extended organisations (in R&D centres) or use the services of food research associations or specialist laboratories. Details of these are provided in the reference section.

1.4

Concluding comments

It is clear from the existence and the contents of this book that the consumer can rely on very great care being taken by manufacturers, distributors and retailers in their efforts to ensure that food is free of unwanted extraneous material. Indeed, when set against the total number of retail transactions, the number of genuine complaints is remarkably small but each one is a cause for concern. The growth of the ‘compensation culture’ has led to a fairly standard letter of complaint, however. To ensure that the manufacturer understands the distress that finding a foreign body has caused, the circumstances of its discovery as reported are calculated to bring tears to the eyes. The eating occasion at which it was discovered was a very important social occasion, the object was snatched from a child’s mouth seconds before it choked to death, granny had a stroke as a consequence and in the confusion the Royal Worcester dinner service was swept to the ground and smashed as the dog panicked and became entangled with the lace tablecloth. Wine ruined the new cream coloured carpet and the family heirloom, the Van Gogh, was knocked from the wall and trampled on. Clearly, only substantial compensation can even begin to alleviate all of this suffering.

1.5

Sources of further information and advice

1.5.1 Organisations which provide facilities to identify foreign objects Campden & Chorleywood Food Research Association, Chipping Campden, G55 6LD Tel: 01386 842017 Email: [email protected] Website: www.campden.co.uk Contact: M Edwards. Special Techniques: SEM, X-ray microanalysis, Fluorescence & Nomarski DIC AAS. HPLC; CE; UV–VIS Spectroscopy; NIRS; glass and other foreign bodies detection Leatherhead Food International, Randalls Road, Leatherhead, KT22 7RY

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Tel: 01372 376761 Email: [email protected] Website: www.leatherheadfood.com Contact: M Saunders. Special Techniques: Foreign body identification, troubleshooting Reading Scientific Services Ltd, The Lord Zuckerman Research Centre PO Box 234, Reading, RG6 6LA Tel: 0118 986 8541 Email: [email protected] Website: www.productcontamination.com Contacts: J Webb and S Deadman. Special Techniques: Contaminant identification in food/pharmaceuticals, (cryo-) SEM; X-ray microanalysis; X-ray microfluorescence; FT-IR RHM Technology The Lord Rank Centre, Lincoln Road, High Wycombe, Bucks HP12 3QR Tel: 01494 526191 Email: [email protected] Website: www.rhmtech.co.uk/pages/general/contact.htm Website: rhmtech.co.uk Contact: Dr Susan Gedney Special techniques: (Cryo-) VPSEM, X-ray microanalysis, FT-IR Fluorescence & Nomarski DIC; glass & other foreign bodies detection 1.5.2 The identification of insects The following institutions identify insects: Central Science Laboratory, Sand Hutton, York, YO41 1LZ Tel: 01904 462000 Email: [email protected] Website: www.csl.gov.uk/prodserv/diag/serv/ent.cfm Insect Information Services, Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD Tel: 020 7942 5726 Email: [email protected] Website: www.nhm.ac.uk/entomology. Contact: The Manager 1.5.3 The identification of pills and drugs Pills and drugs can be identified from a database held by: Reading Scientific Services Ltd (Contact details above)

Identifying potential sources of foreign bodies in the supply chain 1.5.4 The identification of glass Glass can be identified by: Reading Scientific Services Ltd (Contact details on previous page) RHM Technology (Contact details on previous page)

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2 GMP, HACCP and the prevention of foreign bodies R. R. Gaze and A. J. Campbell, Campden and Chorleywood Food Research Association, UK

2.1

Introduction

A foreign body (excluding grease and oil) may be defined as solid matter which is present in a food, but which, whether of intrinsic or extrinsic origin, is undesirable. An ‘intrinsic’ foreign body is derived from another part of the plant or animal than the product that it contaminates, whereas an ‘extrinsic’ foreign body is derived from any origin other than the plant or animal product that it contaminates. There are a number of major sources of foreign bodies in food that could be inadvertently present within raw materials, e.g. stones, wood or soil. These could be inadvertently introduced from processing, handling or packing, e.g. glass, metal or plastic, or from materials entering the food during distribution, or intentionally placed in the food as a result of malicious tampering. Foreign bodies in foods can be safety issues. Typical injuries caused by foreign bodies would include choking, cuts and broken teeth; in extreme cases this may result in hospital treatment. There is no method available that will completely prevent the occurrence of foreign bodies in foodstuffs. However, there is a range of techniques available to reduce their occurrence in raw materials, followed by either manual or mechanical methods to remove those foreign bodies remaining in the intermediate or final product. No method, however carefully implemented, will guarantee complete freedom from foreign bodies. The food industry has recognised that the most effective method of controlling foreign bodies is through the implementation of a hazard analysis and critical control point (HACCP) system. Since its original use by the Pillsbury Company to ensure the production of microbiologically safe food for astro-

GMP, HACCP and the prevention of foreign bodies

15

nauts in the 1960s, HACCP has become the pre-eminent food safety management system. The scope of its use has been subsequently extended by many food operations to include also chemical and physical food safety hazards. HACCP has gained international acceptance with the guidance developed by the Codex Alimentarius Commission in common usage. Codex defines, in its Food Hygiene Basic Texts, seven HACCP principles as stated below: Principle 1 Principle 2 Principle 3 Principle 4 Principle 5 Principle 6 Principle 7

Conduct a hazard analysis Determine the critical control points (CCPs) Establish critical limits Establish a system to monitor control of the CCP Establish the corrective actions to be taken when monitoring indicates that a particular CCP is not under control Establish procedures for verification to confirm that the HACCP system is working effectively Establish documentation concerning all procedures and records appropriate to these principles and their application.

These seven principles can be applied to a food operation through the use of twelve key stages, as stated below: Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 (Principle 1)

Stage 7 (Principle 2) Stage 8 (Principle 3) Stage 9 (Principle 4) Stage 10 (Principle 5) Stage 11 (Principle 6) Stage 12 (Principle 7)

Assemble HACCP team Describe the product Identify intended use Construct flow diagram On-site confirmation of flow diagram List all potential hazards Conduct a hazard analysis Consider control measures Determine critical control points (CCPs) Establish critical limits for each CCP Establish a monitoring system for each CCP Establish corrective actions Establish verification procedures Establish documentation and record keeping

The aim of HACCP is to consider all the steps in the production process, starting with raw material acquisition, through packing, up to distribution and use of the finished product, based on a structured analytical approach. Identification of the origin and manner of access of the foreign bodies can assist in identifying suitable means of prevention or control. The identification of the critical control points (CCPs) for foreign body prevention enables a business to focus its resources at these points in the process and to install procedures or equipment appropriate for the task. Many food operations have now recognised that HACCP cannot work in isolation; it should be underpinned by effective good hygiene practice

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(GHP) and good manufacturing practice (GMP). Within HACCP systems these GMP-type activities are often referred to as prerequisite programmes. In the United States the National Advisory Committee on Microbiological Criteria for Foods (NACMCF) introduced the term prerequisite programmes in its 1997 HACCP guidance to cover the site-wide policies, rules and procedures that food operations should have in place prior to introducing HACCP. The NACMCF defines prerequisite programmes as ‘Procedures, including Good Manufacturing Practices, that address operational conditions providing the foundation for the HACCP system’. These prerequisite programmes manage the basic environmental and operating conditions in a food operation. The NACMCF provides a list of typical prerequisite programmes: these include facilities; supplier control; specifications; production equipment; cleaning and sanitation; personal hygiene; training; chemical control; receiving, storage and shipping; traceability and recall; pest control. Codex Alimentarius, although it does not use the term prerequisite programmes, does state in its Food Hygiene Basic Text that prior to developing a HACCP system the food operation should be complying with relevant legislation, and following appropriate Codex Codes of Practice and the Codex General Principles of Food Hygiene. The use of prerequisite programmes enables the HACCP system to focus on the significant food safety issues that are product or process specific. In particular, it enables focus on the real critical control points of the process, as the prerequisite programmes can manage the low risk hazards as well as many quality and commercial issues. Prerequisite programmes are an important aid in the prevention and control of foreign bodies.

2.2 The role of good manufacturing practice (GMP) and prerequisite programmes The prerequisite programmes provide a solid foundation on which a HACCP system can be developed. They are expected to be in place and working effectively prior to HACCP, hence the term prerequisite, as they provide the basic environmental and operating conditions within a food operation. Typically, prerequisite programmes focus on three key areas, i.e. premises including equipment, personnel and the product including raw materials. Typical prerequisite programmes are given below for these key areas: •

Premises site location; building construction, design and maintenance; equipment construction, design and maintenance, including control of services and calibration;


•

•

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cleaning schedules for the site, plant and equipment; pest control procedures. Personnel control of staff, visitors and contractors; protective clothing rules; jewellery policy; training procedures. Product approved supplier scheme; raw material specifications including packaging; finished product specifications; storage and delivery of raw materials and finished product; traceability system and product recall procedure.

There is considerable general and sector specific guidance available on prerequisite programmes. The sector specific Industry Guides to Good Hygiene Practice provide comprehensive advice on how to comply with The UK Food Safety (General Food Hygiene) Regulations 1995, and where relevant the Food Safety (Temperature Control) Regulations 1995. In addition, the industry guides provide information on good hygiene practice. The Codex Alimentarius Recommended International Code of Practice General Principles of Food Hygiene documents the controls that are needed to ensure the safety and suitability of food for consumption. The British Retail Consortium (BRC) Global Standard – Food and the European Food Safety Inspection Service (EFSIS) Standard and Protocol for Companies Supplying Food Products (Issue 5) are also good sources of prerequisite programme guidance. The Institute of Food Science & Technology (UK) also provides general and sector specific information in their Good Manufacturing Practice guide. 2.2.1 Premises The use of the prerequisite programmes for the prevention and removal of foreign bodies should not be overlooked. The prerequisites can be used during the construction and maintenance of premises as described below: The design and construction of the factory and the design and deployment of the equipment within it need to be considered for the elimination of foreign bodies. Premises need to be designed to prevent the ingress of pests, as well as of dust, dirt and other potential foreign bodies. With new premises this may be relatively easy; however, with older premises it may be more difficult. Ideally, the surrounds should be tidy and free from weeds, stacked pallets, redundant equipment etc., and, where practical, the buildings should be surrounded by a path or hardstanding. Similarly, equipment needs to be designed and built in such a way that the surfaces of all materials and coatings are durable and resistant to cracking, chipping, flaking and abrasion. All the surfaces should be cleanable, have no dirt traps, and

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be capable of being disinfected if necessary. The installation of equipment should be given careful consideration. Clearances around the equipment will be required for cleaning and maintenance purposes and this should not be overlooked at the planning and layout stage. Where possible, machinery and its installation should be designed to preclude foreign bodies falling into the product.

2.2.2 Personnel Personnel are a major potential source for the entry of foreign bodies into food products; examples are jewellery, hair, pens, and tools. This means that those personnel involved in food production need to be trained to prevent foreign bodies from entering the product. It is important that these personnel have a clear understanding of what is required of them, together with the right attitude to personal and operational hygiene. In addition to factory personnel, all visitors and contractors’ workers must be made aware of the potential hazards they may introduce into the production area. Some of the requirements and guidance in respect of personnel issues will now be examined. The UK Food Safety (General Food Hygiene) Regulations 1995 require all personnel working in food handling areas to maintain a high degree of personal cleanliness and wear suitable, clean and, where appropriate, protective clothing. The sector specific Baking Guide advises: ‘Protective clothing for staff handling open food should have no external pockets and should be fastened with press studs or Velcro strips. Buttons should not be used’. The Regulations also require an adequate number of suitably located and designated washbasins. The washbasins must be provided with hot and cold running water, plus materials for hand cleaning and for hygienic drying. Where necessary, hand washing facilities must be separate from sinks used for washing foods and adequate changing facilities for personnel must be provided. The Baking Guide advises: ‘A changing room with facilities for storing outdoor clothing and other belongings should be provided with hanging facilities for outdoor clothing to dry. It is good practice to provide individual lockers. There should be separate changing rooms for each sex. Surfaces should be easy to clean. A receptacle for dirty work wear should be provided’. The Codex Alimentarius Recommended International Code of Practice General Principles of Food Hygiene state that food handlers should maintain good standards of personal cleanliness and, where appropriate, wear suitable protective clothing, head covering and footwear. These requirements also apply to visitors to food-handling areas. Hands should be washed at the start of food handling activities, after using the toilet and after handling raw food or any contaminated food. Personnel hygiene facilities should include adequate means of hygienically washing and drying hands. Wash basins should have a supply of hot and cold (or suitably temperature-


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controlled) water. Adequate changing facilities should be available; these should be suitably located and designated. Personnel should not smoke, chew or eat, spit, sneeze or cough over unprotected food. Jewellery, watches and other items should not be worn or brought into food handling areas if they pose a threat to the safety or suitability of the food. The BRC and EFSIS standards require food-handlers, visitors and contractors entering or working in food-handling areas to wear companyissued protective clothing. The clothing needs to be regularly laundered and must be appropriate so that product does not become contaminated. Where appropriate, hair needs to be fully contained; this includes the use of beard snoods. There are specific requirements for entry to high-risk areas; for example footwear has to be either changed or disinfected and visually distinctive clothing worn. Documented hygiene rules need to be followed by all personnel including visitors. The rules need to cover very specific requirements, for example fingernails, perfume, and plasters for cuts and grazes. Only plain wedding rings and sleeper earrings are permitted. Handwashing facilities need to be suitable and sufficient; the frequency of hand washing needs to be appropriate. Smoking, eating and drinking are only permitted in designated areas. Changing rooms need to be appropriately sited, and work wear needs to be stored separately from outdoor clothing and personal items. The Institute of Food Science and Technology Guide to Good Manufacturing Practice states that manufacturers must provide safety footwear and suitable laundered over-clothing to be worn by food handlers and visitors to food rooms. This should include headgear and, where appropriate, neck covering. Overalls should have internal pockets and non-detachable fasteners. The wearing of jewellery and wrist-watches should be prohibited in open food areas, except for plain wedding rings and secure plain sleeper earrings. Smoking, spitting or taking snuff should be prohibited in any food room or where there is open food. Exposed cuts and abrasions should be covered with a metal-detectable brightly coloured waterproof dressing. It is vital to the success of HACCP and the maintenance of food safety that the prerequisite programmes are part of the scheduled verification activities of the food operation. It is therefore expected that for the prevention and control of foreign bodies in food the prerequisite programmes will be subject to scheduled audits to confirm their effectiveness. Failure to maintain effective prerequisite programmes has often resulted in costly product recalls and in some cases injury to consumers.

2.3 The role of the hazard analysis and critical control point (HACCP) system HACCP provides a structured preventive approach to food safety management and is essentially common sense. HACCP can be successfully used

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to manage physical hazards in a food operation. Although the guidance from Codex provides the basis of many HACCP systems, there are other sources that have provided additional information on how to apply the HACCP principles, e.g. Campden and Chorleywood Food Research Association (CCFRA), International Life Sciences Institute (ILSI) and Mortimore and Wallace. CCFRA in its guide to HACCP suggests that the terms of reference (scope of the study) should clearly define what the study is to cover, whether it is a specific product or process line, or specific range of activities (module). The start and end points of the study also need to be clearly stated. The terms of reference should state the hazards that are to be considered in the HACCP system. Where physical hazards are to be considered they need to be precisely defined; for example the term ‘foreign bodies’ is often used in the terms of reference although its meaning is not precise enough. It is beneficial for the actual hazards to be listed, e.g. metal, glass or hard plastic as appropriate. The more clearly the foreign body is defined then the easier it becomes to put effective controls in place. For example, metal and wood are both unacceptable foreign bodies but the way they are controlled may be very different. Wood may require flotation or other separation techniques whereas most metals can be detected and/or removed by the use of a strategically placed metal detector. Customer complaint records are a useful source of information on the types of hazards to be considered. They will indicate the types of foreign body associated with the manufacturer’s product as well as the rates at which they are occurring. A necessary part of a customer complaint procedure is therefore the accurate identification of these foreign bodies. It is very beneficial to state in the terms of reference the prerequisite programmes that are in place and the hazards they control. By doing this a food operation will have evidence that all relevant hazards have been considered and that they can be effectively controlled. The following examples are used to show the application of the HACCP principles to the management of physical hazards in a food manufacturing operation.

2.3.1 Codex Stage 1 HACCP is best developed by the use of a multi-disciplinary team approach. The team should contain personnel with relevant technical and/or scientific expertise and knowledge of the operation. Typical team members would include representatives from production, quality assurance, the technical function and engineering. Engineers may be a good source of information relating to physical hazards due to their knowledge of the site, e.g. equipment and structure. It is beneficial to include personnel with practical knowledge, e.g. production operatives or supervisors, as well as managers. The team needs to be aware of the types of foreign bodies that could occur


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within the raw materials and the limitations of the equipment available on site to remove them from the food chain. Many food operations have found a team of four to six members to be most effective; additional team members may be co-opted as required. A chairperson will be required to manage the team and ensure that the principles of HACCP have been correctly applied. As notes from team meetings need to be taken, many teams include a technical secretary or scribe. It is recommended that all team members should be trained in the principles of HACCP. In the UK the Steering Group on HACCP standards has developed both introductory and advanced level HACCP training standards on HACCP principles and their application in food safety. Training courses designed to the introductory standard are specifically intended for HACCP team members concerned with developing and implementing HACCP systems. The advanced level training standard is specifically aimed at HACCP team leaders.

2.3.2 Codex Stages 2 and 3 It is vital that the product is fully described by the team to help them understand the essential product characteristics with regard to safety. This description should include details of raw materials, e.g. the use of fresh vegetables or frozen prepared ones and the type of packaging medium, e.g. the product is packed in glass jars with metal lug closures. The intended use of the product by the end-user and/or consumer should be determined, together with the target consumer group. This is of particular importance if the product is specifically aimed at a vulnerable group such as babies or the immuno-compromised. For example, foreign bodies approximately 1 mm in size, although unacceptable, may be safe for adults but may constitute a danger for infants. The development of an accurate product description and intended use will help the HACCP team to determine the significant hazards in the process.

2.3.3 Codex Stages 4 and 5 The HACCP team will need to prepare a flow diagram covering the operational steps of the process under study. The flow diagram should include all process steps in sequence to define the process accurately. In preparing the flow diagram the team will need to include all raw materials, including packaging; waste outputs should also be stated as these may be a potential source of foreign body contamination. The process steps may comprise activities designed to remove or reduce foreign body contamination, e.g. sieving, flotation washing, metal detection or X-ray inspection. There may be the opportunity for the introduction of physical contaminants to the product at many of the process steps. The team may also find

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supplementary information useful; such information could include site plans, equipment layouts and waste material flows. As the process steps detailed in the flow diagram will be those that are used in the hazard analysis stage of HACCP, it is vital that the flow diagram is confirmed by the team as correctly defining the operational steps of the process under study. This is typically achieved by walking the process and checking that the flow diagram is accurate. The team will need to ensure that the flow diagram takes into account potential variations, for example, from different shift practices. The flow diagram will require amending as the operational steps of the process change over time.

2.3.4 Codex Stage 6 The HACCP team should list all potential hazards for each process step, including those that are managed by effective prerequisite programmes. Many HACCP teams use brainstorming as a technique to help identify the potential hazards. The HACCP team must analyse the hazards to determine at which process steps realistic and significant food safety hazards can occur. Each hazard should be assessed with consideration of the risk of it occurring and the severity of the harm it could cause to the consumer. As part of the hazard analysis the HACCP team should consider at least the following five questions: • • • • •

Could any foreign bodies be present in the raw materials? Could any foreign bodies be carried into this step from the previous step? Could any ingredient added at this step include foreign bodies? Could any packaging material applied at this step introduce foreign bodies to the product? Could foreign bodies gain access at this step from machinery, personnel or the environment?

Hazards could occur at raw material intake where foreign bodies could already be present from the supplier, and the introduction of contaminants could occur at a number of different process steps; these need to be determined. In addition the possibility of the failure of a process step that is designed to control a particular hazard should be considered, e.g. the break-up of a metal sieve allowing contaminants to pass through with the additional hazard of the introduction of metal from the sieve. It is important that the HACCP team includes the cause or source of the hazard as this will assist in the determination of appropriate method(s) of control. Control measures will prevent, eliminate or reduce a hazard to an acceptable level. More than one measure may be required to control a specific hazard, although in some cases the same measure might control a number of different hazards. For example, the use of a strategically placed sieve


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could remove wood, glass and plastic from a finely powdered product. Many control measures will form part of the prerequisite programmes; planned maintenance and personal hygiene rules are two examples of prerequisites that can be preventative controls. Using metal as an example of a hazard (contamination of product with metal due to equipment failure), a suitable control measure would be the use of a correctly functioning metal detector with the prerequisite requirement relating to equipment maintenance.

2.3.5 Codex Stage 7 In most food operations there will be a fairly small number of process steps, where control can be applied, that are essential to prevent, eliminate or reduce a food safety hazard to an acceptable level. These process steps are called critical control points (CCPs). The HACCP team should use their professional experience and judgement to determine which process steps are CCPs. Many HACCP teams have found the use of a CCP decision tree, such as that developed by Codex, to be a very helpful tool in this determination. A decision tree is a logical sequence of questions that can be applied to every hazard at each process step unless the hazard is controlled by the prerequisite programmes. A number of different decision trees have been developed and training is recommended for their correct use. Process steps that are specifically designed to detect and/or reject foreign bodies are typically CCPs in most processes. Examples would include sieving, flotation washing, metal detection and X-ray detection. If the Codex decision tree is followed for the process step at which the metal detector is located then the following answers would be obtained. Question 1 Question 2

Are control measures in place? Is the step specifically designed to remove the hazard?

Yes Yes

These two answers would refer the user to the bottom of the decision tree and the conclusion that metal detection is a CCP for the hazard of metal in the product.

2.3.6 Codex Stage 8 Once the CCPs have been determined, the HACCP team will need to set critical limits for the control measures at each CCP. The critical limit is the measurable or observable value or level that separates safe product from unsafe, and should be set on the control measure and not the hazard. Many food operations also set operational target levels; these are set for day-today management of the step and are more stringent than the critical limit and take into account normal process fluctuations. Some critical limits may be stated in legislation and codes of practice documents, others need to be determined, for example through experimental trials.

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Taking metal detection as an example, attempts could be made to set the critical limit as the absence of metal in the product. However, absence is not practical, as the sensitivity of the equipment will limit the size of the test pieces that are appropriate to that particular metal detector. The critical limit will therefore be the test piece size recommended by the supplier of the metal detector related to the product passing through the detector.

2.3.7 Codex Stage 9 Monitoring is a planned sequence of measurements or observations of the control measures at a CCP to check that the critical limit has not been exceeded. The frequency of monitoring should be sufficient to enable corrective action to regain control of the process before potentially harmful product leaves the food operation. Ideally it should be continuous and if it is not it should take place within ‘real time’. The responsibility for monitoring should be assigned. The results of monitoring should be recorded; these records provide evidence that safe food is being produced. Monitoring records should be signed and dated by the person performing the monitoring activity and by a responsible reviewing official of the food operation. Personnel performing monitoring activities will require specific training in how to carry out the task correctly. In the example of metal detection, monitoring would be the check on the metal detector’s performance carried out by a nominated competent member of staff using test pieces at a pre-defined frequency.

2.3.8 Codex Stage 10 The food operation must take corrective action when monitoring indicates that a critical limit has been exceeded. A corrective action plan must be defined that includes what steps must be taken to regain control and to ensure that unsafe food cannot reach the consumer. Product prepared since the last good monitoring result will need to be quarantined before being re-worked, if possible, or disposed of. The cause of the failure must be investigated and full records of all actions kept. In the example of metal detection, corrective actions would need to include repair of the faulty metal detector, quarantine of all suspect product since the last good check and repassing all suspect product through a correctly operating metal detector.

2.3.9 Codex Stage 11 Once the HACCP plan has been developed it needs to be validated. This is a check that the plan can be effective in managing the significant food safety hazards. A team approach is normally used to do this and the responsibility lies with the food operation. The validation should check that all significant hazards have been addressed and will be effectively controlled, in


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particular at the CCPs. For example, if the HACCP team has determined that sieving is a CCP then the sieve mesh size needs to be checked to ensure that it is appropriate for the particular raw materials being sieved and the foreign bodies likely to be present in the raw materials. Once all the CCPs have been validated, the HACCP plan should be signed off as valid and put into practice. As soon as the plan has been implemented it needs to be verified to ensure that there is compliance with it and it is working as planned. Auditing has a major role in verification, focusing on the CCPs and prerequisite programmes. Other techniques may include trend analysis, review of monitoring, corrective action records and customer feedback analysis. Following implementation, the HACCP plan will need to be reviewed regularly to ensure that it is maintained and kept up to date. Changes to the equipment, factory environment and process are examples of some of the factors that should initiate a review of an existing HACCP plan as they may allow the introduction of foreign bodies into the food product. It is beneficial that a review is performed before changes are made. Factors external to the food operation should also initiate a review; examples are changes to legislation or to a code of practice. In addition, there needs to be a scheduled review by the HACCP team; typically this should be performed at least annually. Where metal detection is concerned validation would ensure that the test pieces are the correct size to obtain maximum sensitivity of detecting metal passing through the detector in the named products. For verification to take place the checks should be carried out on a routine basis as defined in the monitoring procedures and there should be a review when there are any changes to the detector and/or products passing through the detector. Additionally, any corrective actions that have been performed should be reviewed to assist in the prevention of any re-occurrence of the metal detection failure in the future.

2.3.10 Codex Stage 12 The final key stage is the preparation of documentation, including procedures and records. Central is the development of the HACCP plan, the written document that shows the application of the HACCP principles by the food operation. The plan would typically contain details of the preparatory stages, including terms of reference, team details, product description, intended use and the flow diagram, together with the details of the hazard analysis and the management of the CCPs. Supporting information such as procedures, work instructions, records of HACCP team meetings and details of prerequisite programmes should be retained for future reference. In the case of metal detection, records would need to include the regular testing of the detector and any actions relating to its failure to detect. Any actions recorded when the metal detector fails should include the actions

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related to the product, how much was quarantined, when it was re-worked, and who authorised its subsequent release or disposal and when the equipment was repaired and serviceable.

2.4

Future trends

2.4.1 Future trends in HACCP The European Commission issued proposals in June 2000 for new regulations on the hygiene of foods and the specific hygiene rules for products of animal origin. The proposals include the requirement that food business operators, other than at the level of primary production, shall have an HACCP system. This shall include documents and records commensurate with the nature and size of the food business to demonstrate effective application of the HACCP measures. Primary production operations must be managed so that hazards are monitored and eliminated or reduced to an acceptable level. It is expected that these regulations will come into effect within the next two years. It is expected that more companies will be seeking HACCP certification; there are already a number of such schemes operating. Many of these schemes make use of ‘national’ HACCP audit standards, such as those developed in Denmark and in the Netherlands. In 2003 CCFRA published its own standard based upon the Codex Alimentarius principles of HACCP. The other standards available are also based on Codex Alimentarius HACCP and this has led to much commonality between the various standards. A development group at the International Standards Organisation (ISO) is currently working on a HACCP standard (ISO 22 000); it is expected that this standard will be published in 2005. The use of such standards and the accreditation of the certification bodies should enhance the quality of HACCP and further promote its use by the food industry. It must be remembered that the auditors working to these HACCP standards are not validating a food operation; they will be performing verification or checking compliance with the HACCP plan. The responsibility for stating that the HACCP plan is correct must be with the food operators. The external auditor’s role is to assess whether the requirements of the standard have been complied with and that the HACCP system is being operated as intended by the HACCP team. This increased focus on HACCP is expected to be additional to existing accreditation schemes such as the BRC or EFSIS standards. The vital role of prerequisite programmes in underpinning HACCP will continue. Many food operations have seen benefits from developing, implementing and maintaining their HACCP systems on the firm foundation of effective prerequisite programmes. The reported benefits include focus on the ‘real’ CCPs in the process and much easier maintenance of the HACCP plan. It must be remembered that effective verification is required for the prerequisite programme as well as for the HACCP plan.


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2.4.2 Future trends in foreign body control systems These systems fall into three categories, namely optical, electrical/magnetic and imaging systems. Optical systems include vision systems, laser systems and NIR-based technologies. Vision systems are continually improving and new developments have focused on mechanical handling systems, optics and microprocessors for rapid data collection and analysis. An inherent limitation with optical systems are that they require a ‘line of sight’ to the foreign body, e.g. for bulk leafy materials, overlapping and folded leaves can result in only 20 % of defects being detected. Therefore development is in the area of optimising the infeed systems so that the material is better presented to the detection system. Further improvements in optical and microprocessor technologies will lead to better size and shape sorting. Other optical foreign body detection systems include the use of lasers as a source of light for inspection systems. Low powered laser equipment has been used to determine the ripeness of fruits. Further development of this technique may lead to the technology being used to examine the internal state of a food product, and thus detect the presence of foreign bodies. Electromagnetic techniques include electromagnetic inspection, capacitive systems, impedance techniques, impedance spectroscopy, electrical resistance tomography, metal detection technologies, microwave techniques and magnetic field detection. Although in its infancy, electromagnetic inspection offers potential for the detection of foreign bodies. The technique relies upon the differences in electrical and/or magnetic properties between a ‘pure’ food product and a food product contaminated with a foreign body. Sophisticated data analysis techniques can quickly compare such electronic images. Impedance spectroscopy uses the large differences in electrical properties of liquids, which depend upon the liquid’s composition. Such methods can be used to determine the presence of unwanted foreign objects, both physical and chemical. The technique is non-contact and non-invasive, and measurements can be taken in real time, making it suitable for in-line process control and monitoring. It is claimed that contaminants can be detected at levels of 10 parts per billion. Microwave energy has also been applied to sensing applications (e.g. radar detection) and can penetrate food materials, often to a depth of several centimetres. This suggests potential for measurements within bulk products. Unlike microwave heating applications, microwave sensors use low-power sources so that there is no heating effect associated with the measurement. It is claimed that glass (10 mg pieces), metal filings (5 mg pieces), plastics, stones, wood and other organic materials can be detected using microwave techniques. Imaging techniques include nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), ultrasound techniques and X-ray methods. NMR is the absorption of high frequency waves by atomic nuclei in a powerful magnetic field. Hence the nuclei are able to absorb and

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re-emit radio waves. NMR can thus be used to identify different compounds quantitatively. MRI is an advanced volume imaging technique and can offer 3D image and position of the structure of the food sample. MRI is being used in medical applications, but is not presently sufficiently robust and fast enough for process line speeds. The potential of NMR and MRI as non-destructive foreign body detection techniques is being investigated. Improvements in X-ray system design include the use of two or more Xray beams to create a 3D image of the food product under investigation. This can be achieved by firing X-ray beams at slightly different angles, to create multiple 2D images which can then be combined into a 3D image. The 3D image provides a ‘depth’ to the analysis.

2.5

Sources of further information

anon (1995) HACCP Principles and their Application in Food Safety (Introductory Level) Training Standard. London, Royal Institute of Public Health and Hygiene. anon (1995) Prevention and Control of Foreign Bodies in Food. Guideline 5. Chipping Campden, Glos, Campden and Chorleywood Food Research Association. anon (1997) Hazard Analysis and Critical Control Point System and Guidelines for its Application, Alinorm 97/13A. Rome, Codex Alimentarius Commission (CAC). anon (1997) A Simple Guide to Understanding and Applying the Hazard Analysis and Critical Control Point Concept, 2nd ed. Brussels, International Life Sciences Institute (ILSI), Europe Scientific Committee on Food Safety. anon (1997) Hazard Analysis and Critical Control Point Principles and Application Guidelines. Washington, DC, National Advisory Committee on Microbiological Criteria for Foods. anon (1998) Food and Drink Good Manufacturing Practice: A Guide to its Responsible Management. London, The Institute of Food Science & Technology (UK). anon (1999) HACCP Principles and their Application in Food Safety (Advanced Level) Training Standard. UK Steering Group on HACCP Training Standards. London, Royal Institute of Public Health and Hygiene. anon (2003) HACCP: A Practical Guide. Guideline 42. Chipping Campden, Glos, Campden and Chorleywood Food Research Association. BRC Global Standard – Food (2002). London, British Retail Consortium. EEC (1993) ‘Council Directive 93/43/EEC of 14 June 1993 on the Hygiene of Foodstuffs’. Official Journal of the European Communities, No L175/1–11. The EFSIS Standard and Protocol for Companies Supplying Food Products (2002, Issue 5) Milton Keynes, Bucks, EFSIS Limited. Industry Guide to Good Hygiene Practice: Baking Guide (1997). London, Chadwick House Group Limited. mayes t and mortimer s (2001) Making the Most of HACCP, Learning from Others’ Experience. Cambridge, Woodhead Publishing Limited. mortimore s and wallace c (1998) HACCP:A Practical Approach, 2nd ed. Gaithersburg, Maryland, Aspen Publishers Inc.

3 Managing incidents involving foreign bodies T. Hines, Leatherhead Food International, UK

3.1 Introduction: managing consumers, manufacturers and retailers 3.1.1 Damage to consumers – health of the nation Legislation exists to protect consumers from illness or harm when they eat or drink. It therefore follows that producers, manufacturers, importers and retailers share the legal responsibility to protect the health of the nation. The legislation in place puts the responsibility squarely on the shoulders of this group to prevent the sale and consumption of food or drink that is unfit for consumption.1 Food containing foreign bodies falls into this category and, under the Food Safety Act 1990, contamination of any unit in a batch, lot or consignment potentially renders the entire batch unsafe unless the manufacturer can demonstrate otherwise.2 Accordingly, one single complaint of a foreign body contamination – glass, for example, made by a consumer to an Enforcement Officer, under the strict interpretation of the Food Safety Act can trigger an immediate recall of the affected batch from consumers, irrespective of where or how the foreign body was introduced. Consumers have a choice of where they do their shopping, accepting that some consumers are constrained by price and availability; in general, consumers buy from a retailer that they trust. High street retailers are trusted by us to provide us with safe wholesome products that we are happy to put into the mouths of our children and other family members. This trust may be worth many thousands of pounds sterling per household per annum and it stands to reason that, if consumers lose this trust in a particular retailer, they will do their shopping elsewhere. Own-label products carrying the name of a retailer are very often multi-million pound ‘brands’ and quite

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rightly no retailer wishes to see the strength and good name of its brand compromised. Retailers will always act speedily and appropriately when faced with a foreign body issue in one of their ‘own-label’ products. Branded products, carrying the name of the manufacture, will not be treated any differently by the retailer except that responsibility for communication to consumers will be the manufacturer’s.

3.1.2 Damage to manufacturer–retailer relationships Partnership is a very important word in manufacturer–supplier relationships and, without doubt, honesty is the best policy. If a manufacturer has a production, quality or contamination issue they must work in partnership with the retailer. That retailer may have spent many years working in partnership with the manufacturer and will appreciate honesty and speed of communication. Naturally, manufacturers will not wish to deliver bad news to their retail partners, but occasionally it must be done – and speedily.

3.1.3 Legal proceedings Enforcement Officers can insist on the removal from sale of any batch of products that is in breach of the Food Safety Act 1990 that represents a significant safety hazard. Failure to act on their professional advice may lead to prosecution and associated negative publicity for the brand or manufacturer. Adverse publicity is inappropriate for any brand or manufacturer, and the publishing of product recall notices of food products due to foreign body contamination from the public domain is, by its very nature, adverse. The nature and reason behind a product recall, the positioning of recall notices in newspapers, the wording, product type, time of year, batch size, how well known are the manufacturer and brand may all have a direct impact on any additional media coverage. How the manufacturer manages the media will have a direct effect on positive or negative coverage. The priority is always to put the safety and interests of consumers first and at no cost to them. Manufacturers must be seen as people who care about their customers. The purpose of this chapter is to assist manufacturers, in making decisions when faced with an issue or crisis. They will want to play for time and may not be familiar with the speed at which the media works or how quickly electronic news will impact on public relations. Web sites and chat rooms are a great source of information for journalists wishing to expand their knowledge of a serious issue or crisis. Both consumers and the media will have more respect for manufacturers if their responses are timely and professional. The first question is always: ‘Is this a routine incident or a crisis?’ It must never be forgotten that what is a routine incident to you may well be a crisis

Managing incidents involving foreign bodies

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or scoop to someone else. One should remember the two young journalists in the Watergate affair, setting out to bring down the President of the United States. Rightly or wrongly, journalists or Enforcement Officers will be looking for their ‘Watergate’. They will want the big story, the big prosecution and the scandal involving the big brand and the president of a household name. It will look good on their CV and get them on the speaker circuit. Understanding the big picture is why crisis management is so important in today’s global, fast-moving, consumer-driven food and drinks industry. Crises are no longer the responsibility of a quality manager or company microbiologist. Brand managers, key account managers, logistics and public relations departments will all have to have an immediate input into a product recall scenario. The big picture in an ideal manufacturing environment will include raw materials supply, manufacture, distribution and retail partnerships. Within each of these disciplines staff at all levels must understand the relationship and importance of other staff and how their role, however insignificant it may seem, has a direct input into the big picture. An example here is a jigsaw; there may be just one small piece missing in a jigsaw, but without it the picture is incomplete and the brand or product could be severely damaged by adverse publicity, retailer de-listing or litigation. Crisis management is all about collecting all the pieces of a jigsaw. You may have one piece or all of them. If you have all the pieces you are very lucky. It is more likely that you will have a few edge bits or a couple of corners. Consumers may have some bits, and so of course may the media. Consumers may have eaten some bits, or are just about to do so. How do you get these pieces back again?

3.2

The crisis management team

The role of a crisis management team is to gather facts, facts and more facts. Some may be gathered quickly and some may take a little time. Whilst these facts are gathered a number of decisions must me made. The skill of crisis management is to make decisions where the facts are unavailable, in a timely manner, and then communicate these decisions to colleagues, shareholders, retailers, the media and consumers. Ambiguity may be present and issues relating to the product communicated when not all the facts are available; for example, the results from analytical testing, foreign body analysis or a statistically inadequate number of samples from which to draw accurate conclusions. Most high street retailers will insist that their suppliers have a crisis management team. What is important is that this team knows and understands why it exists and what it is supposed to do in the event of a crisis occurring or developing. Even more important is that the manager must ‘plan,

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practise and prevent’. Planning must not be ‘if ’ it should be ‘when’. A nonroutine incident may occur and careful planning and an understanding of the big picture will ensure that management of the issue, incident or crisis will be handled in the most positive manner for the management, the brand, the company and, of course, for consumer safety. A crisis management team will have many priorities; consumer safety must always come first and in equal second place will be brand and company survival, which include business continuity. Business continuity will encompass communication internally and externally and will include customers and consumers alike. A resumption in production and a continuity of supply of product are paramount. An often asked question is: ‘What does a crisis management team do?’ The simple answer is that it plans and practises for the unexpected. Systems will be in place to prevent a serious incident from occurring, but occasionally these fail to prevent a catastrophe. Within the food and drinks industry, hazard analysis critical control point (HACCP) and good manufacturing practice (GMP) systems have a preventive role. HACCP will identify all the risks associated with a process that could become a threat, to consumer safety and to the brand. The role, therefore, of a crisis management team is to identify the implications of system failures, which can be described as likelihood versus severity – the likelihood of an issue occurring and the severity or impact on consumer safety or brand security if it did. This is known as a threat assessment and will be explained in more detail later. A useful tool for crisis management planners is ‘horizon scanning’. In its most simple guise this is scanning the horizon, with eyes wide open, and identifying all the issues that could affect the business. It is a negative brainstorming session, where a group of colleagues list every conceivable threat to the business or brand. The starting point should be raw materials supply and the finishing point should be the consumer, and every step in between should be considered. Having identified every possible threat they must be prioritised to obtain a ‘threat rating’, so that one can either live with the threat by managing it or seek to reduce it to a more manageable or acceptable level. Once the threats are prioritised, one must practise how to manage them should they ever become a reality. The best advice is that one should always prepare for the worst case scenario. To non-specialists, this often seems bizarre, but in reality the worst case scenario seldom occurs and therefore the resultant management of the issue will be more competent and professional. The composition of a crisis management team must be decided upon by knowledge and experience, not by rank. All members must have a detailed knowledge and understanding of the business and the implications of a crisis. They must understand the big picture. All members should have a deputy and all companies should practise for a crisis at least annually and preferably six-monthly. The practice should include deputies and must be realistic but imaginatively stretching and intellectually stimulating.


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3.2.1 The actions of a crisis management team Crisis management teams should operate at three distinct levels. These are labelled green, amber and red. The green team will operate at the factory level and include the senior factory staff, quality staff, a food safety expert or microbiologist, someone from distribution and logistics and perhaps someone from sales and marketing. Ideally, the team should number six to eight, all of whom will have deputies. It is likely that contamination with foreign bodies from within the factory will often be recognised first at the factory level – for example, broken sieves, conveyor belts and glass (where used), metal fragments from friction, nut and bolts. The green team must make an initial and rapid threat assessment of such incidents and pass all available information to the amber team. The amber team comprises the decision makers and must expand the assessment made by the green team. Ideally, they will be technical experts with a detailed understanding of food safety legislation and distribution. Their role is to gather facts. Their professional knowledge will enable them to make decisions on the likelihood of an issue materialising and the severity of the impact that the issue will or may have on consumer safety. Their assessment of the issue, which must include where the product is, will enable them to make a recommendation to the red team. The amber team will make the decision on whether to hold stock in the factory, at the distribution level, or at retail or consumer level. They may, of course, recommend that no further action be taken. The red team may well include the MD or chairman of a company and they may vigorously question the recommendation of the amber team. Often, this recommendation will be to recall at various levels, as described above. They may choose to ignore the recommendation of the amber team, but the threat assessment undertaken by the amber team will document their recommendation for future reference and questions relating to due diligence. The red team will ideally consist of logistics and distribution experts, key account managers, sales personnel, PR and media spokespersons. The red team crisis management chairman will initiate the team’s media strategy, and alert insurers, lawyers, and finance directors. The red team makes recalls happen. Very often, the customer care office will be the first point of contact in a crisis, especially if consumers are involved, and its involvement in identifying complaint clusters is paramount to successful crisis management and its value should not be underestimated. The customer care manager should also have a place in the crisis management team. Companies should always have a board level policy on consumer complaints. Such a policy should guide staff on managing complaints that may be fraudulent. No company wants to be involved in a recall at any level due to a complaint that is motivated only by financial reward. The policy must also give guidance on referring such claims to the police. If clear communication channels are established between the customer care

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manager and the crisis management team, confusion and false alarms can be avoided. Customer care staff should be trained in recognising fraudulent claims, both written and over the telephone. Examples include letters that: • • • • • • •

Seek cash or compensation by return. Are badly hand written, with spelling mistakes. Are written on scraps of paper. Refer to non-existent dental or other practices. Contain phantom or photocopied receipts. Refer to a foreign body that is no longer available. Show the wrong date, brand or company.

Naturally, one has to be exceptionally careful not to confuse a genuine complaint with a fraudulent one. The company policy on referring the matter to the police for further investigation must take into account the severity of the claim, the time and trouble to the company in pursuing prosecution and the resultant publicity. However, if the compensation culture is to be tackled, responsible companies will want to play their part in reducing the impact that fraudulent claims have on the food and drinks industry. An appreciation of Section 15 of the Theft Act, 1968, will help in the production of your policy. It says: ‘A person who by any deception dishonestly obtains property belonging to another, with the intention of permanently depriving the other of it, shall on conviction or indictment be liable to imprisonment for a term not exceeding ten years.’ In the UK, food and drinks manufacturers can become members of the Inter Company Consumer Affairs Association (ICCA). The ICCA operates within the scope of the Data Protection Act to identify habitual complainants.3 In the Irish Republic, The Food and Drink Federation is the largest sector within the Irish Business and Employers Confederation (IBEC). It operates a similar service for Irish food and drinks manufacturers within the consumer complaints group.4

3.3

The crisis management plan

The principle of a crisis management plan is to manage communications and make decisions where there may be insufficient data or facts, when faced with a potential risk to the safety of consumers that may have a negative impact on the brand or company. It is important to stress at this point that crisis managers can often be required to manage situations other than consumer safety, examples are industrial action, environmental issues and accidents. For the purpose of this chapter, crisis management plans will incorporate contamination, accidental and malicious, both internally where quality systems have failed and externally where products have been contaminated


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outside the company’s control. This includes accidental and malicious contamination by consumers. To encompass these criteria one must consider issues ‘from farm to fork’, i.e. all those associated with agricultural contamination and product tampering, from within the manufacturing operation including those that are accidental and those deliberately caused by the company’s own staff. The crisis management plan, when prepared, must be regularly updated and practised. It must include who will manage, how they will manage and under what circumstances such plans will be implemented. Practice will, if undertaken with eyes wide open, and challenging questions asked, identify what the plans need to cover. A crisis rarely occurs without the involvement of the media. A chief role in crisis management is therefore communication and at least one member of the team must be trained in handling the media. Alternatively, it is possible to use the services of a public relations company. If this is the preferred option, such a company must be included in the practice to ensure that it can deliver what is expected when it is required. The second most important task of a crisis management team is to be able to answer what may appear to be a very simple question: ‘Where are the products?’ Traceability and the authenticity of raw materials and finished product are key to a successful product recall. A detailed knowledge of the distribution network will make recall by batch far easier, that is if it is known where the products are!5 Crisis management plans must make this a priority – a simple distribution network; for example, sales through one retailer are significantly easier to manage than a complex chain through several retailers, wholesalers and foodservice operations. In practice planning, one should not make the mistake of making the exercise fall within one’s own comfort zone. The practice must be uncomfortable, challenging and realistic and no-one should pretend to know where products are if they do not. Many of the management decisions made in everyday business operations are carefully thought through and researched thoroughly. When an unexpected issue arises, such as foreign body contamination, which is being covered by the national media, there will not be time for detailed assessments and research. A member of the team will have to react very quickly to gather as many facts as possible relating to the contaminant. It is clear from media coverage of the food and drinks sectors, that in the UK consumers are very likely to contact the media directly for their fifteen minutes of fame rather than make initial contact with manufacturers. Recent headlines, such as ‘Condoms in my curry’, and ‘Goldfish in my tin of spaghetti’ are classic examples. It was stated: ‘The goldfish would have been swimming again had a bucket of water been available’. Anyone who knows anything about heat treatment involved in the canning process would have realised the fish had not been cooked, but the story still ran in a red-topped tabloid newspaper!

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Food manufacturing systems do not allow for ‘condoms in my curry’, but these and other foreign bodies still arise from time to time. Some sectors of the media find this an entertaining way of filling the daily papers and it is the brand, manufacturer or retailer that receives the negative exposure. Under such circumstances, when analysts are asked to examine such foreign bodies, questions such as ‘was it worn or used?’ are easy to determine, but of what value is the answer? The important questions are how and when did the object get there? The possibility of malicious contamination by staff should not be dismissed. Equally probable is contamination by third parties within the distribution, retail or home environment. This may be of little comfort; the contamination may still lead to prosecution unless one can prove a due diligence defence. However, there is an old saying, ‘to-day’s newspapers are tomorrow’s fish and chip wrappers’. It is important to try to identify best practice in the matter of recall notices, of which there can be a large variety. The use of the national media is one method of communicating with the general public when members should no longer consume a particular product or batch. It may not be the most appropriate; it is, however, the most wide-reaching and, once implemented, gives journalists the opportunity to file additional coverage or editorial. While it can be argued that this ‘free’ media coverage strengthens the communication process, the one major disadvantage is that food production managers do not write the text, and the opportunity for negative coverage is enormous. Standardisation would be useful, firstly to identify a model of best practice and secondly to avoid confusion by consumers. For example, the national media carry product recall notices, product, announcements and product safety notices. It is doubtful whether many consumers understand the difference between a recall notice and a product safety announcement. If the purpose of a published recall notice is to communicate an important message to consumers, then there are several considerations: which newspapers to use, how large the announcement should be, and on what page and in which position the announcement should stand. For the majority of manufacturers the main criterion is, of course, cost. Can management afford to place display adverts in ten daily newspapers of a size that is appropriate to the level of threat to consumers and therefore be in a position to support due diligence defence or to satisfy enforcement officers or the Food Standards Agency? Equally important is the question regarding the use of display advertising versus classified advertising. There can be a large cost differential between the two and a different cost between newspapers. Management must consider how quickly a recall notice can be placed. Most national newspapers will accept a recall notice that appears the next day if they receive it before five o’clock in the afternoon and they have space available for a display advert. A cheaper option is a classified advert, which will not normally appear in print for 48 hours, but this is hardly ideal


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if the message to consumers is urgent. The author’s review of recall notices highlights differences not only in size but also in positioning. Are consumers more likely to see and read a recall notice if it is on a left- or right-hand page, on pages 2 to 5, or hidden away amongst the financial or sports pages towards the rear of the newspaper? Manufacturers face a dilemma. How much to spend? Is this recall notice based purely on public relations and a need to demonstrate a due diligence defence? Should the notice be placed in one, two or ten newspapers, and which newspapers are the most appropriate? To help in answering some of these questions, the marketing department in most manufacturing companies will be able to identify the socio-economic classification of their customers and therefore which newspapers they are most likely to read. This answers the question in which papers to place recall notices. Multinationals with multi-million pound brands, perhaps with the backing of recall insurance, will place product recall notices in all of the national newspapers. Cost need be no barrier, for example, when manufacturers can demonstrate that contaminants are the responsibility of third party suppliers. Manufacturers are likely to take the view that cost is not a problem; suppliers will have to pay because their ingredients were in breach of specification. On a note of caution, the manufacturer will often bear the initial costs, and in many cases the time to recover such costs through the courts can be lengthy. A crisis management team should address these questions in the planning and practice phase of crisis management. They are not questions that should be considered on the day a serious foreign body contamination issue arises.

3.4

Managing internal and external communications

3.4.1 Internal communications Crisis management teams must control communications both internally and externally. It is important that management and staff are aware that an issue or crisis is ongoing and that their brands are implicated and subject to a recall. They should not read it first in a daily newspaper. However, they should all understand that the media may look to support their stories with additional, unwelcome, inappropriate and untimely quotations. The electronic dissemination of information internally is timely but it is very easy to forward bad news outside business. When using email strict discipline must be observed; in the event of litigation management may be required to disclose all documentation. All contacts with the media must be directed to a trained, designated spokesperson. 3.4.2 External communications The role of a crisis management team is to manage all communications. Specifically, this will include key accounts and retailers, and may include the

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local home authority, trading standards officers and possibly the Food Standards Agency. It is inappropriate for them to first read of a serious safety issue that impacts on consumer safety in the newspapers. Both the media and, of course, consumers may make direct contact with the FSA and ask them to comment. If they can only answer ‘we have no details or information’, this will give a very negative impression. Companies may be asked to give interviews to local, national and international television and radio and to the print media both locally and national. Crisis management teams must ensure that they have suitably trained and experienced staff to fulfil this very often stressful experience. Planning and practice will give the necessary confidence to achieve this. However, this must include answering the most awkward and difficult questions in a manner that enables the delivery of key messages confidently. The national media, especially the tabloid press, have an affinity with foreign body contamination. The story will have currency, picture opportunities and entertainment that are the very reasons newspapers exist. Reference has already been made to ‘fifteen minutes of fame’ and the trend for consumers to take their story of contamination direct to the media. The first that the manufacturer may know of such an incident is when the phone rings and there is a request for comment. Many well-known brands and manufacturers have faced such coverage over the last few years. Glass, beetles, spiders, condoms, frogs, ladies’ sanitary products, goldfish, earrings, nails, snakes, needles, rodents and dogs’ teeth have all provided the media with opportunities to provide entertainment for their readers. What is never known on the day of such stories breaking is what shortor long-term impact such coverage will have on sales. It is probably safe to say that such unwanted media coverage will not have an impact on sales in the long term, despite the protestations in print that ‘it made me feel physically sick and I will never eat another one again’. Such events will cause uproar amongst senior managers, embarrassment to the directors and chairman, and a major internal enquiry under the general heading of ‘has this occurred under our control, accidentally or maliciously?’ Time represents no barrier with such incidents, and print coverage one day can emerge in a television story some months or years later under the ‘Foreign Bodies from Hell’ banner, and will repeat the embarrassment and serve to bring the story back under the spotlight. Recent examples of this would be the ‘condom in my curry’ and ‘rodent in a loaf’ stories. Manufacturers are seldom given by the media little more than a sentence or two to comment. Traditionally, such stories will include ‘a spokesperson for company X has offered a detailed and thorough investigation’. This, of course, may be a précised version of the ten-minute discussion the manager had with a journalist in the hope of convincing him or her that, during the production stage, it would be physically impossible to pass a 4-inch nail through a 2-mm sieve. What is not known is whether such foreign bodies


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were introduced maliciously in the factory, in distribution, in retailing or in the home. The story will run and the manager is to blame. Not commenting is not a sensible option; it implies admission by silence and the manager relinquishes the opportunity of making any positive contribution to the story. Preparation for such incidents is the responsibility of the crisis management team and in particular the media spokesperson and a deputy. The golden rules in crisis communications for such issues may be expressed as follows: ‘our priority is the safety of our customers and, whilst due to the very nature of our production it would be impossible for such foreign material to enter our products, we are undertaking a very detailed review of our production process and distribution chain to ensure that such incidents can never occur (again).’ There is seldom time, nor is there access to the foreign body to determine its source or point of entry. By the time the manager has had the opportunity to determine if a beetle has been subject to heat treatment or if the glass is household Pyrex-type glass, the story has run and been forgotten by the media. They are moving on to the following day’s papers and there will be a struggle to get a published apology when the manager demonstrates beyond doubt that such contamination did not occur when the product was under his or her control. The belief will be upheld that someone put it there, for a joke, maliciously, to seek compensation, to get payment for the published story, or for any number of reasons. Glass contamination carries further problems. Complaints by a consumer to an enforcement officer or to the company have much wider implications. Have glass fragments contaminated only one jar or could the entire batch be affected? Rapid analysis of the glass is desirable if one can have access to it. However, many enforcement authorities will insist on using their analytical facilities; this takes time and meanwhile they want management to recall a product or batch from sale. If the glass belongs to the factory and may have contaminated an entire batch, then there is no other option than to recall the batch or batches. It can only get worse Future European legislation will place an even greater level of responsibility on manufacturers to communicate contamination issues to the general public. Under EC Food Regulation No. 178/2002, a fundamental point of which is that producers must be able to trace their products, manufacturers will have an immediate duty to recall potentially harmful products and pass on information to consumers and immediately inform authorities if a product is potentially injurious to health. The authorities will be obliged to inform the public of the risk, i.e. through the media. Relevant sections will come into force by 2007 and override current general product safety regulations. In addition, under EU Regulations that must be implemented by 2004, enforcement authorities will be able to order a product recall and, to achieve this, distributors must retain and provide all

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documentation relating to traceability. Producers and distributors will be obliged to notify the appropriate authorities of dangerous products. The most suitable vehicle currently available is the print media. As previously discussed, this gives journalists direct access to the situation and may exacerbate any problems. Future and novel methods of communication via text messaging, email and point of sale information may contribute to fewer ‘public’ recalls. The emergence of accurately maintained databases on new mothers, for example, gives manufacturers an opportunity to write directly to consumers who may have responded to vouchers or advertising. However, this form of communication has severe cost implications and should be seen as a tool, not a rule, in the communications portfolio. The compensation culture in which we live and an increase in 24 hours a day, 7 days a week media coverage mean that more news is needed to fill newspapers, magazines, TV and radio news. Consumers who make fraudulent claims in the hope of receiving vouchers, postal orders or direct monetary payment will continue to increase. With foreign bodies, allergies and intolerance to certain foodstuffs, peanuts, shellfish, soya or milk proteins, for example, the new and emerging threat of emotional trauma, and in particular religious trauma, will give manufacturers challenges without boundaries.

3.5

Successful crisis management

3.5.1 Threat assessments Prerequisites to successful crisis management are structure and consistency in decision making. Threat assessments take account of where in the market products are situated. Even severe product contamination, physical, chemical or microbiological, is not a crisis if it is isolated and under your direct control. Corrective measures to remove the contamination may require the safe and supervised destruction of the product, but, as consumer safety is not implicated and the media are not aware of the issue, then the situation is not a crisis. However, there are detailed questions to be asked of the quality systems. Threat assessments use a simple model of severity versus likelihood of an issue compromising consumer safety. Correct and trained interpretation of the model will take into account of where the products are. They can be categorised as follows: products under control, products at distribution level, products at the retail level, and products in the hands of consumers. A detailed understanding of where products are in the distribution food chain will often allow products to be recalled from shelves at the retail level. With the technology in some supermarkets, the product recall can include products at the payment till. In regard to products sold through more complex channels, wholesale/ cash-and-carry/foodservice, it is not generally known where they have gone


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beyond this stage, although the technology to trace consumers via credit card details and internet delivery may soon assist here. Contingency plans for these complex distribution channels must be prepared. Food products sold into the foodservice sector can have complex distribution channels and detailed planning for a product recall must take this into account. The complexity of raw materials supply and traceability has obvious advantages and disadvantages in crisis management. If one can link all raw materials and distribution to a particular batch, the recalls become more manageable, but only if one can access data in a useful and timely manner. The manager will only learn this by imaginative and realistic scenario exercises and planning.

3.5.2 Staff training It is imperative that all staff understand the implications of foreign body contamination and therefore of crisis management. Supervisors and production staff need to understand why there are metal detectors and why HACCP, GMP and other quality systems are in place. Managers need to understand that, although they may not necessarily be members of a crisis team, they may well be asked to contribute to gathering facts and more facts. Directors and the senior management team must be intimately aware of the importance of crisis management and their role, directly through membership of a team, and understand the implications, pressures and problems they may face, not forgetting the cost.

3.5.3 Prevention is better than cure Systems and procedures are in place in all manufacturing environments to prevent contamination by foreign bodies. The management of these procedures to ensure that they never occur is as valuable and important as planning for the occasions when it does. However, where some form of accidental contamination may have occurred, such as a fraying conveyor belt that may have contaminated production, honesty and common sense are extremely beneficial. It is better to speak up and isolate the products before they leave direct control than have to face a product recall.

3.6

Categories of consumer complaints

Food and drinks manufacturers can become aware of contamination through many channels. The customer care line is often the first point of contact. Contamination complaints fall into three distinct categories. Staff in the customer care office are frequently aware of genuine complaints through a careful analysis of historical data and records; complaints in this category, if handled with concern, and, if correctly handled with speed and

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compassion, will not affect consumer confidence in the product. The promise and implementation of an internal enquiry will generally lead to a satisfied customer. The second category of complaints includes the unexplained and unexpected – something outside the usual portfolio of complaints. Frequently, these complaints will require further internal investigation and identification. Acknowledgement of the complaint and the promise of a detailed investigation in a speedy manner are priorities. Often in this category are contamination issues where the complainant is likely to make direct contact with the national newsprint media or enforcement officers. As previously explained, these contamination issues frequently offer newsprint media ‘an entertaining article, with picture opportunities and dramatic headlines’.This category can be further subdivided into accidental, both by consumers and by the breakdown of HACCP procedures. The third category, and there may be some overlap with the second, are malicious and fraudulent complaints from consumers who may be seeking financial gain.Within this category can also be included deliberate and malicious contamination by staff and anyone else with access to the products prior to their being purchased by the end-user. Not all the complaints in this category will have monetary demands associated with them. The complainants can often just be exceedingly mischievous or malicious, or even committing a crime under the Public Order Act.

3.7

Conclusions

Plan, practise and prevent: crisis management teams with appropriately trained and experienced team members who understand their role and responsibilities through detailed scenario planning will be in a strong position to manage the communications surrounding incidents involving foreign bodies. Many such incidents attract short-term media attention but will not have a long and lasting impact on future sales, provided the response is appropriate to the severity of the incident. Management must demonstrate and show real concern and sympathy; they must make it clear that they are doing something, are in control and that they are experts. The crisis management team needs to plan for the expected and the unexpected, and detailed role play or acting out recall scenarios will greatly help the team to prepare and win if faced with a severe crisis. Knowing where the products are, close liaison with the customer care department and the identification of ‘complaint clusters’ are paramount in good crisis management. No-one wishes to recall a product if 99 % of a batch has been consumed without any consumer complaints relating to that batch.


3.8

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Bartlett R, Dancing With The Devil – Crisis Management in the Food and Drinks Industry. Leatherhead Food International. July 1999. ISBN 0 905748 62 X Commissioned, and with a preface by Tony Hines. Bartlett R, Product Recall – An Aid for Management. Leatherhead Food International. July 2000. ISBN 0 905748 59 X Commissioned, and with a preface by Tony Hines. Clare J, Guide to Media Handling, London. Gower, 2001. ISBN 0566 08298 5 Get it Out, Get it Right, Get it Over. Food Products Recall Manual. The Food Institute. NJ. USA Consumer Product Recall. A Good Practice Guide. Department of Trade and Industry. London. November 1999. URN 99/1172 EC Food Legislation Manual. 3rd ed 1993. 2 Volumes. Leatherhead Food International Tony Hines mbe is crisis Management Manager at Leatherhead Food International (LFI), Leatherhead, Surrey. LFI is a membership-based organisation providing research, consultancy, information, training and routine analysis to its 1000 members in 45 countries around the world. Tony has been delivering training courses in crisis management for many years. He introduced the very popular course ‘Dancing with the Devil’ – a training programme for crisis management team members. His current portfolio of courses includes programmes on product recall, crisis management and complaints and media management. Consultancy services and training courses are run around the world, as far afield as South America as well as to many of the major European countries. Private clients include many of the UK’s top ten food and drinks manufacturers. Leatherhead Food International is a world leader in the provision of analytical, microbiological and foreign body analysis. A 24/7 crisis Management Service is offered free of charge to members. For further details of all the LFI services and information on crisis management, contact them on +00 (44) 1372 376761 or [email protected]

3.9 1

References and notes

General principles of food law and the European Food Safety Authority (2002) Off. J. European Communities, 45(L37), 1–24. Food safety requirements (Article 14) Food shall not be placed on the market if it is unsafe. Food shall be deemed unsafe if it is considered to be: (a) injurious to health (b) unfit for human consumption

2

3 4 5

Where any food which is unsafe is part of a batch, lot or consignment of a food of the same class or description, it shall be presumed that all food in the batch, lot or consignment is also unsafe, unless following a detailed assessment there is no evidence that the rest of the batch, lot or consignment is unsafe. The Inter Company Consumer Affairs Association UK (ICCA). For further details and the name of your local Executive Member, contact the Secretary: [email protected] The Irish Business and Employers Confederation. Contact details: IBEC Head Office, Confederation House, 84/86 Lower Baggot Street, Dublin 2. (01) 605 1500 Traceability (Article 18)

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Detecting foreign bodies in food (a) Traceability of food, feed, food producing animals and any other substance intended to be, or expected to be, incorporated into a food or feed shall be established at all stages of production and distribution. (b) Food and feed business operators shall be able to identify any person from whom they have been supplied with a food, feed or food producing animal, or any substance intended to be, or expected to be, incorporated into a food or feed. Such operators shall have in place systems and procedures allowing for this information to be made available to the competent authorities on demand. (c) Operators shall have in place systems and procedures to identify the other businesses to which their products have been supplied. This information is to be made available to the competent authorities on demand.

Reference has been made to the following newspaper articles: ‘Condom in my korma’ (2001) The Sun, 14 June. ‘I’m discustard’ (2002) The Sun, 22 January. ‘Worker put glass in pancakes’ (1999) Daily Mail, 3 November. ‘Mum’s needle horror’ (2001) Daily Star, 13 June. ‘Sweet lover finds dog tooth’ (2001) Sunday People, 1 April. ‘Snake in boy’s pie’ (2002) Daily Mirror, 11 October.

Part II Detection and identification

4 Metal detection J. P. Craig, Thermo Electron Corporation, UK

4.1

Introduction and the history of metal detection

4.1.1 Introduction While metal detection in food is important as a means of protecting consumers and complying with laws and regulations, such as the Food Safety Act 1990, European Council Directive 93/43/EC, and the pending regulation which is due to replace it, perhaps the most important commercial reason for effective metal detection within food processing plants is that of brand protection. Metal contamination in food products can cause concern and even injury to consumers, and compensation may run into tens of thousands of pounds, but the effect of bad publicity arising from such an incident could damage a brand’s reputation beyond repair, causing a loss of business that could be worth millions. In this chapter we will look briefly at the history of metal detection in food processing, the technologies currently in use and consider the factors that are important in the effective operation of metal detection systems to production lines, including ownership of the machine, testing and maintenance records.

4.1.2 History The development of metal detection technology for the food industry started immediately after World War II, and was born of the research that had been done into radar and radio frequency detection. Two UK companies started the work, Goring Kerr (now Thermo Electron Corporation) and Rank Cintel, which presented their first products to the food

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processing industry about 1948. A small number of companies in other countries have become involved in the field, but it remains to this day a predominantly British-based technology. In its early days, metal detection systems were valve-based and had limited applications, but over the last 30 years increasing automation in production lines has forced the technology to advance and improve; today microprocessors allow much greater and faster use to be made of the information generated by the detection systems. In the 1950s and 1960s, when there were several people and as many pairs of eyes involved in running a food production line potential contamination was more likely to be spotted. Today, it is possible to have a food processing line, from raw materials in to packaged product out, overseen by just one person, so the need for high quality detection systems has increased. The instances of checking that raw materials are free from contamination have increased in recent years, driven in part by regulations such as the Food Safety Act 1990, EC Directives and the guidelines of Hazard Analysis by Critical Control Points (HACCPs), as well as by the increasing level of expectation among consumers and the growing likelihood that they will turn to litigation should things go wrong. The increased brand awareness of consumers has also driven the need for all types of contamination to be screened out of products. Some readers may remember an incident where Perrier, the leading brand of mineral water, was contaminated with benzene. The company handled the situation very well in PR terms, withdrawing 160 million bottles of their product from supermarket shelves world-wide almost overnight. The source of contamination was isolated and within days fresh product was on the market; within weeks sales had returned to normal levels. Today Perrier is still one of the leading brands of mineral water, but the cost involved in the recovery process was significant. As more and more automation and mechanisation are involved in food processing, from harvesting right through to final packing, it is recommended that metal detection should take place after every major process. This follows the HACCP guidelines and ensures that any contaminated product is removed before it has any further value added to it. Another reason for placing metal detection systems at several points along a production line is that, as production methods have become more sophisticated, the machinery involved has become more expensive and the need to protect it from damage has increased. Metal contamination may not only affect the product but the machinery too. As well as the cost of repair, should metal find its way into an expensive piece of equipment, the cost of the downtime involved can be enormous. Despite this fact, many companies will only consider approaching a metal detection specialist after a problem has arisen and they become aware of just how expensive an issue it can be.

Metal detection

4.2

49

Types of detection systems

The basic principle of metal detection is based on the transmission and reception of electrical impulses, much like radio waves. All metals have characteristics that will cause an alteration in a transmitted signal, because of their conductivity and magnetic properties. What metal detectors do in food processing lines is to compare the signal received with the expected signal and identify the presence of a contaminant if a variation is observed. There are three types of detection system in use today, one of which comprises over 90 % of those in current use. The systems are: pulse technology, ferrous in foil detection, and the balanced three coil system.

4.2.1 Pulse technology Pulse technology is limited to finding relatively large pieces of metal in raw materials, such as nails or cans in sacks of potatoes. A transmitter is placed on one side of a conveyor and a receiver on the other side. The transmitter sends a pulse signal and the receiver will detect a spike from that signal. Normally that spike will grow and decay in a quick and regular pattern. When metal is present it will cause the decay of the spike to become elongated; it is this change in the pattern of the decay that indicates the presence of metal contamination.

4.2.2 Ferrous in foil detectors Ferrous in foil detectors also have limited applications and are specifically aimed at detecting ferrous metal contamination within products packed in foil trays, such as pies, quiches and casseroles. Ferrous in foil detection is used in cases where there is so much metal passing through the detection system that other types of detection will not give a sensitive enough result or where the change in signal caused by the packaging would swamp any signal change caused by contaminants. This system uses magnetism for detection and is, therefore, only capable of detecting metals with a ferrous content. It comprises upper and lower discrete permanent magnets, between which there is a strong magnetic field, and coils mounted across the centre of the two magnets. Any piece of ferrous metal passing through the magnetic field will be magnetised; as it passes through or under the coils the magnetism will generate a small current within the coils. The coils are connected to amplifiers which produce an output signal to a control device that in turn operates a reject system. Sensitivity, which determines how small a piece of metal might be detected, is relative to the height of the aperture between the magnets. A 100 mm high aperture can generally find a 1.0 mm piece of ferrous metal, so a maximum effective aperture height is generally thought to be 200 mm.

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Ferrous in foil detection systems were particularly popular and relevant when processing machinery contained a high proportion of mild steel. Now that most processing plant is made with 300 grade stainless steel, which has virtually no magnetic potential, the system is becoming less relevant to the industry.

4.2.3 Balanced three coil detectors The balanced three coil detection system is the most common type found today. This system accounts for between 90 and 95 % of all systems in use at the present time because its primary characteristic is a high level of sensitivity. It works by comparing the signal variance received by two receiver coils, placed either side of a transmitter coil, along the length of the conveyor. As Fig. 4.1 shows, the receiver coils are wound in opposite directions so that the values of the signals received from the transmitter coil should balance each other out. Subtracting the value of the signal received at one coil from that at the other should result in a total value of zero. Metal passing through the signal fields will distort the normal pattern of the electrical fields; magnetic metals cause the induced voltage to increase and non-magnetic metals cause the voltage to decrease. As the piece of metal passes through one of the coils this change in voltage will cause the signals at each receiver coil to be different and the ‘out of balance’ signal within the system highlights the presence of a metal contaminant. As Fig. 4.2 shows, when a piece of magnetic metal passes though the coils the field is attracted to the metal, causing a voltage increase. A piece of Transmitter coil RF transmitter

Receiver coil R2 Receiver coil R1

RF receiver

Product passes through aperture

Fig. 4.1

Microprocessor detection and control

Outline construction details of a balanced three-coil detection system.

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51

Three coil system

No metal present

R1

T

R2

R1

T

R2

R1

T

R2

Magnetic metal present field distorted induced voltage decreased

Non-magnetic metal present field distorted induced voltage decreased

Fig. 4.2

Effects of magnetic and non-magnetic metals passing through a balanced three-coil system.

52

Detecting foreign bodies in food R1

Fig. 4.3

T

R2

Detection signal as a piece of magnetic material passes through a balanced three-coil system.

non-magnetic, or conductive, metal will cause the fields to distort away from it, reducing the induced voltage. Figure 4.3 shows an example of how the signal difference will peak as a magnetic metal contaminant passes through the first receiver coil, and then drop away rapidly as it moves towards the transmitter coil. As the metal passes under the transmitter coil the signal will become zero and the inverse pattern will be seen as the metal moves towards, through and away from the second receiver coil. The pattern seen for a piece of conductive metal, rather than magnetic, would be the inverse of that shown, with the voltage at first being reduced rather than increased. The pattern shown on the right-hand side of the graph appears to be negative in this case because the second receiver coil is wound in the opposite direction to that of the first.

4.3

The balanced coil system

4.3.1 Magnetism and conductivity Metals have characteristics of both magnetism and conductivity, with ferrous and non-ferrous metals showing the two in differing and varying

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53

Conductive Product

Stainless steel Ferrous material Non-ferrous material

Magnetic

Fig. 4.4

Reactivity and resistive vectors for various materials.

proportions. Ferrous metals cause the signals from the transmitter coil to be increased; the term used to describe this intensification of the signal is permeability and the unbalanced signal resulting from it is called the reactive effect. Non-ferrous metals have an opposite effect, with a permeability value of 1 and very little reactive effect at low frequencies; they cause energy loss within the signal, reducing the induced voltage in the receiver coil. This current loss is proportional to both the frequency of the oscillating current applied to the transmitter coil and also to the area of the metal object. It is because the current loss is proportional to frequency that non-ferrous metals are easier to detect with higher frequencies. While non-ferrous metals can only cause a decrease in the initial signal current, which is in phase with it, and with a value of perhaps 1, the magnetic effect of ferrous metals can cause increases of possibly 20–30 times, the signals for which are at a 90 ° angle to the transmitted signal. So it can be seen that the conductive effects caused by non-ferrous metals are significantly less than the magnetic effects caused by ferrous metals, making the latter much easier to detect. The issue of detecting non-ferrous metals is further complicated by the fact that the majority of food products themselves have a conductive effect and produce a signal that could swamp the conductive effect of a contaminant. Product conductivity, or product effect, is discussed in more detail in Section 4.3.2. The ratio of conductive to magnetic effect can confirm the nature of a contaminant absolutely. Figure 4.4 shows sample values for the reactiv-

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ity/resistance vectors for a piece of ferrous material, a product, a piece of stainless steel and another non-ferrous material. Reactivity is shown on the x-axis and resistance on the y-axis. The vectors may vary in length as the size of the item varies, but the direction of the vector will remain constant for a given item at a given frequency.

4.3.2 Product effect As already mentioned, many food products are electrically conductive, with blood, salt and moisture being amongst the strongest conductors. Cocoa contains a significant amount of copper, and a number of chemicallybased artificial colourants are highly magnetic. It is quite possible, as is shown in Fig. 4.4, for a food product to produce a far stronger signal vector than any potential contaminants do. This phenomenon is known as product effect. It is possible to overcome product effect and ignore the conductivity of the product being tested. With analogue detection equipment this is done by electronically rotating all the signal vectors, so that the product signal is in line with the y-axis and effectively has no value on the x-axis; this is known as product phasing. However, this also has the effect of significantly reducing the x values of the non-ferrous materials, including stainless steel which is one of the contaminants most likely to occur. With the introduction of microprocessors and, more recently, digital signal processing (DSP) came significant advantages over analogue methods. Not least of these is digital filtering, which allows a particular signal to be extracted from a group of signals and ignored; thus product effect can now more easily be removed from the signal analysis equation. DSP, which was designed specifically for high speed data processing such as that used in mobile telecommunications, has brought significant advantages over microprocessors, not the least of which is speed of operation. Whereas a 386 microprocessor might process a typical function in 800 milliseconds, a 32 bit DSP will perform the same function in about 1 millisecond. The microprocessor and DSP also allow far more analysis to be carried out on a received signal than analogue systems would. The detection system is still a piece of analogue equipment, and the signal has to be converted into a digital format, but DSP allows the contaminant signal to be extracted from amongst the other signals received and the resultant ‘noise’ created by other environmental features such as the transport and reject systems. Conductive products can produce very complex signals, where mass and shape are also influencing factors. DSP allows more signal processing to be done and thus more of the other product-related effects can be eliminated than could be removed simply by product phasing. An interesting aspect of the product effect is that fully frozen foods, i.e. frozen to –18 °C, have almost no conductivity, so the metal detector can act as a secondary check on correct processing of frozen products.

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55

4.4 Factors affecting the application of metal detection systems In this section we will look at some of the considerations that have to be taken into account to obtain the best possible results in an actual applications environment. When considering the installation of a metal detection system it is advisable to partner with a supplier that is expert in providing complete detection solutions. With a number of significant and interrelated factors to be considered when placing a system in its working environment, it is far better to be able to call on a body of expert knowledge, rather than just rely on expertise that is specific only to a particular component of the whole system. The following sections look at some of the more significant factors that need to be taken into consideration when specifying and installing a metal detection system.

4.4.1 Sensitivity The sensitivity of a metal detection system is generally proportional to the height of the aperture through which the product will pass, providing the width is less than about 500 mm. So the size of the products, and of any future products, that will be passed through the detection system should be considered when it is being designed. The sensitivity of the equipment also increases almost exponentially for a distance of about 10 mm around the inside of the casing. This distance has to be taken into account in the design as well, because the product effect signal could swamp any other signals should the product come within the high sensitivity zone, and can affect the overall sensitivity of the equipment to the presence of actual contaminants. Another factor that can affect sensitivity is the ability of the system to phase out product signals. Two machines may be able to demonstrate the same ‘free air’ sensitivity, but the one that has the greater capacity to phase out product signals is going to have a far greater ‘achievable’ sensitivity when actually installed and running.

4.4.2 The orientation effect When one considers sensitivity measurements the contaminant is generally visualised as spherical, but in reality it is a piece of swarf or a length of wire. The sensitivity of the detection system to wire is dependent on orientation, as Fig. 4.5 shows; the signal strength of copper wire decreases as its orientation, relative to the coils increases and for iron wire the signal strength increases. For non-ferrous wires the rate of change in signal strength is greatest when the wire is oriented between parallel to and 45 ° from the direction of the coils. As Fig. 4.6 shows, the length of a piece of wire may have no bearing on its visibility to the detection system. When a piece of 0.5 mm

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re

Relative detection voltage

i Fe w

Cu w

ire

0 15

Signal value scale (log)

Fig. 4.5

30

45 Angle (°)

60

75

90

Variation in signal strengths for copper and iron wire, dependent on their orientation to the detection coils.

Detector with 1.0 mm Fe sphere sensitivity

Best direction

Worst direction 0 10

20

30 40 Wire length (mm)

50

60

70

Detector with 0.7 mm Fe sphere sensitivity

Fig. 4.6

Importance of achievable sensitivity in relation to the size and orientation of a piece of non-spherical metal.

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57

diameter stainless steel wire is in its worst possible orientation to the detector coils its length could be almost infinite without it being large enough to be seen by a detector with a 1.0 mm ferrous sphere sensitivity (top line on the graph), while in the best possible orientation it would be detected if it were only 25 mm long. The lower line on the graph represents a detector with a 0.7 mm ferrous sphere sensitivity. This apparently minor increase in sensitivity would allow the same piece of wire, in its worst possible orientation, to be detected if it were just 20 mm long, or just 5 mm in length if it were in the best possible orientation. This increase in sensitivity of about 25–30 %, and the resultant improvement in detection, is of the same order of magnitude that has been achieved in recent years with the introduction of DSP.

4.4.3 Mechanical considerations The metal detector is a highly sensitive piece of equipment and needs to be protected as much as possible from the effects of outside interference, such as vibration and other electro-mechanical influences that may affect the output signals. Paramount amongst these is the conveyor that carries product through the detector. The sensitivity of the detector extends well beyond the casing in which the system is housed. Because of this a metalfree zone needs to be established at either end of the detector. This zone extends about one-and-a-half times the height of the aperture beyond the casing. Achieving the metal free zone is not as easy as it sounds, because the detection system is also sensitive to static electricity, which excludes the use of some plastics as well. Beyond the metal free zone the loop effect needs to be considered. This occurs when bearings in the rollers of the conveyor system absorb energy and the metal parts in them open and close. This may appear to the detector system like an electrical signal switching on and off. The loop effect can be detected at a distance up to five times the aperture height. To add to the complication, the loop effect may not be seen immediately after installation, when bearings are so tight that no gaps occur; however, after a period of operation the bearing will wear and free up, and may start producing signals. Once the system has been installed major alterations to the local environment must also be taken into consideration. Alterations that include the addition or removal of large metal girders, extra equipment being added to, or alongside, the production line may affect the metal-free zone, induce or remove a loop effect. When installing pipeline systems the size of pipe has to be considered. Laminar flow will cause product in the middle of the pipe to flow much faster than that at the outside. This has significant implications for the design and implementation of the reject system used. One possible method is to divert potentially contaminated product into another, smaller diameter, pipe and retest it. This will allow a smaller amount of product to be

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removed with the contaminant. In gravity feed systems, such as those used for grains and powders, the product is under free fall and accelerating rapidly. The reject mechanism needs to be as close as possible to the detection system, while at the same time being far enough away that it can react in time to remove the contaminated product, without taking too much good product with it.

4.5

Operational and quality control procedures

In this section we will look at the operational and quality control procedures that must be put in place to ensure that a metal detector provides the expected protection.

4.5.1 Ownership Once a metal detection system has been installed somebody has to take ownership of it. It is a sensitive piece of equipment and needs someone to look after its best interests if it is to provide the payback that the investment warrants. The metal detector must function properly to serve a useful purpose and, as described above, the system is sensitive and subject to any number of outside electro-mechanical influences that could affect its operation. Regular testing should be included in the system’s operation and this should form part of the company’s regular QA and ISO 9000 procedures. Alarms should be included within the system and, as a minimum, some form of supervisory intervention should be required to reset them. There should also be a formal procedure in place in response to a system alarm.

4.5.2 Testing procedures The detection system should be checked at regular intervals, using known test pieces. These test pieces should be certified by the manufacturer to confirm their nature. Three test pieces should be used, with materials in them of a similar quality to those likely to be found as contaminants: one containing brass, one with mild steel and one containing 300 grade stainless steel. Each of these test pieces should go through the system three times at each test, to avoid any statistical anomalies, and each piece should pass each test at every pass. The test system should also include some formalised record keeping. The interval between tests can be determined locally, although some supermarkets will dictate testing intervals if a company is supplying them. The rate of testing will be a compromise. The factors that will determine the interval include the time taken to complete a set of tests and the related disruption to the production line. These must be weighed against the cost of dumping or retesting all of the product that has passed through the detec-

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59

tion system since the last set of tests was passed. There are automatic checking systems available, such as Thermo Electron’s AuditCheck, which will pass a test piece through the detection system at pre-determined intervals and record the results. Alarms are triggered automatically if the results are outside approved parameters, and the system can also check the operation of reject mechanisms.

4.5.3 Reject mechanisms Having invested a significant amount of capital in a metal detection system it would be false economy not to install an appropriate reject mechanism which removes contaminated product from the production process. Ideally, these should be automated and secure. There should be no means by which an employee can remove rejected product and replace it on the production line. One of the QA manager’s worst nightmares is being told by the recording system that there are ten rejected items and finding only five in the rejects bin. There are a number of reject mechanisms that can be used to remove contaminated product from a production line. Some are suitable only for particular types, sizes or weights of product. Although a case has been made for an automated system, manual removal systems are still widely in use. Manual systems can still be in place because of the initial cost of installing an automated system; however, product weight or size, or the travel speed on the production line can make an automated system impractical. Air blast systems Air blast systems are relatively cheap, requiring little more than a solenoid to trigger the mechanism, which has little or no inertia. The system can be set very precisely to remove one item from a production line without disturbing other product. Disadvantages of these systems are that they can be noisy, cannot be used in situations where a product might fly, and need a relatively large entrapment area. With a few exceptions, an air blast system is not able to divert a product of more than 500 g in weight. Automatic diverter arms Automatic diverter arms are available in several forms; the simplest and most common being an arm that normally rests alongside the conveyor and swings across it at 45 ° when a reject product needs to be removed to a bin or another conveyor. This system is relatively cheap and can handle packages up to about 1 kg; however, unless the product is well-spaced, the arm may take several good products with the contaminated one. In some cases friction between the primary conveyor and the product packaging may cause the rejected product to be kicked out the line, rather than sliding smoothly across.

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Kicker Another form of diverter arm is the kicker, an arm which flips across the conveyor to knock the product off the opposite side of the belt, rather than scooping it out of line. As with other diverter arms, this method relies on product spacing, as the arm must move across the conveyor, kick the contaminated product off the belt and return to its initial position before the next product passes, otherwise the result could be damaged products and a blocked production line. Flap systems Flap systems rely on gravity to transport the product past the reject system, with a flap being placed across the path of contaminated product to remove it from its journey to the next part of the production line.With gravity playing a significant part in the transportation of product this type of mechanism can only be used with products that will not suffer significant impact damage. Drop end systems Drop end systems are used where the product needs to be handled carefully to prevent wastage, for example trays of chocolates, or where product size precludes the use of other systems, for example large sacks of powders or granules. A system of this type will use sectional conveyors, one of which will have the facility to drop down, allowing product to continue on another conveyor at the same level or be directed to a conveyor at a different level, where it can be removed for inspection. Sweep arm systems Sweep arm systems comprise a pneumatic arm with a paddle attached to it mounted above and across the conveyor system. On rejection the pneumatic arm extends down to the level of the conveyor, while at the same time travelling across the width of the conveyor. This type of system alleviates the interference issues that can be seen with sweep arm systems, and it can handle almost any weight of product, being limited only by the friction between product and conveyor when it is enough to push the belt off track. Retracting band systems The retracting band system can overcome almost all of the issues that affect the other systems described; however, it is the most mechanically complex to maintain, and the most expensive to install. The retracting band system operates at a point where two separate conveyor belts meet. The final roller of the primary conveyor can be retracted along the length of the conveyor, to shorten it, while a mechanism underneath takes up the slack created by the shortening. The effect is to create a hole between the primary conveyor and the next one along the line, into which the contaminated product will fall. This system is generally only capable of handling belt travel speeds of less than 40 m per minute.

Metal detection Inlet

Metal detector

Reject Outlet

Fig. 4.7

Diverter valve for use in a gravity feed metal detector system.

61

62


Gravity feed systems In drop-through or inclined systems used for handling granular or powder materials, a diverter valve can be used, such as is illustrated in Fig. 4.7. Expert knowledge is needed in producing such a system, to take account of blockage due to bridging and the effects of static build up and discharge. Distance between the detector and the valve will be determined on initial speed of fall of the product, response time of the detector and the operation time of the reject valve.

4.6

Future trends and conclusions

Metal detection has been around since the late 1940s and the principal technology is in its maturity. The introduction of DSP has enhanced the ability to make use of the information generated by metal detection systems, and it is likely that further enhancements and advances will be seen in the application of this technology to a number of specific applications. However, there are unlikely to be any major breakthroughs in the existing technology in the foreseeable future. A number of metal detection companies are looking at the application of X-ray technology, particularly as an alternative to ferrous in foil detection, and the future may see some advances here. The importance of application issues, relating to the environment in which a metal detection system is installed, and of system ownership and QA procedures, cannot be over emphasised. Making a metal detection system work efficiently and effectively requires partnership, between production line workers, the QA team and the maintenance team, as well as between the company as client and the supplier as consultant and expert solutions provider. When making a significant investment of this type, you need to partner with an organisation that not only knows how to build metal detectors, but understands all of the factors involved in providing an accurate and effective metal detection system.

5 Magnets E. Apoussidis and I. Wells, Eriez Magnetics Europe, UK

5.1 Introduction: magnetic separators and the principles of magnetism 5.1.1 Magnetic separators Magnetic separators have been commercially available since about 1890 and have been subsequently applied in a wide range of industries for removing unwanted ferrous material, transporting steel products and separating materials such as minerals on the basis of differences in magnetic properties. Until around 1940 magnetic separators were predominately powered by electromagnetic coils as the permanent magnets available until that time were expensive and not considered sufficiently reliable. Subsequent improvements in permanent magnet technology have resulted in their increasing use to provide the power for magnetic separators. In the food industry, magnetic separators that remove ferrous metal from all stages of production have been used since the 1930s. Magnetic separators used in the food industry are invariably powered by permanent magnets of varying magnetic strength to suit the application. The introduction of rare earth permanent magnets that produce 20 times the magnetic power of conventional standard (ferrite) magnets has extended the use of magnetic separators to the removal of very small particles of iron as well as rust, scale, etc. which are below the detection limit of current metal detection technology. Magnets provide a simple and inexpensive method of removing unwanted ferrous particles from both ingredients and products. At the same time, the removal by magnetic separators of ferrous contaminants reduces potential damage and wear to the subsequent processing machinery.

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Magnets are easy to install and can often be retrofitted with minimal disruption and cost. Operatives need little training to maintain magnetic separators successfully as maintenance is usually limited to regular cleaning and checking of the magnet strength.

5.1.2 The basic principles of magnetism A magnetic field exists as a result of the presence of a permanent magnet or a circuit carrying an electric current. A magnetic force, which can be either attractive or repulsive, is exerted by a magnetic field on a material. A magnet will attract any material with a positive magnetic susceptibility. Whilst most materials have a magnetic susceptibility only a few possess one that is sufficiently high to be attracted to commercially available magnets. The majority of materials that can be attracted to such magnets contain iron in some form and so magnets can be used to attract and remove unwanted ferrous objects from food raw materials, semi-processed materials and finished products. The ferrous materials found in food exhibit different magnetic properties and therefore different magnetic susceptibilities. The magnetic susceptibility of a material is the ratio of the intensity of magnetisation produced in a material to the strength of the magnetic field to which it is subjected. Mild (carbon) steel has a very high magnetic susceptibility and therefore even a small magnetic field will magnetise it. On the other hand, rust and scale (oxides of iron) are of much lower magnetic susceptibility and therefore the magnetic field required to magnetise them is correspondingly higher. Stainless steels, which are a class of steel containing varying amounts of chromium (typically 12–20 %), have a range of magnetic properties depending on the grade. Some stainless steels are magnetic and are attracted to a low intensity magnetic field but the majority are nonmagnetic, and are not attracted even to a high intensity magnetic field in normal circumstances. The magnet force acting upon a ferrous particle is the product of the magnetic field strength and the gradient. A magnetic separator must produce a magnetic field of sufficient strength to magnetise the ferrous particle as well as a magnetic gradient to attract it to the magnet itself through the material being processed. The production of a suitable magnet that provides both sufficient magnetic field to magnetise the particle as well as sufficient magnetic gradient to move the particle to the point of collection is the province of the magnet design engineer. In order to separate unwanted ferrous particles it is not sufficient just to magnetise them as they must be moved out of the flow in which they are contained to a point where they can be collected or removed. In order to induce a magnetised particle to move it is essential to produce a magnetic gradient (in other words the magnetic lines of force density must increase towards the point where they are to be collected) to compel the particles

Magnets

65

to move in the desired direction. The magnetic power must therefore be sufficient to magnetise the ferrous contamination at the furthest point from the magnet that it is likely to occur. Furthermore, it must overcome the mechanical force of the movement of the food material past the magnet and, depending on orientation, the force due to gravity. Because the magnetic power varies as the square of the distance between the magnet and material, there is usually more than sufficient force to attract and retain the ferrous contaminant once it is in close proximity to the magnet. However, as ferrous material accumulates on the magnet the magnetic field is shorted out through the material (as it provides an easy path for the magnetic lines of force) reducing the magnetic power and effectiveness of the magnet. Consequently, it is essential for the magnet to be regularly cleaned to maintain maximum effectiveness.

5.2 Methods of producing magnetic fields: permanent magnets and electromagnets A magnetic field can be produced by one of two methods, i.e., by passing a current through an electromagnetic coil or by means of a permanent magnet. Electromagnetic coils can be wound from any conductor; however, for space as well as cost considerations either copper or aluminium are used in practice.

5.2.1 Permanent magnets Standard (low intensity) permanent magnets are composed of either a cast metal alloy or sintered ferrite which, when magnetised, becomes a permanent magnet. Until the 1950s permanent magnets were composed of metal alloys, usually containing cobalt, which is expensive. However, at this time commercial production of ferrites (sintered barium or strontium ferrite) began which resulted in cheaper permanent magnets that did not lose their initial magnetisation over time unless subjected to mechanical or thermal shock. Ferrite magnets had an added advantage in that they could be sintered as well as cut into a wide variety of shapes. Cast alloy magnets have the advantage that they are more resistant to loss of magnetic power due to elevated temperatures. However, in common with other permanent magnets, they have a Curie point above which there is an irreversible loss of magnetic power. Rare earth (high intensity) permanent magnets have been developed since the 1970s and produce magnetic power of up to 20 times that of the low intensity (ferrite) versions. Initially, rare earth magnets were composed of samarium–cobalt. However, subsequently neodymium–iron– boron magnets were developed that are stronger and less expensive. As production volumes have increased, the cost of neodymium–iron–boron

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Table 5.1 The reduction in magnetic field of permanent magnet materials above ambient (20°C) Permanent magnet material

Reduction in magnetic field

Ceramic (barium or strontium ferrite) Rare earth (samarium–cobalt) Rare earth (neodymium–iron–boron)

0.20 % per °C 0.04 % per °C 0.12 % per °C

Table 5.2 Maximum safe working temperature of permanent magnet material Permanent magnet material

Maximum safe operating temperature

Ceramic (barium or strontium ferrite) Rare earth (samarium–cobalt) Rare earth (neodymium–iron–boron)

350 °C 250 °C 120 °C

magnets has diminished to the point where, for an equivalent magnetic power, the cost is approaching that of ferrite magnets. The strength of all permanent magnets is temperature dependent. The performance of permanent magnet materials decreases with increasing temperature. The reduction in magnetic field above ambient temperature (20 °C) for different permanent magnet materials is tabulated in Table 5.1. For temperatures below about 120 °C there is no significant deterioration of the performance of ferrite magnets. However, rare earth magnets of neodymium–iron–boron composition are not suitable at temperatures above 120 °C. Therefore, for higher magnetic intensity applications at temperatures above 120 °C, samarium–cobalt must be used. The maximum safe working temperature of permanent magnet materials are tabulated in Table 5.2. A magnetic separator must produce a magnetic field of sufficient strength to magnetise the ferrous particle as well as a magnetic gradient to attract it to the magnet itself through the material being processed. The magnet force acting upon a ferrous particle is the product of the magnetic field strength and the gradient. Magnetic separators used in the food industry are normally powered by permanent magnets. For attracting larger ferrous particles such as nuts and bolts, low intensity magnetic fields are sufficient. Ferrite magnets generate the magnetic fields for the low intensity magnetic separators. For attracting small ferrous particles as well as iron oxides and certain stainless steels, a higher intensity magnetic field must be used. This is generated by means of rare earth magnets (neodymium– iron–boron for environments up to 120 °C and samarium–cobalt for higher temperatures). In the food industry, permanent magnets are generally preferred as they are fail-safe (electromagnets lose their magnetic power during an electrical

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67

power interruption and any ferrous particles collected could then be reintroduced into the product). However, electromagnets can be switched off, which makes them easier to clean. Permanent magnets cannot be switched off so the removal of the accumulated unwanted ferrous particles has to be undertaken against full magnetic power. The permanent magnets can be contained within sleeves or fitted with non-magnetic covers to ease the removal of the ferrous material but this reduces the magnetic power due to the introduction of a gap between the magnet and the point of collection. Ferrite (ceramic) permanent magnets Commercially developed in the 1950s, ferrite or ceramic magnets are composed of either barium or strontium ferrite that has been sintered. Magnets of this type are universally available in the form of such items as door catches, refrigerator and freezer door seals and loudspeakers. They produce a magnetic field and gradient that is sufficient for the extraction of a wide variety of unwanted ferrous objects such as ferrous nuts and bolts. However, they do not generate sufficient magnetic power to attract and retain smaller items of ferrous material or oxides of iron such as rust and scale. Rare earth permanent magnets Rare earth magnets are very high strength permanent magnets that have been discovered and developed since the 1970s and which contain elements such as samarium and neodymium, two elements within the rare earth group. They are up to 20 times stronger than standard permanent magnets such as the ferrite magnets used in refrigerator doors. Rare earth permanent magnets were originally developed in order to reduce the size and weight of permanent magnet dc motors in cars, i.e. the motors that operate the windscreen wipers, windows and other such parts. Their very high magnetic strength means that very small and light magnets can be used. Today, they are used in computer disc drive motors, miniature loudspeakers, earphones, magnetic couplings and many other applications, as well as in magnetic separators. There are two basic types of rare earth permanent magnet material, samarium–cobalt, which was developed in the 1970s, and neodymium– iron–boron, which was developed in the 1980s. Samarium–cobalt can operate at higher temperatures than neodymium–iron–boron, but is more expensive. Although rare earth magnets are more expensive than ferrite permanent magnets, their use is justified economically in a wide range of applications as a consequence of the greater magnetic power that they produce. All magnets lose their magnetic strength when they get hot and rare earth magnets are not as good as other permanent magnets at withstanding the effect of high temperatures (see Table 5.2). Rare earth permanent magnets are made by a powder metallurgy process. An alloy of the metal elements is milled to a very small particle

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size. The powder produced is compressed under several tonnes of pressure in a mould. This is followed by sintering in a furnace, heat treatment and, after cooling, surface grinding to the required dimensions. Rare earth permanent magnets allow the production of permanent magnetic separators that are up to 20 times as strong as the corresponding separator powered with ferrite or other types of permanent magnets. This means that more weakly magnetic and/or smaller particles can be captured, thereby improving the removal of contaminants from a product. Rare earth magnets also allow the range of effectiveness of many permanent magnetic separators to be extended to new applications such as abraded stainless steel.

5.2.2 Electromagnets A current passed through a coil of any metallic wire will produce a magnetic field. For space and economic factors either copper or aluminium wire is normally used to wind the coils to provide the magnetic field of electromagnetic separators. The magnetic field and gradient of an electromagnetic coil depend on the number of turns and the coil current. Thus to increase the magnetic power of an electromagnet either the number of coil turns or the coil current (or both) must be increased. To prevent the possibility of short circuits the individual wires of the coils must be insulated from one another and from the mechanical components of the magnetic separator itself. Coils for magnetic separators require dc current and therefore a magnetic separator powered by an electromagnet requires a transformer rectifier. Furthermore, an electrical control panel is required for switching the electromagnet on and off.

5.2.3 Relative merits of permanent magnets and electromagnets Electromagnets can be switched off to release the collected ferrous material, which makes cleaning straightforward. Permanent magnets cannot be switched off, although they can be partially or completely de-magnetised if subjected to a mechanical or thermal shock. Because they cannot be inadvertently switched off they are intrinsically fail-safe. Electromagnets can be accidentally switched off and will lose all their magnetic power during an electrical power supply interruption and consequently are not fail-safe. The performance of electromagnets, over the normal temperature operating range of food processing plants, is unaffected. However, the magnetic power of permanent magnets is reduced at temperatures above ambient (see Table 5.1). Certain permanent magnets can lose a significant portion of their initial magnetic power at elevated temperatures. All permanent magnets will lose all their magnetic power above a temperature known as the Curie point. However, this is above the normal operating temperatures encountered in most food processing plants. In food processing plants, permanent

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magnets are invariably more cost effective than electromagnets and, unless self-cleaning, have no electrical power requirement. Permanent magnets will be affected by vibration or mechanical shock and should not be installed in close proximity to other processing equipment such as vibratory screens or feeders as there will be a gradual and permanent loss of magnetic power.

5.3

Safety and environmental issues

Appropriate warning labels should be applied to the magnets and in the area in which they are installed as a reminder to personnel of the precautions to be taken when working and operating in the proximity of magnetic fields. The original suppliers’ advice should be sought on the disposal of permanent magnets that are either redundant or life expired, as they may present a hazard due to their inherent magnetic force. Magnetic separators fitted with permanent magnets must comply with the EEC’s Electromagnetic Compatibility (EMC) Directive EMC89/336/ EEC. For electromagnets, the supplier should supply information concerning the equipments’ compliance with the EMC Directive. Apart from the potential dangers mentioned below there is no concrete evidence to suggest that exposure to magnetic fields is in any way injurious to health. 5.3.1 Heart pacemakers Magnets may disrupt the action of heart pacemakers, even if they are screened to avoid being influenced by external magnetic fields. Consequently, persons fitted with heart pacemakers should not be allowed within three metres of magnetic fields. 5.3.2 Tools As a magnet will attract any ferrous item within the field, non-ferrous tools should be used for working on and in the vicinity of the magnetic separator. If non-ferrous tools are not available then ferrous tools should be used with caution and kept at a distance of at least 500 mm from the magnet to avoid either damage to the equipment or injury to the operative. As the magnetic force acting on a ferrous object is proportional to its volume the magnetic pull on a tool such as a spanner is considerable and consequently great care should be exercised if ferrous tools are used in close proximity to rare earth permanent magnets. 5.3.3 Magnetically recorded information Magnets will disturb or possibly erase magnetically recorded information such as that contained on hard and floppy discs or on credit and debit cards.

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Discs and cards with magnetically recorded information should be kept at least 500 mm away from magnets to avoid disruption of the information.

5.3.4 Watches Magnets will also disrupt the time keeping of both lever-arch and quartz watches. In the case of lever-arch watches the action becomes magnetised and this can only be restored by means of de-magnetisation. In the case of quartz watches time will be lost during the period of time the watch is within the strong magnetic field although no permanent damage will occur.

5.3.5 Medical metal emplacements There is the possibility that the metal (but not nylon) mesh sometimes used for hernia repairs, the screws, pins and plates and other items used for bone fracture repairs, as well as metal adornments such as earrings and body piercing ornaments may be magnetically susceptible. Consequently personnel with such items should be kept at least 500 mm from magnets as a precaution.

5.3.6 TV-monitors The picture on a TV-monitor will be distorted in the presence of a magnetic field, although this does not permanently harm the unit.

5.3.7 Mobile and radiotelephones Some disruption to reception may be experienced to the operation of mobile and radiotelephones (walkie-talkies) in close proximity to magnets. However, the units will still receive and transmit calls.

5.4

Types of magnetic separator used in the food industry

There are five basic types of magnetic separator that are used in the food industry to remove ferrous foreign bodies; these are tube magnets, plate magnets, pipeline traps, drum separators and magnetic pulleys. Tube and plate magnets and pipeline traps generally require manual cleaning although versions of tube and plate magnets are available with automatic cleaning. Magnetic drum separators and pulleys are self-cleaning. As the manufacturers of magnetic separators produce this equipment for a wide variety of applications it is imperative for installations in food processing plants that suitable construction materials are specified.

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5.4.1 Tube magnets A tube magnet is constructed of cylindrical permanent magnets contained within a thin stainless steel tube, generally 25 mm diameter. The length of the tube is usually manufactured to suit the specific location. The tube magnet is either installed singly or in multiples within the process, or built into grates, through which the food material flows.A typical grate composed of tube magnets is illustrated in Fig. 5.1. The grates themselves can be built

(a)

(b)

Fig. 5.1 Grates composed of rare earth tube magnets: (a) model ‘P’ for hopper installation; (b) model ‘GH’ for fitting into ducting/chutework.

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into drawer type assemblies for manual removal to facilitate cleaning or can be arranged for automatic removal actuated pneumatically. For powders with poor flow characteristics, e.g. ground spices, there is the risk that the material might bridge across adjacent tubes thereby causing a blockage. The spacing between the tubes for this type of application should be increased and to compensate for the reduced reach additional rows of magnets should be staggered to increase the probability of capturing the ferrous contamination. Some manufacturers advocate vibration of the tubes to aid the flow of food material through the magnetic system. However, vibration will cause a slow and irreversible loss of the magnetic power and is only recommended if there is no other option; in these circumstances the strength of the magnets should be regularly monitored and the tubes replaced once 20 % of the power has been lost. For higher extraction of ferrous contamination from free-flowing materials, the distance between the tubes can be reduced to increase the probability of removal. Self-cleaning tube assemblies should be considered for hard-to-reach or inaccessible locations and for operations that are continuous and therefore cannot be interrupted for cleaning.

5.4.2 Plate magnets A plate magnet is a rectangular box that contains permanent magnets. Plate magnets are installed above conveyors or in the base of chutes, ducts or pipelines etc. An example of such a magnet is given in Fig. 5.2 (a and b). For the extraction of larger pieces of tramp iron, the plate magnet is constructed from ferrite magnets. Higher strength versions, utilising rare earth magnets, are used for extracting smaller ferrous items or when the depth of material is high. For installations at heights of more than about 150 mm suspension magnets (larger versions of plate magnets) should be used. Plate magnets are available in a wide range of sizes and magnetic strengths to suit different installations. They can be used in sequence to increase the probability of ferrous contaminant capture as well as to increase the interval between cleaning.

5.4.3 Pipeline traps For liquids such as juices, soups, sauces and gravies that are transported in pipelines, magnetic traps containing tube magnets that protrude transversally to the direction of flow are used. The pipeline traps are manufactured in different sizes to suit pipes of different diameter. In Fig. 5.3 a pipeline trap for use with honey is shown. The tube magnets are mounted on a plate so that they can be removed for cleaning. The design of the magnetic tubes should be such that the collected ferrous contamination is held on the downstream side of the tubes preventing the particles being washed off back into the product stream. The design of the housing of the pipeline trap

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(a)

(b)

Fig. 5.2

Plate magnets.

should encourage the flow of material equally around the magnetic tubes without restricting the flow. It is recommended that the pipeline trap is orientated so that the magnets are removed for cleaning above the unit, which avoids spillage of the product. For pipelines carrying liquids that must be maintained above a certain temperature, such as chocolate and honey, a hot water or steam-jacketed trap is essential to prevent solidification of the food within the magnetic trap. Some fluids, such as the tomato sauce for baked beans, are highly corrosive (either due to the salinity or pH or both) and for these applications the trap must be made of appropriate material.

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Fig. 5.3

A rare earth pipeline trap with a hot water jacket.

Liquids that contain a significant amount of solid matter, such as certain soups and meat stews, require a pipeline magnet without any impediment to the flow. This is arranged by replacing the tube magnets with a plate magnet on the outside of the trap, which is incorporated into the body of the trap. To maintain the magnetic effectiveness of such traps the depth of fluid must be reduced by altering the cross-section from circular to rectangular. To increase the probability of capture of ferrous contamination and reduce the frequency of cleaning the traps are often arranged in series. Parallel lines with valves are used when the flow must be continuous for production purposes so that one line can be cleaned whilst the other is in use. The pressure drop across a typical pipeline trap is equivalent to that of a 90 ° bend.

5.4.4 Drum magnetic separators A magnetic drum separator comprises a rotating drum that contains a fixed permanent magnet unit inside, extending approximately 180 ° around the periphery (from top dead centre). Material to be treated is fed by means of a short chute or vibratory feeder to the top of the drum; iron is attracted to the magnet unit and held to the drum as it rotates to the point where the magnetic field ends, approximately 180 ° from the entry point. The material itself is thrown from the drum by the centrifugal action imparted to it by its rotation. A divider or splitter is installed beneath the magnetic drum to divert the product and ferrous contamination into separate chutes or receptacles. The principles of operation are illustrated in Fig. 5.4.

Magnets

Door clamp

75

Hopper

Access and inspection opening Product Adjustable deflector to regulate product flow Shell revolves around fixed magnetic field Door clamp

Drum direction of rotation Drum replacement access panel

Stationary permanent magnetic element

Access and inspection opening Adjustable splitter

Non-magnetic discharge

Fig. 5.4

Removable discharge chute fixing bolts and skirt Magnetic discharge

The operating principles of a magnetic drum magnetic separator.

Magnetic drum separators are used to extract ferrous contamination from dry free flowing ingredients or products such as grain, tea, rice and sugar. For larger items of ferrous contaminants, the magnetic drum separator is built with ferrite magnets. For smaller ferrous particles and iron oxides, etc., higher intensity variants utilising rare earth permanent magnets should be installed. For dusty materials such as tea the magnetic drum should have a dust-tight housing with dust extraction ports. As magnetic drums require an electrical motor to drive the unit this should be outboard of the housing to avoid the possibility of grease, oil or other potential contaminants entering the product.

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Detecting foreign bodies in food Tramp-iron contaminated material

Magnetic pulley

Tramp-iron

Fig. 5.5

Cleaned material

The operating principles of a magnetic pulley for installation at the head of a conveyor.

5.4.5 Magnetic pulleys A magnetic pulley comprises a pulley on a shaft to which permanent magnets have been fixed and surround the entire periphery. The magnets therefore rotate with the pulley which is not the case in the drum magnetic separator. The magnetic pulley is fitted at the head of a conveyor moving a food ingredient or product. As the material approaches the magnetic pulley any items of ferrous contamination are attracted to it. As the magnetic pulley rotates the material is thrown from the conveyor belt by centrifugal action whilst the ferrous contamination continues to be attracted and held to the pulley as shown in Fig. 5.5. At the point approximately 180 ° from top dead centre, where the belt pulls away from the magnetic pulley, the magnetic force begins to diminish. Once the attractive force is no longer sufficient to overcome gravity the ferrous contamination falls from the belt. A divider or splitter is installed beneath the magnetic pulley to divert the product and ferrous contamination into separate chutes or receptacles.

5.5

Factors affecting the use of magnets in food processing

In general, magnetic separators can be used for food ingredients that are dry and free-flowing or in the form of fluids. For materials that are damp the effectiveness of a magnet will be reduced and the advice of a magnet separator manufacturer should be sought. Normally magnetic separators will not extract ferrous contamination when the metal is embedded or contained within a food product such as bread, cake or biscuits or from

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packaged, canned or bottled products. For applications where the amount or incidence of ferrous contamination is relatively low magnets that are manually cleaned such as tube magnets, plate magnets and pipeline traps are suitable. Where the volumes of material or amount of anticipated ferrous contamination is higher, self-cleaning systems should be installed. Also, where the intended location of the magnetic separator is inaccessible or the process is continuous a self-cleaning system should be considered. For dry free-flowing materials, magnetic drums, that can be installed within the process line, are ideally suited. Magnetic pulleys can be installed at the head of conveyors. When the amount of ferrous contamination is relatively low self-cleaning rods actuated pneumatically by pre-set timers can be used.

5.5.1 Guidelines for the selection and application of magnets In general, the following guidelines should be followed. However, for specific applications it is advisable to seek the manufacturers’ advice for guidance in the siting and selection of appropriate magnetic separators. Location The food processing line should be surveyed and locations where ferrous material could be introduced identified. It is important to remove ferrous metal from raw materials and after operations where it is likely to be introduced such as cutting, pulverisation, screening, chopping, crushing and flaking where fragments of the equipment could break off and be introduced into the product. Consideration must also be given to the location regarding the speed of flow of the material, depth of material and other such factors, as this will influence the type, size and cost of the magnet required to provide suitable protection as well as accessibility for cleaning. The site where raw materials are received and/or unpacked is a potential source of ferrous contamination. If the raw materials regularly contain ferrous materials then the supplier needs to be told so that they can take the appropriate action to reduce the amount of ferrous material entering the process. Magnets should precede metal detectors to reduce the number of stoppages induced by the metal detectors. If the product is free flowing such as sugar, tea, grain or breakfast cereals, magnets should be deployed immediately before packing, and in conjunction with a metal detector to eliminate ferrous contaminants prior to despatch. Plate magnets These are usually installed either in the base of chutes and ducts and in contact with the food material or suspended above appliances such as conveyors, that carry the material. The width of the magnet should be matched to that of the chute, etc. The depth of material being carried on the

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conveyor determines the height at which the plate magnet should be installed; the greater the depth of material the stronger the plate magnet must be to extract the unwanted ferrous particle. It must be remembered that a ferrous object is easier to extract from the top of the burden on the belt than through the material and thus the depth also determines the strength of the plate magnet. When installed in the flow the face of the plate magnet should be stepped to increase the retention of spherical contaminants and enhance the extraction of finer particles. A stepped plate provides a higher barrier so that spherical and finer contaminants are not carried by the flow of material. Plate magnets should be hinged when base mounted for ease of cleaning. Tube magnets Tube magnets are normally installed within the actual flow of material. The tubes are usually spaced 50 mm apart (centreline to centreline) to cover the full width of the duct, etc. When installing double, triple or more banks, the tubes should be offset to encourage the material to cascade, thereby increasing the probability of capturing ferrous contaminants. Ferrite magnets should be selected for larger ferrous items and the strongest rare earth magnets for extracting finer particles. It is possible to use ferrite magnets for the first bank to remove larger and more magnetic contaminants and rare earth magnets for subsequent banks to remove smaller less magnetic particles. For dry materials with poor flow characteristics the spacing of adjacent tubes must be increased to reduce the risk of the material bridging. Additional rows of tubes are required to compensate for the reduced reach of the tubes. Pipeline traps A pipeline trap of the same or larger diameter than the pipe into which it is to be fitted, should be selected. For fluids such as chocolate and honey, which are viscous at ambient temperatures, the trap should be fitted with an external steam or hot water jacket to prevent the material solidifying or becoming so viscous that a blockage within the trap occurs. For lumpy materials such as canned meat products, traps with plate magnets must be used to avoid material bridging across the tubes and causing a restriction to the flow or a blockage. Magnetic drums Magnetic drums are suitable for dry free-flowing materials such as grain, rice, granulated sugar and tea. Materials that are either damp (and sticky) or have a significant fine particle (50 mm) content are generally not suitable for this type of separator. A magnetic drum has a relatively high capacity that depends on the bulk density of the material being processed and is therefore selected by reference to the volumetric capacity per unit width of drum. For 310 mm diameter drums, the typical capacity is 100 m3/h m-1 width

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of drum when extracting larger ferrous contaminants such as nuts and bolts and 15 m3/h m-1 for fine ferrous contaminants (a smaller depth on the drum surface ensures that the contaminants in the upper portion of the burden are subjected to a stronger magnetic force). A magnetic drum is normally enclosed in a housing that incorporates an inlet for the material to be processed and outlets for the de-ferrised product and the ferrous contamination. The inlet and outlets should be flanged so that dust-tight connections can be made to the process line. If the material is dusty dust ports need to be incorporated in the housing to facilitate the removal of any airborne dust. For material that has good flow characteristics a simple inclined chute can be used to deliver the product to the separator. For products where the rate of flow needs to be regulated or the flow characteristics are not so good a vibratory feeder should be used. As a consequence of the dimensions of magnetic drums it is not so easy to retrofit them into pre-existing lines. Magnetic pulleys Magnetic pulleys have the advantage in that they can be retrofitted wherever there is an existing conveyor system and require no additional headroom etc. However, they will only remove relatively large pieces of ferrous contamination such as nuts and bolts. The effectiveness of a magnetic pulley is reduced as the depth of material being conveyed is increased, because the ferrous contamination on the top of the burden has to be attracted over a greater distance.

5.5.2 The types, size and shape of adventitious metal that can be removed from food by magnets The sensitivity of magnetic separation depends upon the magnetic susceptibility of the particle, its volume, the shape of the particle, consistency of the material from which it has to be extracted, the speed of flow of the material through the separator and gravity (this component can be positive or negative depending on the location and orientation of the magnetic separator). Additional factors for liquids are the viscosity of the fluid and hydraulic forces. The more free-flowing the material the lower the magnetic force required to extract unwanted ferrous items. If the material has a significant fines content or comprises entangled fibres or is damp then a higher magnetic force is required to overcome the mechanical forces within the material. The higher the rate of flow of material past the magnet the greater must the magnetic force be to arrest the particle and attract it to the collection point. The consistency of the bulk material acts like a drag force on a magnetically susceptible particle, increasing the force required to extract it. For fluids the hydraulic and viscous forces within the material will tend to counteract the magnetic force.

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Types of ferrous particles that can be removed by a magnetic separator The relative magnetic susceptibility of ferrous materials in descending order is as follows: mild steel, magnetic stainless steel, rust, scale, abraded non-magnetic stainless steel. There are various types of stainless steel, most of which are non-magnetic. The majority of non-magnetic stainless steels become magnetic when work hardened. The action of food material passing over stainless steel to create wear in the form of shavings etc. work hardens the stainless steel making it magnetically susceptible. However, as the magnetic susceptibility induced by work hardening is relatively low, rare earth magnets are required to extract abraded stainless steel particles. Size of particle that can be extracted The magnetic force acting on a particle is dependent on the volume – the greater the volume the higher the number of lines of force that can pass through it. Therefore, a larger particle can be extracted more easily than can a smaller particle of the same magnetic susceptibility. The effect of particle shape on magnetic separation The shape of a ferrous contaminant is a factor in the attractive force. For particles where one dimension is much smaller than another, such as a wire, the attractive force is higher. For a sphere of the same mass the attractive force is much lower. Thus the most easily shaped particle for a magnetic separator to extract is a wire, whilst the most difficult is a sphere. 5.5.3 Assessing the appropriate location for magnet installation A full survey of the process line should be carried out to pinpoint locations where ferrous contamination could be introduced. This survey should include the feasibility of installing magnets at the specific locations and the ease of cleaning if the magnets are to be cleaned manually. The survey should also cover ingredients delivered by suppliers to ensure they are free from ferrous contamination. A follow-up survey by a magnetic separator supplier should be undertaken to confirm the findings of the initial in-house survey. 5.5.4 Methods and frequency of cleaning magnets and collecting metal for analysis For the magnetic separators that require manual cleaning, operatives must be trained to inspect and clean them on a regular basis. The method of cleaning depends on the mode of installation and frequency of cleaning. It is imperative to carry this out away from the production line to prevent re-contamination of the product. Methods of cleaning magnets (not self-cleaning) The magnetic force of permanent magnets must be overcome to remove the collected ferrous particles. This can be achieved by a number of differ-

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81

ent methods. Larger ferrous items such as nuts, bolts and wire can be pulled from the magnet manually. For smaller particles, such as rust and scale, the ferrous material can be brushed or wiped to the end or edge of the magnet and dislodged into a bag, which should be labelled as to the source, date and time. Cleaning should be conducted without subjecting the magnets either to a mechanical (e.g. hitting with a hammer) or thermal shock (cleaning with boiling water) as this could lead to a gradual loss of magnetic power. For tube magnets a cloth can be drawn down the tube to remove the accumulated ferrous contaminants. Thin stainless steel sleeves can be fitted over the tubes to aid cleaning although this will reduce the magnetic power. Frequency of cleaning of magnets The frequency of cleaning depends upon the amount of ferrous contamination and operational requirements. Cleaning can be conveniently scheduled between batches or at daily or weekly intervals. However, if the amount of ferrous contamination is relatively high then either the frequency of cleaning must be increased or a self-cleaning separator installed. As ferrous material builds up on a static magnet the magnetic poles are gradually shorted out which reduces the efficiency of extraction. If a large amount of ferrous material is allowed to accumulate on a magnet the ferrous material furthest from the magnet will not be subjected to sufficient magnetic force and will therefore not be held and the magnet will become ineffective. However, this phenomenon can be used to provide a signal that the magnet requires cleaning. As the accumulating ferrous material shorts the poles out the magnetic field at a certain point will diminish. If a probe that measures magnetic field is introduced at this point it can be set to provide an alarm signal at a pre-set reduction to warn the operatives to clean the magnets. Collection and analysis of ferrous contamination The ferrous material collected by the magnet should be placed in bags and labelled with the exact location, date and time. The ferrous material collected should be weighed and the amount recorded so that trends etc. can be detected. For example, an increasing amount of ferrous contamination may indicate that increased wear of preceding equipment is taking place or that the raw material contains more ferrous components. In either event this should be investigated and worn equipment replaced or the suppliers of sub-standard ingredients contacted to improve their metal removal and/or detection. For non-self-cleaning magnetic separators, records should be maintained of the frequency of cleaning with the amount and nature of ferrous material removed annotated. In the case of self-cleaning types, the same information should be recorded but at pre-determined time intervals.

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Calibration and testing of magnets It is recommended that the field strength of a magnetic separator be checked regularly (annually or bi-annually, more frequently if the magnets are subject to thermal or mechanical shock) using a commercial gaussmeter (an instrument for measuring magnetic fields), fitted with a Hall-effect probe. The gaussmeter should be routinely calibrated using a reference magnet. A simple hand-held gaussmeter suitable for use in the food industry will cost approximately £300 and provide an absolute measure of the magnetic field. Reputable magnetic separator manufacturers offer calibration and testing as part of their after-sales service. An alternative method of checking the approximate strength of the magnets is to use a pull-test kit that measures the force required to detach a ball-bearing from the face of the magnet. The cost of the equipment is approximately £30 and can be obtained from the suppliers of magnetic separators.

5.5.5 Limitations to the use of magnets in food processing Magnets will only remove magnetically susceptible material. Consequently, they are suitable for most ferrous contaminants but cannot be used for nonferrous materials such as brass and phosphor bronze. The power of the magnet will determine what type of contamination is removed. Low intensity magnets will be expected to remove larger ferrous objects such as wire ties, nails and screws. The higher power of rare earth permanent magnets will remove smaller sized objects as well as scale and rust. Magnets are not as effective at removing spherical shaped objects. They cannot remove ferrous objects that are imbedded in processed foods such as biscuits, cakes and bread or from packaged, canned or bottled products.

5.6

Examples of magnet use for particular foods

5.6.1 Chopped nuts A UK manufacturer supplies cut nuts (peanuts, almonds, hazelnuts and walnuts) to the producers of breakfast cereals and confectionery. On delivery, the nuts are stored in a silo and fed onto an inspection conveyor table prior to cutting. Before inspection, the nuts pass under a plate magnet to remove any ferrous metal, such as pallet nails, which might come in with the delivery. This ensures that vagrant metal pieces do not damage the razor-sharp knives used to cut the nuts, thereby improving cut nut quality and reducing maintenance costs. After being chopped, the nuts are fed into a hopper prior to final packaging. A single bank of rare earth tube magnets is installed at the outlet of the hopper to remove any fragments of broken chopping blades. Prior to the installation of the tube magnets, stones in the

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raw materials regularly damaged the blades and the batch of nuts often had to be quarantined until all the fragments had been accounted for.

5.6.2 Chocolate In the processing of the ingredients for chocolate ferrous particles are inadvertently introduced due to the wear of the processing equipment. After processing, the chocolate is stored in reception tanks before being pumped through pipework to the final production areas. The introduction of tube magnets powered by rare earth permanent magnets immediately prior to the final processing area eliminates any possibility of the products containing iron or iron oxide during the manufacturing processes. Installation of the magnetic separation system prior to pre-existing metal detectors reduces the number of production line stoppages and dramatically reduces product wastage.

5.6.3 Cereals In the production of breakfast cereals such as muesli ferrous contamination can occur in the raw ingredients and can also be introduced through wear of the processing equipment. Plate magnets are installed above the raw material inspection lines to remove any ferrous contaminants prior to entering the process area. This avoids any ferrous contamination from damaging the processing equipment or causing unnecessary wear. If the ferrous particles are allowed to remain they are likely to be reduced in size by the processing or become embedded in the product making it much more difficult to remove or detect. Prior to packing, the product mix is passed through double bank rare earth powered magnetic tube magnets to remove any ferrous contaminants introduced during processing. These final magnets precede the metal detectors to reduce detector-induced stoppages and product wastage due to the divert systems.

5.6.4 Flour The wheat for flour is flaked using cast-steel rollers, which wear away due to the abrasive nature of the process. The resulting metal fragments fall into the resultant flaked wheat. Rare earth tube magnets are installed at the output from the roller to prevent damage to the subsequent process machinery. Flour is often delivered in bulk by road tankers and stored in silos from which the flour is fed to automatic weighers prior to mixing. Either pipeline or special bullet magnetic separators are fitted in the pneumatic feed line to the bulk flour storage silos. Any metal contamination present will therefore be removed at the point of delivery ensuring that none enters the production line prior to processing. This protects against any metal inflicted damage to the in-process machinery.

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In the UK there is a requirement for the flour used for bread making to contain iron for nutritional purposes. This iron is sometimes introduced in the form of iron filings that have a high magnetic susceptibility. Consequently, the siting of magnetic separators should precede the introduction of the nutritional iron. Even soluble iron that is sometimes introduced in the form of ferrous sulphate, to meet the nutritional requirement, is magnetically susceptible although it requires the strength of a rare earth permanent magnet to extract it.

5.7

Advantages and disadvantages of using magnets

The advantages of (permanent) magnets are that they are a fail-safe method of removing a wide range of ferrous contaminants. Magnetic separators are relatively inexpensive; a modest tube magnet bank costs between £200 and £1000, whilst a typical magnetic drum costs less than £8000. They are lowtech so require very little operator skill to install, operate, clean and maintain. The ferrous material is positively removed from the processed material and can be collected and analysed to determine its source, which means that steps can be taken to reduce the incidence of ferrous metal ingress.

5.7.1 Relative merits of magnets and the interrelation with metal detection In general, magnets utilised in the food industry are suitable for the removal of ferrous objects that range from several centimetres to a few microns in size. However, they will not remove non-ferrous metals and therefore magnets should always be used in conjunction with metal detectors. Metal detectors and magnets are complementary in many respects. As previously mentioned the shape that is most easily removed by a magnet is one in which one dimension is greater than the others such as a wire or bristle. Conversely, objects of this shape are the most difficult to detect. For metal detectors an object the shape of a sphere is the easiest to detect whilst the most difficult to extract magnetically. The lower size limit for a metal detector is generally about 1–3 mm (depending on moisture content) whereas a magnet can remove particles as small as a few microns. An advantage of a magnet system is that the ferrous objects are easily collected for identification to pinpoint the source so that, if appropriate, remedial action can be initiated. Another advantage is that there is very little wastage of product when using a magnet, whereas diverter systems used in conjunction with metal detectors can lead to significant losses of product and add to disposal costs. Metal detection induced line stoppages due to alarms for metal can disrupt production. These can be minimised by using magnets prior to the metal detector to remove the ferrous objects and thereby reduce the frequency of detector induced stoppages or the amount

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of product wastage due to product diversion. A further advantage is the reduced maintenance costs brought about by removing metal before the product passes through crushers, pulverisers, filters, sieves etc., increasing their life and reducing blinding of sieves and screens.

5.8

Future trends

In the next few years, more powerful permanent magnets are likely to be developed by metallurgists and become commercially available. This will result in the availability of even more powerful magnetic separators. Such magnets will offer the prospect of the ability to remove even smaller ferrous particles or less magnetic particles or the reduction in size of the magnetic separators to provide the same magnetic power. It is possible that the use of alternating magnetic field separators (based upon the eddy current principle), which are able to deflect metallic particles irrespective of composition will be extended to particles smaller than 3 mm and find use in removing both ferrous and non-ferrous foreign bodies from food. For electro-magnets it is unlikely that any developments will occur that will affect either their current usage or application within the food industry.Whilst superconducting magnets, which require no electrical power after the initial energisation, have been developed and are now commercially available, the capital and operating costs render them uneconomic for food industry applications.

5.9


In the absence of much published data pertaining to the application of magnets to the processing of food, the principal sources of guidance to their application will be suppliers of magnetic separation systems, local details of which can be accessed from the world wide web (by entering ‘magnetic separators’ in a search engine) or trade directories. However, the following two publications contain sections on magnets. Wallin P and Haycock P (1998), Foreign body prevention, detection and control: a practical approach, Culinary and Hospitality Industry Publication Services. Campden and Chorleywood Food Research Association (1995), Guidelines for the prevention and control of foreign bodies in foods, Guideline No.5. CCFRA Publications.

6 Optical sorting systems S. C. Bee and M. J. Honeywood, Sortex Ltd, UK

6.1

Introduction

Good food can usually be distinguished from bad by colour. This may appear to be an obvious statement, but the implications for the food industry are significant. Human perception of colour has proved very effective in determining food quality. Sorting of food products using the human eye and hand is still widely practised in regions where labour rates remain low. However, where the cost of labour has increased, so automated techniques have been introduced. As a consequence of increasing consumer awareness of food hygiene, it has now become a basic prerequisite for all optical sorting machines to identify and remove all gross contaminants (glass, stones, insects, rotten product, extraneous vegetable matter etc). In addition, optical sorting provides a cosmetic enhancement to the product by removal of blemished, discoloured and mis-shapen product. Contemporary consumers are also demanding increased quality and in conjunction with this, a litigation culture has developed. Especially in recent years, tighter EU and American food and drug administration requirements on food quality have been implemented. Food processors benefit from using automated systems for food sorting, since a machine can maintain greater levels of consistency than hand sorting and frequently offers reduced labour costs. Food processors are able to provide a premium quality product at increased margins, allowing their competitive positioning to be strengthened.

Optical sorting systems

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Hopper Vibrator tray

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Schematic layout of a typical optical sorting machine.

The principal components of optical sorting systems

Colour sorters generally consist of four principal systems: the feed system; optical system; ejection process and image processing algorithms. Figure 6.1 illustrates a typical layout for an optical sorting machine.

6.2.1 The feed system In a bulk sorting system, dry products (rice, coffee, nuts) are fed from a vibrating hopper onto a flat, or channelled, gravity chute. To prevent excessive clumping, fresh or frozen products are fed from a vibrating hopper onto an accelerating belt. Both methods separate the product into a uniform ‘curtain’, or monolayer. This ensures the product is then presented to the optical system at constant velocity.

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6.2.2 The optical system The optical inspection system measures the reflectivity of each item. The inspection components are housed within an optical box and the objects under inspection travel either through, or past, the optical box. Objects should not come into direct contact with any part of the optical box and are separated from it by toughened glass windows. The optical box contains one or more lenses and detector units, depending on the number of directions from which the product is viewed. Early optical sorting machines viewed the product from only one side, which meant that they could only detect surface defects facing the optical system. Today, two or three cameras are used to view the product from different angles as it leaves the end of the chute. Obviously, this increases the efficiency at which the system can identify defects. Lamp units, designed to provide even and consistent illumination of particles, are also usually contained within the optical box.

6.2.3 The ejection system The ejection system must be capable of physically removing unwanted product items from the main accept stream. The ejection process typically takes place while the product is in free fall; accept particles are allowed to continue along their normal trajectory, and rejects are deflected into a receptacle. Deflection is usually achieved by emitting short bursts of compressed air through nozzles aimed directly at the rejects, although large or heavy objects (e.g. whole potatoes) may require some sort of pistonoperated device to deflect the rejects mechanically.

6.2.4 The image processing algorithms The image processing system classifies particles as either ‘accept’ or ‘reject’ on the basis of colour, or both colour and shape. Previously, users were restricted to just one threshold setting for all defect types, be they small spot defects, or larger area colour defects. Sortex’s engineers have recently developed new vision algorithms, which for the first time allow users to set two separate defect thresholds, permitting a far more flexible and application-specific sorting solution. Users can now specify just how far from the average product colour that whole area colour defects can deviate, before they are rejected. This allows precision removal of even the subtlest of colour defects. Similarly, spot defects tend to be very different in colour from the average product colour, but are only present in a few pixels at a time. By setting a separate threshold setting for spot defects, users can now remove grains with fine bran streaks, or tiny peck defects. By setting these two sorting thresholds, users can now easily implement a simultaneous, precision sort, for both small spot and large area colour defects. State of the art electronics hardware is used to implement these new algorithms, initiating new standards in optical resolution.


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6.2.5 Sorting criteria The size, cost and complexity of sorting machines varies, depending on the size range of particles to be handled, the throughput requirement and the complexity of optical measurement. Machines are employed in sorting particles as small as mustard seeds but rice grains are among the smallest particles to be sorted on a large commercial scale. At the other end of the size range are fresh and frozen vegetables (peas, green beans, cauliflower florets etc.) and fruit such as apples or oranges. Seeds are usually sorted on a single- or double-chute machine at a throughput of 60 to 600 kg/hour. A higher throughput can be achieved on a multi-chute or a conveyor belt machine; rice at 16 tonnes/hour (320 000 objects/sec) and peas at up to 16 tonnes/hour are typical examples. The products that can be handled by today’s automatic sorting machines include seeds, coffee, rice, breakfast cereals, nuts and pulses; fresh, frozen and dehydrated vegetables; cherries (with and without stalks); olives; tomatoes; prawns; biscuits and confectionery. Foreign material such as stones, sticks and organic matter can be removed, as well as objects with defects such as discoloration and damaged skin. Figures 6.2 and 6.3 show two typical sorting machines. Spectrophotometry To determine whether a particulate food product is suitable for colour sorting, and which type of sorting machine and optical configuration is most

Fig. 6.2

Sortex Z-Series monochromatic machine, for rice sorting.

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Fig. 6.3

Sortex’s Niagara machine sorting frozen peas at 8 tonnes per hour.

suitable, samples of both acceptable and unacceptable produce must be measured and assessed in the laboratory. The term ‘colour sorting’ arises from the effect on the overall product appearance as a result of optical sorting. Unfortunately, the term is misleading. The criterion the sorting machine measures when it inspects the product is spectral reflectivity at particular wavelengths, rather than the colour as a whole. Figure 6.4 illustrates typical spectral curves obtained from white rice and white rice grains with yellow colour defects. The relative reflectance signal varies from black (zero and therefore no reflectance) to white (100 % reflectance). The wavelengths cover the visible spectrum (400 to 700 nm) and extend into the near infrared (700 to 1100 nm). Optical sorting exploits the region of the spectrum where the reflectance values for all acceptable product are either higher or lower than values for all unacceptable material. If this feature is present, then with the aid of band-pass optical filters, this part of the spectrum can be used as a basis for optical sorting. An optical band-pass filter allows a narrow ‘band’ of optical wavelengths to pass through it (see Section 6.3.3 Optical filters). Conventional spectrophotometry involves the measurement of carefully prepared surfaces under controlled optical conditions and illumination. However, practical industrial, bulk-sorting machines must deal with naturally occurring surfaces, viewed under non-ideal illumination conditions. Computer-controlled reflection spectrophotometers are now widely avail-


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Visible reflectance spectra for white rice. In this example, a blue band-pass filter would be used for monochromatic sorting.

able and enable measurement of the appropriate optical properties of naturally occurring surfaces. Diffuse spherical broadband lighting is used to illuminate uniformly the item under test. The reflected light is then passed through a computer-controlled scanning monochromator, which splits the light into its constituent wavelengths. The output is measured using a suitable detector and sent to the computer. When the equipment is appropriately calibrated, the results can be plotted showing the variation of reflectance (or transmission) with wavelength, for both acceptable and defective product. Monochromatic sorting Monochromatic sorting is based on the measurement of reflectance at a single isolated band of wavelengths. For optical sorting to be effective, there must be a distinct difference in reflectance within the selected waveband between all the acceptable particles and all the reject particles (see Fig. 6.4). Removal of dark, rotten items from product like peanuts, or dried peas and removal of black peck from rice are typical applications of monochromatic sorting. Figure 6.5 shows some typical defects in rice. Bichromatic sorting Sometimes it is not possible to find a single section of the reflectance spectrum where the intensity levels of accept and reject material are clearly separated. Therefore, it becomes necessary to compare simultaneous

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Fig. 6.5

Defects in rice. From top left across to right: black specs, discoloured, yellow, red, bran streaks, immature and chalky.

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Fig. 6.6 Spectral curves for green Arabica coffee. Bichromatic sorting is necessary, since there is no one region of the spectrum where reject material can be successfully separated from the accept material.

measurements at two different wavelength bands; a technique called bichromatic sorting. Figure 6.6 shows two sets of spectral reflectance curves obtained from green arabica coffee. One set of data (solid lines) represents the lightest and darkest of acceptable beans, the other (dotted lines) represents the lightest and darkest of discoloured beans. In this case, no region of the spectrum allows successful separation of the two sets of curves. However,


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between 500 nm and 600 nm, the difference in the gradients of the two sets of curves is at its greatest. This is repeated between 800 nm and 900 nm. If measurement A is taken at 540 nm and measurement B is taken at 850 nm, the ratio A : B can be calculated. This ratiometric approach will yield a distinct signal difference between the ‘accept’ and ‘reject’ reflectance spectra and allow effective optical sorting. (In principle, measurement A could be taken at 510 nm, but this would give a lower signal intensity.) By measuring at two, rather than one, region of band-pass wavelengths, bichromatic sorting involves twice as many optical components. At each optical inspection point, besides simply duplicating many of the optical components (e.g. filters, lenses), additional light-splitting devices and more complex signal processing are also required. Consequently, bichromatic sorting is only used when a simple monochromatic measurement is not adequate for effective optical sorting. Figures 6.6 to 6.12 illustrate various defects that can be removed from coffee using bichromatic colour sorting techniques. Dual monochromatic sorting Dual monochromatic sorting is similar to bichromatic, in that two wavebands are measured but instead of a ratiometric approach, the dual system sorts monochromatically in each of two separate wavebands. This type of sorting is used when it is necessary to reject either two distinct types of defect or defects and foreign material, each of which exhibit different

Fig. 6.7

Amber (yellow) defects – immature berry.

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Fig. 6.8

Fig. 6.9

Brown defects – over-ripe or fermented.

Reddish (‘foxy’) – damage to cherry, or due to staining from liquor.


Fig. 6.10

Fig. 6.11

Black defects (whole or partial) – usually over-ripe.

Brocca/insect bites – antesia bug or coffee bean borer.

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Fig. 6.12

Stones/sticks/foreign material.

spectral characteristics. Dual monochromatic sorting is employed with white beans. Maize is rejected by detecting blue reflectance, and white stones are rejected using near infrared. Some optical sorting applications require both monochromatic and bichromatic decisions to achieve a successful sort. Therefore, bulk optical sorting machines are available which are capable of making both types of measurement simultaneously. Trichromatic sorting Bichromatic sorting techniques can obviously be extended to trichromatic applications. The information gained from the third band-pass filter is often used for detection of gross defects such as the presence of foreign material including glass, stones, thistle heads, caterpillars, insects and mice! Trichromatic sorting almost always uses either green, red and infrared band pass wavelengths. It is unusual for the food industry to use the traditional machine vision community choice of red, green and blue. Trichromatic sorting allows objects to be sorted according to their size or shape, by suitable modifications to the sorting algorithm. In this way, objects of the same colour but different shapes can be separated. For example, pea pods can be distinguished from peas and green stalks, or green caterpillars from green beans. Under- or over-sized objects along with misshaped objects, with holes or cracks can also be detected and effectively removed. The Sortex Niagara machine is capable of simultaneously sorting for both colour and shape at a rate of 40 000 objects/second.


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Fluorescence techniques Obviously, not all bad food is a different colour from good. It has been found that certain non-visible defects (bacteria for example) fluoresce when irradiated with long-wave ultraviolet light (350 nm) and this property may be used as a basis for sorting. This technique was originally developed for removing ‘stinkers’ from green arabica coffee beans, but has found applications in sorting peanuts, almonds and cranberries. However, the fluorescence effects can be short-lived and may also depend on the circumstances and time elapsed since the product was harvested. Infrared techniques Over the last decade, the wavelength range used by sorting machines has been extended from the visible further into the infrared region. Here, both water absorption and other chemical effects play an important part in determining the reflectivity characteristics of food particles. Bichromatic infrared machines are proving particularly effective in removing shell fragments from a variety of tree nuts. Optical sorting with lasers Incorporating the use of lasers into bulk food sorting equipment is a technique that is relatively new to the industry. A laser beam is used to illuminate the product and the reflected light is affected by the amount of laser light that is either scattered from the surface, or diffused within an object. Since the laser produces narrow beams of coherent light at a single wavelength, there is no need to use optical band pass filters. However, a disadvantage with this technique is in maintaining the high capacity demanded by the bulk sorting industry, in conjunction with the necessary resolution to detect defects accurately. The linear scan rate of the laser across the width of the view and the velocity of the product determine the vertical resolution. To date, laser scanning is limited to approximately 2000 scans/second. Therefore, for product travelling at 4 m/s, the vertical resolution is of the order of 2 mm. By comparison, line scan CCD (charged coupled device) technology offers around 5000 scans/second and a resolution of the order of 0.3 mm. Laser scanning also suffers from problems associated with the drop in illumination intensity and therefore, signal-to-noise levels that result by fanning out the laser beam across a line of sight. Mechanically, it can be quite a design challenge to scan reliably a laser, by say a rotating polygon mirror, in the hostile temperature and debris-ridden environments that are often encountered in food processing plants. To a limited extent, some successful sub-surface and texture inspection can be carried out on certain soft fruits and berries with laser light. In fact, the technique has already been commercially deployed for some product areas in the food industry, notably for dried fruits like raisins, or certain vegetables, nuts and tobacco. There is certainly scope for further study and possible wider exploitation of the technique.

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6.3 Inspection systems: selection of wavelength bands, filters and illumination The range of wavelengths measured by an optical sorting machine is defined by the choice of light source, the properties of the optical filters (if used) and the properties of the detector itself. Similarly, at any particular wavelength, the characteristics of the electrical signal from the detector will also depend on these components. Once the optical characteristics of a product which form the basis for optical sorting have been identified and selected, the relevant wavelength bands must be isolated by selection of appropriate filters and illumination. A primary objective of selecting filters and lighting is to obtain the maximum possible signal-to-noise ratio from the detector at the required wavelengths, and the minimum possible signal at all other wavelengths.

6.3.1 Illumination When dealing with irregularly shaped objects, uniform, diffuse illumination is required to minimise highlights and shadows, since these would obviously detract from the measurement of true surface reflectivity. To eliminate shadows and highlights at the point of measurement, the particle should be surrounded by a spherical surface of uniform brightness. However, in practice this is just not possible due to the following constraints: • • •

To allow a path for particles through the optical inspection chamber, there must be entry and exit points. The position of the optical components will result in areas of different brightness, compared with the main chamber wall. The use of light sources of finite size leads to non-uniform illumination.

Specular reflection is almost always a problem, even with a perfect diffuse illumination sphere. If a particle with a diffuse reflective surface is placed in such a sphere, then its true colour will be observed. However, if the particle surface is not diffuse, specular reflection will occur, giving highlights which do not exhibit the true colour of the surface. Clearly, the highlights can adversely affect the optical system and consequently result in the incorrect classification of a particle. The most cost-effective form of illumination involves fluorescent tubes and/or incandescent filament bulbs. A number of lamps are arranged to provide as uniform a distribution of light as possible. With discrete lamps, diffusing windows are used in front of the bulb to diffuse the high-intensity point source of light emitted by the filament. To overcome some of the inefficient heat loss issues associated with incandescent lamps, arrangements using glass rods in conjunction with reflecting ellipses can be implemented. Fluorescent tubes can be manufactured with different spectral characteristics depending on the phosphors used. The spectral range of fluorescent


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Fig. 6.13 Emission spectra for two broad-band fluorescent tubes. Both are attempting to simulate pure sunlight, or natural daylight, where all wavelengths of the visible spectrum are present.

tubes spans the ultraviolet to the far visible red (Figs 6.13, 6.14 and 6.15). The advantages of the fluorescent tube are its long life, diffuse light, low cost and relatively cool operation. Its disadvantages are that it is limited to the visible and UV wavelengths and that it requires a special power supply to prevent flicker. The advantages of incandescent lamps are their inherent broad spectral range, from blue to the near infrared and their DC operation. However, they suffer from being point sources that dissipate large amounts of heat. A typical emission spectrum is shown in Fig. 6.16. In general, for optical sorting applications, fluorescent tubes are the most widely used, except in cases where near infrared or infrared measurements are required. The wider spectral range required for bichromatic machines necessitates the use of incandescent lamps, often in combination with fluorescent lamps.

6.3.2 Background and aperture The simplest form of inspection system views the particles through a small aperture and against an illuminated background. The brightness of the background is adjusted so that the optical system measures the same average value, with or without product. This is known as a ‘matched’ background because it matches the average brightness of product, including any defects. For effective shape sorting, where the boundary of each object must be apparent, an ‘unmatched’ background is used.

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A typical broad-band emission spectrum from an incandescent bulb.

Matching the background offers an advantage in that measurement of reflectance is independent of object size, for example, in the case of a stream of particles containing reject items that are darker than accept items. With a matched background, whenever a dark defect passes the aperture there will be a decrease in signal amplitude and with a light particle there will be an increase in amplitude. Hence an unequivocal decision can be made by the electronics apparatus. However, if the background is lighter than the average of the product, then all product items would give a decrease in signal. In particular, small dark defects would give signals identical with those of large light particles and the two could not be easily distinguished. The intensity of the light reflected from a particle via the aperture is the product of the size of the particle and its reflectivity, including any area of discoloration. The background usually consists of an array of suitably located lamps (or LEDs, light emitting diodes) behind an optical diffusing material. In some cases a white, diffuse reflecting plate is used to reflect light from rear-mounted lamps or LEDs towards the detection optics. The aperture is usually a rectangular slit.The width of the slit must be sufficient to allow for scatter in the trajectories of the particles and for the range of anticipated object sizes.The height of the slit is maintained at a minimum, although allowing for sufficient signal to provide maximum signal-to-noise for detection, in conjunction with resolution and accurate timing of the delay between detection of a defect and rejection of a particle.

6.3.3 Optical filters An optical filter is essentially a piece of coloured glass. An extensive range of optical filters are readily available as off-the-shelf components.

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The transmission spectrum for a low-pass optical filter.

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The transmission spectrum for a high-pass optical filter.

Alternatively, custom filters can be made at a higher cost. Four basic types of filter are used for optical sorting applications: • • • •

Low pass: transmitting only below a certain wavelength (Fig. 6.17). High pass: transmitting only above a certain wavelength (Fig. 6.18). Band-pass: transmitting only within a band of wavelengths (Fig. 6.19). Combinations of the above in a single filter e.g. a double band-pass filter (Fig. 6.20).


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A double band-pass filter.

6.3.4 Detectors Prior to the advent of solid state detectors, the photomultiplier tube was the best detector of visible radiation. The photomultiplier has a good signalto-noise ratio that allows detection at low light levels, and a satisfactory response in the violet–blue (400 to 500 nm) region of the electromagnetic

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spectrum. However, due to their fragile mechanical construction, photomultipliers are not very robust. They also suffer from limited life, high operating voltage and poor deep red and near infrared response (650+ nm). Following photomultiplier tube technology, solid-state technology now dominates the optical sorting industry. Initially, the photodiode was used due to its comparative cheapness, mechanical robustness and almost indefinite life. However, compared to photomultipliers, photodiodes have a poor blue response. Contemporary optical sorting machines now employ high speed line scan CCD (charge coupled device) technology. Silicon CCD technology offers the advantages of high sensitivity, good broad band response (400 to 1000 nm), high spatial resolution and good quantum efficiency. Although CCDs are analogue sensors, their output is easily converted to digital form. Consequently, state of the art, low noise, high speed digital processors can be used for subsequent signal processing. Unfortunately, CCDs continue to suffer from relatively poor response in the blue region. In order to extend the detection range into the infrared domain, other detector materials besides silicon are used. Having a good infrared response (up to approximately 1700 nm), germanium detectors are readily exploited by the food-sorting industry for detection of foreign matter. Historically, single photodiodes were used. However, germanium linear arrays are now commonplace and generally used in combination with silicon CCD technology. In fact, much of the infra-red technology developed for the telecommunications industry is now readily exploited for optical sorting applications.

6.4 The product feeding, ejection, cleaning and dust extraction systems 6.4.1 Feed The product feeding system in a sorting machine should provide three basic functions: • •

•

Metering: to ensure that the optimum number of objects per unit time are fed through the optical inspection area. Acceleration to a constant velocity. The time taken for objects to travel from the optical inspection point to the ejection point must be constant so that activation of the ejector can be accurately coincided with the position of the object. Typically, the velocity of the product is of the order of 4 m/s. The delay between detection and ejection is between 0.5 and 100 ms. Alignment: to ensure a controlled trajectory through the inspection and ejection points.


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In practice, a vibrating feeder tray, situated just below the output of a hopper, meters the product flow. The following feed systems are commonly employed: • • • • • •

An inclined gravity chute. A flat belt. An inclined belt (unique to Sortex). A ‘C’ shaped belt. Contra-rotating rollers. A narrow grooved belt.

To detect small blemishes on fruit and vegetables reliably, it is necessary to inspect the product from two sides. The traditional architecture of an optical sorting machine is to feed the product along a horizontal conveyor and then observe the product from top and bottom views as the product flies off the end of the conveyor. The drawback with this approach is that the bottom camera is soon covered in product. Sortex’s Niagara machine overcomes this problem with the patented PowerSlideTM feed system (Fig. 6.21). The belt conveyor on the Niagara is inclined at 60 ° to the horizontal, such that the cameras can view the product from either side, and remain clean.

Side guide support bar

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Fig. 6.21

The Niagara PowerSlideTM.

Main frame

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Fig. 6.22 The Sortex 3400 machine is a compact design with two single channels. Three sensors surround each of the two channels providing all round inspection and therefore a highly efficient sort. The 3400 is particularly suited to sorting low volumes of high value commodities, since the yield and efficiency of the machine are very high.

The flat belt or gravity chute approach presents the product in a single layer, restricting the view to two sides, enabling a much higher throughput of product to be achieved. In contrast, some feed methods channel and separate the particles into a single stream, each object dropping down after the other in single file. This feed technique allows an all-round view of each object’s surface, since three cameras can be positioned around the foot of the chute. Obviously, three views allow a very high quality sort with an excellent yield. However, one disadvantage is the relatively low throughput of product compared to wide flat belt or chute techniques (a few hundreds of kg/hour compared to several tonnes/hour). As a consequence, single channel feeding is usually only employed for high value products (Blue Mountain coffee, nuts such as almonds or macadamia, selected beans and pulses etc.). Throughput can be increased in single channel feed systems by adding two or more channels to a machine. The Sortex 3400 machine (illustrated in Fig. 6.22) is a compact design with two single channels. Three sensors surround each of the two channels providing all round inspection and therefore a highly efficient sort.The 3400 is particularly suited to sorting low volumes of high value commodities, since the yield and efficiency of the machine are very high.

6.4.2 Ejection The usual method for removing unwanted items from the main product stream is with a blast of compressed air from a high-speed solenoid or piezoelectric valve, connected to a strategically positioned nozzle. Pneumatic ejector valves must have rapid action, reliability, long lifetime (a minimum of one billion cycles) and mechanical strength. The fastest (a


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Sortex patented piezoelectric design) operates at a frequency of 1 kHz, firing a pulse of air for 1 to 3 msec. Ejectors operate at input pressures between 200 to 550 kPa (30 to 80 PSI), depending on the size of the object to be removed. Typically, the ejection point is located outside the optical inspection area, because the action of the air blast on a rejected object could cause dust particles and skin fragments to be blown around that could create false rejections. However, at the same time, it is advantageous to eject objects as soon as possible after the optical inspection point, due to unavoidable variations in the trajectory of each individual item. The appropriate time delay between the inspection and ejection point is generated by electronic circuits. The accurate timing required, to coincide the ejector air blast with that of the object to be ejected, relies on the objects having constant velocity as they fall in front of the ejector nozzle. In practice, the tolerable variation in product velocity is about 5 %. The trajectory of each particle also becomes more difficult to predict, the greater the distance between the viewing point and the ejection point. It can become a major design challenge to position the chute, optics and ejection system as close together as possible. The operational lifetime of the ejectors must be in the region of at least a billion or more cycles. Food processing is usually a 24 hour-a-day, all-yearround operation. Operators cannot afford to shut down a machine regularly for even a few minutes to replace faulty ejectors. Under these circumstances, machine reliability and stability of operation are critical. For certain large or heavy objects a solenoid valve may be used to control a pneumatically operated flap or plunger to deflect rejected items. Specialised ejectors have been developed for pulps or slurries to remove rejects by suction and are mounted above a flat belt, downstream from the inspection unit. Smart Ejection systems To accommodate long, or irregularly shaped objects, Sortex has developed the SmartEjectTM system. With most optical sorting machines, the air blast is fired from one or more ejectors in an array, spanning the width of the belt or chute, just after the optical inspection area. The air blast is aimed solely at the centre of the defect (also known as ‘centroid ejection’). If the defect is a small blemish on a large object, then the blast may not be sufficient to remove the item.To combat this problem, Sortex’s Niagara machine computes the location of the object encompassing the defect and fires the appropriate number of ejectors so as to fire at the entire object – this improved ejection system is known as SmartEjectTM (Fig. 6.23). SmartEjectTM fires one or more of 160 high speed ejectors positioned across the line of view at the profile of an object, rather than at a defect, which improves both accept quality and yield. Three-way separation by two-way ejection Sortex has also pioneered the ability to perform a three-way sort by adding a second bank of ejectors to the Niagara vegetable sorting machine (Fig.

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The SmartEjectTM system for precise removal of larger objects with small defect sites that may not be located at the centre of the object.

6.24). To date it has been the convention in optical food sorting to only have two-way separation, into accept and reject categories. The three-way separation by two-way ejection allows an additional product classification. For example, three-way separation for green beans now enables the following three categories: 1 2 3

Accept. Reject (rots, blemishes, foreign material, stalks etc.). Accept with stalks.

This is illustrated in Fig. 6.25. The advantage of the new third category is to allow recovery of otherwise ‘good’ product that would normally be rejected. Accepted green beans with uncut stalks can be returned to the ‘snibbers’ (stalk cutters). In this way significant savings on recovered volumes can be made. Approximately 2 % of green beans, typically running at 5 to 10 tonnes/hour are rejected on uncut stalks alone.

6.4.3 Cleaning and dust extraction The successful application of optics in an industrial environment pervaded by dust, oil, starch, food debris or water poses major design issues for optical engineers. Considerable expertise is necessary to design an optical sorting machine capable of successful commercial operation under in-plant conditions. The operating temperature range encountered in a food processing plant varies between -5 and +40 °C, making optical, mechanical and electrical tolerances critical to the effective operation of the machine. If the cameras of an optical sorting machine become obscured by debris, then the performance of the machine rapidly deteriorates.


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Fig. 6.24 Three-way separation from two-way ejection. Instead of a single rear firing ejector bank, the Niagara is fitted with a second bank to fire in the forward direction.

Fig. 6.25

Three-way separation by two-way ejection for green beans. From left to right: accept; accept with stalks; reject.

To protect the optical components from dirt or moisture, they are contained in an ‘optical box’ with a glass window. The position of this window in the optical path should be such that any small particles, which may settle on the surface, are out of focus and therefore create minimum noise in the optical signal. However, it is essential that this window is kept as clean as possible and a number of facilities to achieve this may be provided on the machine. Firstly, the product being fed to the machine should be as dry and dust free as possible. However, the action of storing it in a hopper and feeding

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it on a vibrating tray will usually create some dust. Hence, a dust extraction nozzle is often fitted at the end of the vibrating tray. In the case of a chute feed, the top of the chute may be perforated so that air can pervade the product stream to remove dust particles. In addition to dust extraction, the optical box window can be cleaned by means of compressed air jets. These ‘air knives’ as they are also sometimes called, provide a continuous curtain of air to prevent particles settling on the surface of the glass. If necessary, they can also provide a periodic high pressure blast which removes any particles that may have settled on the window. Pneumatically driven blades or brushes can also be used to periodically wipe the window. In some machines this may be combined with an air blow-down facility. As a final precaution, any dirt created by the action of the ejector blast on the particles, may be drawn away from the window area by a dust extraction nozzle, positioned just below the optical box. For wet or frozen product applications water jets and wiper blades can be substituted for air-based systems. Machines for sorting wet product are periodically hosed down with water, so must be water and dust proof to IP65 standards. Similarly, dry product machines are also manually cleaned with an air hose. For hygiene reasons, all potential bug traps must be designed out of all sorting machinery.

6.5

The electronic processing system in sorting machines

The electronic systems in sorting machines have progressed from the simple analogue circuits of the early machines to the advanced digital microprocessor-based circuits found in the present generation of machines. In contrast to many machine vision applications, it is common for the optical data processing system of a bulk optical sorting machine to be hardware, rather than PC-based. At the present time it is simply not practical to process 40 000 objects/s for colour and shape followed by effective control of the ejection process with a PC-based system. Most of the setting up of the sorting parameters can be done by the machine itself, including in some cases the ability of the machine to ‘learn’ the differences between good and bad product. However, the operator is always given the opportunity to finetune the final result. A sophisticated optical sorting machine will track the average colour of the product so that, even though the average product colour may change with time, the machine will continue to remove only the predefined abnormal particles. Optical sorting machines are often provided with a white calibration plate which is either manually or automatically placed in the optical view at user-defined intervals. The machine is then able to correct for any measurement drift that has occurred. Once a machine has been set up for a particular product, all the machine settings can be stored in memory. This can be repeated for a number of dif-


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ferent products and then, at a later time, the machine can be made ready to sort any of these products simply by recalling the appropriate settings from the memory. Alternatively, the settings can be used as a coarse starting point from which to fine-tune a machine towards an optimum setting for a particular set of circumstances. Most food plants sort one particular product type. For example a rice mill may sort different varieties of rice, but would not suddenly switch to coffee. It would be unusual for a food processor to be sorting many different and diverse types of product. Advanced sorting machines have a memory capability that can be exploited to provide information about the product for the operator. For example, this might include details of the number of rejects that have occurred in a certain time, or information about any drifts in colour in a certain batch of product. Information about how the machine itself is operating can also be provided to assist with preventative maintenance.

6.5.1 Optical detection and differentiation by shape There are many applications in food sorting where the defects are similar in colour to the good product. For example: insect larvae amongst blueberries take on the same colour as the berries; the stems on green beans are the same colour as the bean (Fig. 6.26); similarly pea and pea pod; or

Fig. 6.26 The stems found in green beans with stalks present a major problem to freezers and canners. Niagara’s stem recognition system is designed specifically to simultaneously remove stalks, along with foreign and extraneous material, typical defects such as black insect holes and beans outside user specified lengths.

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Fig. 6.27 Advanced shape processing allows removal of many of the problems encountered in sliced carrots such as ‘polos’, ellipses (oblique slices), cracks and those with tangential mis-shapes. This is performed simultaneously with colour defect removal.

green caterpillars among green beans or peas. In order to be able to solve these types of applications, Sortex has pioneered the ability to sort objects on the basis of size, roundness, area, length and, therefore, shape. In addition to colour and shape, the minimum size of the discoloration necessary for a particle to be rejected can also be defined. In the above examples, the larvae are elliptical in shape whereas the berries are round, and the stems on green beans are much thinner than the beans. A major innovation for the food-sorting industry has been the development of new vision algorithms for computing the size and shape of objects. Sortex is an established market leader in terms of the implementation of these algorithms in specialised electronic hardware. The Niagara machine’s ability to sort objects on the basis of shape as well as colour, at high speed, is the basis of one of the major innovations of the machine. Up to 40 000 objects per second can be simultaneously sorted for shape and colour, across an 1100 mm wide line of view. Advanced shape processing allows removal of many of the problems encountered in sliced carrots such as ‘polos’, ellipses (oblique slices), cracks and those with tangential mis-shapes. This is simultaneous with colour defect removal. It is illustrated in Fig. 6.27.

6.5.2 User interfaces A typical machine will have either a keypad and a display unit, or more commonly in contemporary machines, a touch-screen user interface (Figs


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User interface

Controls for conveyors: Emergency stop Run lamp Start Stop

Mains isolator

Fig. 6.28

Detail of the touch screen user interface and typical operator controls.

Fig. 6.29

A user interface consisting of dot matrix alpha numeric text display and keypad.

6.28 and 6.29). A good user interface should allow the operator to set up and control the machine by means of an easy to follow series of menus. In addition, the display unit will provide the operator with information regarding the settings of the machine while it is sorting, together with details of any faults that may occur.

6.5.3 Mapping techniques A bichromatic sorting machine using two band-pass filters, possibly green and red, makes a decision based on the ratio of the two signals in


Colour 2

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Colour 1

Fig. 6.30

A bichromatic map representing the distribution of colour 1 versus colour 2.

conjunction with the intensity of the individual signals. The situation can be represented as a two-dimensional ‘colour’ map by plotting the reflectivity of colour 1 versus that of colour 2 (Fig. 6.30). The bottom left-hand corner of this map represents the reflectivity from a black particle (0 % reflectivity) and the top right-hand corner represents the reflectivity from a white particle (100 % reflectivity). The boundary curve in Fig. 6.30 is the reflectivity map contour, outlining the acceptable product, as seen by the sorting machine, for a typical product. The contour line represents the chosen accept/reject threshold. The ‘+’ within the map contour is the background ‘balance point’, which represents the average colour of the product. A major part of setting up an optical sorting machine is to achieve the best overall accept/reject ratio for the product being sorted. The operator can do this by exploiting the user interface to adjust the shape and size of the map contour (Fig. 6.31), to match as accurately as possible the map contour of the product batch. The sorting sensitivity increases as the machine map contour is decreased in area, as it approaches the area of the map contour of the product batch. Product within the area bounded by the threshold levels is accepted and product outside is rejected. These techniques allow an optical sorting machine to remove a far greater range of defects, with greater accuracy and without the penalty of removing large amounts of acceptable product. Obviously, these techniques can be extended into three dimensions for trichromatic colour sorting.

Optical sorting systems Colour 2

Colour 1 dark

Bias 2

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Colour 1 light

Colour 2 light Bias 1 Black Background (B1,B2)

White Accept Colour 2 dark

Colour 1

Fig. 6.31

6.6

Bichromatic sensitivity thresholds.

Strengths and weaknesses of colour sorting

There is often a misunderstanding that a colour sorter can remove all of the defects from a given batch of product. In practice this is impossible. A colour sorter will reduce the concentration of defects in the product, but it can never be totally effective. All colour sorters will remove some acceptable objects and fail to remove some of the defective objects. There are several reasons for this. Sometimes, the physical size or the colour difference of the defect from the product may be too small for accurate detection. Occasionally, the machine may detect a defect and remove the object, but the object re-enters the accept stream after it has been ejected as a consequence of a random collision. Ejector performance and minimal positional pitch of the ejectors in the array below the optical system can also become a limitation for accurate ejection. At present, the smallest ejectors have a 3 mm pitch. This limits the ejector ‘resolution,’ especially for small products like rice or sesame seeds. Machines can be adjusted by operators to optimise their performance. Sensitivity is one of the principal parameters that the operator can change. Increasing the sensitivity will result in the machine rejecting more defective material. However, a greater proportion of good product will also be rejected as the sensitivity threshold approaches the average product colour. There is normally a compromise point between achieving a high sorting efficiency and optimum yield (the ratio of good to bad material that is

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Table 6.1 Some typical performance figures for a variety of products sorted on different machines. The throughputs are quoted in ranges, since the throughput increases as the level of input contamination decreases Throughput (tonnes/hour) per machine

Product

Machine

Sorting criteria

Whole green beans

Niagara-Trichromatic colour sorter, 1 m wide belt 90003Bi-48 channel, gravity chute bichromatic sorter Z-Series mono-256 channels, 4 gravity chutes, monochromatic sorting machine Niagara-Trichromatic colour sorter, 1 m wide belt

Remove attached stems and blemish

8

Remove defective beans and foreign material

3 to 6

Remove spotted, discoloured rice and foreign material (stones, glass, paddy etc.) Remove foreign material, pea pod, sticks etc. by colour and shape

8 to 12

Green coffee Parboiled rice

Frozen peas

10 to 16

rejected). This compromise point is primarily achieved as a result of operator experience and training. There are physical limits to the product throughput that a sorting machine can successfully achieve. If the product flow is increased above the upper limit, the product sheet will no longer be a monolayer. Objects will overlap and sorting performance will deteriorate, since many defects will be obscured and therefore will not be detected by the optical system. Increasing the flow of product through the machine will also result in increased good product being lost, since overlapping and colliding products are difficult to eject efficiently. Table 6.1 illustrates some typical performance figures for a variety of products sorted on different machines. The throughputs are quoted in ranges, since the throughput increases as the level of input contamination decreases.

6.7

Future trends

Computer vision systems are increasingly being used in general manufacturing, for example in pick and place applications such as printed circuit board (PCB) population and manufacture. However, the demands of the food industry are generally far greater. At present, there is only a limited range of computer vision equipment available for use here. However, in the future this is likely to change.


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Two factors limit the rate at which computer vision systems are being introduced to the food industry: •

•

The data processing rates required in a sorting machine for the bulk food processing industry are very much higher than those in a similar inspection machine for manufactured objects. The development of improved materials handling and separation systems is not keeping pace with the dramatic advances being made in computer hardware technology.

A computer vision system potentially offers many benefits over a conventional colour sorter. The ability to sort objects simultaneously on the basis of several different criteria would be a primary advantage. However, for the immediate future, the most likely application of advances in electronic hardware is the gradual improvement in performance of the present generation of sorting machines. The optical sorting industry readily exploits advances in components, manufacturing processes and designs. At the present time, defect detection is mostly carried out in the visible and near infrared wavelengths, mainly because of the added cost of infrared detector technology. However, other wavelengths are already used in certain areas of the food-processing industry. X-ray techniques are often employed as a final check for foreign material in packed or processed foods, or to detect hollow potatoes, for instance. Ultraviolet light can be used in some nut-sorting applications, especially to detect fungal-infection sites that fluoresce when exposed to UV light. The natural progression of monochromatic to bichromatic sorting may lead towards wider use of trichromatic technology. As the cost of lasers continues to decrease, so laser technology might become commonplace for texture or sub-surface inspection. Meanwhile, advances in detector resolution, valve technology, ejectorduct materials and design, will all help to optimise the ejection process. In the future, unwanted objects may be removed with rapier-like precision. Improvements to the operational stability of the sorting machine are likely to have a big impact, increasing the product throughout and ensuring that the machine optics need to be calibrated less frequently. A consequence of the increased pace of technological advances will be a reduction in the working lifetime of sorting machines. New machines will have to be developed and manufactured under faster cycle times to keep pace with the market. Some components, particularly electronic chips, can quickly become obsolete. Similarly, the falling price of high technology is already allowing new competitors to enter the market place. Any optical sorting company that ignores these factors can only expect reduced profit margins. In some ways, the real future challenge will be to provide integrated solutions that fulfil the demands of the food processing industry, at a price that can be justified.

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6.8


anon (1987) ‘Electronic sorting reduces labour costs’, Food Technology in New Zealand, 22(7), 47. asasent d, talukder a and lee h w (1997) ‘X-ray Agricultural product inspection: segmentation and classification’, SPIE, September, 3205, 46–55. bee s c (2000) ‘Physics sorts the wheat from the chaff’, Physics World, June, 13(6), 24–6. bee s c ‘Optical sorting for the coffee industry’, Association Scientifique Internationale Du Café (ASIC) Conference, Trieste, Italy, 13th–17th May 2001. bee s c (2002) ‘Optical sorting of bulk food products’, Grain Feed and Milling Magazine, Feb/Mar, 10–12. Degrees of protection to EN 60 529/IEC 529 1991 (British Standards Institution). downing d l (1996) A Complete Course in Canning and Related Processes, 13th ed, Timonium, Maryland, CTI Publications Inc. ‘Good Processing Practice Using Colour Sorting’ (1997) Food Technology Europe, October, 54–8. kubiak a and kutzbach h d ‘The application of an artificial intelligence system in the automatic recognition of wheat by the nonlinear approximation technique’, 12th International Congress of Chemical and Process Engineering CHISA, Praha, Czech Republic, 25–30th August, 1996. P9.55 [490]. kubiak a and ojczyk t ‘The automatic recognition of different varieties of wheat grain with the application of the non-linear approximation method’, 12th International Congress of Chemical and Process Engineering CHISA, Praha, Czech Republic, 25–30th August, 1996. P9.56 [489]. nagata m and cao q x (1998) ‘Study on grade judgement of fruit vegetables using machine vision’, JARQ-Japan Agricultural Research Quarterly, October, 32(4), 257–65. nakamura o, kobayashi m and kawata s (1999) ‘Nondestructive inspection of Phaseolus coccineus L. soya beans by use of near-infrared lasers’, Applied Optics, April, 38(12), 2724–7. pearson t c, schatzk i and thomas f (1997) ‘Superior sorter selects stain free nuts’, Agricultural Research, January, 18. wen z q and tao y (1998) ‘Brightness-invariant image segmentation for on-line fruit defect detection’, Optical Engineering, November, 37(11), 2948–52.

Internet Sortex homepage: www.sortex.com Bühler Group home page: www.buhlergroup.com

7 Applying optical systems G. Doménech-Asensi, Polytechnic University of Cartagena, Spain

7.1

Introduction: foreign bodies in fruits and vegetables

Fruits and vegetables contain particular foreign objects that must be dealt with in a specific way. A first classification shows two main classes of foreign objects: those which remain in the surface of the fruit or vegetable, and those which are found inside it. Depending on the kind of foreign object, the method to detect and remove it will vary. From the point of view of its origin, we can make another classification; natural contaminants, such as wood or earth, and artificial contaminants such as plastic or metal. The reasons foreign objects might appear in fruits and vegetables are related to dirty harvesting and packaging equipment, probably due to a poor cleaning after maintenance or repair, or pieces of glass coming from broken bottles, metal staples from packaging equipment or even damaged containers or pallets. In any case, these foreign objects must be removed, mostly by a combination of several technologies. Optical systems can provide a part of the solution; in those cases where foreign objects are on or close to the surface, they can be detected by a vision system. In this chapter, we will first detail how sorting systems are developed for the removal of foreign bodies in fruits and vegetables, and then we will focus on two special cases: olives and potatoes. We will finish the chapter with some comments on future trends to be expected in this field.

7.2 Developing sorting systems for the removal of foreign bodies In general, most of the methods used for the classification of fruits and vegetables are useful for foreign object identification. In fact, when identifying

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such objects we are performing a classification of specific images, and knowledge acquired for classification of these vegetables is necessary to identify those objects which do not fit certain parameters. Any sorting system developed to remove foreign bodies must be designed to take into account the properties of both foreign bodies and fruits and vegetables. It is essential to have a list of characteristics of these items in order to design a proper classification system. For instance, if one is processing oranges, one must take into account the shape, texture and colour of such fruit, and then the possible foreign bodies that can appear and where they can be found, looking for characteristics which make them different from oranges. In (Graves, 1998) we find an overview of several techniques used to detect foreign bodies in foods, depending on the nature of such objects. The first step in the development of an optical sorting system will be the identification of discriminating features for the identification of specific foreign objects of a certain fruit or vegetable. Selection of such features requires some experience because they must be identifiable with available machine vision technology. Optimal features selection will make the automatic sorting easier. Once we have chosen the set of discriminating features, the next step is the configuration of the optical system. The main task is how to obtain the best image to enhance the features of a foreign body, which will depend on the camera used and the lighting equipment arrangement (discussed in more detail later). When the image has been obtained, it needs to be processed, which means some hardware and software. Some equipment offers compact solutions with basic software: however, many applications for complex inspections require much software development. An optical inspection system is composed of three main elements: the illumination system, which provides a suitable environment for the image acquisition, the camera, used to acquire and digitise the images, and finally the processing system. This last is composed of several parts, which can be roughly classified as hardware and software. The hardware involves a digital memory to store the image and a microprocessor to perform its analysis, which can take the form of an image acquisition board placed in a computer, although embedded systems are more popular. The software, finally, is the last step in the vision system and is developed according to user needs or sold as part of a general application inspection system. Software is that part of the system which adds more value to the final system cost, and one can find several companies and vendors which develop both general application and specific tools for visual inspection systems. Before detailing each component of the inspection system, we have to arrange the mechanical structure which will hold the conveyor belt and the optical inspection system. This is a crucial step because, depending of how we design this structure, subsequent tasks will be easier or more difficult. The main restriction comes from the processing speed of fruits and vegetables, which will determine the time available for a single fruit inspection. This implies the use of single fruit inspection or sets of fruits in the same

Applying optical systems

CCD camera

Rollers

Lighting

Belt

CCD camera

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Hopper

Lighting

Rollers Ejector Ejector

Accepted vegetables Rejected vegetables

Fig. 7.1

Rejected vegetables

One side inspection compared with two side inspection.

image. Moreover, depending of the kind of foreign body, we may require a complete surface examination or a more basic inspection. One does not expect to find a staple embedded in a rice grain, but it can appear in an apple. In the first case, it is enough to inspect the rice distributed on a plain conveyor belt: the staples will be seen. In the second case, a complete surface examination is needed which will imply some kind of rotation device for the fruit, by means of rollers or an ‘aerial’ inspection with several cameras (see Fig. 7.1).

7.2.1 Lighting arrangement Illumination is one of the most important issues to think about when developing an inspection system. The type of illumination used is in accordance with the kind of camera employed and with the nature of the inspection performed. White illumination is available for monochromatic or colour cameras and infrared illumination for more specific cameras. Sometimes one needs to combine several sources of illumination such as ultraviolet or infrared with multiple spectra of light, in order to perform difficult operations such as detection of the same colour flaws or of certain bugs that look similar to small fruit. As a general rule, it is very difficult to determine the kind of light which will produce the best result for a new optical inspection application. Experience is needed to arrange, after some tests, the correct light option. Among

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these options, daylight cannot be used, even if this is white, diffuse and clear, because we cannot afford variations in ambient light. The optical inspection system requires some kind of environment, such as an inspection tunnel, where the lights and the camera are placed and where the vegetables flow on a conveyor belt. Another decision involves the speed of fruits and vegetables on the conveyor belt. If they move higher than a certain speed, the camera will take fast images, which usually means shutter speeds shorter than 20 ms. This shutter speed is needed to take clear images of the vegetables, otherwise they will appear to be moving in the picture. The problem related to this speed is an effect called ‘beating’ which may occur when we use light modulated at an electrical network frequency of 50 Hz or 60 Hz. This is the kind of light produced by fluorescent lamps and low-voltage lamps. This effect causes fluctuations on the image intensity due to differences in frequency between camera shutter and light. The effect is a soft variation generally in a sinusoidal form of the light, that makes it impossible to perform a good inspection of the products except for very simple features. To prevent beating, the inspection system will require regulated lights, which means high frequency fluorescent lamps or low voltage DC lights. Once we have determined the light focus which depends on the conveyor belt speed, we have to choose the lighting arrangement. There are several options, some of them with slight differences. For instance, we can use front lighting, which can be direct or diffuse, or back lighting, with one or more sources (diffused). There is also an arrangement called ring lighting or we can use low angle lighting. Special light sources such as polarised lighting or colour filtered lighting are yet other options, and finally we can combine several of the techniques mentioned above with the technology available in the market, like solid state lights, fluorescent lamps, low voltage DC lights or fibre-optic devices. Direct front lighting consists of one or more lamps that shine directly on the vegetable surface. It is useful to inspect textures, and the results obtained depend on the distance from the light source to the object as well as on its angle to the conveyor belt. The smaller the angle is with respect to the belt, the greater the number of shadows that will appear on the surface for the same texture. Also, a more highly textured surface results in more shadow areas. This technique cannot be used for highly reflective surfaces. For identification of foreign bodies based on colour discrimination, the inspection system will require diffuse front lighting. This reduces the contrast and allows a clearer inspection of the surface. If we are processing circular vegetables, such as oranges or apples, ring or circular lighting is a possible option, as it produces a homogeneous illumination on the fruit. With this technique we can detect bugs or even small holes on the surface. For simple operations such as identification of small bodies mixed with the vegetables, one option is the use of back lighting. In this case we will have information only about the contour of the object, which should be enough


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to measure its size and shape. This technique is not compatible with surface inspection. Special sources, such as polarised lights or colour filtered lights can simplify some inspection tasks. Sometimes it is necessary to inspect vegetables with very reflective surfaces which may be wet due to a previous water washing step. The conveyor belt may also be wet. Detection of foreign bodies in these conditions can be very difficult or impossible, even if we use very complex software algorithms. To reduce or eliminate these reflections would constitute a big advance in the development of inspection systems. Polarising filters help to reduce reflections when they are used in combination with polarised lights. In this case, the angle between polarisation axes in both the filter and the light produces gradual reduction of the reflections in the image. The best results are obtained when both filters, the one mounted in the camera and the one set in the light are placed with a 90 ° angle in their respective polarisation axes. Colour-filtered light sources can be used for colour image processing using a black and white vision system. This situation is quite feasible, because the cost of a black and white vision system is much smaller than that of a colour one, and most of the commercial equipment sold as inspection systems works in black and white. Here, the images of the fruits and vegetables appear in a grey tone and different colours appear to have the same intensity. If we need to identify foreign objects by their colour and do not wish to use a colour camera, we can use illumination based on white light with a colour filter placed in the camera. Depending on the type of filter, certain colours will appear lighter and others darker which without the filter would appear similar. One of the applications reported (Johnson, 2000) for special illumination image processing in foreign objects detection consists of the identification of bugs in blueberries; in this case a Japanese beetle, a common field bug in North America. This bug is of a size similar to that of blueberries and is dark, with levels of intensity similar to the fruit. The detection of this bug is important, and after discarding conventional red, green, blue (RGB) colour processing, visible infrared light was used. This gave much better conditions for the detection of the bug than with a conventional colour system. Infrared filters are used in other vegetable and fruit inspection systems (Singh and Delwiche, 1994). Finally, there are even more specific systems, such as the one described by Reid (1976) which uses reflectance measurement in the range of 725 to 800 nm to distinguish apples and stems. In general, the basic theory of interaction of visible light and vegetables or fruits, depending of the wavelength of light used, is described in Gunasekaran et al. (1985). It comprises: a) single wavelength, where the optical reflectance is measured at a single wavelength, b) difference measurements, where two different wavelengths are used to measure the reflectance, c) ratio measurements, which take as criterion the ratio of the

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optical reflectance at two distinct wavelength and d) combination measurements, which are a combination of the aforementioned techniques. In Johnson (2000), another study case is mentioned. Here, the foreign objects are nightshade plants in green peas, which are poisonous and difficult to identify with conventional colour processing because they are also green in colour, i.e. they have a hue identical to a pea. Moreover, discrimination based on geometrical parameters such as size or shape was not possible due to similarity. In this case, an inspection system was developed using ultraviolet light which yielded efficient results. The above is a brief review to show the importance of a proper illumination system as the first step in the development of an optical inspection system for fruits and vegetables. Particular characteristics of these items make it impossible to stipulate a definite illumination structure for a new application. The right decision often involves considerable simplification in the development of the rest of the equipment, especially the discrimination and identification algorithms programmed in the software.

7.2.2 Cameras Although there is a large variety of industrial vision cameras available on the market, choosing one is not difficult if the characteristics of the products to inspect are known. Currently, coupled charge device (CCD) cameras are the more popular in machine vision applications. It is also necessary to decide whether to use a matrix CCD or a linear CCD. Linear CCDs were often chosen for black and white image processing, where economics dictated the choice. A linear CCD is basically a row of pixels for binary image processing, connected to a microprocessor with some options preprogrammed and little flexibility for the user. The CCD is placed to monitor a strip in a conveyor belt, where objects flow in a perpendicular direction to it and so that all the objects can be sequentially monitored. Typical operations performed by linear CCDs are width and area measurements, and this can be sufficient to detect some foreign bodies. A matrix CCD is a matrix of n by m pixels, where m and n are usually between 200 and 1200 units; it takes plain images from objects, such as a regular video camera. The information of each pixel is digitised usually in 8 bits (black and white) or 24 bits (colour) and then sent to a processing system. Machine vision systems mostly use this type of camera; they vary in sophistication, from simple to more complex. Usually, the cameras have controls to adjust shutter speed and gain; the more advanced models offer autofocus and autoiris facilities, although these last options are rarely used in industrial applications. Digital images are transmitted in video signal or RGB information. Black and white cameras are used in such applications where colour is not an important discriminating feature. A black and white system involves cheaper illumination and easier maintenance because camera calibration is simple and no special techniques are needed. Colour cameras are only used


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where colour is necessary to identify foreign objects. Colour cameras involve the use of colour frame grabbers which are more expensive and more time-consuming in image processing. For each bit of information needed in a black and white vision system, three are needed in a colour system, for red, green and blue. The image processing will be more sensitive to the illumination system and the camera will require a white balance before starting the operation. However, some very good colour cameras are available on the market; they have auto-white balance and other automated operations that make maintenance easier. Cameras can be used on their own or in combination with filters, depending on the illumination used and the type of inspection to be carried out as previously explained. A typical filter is the ultraviolet, used to discriminate specific radiations from the analysed object. Ultraviolet cameras are currently available; they are devices that are sensitive to such radiations. The ultraviolet cameras are more compact and could provide an optimal solution for analysis of certain vegetables and fruits because biological material exhibits special reflectance when excited with light. It is also necessary to note the differences between monochromatic and bichromatic cameras. The former are conventional black and white cameras that use a filter to process a certain waveband of light or colour. Bichromatic cameras are devices that can analyse light in two different wavebands, such as red and green, which allows them to identify colours like yellow, orange, green and their combinations. A variety of camera lenses can be used with the same camera, depending on such characteristics as the angle of view or the distance from the camera to the object. Tabulated values that can help when selecting a particular lens are available.

7.2.3 Software Software is a critical part of an optical inspection system. For a same image taken with the same camera and illumination, good software can make the difference between a true classification and a defective one. In foreign object detection, the software is responsible for image enhancement, segmentation, feature extraction and final sorting, which are the aspects developed in digital image processing (Gonzalez and Woods, 1992). Image enhancement consists of the application of filters and other software algorithms to improve the image quality of the vegetable or fruit inspected. Segmentation is the processing performed to individualise objects in the image, whose features are extracted at a later stage. Once these features are known, several criteria and algorithms can be applied to make a final decision in order to sort the object as good vegetable material or as a foreign body. For each processing step, several algorithms have been proposed in the literature which have been developed for different applications in recent years. Some of them are very specific and others allow certain degrees of freedom for a variety of applications.

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Currently, machinery developed for inspection of fruits can be ‘trained’ so that an increasing number of foreign objects can be detected. This is due to good software design which has the capacity of extracting new features from objects. The systems cannot be defined as ‘universal classifiers’ but do include minimum requirements for most of the usual operations in visual inspection systems, such as pattern recognition for several shapes, texture identification and colour processing. An example of a set of parameters used in a general purpose optical inspection system is listed below: • • • • • •

The position of a certain object in the image, usually the first pixel found starting from an image corner. The count of the number of objects in a frame. Area of a single object in the frame, measured as number of pixels. Average brightness of a single object in the image. Inertia momentum of a single object. This measurement is useful to determine how round or long is the object. Angle of the object, measured as the angle of the inertia axis.

The above characteristics have been developed as computer programs in some books (Lindley, 1990). Every feature in the above list (except for the brightness that requires a diffuse lighting) can be performed using backlighting, which simplifies system installation and maintenance. These characteristics can be computed very quickly. Usually, such general purpose inspection systems come as compact systems, which include the camera and the lens, with a microprocessor integrated and already programmed with an easy-to-use menu. They also include the outputs normalised usually to transistor-transistor logic (TTL) levels. Some general purpose systems can be used for foreign object detection in vegetables and fruit. Usually inspection of these is carried out with a single vision system that analyses every step of production from detection of foreign objects to product sorting. However, it is possible that only the foreign body detection is necessary and, in which case, we can add a general purpose system with minimum installation cost. Shape processing can be very effective for some kind of detection. For instance, staples or nails can be easily identified using this technique as they exhibit a very characteristic contour. In an apple processing line, some objects such as small stones can be easily detected by a simple area inspection. Staples that commonly fall off from broken package boxes can be identified because their area is small in comparison with the fruit or vegetable examined, or because their elongation is bigger than that of the product as is the case with pine nuts or sunflower seeds. For simple measures binary images are adequate; they are easy to obtain and quick to process. Other measures such as texture, brightness and colour require diffuse lighting which is a little more complex. Texture analysis identifies a characteristic surface of an object. It differs from colour and brightness and usually requires specialised inspection


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systems. There are available in the market general purpose systems that can process textures. However, they usually only provide a number which is an index of the degree of texture in a surface. For instance, a completely plain surface will have a null index, and a completely rough surface will have the maximum index. This can be used to advantage by using the amount of texture to differentiate between fruits and possible foreign bodies. If a general purpose system does not offer good performance, specific software should be developed. Texture identification software algorithms can be expensive in terms of development time and code complexity, and this means a higher cost for the inspection system. Colour processing in vegetables can be a simple task if we use a proper illumination system. Usually, cameras provide colour images in a coded RGB system, but for colour processing, other codes offer better results. For instance hue, saturation and intensity (HIS) is a more natural code, where hue is the colour itself (red, brown, blue, etc), saturation shows how pure the colour is (i.e. the proportion of pure colour and grey colour – red is a pure colour, while pink is mixed with white) and intensity indicates how light or dark the colour is. A simple analysis of hue should be the starting point for identification of seeds or twigs in vegetables. In order to speed up the processing, some systems are designed to perform parallel operations on the fruits inspected. Aleixos et al. (2002) have described a system where the parallelism starts from the beginning, placing two cameras to take a colour image and an infrared image from a same scene. Two digital signal processors (DSPs) arranged in a master slave architecture process the information coming from these cameras. In most applications, a single feature is insufficient to perform a correct classification so industrial optical sorters combine several features such as colour, size, shape or texture from different vegetables to detect foreign objects. These systems can classify fruits at rates higher than a million objects per minute in certain applications.

7.3 Foreign body detection in the processing of olives and potatoes The processing of olives and potatoes usually includes the detection of foreign bodies. Each crop has the characteristics of its habitat.

7.3.1 Olives Olives may be packaged to be delivered as entire fruits or processed to obtain olive oil, which is their main use. This section focuses on raw olives and the necessary inspection techniques. There are no specific algorithms for foreign object detection in olives, but many techniques used to classify them can be utilised. For instance, Diaz et al. (2003) report on algorithms that classify table olives by means of

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computer vision. These algorithms group olives in four different classes, with the segmentation part common to foreign object detection systems. The olives are placed in a conveyor belt with cylindrical roller which separate them from each other, so they do not overlap, which makes segmentation easier. Some algorithms deal with overlapped fruits, such as the one described in Visen et al. (2001), developed to identify occluding groups of grain kernels that have a shape similar to that of olives. Such algorithms can perform segmentation of olives if they appear in groups with the same image. In the variety of possible foreign bodies that can be present in the vegetable, small stones are the most difficult to identify, due to their similarities to olives. In this case, background illumination is insufficient to obtain a good image to be processed and direct light is needed to identify stones by their colour. However, the combination of several characteristics to identify both stones and olives is more trustworthy and a combination of contour recognition and texture recognition is the optimal solution. Other kinds of foreign object usually present when processing olives are bugs and can be easily identified using shape identification or size measurement. Here, special lighting is very useful.

7.3.2 Potatoes Inspection processes for potatoes depend on the ultimate product. Potatoes may be entire, in the form of potato chips, or products obtained from smashed potatoes, such as purées and potato flour. Below is discussed the inspection of raw potatoes including methods and optical devices used. Human operators can usually differentiate objects in a clustered group easily, while a computer-based system can make mistakes. The first task consisting of the segmentation of potatoes to make them single. This can mean a lower processing speed that might not be acceptable. For automatic inspection at high speeds, a vision system that can perform segmentation of individual items appearing in a clustered group is needed. Marchant et al. (1990) propose a method for potato inspection without singulation using a vision system. The main task of this system is the grading of potatoes into size and shape, working at a speed up to 40 potatoes per second. Once the potatoes have been segmented, they must be classified. For this purpose, the system inspects several attributes such as length, minimum and maximum width and shape, and estimates weight from volume. Except for the last, the features are useful for foreign body identification, specially those that do not appear as objects on the potato surface. As we have mentioned, the simplest identification of foreign objects in potatoes is that which uses area measurement. Because potatoes have a certain size it is possible to determine a threshold between acceptable potatoes and small objects. Such small objects will include both foreign


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bodies and small pieces of potatoes, but only acceptable potatoes will be selected. Commercial systems, past the prototype stage, that can deal with ten tonnes of potatoes per hour using a single camera have been developed. Most of the algorithms used for the classification of potatoes can be used for the identification of foreign objects. Such identification uses a classification of images. Knowledge gained in the classification of vegetables is needed to identify those objects that do not fit certain parameters. Difficulties arise when foreign objects appear on the surface of the potato and a superficial examination is needed. In such cases, peeling defects are difficult to identify and need special lighting techniques like ultraviolet or filtered light. For instance, Palmer (1961) describes a technique based on visible wavelength spectral reflection to distinguish between potatoes and stones or soil clods. More detail about these lighting techniques is given in previous sections of this chapter. Not only do they detect foreign bodies, but also anomalies such as large defects in potatoes or even rot.

7.4

Future trends

For the future, we expect optical inspection systems to become more reliable, especially in the identification of some foreign bodies which require null tolerance. Once this requirement is achieved, other performances should be improved, such as increased processing speed which will allow higher volume processing of fruit and vegetables. In this situation, future trends in new technologies for detection of foreign bodies in vegetables are both anticipated in hardware and software. New hardware consists of the development of new illumination sources, such as ultraviolet, visible infrared or light emitting diodes and also in the use of new solid state array detectors. It is not expected that other advances such as new CCD cameras, or faster microprocessors will produce a large improvement in optical inspection systems, even when they increase the quality of the images, because current devices offer performances much higher than real needs. Possibly the way to real improvements is to be found in new software algorithms, specially in the segmentation task, where combinations with new lighting sources are essential to identify certain bodies. Such new lighting techniques, like ultraviolet or infrared or a combination of these can contribute much of value in the design of new optical inspection systems. Faster microprocessors will help if they are programmed to exploit their speed, and thus handle programs of increasing complexity in real time inspection. One big improvement is expected to arise from the change from conventional black and white image processing algorithms to those for colour image processing, where information comes in three channels (red, green

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and blue). In this way, new correlations appear in the design of segmentation methods which work not with single colour channels, but with a combination of three of them. On the other hand, there are some limitations that cannot be solved using optical inspection systems. This is partly due to the fact that only superficial images can be inspected and for some applications it is impossible to get to inner layers of material or vegetables where the defects may be. For instance inner inspection would be required in the case of a foreign object, like a piece of plastic placed inside a lettuce. Finally, optical inspection systems must grow so that they become easier to use by operators within the industry. Vision systems originally required extreme care in maintenance and often complex procedures for calibration. Currently, compact equipment, ready to use and with many built in autoset functions, is available.

7.5


Comparatively little is published on the identification of foreign bodies but there is more available on optical inspection systems in the food industry. Such information, combined with some general publications on computer vision, gives enough information for further research. For computer vision algorithms and techniques, the Institute of Electric and Electronic Engineers (IEEE) publishes their Transactions that feature several subjects of interest. The Institute also publishes books for general or advanced computer vision applications. For agricultural applications, the Transactions of the American Society for Agricultural Engineering (ASAE), contains valuable information about innovative systems. The Journal of Agricultural Engineering Research and Computers and Electronics in Agriculture are strong sources of information relating new techniques for agricultural uses in the first case and electronic equipment research in the second. For specific information about research on food, Trends in Food Science & Technology is a known publication which offers relevant information on this field. Finally, there are some associations which offer information in the form of publications, conferences, or advertisements of industrial machinery. These associations can be found on the internet and give the possibility of becoming a member, with the added value it means for obtaining publications or information. The ASAE is one of the more important ones, and provides all kind of information regarding agricultural matters. For machinery information, the Automated Imaging Association (AIA) (web site: www.machinevisiononline.org) organises meetings and has some publications available in technical papers and books. FoodProcessingMachinery. com (web site:www.foodprocessingmachinery.com) also organises forums and has links to sorted industrial products.


7.6

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References

aleixos n, blasco j, navarrón f and moltó e (2002) ‘Multispectral inspection of citrus in real-time using machine vision and digital signal processors’. Computers and Electronics in Agriculture, 33, pp 121–37. diaz r, gil l, serrano c, blasco m, moltó e and blasco j (2003) ‘Comparison of three algorithms in the classification of table olives by means of computer vision’, Journal of Food Engineering. (in Press). gonzalez r c and woods r e (1992) Digital Image Processing, Reading, MA, Addison-Wesley. graves m, smith a and batchelor b (1998) ‘Approaches to foreign body detection in foods’, Trends in Food Science & Technology, 9, pp 21–7. gunasekaran s, paulsen m r and shove g c (1985) ‘Optical methods for nondestructive quality evaluation of agricultural and biological materials’, Journal of Agricultural Engineering Research, 32, pp 209–41. johnson t (2000) ‘Bugs, nightshade, & rot. color processing applied to food safety’, Automated Imaging Association Conference. lindley c a (1990) Practical Image Processing in C, New York, John Wiley and Sons. marchant j a, onyango c m and street m j (1990) ‘Computer vision for potato inspection without singulation’, Computers and Electronics in Agriculture, 4, 235–44. palmer j (1961) ‘Electronic sorting of potatoes and clods by their reflectance’, Journal of Agricultural Engineering Research, 6, pp 104–11. reid w s (1976) ‘Optical detection of apple skin, bruise, flesh, stem and calyx’, Journal of Agricultural Engineering Research, 21, pp 291–6. singh n and delwiche m j (1994) ‘Machine vision methods for defect sorting stonefruit’. Transactions of the ASAE, 94, pp 1989–97. visen n s, shashidhar n s, paliwal j and jayas d s (2001) ‘Identification and segmentation of occluding groups of grain kernels in a grain sample image’, Journal of Agricultural Engineering Research (2001) 79(2), pp 159–66.

8 Microwave reflectance R. Benjamin, University of Bristol, UK

8.1

Introduction

Since no one sensor modality can guarantee to detect all possible types of foreign bodies, the food processing industry needs a range of sensing tools that can be used, selectively or in suitable combinations, depending on the particular need. Innovative concepts and advances in technology have recently become available, which jointly make microwave reflectance sensing practical for foodstuff screening. This should add a powerful new modality to the tool kit, with the potential of covering a wider range of scenarios than many alternatives do. Section 8.2 will survey the conceptual evolution of microwave sensing techniques, of progressively more sophistication and power, leading up to combined real and synthetic aperture, synthetically focused microwave inspection of food products as the preferred approach. Section 8.3 then discusses how this technique is matched to various types of foodstuff processing lines. Finally, Section 8.4 considers the strengths and weaknesses of the technique. The fact that this book devotes 11 chapters to detection technologies bears ample witness to the fact that no single sensor modality will meet all requirements. However, microwave imaging has the potential of meeting a wider range of desiderata than most, for the following reasons: •

•

There are very few potential contaminant materials which do not differ noticeably from the subject material in their specific microwave impedance, and hence are amenable to detection. Even very small contaminant particles can be detected.

Microwave reflectance • • • • •

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The contaminants are resolved, and their location is identified, in three dimensions. There are no significant restrictions on the manner of handling of the subject material. There is no significant restriction on the speed of flow of the subject material in its pipe or on its conveyor. There is no radiation or other ‘field’ that could affect the subject material itself, or could constitute a hazard to personnel near the sensor. The cost of the sensor, although large compared to that of a metal detector, is comparable with the costs of other sophisticated food-screening sensors, and very small in comparison with the aggregate value of the product it ‘protects’.

Hence there is a strong case for developing microwave sensing as one of the more important items in the toolkit available to food processors. This raises the question: Why is the microwave sensor not already available and indeed widely used? The answer is that, in order to yield all these desirable characteristics, the microwave sensor has to exploit a number of very recent ideas, and a number of technologies that are only just becoming available, as explained below.

8.2

Microwave imaging techniques

8.2.1 Microwave transmission imaging Perhaps the most obvious technique is the use of microwave transmission through the food product. This entails a physical configuration somewhat analogous to X-ray imaging, with a transmit antenna or array at one side of the product to be tested and a receive antenna system on the opposite side. It is, however, dramatically different in so far as the very small wavelength of X-rays permits the generation of precise projected images, whereas the relatively long wavelength of microwaves (even when compressed by the refractive index of the relevant foodstuff) merely results in a limited modification of the 2D signal pattern observed at the sensor plane. In practice, the principal information so generated is the twodimensional (2D) phase pattern of the received signal, with an inherent unresolvable ambiguity for phase changes by any integral number of complete radio-frequency (RF) cycles. The effect of the contaminant object spreads laterally in the course of onwards transmission, and it gets weakened in magnitude with increasing propagation distance from contaminant to the sensor surface. Thus the technique depends on a highly homogeneous food product, which yields a ‘normal’ 2D received signal pattern of extremely stable phase. This is then used as a reference, and contaminants are detected if they generate a significant observable departure from this reference.As detailed in the next paragraph, such detection, when achieved,

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provides minimal information regarding the nature, magnitude or location of the contaminant object. Let us consider the impact of such contaminants. For objects which are small compared to the in-medium RF wavelength, or for the edges of large objects, the transmitted RF signal experiences diffraction, which results only in a somewhat marginal effect on the phase observed. Large microwave-transparent contaminant objects will only produce the phase change corresponding to the difference of the transit time of the radiation through the contaminant, compared to the transit time through the equivalent volume of the wanted foodstuff. For a microwave-opaque contaminant object we can postulate an obstructed cross-section, normal to the direction of irradiation. We can then model this object by combining the radiation that would have been received in the absence of the contaminant object, with the effect of the above obstructed cross-section radiating a forward signal of the same amplitude but opposite phase to that incident upon its back. It is impressive that, despite these limitations, SIK in Sweden has devised appropriate advanced instrumentation and signal-processing techniques, and has shown that useful results can be obtained, in a number of foodscreening scenarios, by the transmission mode of operation.1 8.2.2 Holography Holography, like the transmission mode, irradiates the subject volume with a plane wave, and records the resulting 2D amplitude and phase pattern, and it suffers from the same inherent unresolvable ambiguity for phase changes by any integral number of complete radio-frequency (RF) cycles. However, in this case it is the reflected or back-scattered signal which is observed, in place of that transmitted. For true holography, the (planar or curved) sensor surface should intercept virtually all the radiation scattered from the contaminant objects. Processing techniques equivalent to back projection of this pattern of scattered radiation can then generate a threedimensional (3D) image of the scattering entities, with a resolution of the order of the microwave wavelength, compressed by the refractive index of the relevant foodstuff (or other embedding medium). Given an appropriate geometric configuration or suitable matching to avoid surface reflection from the specimen volume, or appropriate signal processing to cancel it, any contaminant-free homogeneous foodstuff will then generate no reflected signal, and imperfectly homogeneous foodstuffs will merely generate relatively weak reflected signals, representative of local discontinuities in refractive index. In regard to its intrinsic capability of generating useful information, holography has the following advantages over the transmission mode: •

The phase reference used for signal extraction is independently known, and it does not depend on the nature of the foodstuff being examined.

Microwave reflectance •

•

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The magnitude of the signal exploited is that of the total reflected power, rather than merely the vector difference between the transmitted power in the presence and absence of a diffracting obstruction. The phase of a back-scattered signal is a very, very much stronger function of the range-wise location of the contaminant than the phase of a forward diffracted signal.

However, holography has, compared to reflecting surface-penetrating radar, the weakness that the wanted signal, received at one point in the sensor array from any given direction and range, has to compete with signals scattered in the same direction from all other ranges. This is highly significant since, as already indicated, most foodstuffs are not perfectly homogeneous, and hence generate some scatter from local discontinuities in refractive index throughout their volume. Furthermore, foodstuffs are generally quite heavily attenuating at microwave frequencies. Hence even weakly reflecting innocuous ‘clutter’ backscatter may well mask a more strongly reflecting contaminant at longer range than that clutter. In principle, this limitation could be overcome by combining holography with ‘burst’ illumination by short pulses.This would involve distinctive holographic focusing in respect of each resolvable 3D resolution cell, by the appropriate coherent combination of the signals received at the sensor elements at the relevant times. However, the author is not aware of any attempt to implement such burst-illumination microwave holography. If used, it would endow holography with many of the merits of the realaperture synthetically-focused radar technique described in Section 8.2.5. However, compared to that technique, even burst-illumination holography would have the disadvantage that it benefits from focusing only in the receive direction, whereas the radar described in that section can effectively focus in the transmit direction as well, thus yielding a higher image resolution and better clutter rejection. Furthermore, the radar solution is substantially less demanding in signal processing. 8.2.3 Basic surface-penetrating radar A pipe or conveyor belt could move a foodstuff being processed past a simple stationary radar of high range resolution. The pipe or conveyor (at least if the conveyor is irradiated from underneath) would have to be radar transparent, and the space between the pipe or conveyor and the radar antenna might be filled with a synthetic medium matching the dielectric characteristics of the foodstuff, to avoid interface reflection and refraction problems. A contaminant object would then give rise to an echo signal for as long as, in its passage along the pipe or conveyor, it is within the field of view of the radar. The observed range would clearly be minimum when the contaminant passes vertically above the radar. Hence the succession of these echoes can be exploited to aid detection. The point of minimum range defines the axial location of the contaminant, and the echo delay, at that

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point, defines the depth of the contaminant below the surface (stretched a little if the contaminant is laterally offset from the radar).2 In this or any other reflecting radar, contaminant objects are detected by reflections from the discontinuity in refractive index, arising at their front (and/or occasionally rear) boundaries. Such discontinuities in refractive index can be caused by the inclusion of a ‘foreign’ object, by a void, or by a region of non-standard compaction of the given foodstuff. Material such as potato chips or cornflakes would give a steady bulk echo, due to the aggregate reflections from the set of chips/flakes within the radar ‘footprint’, and this would change with spatial changes of the size and/or packing density of the chips/flakes. With a wide conveyor belt, the contaminant could be significantly displaced laterally from a radar antenna aligned with the longitudinal axis of the conveyor. It may then be desirable to deploy several radar ‘heads’ over the width of the conveyor. To prevent mutual interference, these could operate sequentially, one at a time. In view of the extremely short echoing times from even the maximum relevant range, there is ample time to do such pulse interleaving. If all these effectively parallel radars observe the contaminant object, there will then be a cross-track spatial pattern of variation of the echo delay, equivalent to the temporal along-track pattern described above, with minimum delay for the radar head most nearly vertically underneath the contaminant. Thus the radar yields the location of the contaminant objects in three dimensions, and it is immune to masking ‘clutter’ echoes outside a thin range-dimension shell, whose thickness is half the in-the-medium propagation distance of the wavefront in one pulse length. (The factor 1/2 arises from the fact that we observe the two-way, there-and-back propagation delay.) Considering, for example, a radar frequency of 10 GHz, a 50 % bandwidth, corresponding to a pulse-length of 0.2 ns, and a food-stuff of refractive index 3, the range resolution – and thus also the thickness of the ‘common-range’ clutter shell – is 1 cm.

8.2.4 ‘Conventional’ synthetic-aperture radar If a point object at a given postulated position were carried by the conveyor through the full diameter of a fixed radar’s field of view, the corresponding echo signals from all radar pulses, transmitted during this transit, could be noted, extracted, stored digitally, retrospectively aligned in time and phase, and combined phase-coherently for off-line focusing. If the convergence angle of the directions of view during this transit is q, the resulting along-track resolution can be shown to be l/2q, where l is the wavelength in the propagating medium.3 The depth resolution of this convergence-focusing process is 2l/q2. (For very small values of q, the depth resolution may however be determined by the radar’s pulse length.) Since we have no prior knowledge of the presence or location of contami-

Microwave reflectance

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nants, we have to postulate the possibility of contaminants in all distinct resolvable ‘resolution cells’ of the above dimensions. If the along-track diameter of the radar’s field of view is N times the along-track resolution, we can only obtain independent information on all echoes separated by this along-track resolution if at least N distinct pulses are transmitted and received in the time the conveyor takes to advance through the radar’s field of view. (Although not normally recognised by the synthetic-aperture radar community, the converse also applies: If it can be assumed that there will be not more than P target objects, these can in fact be resolved with little more than P pulses, albeit with a processing gain reduced in the ratio P/N.) Fortunately, this is not a significant limitation in the food-screening scenario, even if each such pulse has to be transmitted M times in succession, for M parallel time-shared cross-track radars. In that case, the echoes of these M such cross-track displaced radars can be similarly combined, in real-aperture synthetic focusing, giving a cross-track resolution determined by the convergence angle achieved in this dimension. The resultant 3D resolution will, however, entail coherent integration of N ¥ M pulses for each of the potential distinct 3D resolution cells: a substantial processing load.

8.2.5 Real-aperture synthetically-focused radar As already noted, the 3D resolution version of the scheme delineated in the preceding section combines synthetic focusing from a synthesised virtual along-track aperture with functionally equivalent synthetic focusing from a real cross-track aperture. However, the designers implementing this scheme failed to recognise and exploit the additional degrees of freedom offered by the fact that, in one dimension, they had the benefit of a real aperture. Generalising this scenario, consider a real transmit antenna of 2D aperture T ◊ (l/2)2, associated with a real receive antenna of 2D aperture R ◊ (l/2)2. Irrespective of whether these antennas are arrays, parabolic reflectors, lenses or some other form of collimators, they may be considered, analysed or modelled in terms of T transmit elements of sub-aperture l/2 by l/2, and R similar receive elements. The coupling from the transmit antenna via a given resolution cell to the receive antenna can thus be decomposed into T ¥ R distinct, coherently-combined constituent paths, each linking one specific transmit element via the given resolution cell to one specific receive element. When the same antenna is used for both transmission and reception, only T 2/2 distinct such paths are formed, since the coupling between a pair of elements A and B is the same irrespective of whether A is transmitting to B or B is transmitting to A. With a synthetic aperture, generated by a movement relative to a single antenna element, there is no option but to exploit the T discrete locations of the antenna, relative to the medium observed, to generate T monostatic paths from and to that antenna. However, in exploiting T real elements

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solely for generating T monostatic paths, system designers have turned a blind eye to the additional contribution potentially available from the T(T - 1)/2 distinct bistatic paths. Since the signals from all these paths add coherently for the wanted signal, but only non-coherently for both noise and clutter, this clearly discards significant potential signal-processing gains. Most practical scenarios tend to be clutter limited, rather than noise limited. Hence even techniques which use RF power inefficiently (i.e. without exploiting the potential additional RF gain) in order to make these multiple path signals separately available are well worth considering.

8.2.6 Time-sharing in real-aperture synthetically-focused radar The two-way propagation times in near-field surface-penetration radar are generally extremely short compared to the required interval between transmissions. As indicated in 8.2.3, this permits ‘parallel’ radar heads to transmit in turn, to avoid mutual interference. Furthermore, it also permits a single transmitter and a single receiver and digitiser to be switched between these multiple antennas, to economise in equipment – and this also reduces calibration requirements. As indicated in 8.2.5, such systems are rarely noise limited. Hence, with a given available total transmitter power, it is generally also permissible for each transmitter to operate repeatedly, once for each associated receive element, so that, even for multi-static operation, a single receiver and digitiser can be switched between the relevant receive antennas, for economy in equipment and calibration. Thus, if all transmit antenna elements and all receive antenna elements share common fields of view, the transmitter is switched in turn to T elements, and it transmits R consecutive pulses from each of these elements, whilst the receiver and digitiser is switched to the R receive elements in turn. Irrespective of such further time-sharing, transmission from only one element at a time permits the independent reception of the signals from all distinct two-way paths: transmit-element A via any given resolution cell to receive-element B. This permits retrospective off-line focusing, using the recorded path signals, to select from each path the signals corresponding to any desired 3D resolution cell, and to endow them with the appropriate time and phase shifts to combine them coherently. This coherent focusing can be undertaken independently for all distinct resolution cells, or for any desired number of selected cells, as first implemented in the mine-detection scheme described in 8.2.7. In fact, the independence of the paths yields the potential for yet further gains. The T · R or T 2/2 paths generally differ in a predictable manner, defined by their geometry, in respect of: • •

Spreading losses. Attenuation.

Microwave reflectance • •

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Clutter susceptibility. Focusing effectiveness.

Hence, in combining the recorded signals corresponding to the distinct paths for any given resolution cell, these signals can also be optimally weighted, limited, or otherwise processed according to the combination of these four parameters and their relative importance – and this is also done in the mine detection application.4

8.2.7 Non-food applications of real-aperture synthetically-focused near-field radar Mine detection The problem for which this technique has been primarily considered to date is the detection of buried landmines. The detection of small plastic antipersonnel mines is almost invariably clutter limited. Hence a technique was devised for this application using a 2D horizontal array, swept across the ground by a motor vehicle, with transmission by one antenna element at a time, and reception (in parallel or sequentially) by all elements sharing (at least) part of their field of view with the transmitting element. Thus the signals for the separate paths could be separately recorded and processed in accordance with all the objectives listed above.4–9 This resulted in a dramatic improvement in capability compared to previous ground-penetrating radars. The detection of buried landmines has many similarities to the detection of contaminants in food, however, with the following main differences: •

•

•

•

•

The medium examined (earth, possibly stratified, and with buried stones, roots, rubble, debris, etc.) is much richer in sources of clutter; and much less homogeneous or statistically ‘well-behaved’ than most foodstuffs. Both the objects sought and the ranges involved in mine detection are much larger, implying the use of longer wavelengths, i.e. lower, more manageable frequencies. The mine-detection radar has to be moved over large areas of possibly rough and overgrown ground, compared to foodstuff in a pipe or on a conveyor belt, moving or flowing smoothly past a fixed radar. It is important to distinguish mines from innocuous objects buried in the ground, whereas virtually any distinct object included in a foodstuff is a contaminant. Such precise classification normally also permits very precise location of a buried mine, whereas, in some food-processing scenarios, classification, size estimation and location may be of secondary importance.

Nevertheless, there is scope for substantial cross-fertilisation from mine detection to food-screening.

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Civil engineering and other variants Variants of the above instrument have obvious applications both to archaeology and to surveying landfill and other sites before committing to the erection of major structures. A more refined variant, currently under investigation, would be optimised for looking for buried pipes or cables, either to help the owners of these systems, for fault location and repair, or to prevent other civil engineering work from damaging such critical infrastructure elements. In due course, experience from developing and operating such systems could also cross-fertilise with synthetically-focused microwave food-screening instruments. Tumour detection Current screening techniques for breast cancer detection are handicapped by the very low X-ray contrast between tumours and healthy tissue, resulting in over 20 % of missed detections, and a similar proportion of false alarms. The author postulated, and experiment confirmed,10 that malignant tumours, being metabolically much more active than the surrounding tissue, will be markedly richer in saline fluid, resulting in a significant dielectric contrast. Hence such tumours should be observable by appropriate microwave imaging. A research project to use real-aperture, syntheticallyfocused, near-field radar to realise this capability has therefore recently been initiated at Bristol University. This work is still in its early stages, but work with both computer simulations and ‘phantoms’ is yielding very promising results. This work has (amongst other work) the following similarities to the detection of contaminants in food: • Both applications involve similar refractive indexes (~3 or 4). • Both can embed the sensor array in a synthetic medium contiguous with the wanted medium and of similar refractive index. • Both seek to detect objects down to sizes of 1 or 2 mm. However, there are also the following main differences: • • • •

The medical sensor array is static over a static but non-planar medium, requiring a compliant, index-matching interface medium. The medical medium is less ‘well-behaved’ than most processed foodstuffs, due to the presence of skin, blood vessels, glands, sinews, etc. The screening time allowable per patient is very substantially longer than that, say, for a can’s worth of baby-food! With lives potentially at stake, the permissible cost per instrument, in the medical application, is larger than may be acceptable to the foodprocessing industry. (See, however, the discussion of permissible cost in Section 8.4 below.)

Nevertheless, there is clearly scope for substantial cross-fertilisation between breast-screening and food-screening.


8.3

141

Microwave inspection of food products

8.3.1 Adaptation to piped, homogeneous food products Many processed food products are reasonably homogeneous and, at some stage in their manufacture, pass through a pipe – or at least could readily be made to do so, if desirable for inspection. Furthermore, the relevant section of pipe can generally be made microwave transparent (e.g. glass, plastic or ceramic) and, for unimpaired transmission, the pipe material can be chosen to have a dielectric constant reasonably similar to that of the foodstuff, or it can be made thin compared to the wavelength in the medium, or preferably both. The relevant section of the pipe may then be surrounded by a solid sleeve, shaped like two cones, base-to-base, separated by a thick disc, as shown in Fig. 8.1. The propagation characteristics of this sleeve would be substantially matched to that of the relevant foodstuff. However, if the foodstuff is highly attenuating, a synthetic medium of equal refractive index but lower attenuation would probably be chosen for this ‘sleeve’. A production line is likely to be used for different food products at different times, particularly where seasonal products are involved. Fortunately, the refractive index is unlikely to vary dramatically between different foodstuffs, and hence a single compromise value can be chosen for the sleeve. On the other hand, the attenuation coefficient is liable to vary more widely. However, the attenuation coefficient of the sleeve will be fixed by design (at a fairly low value), and that for the foodstuff itself can be handled as an input parameter to the signal processing algorithm. Only differences in the aggregate attenuation experienced within the volume of the foodstuff, for

Pipe

Pipe

Sleeve

Sleeve Antennas Y X

Z

Fig. 8.1

Pipe with food-matching sleeve and semi-annulus of antennas.

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the various two-way paths to a given resolution cell, and for all the distinct resolution cells, are relevant. As an illustrative example (near the top end of the range of likely degrees of complexity and sophistication), let us assume that, say, 49 equally-spaced antenna elements are mounted on 180 ° of the circumference of the central disc, alternate ones (25) receive-only and the interleaved ones (24) transmit-only. (The 180 ° of the ‘sleeve’ not associated with antennas may not be essential, but is useful to avoid unwanted short-path reflections from the far surface interfering with wanted long-path reflections from potential contaminant locations.) Due to practical geometrical constraints, the cross-track angle of view of these antennas would be substantially in excess of the diameter of the pipe. However, since the excess volume viewed contains only ‘well-behaved’ synthetic sleeve material, it will not give rise to unwanted clutter. The along-track angle of view would match the taper of the double-conical sleeve. This would permit the distinct generation and reception of sets of 25 ¥ 24 = 600 paths to any resolution cell, from the instant when that cell first enters the field of view of these 49 antennas, until it finally leaves it, i.e. for the full time of its transit through the ‘sleeve’. The rationale for this configuration is explained in the next two sections. As discussed in 8.3.5 below, this is an ‘upper-limit’ design. Nevertheless, the fairly large number of antennas need not imply high cost or complexity. The antennas can be patches or other printed-circuit ones. As detailed below, a single transmitter, single receiver and single digitiser can be timeshared between these antennas. The peak power of the transmitter would be of the order of 1 watt. Even with the very severe time-sharing requirements postulated below, the transmitter would be emitting for only 1/520 of the time: a 0.2 % ‘duty factor’, and hence the mean RF power would be only 2 mW.

8.3.2 Real-aperture cross-track synthetic focusing Each one pulse-length’s t worth of received signal, centred on echo delay t, defines a ‘footprint’ volume, such that all dielectric discontinuities within that volume contribute to the signal and/or clutter received. But the locus of constant go and return path delay between antenna elements A and B is a hyperboloid of revolution, with foci A and B. Hence this footprint volume is a thin curved shell, whose outer and inner surfaces are defined by the hyperboloids for delay t + t/2 and t - t/2 respectively. This volume is further restricted to the area, within this hyperboloidal shell, common to the angular fields of view of antenna elements A and B. For a resolution cell within the vertical plane through the antennas, the cross-track location of that cell is defined by the intersection of these locus surfaces. Within this central cross-track plane, the directions of these locus surfaces are approximately parallel to the axes joining their foci and, with the proposed antenna configuration, these angles are spread in angle over almost 180 °


Fig. 8.2

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The range of sizes and orientations of the axes of ellipsoids arising from the pairing of antennas.

(see Fig. 8.2). The resultant cross-track (horizontal and vertical) resolution d is then la/2rq, where la is the free-space wavelength, r is the refractive index, and q is the convergence angle of the directions of view.3 Thus, for instance, for la = 5 cm (RF frequency of 6 GHz), r ª 3, and q ª p, d ª 2.5 mm. This reduces the locus, common to all these paths, to an elongated ‘stick’, parallel to the axis of the pipe, ª2.5 mm in diameter. Figure 8.3 illustrates the focusing process for a typical resolution cell in the central crosstrack plane, simplified by limiting it to the relevant area, i.e. that within the pipe, and by ignoring the slight curvature of the loci. Normally we require (at least) as many independent input measurements as the number of independent ‘output’ parameters to be derived therefrom. In the present instance, the output parameters are the magnitudes and phases of the signals in all the 2.5 mm diameter resolution cells in the 15 cm cross-section of the pipe: a total of 2800. If the radar has an effective pulselength of 3 cycles at la = 5 cm, the refractive index of the medium is 3, the pulse-length in the medium is 5 cm. Thus independent samples arise when the two-way path-length changes by 5 cm. Clearly, this depends on the angle between the go and return paths, but we may deduce that the average path generates independent measurements for every 3 cm change of range. Hence it generates 5 observations within the 15 cm diameter of the pipe. Hence 560 independent paths are required to define the 2800 independent resolution cells. Thus the 600 paths proposed embody a modest factor of

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Pipe

Within-pipe section of locus, i.e. source of clutter

Area of non-coherent overlap

Area of coherent focus

Fig. 8.3

Cross-track synthetic focusing.

safety. In fact, we can assume that only a small minority of resolution cells are associated with anomalous signals, and these could then be defined with a substantially smaller number of paths. At the lower limit, three paths suffice to define the 3D location of one point target. A practical ‘austerity’ design will be outlined in Section 8.3.5. In the meantime, we shall examine the realisation and implications of an ‘upper-limit’ design, and hence we propose to use the full 600 paths, for the present.

8.3.3 Synthetic-aperture along-track focusing Let us now consider the along-track transit of the resolution cell or point object, over the length of the sensor sleeve. This is conveniently mapped in the plane containing both that transit and the axis of the pipe. Assume that the along-track beamwidth of the antenna elements is ±60 °. When a resolution cell first enters the sensing segment of the pipe, at relatively long range, the locus ‘stick’ there generated will be inclined at -60 ° to the axis, in that plane. As it continues with its transit, this negative angle is progressively reduced, becoming zero at the centre of the transit, and then increasing progressively to +60 °. Thus, for along-track resolution, q is 120 °, and so the resolution, in this direction, using the same formula as above, is ª 3.75 mm. Figure 8.4a illustrates the variation of the radii and crossing angles of the relevant sections of the ellipsoidal loci with axial offset, and Fig. 8.4b shows how these loci intersect in synthesised coherent focusing. Although the size of the 3D resolution cell is 2.5 mm by 2.5 mm by 3.75 mm,


(a)

Fig. 8.4

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(b)

Along-track synthetic focusing (a) Loci for point object at different axial offsets (b) Intersection of these loci in along-track focusing.

subject to reasonable signal/noise and signal/clutter ratios, the smallest detectable objects can be very much smaller than this. Achievement of this resolution – and of the full associated coherentprocessing gain – depends on the phase centre of the relevant reflection remaining effectively constant over the full range of viewing angles. This condition is inherently met when observing foreign objects small compared to the in-medium wavelength, or when observing similarly small distinct features of a larger foreign object. It is not normally met for large objects of reasonably smooth contour. Thus there is a degree of self-optimisation: both the resolution and the coherent-processing gain are maximum where the requirement for high resolution is strongest, and where the intrinsic echo is weakest (and where there is no scope for multi-resolution-cell, noncoherent processing gain). For further illustration, let us assume that the pipe is 15 cm in diameter, the maximum diameter of the ‘sleeve’ is 60 cm, and hence the length of the sleeve is ±30 cm ◊ tan 60 ° = 1.04 m. The maximum 2-way path-length is then approximately 60 cm ¥ sec 60 ° = 120 cm and, with a refractive index of 3, the corresponding propagation time is 1.2 ¥ 10-8 s. For full information, a new set of 600 transmissions has to take place for each advance of a point object by the axial resolution of 3.75 mm. (Once again, if only a small minority of resolution cells are associated with anomalous signals, these could in fact be defined with a substantially smaller number of paths, but we ignore this for the present.) Hence, if the food advances at 1 m/s through the pipe, a new set of transmissions has to take place once per 3.75 ms. In order to timeshare a single transmitter and a single receiver, each set of 600 path signals will entail 600 separate transmissions. Even this will allow 6.25 ms per transmitted pulse, 520 times the maximum relevant path propagation time! With one set of 600 transmit pulses for each 3.75 mm of advance of the foodstuff through the 1.04 m long sensing region, the total number of such pulse sets relevant to a resolution cell or point object is 277. Thus a total of 277 ¥ 600 = 166 200 single-pulse echoes contribute to the eventual signal.

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8.3.4 Instrumentation problems Both the RF technology of the scheme and the signal processing are well within the present state of the art. The main challenge to be met is that of data capture. Wide dynamic-range Nyquist or higher-rate sampling, for digitisation of an RF signal at 6 GHz, is non-trivial. In view of the wide fractional bandwidth, there is little scope or potential benefit in prior down-converting the received signal to a lower frequency, to permit a lower sampling rate. There are several techniques which can be used, jointly or separately, to ease this problem. Perhaps the most obvious is ‘sensitivitytime control’, also known as ‘swept gain’, where the gain of the amplifier is varied inversely with the predicted path attenuation, thus drastically reducing the dynamic range of digitisation. (This might be regarded as an analogue form of ‘floating point’ operation.) Since the gain adjustment for each time delay is known, the swept-gain effect could in principle be allowed for, to restore the true amplitude in subsequent digital signal processing. However, in general, the sensitivity-time control would merely perform, at the front end, a normalising function which would otherwise be required in due course, in any case. Somewhat similar results can also be achieved with a logarithmic amplifier, where the result of a slowly increasing attenuation A and a rapidlyfluctuating clutter-plus-target signal C is log (A ¥ C) = log A + log C. The log A term can then be removed by low-pass filtering before digitising log C. If desired, the log law for C can be re-linearised either before or after digitisation. The value of log A can be noted or separately deduced, for use in subsequent processing, where relevant. The key to other measures for easing data capture is the disparity between the maximum echo delay of 12 ns and the 6.25 ms interval between pulses. (In fact only 7.5 ns out of the 12 ns represent potential echoes from within the diameter of the tube, at any of the relevant angles of incidence into the tube.) In its simplest form, this time margin permits real-time online analogue recording, by a chain of ‘sample-and-hold’ devices, followed by ‘reel-time’ off-line slow-speed analogue-to-digital conversion. With the postulated bandwidth of 6 GHz/3 = 2 GHz, Nyquist-rate sampling would be at a rate of 4 GHz, i.e. samples spaced by 0.25 ns, say 0.2 ns to allow a margin of safety. Hence the relevant 7.5 ns time window would be covered by 38 sample-and-hold devices. A single time-shared digitiser could then operate on these in turn, with 0.16 ms per digitisation. Immediately consecutive sample-and-hold operations might interfere with each other, but this could be prevented by splitting each output into two identical parallel branches, one collecting 19 odd-numbered consecutive waveform samples, and the other collecting the even numbered ones. Sample-and-hold devices for the short samples involved have not long been available, and even now their effective use may depend on the prior gain normalisation, to reduce their dynamic-range requirement, as discussed earlier in this section.


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8.3.5 A practical design Time sharing is conceptually simple and, as shown above, is easily compatible with all the intrinsic time and sampling rate constraints of even an ‘upper-limit’ design. However, the time-share switches are at the limit of current technology, and are quite expensive. They need to: • • • • • •

Have extremely high operating speed. Handle microwave signals. Be of wide bandwidth. Have a small and well-defined insertion loss in the ‘closed’ position. Have a very high minimum assured attenuation in the ‘open’ position. Be capable of an exceedingly large number of operations.

Hence transmitter switching is likely to involve a common low-level coherent driver, fanning out to one switched (i.e. on/off modulated) final-stage amplifier per transmit antenna, and the equivalent receiver switching would involve one switched first-stage pre-amplifier per receive antenna, fanning in to a common main receive-amplifier. (For large fan-out ratios, one ‘trunk’ may feed b ‘branches’ which, in turn, feed t ‘twigs’ each, thus yielding a combined fan-out ratio of b ◊ t, and the same applies obviously for fan-in.) Neither switching time nor the number of switch operations are then significant problems. Nevertheless, there is then a substantial incentive for looking for an implementation concept, requiring fewer transmit and receive antenna elements. (Fewer switch actuations and longer actuation times would also improve the prospect for using micro-electromechanical MEM devices, which promise to improve the performance and reduce the costs of switches. However, one actuation per millisecond entails 3.6 million per hour! Thus even 109 actuations before failure would only give 278 hours of service.) In this context, we note that the upper-limit design entails 166 200 singlepulse echoes combined coherently, whereas the clutter in the much larger individual footprints combines non-coherently. Thus the individual pulses could have a signal-to-clutter ratio as low as -40 dB, and still yield an eventual combined signal/clutter ratio of +12 dB. However, the sub-clutter visibility defines the minimum dynamic range of the digitiser (irrespective of any prior gain normalisation). In the example given here, this is 40 dB, i.e. 14-bit digitisation. This is within the capability of currently available devices, but digitisation of lower dynamic range would permit the use of digitisers of higher speed and/or lower cost. This therefore provides a secondary incentive for a less ambitious implementation. It is in fact exceedingly unlikely that a significant sub-clutter visibility is required in any food-screening application. Thus a number of paths, in excess of those required for resolution purposes, is not warranted by the anti-clutter gain they would generate. Obviously, they also generate a gain against thermal noise. However, that gain would be obtained equally by transmitting repeatedly over each of the smaller number of paths or,

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preferably, by using fewer pulses, of higher peak power but unchanged mean power. Even in the mine-detection problem, where clutter is a serious problem, some hundreds of paths have proved quite adequate, and early results for tumour imaging suggest that of the order of 100 paths are likely to be sufficient. On this basis, it should be ample to use 8 cross-track antenna transmit elements, interleaved with 8 receive elements. Thus this configuration would generate 64 distinct paths in the cross-track dimension, and we would transmit 25 such sets of 64 pulses, a total of 1600 pulses during the onesecond transit of any resolution cell through the 1 m sensing space. This requires only two 8-way switches: one for the transmitter and one for the receiver. It allows 0.625 ms of switching time for these switches. To establish each two-way path once, the 8 transmitting antennas, operating in turn, would each transmit to the 8 receiving antennas in turn, at 0.625 ms intervals. Hence the transmitter is only switched to a ‘new’ antenna once every 5 ms. However, the time available for switching the common transmitter is then still 0.625 ms.With 30 samples per pulse, somewhat above the Nyquist rate, 20.8 ms would be available for each off-line digitisation. Since there is no real problem in performing a digitisation in 1 ms, it would also be possible, within the given antenna-switching scheme and schedule, to transmit each of the previously identified pulses twice (or even n times, n < 21), separated by 1 ms, and to organise the 20 samples as two (or n) distinct interleaved sets of 10 (or 20/n) samples. Thus each time a different set of samples would be collected and then digitised, using the same, timeshared, set of 10 (or 20/n) sample-and-hold devices, thus reducing the number of these devices. During each set of 64 cross-track pulses, each resolution cell within the specimen material will advance by 40 mm within the pipe. For a wavelength, within the medium, of lm = 17 mm, this represents 2.35lm. However, since this movement, at the centre of the pipe, is precisely known, and since the taper of this movement towards the pipe boundary, in a medium of given viscosity, is also known, this effect can be taken into account in signal processing. A lower-limit, ‘austerity’ design might use just 4 cross-track transmit antennas interleaved with 4 receive antennas, yielding sets of 16 paths, and 15 such sets of paths, recorded in the along-track direction, to give a total of 240 paths. The three design options are compared in Table 8.1: Table 8.1 Comparison of design options

Cross-track paths per set Along-track sets Total no. of paths Sub-clutter visibility Switching time

Upper limit

Practical

Austerity

600 277 166 200 30+ dB 6.5 ms

36 25 900 15 dB 1 ms

15 20 300 10 dB 3 ms


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8.3.6 Application to conveyor belts A narrow conveyor belt, carrying a thick layer of foodstuff, poses basically the same problem as a pipe, and can be tackled in the same manner, provided the go and return parts of the relevant section of the conveyor are separated far enough to position the sensor and half the sleeve between them, with the belt sliding over the matching sleeve (see Fig. 8.5). The only significant difference is then that the top surface of the moving foodstuff on the conveyor may have to be kept clear of the stationary matching medium. Hence, we may only be able to use a half ‘sleeve’, as shown in the diagram, or there may have to be an air gap between the foodstuff and top half of the sleeve. In either case, there will be increased interference of short-path signals which have already penetrated the foodstuff with longerpath signals which have not yet penetrated it. With a reasonably homogeneous foodstuff, this effect is predictable, and so the problem can be handled in signal processing, with little degradation of performance. For a wide conveyor belt, possibly carrying a thin layer of foodstuff, the range of angles over which the belt can be effectively viewed is substantially less than 180 °. If all the sensors are to view the full width of the belt, their beam-width may have to vary with their off-axis displacement. In the limit, the body-of-revolution sensor array may have to be replaced by a planar array, and any point object, passing above the sensor array, may be within the field of view of only a subset of the cross-track set of sensors. The reduced number of sensor heads viewing any given resolution cell will then result in a degradation of anti-clutter performance, and the reduced range of viewing angles will degrade the horizontal across-track resolution, to 3.75 mm for q = 120 °. Furthermore, this across-track resolution is then effective only in the horizontal plane. The vertical depth of focus, due to horizontal focusing, is then 2l/q2 = 8.3 mm for q = 120 °.3 Fortunately, this degradation is not serious enough to undermine the viability and utility of the technique. 8.3.7 Conveyors carrying cartons or random chunks of food It will normally be desirable to discover and eliminate any foreign bodies or other contaminants early in the processing chain, to avoid nugatory Foodstuff

Antennas belt

Fig. 8.5

Configuration for moving-belt conveyor.

Index-matched medium

150


processing, and probably also to improve the chance of tracing the source of any contamination. However, some applications may call for a final quality check of the finished product. When this is not in a metallic can but, say, in a plastic, wood or cardboard box, it is still amenable to the sensing technique described above. If the packaging is thin compared to the inmedium wavelength, its effect on radiation near-normal to its surface may be minimal. However, at least some of the surfaces of the container will normally generate a significant radar reflection. If these containers are placed onto the conveyor in well-defined locations and orientations, the standard radar image of the packaging can be pre-stored and subtracted from the observed images, thus generating images of the contents by themselves. If the standard containers are placed onto the conveyor in random locations and/or orientations, more sophisticated signal processing is required to extract the radar image of the container, and then to translate and/or rotate the observed image of the container and its contents electronically, so as to synthesise the condition of the preceding paragraph. At this point, the radar image of the container can be therefore removed, and the contents are viewed in standardised conditions, as before. With soft packaging, there can be considerable tolerance on the shape of the package, which adds to the uncertainty regarding its location and orientation. Nevertheless, it should be possible to define the generic form of the packaging sufficiently well to derive and subtract its contribution to the composite radar echo with reasonable accuracy, leaving a viable radar image of its contents. However, this inevitably will entail some loss in the sensitivity in the detection of any freak anomalies, which this sensor may not be able to distinguish from local thickening of the (largely unspecified) wrapping. However, due to the inherent 3D resolution of this sensor, that limitation is unlikely to prove significant in practice. Any chunks or slabs of meat, at an early point in a food-processing chain, are normally de-boned and otherwise converted to a more standard form before screening. Indeed, one of the objectives of microwave screening may then well be to provide assurance that no bone-fragments remain. However, if examination at an earlier stage is required, it is possible, in principle, to derive the 3D surface of the ‘chunk’ and that of any bone within that chunk, within the dimensional tolerances of the microwave sensor image. However, in most scenarios, an X-ray sensor would be more appropriate for this function.

8.3.8 Detecting thin elongated objects and thin sheets The microwave ‘echoing cross-section’ of bristles, wire-fragments and other thin, elongated objects depends primarily on the resolved component of their length in the direction of the microwave polarisation. In some applications the nature of the process will tend to orient such objects in the direction of flow, or a known source of such potential contamination


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predetermines their orientation. The sensors can then be oriented accordingly. Normally however there is no such a priori information. In that case, sensors of three mutually orthogonal polarisations should be uniformly interleaved – or as uniformly as practical, if the number of transmit and receive antennas are not both divisible by three. In practice, all antenna elements will be flush mounted with the local surface of the ‘sleeve’. Thus, when oriented in the along-track direction (the z-axis, see Fig. 8.1), they will all be parallel to each other, and hence copolar. When oriented cross-track, however, they will vary progressively, between being aligned with the x-axis, at the two ends of the 180 ° arc, to being aligned with the y-axis at its centre. This yields a uniform distribution between X-direction and Y-direction linear polarisation. Hence one element in three should be aligned with the z-axis. (However, one transmit element of polarisation A and three receive elements of polarisations A, B and C will give rise to one co-polar path A–A and two cross-polar paths, A–B and A–C. Generalising this, there will be twice as many cross-polar paths as co-polar ones.) The received path signals to one resolution cell may then be grouped into three sets of mutually orthogonal co-polar echoes and three set of mutually-orthogonal cross-polar ones. The ratios of the means of these six quantities could give a good indication of the orientation and length-todiameter ratio of any elongated object. Similarly to fibres, thin sheets, for example of plastic, are virtually invisible to radiation polarised normally to such a sheet’s surface. However, with the proposed polarisation diversity, they should clearly be detectable. The two-dimensional continuity of such a sheet will then provide a significant (non-coherent) processing gain, thus permitting the detection of even very thin sheets, of low dielectric contrast to the embedding foodstuff medium.

8.4

Strengths and weaknesses of microwave sensors

8.4.1 Capability Microwave sensors can detect any spatial discontinuity in dielectric constant. Obviously, this includes the two extremes: metals and voids. In food products comprised of loosely-packed flakes, chips or fibres, it will also detect discontinuities in their compaction. In virtually all foodstuffs, it will detect glass, wood, stones or bones. As indicated in Section 8.3.3, the size and 3D shape of such inclusions will be delineated with a 3D resolution of 2.5 mm ¥ 2.5 mm ¥ 3.75 mm. However, given an adequate dielectric contrast, much smaller anomalies than this can be detected, and their location is then defined with the above precision. In most foodstuffs, the proposed radar will also detect such items as plastics, extraneous vegetable matter, insects or worms. However, when the

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dielectric contrast between the medium and the extraneous matter is small, the foreign object may have to comprise multiple resolution cells for reliable detection. The scheme can also yield a good ability to detect wires, bristles or fibres of reasonable dielectric contrast to the embedding foodstuff, or thin sheets of embedded contaminant. Thus this sensor can clearly detect a much wider range of contaminant than can metal detectors, and probably also a wider range than colour-based detectors. It also imposes less restriction on the nature of the processing chain than do colour detectors. Unlike a number of other sensors, it imposes no practical limit to the speed of flow through the food-processing system. It generates no X-ray or magnetic fields difficult to contain. It provides good information on the 3D location of a contaminant and, except for very small contaminants, also gives a useful indication of their size and shape.

8.4.2 Status The technical viability of synthetically-focused microwave inspection of food products depends on relatively recently developed4 (and patented11–14) technical innovations, on only recently emerged sampling, switching and digitising technologies, and on signal processing power which, in part, may depend on modern digital signal processing devices and/or fieldprogrammable gate arrays. Hence it is a concept, ready to be demonstrated and implemented, rather than a currently available commercial product.

8.4.3 Cost considerations It is difficult to assign a realistic cost to an instrument that is yet to be designed. Broadly similar laboratory systems, for tumour detection, work off-line, in slow time, have different cost objectives and constraints and can use commercial instruments such as vector network analysers and sampling oscilloscopes. However, it seems safe to assume that the installed cost of a production version of the proposed radar will be more than that of a metal detector, quite possibly less than that of a basic X-ray system, and certainly less than that of a tomographic X-ray system or nuclear magnetic resonance sensor. A more significant criterion (which includes the flow-speed of the processing line) is the following: what is the cost of acquisition, installation and operation over the lifetime of such a sensor, compared to the value of the aggregate product so screened in that time? In other words, what fraction of a per cent does it add to the cost of the product? The justification for that fractional cost increment, in turn, must be framed in terms of an insurance premium, to obviate the cost of possible litigation and the – probably much higher – equivalent cost of possible adverse publicity and any resulting loss of good will and market share.


8.5 1 2 3 4 5 6 7

8 9

10 11 12 13 14

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References berger u k and reimners m (2001) ‘Food radar traces foreign objects in products’, SIK annual report. daniels d j (1998) Surface Penetrating Radar, IEE, UK. benjamin r (1995) ‘Synthetic-aperture antennas’, Microwave Journal, 38(9). benjamin r, craddock i j, hilton g s, litobarski s, mccutcheon e, nilavalan r and crisp g n (2001) ‘Microwave detection of buried mines using non-contact, synthetic near-field focusing’, IEE Proc. Radar, Sonar & Navigation, 148(4). benjamin r (1996) ‘Near-field spot-focused microwave sensing for the detection of buried land-mines’, IEE/EUREL Conference on the Detection of Buried Land-Mines, Edinburgh, October 1996. benjamin r (1996) ‘Post-detection synthetic focusing in near-field radar’, IEE/EUREL Conference on the Detection of Buried Land-Mines, Edinburgh, October 1996. benjamin r, hilton g s, nilavalan r, litobarski s and mccutcheon e (1998) ‘Synthetically-focused surface-penetrating radar for operation from a moving vehicle’, IEE/EUREL Conference on the Detection of Buried Land-Mines, Edinburgh, October 1998. benjamin r, hilton g s, litobarski s, mccutcheon e and nilavalan r (1999) ‘Post-detection synthetic near-field focusing in radar or sonar’, Electronics Letters, 35(8). nivalavan r, hilton g s and benjamin r (1999) ‘A FTTD model for the postdetection synthetic focusing surface-penetrating radar with mine-detection applications’, IEE/URSI Conference on Antennas and Propagation, York, April 1999. pothecary n m, railton c j, johnson r h and preece a w (1994) ‘FDTD analysis of a non-invasive sensor for the detection of breast tumours’, IEEE Microwave Theory and Techniques Symposium Digest, San Diego, May 1994. benjamin r (1996) Near-Field Spot-Focused Microwaves, European Patent GB 9 611 801.3; 6/6/1996. benjamin r (1999) Apparatus for and method of detecting a reflector within a medium. US Patent 5 969 661, 19 October, 1999. benjamin r (1996) Synthetic Post-Reception Focusing, European Patent GB 9 611 800.5; 6/6/1996. benjamin r (1999) Post-Reception Focusing In Remote Detection Systems, US Patent 5 920 285, 6/7/99.

9 Nuclear magnetic resonance imaging B. Hills, Institute of Food Research, UK

9.1

Introduction

The early pioneers of nuclear magnetic resonance (NMR) could hardly have anticipated the enormous commercial impact of their seemingly obscure and academic observations of a nucleus resonating in a magnetic field. Yet over half a century later, their first observations have given birth to major industries manufacturing whole-body medical scanners and NMR spectrometers for research and analysis. Moreover, even half a century of research seems only to have skimmed the surface of the enormous hidden potential of NMR and this is reflected in the hundreds of research publications appearing every year. From a commercial viewpoint NMR also continues to be a major growth area. The relationship between field strength and spectral resolution is driving the development of ever-stronger superconducting magnets, which, in turn, creates the need for new generations of RF probes, shim and gradient coils and the software to drive them. The potential of functional MRI which indirectly monitors brain activity is also driving the development of new generations of clinical imagers. There does, however, remain one obvious gap in this exciting scenario. Despite all this ongoing research, NMR has failed to be developed as a sensor in the industrial manufacturing sector. This is surprising because, as this book testifies, almost every other type of spectroscopy has given rise to commercially viable sensors on the production line. Near infrared (NIR) sensors, X-ray sensors, ultrasonic sensors and optical sensors, to name but a few, are routinely found on production lines where they are used in a fully automated mode to detect foreign bodies and product defects. Yet, apart from a few rare examples in

Nuclear magnetic resonance imaging

155

niche applications, there are no automated NMR or MRI sensors on the production line. As an NMR specialist, this situation has always struck me as somewhat bizarre. It is true that simple, low-field NMR spectrometers are often used in an off-line mode to measure properties such as solid–liquid ratios in quality assurance but this is a pitiful state of affairs for a mature technique that has been researched for over 50 years. There are hundreds of research papers in the NMR literature showing useful correlations between NMR properties and the internal quality of materials as diverse as food, wood, minerals and concrete, yet most of these correlations are unexploited in on-line quality assurance. In this chapter we therefore review some of the technical and theoretical reasons why NMR has, so far, failed to penetrate the industrial production line as a foreign body and quality control sensor and, on a more optimistic note, point out some of the ongoing research directed at overcoming these limitations.

9.2 Principles of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) Polarisation is a necessary first step in any NMR or MRI experiment and involves inducing magnetisation in the sample by placing it in a magnetic field. An unmagnetised diamagnetic sample, such as an apple, placed in a static magnetic field, will develop magnetisation, M(t), at a rate that depends on the longitudinal relaxation time, T1, such that, M(t) = M(•)[1 - exp(-t T1 )]

[9.1]

The longitudinal relaxation time, T1, is characteristic of the nature of the sample and depends on factors such as its chemical composition, microstructure and molecular mobility as well as on temperature and magnetic field strength, B0. Pure water has a proton T1 of the order of 2–3 seconds. In equation [9.1] M(•) is the equilibrium longitudinal magnetisation given by Curie’s law, M(•) = c 0 B0

[9.2]

Here c0 is the static nuclear susceptibility, which depends inversely on the temperature, and B0 is the magnitude of the applied field. Naturally, magnetisation will be developed in all the different types of nucleus throughout the sample, provided that they have a non-zero nuclear spin quantum number, and this includes protons, natural abundance carbon-13 nuclei, sodium-23, phosphorus-31 nuclei and so on. However, the proton magnetisation is by far the most important for foreign body detection and originates mainly from protons in water together with contributions from the protons in lipids, carbohydrates and biopolymers. Once the NMR system has been tuned to detect the proton resonance frequency no other type of

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nucleus can be detected and this provides one possible basis for foreign body detection. A piece of glass in a jar of jam will be detected because, lacking protons, it gives no NMR signal in contrast to the proton-rich jam itself. The glass would therefore appear as a zero intensity region surrounded by a high intensity signal from the jam. Equation [9.1] is somewhat of a simplification in many real objects, such as an apple, because water in different parts of the sample, such as the apple core, the pips, parenchyma tissue and skin will have different intrinsic longitudinal relaxation times, T1, and even within a single type of tissue, there may be separate proton pools with different values of T1 associated with the microscopic and molecular structure of the tissue. In general, therefore, we should replace equation [9.1] with a multiple exponential decay function. The possibilities of exploiting these different intrinsic relaxation times for image contrast will be discussed later. However, for many applications it is sufficient to approximate the development of magnetisation as a single exponential with an effective longitudinal relaxation time. As mentioned, T1, in general, depends on B0 and usually decreases at lower field strengths. For example, human kidney cortex tissue has a T1 of 827 ± 26 ms at 100 MHz but only 206 ± 27 ms at 2.5 MHz. This is an important consideration because shorter values of T1 permit faster polarisation. This can be seen from equation [9.1] which shows that the sample will be fully polarised (i.e. contain the equilibrium magnetisation, M(•)) after times longer than about 5T1. This is not usually a problem when the sample is placed in the magnet of a conventional NMR spectrometer because it takes several seconds to set up the experiment. For example, T1 for water is about 2 s so 10 s is sufficient to fully polarise water and most biological samples. Unfortunately the situation is not so straightforward if we wish to develop NMR as a foreign body detector on samples moving on conveyors at speeds of several metres per second. Consider, for example, a bottle of water moving at 2 m/s on a conveyor. In the 10 s required to fully polarise the water the bottle has travelled 20 m. Creating a magnetic field extending 20 m over a conveyor belt is possible, but would not be commercially viable. Inducing sufficient magnetisation in the sample is therefore a significant problem to be overcome in on-line NMR foreign body detection. One mitigating feature is that the magnetic field used to polarise the sample need not be highly homogeneous. A homogeneity of 1 % would be sufficient to polarise the sample, especially if the sample is rotated in the field to average out the variations, but it would probably not be good enough for reliable NMR. This suggests that the best strategy for on-line foreign body detection in samples with long T1 values is to use separate polarising and detection magnets. The first, polarising magnet need only have low homogeneity, but preferably be of high field to achieve maximum polarisation, which suggests some type of permanent magnet. However, the second, detection magnet needs to be highly homogeneous and this require-


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ment is another difficulty when developing low cost NMR foreign body detectors. Having polarised the sample, the next step involves acquiring an NMR signal from the sample. For this the polarised sample needs to be irradiated with radiofrequency (RF) radiation having a frequency, w0, set equal to gB0. This is the so-called resonance frequency. There is one other essential requirement, namely that the magnetic field component, B1, of the RF must be perpendicular to B0 if it is to induce resonance. The usual way of achieving this condition is to surround the sample with carefully designed coils of copper wire. The RF is then created by passing pulses of AC current at the resonance frequency through the coil. In the simplest NMR experiment (the free induction decay, or FID) the RF frequency is set to resonance (w0 = gB0) and is turned on for just sufficient time (usually just a few microseconds) to make the sample’s magnetisation rotate through 90 degrees so that it ends up at right angles to B0. In this perpendicular orientation the magnetisation vector rotates (or, more precisely, precesses) at the resonance frequency around B0 and, this precession can be detected with the same coil of wire used to produce the RF. This is because, by Faradays Law of Induction,1 the magnetisation vector cutting through the loops of wire induces a small voltage oscillating at the resonance frequency and this can be filtered, amplified and recorded as the NMR signal. Obviously the precessing magnetisation does not continue precessing and generating NMR signal for ever and there are numerous mechanisms that cause the magnetisation to decay to zero, processes usually described as ‘transverse relaxation processes’ because they affect magnetisation that is in a transverse direction to B0 to distinguish them from longitudinal relaxation processes characterised by T1 which relate to the magnetisation parallel to B0 (see equation [9.1]). The net result of transverse relaxation is that the NMR signal decays with a time, T2, called the transverse relaxation time, such that, M(t) = M(0) exp[-t T2 ]

[9.3]

Like the longitudinal relaxation time, T1, the transverse relaxation time, T2, is a characteristic of the sample and depends on state variables such as composition and temperature. For foreign body applications it is sufficient to know that, in general, T2 gets shorter as the molecules giving rise to the proton signal become less mobile and more rigid. For example, the highly mobile water molecules in liquid water have a long T2 equal to T1 and about 2–3 seconds (depending on the water’s purity), whereas ice has such a short T2 of just a few microseconds that it cannot usually be detected in the time that most NMR spectrometers can respond (a few tens of microseconds). Ice therefore gives no detectable signal in NMR which is useful if ice crystals are regarded as a type of undesired ‘foreign body’ which may be the case in some foods. It also means that unfrozen regions in otherwise frozen foods are easy to detect. Such unfrozen regions could be a source of

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microbial contamination so NMR detectors could be extremely useful in the frozen food industry. Plastics, being semi-rigid materials, are also associated with very short values of T2 and this also means that plastic foreign bodies can be detected by NMR from their lack of signal. This is a unique advantage of NMR over detectors based, for example, on X-rays, which, because of the low atomic masses of carbon and hydrogen, struggle to detect most synthetic plastics. The relationship between resonance frequency and field strength, w0 = gB0, is also the basis of MRI. All that is needed to create a one dimensional profile of the sample in, say the X direction, is an additional externally imposed linear field gradient, Gx, where Gx is dBz/dx, because the resonance frequency now depends on the position of the nucleus in the sample: w(x) = gB0 + gxG x

[9.4]

The signal distribution plotted against frequency is therefore the projection of the sample along the x-axis. Imposing gradients along the Y and Z directions permits three-dimensional imaging. Naturally, the practice of MRI is considerably more complicated that this, but, fundamentally, it always relies on the imposition of external linear magnetic field gradients to make the resonance frequency spatially dependent. The interested reader will find more details of MRI in the lucid book by PT Callaghan, Principles of Nuclear Magnetic Resonance Microscopy.2 The principles of non-spatially resolved NMR can be found in numerous texts including the now classic work of A Abragam, The Principles of Nuclear Magnetism, published by Oxford University Press.3 Contrast in NMR images arises from differences in proton number density (which relates to the internal distribution of air, water, lipid and biopolymers) and from differences in the relaxation times, T1 and T2, which relates to more subtle factors such as molecular mobility and proton exchange rates, and to molecular diffusion. The dependence on molecular diffusion arises because the diffusion of molecules through an externally imposed field gradient alters their resonance frequency and causes signal attenuation to an extent that depends on the distance travelled in the direction of the field gradient. The dependence of image contrast on so many subtle molecular factors makes NMR an extremely versatile detector of internal features in the sample and this versatility is further enhanced by the fact that the time course of the radiofrequency excitation is under the operator’s control, so that the potential for creating new ‘RF pulse sequences’ to highlight desired features is enormous and limited mainly by our ability to understand and manipulate the underlying spin physics. This diversity sets NMR and MRI apart from all other forms of spectroscopy and imaging and also suggests that the commercial viability of an NMR foreign body sensor can be increased by its ability not only to check for foreign bodies but also to provide useful information about the internal quality of the sample. As an


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example, consider the sorting of apples on a conveyor. While an MRI sensor could pick out foreign bodies such as worms or insects, as well as their holes, the same sensor could, in principle, detect bruises, watercore as well as degrees of mealiness and ripening (the Brix value). This makes the sensor a far more attractive commercial proposition. In the next section, where practical applications are discussed, the multifunctional ability of NMR is therefore emphasised. Besides its multifunctional characteristic, NMR has several other unique features. It is non-invasive and non-destructive; it is safe in the sense that human operators are not in danger of being irradiated and the samples are unaffected by NMR. As we have seen, it is versatile, and can be tailored to pick out desired internal features of the sample. It is able to detect low atomic mass species, such as plastics and organic foreign bodies such as cigarettes and insects as well as dense materials such as bone and metallic objects. The disadvantages of NMR also need to be considered. Magnets become very expensive as their field strength increases, so there is an advantage in developing low field NMR sensors. However, equation [9.2] shows that the sensitivity also decreases with lower magnetic fields and low sensitivity will set a lower limit on the spatial resolution and on the size of the foreign body that can be detected. It is therefore very unlikely that small foreign bodies such as a human hair could be detected in an on-line situation by MRI. Obviously, because the technique depends on magnetic and RF fields, it is not suitable for detecting foreign objects in metal containers such as tin cans, although metal itself will be detected and could cause operational problems.

9.3 The use of NMR and MRI techniques in food processing A number of publications have shown the potential of NMR for foreign body detection.4 However, most of them have used stationary samples and conventional NMR equipment, so it remains to be seen whether similar correlations can be established with moving samples in an on-line situation.

9.3.1 Detecting bone fragments in meat Because the proton density inside bone is lower than that of the surrounding meat, bone fragments can be readily detected using conventional MRI, provided the spatial resolution is adequate. Besides bone fragments, NMR has the potential of quantifying other quality factors of the meat, which makes it commercially more attractive. One such quality factor is the amount and distribution of solid fat. This was shown as long ago as 1984 by Fuller, Foster and Hutchingson at Aberdeen using a low field imager.5 More

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Rib Cut

Loin

Belly

Shoulder Blade

Cut

Rib

12 14 16 18 20 22 24 26 28 30 32 SLR % Fig. 9.1. NMR mapping of adipose tissues in pork loin, belly and shoulder. (Reproduced from Davenal, A, Marchal, P, Riaublanc, A and Gandemer, G (1999), Advances in Magnetic Resonance in Foods, p 272).

recently, Davenal et al.6 have mapped the distribution of solid fat in pork and presented a calibrated solid fat level (see Fig. 9.1). They also showed that the average solid fat content could be measured from the non-spatially resolved free induction decay (FID). Meat texture is a more subtle quality factor that relates partly to the extent of fat ‘marbling’ in the meat.7 This can also be determined with a combination of off-line MRI and image texture analysis although it remains to be seen whether such detail can ever be achieved in on-line situations. The possibility of measuring the fat content of minced meat as it is being extruded is being researched by a French team at Institut National de la Recherche Agronomique (INRA).8 It is of interest to note that phosphorus31 NMR spectroscopy has been used to detect the phosphate in bone (see Fig. 9.2), the bone component appearing as a broader peak at lower field compared to the narrow resonance of smaller molecule components dissolved in the cellular tissue of the meat.9 This therefore offers additional potential for on-line detection of bone fragments.

9.3.2 Foreign bodies in fish Nematode worms are a troublesome foreign body in some species of fish and MRI can, in principle, detect these. As with bone, additional quality


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Narrow meat resonance

Broad bone resonance

10.0

0.0

–10.0

ppm

Fig. 9.2. 31P spectrum of chicken with bone. (Reproduced from Bellmer, D D and Morgan, M T (1995), Proceedings of the 4th Conference on Food Engineering, Chicago, IL).

factors are also available. For example, the lipid distribution in fish can be determined.10 Whether more subtle quality factors such as the freshness of the tissue or whether the fish has been frozen-thawed or just chilled can be detected by MRI remains to be investigated. Again it should be noted that these MRI studies were all undertaken off-line in conventional high-field NMR equipment. Whether similar results can be obtained on-line with lowcost NMR sensors remains to be determined.

9.3.3 Foreign bodies in horticultural products As long ago as 1989, Chen and co-workers11 showed that MRI can detect holes in fruit and vegetable as well as distinguish worms and other insects such as wasps in fruit. Figure 9.3 shows an example of holes left in a pear. Zion and co-workers12 have also investigated whether one-dimensional MRI profiles could be used to detect the presence of ‘pits’ in cherries and olives. These may not qualify as the usual foreign body but a pit left in a cherry or olive during an industrial depitting process is highly undesirable. A commercial depitting machine works by pushing a plunger through the fruit thereby displacing the pit. This process also damages the fruit so care must be taken to distinguish damaged depitted fruit from pitted fruit in the image profiles. Foreign bodies, such as insects in horticultural products are not the only concern, and additional quality factors such as the degrees of ripeness, the Brix (soluble solid) content, bruising, and physiological disorders such as

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(a)

Fig. 9.3.

(b)

Worms and holes in Pears. (Reproduced from Chen, P, McCarthy, M J and Kauten, K (1989), Am. Soc of Agric. Engineers, 32, 1747).

mealiness, watercore or brown heart have also been studied in off-line NMR measurements at a variety of field strengths. Table 9.1 lists a number of these studies where correlations between NMR parameters and internal fruit quality attributes have been established. The second column gives the field strength (B0) and the last column the type of NMR pulse sequence used in the study. Table 9.2 does the same for some commercially important vegetables. More details can be found in a recent review to be published in Annual Reports on NMR spectroscopy.13

9.3.4 Foreign bodies in well-defined containers Acquiring a full three-dimensional image of a sample is necessarily timeconsuming, although some fast imaging pulse sequences such as FLASH and EPI succeed in reducing imaging time to a few tens of milliseconds. The price that is paid for this increased speed is a lower signal-to-noise ratio and therefore poorer spatial resolution. However, there are circumstances where full three-dimensional imaging is not necessary for foreign body detection. For example, a conveyor carrying rows of identical jars of some semi-liquid product, such as baby-food, would require only direct comparison of a one-dimensional profile with the ‘ideal’ sample to detect the presence of foreign bodies such as glass or plastic. One-dimensional profiling can be done at high speed without loss of resolution so is perfect for online applications.


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Table 9.1 NMR methodology used to study quality attributes or disorders in fruit Product

Attribute/disorder

Field strength (tesla)

Apple

Bruising Bruising Bruising Internal browning Internal browning Internal browning Mealiness Mealiness Superficial scald Watercore Watercore Watercore Watercore Ripeness (Brix) Dry matter

2 2 4.7 1.5 0.6 0.13 4.7 2.3 4.7 1.5 0.5 0.5 0.13 0.13 2

Dry matter

2 0.13

Banana Cherimoya Durian

Dry matter Ripening Maturity Maturity – rots Maturity (?)

Kiwi fruit

Sugar content

Avocado

Mandarin Mango

Mangosteen Melon

Nectarine Orange Papaya

Maturity Maturity Heating injury Insect infestation Maturity Maturity (?) Maturity – rots Internal necrosis Sugar content (water suppression) Woolly breakdown Fungal infection Dehydration Sugar content Heating injury

4.7 0.5 2 Earth’s field 2 4.7 4.5 4.5 4.5 2

Methodology se imaging se, ge imaging se imaging se imaging T1, T2, PD (se mapping) T2-CPMG T2 (se mapping) T2 se imaging se imaging se imaging se imaging T2-CPMG and PFGSE T2-CPMG and Dw se imaging; MRS – T1, IR; T2 – se. Use of surface coils. FID, fruit moving over surface coil at 0–250 mm/s. FID T2-CPMG T2-CPMG, Dw T1–IR, T2-se imaging se imaging FID spectrum from surface coil, at speeds up to 300 mm/s. FID T1 and T2

0.5 2.0 4.7

T1, T2 (se mapping) T1, T2, PD (se mapping) se imaging se imaging T2, (se mapping) FID spectrum with surface coil, speeds up to 300 mm/s se imaging se and ge imaging single pulse and IRFT

0.5 4.7 2 0.13 1.5

se, ge imaging T1, T2, PD (se mapping) se imaging FID, T2-CPMG se T1-, T2-PD-weighted imaging

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Table 9.1 Continued Field strength (tesla)

Product

Attribute/disorder

Peach

Bruising Bruising Mealiness Bruising Core breakdown Core breakdown Firmness (?)

2 2 4.7 2 0.5 4.7 2

Maturity Ripening Quality disorders Void detection

2 2 0.5 0.5

Sugar content Void detection

0.5 0.5

Freezing effects Sugar content Fungal infection

7.1 0.2 4.7

T1-null imaging FID T2-CPMG IR se imaging; T1-, PDweighted imaging

2

1-D se imaging projections

0.2 2

FID T2-CPMG 1-D se imaging projections at speeds up to 250 mm/s FID spectrum with surface coil

Pear

Pineapple Tangerine Watermelon

Berries Blueberry Grape Strawberry

Small, stone fruits (drupes) Cherry Quality evaluation (pits) Cherry Sugar content Olive Quality evaluation (pits) Plum Sugar content (prunes)

2

Methodology se imaging se imaging T2 (se mapping) se imaging se imaging T1-weighted se imaging FID spectrum with surface coil, speeds up to 300 mm/s se imaging T2 se mapping se imaging 1-D projection profiles on static samples T1 and T2 volume-selected se imaging at speeds up to 350 mm/s

Abbreviations: Dw, self-diffusion coefficient; ge, gradient-echo; IR, inversion recovery; IRFT, inversion recovery Fourier transform; MRS, magnetic resonance spectroscopy; PD, proton density; PFGSE, pulsed field gradient spin echo; se, spin-echo.

9.4 Factors affecting the development of low-cost on-line MRI foreign body sensors Almost all the feasibility studies of the previous section were undertaken with conventional NMR equipment that, because of its high cost and lack of robustness is not suitable for use as a foreign body detector on a factory


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Table 9.2 NMR methodology used to study quality attributes or disorders in vegetables Product

Attribute/disorder

NMR field strength (tesla)

Courgette/ zucchini

Chill injury

4.7

Cucumber

Pathogen invasion Chilling injury Bruising Disease Sensory texture Dry matter Hollow heart, brown core

2 0.6 2 7 0.5 7 4.7

Maturity Maturity Maturity Maturity Firmness

2 6.3 6.3 2 0.2; 0.6

Onion Potato

Tomato

Methodology T1 and T2 weighted se imaging se imaging T1 se mapping se imaging se, ge imaging T2 T1-IR, T2-CPMG. se imaging, 1-D se imaging projections se imaging se imaging T1 (se mapping) se imaging MRI – se, ge; MRS – T1 IR, T2

Abbreviations: ge, gradient-echo; IR, inversion recovery; PD, proton density; se, spin-echo.

production line. In this section we therefore consider some of the technical hurdles that need to be overcome before low-cost on-line MRI foreign body detectors can become commonplace. The high cost of commercial NMR spectrometers is certainly one of the major factors hindering their development as on-line sensors. Most modern systems are based around expensive superconducting magnets because they provide high field strengths and do not require a power source. However, the high cost of these magnets and the need to keep them filled with liquid helium and nitrogen means this technology is inappropriate for most factory situations. Permanent magnet systems are more robust and cheaper but the difficulties in creating a highly homogeneous magnetic field using permanent magnet systems over the large volumes needed when a sample is moving at speed through the magnet are prohibitive. This leaves only resistive solenoid magnets, which were first used in the early pioneering days of NMR. The problems associated with resistive magnets are well known. The high currents needed for creating substantial magnetic field strengths deposit large amounts of heat in the coils, which necessitates expensive water-cooling. Special steps are also needed to avoid current fluctuations and temperature variations in the magnet. As mentioned previously, perhaps the best compromise configuration for on-line applications uses a permanent magnet array with relatively

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low field homogeneity to polarise the sample followed by a resistive solenoid magnet of higher homogeneity for NMR detection. Perhaps the most serious hurdle to be overcome in the development of on-line NMR sensors is the requirement for a magnetic field of sufficiently high spatial homogeneity for meaningful NMR. In commercial spectrometers high homogeneity is achieved by ‘shimming’ which uses small currents through sets of ‘shim coils’ to create small magnetic fields in the sample volume to compensate for imhomogeneities in the main field. Designing such sets of shim coils is a highly specialised and time-consuming task. A shimmed field can be adequate for a stationary sample placed in the shimmed field but additional difficulties can arise if the sample is moving and multiple acquisitions are required. We are then faced with the problem of shimming quite large volumes. This is technically possible and is done in whole body imagers, but the design of magnet and shim coils that permits this high degree of field homogeneity is an expensive and technically demanding exercise, and this hurdle has to be faced by anyone aspiring to design a commercially viable on-line NMR sensor. It is therefore worth analysing in some detail the effects of field inhomogeneity on the NMR of a discrete sample, such as an apple, moving at constant velocity through the field. As a first step let us expand the inhomogeneous magnetic field at a point, r, as a Taylor series: B(r + Dr) = B0 (r) + G eff (r)Dr + . . .

[9.5]

This shows that the field inhomogeneity can, to first order, be represented as an effective local gradient, Geff(r), whose direction and magnitude will vary with position, r. Let us therefore analyse the effect of these local gradients on the FID of a rigid sample, such as an apple, moving at constant velocity, v, through the sensor field. For the time being effects of spin relaxation and spin diffusion are ignored. Consider spins in the volume element dV at position vector r(0) at time 0. After a time t, this element will have moved to r(t) = r(0) + vt, where v is the linear velocity. For notational convenience we drop the subscript ‘effective’ in the field gradient, Geff. The precession frequency at r(t) will then be w(r(t), t) = 2 pf (r(t), t) = gB0 + gG.r(t) = gB0 + gG.r(0) + gG.vt

[9.6]

As the spins initially at r(0) are carried along in the field gradient, they will accumulate a net precessional phase angle in the transverse plane of t

j(r (t ), t ) = Ú dt ¢2 pf (r(t ¢), t ¢) = gB0 t + gG.r(0)t + gG.v t 2 2 0

[9.7]

The spin density r(r(t),t) of spins at r(t) at time t is clearly equal to r(r(0),0) if we ignore diffusion and bulk motion of the sample other than the linear translation, v. The contribution dS(r(t),t) to the total signal S(t) from spins at r(t) is therefore given by


167

dS(r (t ), t ) = A. r(r (t ), t ). exp[ij(r (t ), t )] = A. r(r (0), t ). exp[igB0 t + igG.r(0)t + igG.v t 2 2]

[9.8]

The phase factor exp(igB0t) is eliminated by setting the RF coil ‘onresonance’ so, neglecting the constant of proportionality, A, and writing r(0) simply as r, and r(r(0),t) simply as r(r) this becomes, dS(r(t), t) = r(r). exp[igG(r).rt + igG(r).v t 2 2]

[9.9]

The total signal S(t) is obtained by integrating over the whole sample volume, S(t) = Ú dV r(r). exp[igG(r).rt + igG(r).v t 2 2]

[9.10]

Various limits of this expression can be considered. If the sample, such as an apple, is stationary and G is an applied linear imaging gradient, then the r-dependence of G(r) can be dropped and the expression reduces to the conventional imaging algorithm S(t) = Ú dV r(r). exp[igG.rt]

[9.11]

Fourier transforming the signal, S(t) then gives the image, r(r). But now suppose the sample is stationary and the field gradient, G(r), is not a linear applied imaging gradient, but rather represents a part of a non-linear field gradient resulting from inhomogeneity in the main field, B0. In this case the exponential term will cause dephasing of the spins in different parts of a finite-sized sample such as an apple, simply because spins in different parts of the sample experience different local fields and field gradients. The effect will be reflected in a decay of the FID with a time constant, T2*, which is much shorter than the true intrinsic T2 and which contains no useful information about sample quality, merely the magnitude of the local field inhomogeneity. Of course, with a stationary sample this type of dephasing can be refocused in a spin-echo, and this permits meaningful measurement of transverse sample relaxation times, T2. However, if the field inhomogeneity is very severe then dephasing will be so rapid that T2* is short compared to the ring-down time of the RF probe so no FID will be observed, though it may still be possible to see a spin-echo if a very short echo time is used. Additional complications arise if the sample is travelling with velocity, v, through an inhomogeneous field. Not only is there additional dephasing from the term, gGeff(r).vt2/2, where Geff(r) differs from point to point in the magnet, there is an even more serious problem arising from the sample motion because the dephasing cannot necessarily be refocused as a meaningful spin-echo. This is because each moving spin will experience different local effective field gradients before and after the refocusing 180-degree pulse, so that it will no longer be returned to its initial phase at the expected echo time 2t. Even if a spin-echo can be observed from a moving sample in an inhomogeneous field it will not necessarily contain any useful

168


information about the nature of the sample, such as its transverse relaxation time, T2, but only information about the local magnetic field inhomogeneities the spins have experienced in their journey during the echo time! This, no doubt, accounts for the observation that echo attenuation increases with increasing speed as a sample moves through a laboratory horizontal bore NMR spectrometer. Rapid dephasing of moving samples by field inhomogeneities also means that we require RF coils with short ring-down times if we are to have any hope of picking up NMR signals. For those readers familiar with electronics, the probe ring-down time is 2Q/w0, where the Q factor refers to the probe circuit. This means we must design RF probes with low Q factors. Low Q factors are also needed to avoid over-sensitive probe tuning whereby the tuning varies from one sample to the next, which is potentially disastrous in an on-line situation. There is therefore a compromise to be made between high signal noise requiring a high Q factor, and short probe ringdown times and low tuning sensitivity which requires a low Q factor. These various considerations illustrate some of the major scientific and technical problems to be overcome if conventional laboratory NMR is to be adapted as a sensor of food quality in an on-line situation. Of course, all of the technical problems can be overcome at a price. Indeed, apart from the price tag, there is nothing preventing a whole body clinical imager being used in a pack house to monitor, for example, apple mealiness on a conveyor. However, even with a whole body scanner one would be restricted by sample speeds. An apple moving at 2 m/s through the imager would not be fully polarised by the time it reached the central homogeneous part of the magnet and a very fast imaging sequence such as EPI or FLASH would be needed to acquire a 3D image before the sample departs. Unfortunately such rapid acquisition sequences require expensive and very carefully adjusted gradient controllers and coils. Indeed, just one standard gradient power amplifier costs about £10 000 and 3 would be needed for 3D imaging, which illustrates the need for developing new generations of low cost hardware for on-line applications. Few fruit or vegetable pack houses would be prepared to pay the prices of conventional NMR hardware for marginal improvements in sorting efficiency! All this points to the need for novel approaches to on-line NMR if commercially viable NMR sensors are to become commonplace on the factory floor. We therefore turn to some more recent technical developments that may well succeed in breaking through these various technical hurdles.

9.5

Future trends

The development of high temperature superconductivity is the single breakthrough that would permit the development of low cost high field magnets suitable for on-line MRI sensors. It would, of course, also revolu-


169

tionise many other areas including power transmission technology. For this reason considerable resources are being devoted to the development of high temperature superconductivity, so far with only limited success. Nevertheless there are other, less conventional, ways of carrying out NMR which could, perhaps, be developed for on-line applications. The first is the commercially available NMR-mouse.

9.5.1 The NMR-mouse The mobile universal surface explorer is described as a new-generation, hand-held instrument with exciting potential for use on-line.14 Mouse devices are based on the principles of ‘inside-out’ NMR where open magnet designs are used for measurements within the field. Essentially the unit consists of two permanent magnets side by side, separated by a surface coil. Sample-size restrictions are thus reduced and samples are probed by relaxation time measurements. By employing magnetic field gradients it is claimed that spatially-resolved characteristics of materials can be obtained with inhomogeneous magnetic fields without the need for superconducting magnets. In principle, any of the current echo-based excitation schemes employed in conventional NMR can be used with inhomogeneous fields. A disadvantage is that the sampling region is quite close to the surface which limits the technique to foreign bodies and defects that are near the surface or adhering to the surface. Rotating the sample through an array of such mice would extend the range of applications. Instruments weigh around 5 kg and have field strengths of around 0.5 T.

9.5.2 SQUIDS SQUIDS (or superconductive quantum interference devices) are superconducting devices that directly transform changes in sample magnetisation induced by spin transitions into voltages.15 They therefore act essentially as linear magnetic flux to voltage converters and replace the conventional tuned RF coil for detecting NMR signals. They can detect both changes in longitudinal and transverse magnetisation so are more flexible than conventional RF coils that detect only transverse magnetisation. Because SQUIDS detect changing magnetisation their sensitivity does not depend on resonance frequency so they can operate at ultralow frequencies below 200 kHz. This has the outstanding advantage that highly inhomogeneous magnetic fields do not cause large frequency differences and therefore loss of resolution. In other words, for a given field inhomogeneity the width of the NMR line scales linearly with the measurement field and so peak height and signal/noise are enhanced as the measurement field is reduced. For the same reason SQUIDS do not need to be tuned. To detect an NMR signal the sample still needs to be polarised and the small polarisation at very low fields is a potential problem in lost

170


signal/noise that can be overcome with some form of rapid prepolarisation. The sample also needs to be excited with RF radiation, but this can be done in a small static magnetic field of less than 10 mT with conventional RF coils. This means that the SQUID is usually spatially separated from the sample environment to avoid direct RF irradiation. Recently an MR image of a small tube of water has been obtained at field-strengths only a small fraction of that of the earth’s field using a SQUID sensor. The SQUID obviously has several potential advantages for on-line applications. It can operate at very low inhomogeneous magnetic fields, avoiding the need for large expensive shimmed magnets. It also does not require tuning and has very low power requirements. However, there are also severe technical limitations to be overcome before it can be used in on-line commercial operations. The whole SQUID needs to be immersed in liquid helium and a conventional RF coil is still needed to excite the NMR resonance. It remains to be seen whether such devices can be adapted as on-line foreign body detectors. 9.5.3 Multispectral fusion It is very unlikely that any single spectroscopic or imaging technique will be sufficiently versatile to detect all types of foreign body in the multitude of potential samples and applications. It is more probable that combinations of several different types of sensor will be required. Attractive propositions include a combined Optical–X-ray–NMR sensor for meat and fish products, but many other combinations can be considered. These sensors would not work in isolation but would use the information gained from one technique to help interpret data from another, thereby increasing the degree of confidence in the analysis. One particularly simple combination would be ‘structured light’ technology and MRI. Structured light generates a three-dimensional surface rendering of a sample by analysing the distortion of a line of laser light as the sample moves under it. This information would greatly facilitate the detection of foreign bodies with fast onedimensional MRI profiling.

9.6


A general discussion of on-line MRI for process control and quality assurance may be found in Chapter 6 of the author’s book, Magnetic Resonance Imaging in Food Science4 as well as in the review article by P J McDonald listed in the references.16 A recent review of NMR studies of the quality of horticultural products has been published in the Annual Reports of NMR Spectroscopy.13 Specialised journals on NMR include Magnetic Resonance Imaging and the Journal of Magnetic Resonance. A comprehensive review of most aspects of NMR is to be found in the nine volume series Encyclopaedia of Nuclear Magnetic Resonance.17


9.7 1 2 3 4 5 6

7

8

9 10

11 12 13 14 15 16 17

171

References lorrain p. and corson d (1970) Electromagnetic Fields and Waves, San Francisco, W H Freeman. callaghan p t (1991) Principles of Nuclear Magnetic Resonance Microscopy, Oxford, Oxford Science Publications. abragam a (1961) The Principles of Nuclear Magnetism, Oxford, Oxford University Press. hills b p (1998) Magnetic Resonance Imaging in Food Science, New York, John Wiley. fuller m f, foster m a and hutchingson j m s (1984) Nuclear magnetic resonance imaging of pigs’ in In-vivo estimation of body composition (ed: D Lister), London, Elsevier. davenal a, marchal p, riaublanc a and gandemer g ‘Magnetic resonance mapping of solid fat content of adipose tissues in meat,’ in Advances in Magnetic Resonance in Foods (eds P S Belton, B P Hills and G A Webb), p 272, London, Royal Society of Chemistry. cernadas e, antequera t, rodriguez p g, duran m l, gallardo r and villa d (2001) ‘Magnetic Resonance Imaging to classify loin from Iberian pig,’ in Magnetic Resonance in Food Science (eds G A Webb, P S Belton, A M Gil and I Delgadillo), p 239, London, Royal Society of Chemistry. donnat j p, beauvallet c, foucat l, veyres f, martin g and renou j p ‘On-line NMR determination of fat content in ground beef,’ Proceedings of the Fifth International Conference of Magnetic Resonance in Food Science, Portugal 2000, London, Royal Society of Chemistry. bellmer d d and morgan m t (1995) ‘Distinction between bone and meat using 31P NMR,’ Proceedings of the 4th Conference on Food Engineering, Chicago, IL. collewer g, toussaint c, davenal a, akoka s, medale f, fauconneau b and haffray p (1999) ‘MRI as a tool to quantify the adiposity distribution in fish’ in Magnetic Resonance in Food Science (eds G A Webb, P S Belton, A M Gil and I Delgadillo), p 252, London, Royal Society of Chemistry. chen p, mccarthy m j and kauten r (1989) ‘NMR for internal quality evaluation of fruits and vegetables,’ Am Soc of Agric Engineers, 32, 1747. zion b, kim s m, mccarthy m j and chen p (1997) ‘Detection of pits in olives under motion by Nuclear Magnetic Resonance,’ J. Sci. Food Agric., 75, 496. hills b p and clark c j (2003) ‘Quality assessment of horticultural products by NMR,’ Annual Reports in NMR spectroscopy (ed G A Webb) Royal Society of Chemistry, 50, 75–120. kühn h, klein m, wiesmath a, demco d e, blümich b, kelm j and gold p w (2001) ‘The NMR-MOUSE: quality control of elastomers,’ Magn. Reson. Imaging, 19, 479–99. mcdermott r, trabesinger a h, mück m, hahn e l, pines a and clarke j (2002) ‘Liquid-state NMR and scalar couplings in microtesla magnetic fields,’ Science, 295, 2247. mcdonald p j (1995) ‘The use of NMR for on-line process control and quality assurance, in Food Processing: Recent Developments (ed. A G Gaonkar), page 23, chapter 2, Amsterdam, Elsevier Science. Encyclopedia of Nuclear Magnetic Resonance, Volumes 1 to 9 (1996) (eds D M Grant and R K Harris) New York, John Wiley.

10 Surface penetrating radar U-K. Barr, SIK and H. Merkel, Chalmers University of Technology, Sweden

10.1

Introduction

The common need presented by the food industry is a cost-effective, rapid, non-destructive and on-line method to detect foreign bodies in food products. In order to promote development in this direction, The Swedish Institute for Food and Biotechnology (SIK) performs club research projects in collaboration with Chalmers University of Technology and with food and food equipment manufacturers. The declared goal is to arrive at commercially available equipment for touch-free foreign body detection and thereby focusing on low intensity microwave transmission techniques. Such techniques have a proven potential to meet a considerable set of demands in various industrial food chains. In contrast to heating applications, low power microwaves are used for detection purposes. The power levels chosen are far below any radiation emission limits and far below the power levels required for local heating. The main physical effect used in the technique consists of measuring the transmitted microwave field passing through a product and the local variation in dielectric properties between the product and the different materials that constitute the foreign bodies. Different dielectric features are seen as a change in phase velocity and absorption in a foreign body. These in turn are measured as diffraction – a change in the expected damping and delay of the microwave signal in comparison with a product free of foreign bodies. The basis of the method is summarised in this chapter.

Surface penetrating radar

10.2

173

Principles of surface penetrating radar

James Clark Maxwell described the properties of electromagnetic radiation mathematically (Maxwell, 1873) assuming freely propagating waves at a constant velocity. Heinrich Hertz gave the first experimental evidence for radio waves in 1888. (Hertz, 1888) Electromagnetic waves comprise radio waves, microwaves, millimetre waves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays and gamma rays (Balanis, 1997). All these wave phenomena are described by a single set of equations – Maxwell equations with only one free parameter – the frequency indicating how many wave cycles are counted each second. Hence, for a constant propagation speed in m vacuum (c0), c0 ª 2.998 ¥ 10 8 , the wavelength (l) is uniquely determined s by the frequency ( f) as: l=

c0 f

[10.1]

The electromagnetic spectrum in a vacuum is depicted in Table 10.1; it shows frequency and wavelength. Electromagnetic waves have been used as a means of ranging and detection (e.g. radar) since the 1930s (Hollmann, 1933) (Silver, 1949) in typical military applications. Microwave technology has been used to measure selected quality aspects in food since the 1960s. Measurements of water content in different products have gained most attention. The water content is an important parameter in food processing since many microbial and chemical processes depend on the amount of available free water in the material. (Kraszewski, 1996) Further, measurements of salt content (Shiinoki et al., 1998) and maturity of fruits (Nelson et al., 1995) have been carried out with good results. However, the application of detecting foreign bodies by the use of electromagnetic radiation in foodstuffs has only recently been undertaken, primarily due to the availability of cheap and Table 10.1 Spectrum of electromagnetic radiation in vacuum f (frequency)

l (wavelength)

0–300 MHz 300 MHz–300 GHz 300 GHz–300 THz 300 THz–375 THz

•–1 m 1 m–1 mm 1 mm–1 mm 1 mm–800 nm

375 THz–750 THz 750 THz–3 000 THz 3 000 THz–30 000 THz +30 000 THZ

800 nm–400 nm 400 nm–100 nm 100 nm–10 nm 10 nm–0

Radio Microwave IR (infrared) NIR (near infrared) VIS (visible) UV (ultraviolet) X-ray g-ray

174


precise circuitry that makes measurements possible that were considered impossible just a decade ago. 10.2.1 Electromagnetic waves As mentioned above, electromagnetic waves travel in free space at a velocity identical to the speed of light, independent of how fast the reference frame used for its measurement is moving. Electromagnetic waves are most suitably described as a propagating vector potential requiring four independent variables. Depending on the reference frame, this potential expresses in electric and magnetic fields propagating at speed of light and oscillating at the wave frequency. The theory of electromagnetism has been summarized in terms of four field equations developed by Maxwell (Harrington, 1962) involving E (H), electric (magnetic) field strength, as variables. These four relations must be completed by two material equations. Inserting these six equations and eliminating all but one field variable, one is left with two identical equations for the electric or magnetic fields that take the form of a Helmholtz partial differential equation. The relation for the electric field is: — 2 E - e 0 e r m 0m r

∂2 E=0 ∂t 2

[10.2]

The propagation operator k is given by: k 2 = -e 0 e r m 0m r

∂2 ∂t 2

[10.3]

It collapses to a constant: k = w e 0 e r m 0m r

[10.4]

for any given angular frequency, w. In the above relations e0 denotes the vacuum permeability (e0 = 8.854 ◊ 10-12 F/m) and m0 the vacuum permittivity (m0 = 4p ◊ 10-7 H/m) in SI units. The absolute values of these constants depend on the unit system chosen. A common fact in all different unit systems is that the product e0m0 is equal to the inverse square of the speed of light. The coefficient functions er and mr denote the complex relative permeability and permittivity, er and mr depend on the material in which the wave is propagating. A more detailed description of the mechanisms behind the relative dielectric function er is found in the following section. 10.2.2 Interaction between electromagnetic fields and matter Electromagnetic waves interact with matter by exerting forces on the charges present in the material. There are two distinct types of forces that are caused by electromagnetic waves: ‘electric’ forces accelerating charged


175

particles in the direction of the field and ‘magnetic’ forces that accelerate charged particles in a direction orthogonal to the field and to the present velocity of the particle. The latter type of interaction is generally not relevant in foodstuffs and is not treated here. In the case of ‘electric’ interaction, there are again two situations that must be taken into account. The first situation occurs if the electric field interacts with free charged particles (e.g. free electrons in a metal, ions in an aqueous solution); here electric conduction results. The particles are accelerated and displaced until they collide with other particles and transfer their accumulated momentum. Conduction effects and the associated losses are always present in electromagnetic wave interaction with matter as soon as free charged particles are present. The importance of the loss mechanism depends on the particle’s mobility and the applied wave frequency. Alternatively, the electric field may interact with bound particles (e.g. bound electrons in an atom or molecule, polarized molecules such as water) and then there is only displacement on an atomic or molecular scale. This in turn gives rise to polarization effects. There is no conduction loss but there can be losses when the electromagnetic wave is in resonance with the bound particle eventually creating an excited state. For the interaction with bound particles, there is always an upper frequency at which the bound particle can no longer follow the electromagnetic waves. The intensity of these wave material interaction processes are expressed in terms of a relative dielectric function er which describes the interaction with matter by a macroscopic view, i.e. integrated over many individual molecules or ions giving rise to average interaction strength. For a given frequency the relative dielectric function is a complex number where polarization effects are summarized in its real part and conductivity effects give rise to a negative imaginary part. The relative dielectric function is connected with the dielectric function of the material e = ere0. In the physics community e is referred to as dielectric function to indicate its dependence on frequency. e indicates a property of matter resulting in the rotation, dielectric property to be found in the chemistry community. In engineering textbooks it is usually denoted dielectric constant implying monochromatic models. The dielectric function is related to the polarizability c = 1 - er implying, er = 1 and c = 0 for a vacuum. When investigating foodstuff four important polarization and loss mechanisms with characteristic frequencies require attention: 1

The slowest process (under or around 1 Hz) is determined by the motion of charged cells and cell parts giving rise to cellular diffusion seen in probes subject to low frequency fields for several weeks. 2 The next process is usually ionic conduction by dissolved salts and other charged molecules. This loss mechanism is visible up to several kHz for long molecules and up to the millimetre wave range (some hundred GHz) for single atom ions.

176 3

4

Detecting foreign bodies in food A slow but very efficient polarization effect present in water is given by the interaction of electromagnetic waves with hydrogen bridges. At room temperature this effect cuts off at 5.8 GHz. For electromagnetic waves ranging from 0 to 2 GHz, water exhibits a relative dielectric function between 65 and 80 depending on the temperature. Resonant rotation of the –OH bonds of a water molecule itself results in a much faster polarization process expressed as series of strong water absorption bands (e.g. at 600 GHz and 1.3 THz). While the latter process can be present in liquid and gas phases the hydrogen bond interaction requires liquid water. For optical frequencies, water has an index of refraction n ª 1.5 implying a relative dielectric function of 3 (for these frequencies the index of refraction n is given by n = e r ). This is due to the remaining vibrational polarizability of the –OH bond in the water molecule that finally cuts off around 200 nm wavelength.

Most molecular polarization effects cut off in the upper microwave range (5 to 60 GHz) and give rise to a (relative) real part of dielectric functions of 10. Using microwaves for food quality determination, the measurement retrieves the presence and intensity of the water interaction only. Water plays a key role in foreign body detection, the behaviour of antennas and frequencies is inherently different depending on whether it is present or not. 10.2.3 Solution of Maxwell’s equations in materials Solving equation [10.2] for a homogeneous medium in an infinite halfspace, one obtains a solution template of the following form: e real ¸ ÏE0 cos(wt - bx) E ( x, t ) = Ì ˝ - ax ÓE0 e cos(wt - bx) e complex˛

[10.5]

In the above relation E0 indicates the field amplitude at the co-ordinate origin as an arbitrary constant of renormalization. The circular frequency w is related to the signal frequency f by w = 2pf. The propagation constant k is split into a real (b) and imaginary part (a). It is linked to the dielectric and magnetic properties via the equation: k = w em = b - ia

[10.6]

Care must be taken to ensure that the exponential function in [10.5] remains bounded requiring a ≥ 0 for positive spatial coordinates x. It is important to note that a wave propagates in a medium at a speed different from that which it does in a vacuum. The wave is damped exponentially as soon as losses are present. A schematic view of a solution of waves travelling through a lossy material is given in Fig. 10.1. On the left-hand side of the coordinate, a vacuum is present and on the right of the origin, the medium. The polarization of the electric field vector is parallel to the surface. The shortening of the wavelength in the medium compared with that in the vacuum can be observed clearly.


177

1

0.5

0

–0.5

–1 –4

Fig. 10.1

–2

0

2

4

Propagation of electromagnetic radiation in a medium.

10.2.4 Scattering The scattering field caused by a foreign body is related to its dielectric contrast being the difference of its dielectric function and the dielectric function of the surrounding medium. Without dielectric contrast there are no scattering effects. A foreign body can cause a scattering signature depending on the volume of the object and its dielectric contrast in relation to the product. Scattering effects can originate from sharp edges. Scattering dominates over volumetric effects for small objects in the near field. The field caused by a foreign body is given by the field of an elementary current source. For such a source (pointing in a direction given by the dominant edge of the object), Green’s function of the electric field is given by the following expression: - i◊k r - r ¢ 1 È ˘ e G = Í1 - 2 —¢—¢ ˙ ◊ r Î k ˚

[10.7]

In equation [10.7], r stands for the observation point where the electric field is measured r¢ denotes the place where the foreign body is situated and k is a shorthand notation for the wave vector. The first term in the square brackets stands for the well known far-field expression and the second term takes care of the near field of a source; ¢ is the del operator acting on the primed (foreign body) co-ordinates only. Equivalence between a dielectric disturber and a scattering source As mentioned above, without dielectric contrast there is no scattering effect. The scattering effect caused by a foreign body is therefore given by

178


the net excess displacement current in the foreign body compared with the surrounding material. Knowing the incident field, the scattered field of a foreign body with a given contrast becomes: E (r ) = Einc + Ú G(r - r ¢) ◊ (e(r ¢) - 1) ◊ E ◊ dr ¢

[10.8]

R¢

This relation links the dielectric contrast of any object (here assumed to be embedded in air) with the scattered field it causes. A foreign body causes a scattering signature depending on the volume of the object and its dielectric contrast. There is a strong edge-dependent response caused by the field singularity at a sharp edge. This effect is builtin in the singularity in the Green’s function. Scattering field for a set of frequencies Using the described method one measures the transmitted field response at a set of frequencies. It is therefore suitable to plot the measured field as a function of the translation of the object and the antenna and as a function of the frequency. In Fig. 10.2 the calculated difference of the scattered field with respect to a reference is summarized for a set of frequencies. In this figure, the scattered field caused from stochastic variations in the dielectric function of the food material (gray curves) is compared to the scattered field where a single foreign body is present in addition to the

20

E (dB)

0 –20

3.8 GHz, +fb

–40

1.3 GHz, +fb 2.4 GHz, +fb 3.8 GHz, ref

–60

5.8 GHz, +fb 7.4 GHz, +fb

–80

5.8 GHz, ref 7.4 GHz, ref –1

–0.5

0 x (l)

0.5

1.3 GHz, ref 2.4 GHz, ref

1

Fig. 10.2 Scattered field amplitude as a function of the object translation in normative units for typical microwave frequencies channels (1.3 GHz, 2.4 GHz, 3.8 GHz, 5.8 GHz, 7.4 GHz). Foreign body free food material with dielectric noise (gray curves) and the same product with foreign body (black curves) are presented.


179

dielectric noise (black curves). It is assumed that the dielectric function of the food product in the above calculations deviates locally by 1 % from the reference. The dielectric noise is 1 % of the embedding material’s dielectric function. 10.2.5 Model for the signal-to-noise ratio A measurement performed on a reference object with a dielectric function eref and using Green’s functions yields the following dataset: Eref (r ) = Einc +

Ú

G(r - r ¢) ◊ (e ref (r ¢) - 1) ◊ E ◊ dr ¢

[10.9]

R = body

Repeating the measurement for an object with a foreign body denoted FB hidden at the position b with a dielectric function eFB. E (r ) = Einc +

Ú

G(r - r ¢) ◊ (e food (r ¢) - 1) ◊ E ◊ dr ¢

R ¢ \ FB

+ Ú G(r - b) ◊ (e FB - 1) ◊ E ◊ db

[10.10]

FB

The difference between the two measurements then becomes: ED (r ) =

Ú

G(r - r ¢) ◊ (e food (r ¢) - e ref (r ¢)) ◊ E ◊ dr ¢

R ¢ \ FB

+ Ú G(r - b) ◊ (e FB - e ref (b)) ◊ E ◊ db

[10.11]

FB

It is suitable to introduce a shorthand notation for the difference in the dielectric function De, for the reference object and for the food material tested with the foreign body. By assuming that the foreign body has a homogeneous dielectric function eFB and a volume VFB and by adding a set-up noise due to the measurement hardware Nsetup, the difference between the foreign body and the surrounding material is obtained as: ED (r ) =

Ú

G(r - r ¢) ◊ De(r ¢) ◊ E ◊ dr ¢

R ¢ \ FB

+ G(r - b) ◊ e red,FB (b) ◊ E ◊ VFB + N setup

[10.12]

The first term in the above relation is an inconvenience and must be kept as small as possible. The second term is the measured signal. The ratio between both terms indicates the detectability of an object. The noise generated in the measurement set-up must be added as follows: S N

= E

(G(r - b) ◊ e red,FB (b) ◊ E ◊ VFB )bÆmax Ú G(r - r ¢) ◊ De(r ¢) ◊ E ◊ dr ¢ + N setup

[10.13]

R ¢ \FB

This relation denotes the field ratio between the scattered field of the foreign body and the scattered field of the reference versus food material

180


error. Obviously, there is no detection possible wherever the electric field E incident to the body is zero.

10.2.6 Detection limit for a multi-frequency measurement set-up For a measurement set-up comprising a set of frequencies, the signal-tonoise relationship must be a frequency-dependent entity: S N

= E

(G(r - b; w) ◊ e red,FB (b) ◊ E(w) ◊ VFB )bÆ max,E (w )Æ max

Ú

G(r - r ¢; w) ◊ De(r ¢, w) ◊ E (w) ◊ dr ¢ + N setup (w)

[10.14]

R ¢ \FB

S/N|E should be larger than 0 dB to ensure proper detection in all circumstances. Using pattern recognition there is still a reasonable probability of detecting foreign bodies up to a S/N|E of -10 dB in special cases. Ways to increase S/N|E: • • • • • •

Good repeatability of product placement. Good repeatability of the dielectric properties of the analyzed food material. High homogeneity of the analyzed food. Foreign bodies should be close to the antenna. Large dielectric contrast of foreign bodies compared to the food material. Foreign bodies with significant diffraction edges (i.e. razor blade-like objects, breaking surfaces, non-spherical objects) are easier to detect.

10.3

Detecting foreign bodies using microwaves

Foreign bodies are detected in an embedding material by transmitting low power microwaves through the material. The transmitted microwaves are detected in such a way that the damping and the runtime of the microwaves are available as measurement data. The material under test is moved relative to the antennas. Comparing a measurement performed with a reference measurement one obtains a difference data set indicating the presence or absence of a foreign body, as well as the corresponding deviation. The signals generated by microwave scattering are strong if the foreign body contains a large number of sharp edges, i.e. broken glass, broken plastic or stones and when the ability to generate bound polarized charges on the body’s surface, i.e. dielectric function, is inherently different from that generated in the embedding material. This is always the case for wet food such as cheese spread, where the water content causes a very high value of the real part of the relative dielectric function of about 80. That for plastic is about 2.5 and glass up to 10. Dry foods such as spices have a real part of the relative dielectric function of about 1 to 2. In these cases a substantial set of foreign bodies can be detected successfully. The method is very well


181

suited for homogeneous and well-controlled food material, as described in Section 10.4.2. Metal objects are detected as well as nonmetallic objects as soon as the dielectric contrast is sufficient. The detection of metallic objects does not require a total metal-free surrounding as the metallic parts of the product are easily included in the reference data. The dielectric contrast of a metal body is generally sufficient to allow detection in all cases. Working in the antenna’s near field enables the method to detect objects far below the traditional radar limit of a half wavelength (1 wavelength at 3 GHz is 10 cm in air and 3 cm in water). For an electromagnetic wave with wavelength l the near field distance q is given by: q
1 in material observe the minus sign in er

1 GHz to 18 GHz

i2 = -1 k = w e 0e g m 0 m g c0/f, 10 cm at 3 GHz in vacuum 4p ◊ 10-7 H/m 1 in food stuff

S/N

permeability relative permeability noise level in the measurement (average field amplitude) distance of the disturbing object, near field distance angular frequency region where the food material is found in Cartesian coordinates point in space, where the fields are evaluated point in space, where the food material and the foreign body are located signal level in the measurement (average field amplitude) signal to noise ratio

c x¢,y¢,z¢ x,y,z

polarizability of a medium Cartesian coordinates for the dielectric body Cartesian coordinates for the electromagnetic fields

q w R r r¢ S

w = 2pf

>1 for a physically useful measurement c = 1 - er

192

10.8


References

balanis c a (1997) Antenna Theory (2nd ed) Appendix IX, J. Wiley and Sons. harrington r f (1962) Time-harmonic Electromagnetic Fields, IEEE Press Series on Electromagnetic Wave Theory, New York, Wiley. hertz h r (1888) ‘Über die Ausbreitungsgeschwindigkeit der electrodynamischen Wirkungen’ and ‘Über die Einwirkung einer geradlinigen electrischen Schwingung auf eine benachbarte Strombahn’, Hertz Heinrich R and Fitzgerald, George, to the Mathematical and Physical Section of the British Association 1888. hollmann h e (1933) ‘Self activated registration of the heavy side layer’, Zeitschrift für Hochfrequenztechnik, 19, 392. hollmann h e (1935) Multicavity Magnetron, U.S. Patent 2151766. kraszewski a w (1996) ‘Microwave aquametry: electromagnetic wave interaction with water-containing materials’, IEEE Press, 177–203. maxwell j c (1873) A Treatise on Electricity and Magnetism, London, O.U.P. nelson s o, forbus w r and lawrence k c (1995) ‘Assessments of microwave permittivity for sensing peach maturity’, Transactions of the ASAE, 38(2), 579–85. shiinoki y, motouri y and ito k (1998) ‘On-line monitoring of moisture and salt contents by the microwave transmission method in a continuous salted butter-making process’, Journal of Food Engineering, 38(2), 153–67. silver s (1949) Microwave Antenna Theory, MIT Radiation Lab Series, New York, McGraw Hill. wolfram (2003) ‘Electromagnetic Radiation’ http://scienceworld.wolfram.com/ physics/ElectromagneticRadiation.html

11 Electrical impedance R. Dowdeswell, Kaiku Ltd, UK

11.1 Introduction: measuring the electrical properties of materials Impedance is a commonly used term in various branches of science and relates to the electrical properties of materials. Various measurements come under the term impedance and this chapter will look at foreign body detection systems, which work on the principle of measuring these changes in electrical properties as a means of detection. It must be said that at the time of writing no commercially available systems for foreign body detection based upon impedance methods are available and the devices described in the following sections are either prototype systems or those which can be modified to act as a foreign body detector. The field of impedance measurements sits between conventional metal detection systems and those using microwave-based approaches. Conventional metal detection systems generally rely on differences in electromagnetic fields and this overlaps with impedance measurements. In terms of microwave-based approaches there is a continuum from the frequencies used in impedance spectroscopy, which are generally referred to as radio frequencies, up to those that are classified as microwave. However, compared with conventional metal detection systems an approach based upon measuring changes in the electrical properties of the food opens the possibility for detecting a much broader range of potential materials such as glass, plastic and wood as well as both ferrous and non-ferrous materials, all by a single instrument. The use of these electrical-based approaches seems not to have received the attention of developers which other systems have, and as such there is only a very limited body of work applying this technique to foreign body detection.

194


Using measures of the electrical properties of materials is just another tool in the potential arsenal which can be applied in the investigation of food. If we take, for example, two cans of soup: one can of tomato and one can of chicken. If we were asked to tell the difference between them we would probably base our decision upon factors such as colour, taste and smell. Equally, we could be more advanced and take measurements of their physical properties such as colour, density or viscosity or even conduct measurements of chemical composition such as salt concentration.They can also be characterised in terms of electrical properties and here there are number of potential measurements which could be made. This chapter is devoted to inspection systems, which utilise changes in the electrical properties of food as a means of detecting foreign bodies.

11.1.1 Background to electrical measurements Let us start by looking at what electrical properties we can measure and what they are. A well-established measurement parameter in the food processing industry as well as in many others is that of conductivity, and probes to measure conductivity have been used for many years. Conductivity or conductance is a measure of how easy it is for an electric current to flow through a material between the two measuring points and is expressed in units of siemens (S) and given the symbol G. The more the material between the measurements points can conduct the easier it is for electricity to flow. The inverse of conductance is resistance. The term resistance, represented by the upper case letter R, is a measure of how difficult it is for electricity to flow through a material. This measure of difficulty is expressed in ohms and is denoted by an upper case omega (W) from the Greek alphabet. When conducting measurements of either conductance or resistance a voltage signal is applied to the material that is being measured. If this voltage is of a constant level and does not change direction over time then this is said to be a direct current (dc) signal. If this measuring signal changes direction with time this is then described as an alternating current (ac) signal. When describing direct current measurements the term impedance which is denoted by the uppercase Z* is equal to the value of resistance and the two terms are interchangeable. Z* = R

[11.1]

When an alternating signal is applied to our test material, that is a signal whose level varies as a function of time such as a sine wave, the situation becomes slightly more complicated. Our material is said to become reactive in that the signal, which is applied to test materials, becomes corrupted. An example of this is shown in Fig. 11.1, which illustrates the effect of passing an alternating electrical signal through a material. This corruption can manifest itself in two forms. Firstly, the amplitude of the signal that we

Electrical impedance Amplitude +

Phase Input signal

Output signal –

Fig. 11.1

195

Amplitude Time

Phase and amplitude variation of a signal passing through a test material.

get from our test material will be lower than that applied. This amplitude difference is caused by the ‘resistive’ element of our test material. Secondly, our output signal can differ from our input system in terms of their phase. This phase difference between input and output signals is due to the reactive properties of the material to an alternating signal. These ‘reactive’ elements are measured by the term reactance, denoted by the upper case letter X, and it is a measure of how difficult it is for the passage of an alternating current through our material. This difficulty is caused by the capacitive or inductive properties of the material. The capacitive properties are a measure of the material’s ability to store energy in the form of an electrostatic charge and are measured in farads (F). Electrostatic charges arise when there is a difference in voltage between two points. In its simplest form, a capacitor consists of two conducting plates separated by a material called the dielectric. The inductive properties are a measure of the material’s ability to store charge in the form of a magnetic field and are measured in henrys (H). The reader is directed to Horowitz and Hall (1989) for a more detailed text on electrical terminology. As such with an alternating measurement signal the impedance of our material becomes a combination of both of these factors; Z* = R + X

[11.2]

One key point to note is that both the resistance and the reactance of a material are dependent upon the frequency of the measurement signal, and different values will be obtained at different frequencies. By conducting these measurements at a range of frequencies an impedance spectrum can be generated which is unique for the material being tested. The term impedance therefore encompasses a variety of measurements that can include the resistance, capacitance and inductance of a material, generally referred to as the material’s dielectric properties. The following

196


sections are divided according to those systems based around either a direct current approach, measuring resistance or capacitance, or an alternating approach measuring changes in impedance. Although inductance should be included within the scope of this chapter it is more thoroughly covered in the discussion on electromagnetic metal detection. For a more detailed explanation of impedance spectroscopy the reader is directed to Ross (1987).

11.2

Capacitance, resistance and impedance-based systems

11.2.1 Capacitance-based systems A number of sensing systems based upon changes in capacitance have been developed over the years, although not specifically for use in foreign body detection applications. They have been primarily aimed towards the detection of missing parts from discrete packets or containers transported on a conveyor belt and the literature mentions no systems for use in pipeline applications. One of the early systems developed was by Laetus Systems Limited (Faruq, 1991) and is described in detail in US patent 4 922 181. This system was developed to monitor the absence of pills contained within blister packs and worked by passing the packets between a pair of parallel electrodes, one of which was segmented. As the packets passed between the electrodes changes in capacitance reading were used to determine if the blister contained a pill or not. The use of a segmented sensor increased the sensitivity of the system as small changes in the capacitance of the packet could be detected by smaller electrodes. The electrodes used in this system were typically 0.4 cm2 which gave a resolution of 2 mm when the top of the packet was 2 mm from the electrode. This approach was based upon earlier work that used a similar approach for detecting knots in pieces of wood. A similar approach has been adopted by Detection Systems Pty Limited who manufacture a system called PCIS (Package Content Inspection System). This system consists of the same general arrangement as that described by Faruq and it is primarily aimed at detecting damaged or broken boxes, cartons or packages as they pass through the sensor on a conveyor belt. This system is being distributed in North America by Spee-Dee Packaging Machinery Division and publicity material describes the system as being capable of inspecting up to 1000 chocolate bars per minute. A similar approach is reported by Process Tomography Limited. Using their PTL110 capacitance transducer, measurements have been conducted on chocolate finger biscuits to detect the moisture level of the wafer. This technology has also been applied to detecting whether or not the biscuit

Electrical impedance Parallel electrodes

Adjacent electrodes

Test sample

Test sample

197

Sensing electrodes

Fig. 11.2

Arrangements for capacitive-based electrodes.

wafer is present within the chocolate finger. The sensor arrangements described to conduct these measurements consist of either a parallel plate system similar to the Laetus and Detection Systems configurations or two adjacent sensing elements with the test sample above them as illustrated in Fig. 11.2. Both are used in conjunction with a conveyor system and measurements conducted as the material passes through the sensing area. This approach has been applied to the detection of plastic foreign bodies in the biscuits but no promising results have been obtained. All of these systems have the potential to be used in foreign body detection applications. However, their effectiveness will be governed by a number of factors. For a detector system to work it is necessary to determine a ‘reference’ capacitance value for the packet or carton in question and compare all subsequent readings to this reference value. This would assume that the variability between samples both dimensionally and in their contents would be very small. It is also necessary to detect the change in the capacitance of the packet caused by the inclusion of a foreign object and this could easily be masked by variability in the product. As such this approach may have limited success and at the time of writing no commercial instruments have been produced aimed at foreign body detection.

11.2.2 Resistance-based systems In a similar way to the capacitive-based systems described above it should be possible to determine the presence of a foreign body based upon measurements of the resistance or conductivity of a sample. The literature does not describe any inspection systems for packages based upon resistance but there is an interesting application for use as a pipeline sensor based upon a tomographic measurement system. (Bolton and Sharif, 2001) Electrical resistance tomography is used to obtain information about the spatial distribution of a mixture of materials inside a vessel, by measuring the electrical resistances between sets of electrodes placed around its periphery and converting them into an image showing the distribution of the resistance. This approach has been researched for a number of years and has been applied to a range of applications such as imaging two-phase

198


liquid/gas mixtures in oil pipelines and solid/gas mixtures in fluidised beds and pneumatic conveying systems. Bolton and Sharif describe the development of a prototype system which was based upon an 80 mm diameter pipe. Static tests were conducted with the pipe full of cherry jam and a 4 mm diameter plastic sphere was used to simulate a cherry pit. It was reported that the system detected the presence of the plastic sphere although the differentiation was small. It was reported that the performance of tomographic systems for metal detection depended on the shape of the metal object. A series of tests was described whereby different sizes of stainless steel spheres and wires were placed in the centre of a 100 mm diameter pipe with a 16-electrode array spaced around the outside. It was noted that the detection of metal wires was possible if the wires were parallel to the sensor plane; however, detection was poor when the metal wires were perpendicular to the sensor plane. The wire used for these tests was 0.6 mm diameter and 25 mm in length. The paper reports that further development work is continuing. In order to conduct measurements of the foodstuff resistance the sensors need to be in direct contact with the material and the pipe housing made from a non-electrically conducting material. By having electrodes in direct contact this can lead to problems associated with sensor fouling and cleaning issues.

11.2.3 Impedance-based systems Impedance-based systems have for a number of years been used in the food industry for a variety of purposes; measuring changes in the dielectric properties of food is a potentially promising route as the basis of a foreign body detection system. One current application of impedance-based systems is the KLD 2042 leak-detecting machine manufactured by Bosch. This machine is designed to detect leaks or damage in bottles and does so by applying a high voltage electric field to four points around the circumference of the bottle. If the bottle is either damaged or empty this causes changes in the electrical impedance of the bottle. These readings are then compared to a reference sample and the bottle is rejected if they do not match. It is claimed that the system is capable of detecting hairline cracks, leaking seals or thin walls at up to 10 000 units per hour. By utilising a similar approach it should also be possible to scan the contents of the bottle and so yield information as to the bottle’s contents. It is possible that this approach could be adapted to other forms of package although the limitations discussed for capacitive-based systems on sample reproducibility would also be applicable. This approach would only be applicable to nonelectrically conducting containers as the electrical field generated would not penetrate through the container’s wall. The determination of moisture levels in various foodstuffs such as grain (Nelson et al., 2001) has been conducted for a number of years based upon

Electrical impedance

199

measurements of dielectric properties. This is again a cross-over area because at higher measurement frequencies one enters the realm of microwave-based instrumentation and this is discussed elsewhere in this book. Some of the commercially available moisture measurement systems do operate at lower frequencies and again this opens a possible area for experimental work to be conducted, although to date it appears that no research work has been done here. A development of the impedance measurement described above has been made by Kaiku Limited which has modified its existing in-line chemical analysis system for use as a foreign body detector for pipeline applications. The system is currently undergoing performance evaluation. Kaiku’s technology consists of a non-electrically conducting pipe that has a pair of electrodes on the outside of it forming a flow cell. Figure 11.3 depicts the flow cell and shows the electrodes as a pair of plates that are separated by a small gap. One of the key elements of the system is that the electrodes are located on the outside of the pipe and so do not suffer any of the problems associated with sensor fouling or cleaning issues.The sensor has been certified to European Hygienic Design Group (EHDEG) test procedures.The only element of the system that changes is the fluid itself, which is what the system is trying to measure. As the fluid flows through this cell an alternating electrical signal is applied to one of the electrodes which passes through the liquid and is

Non-conducting pipe

Gap

Input working electrode

Output working electrode

Insulating layer

Shielding electrode

Fig. 11.3

Internal representation of Kaiku sensor.

200


detected at the other electrode. The Kaiku measurement system uses this flow cell to form part of an electrically resonant circuit. At a particular frequency, known as the resonant frequency, there is no phase difference between the input and output signals. This is a finely balanced point so that the slightest change in the chemical composition causes much larger changes in the electrical properties, which can be tracked. Compared with measurements of liquid conductivity this approach is more sensitive and can give rise to detection of contaminants down to parts per billion levels. Whenever the composition of the fluid changes this has an effect upon the electrical properties of the fluid. When part of the fluid is displaced within the measuring chamber this also has an effect upon the electrical properties of the fluid and so it is possible to determine the presence of a foreign body within the flow cell. A clear advantage of this approach is that both metallic and non-metallic objects can be detected as they both disturb the reference point. A number of factors will affect the performance of this instrument. Firstly, the design of the electrodes needs careful consideration. For chemical composition monitoring the fluids are generally homogeneous and so the configuration of the electrodes is such that it does not need to generate an electric field that encompasses the whole of the flow cell. For the detection of foreign bodies it is vital that as the fluid passes through the flow cell all of the fluid comes within an active measuring zone. If this were not the case then it is possible that an object could be missed by the system. Secondly, the speed at which measurements are made needs to be related to the speed of the fluid flow through the pipe so that gaps in the measurement do not allow for objects to pass unnoticed. Figures 11.4 and 11.5 illustrate the response from the Kaiku system for typical foreign bodies, which were introduced into a 50 mm diameter sensor. Figure 11.4 illustrates the signals received from a stainless steel ball bearing of 1 mm diameter and an aluminium shaving 5 mm by 2 mm in size carried in water. Figure 11.5 illustrates the response for three pieces of a rubber DIN seal of two different dimensions. The spikes illustrated on the plots show the change in the fluid’s electrical properties while the foreign body was in the detection area. The two plots illustrate the magnitude of the signal change but not its polarity. As metallic and non-metallic materials change the fluid’s electrical properties in different ways, making it either more or less conductive, it is possible to display the data in such a way as to give some degree of indication as to the nature of the foreign body. One of the design considerations of the Kaiku system is to alleviate the possibility of false positive or false negative readings. As the system is measuring sudden small changes in the active field area it is necessary to confirm whether these detected signals are caused by a foreign body or are indeed a glitch in the measurement system. This is addressed in two ways: firstly, multiple sets of electrodes are used within the same sensor head. In such an arrangement, as the foreign body passes between the different

Response (arbitrary units)

Electrical impedance 3500 3000

1 mm stainless steel ball bearing

2500 2000 1500 1000 500


0 40 000 35 000 30 000

Aluminium shaving 5 mm by 2 mm

25 000 20 000 15 000 10 000 5 000 0

Fig. 11.4

Response of the Kaiku sensor to metallic foreign bodies.

30 000 Rubber DIN seal 25 000


7 mm ¥ 5 mm ¥ 2 mm 20 000

15 000 4 mm ¥ 4 mm ¥ 2 mm 10 000

5 000

0

Fig. 11.5

Response of the Kaiku sensor to non-metallic foreign bodies.

201

202


measuring regions, it triggers a response in each one. In this way a positive result would be detected once a response had been triggered in a number of regions. Secondly, by utilising a fast measurement system ‘hits’ are recorded for an object many times as it passes through an active sensing region.

11.3

Conclusions and future trends

The challenge for any impedance-based foreign body detection system, or in fact for any method of detection, is to apply a measurement technique to a continuously changing baseline product where you are required to identify the occurrence of a rare event. Due to the wide range of material types and size that constitute foreign bodies, there is a considerable challenge in defining the change in measurement in the signal that can be defined as a positive ‘hit’. It is therefore necessary to define what the normal variability of the product is and base the rejection on that. However, with very variable products, for example a chunky vegetable soup, this base line will be large and there exists the possibility of objects passing through unnoticed. From the systems described it seems likely that both the resistance-based tomography approach and the use of resonant frequency interrogation systems offer ample scope of development towards commercial instrumentation directed towards foodstuffs conveyed in pipelines. The resonant frequency approach may offer advantages as its measurements are conducted in a non-contact fashion and so cleaning is not an issue. Capacitive-based instruments do offer the possibility for inspection of package-based products although this is already a fiercely contested market dominated by X-ray-based inspection systems. However, this approach still offers the potential for the detection of those materials such as glass, plastic and bone that are considered difficult to find by other measurement techniques while metallic material is still detected.

11.4


Detection Systems Pty Ltd, 15 Clare Street, Bayswater, Victoria, 3153, Australia Tel: +61 3 9720 3399 Fax: +61 39720 5426 http://www.detection.com.au Spee-Dee Packaging Machinery, Inc. 9950 Durand Ave. Sturtevant, WI 53177 Tel: +1 (262) 886-4402 Fax: +1 (262) 886-5502 http://www.spee-dee.com Process Tomography Ltd, 86 Water Lane, Wilmslow, Cheshire. SK9 5BB UK

Electrical impedance Tel/Fax: +44(0) 1625 549021

203

http://www.tomography.com

Robert Bosch Packaging Machinery Ltd, Denham, Suffolk, UK Tel: +44 (0) 1895 83-89 10 Fax +44 (0) 1895 83-89 20 http://vt.bosch.com/en/index.asp Kaiku Ltd, Greenheys Centre, Manchester Science Park, Manchester, M15 6JJ Tel: +44 (0) 161 227 8900 Fax: +44 (0) 161 227 8902 http://www.kaiku.co.uk

11.5

References

bolton g t and sharif u (2001) ‘Process tomography for contamination detection in liquid foods: a feasibility study’, Proceedings of the 2nd World Congress on Industrial Process Tomography, Hanover, Germany, 29th–31st August 2001. faruq a (1991) ‘Closed pack contents inspection – without X-rays’, Sensor Review, 11(4), 26–7. horowitz p and hall w (1989) The Art of Electronics, Cambridge, Cambridge University Press. nelson s o, trabelsi s and kraszewski a w (2001) ‘RF sensing of grain and seed moisture content’, IEEE Sensors Journal, 1(2), 119–26. ross j (1987) Impedance Spectroscopy – Emphasizing Solid Materials and Systems, New York, John Wiley.

12 Ultrasound O. A. Basir and B. Zhao, University of Waterloo and G. S. Mittal, University of Guelph, Canada

12.1

Introduction

A ‘foreign body’ (FB) is any undesirable piece of solid matter present in a food product. An FB could be a piece of metal, glass, stone or plastic. It can also be an insect, dirt, nail or hair. FBs are different from food in terms of acoustic impedance, which is the product of density and sound velocity in a given material. Reflections, refractions and scatterings take place due to discontinuity in acoustic impedance along the sound propagation path through the food medium. In the ultrasound frequency range, frequency higher than 20 kHz, FBs can be detected by sensing the reflected, refracted or scattered acoustic signals due to a FB’s impedance discontinuity in the food. In practice, the frequency used in ultrasound detection is normally above 1 MHz with intensity less than 1 W/cm2, as such this range is referred to as low intensity ultrasound. Typically, high intensity ultrasound ranges from 10 to 1000 W/cm2 (McClements, 1995), which is not within the scope of this chapter. Food processing is an industry where small and medium enterprises (fewer than 250 employees) represent a significant proportion (90 % in the UK). (Wallin and Haycock, 1998) For such enterprises cost is an important factor when purchasing equipment. A decision to invest $50 000 in equipment can be more challenging for an enterprise with sales of $1 million than it would be to an enterprise with sales of $50 million or more. Installing equipment to maintain the quality of its products, for instance to detect FBs, may turn out to be an expensive endeavour for small scale enterprises. Some FB detection equipment, such as metal detectors, can be inexpensive; however, their applications are quite limited. The price of optical/vision

Ultrasound

205

detection systems ranges from $10 000 to $0.5 million, depending on their functionality and desired performance. Again, this type of equipment is not applicable to optically opaque materials. From the point of view of broad range material detection capability, X-ray systems seem to be well suited. However, their cost, ranging from $50 000 to $340 000 (Wallin and Haycock, 1998), makes these systems less affordable to small scale enterprises. Furthermore, long time exposure to X-rays is hazardous to operators, which raises concerns regarding personnel safety. In comparison with X-ray systems, low intensity ultrasound-based systems are non-hazardous with relatively low costs. They are widely used in medical diagnosis and in industrial flaw detection, and as such they are considered to be a highly promising candidate for foreign body detection in foods. A salient feature of ultrasonic imaging is that a quantitative characterization of materials is realizable as the result of the complicated manner in which the ultrasound signal interacts with the material. A number of ultrasound propagation parameters can be used to characterize a medium such as food. These parameters include attenuation/absorption, backscatter, velocity and nonlinearity. (Shung et al., 1992) Another advantage of ultrasonic-based systems is that they are more adaptable to various test environments and conditions than systems based on other techniques. Ultrasound sensors can be made as small as a catheter tip of 1.2 mm diameter, on which a circumferential array of 64 elements can be formed and placed in the coronary arteries or veins of the heart to image blood flow. (Crowe et al, 2000) Food processing requires high hygienic standards. These standards are achievable as ultrasonic inspection is non-destructive and non-intrusive to the food product. Ultrasound generation and reception can also be performed in non-contact or remote mode using air-coupled transducers, electromagnetic acoustic transducers (EMAT), optical fibers and laser techniques. Last but not least, it is crucial to recognize that the food industry is cost sensitive and hence it is desirable to maintain a high production rate. Thus, the speed of inspection is a determining factor, and on-line inspection speed is important. Using synthetic transmit aperture (STA) or limited diffraction beams imaging, a linear transducer array system has the capability of a rate of from 1000 frames per second (Misaridis and Jensen, 2002) to 3750 frames per second. (Lu and He, 1999) This means that ultrasound-based systems can be used to perform on-line inspection.

12.2

Principles of ultrasound

12.2.1 Acoustic waves There are four types of wave propagation in acoustic energy transmission: compressive (or longitudinal), shear, Rayleigh and Lamb waves. The

206


direction of particle displacement is parallel to the wave propagation for the compressive wave. Particle displacement of the shear wave lies in any direction in the plane perpendicular to wave propagation. The Rayleigh wave can be considered propagating only on the surface of a material, since its amplitude decreases exponentially with depth. The Lamb wave propagates on a plate or filament whose transverse dimension is of the order of millimetres. The velocity of ultrasound wave is different for the four types. In pure materials, the compressive wave velocity (cp) and shear wave velocity (cs) can be calculated as c p = (K + 4G 3) r

[12.1]

cS = G r

[12.2]

where K and G are bulk and shear modulus, respectively; r is the density of the material. Compressive waves propagate faster than do shear waves. Approximately, the former is two times faster than the latter. The velocity of the Rayleigh wave is expressed approximately as: cR =

0.87 + 1.12n 1 -n

E 1 r 2(1 + n )

[12.3]

(Krautkramer and Krautkramer, 1969) where E and n are respectively the Young’s modulus and the Poisson ratio of the material. The Rayleigh wave velocity is the slowest compared with other waves, typically, cR = 0.92 of cS for steel, and 0.93 of cS for aluminum. There are two Lamb wave propagation modes, namely, symmetric and asymmetric. Nevertheless, there is no explicit analysis solution for Lamb wave speed. Such velocities depend not only on the material properties but also on ultrasound frequency. Telschow and Deason (2002) summarized the derivations of the Lamb wave mode dispersion relationships used in the paper quality inspection. Krautkramer and Krautkramer (1969) showed a diagram of this relationship for steel. Few foods are single phase materials; most tend to be multiphase. The velocity of sound in a food is a complicated function of a large number of physical, chemical and biochemical properties. Povey (1998) proposed a volume-average method using density and adiabatic compressibility of each food component. The ‘ultrasound velocity’ is, to some extent, a vague concept. As we indicated, the speed of ultrasound propagation can be a function of frequency (dispersive). The ‘ultrasound velocity’ calculated using equations [12.1], [12.2], and [12.3] is phase velocity (Kinsler et al., 1982), which is frequency dependent. However, wave velocity is estimated by detecting the time of flight (TOF) of an ultrasound pulse propagating through a sample of a given thickness. This velocity is called group velocity (ng) (Povey, 1997), and related to phase velocity (np) as:

Ultrasound n g = n p + f ◊ dn p df

207 [12.4]

where f is the frequency of the ultrasound wave in Hz. For pure materials, the dispersion effect is very small (dnp/df ~ 0 ). For instance, the phase velocity is 1480.00 m/s at 24 MHz and 1480.27 m/s at 60 MHz for pure water (Wang et al., 1999). In such a case, it is acceptable to approximate group velocity by phase velocity, and vice versa. Therefore, ‘ultrasound velocity’ is used in flaw and tumor detection in industry, where, regardless of the frequency of the ultrasound wave, one will get the same results in terms of flaw and tumor’s positioning and sizing. In contrast, for many suspensions, emulsions or semi-fluid foods, phase velocities are different at different frequencies. It is then imperative that the frequency of used ultrasound signal be known when dealing with these types of materials. Table 12.1 lists typical ultrasound velocities along with frequency conditions for some foods and materials used in food processing and packaging. The frequency condition is not always available in the cited sources. Center frequencies of transducers used by those authors are listed as a reference.

12.2.2 Ultrasound waves applications The Rayleigh wave is often used to detect cracks along the surface of metallic components, while the Lamb wave is often used to detect defects in a thin sheet. Recent research publications have reported the use of Lamb wave in air transducers design to increase the efficiency of wide bandwidth ultrasound radiation to air (Degertekin et al., 1998). The two widely used wave modes in foreign body detection in food products are compressive and shear waves. Compressive waves can be used to detect foreign bodies in liquids, solids or semi-fluid foods. A compressive wave is effective only for inspection applications where strict transducer orientation conditions can be readily realized to avoid refraction issues. Shear waves can be used to inspect solid and semi-fluid foods. An advantage of shear wave is that it has larger tolerance for sensor alignment than compressive wave owing to its lower propagation velocity. We will elaborate more on this point later. However, shear waves cannot be used to inspect liquid foods such as beverages since the shear modulus of a liquid is zero and no shear wave can propagate as a consequence. Acoustic energy decreases in the course of propagation in materials as do other forms of energy. The rate at which the wave energy decreases differs for different materials. It is quantitatively described by the attenuation coefficient in units of nepers per meter [Np/m]. The attenuation coefficient is frequency dependent: a high frequency acoustic wave attenuates faster than a low frequency acoustic wave. Some attenuation coefficients are listed in Table 12.1.

3

9.05 ± 0.37 3.72 ± 0.34 690 ± 45

1.38 ± 0.02 1.32 ± 0.04 2.02 1.69

8.93 ± 0.46 2.36 ± 0.44

7.97 ± 0.22 29

1 MHz 50 kHz transducer, first day fruit 1 MHz transducer, 250-day average 5 MHz transducer 5 MHz transducer 5 MHz transducer 3 MHz, 20 wt % oil droplet mean diameter 1.5 mm 5 MHz transducer 5 MHz transducer 1 MHz transducer, 60-day average 1 MHz transducer, 90-day average 1 MHz transducer, 240-day average 5 MHz transducer 5 MHz transducer Mean value of frequency 4–12 MHz, dvp/df ~ 0 10 MHz 1 MHz transducer, 80 day average 2.25 MHz transducer, 100 wt % concentration

24 MHz

576 0.025 258 ± 43

1 MHz 1 MHz 10 MHz 5 MHz 1 MHz 5 MHz 2.5 MHz

Note

138 0.21 2 148.5 23 58.7 23

Attenuation [Np/m]

1.21 ± 0.02 1.28 ± 0.03

1.63 ± 0.03 1.44 ± 0.03 1.73 ± 0.06

0.0004 17 14.15 2.9 3.2 2.4 2.47 4.2 31.4 45.45 1.48

Impedance [kg/m2 s] ¥ 106

Shung et al. (1992), 2Birks and Green (1991), 3Onda Corporation (2003), 4Kaye (1986), 5Wang et al. (1999), 6Mizrach and Flitsanov (1999), 7Benedito et al. (2002), Haeggstrom and Luukkala (2001), 9Coupland and McClements (2001), 10Benedito et al. (2001), 11Chivers et al. (1995), 12Llull et al. (2002), 13Saggin and Couplant (2001).

8

1

1575 ± 5 1626 ± 6 1658

Peach syrup11 Sobrassada from Mallorca12 Tomato ketchup13

± 20 ± 130 ± 20 ± 12 ± 25 ± 20 ± 50 ± 30

1118 1145 1645 1676 1715 1323 1311 1550

343 6320 5660 2600 2670 2740 2350 2380 4175 5660 1480 338.1 ± 55.7 1669 ± 10 1420 ± 30 1330 ± 30 1842 ± 70 1477

Velocity [m/s]

Margarine: extra salty8 Margarine: normal salty8 Mahon cheese (fresh)10 Mahon cheese (half ripened)10 Mahon cheese (ripened)10 Processed cheese: Grevé8 Processed cheese: Naturelle8 Canned peach11

Air1 Aluminum1 Glass (crown)4 Oriented nylon3 Plexiglass1 Polypropylene3 Polystyrene4 Polyvinylidene chloride (PVDC)3 PZT-5H (Lead zirconate titanate)5 Stainless steel2 Water1 Avocado6 Cheddar cheese7 Cherry marmalade: processed8 Cherry marmalade: unprocessed8 Chunk cheese: Jämtgård8 Corn-oil-in-water9

Material

Table 12.1 Acoustic properties of various materials and foods at room temperature

208 Detecting foreign bodies in food

Ultrasound

209

12.2.3 Transmission, reflection, refraction and mode conversion When an ultrasound wave is transmitted from one material to another, part of the energy is reflected at the interface and part of it is transmitted through the second material.The amplitude ratio between the reflected part and the incident ultrasound wave is referred to as the reflection coefficient (R). The ratio between the transmitted part and the incident ultrasound wave is referred to as the transmission coefficient (T). They are calculated using the following equations: R = (Z2 - Z1 ) (Z1 + Z2 )

[12.5]

T = 2Z2 (Z1 + Z2 )

[12.6]

where Z1 and Z2 are, respectively, the acoustic impedances of the first material and the second material. If the incident ultrasound is not normal to the interface, the angles between the incident, reflection and refraction obey Snell’s law, which states: sin(f ) sin(q ) = c1 c 2

[12.7]

where f is the incidence angle, q is the refraction angle, and c1 and c2 the ultrasound velocities in the first and the second materials, respectively. An incident angle is critical (hence critical angle) when the refraction angle is 90°. In this case, there is no transmission of this ultrasound wave through the second material. If the ultrasound is transmitted from a liquid to a solid, a shear wave is produced in the solid, in addition to a compressive wave. In this case we have: sin(f ) cL = sin(q p ) c p = sin(q s ) c s

[12.8]

where cL is the ultrasound velocity in the liquid, qp and qs are, respectively, the refraction angles of the compressive and the shear waves. In particular, when the incident angle is larger than the first critical angle (i.e., the refraction angle of the compressive wave is 90°) and less than the second critical angel (i.e., the refraction angle of the shear wave is 90°), only a shear wave will develop in the solid. This phenomenon is called mode conversion.

12.3

Types of ultrasonic transducer

12.3.1 Piezoelectric transducers Piezoelectric materials are used to make ultrasonic transducers. As the name indicates, electricity is developed when pressure is applied to the material. Conversely, when an electric field is applied to the material, the material rapidly changes shape. Originally, an ultrasonic piezoelectric transducer was designed to generate and/or receive compressive wave:

210


change in thickness of a piezoelectric disc or film is related to electric signal change. An ultrasound beam produced by a disc or film is unfocused and can be focused in a manner analogous to focusing light, since geometric acoustic analysis applies when the wavelength is less than the object dimension. An acoustic lens can be made of a solid or a liquid (Birks and Green, 1991). They are normally integrated in the transducer. The main disadvantage of traditional piezoelectric transducers is that they need contacts to transmit acoustic energy to the object to be inspected. The contact mode limits the scope of application and the detection speed of such transducers. Transducers are easily worn due to transducer-to-object contact. When allowed, water can be used as a coupling medium to overcome this problem. However, high speed movement between the transducer and the objects can induce serious water turbulence in scanning. This turbulence deteriorates the signal-to-noise ratio. Some non-contact ultrasound transducers have been developed. These non-contact transducers include air coupled transducers, electromagnetic acoustic transducers (EMAT) and laser-ultrasound systems.

12.3.2 Air coupled transducers There are two types of air coupled ultrasonic transducers depending on the specific mechanism they employ. One type uses a piezoelectric mechanism, and the other type uses an electrostatic (capacitance) mechanism. In piezoelectric designs, the key problem is to eliminate the acoustic impedance mismatch between the piezoelectric ceramic and air. Equation [12.6] depicts how the transmitted energy is very small when an ultrasound wave is transmitted from a piezoelectric transducer to the air due to small impedance and high attenuation of the air (Table 12.1). Researchers and transducer experts have been designing piezoelectric devices by manipulating the acoustic impedance transitional layers in front of the piezoelectric element. Typically, these layers have low impedance (less than 1 ¥ 106 kg/m2 s). Using compressed fibers as the final matching layer, a piezoelectric air coupled transducer can propagate up to 5 MHz ultrasound through nearly all materials in non-contact mode. (Bhardwaj, 2002) Early electrostatic ultrasound transducers were composed of a thin membrane film and a rigid conducting backplate to form a capacitor. Applied voltages cause the membrane to vibrate, and hence generate ultrasound wave, whereas a change in charge across the membrane can be used to detect ultrasound. The surface of a backplate is an important factor for sensitivity. Recent electrostatic ultrasound transducers are made using micromachining techniques to form optimized surface features on silicon wafers (Gan et al., 2001). This type of transducer has been used to detect defects in Teflon, aluminum, brass, and carbon-fiber-reinforced polymer composites (Hutchins et al., 1998; Gan et al., 2001). Electrostatic air coupled

Ultrasound

211

transducers work at lower frequency (around 1 MHz) compared to the piezoelectric type which operate at higher frequency bands. A drawback of air coupled transducers is their operation mode. They are operated either in thru-transmission mode, or in the form of a separate transmitter and receiver on the same side of the object to be inspected. No concrete proof of using single transducer mode such as the pulse/echo has been reported for high acoustic impedance materials such as glass, metals and ceramics. In applications the thru-transmission mode is unsuitable for detecting FBs settled at the bottom of the food container, due to the container shape, for instance a converged/extruded bottle neck. Same-sideseparate mode is suitable for defect detection in materials but not suitable for FB detection in containers. The high signal attenuation in air limits this mode to low frequency bands (i.e., less than 5 MHz), and as a result aircoupled transducers tend to suffer from low spatial resolutions. Furthermore, air coupled transducers need accurate orientation to the surface of containers of high acoustic impedances (this issue will be explained in more detail later). Accurate transducer orientation is not guaranteed in production lines, and hence may give rise to high failure or to false detection rates.

12.3.3 Electromagnetic acoustic transducers (EMAT) EMAT is applied to excitation and detection of ultrasound waves in conductive or magnetic materials. EMATs are basically composed of an electric wire coil and a bias magnet. Suppose the coil is placed adjacent to a metal surface and driven by a current at an ultrasound frequency. Eddy currents will be induced within the metal. These eddy currents will experience Lorentz forces in the presence of a static magnet field. The Lorentz forces on the eddy currents are transmitted to the solid by collisions with the lattice or by other microscopic processes (Birks and Green, 1991). Reciprocal mechanisms also exist whereby waves can be detected. In addition to non-contact ways, EMATs are conveniently able to excite horizontally polarized shear waves or other special wave types that provide test advantages in certain applications. In contrast, piezoelectric transducers have to use coupling wedges to convert compressive wave into shear waves. Another advantage of EMAT is that the transducer orientation issue does not arise since there is no need to ensure coupling. A drawback of EMAT is that it is suitable only for foreign body detection in electrically conductive food products. Thus, the EMAT is less efficient and versatile compared to a piezoelectric transducer.

12.3.4 Laser-ultrasound systems Laser-ultrasound systems use the thermoelastic or ablation effect of material for ultrasound excitation. When a pulsed laser beam is focused on a material surface, the resulting thermal shock excitations are transformed

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into mechanical vibrations, as a result of expansion and subsequent contraction. This provides a source of ultrasonic waves. The ultrasound signals can be received using a contact piezoelectric transducer or a non-contact mode transducer such as an air-coupled transducer or EMAT. This ultrasound signal can also be sensed using an interferometer which quantifies the material surface vibration by monitoring the shift of interference fringes. Laser-ultrasound systems provide a number of advantages over conventional ultrasonic techniques, including advantages such as non-contact, high spatial resolution, curved surface applications, ability to reach hard-toaccess areas, ability to generate both narrowband and broadband scanning, and large tolerance to transducer orientation. Disadvantages include low ultrasound wave amplitude when the laser intensity is in the thermoelastic region, they are expensive, and they tend to be bulky.

12.3.5 Transducer arrays Transducer arrays offer more capabilities than single transducers in terms of inspection speed, shape determination, size and orientation of discontinuities. There are three basic types of arrays: linear, planar and annular. Piezoelectric arrays are composed of multiple transducer elements. Arrays of laser sources use optical diffraction gratings, lenticular arrays, and interference patterns, multiple fiber-optical delays, multiple laser cavities and White cell optical delay cavity systems (Murry et al., 1996). While transducer arrays may seem similar to each other in construction, they may be quite different in operation. A linear switched array is operated by applying voltage pulses to groups of elements in succession. In this way, the ultrasound beam is moved electronically as one moves a single transducer manually.A phased array can focus or steer the ultrasound beam by properly delaying firing time of each element in the array.

12.3.6 Peculiarities of FB detection in food containers Compared with A-Scan in non-destructive testing (NDT) for mechanical parts inspection and B-Scan in medical diagnosis, foreign body detection in food containers has some peculiarities. A-Scan uses the echo signal information in a certain time zone (time gating) to detect flaws in a mechanical part (see Fig. 12.1): (i) when an ultrasound is transmitted to a clear portion, an echo signal from the back surface of the part can be received; (ii) When a small flaw is on the path of the ultrasound beam, echo signals will appear ahead of the back surface echo in the time domain; and (iii) the back surface echo will disappear if the flaw is large enough to intercept most of the beam from the transducer. In both cases, A-Scan uses the back surface echoes of a mechanical part as a reference signal. For an FB in a food container, the back surface reference is not

Ultrasound Part

Part

Incident

Incident

Front surface reflection

213

Part Flaw


Incident Front surface reflection

Flaw

Flaw reflection Back surface reflection

Back surface reflection

Front surface reflection Back surface reflection

Front surface reflection Flaw reflection Back surface reflection

t

(a)


t

(b)

t

(c)

Fig. 12.1 Illustration of the principle of flaw detection in a mechanical part using an ultrasound signal: (a) good part and echo signals, (b) small flaw in the part and echo signals, and (c) big flaw in the part and echo signals.

available in most cases. This is because the FB is behind the inner surface of the container’s front wall rather than inside the wall. Figure 12.2 is an illustration of this circumstance where a glass fragment is on the glass bottle bottom. The back wall of the bottle does not provide a reliable reference due to the fact that it is not parallel to the front wall and the wall’s nonuniform thickness. On the other hand, a food normally has a high ultrasound damping factor and its attenuation coefficient is up to a thousand times larger than package materials (see Table 12.1). The back wall echo may be too weak to be detected and thus does not provide a reliable reference. A B-Scan transmits the acoustic energy into a human body which has small variations of ultrasound velocity (> XF) and absorption coefficient mB. The intensity of the emergent beam is I F = I 0 exp[- m B ( X B - X F )] exp(- m F X F )

[13.3]

If no foreign body is present, the emergent beam intensity is I B = I 0 exp(- m B X B )

[13.4]

Comparing [13.3] and [13.4], we obtain a measure of the local contrast (C), introduced by the presence of the foreign body. We define C as C = IB - IF IB

[13.5]

Since XF

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