New Methods of Food Preservation New Methods of Food Preservation

Edited by

G.W. GOULD Unilever Research Laboratory Bedford

SPRINGER-SCIENCE+BUSINESS MEDIA, B.V. First edition 1995 © 1995 Springer Science+Business Media Dordrecht Originally published by Chapman & Hall in 1995 Softcover reprint ofthe hardcover lst edition 1995 Typeset in 1O/12pt Times by Acorn Bookwork, Salisbury, Wiltshire

ISBN 978-1-4613-5876-3 ISBN 978-1-4615-2105-1 (eBook) DOI 10.1007/978-1-4615-2105-1 Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case ofreprographic reproduction only in accordance with the terms of the Iicences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of Iicences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the Glasgow address printed on this page. The publisher makes no representations, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or Iiability for any errors or omissions that may be made. A catalogue record for this book is avaiJable from the British Library Library of Congress Catalog Card Number: 94-79139

t§Printed on acid-free text paper, manufactured in accordance with ANSIINISO Z39.48-1992 (Permanence of Paper) Preface

The majO.r techniques emplO.yed fO.r fO.O.d preservatiO.n have a 100ng histo'ry O.f use. They include chilling; freezing; drying; curing; cO.nserving; fer• menting O.r O.therwise acidifying; the additiO.n O.f preservatives; heat• pasteurisatiO.n and sterilisatiO.n. Newer techniques mO.re-O.r-less derived frO.m these traditiO.nal proce• dures include the successful applicatiO.n O.f cO.mbinatiO.n. preservatiO.n O.r 'hurdle' methO.ds, vacuum- and mO.dified atmO.sphere-packaging, and cO.ntinuO.us sterilisatiO.n cO.upled to' aseptic packaging. More innO.vative techniques, such as the use O.f iO.nising radiatiO.n, are increasingly being emplO.yed. At the same time, there is a reawakening O.f interest in even mO.re radical apprO.aches. The reaSO.ns fO.r this derive principally from cO.nsumers' requirements fO.r fO.O.ds that are higher in quality, so. less severely prO.ces• sed; mO.re natural, so. less heavily preserved; nutritiO.nally healthier, so. cO.ntaining less salts, sugars and fats; and, with respect to' fO.O.d pO.isO.ning, with retained, O.r preferably improved, assurance O.f safety. SO.me O.f these mO.re radical apprO.aches are chemically-based, SO.me biO.lO.gical and SO.me physical. A number O.f them build O.n current technO.- 100gies whilst O.thers are cO.mpletely new. They include, fO.r example, a cO.ntinual widening O.f the cO.mbinatiO.n prO.cedures that can be effectively and safely used; new applicatiO.ns O.f mO.dified atmO.sphere packaging; use O.f naturally O.ccurring antimicrO.bials that are animal-derived (e.g. lysO.zyme, lactO.perO.xidase, lactO.ferrin), plant-derived (e.g. herb, spice and O.ther plant extracts) and microO.rganism-derived (e.g. bacteriocins); new and improved means fO.r the accurate delivery O.f heat to' fO.O.ds (e.g. by micrO.waves, by O.hmic heating) so. as to' achieve the minimal prO.cesses necessary to' ensure stability and safety; the use O.f high hydrostatic press• ures to' inactivate microO.rganisms in fO.O.ds withO.ut the need fO.r substantial heating, and with cO.nsequent minimal damage to' product quality; the use O.f high vO.ltage electric pulses fO.r similar purpO.ses; the direct and synergis• tic applicatiO.n O.f ultrasO.nic radiatiO.n to' pasteurise and sterilise fO.O.ds with the minimal applicatiO.n O.f heat; innO.vative fO.O.d surface decO.ntaminatiO.n prO.cedures aimed at greatly improving the safety O.f SO.me fO.O.ds O.f animal O.rigin; radically new apprO.aches to' aseptic prO.cessing. This bO.O.k CO.vers these majO.r trends in such a way as to' summarise prO.gress already made and alSo. to' indicate pO.tential fO.r the future. It is directed at fO.O.d cO.mpanies invO.lved in prO.ductiO.n, distributiO.n and sale. vi PREFACE

It will be of value to those in government and industry-sponsored food research institutes around the world. It will be useful for teaching courses in food science, home economics, microbiology and process engineering, etc., and for those engaged in food-related research in industry and academia. I would like to thank the authors of the various chapters for their contributions, and also for their patience and cooperation during the preparation of this volume.

G.W.G. Contributors

R.G. Board School of Biological Sciences, University of Bath, Claverton Down, Bath BA2 7A Y, UK J. Burgos Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain s. Condon Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain A.R. Davies Department of Food Microbiology, Leatherhead Food Research Association, Randalls Road, Leatherhead, Surrey KT22 7RY, UK P. Fryer School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham B15 2IT, UK G.W. Gould Unilever Research Laboratory, Colworth House, Sharnbrook, Bedford MK441LQ, UK C.lliU National Food Biotechnology Centre, University Col• lege, Cork, Ireland D. Knorr Department of Food Technology, Berlin University of Technology, Konigin-Luise-Str. 22, D-14195 Berlin, Germany L. Leistner An den Weinbergen 20, D-95326 Kulmbach, Germany P. Loaharanu Food Preservation Section, International Atomic Energy Agency, Wagamerstrasse 5, PO Box 100, A-1400 Vienna, Austria P. Lopez Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain B. Mertens Breedstraat 3, RMC Corporation, B-9100 Sirnt• Niklaas, Belgium J. Mullin Unilever Research Laboratory, Colworth House, Sharnbrook, Bedford MK441LQ, UK G.J.E. Nycbas National Agricultural Research Foundation, Institute of Technology of Agricultural Products (ITAP), Sof. Venizelou 1str., Lycovrisi 14123, Athens, Greece viii CONTRIBUTORS

J. Raso Facultad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain D.Rose Department of Produce and Packaging Technology, Campden Food & Drink Association, Chipping Camp- den GL556LD, UK F.J. Sala Facu1tad de Veterinaria, Universidad de Zaragoza, Miguel Servet 177, 50013 Zaragoza, Spain W. Sitzmann Krupp Maschinentechnik GmbH, Division Extrak• tionstechnik, PO Box 90 05 52, D-21045 Hamburg, Germany F.J.M. Smulders Department of the Science of Food of Animal Origins, The University of Utrecht, Bilstraat 172, PO Box 80 175, 3508TD Utrecht, The Netherlands Contents

Overview xv G.W. GOULD

1 Principles and applications of hurdle technology 1 L. LEISTNER

1.1 Introduction 1 1.2 Examples of the hurdle effect 1.2.1 Fermented foods 1 1.2.2 Shelf stable products (SSP) 3 1.2.3 Intermediate moisture foods (IMF) 5 1.3 Behaviour of microorganisms during food preservation 8 1.3.1 Homeostasis of microorganisms 8 1.3.2 Multi-target preservation of foods 9 1.3.3 Stress reactions and metabolic exhaustion 9 1.4 Total quality of foods 10 1.4.1 Optimal range of hurdles 11 1.4.2 Potential safety and quality hurdles 12 1.4.3 User guide to food design 12 1.5 Application of hurdle technology in less developed countries 15 1.5.1 Fruits of Latin America 15 1.5.2 Dairy product of India 17 1.5.3 Meat products of China 18 1.6 Future potential 19 References 20

2 Bacteriocins: natural antimicrobials from microorganisms 22 C. HILL

2.1 Introduction 22 2.1.1 Historical 23 2.2 Bacteriocin structure and function 23 2.2.1 Lantibiotics 23 2.2.2 Small heat-stable bacteriocins 26 2.2.3 Large heat-labile bacteriocins 28 2.3 Genetics of bacteriocins from LAB 29 2.3.1 Genetic organization of bacteriocin operons 29 2.3.2 Genetic location of bacteriocin genes 31 2.4 Application of bacteriocins in food systems 31 2.4.1 Dairy industry 32 2.4.2 Canning industry 34 2.4.3 Meat industry 34 2.4.4 Wine and beer 35 2.4.5 Sauerkraut 35 2.5 Future prospects for bacteriocins 36 References 38 x CONTENTS

3 Natural antimicrobials from animals 40 R.G. BOARD

3.1 Introduction 40 3.2 The phagosome 44 3.3 Antibiotic peptides 45 3.3.1 Biological role 45 3.3.2 Chemical attributes and spectrum of action 46 3.4 Protein amendment and production of antibiotic peptides 49 3.4.1 Iron 50 3.4.2 Avidin 51 3.5 The lactoperoxidase system (LPS) 52 3.6 Lysozymes 53 3.7 Prospects 54 References 55

4 Natural antimicrobials from plants 58 G.J.E. NYCHAS

4.1 Introduction 58 4.2 Phytoalexins 59 4.3 Organic acids 60 4.4 Essential oils 60 4.5 Phenolics, pigments and related compounds 67 4.5.1 Factors affecting antimicrobial action 75 4.6 Modes of action 77 4.7 Health and legislative aspects 81 4.8 Conclusions 82 References 83

5 Food irradiation: current status and future prospects 90 P. LOAHARANU

5.1 Introduction 90 5.2 Development of national regulations 90 5.3 Technical advantages and limitations of food irradiation 91 5.3.1 Techno-economic advantages 92 5.4 Limitations of food irradiation 98 5.4.1 Technical 98 5.4.2 Infrastructure and economics 99 5.4.3 Consumer concerns 99 5.5 Consumer acceptance of irradiated food 100 5.5.1 Consumer attitude surveys 100 5.5.2 Market testings and retail sales of irradiated food 102 5.6 Commercial applications of food irradiation 103 5.7 International co-operation in the field of food irradiation 105 5.7.1 Co-operation among FAO, IAEA and WHO 106 5.7.2 Co-operation with the Codex Alimentarius Commission 107 5.7.3 Co-operation leading to international trade in irradiated food 108 5.8 Conclusions 109 References 109 CONTENTS xi

6 Microwave processing 112 1. MULLIN

6.1 Introduction 112 6.2 Introduction to microwaves and their interaction with food materials 113 6.2.1 Basics 113 6.2.2 How microwaves heat 114 6.2.3 Power absorption 115 6.2.4 Uniformity of heating 115 6.2.5 Material properties 116 6.3 Microwaves and microorganisms 117 6.3.1 Early work (1940-55) 118 6.3.2 Renewal of interest in the 1960s 118 6.3.3 Conclusion 120 6.4 Microwave processing equipment 120 6.4.1 The benefits of microwave processing 120 6.4.2 Current status of microwave processing in food industry applications 121 6.4.3 Microwave patents in preservation 122 6.5 Case histories 123 Case history 1 Green tea drying/roasting system with microwave and far infra-red techniques 123 Case history 2 Drying of pharmaceuticals 125 Case history 3 Pasteurisation of fruit and sugar mixture 127 Case history 4 Sterilisation after packaging of pasta products 128 Case history 5 Pilot plant microwave steriliser 129 6.6 The future 133 References 133

7 Hydrostatic pressure treatment of food: equipment and processing 135 B. MERTENS

7.1 Introduction 135 7.2 General description of an industrial high pressure system 136 7.2.1 The high pressure vessel and its closure 136 7.2.2 Pressure generation 139 7.2.3 Temperature control 142 7.2.4 Material handling 143 7.3 Current commercial applications of high pressure technology 144 7.3.1 Isostatic pressing 144 7.3.2 Quartz growing 145 7.3.3 Chemical reactors 146 7.4 Current status of high hydrostatic pressure technology with a view to food processing 146 7.4.1 Introduction 146 7.4.2 HHP food processing conditions: time, temperature and pressure 147 7.4.3 Capacity requirements 147 7.4.4 Fast cycling in combination with three shifts per day, 300 days per year operation 149 7.4.5 Process control 150 7.4.6 Safety 150 7.5 The challenges of the commercial application of high pressure technology in the food industry 151 xu .CONTENTS

7.5.1 Technical challenges 151 7.5.2 Economic and commercial challenges 156 7.6 Outlook 157 Acknowledgements 158 References 158

8 Hydrostatic pressure treatment of food: microbiology 159 D.KNORR

8.1 History and key issues of high pressure application 159 8.2 Current applications 160 8.3 Pressure effects of microorganisms 162 8.3.1 Possible mechanisms of action 162 8.3.2 Pressure inactivation of vegetative cells in food systems 164 8.3.3 Pressure effects on bacterial spores 166 8.4 Combination treatments 167 8.5 Conclusions 172 Acknowledgements 172 References 172

9 Effect of heat and ultrasound on microorganisms and enzymes 176 F.J. SALA, J. BURGOS, S. COND6N, P. LOPEZ and J. RASO

9.1 Historical perspective 176 9.1.1 Heat inactivation of microorganisms and enzymes 177 9.1.2 Destructive effect of ultrasound waves on microorganisms and enzymes 182 9.2 Destructive effect of combined treatments of heat and ultrasound under pressure: Mano-Thermo-Sonication (MTS) 190 9.2.1 Effects of MTS on microorganisms 192 9.2.2 Effect of MTS on enzymes 195 9.3 Conclusions 199 References 200

10 Electrical resistance heating of foods 205 P. FRYER

10.1 Introduction 205 10.1.1 The thermal sterilisation of foods 205 10.1.2 Heat generation: electrical resistance heating 207 10.1.3 APV Baker ohmic heater 209 10.1.4 Preservation by electrical heating 210 10.2 The physics of electrical heating 211 10.2.1 Governing electrical equations 211 10.2.2 Thermal properties of foods 215 10.2.3 Food mixtures: flow and heat generation 215 10.3 Models for electrical heating 216 10.3.1 Electrical conductivity of foods 216 CONTENTS xiii

10.3.2 Electrical conductivity of solid-liquid mixtures 217 10.3.3 Flow and heat transfer 219 10.3.4 Holding and cooling systems 224 10.4 Electrically processed foods 225 10.4.1 Frequency effects in electrical processing 225 10.4.2 Enhanced diffusion in electrical processing 226 10.4.3 Differences between diffusion in conventional and electrically processed foods 228 10.5 Conclusions 231 Acknowledgements 232 References 232 Nomenclature 234

11 High-voltage pulse techniques for food preservation 236 W. SITZMANN

11.1 Introduction 236 11.2 Cell count reduction by using electricity: a historical review 237 11.3 The Elsteril Process 242 11.4 The influence of high-voltage pulses on microorganisms 244 11.5 The influence of electric high-voltage pulses on food ingredients 247 11.6 Mathematical modelling of cell count reduction 248 11.7 Conclusions 250 References 250

12 Preservation by microbial decontamination; the surface treatment of meats by organic acids 253 F.J.M. SMULDERS

12.1 Introduction 253 12.2 Critical control points in carcass contamination 254 12.2.1 Material: the animal 254 12.2.2 Machine: equipment and utensils 255 12.2.3 Method: slaughter and fresh meat processing 255 12.2.4 Man: the slaughter personnel 256 12.3 Organic acids as meat decontaminants 257 12.3.1 The antimicrobial properties of organic acids 257 12.3.2 Factors influencing the efficacy of meat decontamination by acids 259 12.4 Effects of acid treatment on sensory properties 268 12.4.1 Effects on colour 268 12.4.2 Effects on flavour and odour 270 12.4.3 Effects on drip loss 271 12.5 Mode of application of acids; technologies available 272 12.5.1 Spraying and spray cabinets 272 12.5.2 Immersion 274 12.5.3 Other methods 276 12.6 Acceptability of acid treatment 277 12.6.1 Toxicological considerations 277 12.6.2 Legislation and regulations 277 12.7 Conclusions and actions needed 277 Acknowledgements 278 References 278 xiv CONTENTS

13 Advances and potential for aseptic processing 283 D. ROSE

13.1 Aseptic technology 283 13.2 Regulatory effects 284 13.3 Aspects of food manufacturing practice 285 13.3.1 Scheduled processes 285 13.4 GMP guidelines 289 13.5 Design and development 289 13.5.1 Food contact surfaces 291 13.5.2 Food process 291 13.5.3 Non-food contact surfaces 293 13.5.4 Decontamination of packaging 293 13.5.5 Aseptic filling zone 294 13.6 Commissioning tests 295 13.7 Manufacturing directive 296 13.8 Economics and market trends 296 13.8.1 Bulk packaging 298 13.8.2 Commodity, added value or niche product? 298 13.9 Conclusions 300 References 301

14 Advances in modified-atmosphere packaging 304 A.R. DAVIES

14.1 Introduction 304 14.1.1 Role of gases 305 14.2 Market status and potential 306 14.3 Microbiology of MAP 308 14.3.1 Microbial spoilage 308 14.3.2 Microbial safety 310 14.3.3 Clostridium botulinum 310 14.3.4 Other pathogens 311 14.4 Developments in MAP 312 14.4.1 Intelligent packaging 312 14.4.2 Predictive, mathematical modelling 315 14.4.3 Combination treatments 316 14.4.4 Packaging films/equipment 316 14.4.5 Indicators 317 14.5 The future 317 Acknowledgement 318 References 318

Index 321 Overview G.W. GOULD

Introduction

The major technologies that are employed to preserve the quality and microbiological safety of foods include: (i) procedures that prevent the access of microorganisms to foods in the first place; (ii) procedures that inactivate them should they nevertheless have gained access; (iii) procedures that prevent or slow down their growth should they have gained access and not been inactivated. Whilst the currently used traditional preservation procedures continue to act in. one of these three ways, there has recently been a reawakening of interest inthe modification of these technologies, mainly in the direction of reducing the severity of the more extreme techniques. These modifica• tions are being sought primarily to improve the quality of food products, and principally in order to meet the requirements of consumers through the avoidance of the extreme use of any single technique. In addition to the modified techniques, but with the same objective of improving food quality, radically new techniques are also being researched and applied. For both modified and the new techniques it is imperative that they deliver not only the promised improvements in quality but also an equivalent, or preferably an enhanced, level of safety compared with the procedures that they replace. For these reasons, the summaries of new and improved methods of preservation in the following chapters are opportune.

Consumer requirements

Consumers' requirements constantly change and, with respect to foods in recent years, have encompassed desires for foods that are convenient to store and use and yet have higher quality, are 'fresher', 'more natural' and 'h~althier' than hitherto. At the same time, increased awareness of the risks of food poisoning has ensured that a high degree of assurance, and indeed improvement, of safety are perceived as key requirements as well. xvi OVERVIEW

Table 1 Consumer requirements impacting on the development of preservation technologies

Major requirements More convenience ease of storage satisfactory shelf-life Higher quality better flavour, texture and appearance Fresher More natural Nutritionally healthier Safer

Means of achievement Less severe processing less intensive heating minimal over-heating minimal freeze damage Less use of artificial additives More use of natural preservation systems Lower levels of salt, fats and sugars Elimination of food-poisonoing microorganisms from the most often contaminated foods and raw materials

Table 1 summarises these consumer requirements and indicates some of the likely means for satisfying them. It will be apparent that a number of these means of achievement (e.g. less heat, less salt, less use of preserva• tives) may actually lead to a loss in the intrinsic preservation and safety of a food. It is therefore important that the new and improved technologies effectively build back the preservation that may otherwise be lost.

Existing technologies

The existing technologies for food preservation are summarised in Table 2. Few of these act primarily by restricting the access of microorganisms to foods (item (i) above) except at the terminal phase of production of thermally processed foods, and in the sense that packaging restricts access. There are more procedures that act via inactivation (item (ii) above) but still, considering the tonnages of foods treated, only heat is used substantially. Turning to procedures that slow down or prevent the growth of micro• organisms in foods (item (iii) above) there are many more procedures available for use, including those that rely on control of the environment (e.g. temperature control), those that result from particular methods of processing (e.g. microstructure control) and those that depend on the intrinsic properties built in to particular formulated foods (e.g. control by the adjustment of water activity or pH value). OVERVIEW xvii

Table 2 Major existing and new technologies for food preservation

Restriction of access of microorganisms to products Aseptic packaging of thermally processed foods Packaging Inactivation of microorganisms in products Heat pasteurisation and sterilisation Ionising radiation Addition of enzymes (e.g. lysozyme) Application of high hydrostatic pressure Electric shock treatments Slowing down or prevention of growth of microorganisms in products Lowered temperature-chilling and freezing Reduced water activity-curing, conserving, drying Acidification Fermentation Vacuum and modified atmosphere packaging Addition of preservatives Microstructure control in water-in-oil emulsions

It is against this background that the new and improved techniques are being developed.

New and improved techniques

With respect to the procedures that restrict the access of microorganisms to foods, the employment of aseptic packaging techniques for thermally processed foods has expanded greatly in recent years, both in the numbers of applications and in the numbers of alternative techniques that are commercially available (chapter 13). With respect to the improvement of techniques for the inactivation of microorganisms in foods, most effort and new application has concerned thermal processing. A particular aim has been to minimise damage to product quality. This is being pursued in two, often complementary, ways. Firstly by the wider application of more high temperature-short time processing, with associated aseptic packaging where relevant (chapter 13). Secondly, by delivering heat in new ways, e.g. by microwaves (chapter 6) or by electrical resistance ('ohmic') heating of foods (chapter 10), which allow better control of heat delivery and minimise the over-cooking that commonly occurs in more conventional thermal processes. An important safety consideration that must be borne in mind is the overall reduction in total heat delivery to foods that will result from the wider application of these techniques, as target Fo values are more and more tightly achieved. The use of ionising radiation to preserve foods or to eradicate pathogens from them, is already well established. In addition to its value as a preservation technique, it offers a very effective route for the reduction in xviii OVERVIEW food poisoning, e.g. via the irradiation of the often Salmonella- and Campylobacter-contaminated foods such as poultry and other foods of animal origin. Whilst the use of radiation continues to grow worldwide (chapter 5), negative consumer reaction in many countries holds back its wider use. Radically new procedures for the inactivation of microorganisms in foods include two other physical procedures that offer alternatives to heat: the use of high hydrostatic pressure (chapters 7 and 8) and the use of high voltage electric pulses (chapter 11). Both techniques are highly effective in inactivating vegetative cells of bacteria, yeasts and filamentous fungi, at pressures and at voltage gradients that are compatible with the retention of high quality in some foodstuffs. However, bacterial spores remain more difficult to control by both these procedures, so that their use for the preservation of foods other than relatively short shelf-life or products in which spores are not a problem because they are inhibited by the intrinsic properties of the food (e.g. low pH or low water activity) must await further research. Finally, concerning novel inactivation procedures, the effectiveness of ultrasonic radiation in inactivating the vegetative forms of microorganisms has been well known for many years. However, the recent research work that has shown that its efficacy can be enhanced by the simultaneous application of (relatively low) hydrostatic pressure is leading to a re• evaluation of its potential as a food preservation aid (chapter 9). A particularly important new inactivation technique has resulted from the development of surface decontamination procedures that can be applied to meat and poultry carcasses, and to other animal-derived foods which are known to be potentially contaminated with enteric pathogens (chapter 12). In many countries, unacceptably high levels of enteric infection in the human population still occur, and the situation is getting worse rather than getting better. Many food microbiologists have come to realise that although improved hygiene education and the application of Hazard Analysis and Critical Control Point techniques etc. may all help to improve food poisoning statistics, a major reduction will only be achieved if such new elimination techniques are employed. If the organisms of concern did not enter the home or the catering establishment etc. in the first place, then the momentary lapses of hygiene that will always occur, at some frequency or other, would be of little consequence. With respect to procedures that slow down or prevent the growth of microorganisms in foods, major successes have been seen, and new appli• cations are steadily being made, in the use of 'combination preservation' techniques or 'hurdle technology' (chapter 1). This has been supported by a greatly improved understanding of the principles underlying the stability and safety of an enormous number of combination-preserved foods that are traditional and indigenous to different parts of the world. It has also been supported by the beginning of an understanding of how many of these OVERVIEW xix combination procedures act at the cellular level, which often seems to involve 'multitarget' interference with the various homeostatic mechanisms that are fundamental to the reaction of microorganisms to the stresses to which the food technologist exposes them in foods (chapter 1). Though still a relatively new technology, modified atmosphere pack• aging has grown rapidly in use in some countries, particularly for the extension of the high quality shelf-life of certain chill-stored foods. It remains, however, little used in other countries (chapter 14). Again, considering its wide use, it is surprising that a full understanding of how modified atmospheres (particularly the carbon dioxiHe component that most of them contain) exert their inhibitory effects at the level of cell biochemistry have not yet been worked out. Elucidation of the mechan• isms of action could lead to improved means for effective application. To some extent, interest in naturally occurring antimicrobial systems has expanded in recent years in response to consumers' requirements for fresher, more natural additive-free foods. With a few notable exceptions, very few of these systems have yet been taken through to application. However, the substantial research efforts underway on animal-derived (chapter 3), plant-derived (chapter 4) and microorganism-derived anti• microbial systems (chapter 2), are demonstrating the efficacy of a wide range of natural mechanisms, many of which have potential for use in food preservation. So far, few such natural systems have been included as components in combination studies, i.e. as additional 'hurdles'. This is a pity, because the food technologist has important opportunities to use these systems in a wide range of combinations with other potential inhibi• tors. Furthermore, it is arguable that although in vitro studies are necessary to investigate mechanisms of action and for intense genetical and bioche• mical studies, too few studies of natural systems have still been undertaken using actual foodstuffs. Sound and extensive food studies are essential prerequisites before food manufacturers will expend the effort or make the investment necessary to bring new preservation systems into successful application.

Conclusions

This overview serves to highlight the fact that although there is a large and more-or-less traditional and stable range of food preservation tech• niques available, there is also a surprisingly large and growing number of improved technologies, and also radically new ones, that are being researched or are in the early stages of application. This is heartening news for food technologists and for research workers, and hopefully for consumers as well, so that their developing requirements can continue to be effectively and safely satisfied in the future.