Principles and Applications of Modified Atmosphere Packaging of Foods

Principles and Applications of Moditied Atmosphere Packaging of Foods

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Principles and Applications of Modified Atmosphere

Packaging of Foods Second Edition

Edited by

B.A. BLAKISTONE Senior Scientist

Food Chemistry and Packaging Department National Food Processors Association

Washington DC USA

Springer Science+Business Media, LLC

First edition 1993 Second edition 1998

© 1998 Springer Science+Business Media New York Originally published by Thomson Science in 1998. Softcover reprint of the hardcover 1 st edition 1998

Typeset in 1O/12pt Times by Cambrian Typesetters, Frimley, Surrey

Thomson Science is a division of International Thomson Publishing I(j)p· ISBN 978-1-4757-6254-9 ISBN 978-1-4757-6252-5 (eBook) DOI 10.1007/978-1-4757-6252-5

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Contents

Contributors xi

Preface xiii

1 Introduction 1 B.A. BLAKISTONE

1.1 Definitions, terminology and abbreviations 1 1.1.1 Modified atmosphere packaging (MAP) 1 1.1.2 Controlled atmosphere packaging (CAP) 1 1.1.3 Gas packaging 2 1.1.4 Vacuum packaging (VP) 2 1.1.5 Gas cocktail 2 1.1.6 Controlled atmosphere storage (CAS) 2 1.1.7 Hypobaric storage 3

1.2 Methods of atmosphere modification in packaged foods 3 1.2.1 Vacuum packaging 3 1.2.2 Gas packaging 3

1.3 Gases used in MAP 7 1.3.1 Oxygen 7 1.3.2 Carbon dioxide 8 1.3.3 Nitrogen 8 1.3.4 Carbon monoxide 8 1.3.5 Sulphur dioxide 9

1.4 Microbiology of MAP 9 1.4.1 Effects of spoilage microorganisms 9 1.4.2 Effects on pathogenic microorganisms 11 1.4.3 Storage temperatures 12

References 13

2 Markets for MAP foods 14 A.L. BRODY

2.1 Introduction 14 2.2 History of CAP, MAP and vacuum packaging 15

2.2.1 Tectrol 15 2.2.2 Transfresh 16 2.2.3 Cryovac 16 2.2.4 Processed meats 17 2.2.5 Bakery goods 17 2.2.6 Retail red me at 17 2.2.7 Sous-vide 18 2.2.8 Pasta 18 2.2.9 Microwave pasteurization 19

2.3 Europe 19 2.3.1 Uni ted Kingdom 20

VI CONTENTS

2.3.2 France 21 2.3.3 Germany 23 2.3.4 Italy 23 2.3.5 Other countries 24 2.3.6 Fresh meats 24

2.4 USA and Canada 25 2.4.1 Red meat 25 2.4.2 Pork 29 2.4.3 Poultry 29 2.4.4 Fish 30 2.4.5 Fruit and vegetables 30 2.4.6 Soft bakery goods 32 2.4.7 Prepared foods 34 2.4.8 Delicatessen salads 36

2.5 Contemporary issues in MAP technologies 36 2.6 Conclusion 37

Further reading 38

3 MAP machinery 39 M.J. HASTINGS

3.1 Historical development 39 3.2 Gases 39 3.3 Packaging material 41 3.4 Packaging machines 41 3.5 Chamber machines 41

3.5.1 Thermoforming system 41 3.5.2 Pre-formed container machines 45

3.6 Flexible form-fill-seal machine systems 48 3.6.1 Horizontal form-fill-seal machines systems 48 3.6.2 Inverted horizontal form-fill-seal machine systems 53 3.6.3 Vertical form-fill-seal systems 55

3.7 Fail-safe assurance 56 3.8 Automatie product feeding systems 58 3.9 Conclusion 59

Appendix 3.A Packaging systems for MAP 61

4 Packaging materials for MAP of foods 63 1. GREENGRASS

4.1 Introduction 63 4.2 Plastic films commonly used in MAP 67

4.2.1 Polyolefins 67 4.2.2 Vinyl polymers 71 4.2.3 Styrene polymers 73 4.2.4 Polyamides 73 4.2.5 Polyesters - polyethylene terephthalate (PET) 74 4.2.6 Otherfilms 74

4.3 Combination of films 75 4.3.1 Laminates, coextrusions and extrusion coating 75 4.3.2 Specifications 80

4.4 Typical specifications for MAP use 81 4.4.1 Horizontal and vertical form-fill-seal systems 83 4.4.2 Bulk gas packaging 84 4.4.3 Microwavable packs 85 4.4.4 Technical problems 85 4.4.5 Testing MAP packs 86

4.5 Seal system and quality 4.5.1 Peelable seals

CONTENTS

4.5.2 Antifog (AF) properties 4.5.3 Printing and labelling 4.5.4 Supplier/packer relations

4.6 Legislation and the environment References Appendix 4.A Film calculations Appendix 4.8 Antifogging properties Appendix 4.C Recommended storage conditions for MAP materials Appendix 4.D Draft specification for MAP reels Addendum to chapter: Specification development

5 Quality assurance of MAP products I. ALU and L.M. WEDDIG

5.1 Introduction 5.2 Safety and quality of MAP foods 5.3 Application of HACCP to MAP foods

5.3.1 Revisions to HACCP Guidelines 5.4 Total quality management and quality of MAP foods

vii

87 89 90 91 93 93 96 96 98 99 99

101

102

102 102 103

see page xiv 110

5.5 Combining hazard analysis with both critical control point and total quality control 111

5.6 International Organization for Standardization and ISO 9000 se ries as a quality management tool

5.7 Inspection and testing methods 5.7.1 Film faults 5.7.2 Headspace gas analysis 5.7.3 Seal strength 5.7.4 Temperature checks

5.8 Regulatory aspects of MAP foods 5.8.1 HACCP 5.8.2 Labeling

5.9 Summary References

6 Fresh-cut produce E.H. GARRETI

6.1 Introduction 6.2 Product respiration and MAP 6.3 Quality maintenance 6.4 Safety of MAP produce 6.5 Packaging materials 6.6 Future industry needs

References

7 Bakery products D.A.L. SEILER

7.1 Introduction 7.1.1 Types of wrapper 7.1.2 Reasons for improving shelf-life

7.2 Factors governing shelf life 7.2.1 Microbiological spoilage

111 114 114 114 114 119 121 121 121 123 123

125

125 126 128 129 130 131 133

135

135 135 135 136 137

viii CONTENTS

7.2.2 Staling 7.2.3 Moisture loss or gain

7.3 Methods of extending mould-free shelf-life 7.3.1 Hygiene considerations 7.3.2 Preventive measures 7.3.3 Destructive measures 7.3.4 Measures involving growth inhibition

7.4 MAP 7.4.1 Gas packaging 7.4.2 Oxygen scavengers 7.4.3 Ethanol 7.4.4 MAP methods and materials

7.5 Conclusions References

8 Dairy foods, multi-component products, dried foods and beverages P.J. SUBRAMANIAM

8.1 Dairy products 8.1.1 Cheeses 8.1.2 Hard cheeses 8.1.3 Mould-ripened and soft cheeses 8.1.4 U nripened cheeses 8.1.5 Yoghurt 8.1.6 Milk 8.1.7 Milk powders

8.2 Coffee 8.2.1 Whole be ans 8.2.2 Ground coffee 8.2.3 Instant coffee

8.3 Tea 8.4 Snacks

8.4.1 Nuts 8.4.2 Crisps and other snacks

8.5 De\icatessen/multi-component products 8.5.1 Sandwiches 8.5.2 Dressed salads 8.5.3 Breaded and batter-coated products 8.5.4 Pastry-based products 8.5.5 Safety concerns

8.6 Fruit juices and other beverages 8.7 Use ofMAP in combination with other processes

References

9 Fish and shellfish H.K. DAVIS

9.1 Introduction 9.2 Spoilage of fish

9.2.1 Effects of temperature on fish spoilage 9.2.2 Measurement of spoilage 9.2.3 Hazards associated with stored fish

9.3 Gaseous preservatives used in modified atmosphere storage offish 9.3.1 Properties of the principal components 9.3.2 Proportions of gases recommended for use in MAP 9.3.3 Vacuum packaging

140 140 141 141 141 142 143 145 146 151 152 153 155 156

158

158 158 159 165 167 169 169 171 171 171 172 174 174 175 175 176 178 181 182 183 184 185 185 188 190

194

194 196 199 199 202 202 202 204 205

CONTENTS

9.4 Changes occurring during storage of fish products in MAP 9.4.1 Composition of the headspace gas mixtures 9.4.2 Effect of MAP on the pH of fish products 9.4.3 Bacteriological changes 9.4.4 Effects of MAP on microbiological hazards 9.4.5 Sensory properties 9.4.6 Reference materials 9.4.7 Effects oftemperature rises on MAP products 9.4.8 MAP and chemical indices offish spoilage 9.4.9 Fish products in bulk MAP 9.4.10 Residual effects

9.5 Adjuvant treatments 9.5.1 Chemical additives 9.5.2 Physical treatments

9.6 Conclusion References

10 Meats and pouItry B.A. BLAKISTONE

10.1 Introduction 10.2 Microbiology of red meats

10.2.1 Microbiology of red meat 10.2.2 Colourofred me at

10.3 Packaging of meats and poultry 10.4 Vacuum packaging 10.5 Gas atmospheres 10.6 MAP storage of poultry 10.7 Meat products

10.7.1 Important considerations 10.7.2 Cured colour stability 10.7.3 Water activity, pH and microbial spoilage

10.8 Effects of MAP on selected me at products 10.8.1 Bacon 10.8.2 Beef jerky 10.8.3 Cooked beefroasts 10.8.4 Ground beef patties 10.8.5 British fresh sausages 10.8.6 Cooked meat loaves 10.8.7 Frankfurters 10.8.8 Harn 10.8.9 Meat pies 10.8.10 Pastrami 10.8.11 Wieners in natural casings 10.8.12 Poultry products 10.8.13 Cook-{:hill systems 10.8.14 Sous-vide cook-{:hill system

10.9 Safety aspects of MAP 10.10 The future References

Index

lX

206 206 207 207 213 216 220 221 221 222 224 224 224 226 227 228

240

240 241 241 243 245 246 250 253 258 258 261 262 264 267 268 268 270 270 272 272 273 275 276 276 276 277 278 278 281 283

291

Contributors

I. Alli Faculty of Agricultural and Environmental Sciences, McGill University, Macdonald Campus, 21, 111 Lakeshore, Ste-Anne-de-Bellevue, Quebec, Canada H9X3V9

B.A. B1akistone Senior Scientist, National Food Processors Association, 1401 New York Ave, NW Washington, DC 20005, USA

A.L. Brody Rubbright Brody, Inc., 4981 Trevino Circle, Duluth, GA 30136, USA

H.K. Davis Central Science Laboratory, Sand Hutton, Nr York Y041LZ, UK

T. Frey Packaging Partners, 5859 Buena Vista Ave, Oakland, CA 94618, USA

E.H. Garrett International Fresh-Cut Produce Association, 1600 Duke Street, Alexandria, VA 22314-3400, USA

F.R. Grabiner Lifelines Technology, Inc., 116 American Rd, Morris Plains, NJ 07950, USA

J. Greengrass J.G. Associates, Farthings, Swan Lane, Draycott, Somerset BS27 3SS, UK

M.J. Hastings Owls, 1 Melbourne Park, York Y04 4QE, UK

D.L. Newslow Lloyd's Register QA, 8260 Cathy Ann St, Orlando, FL 32818, USA

T. Prusik Lifelines Technology Inc., 116 American Rd, Morris Plains, NJ 07950, USA

D.A.L. Seiler 27, Long Park, Chesham Bois, Amersham, Bucks HP6 5LA, UK

P.J. Subramaniam Leatherhead Food RA, Randalls Road, Surrey KT22 7RY, UK

L.M. Weddig National Food Processors Association, 1401 New York Ave, N.W., Washington, DC 20005, USA

T.C.S. Yang Army Natick Research, Development and Engineering Center, Natick, MA 01760-5018, USA

Preface

Modified atmosphere packaging may be defined as an active packaging method in which an altered atmosphere is created in the headspace that retards chemical deterioration while simultaneously retarding growth of spoilage organisms. Shelf lives of perishable products, such as dairy products, meat, poultry, fish, fruits and vegetables, and bakery items are limited by biochemical changes in the product catalysed by exposure to the normal atmosphere (21 % oxygen, 78% nitrogen and less than 0.1 % carbon dioxide) and growth of spoilage organisms. Modification of the atmosphere within a package containing these products helps to better maintain the quality of the food under longer storage conditions and retards the growth of undesirable organisms. Of course, deterioration is also slowed by chilling, which is required for the transport to market of highly perishable items like meat, poultry and fish that would either spoil or have the potential for contamination by certain food pathogens. Chilling plus a modification of the atmosphere optimizes the keeping quality of food.

Modification of the atmosphere has been known for over a century as a means of food preservation and has become a very popular means of food preservation in the latter part of the 20th century. Modified atmosphere packaging (MAP) is practised extensively in Europe, Canada and the USo Both vacuum packaging (rem oval of air from the package) and addition of gases within the package are considered MAP. The gas mixt ure used is dependent on the type of product being packaged. The exact percentages of the gases at any time during the shelf life of a product will change depending on the rate of respiration of the product (e.g. fruits, vegetables or meat) , biochemical changes and the permeation/transmission characteristics of the package, which permits influx and outflow of all gases.

MAP has become apart of the food industry as consumers demand fresh products that retain quality throughout their shelf life. Foods that are commonly seen in the grocery packaged under MAP in the USA include an array of pre-cooked pastas, pre-cooked poultry, some sausage biscuits, most of the California strawberries and prepared salads. All cured or processed meats and cured cheese products are either vacuum packed or MAP packed. In New York State, at least one major dairy is using MAP in the packaging of cottage cheese and will soon be using MAP in ice cream mixes and fluid milk. Products subjected to MAP in Europe and Canada include those mentioned for the USA, with fresh and fresh-cut fruits and vegetables substituting for California strawberries, and additionally retail meats and bakery items.

xiv PREFACE

Much research continues to be devoted to quality changes during the extended shelf life of foods packaged under MAP and, quite properly, to published papers focusing on food safety concerns. This edition presents the latest information on the world-wide markets and research on-going on several continents. Hazard Analysis Critical Control Point (HACCP) and ISO 9000 series are specially presented as structured food safety and quality management programmes, respectively, that are available to food manufacturers. The objective of this second edition of the text is to remain a comprehensive examination of MAP from scientific, technological and commercial aspects.

B. A. BLAKISTONE

Note added in proof

5.3.1 Revisions to HACCP Guidelines

On August 14, 1997 the National Advisory Committee on Microbiological Criteria for Foods (NACMCF) released the review of its 1992 HACCP document, comparing it to the guidance document of the Codex Committee on Food Hygiene. (The document titled 'Hazard Analysis and Critical Control Point Principles and Application Guidelines' is available from the USDA, Washington, DC.) NACMCF made the HACCP principles more concise, revised and added definitions, inc1uded sections on prerequisite programs, education and training, revised and provided a more detailed explanation of the application of HACCP principles and provided an additional decision tree for identifying critical control points. HACCP has been re-defined as 'a systematic approach to the identification, evaluation, and control of food safety hazards.' Especially important are the revised Principles of HACCP.

1. Conduct a hazard analysis. 2. Determine critical control points. 3. Establish critical limits. 4. Establish monitoring procedures. 5. Establish corrective actions. 6. Establish verification procedures. 7. Establish record-keeping and documentation procedures.

1 Introduction B.A. BLAKISTONE

Changing lifestyles have dicta ted the need for foods that offer convenience to the consumer in a myriad of ways, such as minimizing preparation time, easy opening of the container and single service, while also offering high quality throughout an extended shelf life. Plastic packaging has responded to these demands, and creativity with plastics has been limited only by the imagination of the designer. Modified atmosphere packaging (MAP) is not a new concept; it has been used in one form or another for over a century, but research on MAP has advanced methods substantially way beyond the simple but elegant idea of changing the normal composition of air within a package from 21 % oxygen, 78% nitrogen and less than 0.1 % carbon dioxide. Modification of the package headspace by reducing the oxygen content while increasing the levels of carbon dioxide and/or nitrogen has been shown to significantly extend the shelf life of perishable foods at chill temperatures (Table 1.1). MAP has evolved sufficiently to require definitions of the terminology used to describe the many variations and the multiple means of generating the atmosphere.

1.1 Definitions, terminology and abbreviations

1.1.1 Modified atmosphere packaging (MAP)

MAP is a form of packaging that involves the removal of air from the pack and its replacement with a single gas or mixture of gases. The gas mixture used is dependent on the type of producL The gaseous atmosphere changes continuously throughout the storage period owing to factors such as respiration of the packed product, biochemical changes and the slow permeation of gases through the container.

1.1.2 Controlled atmosphere packaging (CAP)

Controlled atmosphere packaging is a term often used synonymously for MAP. Its use is, however, incorrect as it is not possible to control the atmosphere within the pack once it has been sealed.

2 PRINCIPLES AND APPLICATIONS OF MAP OF FOODS

Table 1.1 Estimated shelf life of MAP products

Product Air packaged

Beef" Porka Chickena

Cooked meatsa

Fisha

Breadb

Coffeeb

"Refrigerated storage. b Ambient storage.

1.1.3 Gas packaging

(days)

4 4 6 7 2 7 3

MAP (days)

12 9

18 28 10 21

548 (18 months)

An alternative term commonly used to describe MAP is gas packaging. It is a misnomer since atmosphere modification can be achieved by simple vacuum or evacuation of the air. It is also feit to have adverse emotive connotations for the consumer and is consequently a term avoided by many manufacturers and retailers.

1.1.4 Vacuum packaging (VP)

The simplest and most common means of modifying the internal gaseous atmosphere in a pack is by vacuum packaging. The product is placed in a pack made from film of low oxygen permeability, air is evacuated and the package sealed. An evacuated pack collapses around the product so that the pressure inside is seldom much less than atmospheric.

1.1.5 Gas cocktail

The gas cocktail is a term sometimes used to refer to the gas mixture used to modify the atmosphere within a package.

1.1.6 Controlled atmosphere storage (CAS)

Controlled atmosphere storage is a form of bulk storage where the concentration of gas initially introduced to modify the atmosphere is maintained throughout the period of storage by constant monitoring and regulation. The stores are also refrigerated. CAS has been used since the 1930s for the storage of fruit in large sealed chambers in which the levels of carbon dioxide and oxygen can be controlled. More recently it has been successfully used to extend the life of fresh poultry carcasses stored in bulk.

INTRODUCTION 3

1.1.7 Hypobaric storage

Hypobaric or low-pressure storage is another form of controlled atmosphere storage where pressure, temperature and humidity are accurately controlled. It has been used for the storage of soft fmits.

1.2 Methods of atmosphere modification in packaged foods

1.2.1 Vacuum packaging

Vacuum packaging was the earliest form of MAP developed commercially and it is still extensively used for such products as primal cuts of fresh red meat, cured meats, hard cheeses and ground coffee. It is not suitable for soft products or bakery products since the vacuum process causes irreversible deformation of the product.

The process involves packaging the product in film of low oxygen permeability and sealing it after first evacuating the air. Under good vacuum conditions the oxygen level is reduced to less than 1 %. The barrier properties of the film used restricts entry of oxygen from outside. In the case of vacuum-packed meat, respiration of the meat quickly consumes the residual oxygen replacing it with ca rb on dioxide, wh ich eventually increases to 10-20% within the package.

Unfortunately vacuum-packaged meat is unsuitable for the retail market because the depletion of oxygen coupled with the low oxygen permeability of the packaging film causes a change of meat colour from red to brown owing to the conversion of myoglobin to metmyoglobin. This is not an acceptable meat colour to the consumer. A further disadvantage is the accumulation of drip during prolonged storage of meat in vacuum packs.

1.2.2 Gas packaging

The desired headspace atmosphere in a modified atmosphere pack can be achieved in two fundamental ways. These are the mechanical replacement of air with agas or gas mixture or by generating the atmosphere within the package either passively, as in the case of fmit and vegetables, or actively by using suitable atmosphere modifiers such as oxygen absorbents.

Mechanical air replacement. mechanical air replacement compensated vacuum.

There are two different techniques for in a package: (i) gas flushing and (ii)

Gas [lushing. The gas-flush process is usually performed on a form-fillseal machine. A continuous stream of gas is injected into the package to

4 PRINCIPLES AND APPLICA TIONS OF MAP OF FOODS

replace the air. This dilutes the air in the headspace surrounding the food product. When most of the air has been replaced, the package is sealed. There is a limit to the efficiency of this system since replacement of the air in the package is accomplished by dilution. Typical residual oxygen levels in gas-flushed packs are 2-5%. This means that the gas-flush technique is not suitable for packaging very oxgyen-sensitive foods. The great advantage of the gas-flush process is speed, since it is a continuous operation.

Flushing with nitrogen is a common industry practice to extend the shelf life of beverages, juices and juice drinks. A drop of liquid nitrogen is injected into cans containing beer or carbonated soft drinks immediately before seaming. The liquid nitrogen rapidly evaporates into gas, flushing out oxygen taken up during the filling process. The benefits of flushing cans with nitrogen include increased shelf life, retention of product aroma and reduced can corrosion.

Compensated vacuum. The compensated vacuum process first applies a vacuum to remove the air from inside a pre-formed or thermoformed container holding the food and then intro duces the desired gas or gas mixture via lances or ports. Machines designed to perform this operation are of the chamber variety. Since this is a two-stage process, the speed of operation of the equipment is slower than the gas-flushing technique. Because the air is removed by vacuum, however, the efficiency of the process with respect to residual air levels is much superior.

Modified atmosphere generation

Passive atmosphere modification. Vegetables and fruits continue to respire after harvest, consuming oxygen and producing carbon dioxide and water vapour. If the respiration characteristics of the commodity can be accurately matched to the permeability of the film used for packaging, then a favourable modified atmosphere can be created passively within the package when an equilibrium concentration of oxygen and carbon dioxide is established. Equilibrium modified atmospheres (EMA) containing 2-5% oxygen and 3-8% carbon dioxide have been shown to delay maturation and softening of vegetables, and reduce chlorophyll degradation, microbial spoilage and enzymic browning.

An intriguing approach to controlling the porosity of films used in passive atmosphere modification was developed in the late 1980s by Courtaulds Flexible Packaging, prior to its acquisition of Sidlaw Flexible Packaging Ltd (Bristol, UK). Sidlaw has since patented a method of creating holes of 20-100 flm in films to achieve high permeability while minimizing moisture loss. The method is microperforation technology or the P-Plus process. The process can be adapted for lower-respiring

INTRODUCTION 5

produce, but it functions best on produce with higher rates of respiration, such as be an sprouts, fresh podded peas, sliced mushrooms and strawberries (Frey, 1997).

Active packaging. The incorporation of certain additives into packaging film or within packaging containers to modify the headspace atmosphere and to extend product shelf life is referred to as active packaging. Under this definition can be classified oxygen absorbents, carbon dioxide absorbents/emitters, ethanol emitters and ethylene absorbents. This relatively new technology has considerable potential but is currently expensive. In the ca se of oxygen absorbents, the cost of the additive is 3p to 5p per pack plus the cost of automated insertion equipment. The subject has been extensively treated by Rooney (1995). An excellent summary of the status of active packaging effectors is given in Table 1.2.

Oxygen absorbents. According to Rooney (1995), the market projection for global use of oxygen-scavengers has suggested that, provided that the oxygen-scavenging plastic formulations reach the marketplace, the annual production of oxygen-scavenging packaging will increase from the current $200 million annually to at least $1 billion within five years. The most frequently used oxygen scavengers are in the form of small sachets containing metallic reducing agents such as powdered iron, which in suitably humid conditions (water activity (aw ) > 0.85) uses up residual oxygen to form nontoxic iron oxide. 'Ageless', a range of gas-scavenger products made by the Mitsubishi Gas Chemical Company, are the most widely used. The sachet is designed to reduce oxygen levels to less than

Table 1.2 Active packaging commercialized or researched

Effect Media Commercial Research

Oxygen scavenging Iron in sachetsllabels Global USA, Japan, France Oxygen scavenging Iran in plastics USA, Japan USA, Japan Oxygen scavenging Reactive plastics n.a. Australia Ethanol release Sachets Japan n.a. Carbon dioxide release Sachets Global n.a. Carbon dioxide release Plastics n.a. Australia Antimicrabial Plastics n.a. Australia Permeability balance Plastics with holes/ Global Global

powders Taint removal Plastic/powder Japan, USA USA Water buffering Plasticlfibreboard Japan, USA Australia Permeability contral Liq uid crystal plastic n.a. USA Ethylene removal Plastics n.a. Japan/ Australia Ethylene removal Pellets Global n.a.

n.a., not applicable. Reprinted with permission fram Rooney (1995).


0.1 %. Oxygen comes from the package headspace and from dissolved oxygen in the food, or it may enter the package by diffusion during the distribution process.

To avoid problems with metal detectors or the potential for metallic taints or odours, non-metallic oxygen scavengers have been developed. They employ ascorbic acid or ascorbate salts. Rooney (1995) notes particular improvements in oxygen-scavenging sachets during the previous three years, not only in the composition of the sachet contents and their materials construction, but also in development of adhesive labels for the inner wall of packages. This concept was first used by Multisorb Technologies (Buffalo, NY) in their FreshMax ™ labels. The company has enhanced the design in the form of full package labels that include transparent areas to allow food product visibility while at the same time providing sufficient area of iron-containing label.

Carbon dioxide absorbentslemitters. Several commercial systems exist that can be used either to scavenge or to emit carbon dioxide. One of the 'Ageless' products, type E, contains calcium hydroxide, which at a sufficiently high humidity reacts with carbon dioxide to produce calcium carbonate. It is used to prevent packaged fresh-roasted ground coffee, which pro duces significant volumes of ca rb on dioxide, from bursting. It is also used in the USA for packaging beef jerky and similar dehydrated poultry meat products.

In the Freshilizer Series made by Toppan Printing, types C and CW absorb oxygen and generate carbon dioxide, which simultaneously inhibits microbial growth while reducing pack collapse caused by the removal of oxygen. Type C is treated to prevent generation of water and is, therefore, suitable for low aw « 0.8) foods while type CW is suitable for highermoisture foods.

Ethanol vapour generators. Ethanol has well-known anti-microbial properties and can be sprayed directly onto food products prior to packaging where surface contamination is likely to be the principal cause of spoilage. However, more sophisticated systems for delivering ethanol to foods now exist whereby controlled release of ethanol vapours from films or sachets after packaging is achieved. The main commercial system is the Japanese Freund ethanol emitter. The ethanol is entrapped in silica within a sachet made of a film highly permeable to ethanol vapour. Tbe system is being used for bakery, cheese and semi-dried fish products.

Ethylene absorbents. Ethyle~ 'is a growth-stimulating factor that is produced by fruits and vegetables during storage. If it accumulates in the package it speeds up respiration rates and reduces shelf life. Numerous ethylene absorbers are available. Several commercial systems available in

INTRODUCTION 7

Japan use a silica gel that contains permanganate. This is contained in a sachet that is highly permeable to the ethylene. The system has been used commercially with success for many fruits, including kiwis. Silicon dioxide is another ethylene absorber, without the toxicity problem of permanganate and which additionally acts as a desiccant when combined with an appropriate catalyst. They can be incorporated into packaging films during lamination or coextrusion.

1.3 Gases used in MAP

MAP may be defined as an active packaging method in which an altered atmosphere is created in the headspace that retards chemical deterioration while simultaneously retarding growth of spoilage organisms. Gases primarily used include oxygen, carbon dioxide, and nitrogen. Carbon monoxide has limited application in MAP, and sulphur dioxide is commonly used. Table 1.3 presents some recommended gas mixtures for extending the shelf life of a variety of products.

1.3.1 Oxygen

Reduction of oxygen delays oxidative reactions such as lipid rancidity in meats, fish, prepared foods and baked goods, wh ich results in off odours and flavours, or the browning reaction occurring in cut surfaces of fresh fruits and vegetables by the action of polyphenol oxidase. Improved quality throughout the extended shelf life can be maintained by reduced

Table 1.3 Recommended gas mixtures for MAP

Product Oxygen (%) Carbon dioxide (%) Nitrogen (%)

Red meat 60-85 15--40 Cooked/cured meats 20-35 65-80 Poultry 25 75 Fish (white) 30 40 30 Fish (oily) 60 40 Salmon 20 60 20 Hard cheese 100 Soft cheese 30 70 Bread 60-70 30--40 Non-dairy cakes 60 40 Dairy cakes 100 Pasta (fresh) 100 Fruit and vegetables 3-5 3-5 85-95 Driedlroasted foods 100

Taken from Parry (1993) with permission.


oxygen. Complete absence of oxygen is usually avoided, particularly in white fish, because such conditions can foster the growth of pathogens such as Clostridium botulinum. The exceptions occur where oxygen is needed for fruit and vegetable respiration, colour retention (as in the case of red meat) or to avoid anaerobic conditions in white fish.

1.3.1 Carbon dioxide

Carbon dioxide is useful as areplacement gas in MAP foods because it particularly inhibits Gram-negative, aerobic spoilage bacteria such as Pseudomonas spp., which cause flavour and odour changes in meat, poultry and fish. The gas is highly soluble in the aqueous phase of foods, thus acidifying them through the production of carbonic acid. 1t also has some direct anti-microbial effects (Zagory, 1994). The gas acts to suppress respiration in fruits and vegetables and at levels above 1 % can render plant tissues insensitive to the ripening hormone ethylene. However, an excess of carbon dioxide can damage plant tissues. Higher concentrations can damage muscle foods, producing excess drip, and cause off flavours in fats and oils, discoloration of fresh produce and package collapse when the gas continues to be absorbed by the aqueous portion of the packaged product (Zagory, 1994).

1.3.2 Nitrogen

Nitrogen, an inert gas, is used in MAP and other food packages to displace atmospheric air, especially oxygen, thus extending shelf life. It retards the growth of aerobic spoilage organisms and prevents package collapse because of its low solubility in water and fat phases of food.

1.3.4 Carbon monoxide

Carbon monoxide is very effective in maintaining the red colour in fresh meat through the formation of carboxymyoglobin. It has not been used commercially for this purpose, however, since carbon monoxide (a highly toxic gas and explosive at concentrations of 12.5-74.2%) is not approved by the regulatory authorities owing to the possible health hazard to packaging machine operators. Its use has, however, been sanctioned in the USA to prevent browning in packed lettuce. When it is used in combination with a controlled atmosphere of low oxygen, Clark et al. (1976) have noted an inhibitory effect on psychrotrophic bacteria. Carbon monoxide has been reported to be effective as a fungistat in fruits (Sommer, 1981; El-Kazzaz et al., 1983). Zagory (1994) noted its effectiveness against many bacteria, yeasts and moulds in concentrations as low as 1%.

INTRODUCfION 9

1.3.5 Sulphur dioxide

Sulphur dioxide is anti-microbial in its unbound non-ionized molecular form (ICMSF, 1980) and, therefore, treatments are most effective at pH va lues below 4. It is used to control the growth of mould and bacteria on a number of soft fruits, especially grapes and dried fruits (Zagory, 1994). It is useful in the control of microbial growth in fruit juices, wines, shrimp, pickles and some sausages. Sulphur dioxide is selective in its toxic action (ICMSF, 1980). At low concentrations (e.g. 25 ppm) it is fungicidal, but at 1-2 ppm, it is bacteriostatic. Its effectiveness is greater against Gramnegative rods such as Escherichia coli and Pseudomonas spp. than against Gram-positive rods such as lactobacilli.

1.4 Microbiology of MAP

Microorganisms require certain definable conditions for growth and reproduction. In a food product, these conditions are either intrinsic properties of the food, such as pH and aw or extrinsic factors associated with the storage environment. Among the relevant extrinsic factors are the gaseous composition of the environment and the temperature. It is these two extrinsic factors that can be controlled with MAP to retard spoilage and extend shelf life.

Contrary to popular misconception, MAP is not a panacea for hygiene abuse during production or handling of a food product. There is no enhancement of product quality. MAP simply arrests the natural deterioration process. It requires a good, clean product to increase shelf life significantly. A strong quality management programme such as ISO 9000 or Total Quality Management is needed in a food processing plant to ensure good manufacturing practices are being followed. A food safety programme such as hazard analysis critical control point (HACCP) is required to identify microbiological, chemical and physical hazards at every stage of the production and packaging processes. Some of these hazards will be classified as critical control points (CCPs), and their monitoring will be extremely important so that immediate corrective action can be taken to avoid a food safety incident.

The specific microbiology of various foods and food products is dealt with later in this book. However, general comments on the effects of modified atmospheres on the food spoilage and pathogenic 'food poisoning' bacteria are pertinent.

1.4.1 Effects on spoilage microorganisms

Microbial food spoilage is caused by the growth of microorganisms that render the food unsaleable or inedible. It is characterized by undesirable

, sensory changes in colour, texture, flavour or odour.


Concentrations of carbon dioxide in excess of 5% (v/v) inhibit the growth of most food spoilage bacteria, especially psychotrophic species, which grow on a wide range of refrigerated foods. In general Gramnegative bacteria are more sensitive than Gram-positive ones.

The common aerobic spoilage organisms of fresh meat and poultry, the pseudomonads and the Acinetobacterl Moraxella spp., are readily inhibited by carbon dioxide. Other common food spoilage bacteria such as Micrococcus and Bacillus spp. are also very sensitive to carbon dioxide. The lactic acid bacteria, by comparison, are very resistant to carbon dioxide and replace aerobic spoilage bacteria in modified atmosphere packs of fresh meat. They are slow growing and do not produce offensive spoilage effects until their numbers are very high.

Most food spoilage mould species have an absolute requirement for oxygen and appear to be sensitive to high levels of carbon dioxide. Consequently foods with low aw values, such as bakery products, that are susceptible to spoilage by moulds can have their shelf lives extended by MAP. Many yeasts are capable of growing in the complete absence of oxygen and most are comparatively resistant to carbon dioxide. The oxygen requirements of some common food spoilage and pathogenic microorganisms are listed in Table 1.4.

TabIe 1.4 Oxygen requirements of some common food-spoilage and pathogenic microorganisms

Anaerobes - require atmospheric oxygen for growth Spoilage organisms Pseudomonas spp.

Pathogens

Acinetobacterl Moraxella Micrococcus Moulds Bacillus cereus Yersinia enterolitica Vibrio parahaemolyticus

Microaerophiles - require low levels of oxygen fOT growth Spoilage organisms Lactobacillus Pathogens Campylobacter jejuni

Listeria monocytogenes Facultative organisms - grow in the presence or absence of oxygen

Spoilage organisms Brocothrix thermosphacta Shewanella putrifaciens Bacillus spp. Enterobacteriaceae Fermentative yeasts

Pathogens Salmonella spp.

Anaerobes - inhibited/killed by oxygen Pathogens

Reprinted with permission from Parry (1993).

Staphylococcus spp.

Clostridium perfringens Clostridium botulinum

INTRODUCTION 11

Spoilage is an important safeguard in preventing food poisoning outbreaks since it is deterioration in the food that wams the consumer that it may be unsafe.

1.4.2 Effeets on pathogenie mieroorganisms

Knowledge of the effects of modified atmospheres on food pathogens is incomplete particularly for the pathogens such as Listeria monoeytogenes and Yersinia enterolitiea.

High levels of carbon dioxide have generally been found to have an inhibitory effect on Staphyloeoeeus aureus, Salmonella spp., E. eoli and Y. enterolitiea. The degree of inhibition increases as temperature decreases (Hintlian and Hotchkiss, 1986).

There are five food-bome pathogenic bacteria known to be capable of growth below 5°C (Table 1.5): Bacillus eereus, C. botulinum type E (group 11), L. monoeytogenes, Y. enterolitiea, and Aeromonas hydrophilia.

Table 1.5 Pathogenic microarganisms and their growth characteristicsa

Microorganisms Minimum Minimum Minimum Aerobic/ temperature pHb awb anaerobic

(0C)b

B. cereus 4 4.3 0.95 Facultative Campylobacter jejuni 32 4.9 0.99 Microaerophilic C. botulinumC

Group I 10 4.6 0.93 Anaerobic Group Ir 3.3 5.0 0.97 Anaerobic

(or;;. 5.5%) C. per/ringens 12 5.0 0.95 Anaerobic E. coli 7 4.4 0.95 Facultative E. coli 0157 6.5 4.5 0.95 Facultative L. monocytogenes 0 4.3 0.92 Facultative Salmonellad 7 4.0 0.94 Facultative S. aureus 6 4.0 0.83 Facultative

(10 far toxin) (4.5 far toxin) (0.90 far toxin) V. cholerae 10 5.0 0.97 Facultative V. parahaemolyticus 5 4.8 0.94 Facultative

(halophile) Y. enterocolitica -1 4.2 0.96 Facultative

Reprinted with permission from Anon. (1996). aNew scientific da ta become available on a regular basis. European Chilled Food Federation is keeping this table under review. bGrowth limits. Under otherwise optimal conditions, limits will vary according, for example, to strain, temperature, type of acid, solute and other factors, and they will normally be higher in foods. However, variabilities in measurements etc. must be allowed for: a margin of error must be incorporated. These figures are indicative only and are not necessrily representative of all strains of microorganisms in foods. cGroup I: mesophilic/proteolytic; Group II: Psychrotrophic/non-proteolytic. dMinimum temperature for most strains of Salmonella is 7°C; some strains, however, can sometimes grow at 5.2°e.


(Enteropathogenicity of A. hydrophilia is controversial and not accepted by the European Chilled Food Federation (Mossei et al., 1995).) Four others are capable of growth at temperatures just above 5°C: enterotoxigenic E. coli, S. aureus, Vibrio parathaemolyticus and Salmonella spp. Consequently the ability of modified atmospheres to inhibit the growth of these organisms in foods under refrigerated storage is of vital importance. Fortunately most of these organisms do not compete weil with harmless bacteria such as the lactic acid bacteria, which grow rapidly if temperature abuse occurs.

Because L. monocytogenes is facultative as weil as being capable of lowtemperature growth, its potential import an ce in modified atmosphere packs needs to be clearly established since it is a common contaminant of vegetables and poultry.

The main cause for concern, however, is the possible growth of C. botulinum type E, because this is both an anaerobe and low-temperature tolerant. Of particular concern is the fact that it may grow and pro du ce toxin on the product before spoilage is detectable to the consumer (Kautter et al., 1981; Genigeorgis, 1985; Post et al., 1985). Conflicting results on the effect of modified atmospheres on the growth and toxin production of C. botulinum make it difficult to draw valid conclusions. Rowever, storage at low temperatures (below 3.3°C) and atmospheres containing at least 2% oxygen should provide an adequate safeguard for products susceptible to contamination with C. botulinum.

J. 4. 3 Storage temperatures

Proper temperature management is the most important factor in maintaining the quality of fresh packaged foods (Zagory, 1994). Zagory (1994) notes that low temperatures slow down oxygen-requiring reactions and the metabolism of spoilage organisms and pathogens producing toxins and reduce the rates of permeability of films. With the exception of bakery goods and some dried products, MAP products must be refrigerated. Table 1.5 details minimum temperature, pR and aw values for growth of pathogenic microorganisms.

Acknowledgement

Parts of this introduction have been based on the excellent material prepared by R.T. Parry (1993) for the first edition of this book. Modifications and additions have been made to reflect current information.

INTRODUCTION 13

References

Anon. (1996) Appendix D, in Guidelines for the Hygienic Manufacture of Chilled Foods 1996. European Chilled Food Federation, Hillevi Latvalahti, Secretary General, Finnish Food and Drink Industries, Chi lied Food Industries Association, P.O.B. 115, FIN-00241 Helsinki,62.

Clark, D.S., Lentz, C.P. and Roth, L.A. (1976) Use of carbon monoxide for extending shelf life of prepackaged fresh beef, Can. Inst. Food Sei. Tech. J., 9,114-117, as seen in Ch. 10, Gases as preservatives, in Microbial Ecology of Foods, Vol. 1 (1980). The International Commission on Microbiological Specifications for Foods, p. 17l.

El-Kazzaz, M.K., Sommer, N.F. and Fortlage, R.J. (1983) Effect of different atmospheres on postharvest decay and quality of fresh strawberries. Phytopathology, 73(2), 282-285.

Frey, T.D. (1997) High gas permeability key to 'passive' CAP for produce. Packaging Technol. Eng., 6(3), 40-45.

Genigeorgis, C.A. (1985) Microbial and safety implications of the use of modified atmospheres to extend the storage life of fresh meat and fish. Int. J. Food Microbiol., 1(5), 237-251.

Hintlian, C.B. and Hotchkiss, J.W. (1986) The safety of modified atmosphere packaging: a review. Food Technoi., 40(12), 70-76.

ICMSF (1980) Microbial Ecology of Foods, Vol. 1: Factors Affecting the Life and Death of Microorganisms. Academic Press, London, pp. 180-184.

Kautter, D.A., Lynt, R.K., Lilly, R. Jr and Solomon, H.M. (1981) Evaluation of the botulism hazard from nitrogen-packed sandwiches. J. Food Proc., 44, 59-<il.

Mossel, D.A., Corry, J.E.L., Struijk, C.B. and Baird, R.M. (1995) Diseases of microbial origin transmitted by foods, Ch. 4 in Essentials of the Microbiology of Foods, Wiley, Chichester, UK.

Parry, R.T. (ed.) (1993) Introduction, in Principles and Applications of Modified Atmosphere Packaging of Food, 1st edn (ed. R.T. Parry). Blackie (Chapman & Hall), pp. 1-18.

Post, L.S., Lee, D., Furgang, D., Specchio, J. and Graham, C. (1985) Development of botulinal toxin and sensory deterioration during storage of vacuum and modified atmosphere packed fish fillets. J. Food. Sci., 50(4), 990-996.

Rooney, M.L. (ed.) (1995) Active Food Packaging. Blackie (Chapman & Hall), London. Sommer, N.F., Fortlage, R.J., Buchanan, J.R. and Kader, A.A. (1981) Effect of oxygen on

carbon monoxide suppression of postharvest pathogens of fruits. Plant Dis., 65(4), 347-349.

Zagory, D. (1994) Fundamentals of reduced-oxygen packaging, in Modified Atmosphere Food Packaging (ed. A.L. Brody). Institute of Packaging Professionals, Herndon, VA, pp. 9-17.

2 Markets for MAP foods A.L. BRODY

2.1 Introduction

Although modified-atmosphere and vacuum-packaged foods are not highly visible in world food marketing and technology, they constitute a substantial and growing proportion of North American and European food supplies - albeit mostly in distribution packaging. Vacuum packaging, MAP and CAP are all regarded by most observers as part of the same technology. In this definition, therefore, the market for MAP food includes fresh and minimally processed foods packaged under vacuum or altered gaseous environment. More than 80% of the beef in the USA and Canada is shipped from me at packers to retailers and hotel/restaurant/ institutional (HRI) (catering) operations in the form of vacuum-packaged primal cuts. Similar proportions are valid for Europe. About half of all fresh poultry in North America is master packed in bulk under modified atmospheres for distribution to retail grocery and HRI outlets. A growing retail category in the USA is precooked poultry packaged under modified atmosphere or vacuum, and marinated and precooked poultry packaged under vacuum. Virtually all cured or processed meat and cured cheese products in retail distribution are packed under either vacuum or inert (i.e. nitrogen) atmosphere. Most cured and cooked meats for in-store delicatessen use are vacuum packaged. Vast quantities of fresh and fresh-cut fmit and vegetables - 15% of all lettuce in the USA and more than 80% of all California strawberries - are distributed under modified atmosphere conditions throughout the Western world. In Europe, dozens of meat packers and thousands of retail stores employ MAP to distribute retail cuts of red meat to retail stores and consumers. In the USA and Canada, about 1000 stores (of a total of 35 000) are displaying centrally packaged cuts of fresh beef, mostly ground beef, but more are using retail cuts of case-ready pork. In Europe, several hundred bakeries employ CAP to extend the distribution cycle for soft bakery products such as breads and cakes. In Canada, several bakeries and sandwich-makers are using this technology commercially. A unique category of sausage and biscuit sandwiches is modified atmosphere packaged in the USA. Throughout the industrialized world, fresh pasta is distributed in MAP form. Large quantities of sousvide (vacuum packaged and postfill thermally pasteurized) precooked

MARKETS FOR MAP FOODS 15

foods are used in restaurant chains in France. More than 500 installations in the USA employ cook--<:hill processing/packaging technology for pumpable bulk foods such as soups and sauces.

Therefore, the spectrum of vacuum packaging, CAP and MAP extends from fresh red meat through precooked meals and embraces most of the categories of perishable and minimally processed products currently in the food chain.

2.2 History of CAP, MAP and vacuum packaging

For decades, food and food-packaging technologists have been taught the principles of microbiological growth and retardation, including lowering temperature to reduce activity rates and applying heat to destroy microorganisms. Included in such teaching has been the nature of microorganisms to slow their respiratory and growth processes when oxygen is reduced and the respiratory gases such as carbon dioxide are increased. Aerobic respiration is the basis for the degradation of plant materials after removal from the growing plant. In growth, the plant uses oxygen and produces carbon dioxide and water. After growth, the process is reversed. The slowing of plant-product respiration by increasing carbon dioxide and water conte nt coupled with reduction of oxygen and possible removal of ethylene has been known for many years.

In the early years of the twentieth century, fresh meat shipped from the Antipodes to England was sometimes chi lIed by solid carbon dioxide. Mutton, beef and lamb so held were noted to have shelf-lives longer than carcass meat held under wet ice only, a phenomenon attributed later to the upset of the gaseous atmosphere. These observations provided scientific bases for increasing carbon dioxide and reducing oxygen in transport vehicles and storages for fresh meats.

Beginning in the 1930s, fresh apples and pears were placed in enclosed warehouses. The natural respiratory activities of the fruit reduced the oxygen and increased the carbon dioxide within the storage areas sufficiently to slow respiration markedly. The stored apples or pears could be consumed as much as six months after the original harvest: an extension of about double the normal chilled storage shelf-life. The use of natural respiratory-controlled storages for apples and pe ars expanded rapidly during the 1950s in both New York and the Pacific Northwest.

2.2.1 Tectrol

During the 1950s and 1960s in the USA, Whirlpool Corporation's food scientists developed methods to control directly the atmospheres surrounding meat, fruit and vegetable products. The concept was adapted to bulk


and industrial distribution of fresh fruit and meat products. The name Tectrol (Total Environmental ConTROL) was applied to total control of warehouses. By employing gas burners to reduce the oxygen, valving to the exterior to permit in air to prevent oxygen extinction, and filters and scrubbers to remove excess carbon dioxide, total temperature and gaseous environmental control were effected. By the mid- to late-1960s, hundreds of apple and pear warehouses throughout the USA and Europe were equipped with Tectrol systems to extend the shelf-life of apples and pears.

2.2.2 Transfresh

Because of Whirlpool's dedication at that time to domestic appliances, the concept was spun off to the produce grower, Bruce Church, in California and reorganized under the name Transfresh. Virtually all developments since the 1960s have emanated from the Transfresh organization. The Tectrolffransfresh concept has since been expanded to transport containers in order to control partially the internal gas content surrounding the contents. The Tectrolffransfresh system is now used to deliver bulk and consumer-size packages of fresh-cut vegetables to HRI and retail outlets, and pallet loads of strawberries to sub-distributors. In 1990, the process was refined into complete control of shipboard container loads. A further spin-off from Transfresh is Fresh Express, wh ich produces and packages fresh-cut vegetables for the retail market.

2.2.3 Cryovac

During the 1960s, barrier shrink film vacuum packaging used to protect frozen turkeys in distribution was applied to fresh red meat by the Cryovac organization, a name also applied to the process. The Cryovac vacuumpackaging process is based on the fact that in the absence of oxygen, microorganisms responsible for meat spoilage are retarded. Simultaneously, however, the original purple myoglobin color of fresh red meat is retained. In the mid-1960s, Cryovac and Iowa Beef Packers (now IBP) joined in a new concept of slaughtering cattle at a central location and, rather than shipping in hanging carcass form, breaking the meat into primal and sub-primal cuts, these were subsequently vacuum-skin packed into high gas barrier multilayer plastic film bags. The reduced oxygen of the vacuum retarded microbiological growth and oxidative changes in fat. Further, the bag did not permit the passage of water vapor, and so weight loss owing to evaporation was significantly reduced. These filled bags were then packed in water-resistant corrugated fiberboard shipping cases (hence the name boxed beef) for shipment to hotels, restaurants and institutions. By the mid-1970s, the process was successfully introduced to retail


supermarkets to supply me at for butcher reduction to consumer size cuts and subsequent packaging.

2.2.4 Processed meats

During the early 1950s, vacuum packaging of cured meats such as frankfurters, harns, harn slices, bologna, etc. was developed using thermoforming webs of flexible oxygen-barrier nylon-based flexible packaging materials, placing the processed meat within the formed web and heat sealing with a second gas-barrier material while drawing a vacuum or, later, back-flushing with inert gas. Tbe American companies Standard Packaging and DuPont were among the pioneers in this activity. This concept employing carbon dioxide flushing to displace oxygen was later applied to prolong quality retention of cured cheese. These widely used applications of reduced oxygen packaging are not usually included in discussions of CAPIMAP/vacuum packaging.

2.2.5 Bakery goods

In the mid-1960s, the British Flour Milling and Bakery Research Association investigated the use of elevated carbon dioxide to retard mold growth on surfaces of perishable bakery products. Tbeir results did not culminate in commercial activities, and so the publications remained dormant until the late 1970s when the West German government instituted a regulation to declare all food additives on the package label. Rather than communicate the presence of chemical preservatives to consumers, many commercial bread bakers opted for sealed gas-barrier packages containing carbon dioxide and reduced oxygen to permit the desired shelf-life. In the mid-1980s, the principles were applied to meat-filled sandwiches in both the USA and Canada. Those sandwiches containing cured meat fillings, e.g. sausage, retained their quality under modified atmospheres for many weeks (at chilled temperatures), stimulating a sm all , but solid and growing, product niche. Instructions to consumers are to heat the product before eating, a simple process that partially overcomes the staling effects in the bakery product over long storage. A similar process (i.e. directions for heating before eating) was developed for MAP French-type breads in the early 1980s in England, but this concept was not attractive in the UK. It has since been reintroduced with modest success on Continental Europe and in Australia.

2.2.6 Retail red meat

During the 1960s, investigations by Kalle, a German plastic film converter, demonstrated that fresh red meat could be preserved under refrigeration


with its desirable cherry red oxymyoglobin color if a high carbon dioxide/ high oxygen environment were present. The presence of high oxygen levels was contrary to normal practice because common belief had been that high oxygen accelerates microbial growth, respiration and fat oxidation. Elevated carbon dioxide was shown to be sufficiently effective in retarding microbiological growth, and so the adverse oxidative effects of the high oxygen could be compensated. These findings were put into practice in West Germany employing thermoform/vacuum/gas flush-seal packaging systems and high oxygen-barrier materials. A number of German and other European packaging firms - including Sepp Hagenmuller (Multivac), Kramer & Graebe (now TiromaT) and Dixie-Union in West Germany, Akerlund & Rausing in Sweden, Ono in France, and Otto Nielsen in Denmark - developed total thermoform/vacuum/gas flush-seal systems under various trade names. In this way, centralized red meat or case-ready packaging of retail cuts of fresh meat was introduced in Europe and Canada.

2.2.7 Sous-vide

During the 1980s, M. Georges Pralus, a French chef, studied the relationship of total thermal input to food quality and developed the sousvide or vacuum heating process for internal restaurant use. In this comprehensive system, the total heat of precooking, followed by postpackaging cooking to pasteurize, chilling and then reheating produces the same effect as if a skilled chef had cooked the original fresh ingredients. One of the heating inputs, however, is intended to reduce the microbiological count of carefully selected and processed food components. Vacuum is employed to reduce oxidative deterioration and aerobic microbiological growth and to compact the package and permit rapid heat transfer in and out. Rigid temperature control at less than 3°C and very short distribution cycles -less than 21 days - significantly reduce the risk of microbiological hazard. Thus, sous-vide and related processes have gained significant positions in France for restaurant food. In the USA, a related technology, cook--chill, has been growing to supply bulk precooked readyto-heat sauces, soups and pumpable entrees for HRI outlets. The notion that thermal pasteurization destroys aerobic spoilage microorganisms but not more heat-stable an aerobic pathogenic spores, leaving distribution temperature control as the sole barrier to growth and toxin production in vacuum packages of many foods, surfaced with sous-video This issue has received much discussion in scientific and regulatory circles.

2.2.8 Pasta

During the early 1980s, a sm all New York store initiated the integration of excellent sanitation of both ingredients and process plus modified


atmosphere with sealed plastic packages to enable it to distribute 30% moisture pasta under chilled conditions. These measures permitted prolongation of safe quality up to 40 days. More importantly, the concept triggered others to apply the principles for entrees, side dishes and meatl fish salads.

2.2.9 Microwave pasteurization

In Sweden, the linkage of sanitation, vacuum packaging, microwave pasteurization, rapid chilling and controlled distribution at temperatures below 4°C resulted in the delivery of precooked pastas, fish dishes, omelets, etc. to retail convenience stores in 1988 and 1989 with no untoward incidents. The factory in central Sweden and the retail outlets were all owned and operated by a single firm. This ready me al system was sold to Tetra Pak in 1990. Meanwhile, microwave plus hot-air pasteurization was developed in Italy and later in the Netherlands during the late 1980s to supplement modified atmosphere and refrigerated distribution to prolong quality retention time of 30% moisture pasta and preprepared meals. These activities followed closely the use of steam on the exterior of the sealed packages to achieve the microbiological load reduction effect.

2.3 Europe

From a marketing perspective, vacuum packaging and MAP appear to be more successful in Europe than in the USA for distribution of foods at retaillevel, while the reverse is probably true for distribution packaging to deli ver foods to retailers and HRI operations. The most significant driving force in Europe has been the CAP/MAP/vacuum packaging of chilled foods, influenced by retailers whose objective is to seil more high-margin fresh or chi lied products and reduce losses and in-store waste caused by limited shelf-life. Further , geographic distribution distances gene rally are shorter in Europe than in North America. European retailers demanded the development of total systems to maintain quality and deliver the desired quality and shelf-life results. European retailers have been successful in using their singular influential positions in the market-place to dictate product quality specifications and distribution requirements to their packaged food suppliers. Giant retail chains such as Sainsbury, Tesco, Safeway, Asda, and Marks & Spencer in the UK; Euromarche, Carrefour and Lenor in France; Irma in Denmark; Spar in the Netherlands; and Tengelmann in Germany have dominated the retail food market with their activities in the area of chilled foods and their derivative CAP/MAP/ vacuum-packaged foods. CAP/MAP/vacuum packaging of fresh and freshcut fruits and vegetables permits distributors and retailers to reduce the


fresh produce shrink, including evaporation, and microbial and enzymatic spoilage that otherwise can generate losses that exceed 10% of the original product weight. It affords retailers the opportunity to increase the profitability of their fresh produce departments, which usually represent about 10% of the total grocery sales but may generate more than onefourth of the entire store profits. It also permits expansion of the chilIed prepared foods and delicatessen departments, which genera te profit margins of over 50% and net profits of up to 6%, often 6--60 times the net profit of canned, dry and frozen foods departments.

During the 1980s, the commercialization of CAP/MAP/vacuum packaging for chilled food products was most rapid in the UK, which accounts for about half of the European market for this category. France appears to be second with about one-fourth of the European CAPIMAPI vacuum packaging market. The success of CAP/MAP/vacuum food packaging in both the British and French retail food markets is usually attributed to the presence of tightly structured retailing organizations. Although the proportions of retail food sales through chain retailers in Belgium, Denmark and the Netherlands are comparable to those in most other European countries, the relatively low populations in these nations might account for their low shares of the total European CAPIMAPI vacuum packaging market compared with the UK and France.

2.3.1 United Kingdom

Almost all major retail grocery chains in the UK today offer foods under MAP. The total UK market has grown by over 300% since 1982 and was still expanding at about +9% annually by 1991. Since then, the growth rate has slowed to about 5-7% per annum (Table 2.1).

Up to 38% of red meat sold in the UK is now sold in MAP. In 1994--5, Tesco converted virtually all of its red meat to case-ready under modified

Table 2.1 The UK market for MAP foods in 1992

Product

Red meat Cooked meat Fish and seafood Snack and dried food Fruit and vegetables Dairy Poultry Bakery Fresh pasta Other

Units (%)

30 13 10 14 7 5 5 5 2 9


atmosphere using gas-barrier foam polystyrene trays with gas. The fastest area of growth in MAP from 1990 through 1993 was in fresh fmit and freshcut vegetables. The Tesco chain led in fresh-cut vegetables. The Asda chain has also developed an extensive range of prepared vegetables in MAP since 1992.

The bakery goods market has begun to develop as, following the fresh example, MAP has diversified into products such as baguettes, croissants, muffins, par-baked pizzas and cmmpets. More recent additions to the bakery products range in the UK include par-baked bread, pizza bases, topped pizzas and quiche. The use of MAP for cakes also continues to expand, boosted like many areas of the bakery sector by the recent increase in horizontal form-fill-seal wrap packages.

Many more new 'added-value' prepared products have be gun to appear in the UK, including meat and gravy, steak and butterpat, prepared vegetables and dressing, medallions of salmon, tropical fmit salads, parfried chips and fresh herbs.

2.3.2 France

MAP in France was pioneered by SOCOP A and the industrial gas company Air Liquide during the 1970s and has now spread to over 30 companies and cooperatives through the country.

The baguette is a traditional bakery product and forms a large sector of the French fresh food market in which MAP is particularly successful. Champagne Viande was the first company to introduce MAP of red meat, but they have now been joined by many others including Casino, Werbert Ricoeur, Sicada, Brunet and Sovico. Poultry is another traditionally large fresh food area in France and one in which MAP has grown since 1983.

Between 1985 and 1990, the largest area of growth on the French MAP market was that of fresh-cut vegetables. The tonnage of product packed in this area moved from virtually zero in 1985 to 36 000 tonnes in 1988, about 80% of which was ready-to-eat salads. Since 1990, sales have stabilized at about 40 000 tonnes. In general, the French eat far more salads than the British and this, in part, at least explains the early and enormous success of MAP in this area. The brand leaders in the French ready-to-serve vegetable market in the early 1990s were '5eme Saison' (20%), Cmdi Frais (15%) and La Florette (15%). The leading brand '5eme Saison' was formed in 1985 by four food companies - with the specific intention of developing this market.

During the early 1990s, the French market continued to diversify with a significant shift from lettuce and fresh-cut salads to heavier vegetables such as potatoes, cabbage, carrots, swedes and leeks. MAP is also weil developed in the French market for pizzas and ready-cooked meals. Tbe ready-to-cook meals sector increased almost four times between 1985 and


1990 and is now almost exclusively packed in PVC, with MAP estimated to account for about 70% of total demand.

Another area of more recent development is that of dairy desserts. About 34 000 tonnes of dairy desserts are produced in France every year and the market leaders (Gervais Oanone, Chambourcy and Yoplait) have recently started to use nitrogen and carbon dioxide mixes to increase the shelf-life of their products. This has led to increased demand for gas-tight lidding and tubs.

France is the world leader in sous-vide packaged foods, which are prepared, vacuum packaged, thermally pasteurized, and distributed refrigerated. Most sous-vide food products in France are for HRI distribution. French legislation permits expiration times of up to 21 days for sous-vide products heated to an internal core temperature of 65°C and with a pasteurization value of over 100; and 42 days for products heated to 70°C with a pasteurization value of over 1000, always with distribution temperature below 3°C. French sous-vide food producers are under government regulation, notably le Oirecteur General de l' Alimentation! Service Veterinaire d'Hygiene Alimentaire. Each producer is mandated to control its own production and safety. In 1988, SYNAFAP, a trade association for producers of prepared ready meals, was formed by 26 private companies to help establish guidelines for the production of sousvide and to ensure safety and quality throughout distribution. Ouring its earlier years, sous-vide was used almost exclusively in French restaurants, which packaged and cooked individual portions under vacuum in their own kitchens and stored them under refrigeration for later use. One-third of French restaurants are reported to have either sous-vide packaging or cooking equipment or both. The real development in sous-vide packaged foods occurred during the mid-1980s. In the late 1980s, growth declined to ab out 40% annually. In 1988, HRI total sous-vide production in France was reported at 11 300 tonnes, with retail packaged items constituting about 5700 tonnes, a 25% increase over 1987. In 1988, sous-vide represented more than 15% of refrigerated prepared foods sold by French hypermarkets and supermarkets. Larger stores of over 800 m2 size sell over three-quarters of all sous-vide consumer prepackaged production. Owing to a slower turnover, however, smaller stores manage with low stock because of the product's relatively short shelf-life. Oepending on the type of store and product, consumer prices range from 20-30 francs (about US$4--6) per portion, reinforcing the product's positioning as higher-end, targeted to higher-income buyers. Cafeteria-style chain restaurants are major users of sous-vide meals, having consumed 3200 tons (US) (2900 tonnes) in 1988. Flunch, a major chain restaurant, fed more than 110000 French persons per day, with one-third of their sales from sous-vide products. Flunch was one of the first institutional users of sous-vide products in the mid-1980s. As the benefits of this technology became more


evident, their parent company, Auchan, created Le Petit Cuisinier in 1986 as a primary supplier of sous-vide to Flunch. Le Petit Cuisinier distributes nearly 80% of the 3000 tons (US) (2700 tonnes) it produces to Flunch. Another major sous-vide producer is SABIM, producing for Casino, another self-service retail chain. A number of sm aller companies have developed niches in either fish or other special dishes. About 35% of sousvide meals are bought by single people and 33% by couples without children. As of mid-1988, sous-vide products had penetrated only 5% of French households. Fleury Michon, the current overall retail market leader, entered solely in the consumer segment. Individual portions represent about 65% of the volume sold, and multiportions 35%; tradition al recipes were 60% of volume, and 'light' or lower calorie versions were 40%. Other brand names have included Les Freres Matt, Marie, and Olida and Pere Dodu.

Both sous-vide and cook-chill foods are employed in HRI operations in England and other count ries of Europe. Cook-chill is related to sous-vide but involves cooking and 'pasteurizing' before packaging hot and then chilling to create a partial vacuum within the package. Generally , the technology is applied to bulk bag packaging for pumpable soups and sauces.

2.3.3 Germany

In Germany, the initial emphasis was placed upon fish and prepared salads and pizzas, but none achieved significant success. A serious obstacle for MAP in Germany and in other parts of Northern Europe in the 1980s and 1990s has been fierce resistance to the use of one-trip plastics and especially PVC-based packaging materials, which have a very poor environmental image. Although alternatives in PE and PET have been tried on the market and all packaging must now be recycled in Germany, prejudice remains strong and will be difficult to overcome.

Some signs of a slow rise in interest in MAP in Germany have been in products such as ready meals, pizzas and fresh-cut salads.

2.3.4 Italy

In Italy the relatively sm all share of chain food retailers combined with the wider availability of good-quality, unpackaged fresh produce has been a major factor limiting the potential for MAP. Since the end of the 1980s, there has been renewed interest in MAP, with the emphasis placed on delicatessen products and pasta. MAP is now estimated to account for about 10% of the presalted meat market and 72% of the industrially produced pasta market (50000 tonnes in 1992). Within the pasta sector,


the market is divided between vertical form-fill-seal pillow pouches (12% of sales), thermoformed containers (85%) and envelope bags (3%).

The long-term development of MAP in Italy appears fundamentally limited because of the hotter climate (which does not favor chilled products) and the absence of a concentrated food retailing structure.

2.3.5 Other countries

In the Netherlands and Belgium, MAP was initially developed for vegetables and salads rather than for meats, which are normally retailed as fresh, unpackaged products. The market remains relatively undeveloped particularly in the Netherlands, where there is continued strong resistance to the use of one-trip plastics packaging.

In Spain, MAP has been targeted at the bulk counter-cyclical fruit and vegetable trade with Northern Europe.

Norway was one of the first European markets to pack whole lettuce in MAP and to use MAP for fresh fish.

2.3.6 Fresh meats

In Europe, the drive to centralized prepackaging of fresh meat cuts was sparked almost entirely by retailer chains. Europe's first successful centralized fresh red meat prepackaging began in the late 1970s in Denmark with the Irma cooperative chain, which owns and operates a me at processing and packaging plant near Copenhagen to supply its retail outlets throughout Denmark. Irma applied sanitation plus MAP (high oxygen) to extend refrigerated shelf-life of its red meat cuts. Irma allows five days for distribution and sale of its branded fresh meat from the date of packaging. All retail outlets are within 250 miles of the packing plant. The chain employs its own vehicles for daily delivery to its retail outlets and thus maintains total distribution contro!. Following Irma's lead, Marks & Spencer introduced MAP of fresh meat cuts to its outlets in the late 1970s under its St Michael brand. Marks & Spencer utilizes high-oxygen MAP for fresh meats in its outlets throughout the UK. This system is now the standard for UK fresh meat packaging, which in all shops represents over a third of all retail fresh meat in the UK. Marks & Spencer does not maintain ownership of its cut meat supplier but assurnes an active role in product development, processing and packaging specifications, quality control and inspection. The products, including minced meat, are shipped directly from the supplier to their retail outlets daily and carry a 'sell-by date' of six days from packaging. No Marks & Spencer retail outlet is more than 300 miles from the packer. Tesco, Sainsbury and Safeway, three major UK grocery retailers, followed Marks & Spencer's lead in case-ready CAP


fresh meat, although, in Sainsbury and Safeway stores, about half of their fresh meat at retail is still conventionally cut and wrapped in the backrooms of their stores. At the outset in the UK no retailers using MAP for fresh meat undertook any special marketing program for introduction of the product. At the other end of the spectrum, no major me at packer has made a commitment to MAP or to any other centralized prepacking me at system in Europe, except as a contract packer for a retailer.

During the early 1990s, the 500-store British Tesco chain undertook a case-ready me at program culminating in the virtual conversion of all of their stores by the end of 1995. A small fraction of beef, pork, veal, lamb and mutton is still sold by butchers who custom cut according to direct consumer request. The great majority, however, is centrally packaged at one of eight independent plants operating under tight sanitation and temperature control. Distribution is also under low temperature, with retail display cabinets having been re-engineered to ensure product temperatures below 3°C. The packaging system came from the UK firm Linpac and employs preformed barrier foam polystyrene trays sealed with flexible barrier film on Ross deposit/gas flush-seal equipment from the USA. The internal gas environment is high oxygen/high carbon dioxide to retain the oxymyoglobin red color while suppressing microbiological deterioration. The integration of raw-material quality, in-plant sanitation, total system temperature control, including retail display, and gas mixture delivers sufficient quality retention for the product to reach consumers.

Distribution of MAP meat in Europe is usually within a 100 mile radius and only infrequently exceeds 250 miles. European chilled meat distribution systems are gene rally very efficient, cost effective and weIl controlled. Retailers require only about six days of shelf-life from packaging to last sales date, but in many instances, the indicated expiration date exceeds six days. European meat consumers typically purchase sm aller portions than shoppers in the USA, shop more often and pay more for their meat. Less than 15% of European meat purchases subsequently are frozen by the consumer; by comparison, in the USA, up to three-quarters of all fresh me at purchases are frozen at horne before use. MAP me at generally freezes poorly because of the large headspace volume that fosters the formation of unsightly frost and eventual product freezer burn. Therefore, MAP is weIl suited to general European meat consumption patterns.

2.4 USA and Canada

2.4.1 Red meat

Of the 39 billion pounds of red meat produced in the USA annuaIly, approximately 63% is beef, 36% is pork and the remainder is lamb, veal,


etc. About two-thirds of American pork enters further processing, i.e. to become cured meats such as harns, bacon and sausages. Only 12% of beef is further processed. Of the 21 billion pounds of beef not further processed, approximately 40% or 8.5 billion pounds becomes ground beef. Therefore, about 5 to 6 billion pounds of beef cuts are generated and overwhelmingly cut and packaged at retail grocery level in the USA. Almost all of this beef is cut and air-packaged in the supermarket backroom, employing foamed polystyrene trays with plasticized PVC flexible film overwraps plus paper labels. In addition, nearly 5 billion pounds of ground beef is marketed through retail stores in the USA, largely in foamed polystyrene trays with PVC wraps. Almost all of this nearly 5 billion pounds of beef is coarsely ground beef at federal-government-regulated factory level; it is distributed under low-oxygen conditions, packaged in flexible film keeper barrier chub casings and sent under refrigeration to retail stores where final grinding and retail packaging occurs. Almost 500 million pounds are finely ground in factories into barrier-film chub packaging, also under low-oxygen conditions, for retail sale.

Centralized prepackaging of red meat experienced a surge in the 1980s with the stimuli of Excel, the country's second largest meat packer, and the Kroger supermarket chain. Using vacuum shrink packaging, the factory centrally packaged red meat cuts were distributed under refrigeration to nearly 1000 of Kroger's 1700 retail supermarket outlets. The absence of desirable cherry oxymyoglobin red color on the packaged beef was among many factors that caused a decline to fewer than 200 stores by 1991 and the practice to end in 1994.

This activity was accompanied by several ventures by other packers into vacuum-packaged fresh pork, many of which have been discontinued, and by several in high-oxygen MAP of beef cuts, particularly in Canada.

Among the more significant driving forces for central packaging of fresh me at in Europe has been retailer control, while in the USA, interest is packer driven and controlled. Packer interests apparently dictate extended shelf-life beyond the six days or so common in Europe. For a packer shipping fresh meat cuts across the USA from a central location to distribution centers where the product is to be trans-shipped to retail outlets, a minimum of 15 days is reported to be required: nine days to re ach the retail display case, four days at retail and two days before use. Modified atmosphere/vacuum technology is at its marginal limits for this time period.

In the late 1980s, H.E.B. (H.E. Butt), a small Texas retail food chain, installed a 25-per-minute Cryovac vacuum skin packaging machine in its Austin, Texas meat fabrication plant to supply its 180 stores with 25 different retail red meat cuts ranging from filet mignon to boneless pot roasts. Prior to its venture in vacuum skin packaging (VSP), H.E.B. supplied primal cut beef to its stores for further back-store processing and


packaging. H.E.B management reported their reasons for testing the VSP system:

• better utilization of in-store labor; • shortage of skilled meat cutters; • reduction of stock-outs.

In initiating this system, H.E.B. cited several advantages over other retailers:

• their in-store meat cutters were non-union and unlikely to oppose central processing and packaging;

• they had an existing central beef fabrication facility and distribution system;

• all their retail outlets were within 200 miles of the processing plant.

In 1989, H.E.B. began packaging with a peelable oxygen barrier that could be removed just prior to shelf display to allow the meat to be exposed to air to bloom and thus regain its cherry-red color. Actual experience indicated that after the barrier layer was peeled away, oxygen diffusion through the remaining film was irregular and not all surfaces were exposed equally, resulting in a blotched appearance. Therefore, H.E.B converted to oxygen barrier film to retain internal vacuum. Each package carried a 'freeze by' date, wh ich was 25 days from packing. H.E.B. management cited product color as the major drawback of the program. Repeat sales were high but consumer initial trials remained a barrier. Less than 10% of H.E.B.'s beef sales were in vacuum packaging.

The H. E. Butt retail chain centralized packaging fresh red meat venture from its own factory was discontinued in the 1990s.

Despite the 5+ billion pounds of barrier-bag-packed primal cuts and nearly 5 billion pounds of coarsely cut chub-packaged ground beef handled by retail butchers in the USA, the proportion represented by case-ready beef in 1995 is disappointing, but of even more concern are the prospects, which, despite considerable recent publicity, remain questionable.

The few isolated instances of so me form of case-ready beef are largely master pack with PVC film-wrapped foamed polystyrene trays from CVP Systems using high carbon dioxide and depending on the air after opening to regenerate the red color. Two companies are employing vacuum skin packaging technologies for beef cuts. Several packers are packaging finely ground beef on Ross deposit/vacuum/gas flush-seal machines using barrier polystyrene foam trays plus heat-seal flexible closures to retain a high oxygen/high carbon dioxide internal environment. There is some use of master packaging of ground beef with high oxygen/high carbon dioxide in the bag atmosphere where conventionally PVC-wrapped polystyrene foam trays contain the consumer sale units of ground beef, but these examples


are too few to constitute a trend, especially since they are being employed for specialty low-fat ground beef.

As indicated above, possibly 100 million pounds of ground beef is marketed at retail level in pressure-stuffed chubs, a decline in both poundage and percentage from as recently as 1990.

The total of case-ready beef in any form, except for the chub packages, is less than 100 million pounds of ground beef and possibly 10 million pounds of beef cuts, from fewer than a dozen packing organizations. Case-ready beef, therefore, comprises less than 2% of all retail beef, nearly the lowest percentage since the mid-1970s. These figures might be a signal to all that case-ready beef may not yet be quite ready for the USA retail grocery scene. Concern about meat microbiological safety has, however, been generating additional interest and trial during the 1995-1997 period. Significant penetration will require a long time and capital investment ranging into the billions of dollars for packaging and distribution equipment alone.

Despite the promises that the mammoth warehouse club stores would convert into centralized packaging for their retail fresh meat cuts, the situation in the mid-1990s is very similar to that in mid-1980s. One or two packers are supplying one or two warehouse club store operations in specific geographic locations, but these are exceptions.

Currently there are more technologies - some proven but many laboratory bench variety - than there are actual installations of centralized meat packaging systems. The fact that the proven European high oxygen/ high carbon dioxide system is being used in the USA only slightly should illustrate that the resistance is not technical. Rather, something far deeper is involved: fear by retailers and their employees and of meat packers of losses of jobs, customers, competitive edge and price advantage; this results from a paucity of packer culture centered about marketing.

An interesting trial has been underway in Canada since 1993 involving Cryovac's VSP and Lucerne, a Safeway grocery store operation. One of the factors distinguishing this test in the province of Alberta is that it has been developed and managed by responsible technologists and business professionals with comprehensible objectives and plans. About 200 Safeway stores were involved in early 1997. The package consists of a thermoformed semi-rigid PVC base and a flexible coextruded film top that drapes snugly to the red meat surface. About a dozen beef cuts are involved, being placed on the trays prior to vacuumization. Color of the meat surface is purpie myoglobin during distribution because of the gas barrier in the film. At retaillevel, the top gas-barrier layer of the top skin film is manually removed by the meat clerk exposing agas-permeable membrane through which air passes to help convert the pigment to the desirable oxymyoglobin.


This is not the first manifestation of peelable film for red meat packaging, but is certainly the first time in North America that two dedicated large organizations have worked together to attempt to determine the commercial viability of an older technology.

During the late 1960s and early 1970s, based on sheer logic and fact and on intensive review of the scientific literature plus surveys of meat packers and retailers, this author believed that centralized packaging of beef in the USA was an inevitability. Despite the periodic forays, little has resuIted, except a steady decline in red meat consumption. In the future, we might see a race between the eventual extinction of fresh beef as we know it today and the rise in prepared (i.e. added value) beef and its adjuncts.

2.4.2 Pork

Meanwhile, fresh pork consumption has crept upward to a total retail volume of 1.5 billion pounds. Perhaps up to 50 million pounds annually from all the major pork packers, or about 3% of the total, is centrally packaged, including master packaged and vacuum packaged, some with added antioxidants. If there is to be penetration of the case-ready concept in the USA, it is in the relatively minor market sectors such as lamb, veal and pork that it will occur, but probably with no new MAP technologies.

2.4.3 Poultry

What is remarkable about the virtual absence of case-ready red meat is the effective penetration of centralized packaging into poultry. Nearly onethird of the rapidly expanding USA pouItry market - now alm ost the same size as beef - is centrally prepackaged and branded, i.e. marketed much like Coca-Cola or Kellogg's corn flakes. Until the early 1980s, nearly all pouItry was whole bird packed in ice in wire-bound wood crates. In the retail store backroom, the poultry was removed from the chopped ice by the butcher, cut and repackaged in foamed polystyrene trays with plasticized PVC, and a store label was adhered. As a resuIt of the introduction of chili pack (crust frozen) followed by dry pack and MAP in master packs, the concept of 'shelf-ready' pouItry emerged. In effect, the retail 'butcher' removes the factory prepackaged (often prepriced) poultry from the master pack and places it on the shelf, and hence the terminology, 'shelf ready'. Today 2 billion pounds of poultry are master packed, about half with overt modified atmosphere surrounding the PVC film-wrapped foamed polystyrene trays.

From a MAP perspective, precooked and fresh-ground poultry represent a 200+ million pound market, 20% of which is produced on thermoforml


vacuum/back-gas flush machines such as Multivac or Tiromat, using barrier polystyrene foamJethylene vinyl alcohol film-Iaminated trays with heatsealed flexible dosures. Most of the remainder is packaged in conventional foamed polystyrene trays overwrapped with coextruded barrier film on Ilapak or similar horizontal flow wrapping equipment. The internal atmosphere for precooked poultry is high carbon dioxide and for fresh ground poultry high oxygen/carbon dioxide to retard microbiological growth and retain red color. Partly as a result of employing centralized MAP coupled with controlled distribution and modern marketing, poultry has overtaken red meat as the animal protein source of choice in the USA.

2.4.4 Fish

Per capita consumption of fish in the USA remains low at dose to 14 pounds, with relatively IeW prospects for major growth in the ne ar-term future. Owing to grave concerns about the potential for growth of pathogenic anaerobic microorganisms such as type E Clostridium botulinum, which can grow and produce toxin at low temperatures, regulatory officials have discouraged reduced oxygen packaging of fish. Some seafood products are vacuum skin packaged and frozen for retail slackout, a practice that gains some of the benefits of low-oxygen packaging and somewhat skirts the regulations. Unlike the situation in European countries, little can be expected for MAP in this category in the USA.

2.4.5 Fruit and vegetables

The strongest growth segment for MAP in the USA in the 1990s has been fresh-cut produce, also universally regarded as the fastest growing of all food products in the country. Begun in the mid-1980s as vacuum packaging of fresh-cut lettuce for use by fast-food chains in their penetration into the salad market, expansion has been into the retail market with rates of 75 and 93% (sales volume increase over the previous year) for the 1992 and 1994 period, respectively, to a 1996 US$l billion retail value, representing nearly 10% of fresh produce sales. The HRI market is valued at about $2 billion with growth to as much as $8 billion total by the end of the 20th century. The major product within the category, with about two-thirds of the volume, is fresh-cut lettuce, with up to 15% of the crop in so me form of fresh-cut under MAP. Other products of importance indude broccoli and cauliflower florets, diced onions, baby carrots, sliced peppers, and aseries of fresh-cut vegetables packed with meat, pasta, croutons, salad dressing, etc. to create a complete salad-based meal.

More than 200 companies participate in the fresh-cut business in the USA, with about four or five dominating on anational scale and the


remainder competing largely on regional and local levels. Retailers are noticing cannibalization of their whole-produce sales by the fresh-cut, and so the product category is establishing itself among consumers.

From a technological perspective, most fresh-cut produce is cut, cleaned and cooled using chlorinated water, and packaged on vertical form-fill-seal machines such as Hayssen for retail distribution, and on preformed bag fillers such as CVP Systems or M-Tek for HRI distribution. Relatively little is packaged with gas mixtures, but the CVP and M-Tek systems evacuate the air so that the product contents consume the oxygen and produce carbon dioxide. Respiratory anaerobiosis is minimized by employing high surface-to-volume ratio pouches in the case of bulk packages and high gaspermeability plastics for both bulk and retail size pouches. Gas permeabilities are around 3000 ml m-2 . It is axiomatic that the materials have low water vapor permeabilities and that generally, for the retail sizes, antifog agents are incorporated in the film to permit visibility.

One major category of MAP produce is pallet wrapping of strawberries in both Florida and California. In the latter, alm ost all of the crop exported from the state, either to other states or to other countries, is shrouded in high-density ethylene vinyl acetate/polyethylene film and evacuated with the vacuum replaced by a mixture of carbon dioxide and nitrogen to suppress mold growth. Probably as many as 100000 pallets annually are treated to effect the modified atmosphere condition within the pallet wrap.

For the bulk sizes, monolayer high-density ethylene vinyl acetate films are often employed although CVP Systems uses coextrusions to achieve their requisite gas permeabilities. For consumer sizes, laminations of styrene block copolymer and metallocene-based low-density polyethylene, or polypropylene plus metallocene-based polyethylene are used. The California packer Fresh Western uses FreshHold mineral-filled polypropylene over large orifices in the polyethylene film substrate to achieve their high oxygen permeation. Increasingly coextrusions incorporating metallocene polyethylenes are being applied to gain some economies. The general range of film gauges is about 0.002 to 0.0025 inches (0.005-0.00625 cm).

A shadow over fresh-cut produce is the regulatory concern over the possibility of growth of pathogenic an aerobic microorganisms in the reduced oxygen environments of the package interiors. Proven to be possible under laboratory conditions, pathogenic anaerobic growth has occurred only once in commercial practice, and this event has been so clouded with legal complexities that its reality is somewhat doubtful. To generate the toxin even under optimum conditions requires a large initial loading of spores plus a long time, measured in weeks, at relatively high temperatures, all together improbable but, of course, not impossible. No regulatory restrictions apply to fresh-cut produce in spite of several attempts, particularly in New York State, to im pose them. Responsible


operators functioning under knowledgeable technology generally do not encounter such public health problems.

Just beginning in the USA is the long-awaited cut-fruit packaged product, today mostly cut pineapple packaged with high carbon dioxide levels, or vacuum packaging or, too often, with air followed by passive equilibrium atmosphere. In test are cut melons (which are low acid), table grapes for outdoar recreational activities such as sports events and freshcut apples with chemical antioxidants such as ascorbic acid to retard browning. Under test with some sort of modified atmosphere is almost every fruit that might be seriously considered for convenience snacking, such as strawberries, peaches and pears, but, to date, the technical problems are challenging. As these are satisfactorily resolved, cut fruits under modified atmospheres will replace the store back roorn/restaurant same-day cutting and simple packaging currently being employed.

Not truly CAP/MAP but a related technology is the 'Natural Pak' process for a commercial fresh product dubbed 'Tom AH Toes'. Unripe tomatoes are stored under 3-5% oxygen and elevated carbon dioxide at 18-22°C until they are ripe. Under the altered atmosphere, the unripe tomatoes may be held far extended periods without suffering chill damage and then packaged in air for retail distribution.

2.4.6 Soft bakery goods

Soft bakery goods, such as breads, rolls, cakes and muffins, are subject to rapid quality loss, with the most prevalent deteriorative vectors being moisture loss, staling and mold growth. Staling is a partially reversible crystallization of starch leading to text ure hardening. Moisture loss is drying independent of staling. Microbiological deterioration in soft bakery goods is mostly surface and crumb mold growth. Moisture loss can be retarded by packaging in low water-vapor permeability materials such as low-density polyethylene film. Use of such materials retains water vapor within the package but can create conditions optimal for mold growth. Extending the shelf-life of soft bakery products can:

• reduce the number and magnitude of returned goods; • extend the distribution range of bakeries; • reduce the frequency of deliveries; • further extend shelf-life; • permit a bakery to produce a broader range of products.

Reduction of oxygen and replacement with inert atmosphere is ineffective unless the oxygen content of the internal atmosphere is reduced to below 1%. The most common reduced-oxygen packaging system for bakery goods is horizontal form-fill-seal using PVdC-coated oriented


polypropylene film in a single web. This system requires extensive gas flushing while forming and sealing the flexible pouch. The system is used for relatively simple breads such as German rye meal. In vacuumpackaging systems, twin-web thermoform/vacuumlgas flush-seal equipment is used. After formation of the bottom cavity, the product is inserted and the filled cavity is evacuated and back-flushed with carbon dioxide or carbon dioxide plus nitrogen. Thermoform vacuum/gas flush-seal systems require more expensive packaging materials and equipment than horizontal fill-seal systems but deli ver better seals. Multivac and Tiromat thermoforml vacuum/gas flush-seal equipment is used in Europe and Canada for CAP of specialty goods such as crumpets. An interesting product concept made possible by MAP is brown-and-serve French bread. The commercial baker partially bakes the loaves and places them into a thermoformed plastic cavity on a thermoform/vacuum/gas flush-seal machine. Still hot and expelling the carbon dioxide of baking, the packages are closed and cooled. The carbon dioxide-saturated packages have over a three month ambient temperature shelf-life. The consumer places the loaf in a conventional oven where a brown crust is developed and the crumb remoisturized. The result is comparable to freshly baked French baguette. This system is commercially used in Europe and Australia in niche markets.

In Canada, crumpets, similar to English muffins, are being modified atmosphere packaged on thermoform/vacuum/gas flush-seal equipment with carbon dioxide back-flushed, on a commercial scale. Sandwiches are being packaged under vacuum using similar systems, but the sandwiches are distributed under refrigeration to preserve the fillings. Fewer than a half-dozen small Canadian firms are involved.

Several American companies are packaging sandwiches for refrigerated distribution on a regional basis. These products may appear in cured meat or special refrigerated displays with or without expiration dates. Perhaps the most widely commercialized applications are the little-known MAP of sausage and biscuits, and of precooked hamburgers. About 90 days actual refrigerated shelf-life is achieved by MAP on Multivac thermoforml vacuum/gas flush-seal equipment - with a target expiration date of about 45 days. Since sausage is cured meat, problems of microbiological safety are minimized. The instruction to the consumer to heat the product drives moisture from the meat into the biscuit to refresh it. All such products are nationally distributed.

Probably fewer than a half dozen small American bakers are using MAP for soft bakery goods, largely specialties. Although the concept makes eminent sense and was heavily marketed in the USA, MAP for soft bakery goods is a rarity in this market. The situation in Europe is somewhat better. More than 300 packaging machines are operating in bakeries in Europe. In addition, soft bakery goods shelf-life in Europe is now being


prolonged by complementing MAP with steam or microwave pasteurization. Bulk quantities up to pallet loads are heated by steam or microwave energy to destroy mold spores present within packages. The application of modified atmospheres with or without heat pasteurization to extend shelflife of soft bakery goods is technologically and economically very sound. The reasons for the paucity of commercial applications in North America cannot be explained on a rational basis.

2.4.7 Prepared foods

The arena of prepared foods under MAP is led by the very successful highmoisture pastas that have now been joined by pasta sauces packaged under reduced oxygen by virtue of hot filling. Probably $200 million worth of such products are in wide-spread chilled distribution in the USA, but after significant growth for several years, the category appears to have reached its maturity. The three major technologies employed include oxygen replacement in which nitrogen is the sole gas injected after evacuation. This system, used by Kraft for their DiGiorno brand pastas, also incorporates an internal oxygen scavenger sachet affixed to the interior of the bottom tray to remove residual oxygen and any that is transmitted into the package during chilled distribution. The claim is for up to 90 days actual chilled shelf-life for packs containing oxygen-removal systems.

The second system, used by most of the producers, is displacement of the internal environmental oxygen with a mixture of at least 25% carbon dioxide plus nitrogen with no further active packaging. This system is also claimed to be satisfactory for up to 90 days of refrigerated shelf-life, on a microbiological basis.

All systems rely on initial product, process and packaging sanitation with chilling an essential element. They further integrate product formulation to ensure a sufficiently low water activity to minimize the prob ability of growth of pathogenic an aerobic microorganisms. This product category is one of several in which natural antimicrobials and/or natural antioxidants are incorporated into the formulation to complement the basic food preservation actions.

The third system, applied to the sauces, so me of which are low pH cream style, is hot filling in which, of course, the condensation of steam generates a vacuum that, together with the heat and subsequent refrigerated distribution, inhibits aerobic spoilage microorganisms. Obviously, there is an imperative to maintain outstanding sanitation to minimize the prob ability of growth of an aerobic pathogenic microorganisms. To date, there have been no adverse incidents as a result of pasta sauces, but there has been one tragic event from the chilled distribution of hot filled low-acid soup, which, of course, may be similar to low-acid pasta sauce from a microbiological standpoint. The direct cause was severe consumer abu se of


the packaged product, in which the product was stored under ambient temperature conditions for more than one month prior to consumption.

Most of the pasta is packaged on thermoform/vacuum/gas flush-seal machines such as Multivac or Tiromat using semi-rigid PVC as the thermoformed web and a lamination of polyester/PV dC and polyethylene as the flat sealing web. The pasta sauces are usually in polypropylene tubs with flexible polypropylene heat-seal closures. Some pasta sauces, as weil as many other soups and pumpable foods, are packaged in flexible barrier pouches fabricated from stmctures of polyester and/or nylon plus linear low-density polyethylene. These pouches may or may not be distributed in paperboard sleeves to help protect them from abuse and damage.

It is difficult to estimate how large the market volume for hot-filled and MAP prepared foods is in the USA since alm ost all the market comprises regional and local producers. Beyond pasta and pasta sauces, a conservative estimate would place the size of the market at weil over 100 million units and growing at more than 9% annual rate. Included in the category are pumpable foods such as soups and Chicken a la King, pasta and sauces mixed, Chinese foods such as egg rolls, Mexican foods, etc.

Another category of foods under reduced oxygen and chilled distribution is cook-chill bulk packaging for pumpable foods such as pasta sauces, soups and chilli. The product is packaged hot into nylon/polyethylene pouches capable of holding about one gallon. The hot packaged product is immediately chilIed in cold water and distributed at temperatures of about O°C to satellite HRI outlets for reheating and serving. This 10+ million unit market is technologically very similar to the ill-fated sous-vide, which is not used to any extent on the USA market because of the furor raised ab out its microbiological safety in the mid-1980s. Regulatory officials have not raised the same objections over cook-chill and so there are more than 300 installations using this technology around the country in 1996 with more entering. Of course, almost all cook-chill is made and applied internally by mass feeding operations and central commissaries and so is under a single control throughout its short distribution cycle. Retail consumers almost never see or are exposed to cook-chill pouches of food, and so specific mies governing this technology alone are not in effect. Obviously, there are very good microbiological safety reasons for adhering to codes to minimize the risk of microbiological problems.

Related to cook-chill technology are an entire family of products that are inserted into high oxygen-barrier pouches, sealed, cooked to pasteurization temperatures, chilIed and subsequently distributed under chi lied conditions to the many delicatessen operations. These products are vacuum packed and must be chilled to ensure microbiological safety and retail quality. The product category includes roast beef, turkey and similar solid pack products in which cooking and pasteurization are effected by the same cook cycle.


2.4.8 Delicatessen salads

Prepared salads were nearly a $1 billion market in 1994 with continued growth predicted. Most mayonnaise-based meat, seafood and vegetable salads are produced by local and regional companies and packaged in air in bulk for delicatessens in HRI outlets. Salads for supermarket delicatessens are often prepared in-store by central commissaries or by one of more than 100 local and regional suppliers; there is one national supplier which packages in bulk. About 5% of the total USA market is prepacked in unit portion size for retail sale. To date, there has been almost no actual implementation of MAP for prepared protein or mayonnaise-based salads. Until effective distribution temperature control is implemented commercially, most producers will probably not attempt reduced oxygen packaging.

2.5 Contemporary issues in MAP technologies

MAP technology is aglobai venture. Issues of microbiological safety continue to concern regulatory officials and to deter some potential users. Although these concerns have validity and must not be overlooked, they should no more be a roadblock than was the absence of quantitative knowledge of the thermal death of spores in heat sterilization processes a half century ago. There are few or no technical means known today to overcome the microbiological issues involved in these new technologies: no chemical additives or special gases or gas blends, at least that can be employed safely; no special package films that have been proven to emit microbicidal power, etc. The scene in the USA shows little that will resolve the microbiological safety issue any better than the well-known precautions plus HACCP, founded in distribution temperature control.

Plastic resin suppliers and the package material converters have been developing new structures incorporating their singular offerings such as metallocene catalyst-based polyethylenes. These intriguing new polymers and polymer derivatives pro mise and deliver higher degrees of specificity on gas and water vapor permeability in lower gauges than ever before available. The ability to deliver lower densities ofpolyolefins (polyalkenes) means that high er gas permeabilities can be achieved with the same or lower masses of package materials. All of these will soon address the questions of variabilities of gas and water vapor permeabilities that have concerned fresh-cut produce packagers in the 1990s. Because of the nowvisible growth of MAP of fresh-cut vegetables, we must expect greater attention to the package materials and structures and the gauge uniformities offered to the market by plastics suppliers and converters. However, it is important to note that these firms are not addressing basic issues such as


prolonging food-quality retention, but rat her they are deciding how to deliver more efficient structures to achieve today's objective of minimizing the adverse effect of respiratory anaerobiosis. Universities are developing quantitative me ans for predicting optimal conditions to avoid respiratory anaerobiosis, as if this were the true objective.

Answers to far more fundamental questions on extending shelf-life must be addressed by something more than polymers or predictive models. Just as modified atmosphere offered the opportunity of doubling or tripling quality retention times and reducing spoilages by factors of two to four, so also must the next generation offer yet another order of magnitude improvement to bring us commercially far beyond the marginal performance range in which we are now operating.

One technology exists and can deliver: temperature contro!. Currently, 8°C, 7°C, etc. are the benchmarks, but the scientific evidence cIearly demonstrates that -1 to -2°C can double or more our current best. If a select few US processor/packagers can opera te their plants at -1°C or even lower, there is every reason to extrapolate into total system temperature contro!. This paradigm, this shift from incremental improvements to major leaps using the answers we already have, should be the challenge of the next few years. Just because the challenge lies beyond the laboratory bench and in the realms of distribution professionals should not be a deterrent but rather a spur to thrust us into the means to apply our knowledge. Temperature control at -1 to -2°C is the route to major enhancements, where shelf-life doubles again, where pathogenic anaerobic microorganisms do not grow, where even the pathogenic aerobes have difficulty, where me at color alterations are significantly slowed, and even where some fruit chiIl damage is obviated.

MAP and food preservation is still in its infancy, still not in the mainstream university curricula or textbooks. It is the responsibility of food scientists to use our resources to communicate and to apply our good knowledge to elevate this technology to its proper level in the realm of food preservation. Our message is: investigate, understand, develop and apply that which truly achieves the goal of enhanced food preservation through controlled and modified atmospheres.

2.6 Conclusion

Depending on the definitions accepted, the magnitude of MAP foods can range in market size from one billion to 10 billion package units and from ten billion to nearly 100 billion pounds of food contained annually. The larger figure would prevail if vacuum fresh-packaged red meat primal cuts for distribution are incIuded. The lower figures would encompass only retail packages that have been flushed with mixtures of carbon dioxide,


nitrogen and oxygen as required. If only the last, astriet definition, is included, and figures are restrieted to retail groeery stores, then the growth rate from a virtually zero base in the 1980s is 5-10% annually. The most signifieant (striet definition) MAP items in North Ameriea are fresh-eut vegetables, 30% moisture pasta, sausage and biseuits, and preeooked poultry. In Europe, the most signifieant produets are fresh 'red meats, fresh salad vegetables, 30% moisture pasta, eured and eooked meats, fish and seafood, dried foods and soft bakery goods. By any measure, therefore, both volume and growth are present, being driven by eonsumer demand for freshness and by awareness of the teehnology.

Further reading

Anon E. (1988) Modilied Atmosphere Packaging: The Quiet Revolution Begins. Packaging Strategies, West Chester, PA.

Anon. (1990) Modilied Atmosphere Packaging. Food Development Division, Agriculture Canada, Ottawa, Canada.

Brody, A.L. (1989) ControlledlModilied AtmosphereiVacuum Packaging 01 Foods. Food & Nutrition Press, Trumbull, CA.

Brody, A.L. (1993) in Proceedings 01 the MAPack Leading Edge Conlerence on Modilied Atmosphere Packaging 01 Foods, /993, Greenville, Sc. Institute of Packaging Professionals, Herndon, VA.

Brody, A.L. (1995) in Proceedings 01 the MAPack Leading Edge Conlerence on Modilied Atmosphere Packaging 01 Foods, /995, Anaheim, CA. Institute of Packaging Professionals, Herndon, VA.

Brody, A.L. (1997) AVision of Packaging for the 21st Century. Paper presented at Relrigerated Foods Association Annual Meeting, Orlando, FL.

Brody, A.L. (1997) The impact of minimally processed - and often minimally packagedfoods on packaging, in Proceedings 01 Packaging Strategies Conlerence, Atlanta, GA.

Brody, A.L. and Shepherd, L. (1987) ControlledlModilied Atmosphere Packaging: An Emergent Food Marketing Revolution. Schotland Business Research, Princeton, NJ.

Bruce, J. (1991) Is European fresh produce packaging transferable to North America - the meat market analyzed, in Proceedings 01 PACK AL/MENTA/RE '9/, Food and Beverage Packaging Expo and Conlerence, New Orleans, LA. Schotland Business Research, Princeton, NJ.

Cakebread, D. (1993) European market developments and opportunities for MAP, in Proceedings 01 MAPack 93, Leading Edge Conlerence on Modilied Atmosphere Packaging 01 Foods, Herndon, VA. Institute of Packaging Professionals, Herndon, VA.

Day, B. (1990) Perspective of modified atmosphere packaging of fresh produce in Western Europe, in Proceedings 01 CAP '90, Filth International Conlerence on ControlledlModilied AtmospherelVacuum Packaging, San Jose, CA. Schotland Business Research, Princeton, NJ.

Day, B. (1995) in Proceedings 01 the International Conlerence on Modilied Atmosphere Packaging and Related Technologies. Campden & Chorleywood Food Research Association, Chipping Campden, Glos, UK.

Parry, R.T. (ed.) (1993) Principles and Applications 01 MAP 01 Food. Blackie, Glasgow.

3 MAP machinery M.J. HASTINGS

3.1 Historical development

Machinery used for MAP did not require invention but is the result of the development of established packaging systems. In the late 1950s and early 1960s, experimental work in the commercial use of MAP was being undertaken. The horizontal form-fill-seal or pillow pack type of packaging machine was becoming established, setting a packaging style that is now commonplace. In Italy, a manufacturer of processed cheese attempted to introduce an inert gas into the packs produced on a pillow pack machine by flushing out the air prior to the pack being sealed. Given the unsophisticated type of sealing system then employed and the simple film structures available, it is unlikely that a successful result was achieved. However, this was the beginning, and as the demand for improved seals and efficiency of flushing increased, so manufacturers of machinery turned their attention to developing the pillow pack machine into a specialist MAP machine.

During this period, vacuum packaging of food products was also becoming universally acceptable. The virtual total exclusion of air from a pack allowed manufacturers of food products to offer a considerable shelflife for many products without the degradation experienced with other forms of preservation, particularly those involving the use of high temperatures. The process became commonly used for the packaging of cheese and meat. Unfortunately, some products lost their fresh appeal when vacuum packed. Natural cheese, for example, appears wet and sticky when the pack is first opened, fresh meat loses its red colour because of the exclusion of oxygen, and slices of cooked meats fuse together because of the press ure exerted by the packaging material created by the vacuum process. It is not surprising, therefore, that the parallel major development in MAP machinery should come from vacuum packaging in the style of thermoforming or chamber machines.

3.2 Gases

A successful MAP system is determined by the interrelationship between gas, packaging material and packaging machine. The choice of gas or gas


mixture, sometimes referred to as the gas cocktail, used to replace the air atmosphere depends entirely on the nature or recipe of the producL Many food products undergo spoilage through the process of oxidation. This can be retarded by the removal of the oxygen and replacing it with an inert gas. The most commonly used gases are carbon dioxide and nitrogen.

Carbon dioxide is widely employed in sections of the bakery industry. The removal of oxygen and replacement of it with carbon dioxide successfully inhibits mould growth but does not prevent the natural process of staling. One approach has been to pack partially baked bread products in an atmosphere of carbon dioxide and nitrogen mixture to extend the shelf-life, leaving the consumer to complete the baking process, wh ich overcomes any staling. Because carbon dioxide is water soluble, its absorption by the moisture held within a sealed pack creates a partial differential press ure between the gas held inside the pack and the outside surrounding atmosphere. The resultant partial vacuum causes a collapsing of the pack, frequently referred to as 'snugging down' or 'soft vacuum'.

The small amount of carbonic acid (H2C03) formed in the equilibrium reaction

CO2 + H20<------>H2C03

helps in inhibiting mould growth and has no discernible affect on the taste of the producL

A visual indication of a failed pack seal may be indicated by the noncollapse of the packaging material onto the product being packed. The results may take some time to show, depending on factors such as the volume of gas relative to the volume of the product and the amount of water vapour present, both of wh ich have a bearing on the absorption rate of the carbon dioxide gas.

The production of a partial vacuum by packing in a carbon dioxide atmosphere on horizontal form-fill-seal machines has been used to advantage in the packaging of salami and similar meat products. The previous practice of vacuum packing caused juices and oils in the meat to be squeezed out of the containing skin through the dipped ends and thus to contaminate the inner surface of the packaging material. A change to MAP using carbon dioxide as the purging gas resulted in the production of a partial or soft vacuum over time without undue pressure on the product and, therefore, not causing contamination of the pack.

Nitrogen gas is also widely used because of its inert quality. It is frequently used together with carbon dioxide in proportions up to 20% by volume to control the amount of pack collapse. When employed as the only purging gas, as is sometimes used in packs of coffee produced on vertical form-fill-seal machines, nitrogen is better retained in the sealed pack because the high concentration within the pack is dose to equilibrium with the amount by volume in air atmosphere (80%) surrounding the pack.

MAP MACHINERY 41

Oxygen is used in typical mixtures of 80% oxygen with 20% carbon dioxide for packing red meat under chill conditions of 4°C and lower. Shelf-life can be extended to 7 days with the red colour, seen as a measure of freshness, maintained.

3.3 Packaging material

The choice of packaging material is an extremely important part of the MAP operation. Considerable research has been and is being undertaken by manufacturers to find improved laminations and structures. Costeffective materials must have the essential physical properties of low watervapour transmission, high gas barrier, mechanical strength to withstand machine handling and subsequent storage and distribution in the finished pack format, as weil as having the capability of giving high integrity seals to ensure retention of gas within the pack until it is opened by the consumer.

Laminations of polyester and polyethylene, nylon and polyethylene, polyvinyl dichloride, orientated polypropylene and many more are in common use. The requirements of the packer often call for solutions that test the ingenuity of the machinery designers. With the fundamental need to retain gas in a pack, contradictory demands are made, for example, in creating deeply formed trays from relatively thin materials in the case of the thermoforming style of pack, and for peelable seals, antifog coatings and recloseable seals.

3.4 Packaging machines

Machinery available for MAP falls into two main categories: chamber and pillow wrap. Chamber machines can be subdivided into those employing the thermoforming technique and those using pre-made containers. Pillowwrapping machines are either of the horizontal form-fill-seal or vertical form-fill-seal type.

3.5. Chamber machines

3.5.1 Thermoforming system

The thermoforming method (Figure 3.1) involves the use of a flexible or semi-rigid base material that is fed from a ree! into grippers or clamps he!d on chains running either side of the web of material and the length of the machine. The material is drawn from the ree! into a heating station where it is softened and thence to a forming station where, by use of vacuum, air


Longitudinal Cutting Unit

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Base Web Material

Figure 3.1 Thermoforming system.

pressure or a combination of both, the plasticized material is formed in a mould to the required shape of container. In some cases, depending upon the complexity of the forming and particularly where a deep container has to be drawn, some mechanical assistance is provided by an integral plug. This helps to reduce thinning of the material in the base corners, which would otherwise cause a weakness in that area of the container.

This type of machine normally operates by intermittent motion and, subject to the size of the individual container being produced and the size range capabilities of the machine, a number of containers can be produced across the web per machine cycle. The web of formed containers is indexed forward to a product loading area. Loading may be accomplished manually or by automatic means depending upon the nature of the product. The filled containers are indexed forward to the chamber station and simultaneously a flexible top web of material is unwound and placed over the unsealed containers. The chamber is closed, air atmosphere removed by the vacuum process and the modified atmosphere introduced through specific ports. The top web is then sealed to the container by heat and pressure. The completed web of sealed packs passes to a cutting station where rotary knives and a cross-guillotine or a punch and die arrangement separates the packs from the web, the remains of which are removed by either winding onto a spool or by vacuum li ne into a waste collection tank. Examples of products packed in this manner are shown in Figure 3.2.

Thermoformillg machines, as illustrated in Figure 3.3, are being continuously developed to give more efficient vacuum gassing operations and increased outputs. One manufacturer will claim improvements over another and systems are being refined to enable more complex packaging materials to be handled. Recent developments include the provision of a packaging materialloading unit, allowing large diameter reels of material, too heavy to be lifted manually, to be positioned on the machine ready for use. Reel diameters up to 1000 mm have increased the length of the operating and, therefore, the production time between reel changes.

MAP MACHINERY 43

Figure 3.2 Tray packs produced by the thermoforming system .

Figure 3.3 Multivac R530 thermoforming machine.

To reduce the machine cyc\ing time, which is governed by the constraint imposed by the time taken to evacuate the chamber and introduce the gas, one manufacturer has incorporated a punching unit that produces large evacuation and gas introduction holes in the edge of the web of film. The


time taken to remove the air and introduce the gas into the pack has been shortened and overall output increased.

The introduction of micro-electronic systems permits accurate control of all machine functions, including temperataure settings and movement of the packaging material through the machine, so that the optimum results can be achieved. Such controls have enabled some very complex material structures to be handled.

The thermoforming system offers variations to the standard pack style of rigid or semi-rigid base tray with sealed flexible material lid. One is to use a similar material for the top web as used for the base and to form the top web as a mirror of the base prior to the sealing station. The resultant pack, which is used for sliced meat products, can be made with a header section incorporating an easy-open device and a hole or Euroslot to permit the pack to be hung vertically in a chilled display cabinet.

The requirements for microwavable packs has led to the development of thermoforming machines capable of forming containers from expanded polystyrene film in lamination with high-barrier materials. By combining with a flexible formed top web material, high products that extend above the height of the side wall of the base tray can be accommodated. In some cases, the packs can be passed through a shrink tunnel to produce a tight pack.

The top forming of semi-rigid materials, as used in the container fraicheur in France for packing delicate bakery products, produces a rigid MAP pack with good display and stacking features.

Developments have taken place leading to the combination of vacuum skin packaging with MAP. Ideally suited for packing meat and fish, the product is placed in a formed tray and a gas-permeable film is shrunk over so that it is held firmly in the tray, improving presentation and containing any seepage of juices. Gas is allowed into the headspace before a barrier film is sealed to the top of the tray. Darfresh and Flavoloc are two such systems.

A further innovation, developed in France, is the 'Gemella System' combining both carton board and plastic materials. A printed board blank is placed into a carrier infeed prior to a suitable plastic material being heated and formed to line the cavity of the carton tray. The resultant composite structure is loaded with a product, evacuated and gas flushed before a lid is sealed to the top.

There are several advantages of the thermoform style of pack for MAP. Evacuation of the air from the pack before flushing with the required gas can achieve a low residual oxygen content in the finished pack when packing products with a cellular structure such as bread rolls. The pack is attractive and convenient to use. The system manufactures its own tray where necessary or desirable for containing a product. This will give a cost advantage against having to use apre-made tray. There are also

MAP MACHINERY 45

disadvantages. A pack size change may require the changing of expensive dies and tooJing, which can be time consuming and require the presence of skilled personnel. Machinery manufacturers are addressing these issues and recent developments have, in some circumstances, resulted in considerable improvements in the flexibility of the equipment. Machines operating by intermittent motion tend to be slow in terms of their cycie times, although the output of finished packs will depend on the number that can be accommodated into the area of the forming head. Automatic feeding of products is also difficult but advances are being made with the development of robotic feeding systems.

3.5.2 Pre-formed container machines

Automatie machines. The most common type of pre-formed container is a tray, although this could equally weil be a pre-formed pouch. The thermoforming system, compared with that using pre-formed trays, is likely to be the more cost-effective MAP solution for the food packer where production volumes are high and pack size changing is infrequent.

Employing the machine to manufacture its own tray from reel stock is more economic than buying in trays from an outside source. However, buying in trays relieves the packer of so me of the engineering and housekeeping skills necessary in operating an efficient thermoforming system. Trays can be purchased to individual designs and requirements. They can be produced by the process of thermoforming or injection moulding, where greater rigidity is desirable. Trays can also be produced with a turned down top flange (Figure 3.4), which also improves rigidity, particularly at the high temperatures experienced when the tray containing a product is subjected to heating in a conventional or microwave oven.

Machines used for the MAP of pre-formed trays are often referred to as tray sealers and have many similarities with thermoforming machines.

(a) (b)

Figure 3.4 Styles of tray: (a) standard flat flange; and (b) turned down flange.


Instead of employing reel-fed material and heating and forming it into the required shape, the automatic tray seal er accepts a pre-formed tray, either by manual means or automatically from a magazine, into one of aseries of carriers held on chains that convey the trays throughout the length of the machine. The product is loaded into a tray, which then passes into a chamber together with the top lidding material. The chamber doses, air is withdrawn, the flushing gas allowed in, the lidding material is sealed to the top flange of the tray and the chamber is vented and opened to allow the machine to index forward another cyde (Figure 3.5). In subsequent stations, the lidding material is cut from the web and the sealed packs are removed from the carriers. It is possible to feed pre-made lids from a magazine but this can prove difficult and unreliable and the preferred option is to use lidding material from the reel. Outputs are similar to those produced by thermoforming machines and are controlled by the vacuum

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MAP MACHINERY 47

and gas flushing cycle time, the number of trays held in the carriers and the indexing time.

Semi-automatic machines. Where the need is for low production rates and high flexibility, sem i-automatie machines are available. These units are relatively smalI, some being designed for table top use. They comprise a chamber and a means of feeding a top lidding material. Manual loading and unloading of the trays is necessary. The top section of the chamber may be a hinged hood that is closed to seal the chamber, wh ich is then evacuated, flushed with the desired gas and the lidding material sealed to the flanges of the tray with final cutting to separate it from the web.

Small machines with similar design parameters are also available for handling pre-made flexible pouches instead of trays. Here, filled pouches are manually placed on to a platen in achamber. Closing the lid begins the process of evacuation and introduction of gas before the top of the pouch is sealed and, if necessary, any waste material trimmed off. Some models incorporate two platens and chambers with a single hood, which is hinged to allow it to seal either chamber. While the operation takes place on one side, the other may be prepared with the loading of product, thus saving some time between machine cycles.

In Europe, there is considerable interest from the multiple retailers in the 'bag-in-box' concept for MAP. Unwrapped products or those already prepared in retail packs are loaded into barrier bags contained in a corrugated outer case used as the shipping unit. In the machine (Figure 3.6), snorkels enter the bag through the stretched opening. As with other systems, air is withdrawn followed by back-flushing before sealing. The completed bulk pack can be shipped to the point of sale where, on opening, the individual units can be offered immediately for sale to the consumer.

Semi-automatic machines are relatively inexpensive when compared with automatie versions, particularly as they require only a single set of sealing dies. They are ideally suited for use by the sm all food retailer with a need for in-house packing facilities or by the packer wishing to conduct a market test or carry out experimental work before making the commitment to the purehase of a fully automatie system with its associated high capital investment.

Gas flushing without evacuation. The types of system described so far all involve creating a vacuum in the pack before flushing with gas. When the level of residual oxygen is to be as low as possible, forming a vacuum followed by gas flushing can give very good results. Depending on the nature of the product being packed, residual oxygen levels in packs discharged from the machine are minimal and can be expected to be below 1 % by volume. Not all products are suitable for withstanding the vacuum


Figure 3.6 CVP Systems A200 bag-in-box machine.

process. Those of a delicate and friable nature are liable to damage and are better suited to gas flushing only .

As has been seen, chamber machines handling pre-made containers operate by intermittent motion, which limits the output. Continuous motion machines are available for gas flushing and top-sealing pre-made trays but without initial vacuumizing. Trays are loaded into an infeed section of the machine, filled with product and pass through a tunnel that is purged of air by the gas being used. Subsequently, lid sealing takes place before the finished packs are discharged from the machine. It is more difficult to achieve the very low residual oxygen levels that the vacuum/gas flushing systems can give but, once again, so much depends on the nature of the product. Unless, for reasons of marketing, a tray with heat-sealed lid is required for presenting a particular product , the packer should be advised to consider the horizontal form-fill-seal type of machine.

3.6 Flexible form-till-seal machine systems

3.6.1 Horizontal form-fill-seal machine systems

The continuous motion horizontal form-fill-seal type of machine is used throughout many industries to pack a wide variety of products in the

MAP MACHINERY 49

familiar pillow-pack style. Based on established principles of operation, machines have been developed for the specific requirements of MAP while retaining the considerable flexibility associated with non-MAP machines. The method of operation (Figure 3.7) is to employ a single reel of flexible packaging material that is passed through a forming tool, usually referred to as the folding box, which forms the material into a tube, the two edges of which are sealed together by heated rollers under pressure. The product is passed into this tube from an infeed designed to suit the particular handling characteristics of the product in question. If the product is not susceptible to damage by being pushed or is contained in a tray, the infeed can consist of aseries of paddles attached to a chain and set at a regular pitch position. If the product is of a delicate nature, such as a portion of cheese, it can be carried along the infeed section on conveyor slats. A lance protrudes into the tube of wrapping material and through this the flushing gas is passed, venting the tube of air while the machine is in motion.

Any successful MAP operation must be assured of pack integrity and the avoidance of leakers. On standard horizontal-form-fill machines, the cross seals are made by rotating jaws that give very limited contact time in which to achieve a good quality of pack seal. Machines with this type of crosssealing system are in use in combination with MAP where the packaging material is capable of being sealed at low temperature and high pressure. Such materials rely on a wax constituent to act as asealant, rather than the more usual polyethylene, as weil as as a barrier to the outside air atmosphere. The low temperature required to melt the wax means that the transfer time for the he at to pass from the machine sealing jaws into the material is short and hence a shorter contact time is required. One advantage of being able to use a rotary jaw action is the higher output that can be achieved over systems using orbital-motion cross-sealing units.

The orbital-motion cross-sealing unit is designed to extend the contact time for sealing during each machine cycIe by moving the jaws, with the product encIosed in the tube of wrapping material, so that they describe a reciprocating or box motion. A knife, integral with the sealing jaws, cuts through adjacent packs to produce the separation.

The need for very high standards of hygiene is essential in any food packaging environment and particularly so with MAP. The machinery must be easily accessible for cIeaning and be capable of withstanding washing down during the sanitizing process. Areas of direct contact between product and machine, such as the infeed, should be manufactured in stainless steel, or alternatively, in food-grade plastic material. Painted surfaces should be avoided, especially if located above the product wrapping line. Manufacturers of horizontal form-fill-seal machines have recognized this need and many offer machines to either a wash-down or a hose-down specification. The distinction between the two is generally not recognized. A wash-down machine specification is interpreted as one

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where the machine is constructed with a stainless steel infeed that can be cleaned with hot water and detergents. Other parts of the machine, such as guards, are covered in stainless steel panels. The long seam-sealing rollers and cross-sealing jaws will not withstand cleaning with copious amounts of water and should be wiped clean with damp cloths. A machine to a hosedown specification should be able to withstand cleaning with a high pressure hose and, therefore, all electrical items including motors need to be shrouded to a level of protection of at least IP65INEMA 4X standard.

With parallel improvements in packaging materials and machinery, the capability of achieving high wrapping speeds is possible. It has been necessary to give particular attention to the material-sealing system. Accurate temperature controls are essential to monitor the preliminary heated cleft, where applicable, and the sets of high-pressure heated sealing rollers in the long seam unit. To avoid burning the film when the machine is stopped, the heated sections should open automatically and re-engage when the machine is started. Potential damage to he at-sensitive products passing over the heated rollers can be eliminated by using refrigerated cooling plates.

The work done by the cross-sealing jaws requires accurate control to ensure that the strength of each seal reaches the required standard, especially when sealing through four thicknesses of material in the centre of the pack where the longitudinal seal is laid over flat and when side tucking is used. A long jaw profile, necessary when handling a wide product, is difficult to heat evenly along its length. Temperature distribution has been improved by the incorporation of heat pipes in the jaws, replacing cartridge heater units.

A useful innovation has been the development of a detection system that can determine the position of the product in the tube of packaging material. Should the product be out of its correct position, the closing motion of the sealing jaws is interrupted; this prevents the jaws from striking the product, with the inevitable contamination that would follow, necessitating cleaning of the jaws and loss of production time. The packaging machine continues to operate and the misplaced product or products can be automatically rejected from the line at the discharge end of the machine.

When the system is required to operate at high outputs, which may be gene rally indicated as being greater than 100 cycles per minute, there is some advantage to be gained by withdrawing air from the wrapping tube simuitaneously with the flushing operation. The introduction of another rigid lance into the wrapper tube and continuously evacuating gains little and may require the use of dimensionally wider wrapping material to compensate for the additional girth because of the presence of the extra lance. This will increase packaging costs.


Machinery manufacturers of the more sophisticated horizontal form-fillseal machines favour evacuation by using a narrow port located prior to the first pair of bottom sealing rollers before the two edges of the web of wrapping material are brought together. By careful control of the pressure of evacuation and pulsing the action with the movement of the crosssealing jaws, improved efficiency of flushing combined with a tight pack appearance can be achieved. The latter point is important as ballooned packs are difficult to control during automatie weighing, labelling or secondary packaging.

A technique that has been tried in order to accelerate the collapsing of the pack is initially to heat the carbon dioxide gas to expand it before it passes into the flushing lance. This has been done by passing the gas through a copper coil contained in a hot-water bath. On cooling to the ambient temperature within the closed pack, the gas decreases in volume and the pack shows signs of collapse. Such a me ans is overly complicated and relies on additional controls that should best be avoided.

The horizontal form-fill-seal machine (Figure 3.8) is easily changed for different product sizes and can be rapidly tuned during production without the requirement of highly skilled engineers. Machines may be equipped with an adjustable folding box, the operation of which can be automatically controlled. The reliability of multi-axes microprocessor drive units over tradition al mechanical drive systems is proven and the ability to programme in size details of various products and associated information and recall this from a memory unit makes the horizontal form-fill-seal machine an extremely flexible packaging machine.

•

Figure 3.8 GEI Autowrappers Flowtronic 410 microprocessor-controlled horizontal form-fillseal machine.

MAP MACHINERY 53

3.6.2 Inverted horizontal form-fill-seal systems

The inverted horizontal form-fill-seal (pillow wrapping) machine (HFFS) (Figure 3.9) is used in the MAP of cheese and sliced meats. The packaging material is fed from below the machine infeed and the two edges brought together on the top of the pack to form either a fin or an overlap seal. Products can be loaded directly onto the film, which is an advantage when handling anything of a sticky nature that might prove difficult to transfer from an infeed conveyor onto the web of film. In such cases and when an unprinted film is used, a marker device prints a spot on the material at the web unwind position to indicate to the person or persons feeding the machine with products where to place those products on the film in the infeed section.

The film is formed into a tube as on conventional HFFS machines. Gas is introduced through a lance and the guides that assist in forming the tube. Heated roIlers mounted above the tube seal the edges of the film before it passes to the cross-sealing unit. Aseries of sealing jaws, each with an integral knife, is held in a circular cam track. The trailing edge of each product is detected by a photoelectric cell that signals the release of the leading set of sealing jaws queued in a holding position. By this means, any variation in product length is accommodated by a corresponding variation in the pack length, provided the film is not printed and registered.

Sealing is effected over a long period of approximately 120 degrees of travel around the cam track, equivalent to about three machine cycIes. By comparison, the best sealing time that can be obtained on an orbitalrnotion machine is about one-third of the machine cycIe. However, each machine system has its optimum sealing time dependent on temperature and pressure and increasing that contact time does not necessarily improve seal quality. The objective of the designers of this type of machine, as dicta ted by market demand, is to increase machine wrapping speed at the same time as maintaining optimum sealing jaw dweIl time.

Another factor affecting a machine's output is the efficiency of the gasflushing unit. The faster a machine runs the more gas that must be pumped into the wrapper tube to displace the air drawn in by the passage of the product.

A further advantage of the inverted machine is noted when packing products of a crumbly or dusty nature. Any debris falling from the product will remain within the pack and not contaminate the longitudinal seal area, which might otherwise become the cause of a poorly sealed and, therefore, a leaking pack.

Inverted machines with an orbital-motion cross-sealing head are available from a few machine manufacturers. The degree of flexibility is similar to conventional bottom longitudinal se am machines. The simpler cross-sealing system compared with that employed on the cam track type of

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· MAP MACHINERY 55

machine requires less maintenance and time for cleaning as wen as being much faster in making a three-dimensional product size change.

One modification has enabled the HFFS machine to produce a fantail bag style of pack. By extending the pack material cut length relative to the product length and by using a specially designed set of sealing jaws on the orbital-motion type of machine, an overlong pack is produced. This can be transferred to a suitable neck-tying unit that completes the operation to give the fantail style. Typical applications are in the packing of smoked sausages in Scandinavia and bakery products in Germany.

3.6.3 Vertical form-fill-seal systems

The vertical form-fill-seal machine is one of the most widely produced machines of the packaging industry, second only to the number of HFFS machines of all types purchased each year. The principles of operation are common to an and the difference between models is in the method of transporting the packaging material through the machine.

In MAP, machines are used for packing granular and free-flowing products such as coffee, nuts and snacks. A typical sequence of operation (Figure 3.10) involves the single web of packaging material being drawn from areei, registered if printed, and passed over a forming shoulder and around a metal tube mounted in the vertical plane. The tube acts like the folding box on HFFS machines and the two edges of the film are guided to form either a fin, which passes through heated rollers, or, more commonly, an overlap seam that is sealed by a heated bar applying pressure to the se am area. In the majority of cases, machines operate by intermittent

Figure 3.10 Vertical form-fill-seal machine.


motion and the film is driven by draw down belts that engage on either side of the forming tube. In some cases, the belts move continuously and engage onto the film to drive it down on a signal that a product has been fed into the tube. The belts can also be perforated and a vacuum drawn to initiate the drive of the film. Alternatively, the belts can be permanently in contact with the film and move intermittently when signalIed.

Product fed from a suitable weighing, auger or volumetric feeding system is dispensed down the tube and into the bag formed at the base of the tube. Cross-sealing jaws incorporating a knife move in and out to make the cross-seal and separate successive packs. When agas is to be introduced, two tubes are used, one inside the other. The packaging material is drawn around the outside of the outer tube, the product falls inside the inner tube and the gas is introduced into the space between the walls of the two tubes so as to flush out the air in the pack being formed.

3.7 Fail-safe assurance

It is important that, whatever the type of MAP machine being employed, downtime be reduced by the inclusion of diagnostic aids and prompts that can be readily understood and acted upon by the machine operator. Warning devices that detect and shut down the system and alert the operator are essential for determining a failed heater unit, affecting the quality of sealing, and a fall in gas pressure, which will determine the efficiency of the flushing operation.

The integration into the machine of gas mixing, feeding and analysis equipment is recommended. The ability to mix carbon dioxide, nitrogen and oxygen gases from single sources of supply and to control the pressure and volume of each gas adds further flexibility to the operation. It is essential to analyse the gas or gases within a pack to ensure that the desired product shelf-life is obtained. On HFFS machines where the elimination of oxygen is sought, continuous sampling of the gas in the wrapper tube is possible by using a narrow bore tube of 1-1.5 mm internal diameter attached to, or mounted in parallel, to the main gas-flushing lance. A small volume flow of gas is continuously extracted and fed to an oxygen analyser (Figure 3.11) or aseries of analysers checking different gases. If apre-set gas level is not recorded, automatic adjustment of flow rates can be made or, if a fail-safe level exceeded, warning indicators illuminate or sound and the machine is stopped.

Total reliance on machine performance units should not be made and it is necessary to conduct both on-line and off-line checks for gas content and seal integrity. Gas analysis of packs coming off the end of the packaging machine can be made by the incorporation of a random sampling unit into the gas mixer/analyser. Alternatively, a sm all portable unit (Figure 3.12)

MAP MACHINERY 57

[ KG 1550 Gas Analyser )

Figure 3.11 Hitech Instruments KG 1550 gas analyser facia mounted.

can be kept dose to the packaging machine to give fast results, with the advantage of a printout record for quality-control purposes.

A number of methods is available for the checking of pack seal integrity. One such, which is a IittIe more scientific than the common practice of squeezing a pack by hand under water , is to employ a leak tester. The pack to be tested is placed into a water bath made of a suitable plastic transparent material, a lid is sealed on and the air in the free headspace above the water line slowly pumped out. The pack is held below the water line and, as the outside pressure falls, the pack inflates. A seal failure will be indicated by aseries of bubbles of gas escaping from the pack at the leak point. Apressure gauge attached to the tester indicates the press ure at which the leak has occurred.


Figure 3.12 Hitech Instruments MAPtest 4000 portable gas analyser with printer.

Regular on-line checking of the gas content of packs and seal integrity indicates at an early stage areas of the packaging machine requiring attention. Sealing temperatures may require adjustment or sealing surfaces may be contaminated by product debris and require cIeaning.

3.8 Automatie product feeding systems

Where possible, it is desirable automatically to feed products into packaging machines in an effort to minimize labour costs and obtain optimum productivity. The feeding of chamber and flexible pillow wrapping machines require different approaches.

Thermoforming machines that produce their own tray as the product

MAP MACHINERY 59

container are difficult to feed automatically. In many cases, a number of trays are formed in a matrix per machine cyde and this requires a multilane feeding system. Liquids and semi-liquids can be pumped and dispensed. Sliced meats can be fed automatically by linking with slicing machines and shingling and batching conveyors. Solid products with a regular shape can be selected and lifted by robotic means. Capital costs for this type of equipment remain high but will inevitably become more attractive as labour costs increase. Where pre-formed trays are used, these can be automatically dispensed from magazines provided that the trays will nest one inside another.

Pillow wrapping machines are much easier to feed because they are single lane machines. The task is even easier if the product is contained in a tray that has the advantages of fixed dimensions and can be pushed and queued. Feeding units are available that are capable of taking a randomly pitched stream of products and feeding them into the wrapping machine infeed either from the side or in-line. In the latter case, electronically controlled timing conveyors handle delicate products, for example biscuits and cakes, monitor their position and pass them into the infeed of the wrapping machine without them touching one another and without violent changes in direction or velocity that might cause damage to the product. Microprocessor-controlled pillow-wrapping machines also offer the facility of no product/no wrap, a distinct advantage in saving on wastage of wrapping material.

The complete MAP system must take account of the next operation in the packaging line. Many pre-packed fresh food products are required to be weighed, priced and labelIed accordingly and consideration must be given to the interface linkage with automatic machines for weighing, pricing and labelling.

3.9 Conclusion

The benefits of MAP are an improved product quality at the point of sale, removal of chemical preservatives, satisfying customer demands and control regulations, and cost savings arising from more efficient production and distribution channels. Product returns are reduced and the opportunity is presented to introduce new products and enter new markets.

The choice of packaging machine will be dependent upon many factors, determined by the nature of the product and the market requirements. The chamber type of machine will give a better minimum residual oxygen content in the pack where this is desirable by the process of evacuation before back-flushing with gas, compared with the continuous flushing systems. However, this method is unacceptable when handling fragile products. The degree of machine flexibility must be considered. Chamber


machines lack the flexibility of product size changeover that is offered by pillow wrapping machines. Where there is a need to use high levels of oxygen, above the 20% in normal air atmosphere, as when packing fresh red meat, only chamber machines should be considered. Pillow wrapping machines and similar that employ continuous gas flushing pose a fire hazard because of the venting of high levels of oxygen into the area surrounding the machine. Sparking from electrical components and the presence of combustible items such as grease, oil and product debris should be avoided in an oxygen-rich atmosphere.

The packaging machine should be able to give consistent and reliable seals, minimizing pack leakers, achieve the desired gas conte nt of the pack and be quickly and efficiently adaptable to meet production and changing market requirements. A wide choice is available to the food packer intending to enter MAP production.

Acknowledgements

The author wishes to express his thanks to those companies and individuals who have assisted hirn by supplying information and material necessary in the preparation of this chapter: Brian Day, Campden & Chorleywood Food Research Association, Chipping Campden; Steve Scully, CVP Systems (UK) Ltd, Uxbridge; Tony Trocian, Hitech Instruments Ltd, Luton; David Reed, Multivac UK Ltd, Swindon; Chris Hemming formerly of Raackmann UK Ltd.

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4 Packaging materials for MAP of foods J. GREENGRASS

4.1 Introdnction

Whenever the marketing of almost all produce and food products is under consideration, it is essential to remember that packaging is a very important part of the overall marketing operation. Marketing may be defined as 'the identification, anticipation and satisfaction of customer need profitably'. Properly designed food packaging mnst not only preserve the quality of the food but also ensure its safe delivery to the consumer at an economic cost. Because of this it is worthwhile considering the basics of food preservation in some detail before discussing the virtues and disadvantages of the available packaging materials. Table 4.1 outlines the six most important methods of preserving foods and points out that an essential seventh requirement in all instances is suitable packaging to prevent microbial entry and retard chemical deterioration. Table 4.2 defines the multiple functions of packaging that must be borne in mind when materials are selected to contain and protect a specific product.

The need for energy sources have driven humankind to find means of food preservation since the earliest of times. The drying of solid foods naturally by wind and sun and the concentration of liquid foods by evaporation were, together with fermentation processes, among the earliest methods employed. Reat processing (canning and gl ass packaging) later followed by chilling and freezing processing techniques came to the fore because they offered longer-life products and better and more healthy eating.

This trend towards more healthy eating coupled with demands for convenience, the greatly increased use of plastics-based packaging and the availability of new cooking equipment, such as microwave ovens, have revolutionized food and beverage packaging techniques since the 1950s. The trend has led to several newer food preservation processes included in Table 4.1 and novel packaging technologies such as MAP.

To understand the role of packaging film in the design of a modified atmosphere pack, it is important to consider the inherent potential variability of a modified atmosphere system. Any accurate description of MAP recognizes that, while a specific atmosphere might be achieved initially, changes start to take pi ace immediately as a result of the


Table 4.1 Methods of preserving foods

Methods

Traditional commercial sterility processes Non-traditional processes

Chilling and freezing Dehydration, curing, salting, sugar

conservation Acidification and fermentation Addition of preservatives Modification of package atmosphere

Packaging with hermetic seal

Comment

Retorting, hot packing Ultra-high hydrostatic pressurea Pulsed electric fielda Pulsed lighta Irradiationa Pulsed magnetic f1eldb Microwavec Ohmicc May be preceded by blanching Reduction of available water

Nitrites, sulphites, benzoates, BHA, BHT Vacuum packaging, addition of gas blends, gas scavengers

Microbiological and oxidative barrier

aDestroys vegetative cells, but sporicidal effectiveness is under investigation. bEffectiveness unknown. cSimilar to tradition al thermal processing methods in achieving commercial sterility.

Table 4.2 Basic functions of food packaging

• Identify the product • Be compatible with the food • Maintain quality while carrying the food safely through the distribution system • Meet production and packaging line requirements • Provide information on the food, its shelf-life, preparation, users and nutritional value • Inform the user about how to open and rec\ose the packaging and finally dispose of it in an

environmentally responsible mann er • Conform with all necessary legal and regulatory requirements • Ensure safe delivery of the food to the consumer in good condition at an economic cost

respiration of packaged produce such as leafy vegetables or fruits and even fresh cuts of meat. More significantly, the gases of the contained atmosphere and the external ambient atmosphere tend to equilibrate by permeation through the package walls at a rate dependent upon the differential pressures between the gases of the headspace and those of the ambient atmosphere. It is in this context that the barrier to gases and water vapour etc. provided by the packaging must be considered. The barrier material used is, therefore, crucial to success. Different potential packaging materials for MAP of foods have differing properties.

Glass and metal containers. These are excellent barriers to gases but are not generally suitable for MAP because the quality of foods processed and packed in these containers are not enhanced by gas introduction.

PACKAGING MATERIALS FOR MAP OF FOODS 65

Sem i-rigid and plastic containers. If permeability is suitable and sealing is reliable, a combination of low-cost semi-rigid containers with flexible lidding or overwrap can be suitable for products needing physical protection.

Flexible films. These, almost to the exclusion of other materials, provide the range of permeability to gases and water vapour (see Tables 4.3 and 4.4), together with the necessary package integrity, needed for MAP of meats, fish and bakery products.

The major factors to be taken into account in selecting the packaging are: (i) the type of package (i.e. rigid or semi-rigid lidded tray or flexible film pouch); (ii) the barrier properties needed (i.e. permeabilities of

Table 4.3 Permeability of common polymers to gases and vapours, together with corresponding diffusivity and solubility data at 25°C

Polymer Permeant 1015 Permeability 1012 Diffusivity 103 Solubility (kg rn-I kPa-1 S-I) (m2 S-I) (kg m-3 kPa-1)

LDPE Isobutene" 680 4.7 140 n-Hexane" 6200 2.5 2500 Water vapour 540 23 24

HDPE Heliuma 1.9 360 0.0054 Oxygena 5.4 22 0.25 Nitrogena 1.7 12 0.14 Carbon dioxidea 31 16 2.0

PVC Carbon dioxide 0.52 0.21 2.5 PA6 (Nylon 6) Nitrogen 0.023 0.025 0.94 PETP Nitrogen 0.063 0.013 0.48

Note: 1kPa = 1/101.3 atmosphere -0.01 atmosphere. a30°c.

Table 4.4 Permeability of films to gases (at 30°C) and water vapour (at 25°C, 90% RH)

Material Permeability Ratio to N2 as 1.0 (mI m-2 MPa-1 per day)

N2 O2 CO2 H20 PO,IPN, PCO,IPN , PH,dPN,

PE (0.922) 120 300 2300 5300 2.9 19 44 (7.6)a (0.954-0.960) 18 71 230 860 3.9 13 47 (3.3)

PP (0.910) 150 610 4500 - (4.0) UPVC 2.7 8.0 67 10 000 3.0 25 3800 (8.1) PVdC 0.07 0.35 1.9 94 5.0 27 1 400 (5.4) PS 19 73 590 80000 3.8 32 4200 (8.1) PA (Nylon 6) 0.67 2.5 10 47000 3.7 15 70000 (4.0) PET (Mylar A) 0.33 1.47 10 8700 4.5 31 27000 (6.6)

RH, relative humidity. aFigures in parentheses in the last column are ratios of permeabilities for carbon dioxide and oxygen.

66 PRINCIPLES AND APPLICA TrONS OF MAP OF FOODS

individual gases and gas ratios when more than one gas is used); (iii) the physical properties for machinability and strength; (iv) integrity of closure (e.g. he at sealing), fogging of the film as a result of product respiration; and (v) printability.

It should be emphasized that the correct atmosphere at the start will not last long if the permeabilities of the barrier material(s) allow(s) rapid changes to occur. Few single materials have the properties required to make them suitable, and often a laminate or a coextrusion is needed. The transmission rate of the films used will be proportional to the surface area of the package and inversely proportional to the film thickness. Even though permeability rates through the film can be easily measured, shelflife performance of the films with the product necessitates shelf-life testing because of residual oxygen within the producL This is not always eliminated by flushing or any gas curtain techniques used when filling. Therefore, dependent upon the rate of release, residual oxygen modifies the original gas mix, as distinct from modification resulting from the film performance.

MAP is a concept that can be utilized on a grand or minor scale. Consider a gas-filled transparent container with several respiring fruits stacked inside and a large free volume. Then, compare it with 100 g of a product in a dose-fitting thermoformed tray with a peelable lid and the minimal practical headspace. Note that in both cases the seal strength and barrier properties of the container are the controlling factors, assuming produet stability onee the pack is closed and ambient eonditions established. It is important to recognize that the conditions do affect the rate of change in the producL For example, food-storage conditions for modified atmosphere packs are usually considered to be in the range of 0-5°C. The rate of product respiration and the rate of film permeability are both affected by storage temperature.

Sometimes high-barrier materials are essential, yet with some products that respire it is important that high-barrier films are not used as undesirable, potentially hazardous anaerobiosis can be initiated. From this it can be seen that the choice of film for any particular pack must take into account a wide range of factors.

Additionally, while so far the relationship of the barrier properties of a particular film, the product and the contained atmosphere have been discussed, a modified atmosphere pack must also be machineable, capable of withstanding transport and handling, attractive to the prospective purchaser, capable of carrying a message (if only to meet legal requirements) and all this at an acceptable cost. Technically, it is easy to ignore the cost factor - does it matter that it costs 30-40% more to provide a high barrier? It is a fact of life that while certain products with a prestigious image can command a premium, most products seil competitively. A pack suitable for an expensive cut of meat or a premium pasta line might also be


the most suitable for a low-margin bakery product, but there is no way the bakery could afford to use it. The best compromise must be achieved.

Before considering the properties and features of individual films, the range of relevant characteristics warrant review. It can be seen that many of the films used in MAP do not meet the requirements in themselves, hence combinations of films are produced by lamination, coextrusion or extrusion coating as necessary.

4.2 Plastic films commonly used in MAP

There are several groupings in MAP films. The following comments will indicate the necessity to utilize complementary properties to provide the features particularly desired for the intended use. It will be seen that polyethylene in one of its forms is common to all but one area, polyethylene being the common element used to provide a hermetic seal, but it also provides a medium of control for characteristics such as antifogging abilities, peelability and the ability to seal through a degree of contamination.

4.2. I Polyolejins

Low density polyethylene (LDPE). Extremely versatile, this accounts for the biggest proportion of packaging plastics. LDPE is relatively inert chemically; its permeability is moderately low for water vapour, but the gas permeability is high with a poor odour barrier. Essential oils pass rapidly through LDPEs. Related to LDPE is ethylene vinyl acetate, a copolymer of ethylene and vinyl acetate (usually with up to 4% vinyl acetate). The copolymer has superior sealing qualities, i.e. lower he at seal threshold, allowing a seal to be made through a level of contamination, such as traces of water or condensation or fats from the product packaged. The performance does not equal that of a linear low-density polyethylene or Surlyn but is an improvement on standard low-density polyethylene.

The use of blended polyethylene on both sealing faces with different selected additives allows a peelable seal to be made: a strong, valid, adequate barrier in practical terms, yet peelable. When used with other films for lidding, base webs, preformed trays, bulk packaging bags or horizontal/vertical form-fill-seal webs, low-density polyethylene can be laminated, extrusion coated or, in some cases, coextruded. The performance possibilities become apparent when a specific pack lidding web is considered.


Lamination. Lamination allows sandwich printing (the print being trapped between the two webs), high surface gloss and no print contact with the sealing face.

Extrusion coating. Extrusion coating commonly utilizes a surface print on the face. However, with high gloss inks excellent print results can be obtained.

Clarity. Both lamination and extrusion coating can produce high-clarity lidding webs.

Antifog characteristics. Antifogging properties vary slightly between the processes. However, the actual performance always depends upon product and storage/presentation conditions. Both systems produce commercially acceptable results.

Bond strength. Both lamination and extrusion produce a strong bond, capable of withstanding the loads imposed by a top lidding web and a formed base web. However, simply achieving the highest bond under laboratory conditions is not necessarily ideal. Both top lidding and base are stressed during MAP processing. The top web finds linear stress during registration and/or application, then surface area stress during gas flushing and sealing. The base web is stressed and deformed during the trayforming process when it is significantly heated (i.e. between 120 and 140°C). Therefore, the adhesive used in lamination, and indeed the bond achieved by extrusion coating, must be inherently resilient and extensible.

Linear low-density polyethylene (LLDPE). Long before this film became freely available in the UK and Europe, it was available from DuPont of Canada. Later Union Carbide announced its new process for making LLDPE, stating its advantages over low-density polyethylene. Subsequently, resin producers such as Exxon took out a licence to produce it. Dow Chemical also entered the market with a product based on their own technology. Claimed advantages over LDPE are:

• allows down gauging; • better impact strength; • better tear resistance; • higher tensile strength; • higher elongation potential; • greater resistance to environmental stress cracking; • higher head resistance; and • better puncture resistance.


Disadvantages are:

• higher heat seal temperatures required; • inferior gloss; • inferior transparency; • requires more energy to extrude than standard LDPE; • difficult to incorporate additives; and • increased cost.

From these, it can be seen that LLDPE can meet certain special requirements but at some cost, both financially and technologically. While it remains a viable choice, variations on a basic low-density polythene remain by far the most common films in use.

High-density polyethylene (HDPE). This has a high er softening point than the lower density polyethylenes, provides superior barrier properties and is a harder film. It is not suitable as a sealing layer. Therefore, it is found not in thermoformable base webs, but as one of the webs in a coextruded form composing a top or lidding web. It has better gas-barrier properties than LDPE but its cIarity is poor.

Polypropylene (PP) and Oriented Polypropylene (OPP). pp is chemically similar to polyethylene and can be extruded or coextruded to provide a heat sealable characteristic. OPP, while providing higher ranges of moisture vapour barriers, also provides a much greater barrier to gases: seven to ten times that of polyethylenes. In addition it has excellent grease resistance. OPP coated with polyvinylidene chloride (PVdC) in low gauge form provides high barriers to moisture vapour and gases; it is preferred for non-modified atmosphere lidding formats where total seal integrity is less stringent. A possible format for a relatively high-permeability lidding web would be 15 11m OPP to 25 11m LDPE.

Coextruded orientated polypropylene (COPP). A primary use of CO PP is in the vertical form-fill-seal machinery format, e.g. for salads.

Film production. This film is produced in one of two ways: 'blown' in an inflated bubble system or cast and drawn via astentered system. Although it is a good moisture vapour barrier, the gas-barrier properties are improved by coating with PVdC. Used mainly in variations of the basic film, it can be laminated as part of a lidding web. It can be produced by coextrusion with the basic barrier properties, or it can be perforated mechanically to provide a greatly reduced resistance to gas f10w on respiring products. CO PP is supplied with fine, controlled, laser-produced perforations to suit a particular set of values.

These are the only commercially available films when agas f10w is


desired in the 'modified atmosphere' context, although there are other highly permeable specifications, available for medical uses for example.

Microperjoration technology (Frey, 1997). Two factors are pivotal to understanding the operating mechanisms behind 'breathable' packages for fresh cut produce. The first is the rate at which the produce breathes. The second is the rate at which oxygen enters the package.

Some cut produce products such as Romaine lettuce respire at relatively low rates and, therefore require a low-permeability package. Others such as broccoli, cauliflower and many cut-fruit items respire at very high rates. Since the purpose behind breathable packages is to match the package's permeability to the product's respiration rate, a broad range of permeability levels are necessary to meet the needs of the wide variety of packaged produce items.

The permeability requirements for retarding the ripening of produce with low respiring rates can generally be matched by integral films. Polyolefin films in the range around 2 mil offer permeability levels sufficient for most prepackaged salads. For the high respiring rate products, however, such films would need to be so thin to match their permeability requirements that machinability and transportation problems could arise. To rectify this deficiency, many attempts have been made to develop more porous structures in films by creating discontinuities in the packaging membrane. In general, there are three generic approaches that have been pursued to create additional porosity beyond that provided by integral films. The first method incorporates inorganic fillers such as mica or calcium carbonate into the film itself. This method does increase porosity but at the expense of clarity. Such films have failed to achieve commercial success in retail packaging because of the haziness inherent in filled polymer films and the li mi ted increase in porosity that can be achieved.

The second method is to insert a porous patch somewhere into the package. Instead of changing the permeability of the film, the packager die-cuts a hole in the package and adheres a permeable membrane to it. This membrane is so porous that its permeability overrides that of the base packaging film. By changing the porosity of the patch, the packager can respond to the different respiration needs of various fresh produce items. Because the application technology and patch manufacturing processes add substantively higher costs to the package, however, patches are currently viewed as niche technologies.

The third generic method is to puncture the film itself by making one or more holes. There are a number of approaches that have been tried to perforate a package without adding the extra costs of die-cutting a hole and adding a porous patch. The approach es fall into two categories, which can best be described as macroperforation and microperforation.


In general, it is not sufficient just to punch a few holes into the package. So-called macroperforation technologies, which can be accomplished with hot needles or on-machine syringes, produce large, imprecise holes because of irregular puncture slits and blunting of the needles. They lack the sophistication to deliver the precise permeability level that needs to be achieved to sustain the ideal atmosphere within the package. It is important to remember that excessive oxygen in the package defeats the purpose of providing a 'breathable' film. Although they do ensure sufficient oxygen within the package to avoid anaerobisis, macroperforation technologies generally make holes that are too large to be useful in creating a specially designed atmosphere.

Microperforation technology, by comparison, depends on numerous, smaller, more precise holes for delivering the targeted atmosphere. Such holes are typically in the range 50-60 {im diameter. The number of microperforations designed for any particular produce item will be determined by the product's respiration rate, the desired balance between oxygen and carbon dioxide in the package, the diameter of the holes and the thickness and permeability of the packaging medium.

Microperforation technology can be applied to a wide variety of substrates. Commercial applications exist using both monolayer and coextruded films. Such films are selected for properties totally different to their inherent permeabilities. Although most of today's commercial products based on microperforation technology are packaged in OPP, they are not barriers to working with other resins irrespective of their level of orientation and the manufacturing process used to extrude them. Effectively, microperforation is a postextrusion converting process such as embossing or printing wh ich is designed to add value to the extruded base film.

Sidlaw Flexible Packaging Ltd developed microperforation technology by creating a process for making consistent, ti ny holes at commercial rates of speed. Though the two patents issued were in the UK, the technology is being commercialized worldwide as P-Plus.

Ionomers. The first commercially available ionomer was a polymer of ethylene, Surlyn A. It is similar in many properties to polyethylene but has several advantages, such as it develops a high tack and will seal through a level of surface contamination. Ionomers can be extrusion coated. Very thin coatings can be obtained but unless these particular properties are necessary, the cost may preclude use.

4.2.2 Vinyl polymers

Ethylene vinyl acetate copolymer (EVA). EVA is a polymer with high flexibility in sheet form and with permeability to water vapour and gases


that is higher than that of LDPE. In both lidding and base films its main value is as a component of the sealing element of the primary films. For example, a 4% addition to the polyethylene laminant improves seal ability both in tolerance and ability to cope with a minor level of surface contamination with product or water in the sealing area.

Polyvinyl chloride (PVC). In unplasticized form (UPVC), this is the most widely used thermoformable base web for MAP. Polyesters or polystyrenes have been developed as replacers, including aversion of foamed polypropylene, but PVC's suitability both technically and commercially is paramount. PVC provides good gas barrier and a moderate barrier to moisture vapour. It has excellent oil and grease resistance and, in its unplasticized form, is capable of smooth, even forming into shallow or deep trays. To maximize the forming potential, even heat distribution over the greater depths, 'plug-assisted' forming machine techniques are recommended.

Both barrier properties and strength characteristics vary with thickness. Therefore, the particular gauge and the essential polyethylene laminant gauge must be chosen with the depth of form in mind, and most particularly the final formed surface area in relation to the initial unformed surface area. In practice, many regard a pack corner thickness of 15 11m, reduced from any initial thickness, as the effective minimum. At this gauge, with strong pack form sides, possibly fluted for strength, the lower form corners would be thin and relatively soft. In conventional terms, the pack would be valid, with areas of highest permeability to moisture vapour and gases in the areas of lowest gauge.

UPVC film is manufactured in a different way to the 'blown' or 'stentered' techniques used for other MAP films; PVC is milled and calendered with the mix of constituents required to arrive at the desired blend to suit the market or markets perceived by the manufacturer. This provides a range of properties from which the laminate manufacturer or converter can select a suitable grade, for example a deep draw/narrow pack. As a base web, UPVC can be coloured or printed. But clear film is more frequently used.

Polyvinylidene Chloride (PV dC) copolymer. This copolymer of vinylidene chloride with vinyl chloride is used in MAP as a gas-barrier coating for lidding films and in film form as a sandwiched barrier web. It has outstanding barrier properties with low permeability to water vapour and gases. It is used mainly as a coating on polyesters and OPP for lidding films. Although it is available with polystyrene as a base web, this not in common use.

Current legislation and prejudice against the use of chlorine-containing compounds in contact with food or with constituents that may mi grate


within the laminate has resulted in some re action producing a move away from the use of films or coatings with any significant chlorine content. However, a move away from the only really suitable coating for the required barrier purposes means the development and adoption of other film combinations with a 'buried' gas barrier.

Ethylene vinyl alcohol (EVOH) (EVAL®). As a film this is a moisturesensitive, very high gas-barrier material that is sandwiched between the main formable and sealing element to provide protection. It is expensive and is, therefore, used at the lowest gauge that provides adequate barrier properties for the intended laminate.

4.2.3 Styrene polymers

Polystyrene (PS). This is a clear thermoplastic with a high tensile strength but is a poor barrier to moisture vapour and gases. Alone, PS is intrinsically brittle but by blending styrene-butadiene or styrenepolybutadiene, the properties required for thermoforming can be obtained, but some of the original clarity is lost. It is common to sandwich a barrier layer - possibly EVOH - between PS and polythene sealant.

High-impact polystyrene (HIPS). This is an opaque, thermoformable, moderately low gas-barrier film, and consequently it is used as a component of a laminate or coextrusion.

4.2.4 Polyamides

Nylons are manufactured in various ways and characterized by a number derived from the number of carbon atoms in the molecule. Hence a commonly used nylon-6 would refer to the number of carbon atoms in the parent amino acid, e.g. PA6.

Nylons are tough films with high tensile strength and good resistance to abrasion, but their mechanical properties are altered by water absorption. These properties are recovered on drying, and this fact must be taken into consideration for storage and handling. Nylon transparency is very good, biaxially oriented better than other forms. In MAP, this film, when laminated or extrusion coated with polyethylene, is suitable as a lidding material, producing physically the strongest web, gauge for gauge. Printing is possible, but the results are poorer than with PET (polyethylene terephthalate) film.

When fabricated into bulk gas pack (BGP) bags, nylon provides the greatest flexible resistance to puncture, hence with meat products potentially containing bone edge or splinters, a nylon laminate is preferred.


4.2.1 Polyesters: polyethylene terephthalate (PET)

PET is used in various forms in MAP: as a low-gauge orientated film of high clarity for lidding films and in crystalline or amorphous form as in-line preformed or thermoformed trays. In the lidding format, it is commonly used with a gauge of 12 flm, which can carry as a high-barrier web 3 flm PV dC. It has a high tensile strength and a high softening point.

Except in one form, where it is used as a non-MAP lidding material in its own right, PVdC is incapable of producing a valid hermetic seal without lamination to a coating of polyethylene or polypropylene. Conventionally, orientated PET is used for MAP. It is capable of accepting a full range of printing or coding inks as weil as the commercially available label range. Additionally it can be metallized to allow the commercial and technical advantages of the silver face to be fully utilized.

While lidding film is conventionally orientated and 12 to 19 flm thick, a special limited form ability grade is produced suitable for lidding deformation or minor base formation. As a truly formable base web, the gauges are higher and the film is not orientated. It can also be coloured rather than the more commonly used clear or white. Gauges up to 750 flm are available in both amorphous and crystalline form, the latter film being suitable for dual ovenable/microwavable use, albeit a more expensive packaging form. CPET (crystalline polyester) is not suitable for conventional linear thermoform machinery. It is, however, eminently suitable for use in preformed trays for subsequent lidding.

Amorphous polyester (APET) is potentially a significant contributor to the European MAP market, in that arecent trend away from UPVC (because of the move from products with a vinyl chloride content) necessitated a commercially comparable alternative. However, the commercial pressure to develop an alternative base to UPVC has now receded in the UK, and the substantial use of PS in France, a big MAP user, has slowed market development. Nevertheless, it is probable that base web usage over the next few years will be in the descending order of PVC/PE, APETIPE and PSIEV AL/PE.

4.2.6 Other films

There are other films available such as polycarbonate (PC), wh ich could be areplacement for PET in some areas, and acrylonitrile butadiene styrene (ABS), wh ich can be used as a thermoformable base web, having very good chemical and deep-freeze properties. However, these films are not in demand currently in the broad market of MAP.


4.3 Combinations of films

For clarity it is advisable to separate the film characteristics required for marketable MAP into groups. It is possible to concentrate on the technical attributes without considering the marketability of the product, but modern commercial economics must be considered seriously. To summarize performance characteristics required:

• permeability to the atmospheric gases carbon dioxide, oxygen and nitrogen;

• water vapour permeability; • sealability characteristics; • ability to thermoform; • ability to form from a flat web to a pouch in a linear motion; • resistance to puncture; • clarity; • antifog properties; • required rigidity level; • cost per square metre; • ability to sandwich/surface print; • ability to accept coding or labels; • availability of film; and • lead time for supply.

These are the requirements for a wide range of package types, but all of them are not necessary for every pack. Films for thermoformed BGPs or horizontal or vertical flowpacks have different machineability characteristics. Hence not all characteristics are relevant. Nevertheless, suitable factors from the list must be selected to arrive at the correct specifications.

4.3.1 Laminates, coextrusion and extrusion coating

Tables 4.5 and 4.6 indicate the properties of the various films with respect to relative barrier values and the comments under the film headings give a guide to the types of film discussed. However, in addition to barrier properties, films require the properties of formability and/or seal ability , depending upon their functions. As already stressed, one inherent requirement of all MAP packs is the ability to retain the desired atmosphere as long as possible. This is achieved first by choosing a film or films to provide the required gas and moisture vapour permeability characteristics, and second by ensuring efficient sealing of the package. With thermoformed and lidded preformed trays, the interface seal is critical. With horizontal and vertical form-fill-seal packs, valid end and back seals are essential, as are the premade end and side seals of the BGP


Table 4.5 Basic film barriers

Gas transmission rate WVTR (cm3 m-2 atm-1 per 24 h for 1 ml film at 25°C)

(g m-2 per 24 h) at 38°C,

Film 90% RH) Oxygen Nitrogen Carbon dioxide

LDPE 18 >800 2800 42000 HDPE 7-10 2600 650 7600 PPcast 10-12 3700 680 10 000 OPP 6-7 2000 400 8000 OPP coated with PVdC 4--5 10-20 8--13 35-50 Ionomer 25-35 6000 6000 EVA 40-60 12500 4900 50000 UPVC 30-40 150-350 60-150 450-1000 Plasticized PVC 15-40 500-30000 300-10 000 1500-46000 PVCIPVdC copolymer 1.5-5.0 8--25 2-2.6 59-150 EVOH 16-18 3-5 PS oriented 100-125 5000 800 18000 Nylon 6 84--3100 40 14 150-190 Nylon 6,6 45-90 78 6 140 Nylon 11 5-13 500 52 2000 APET 40-50 110-130 CPET Permeabilities change according to degree of crystallinity.

For each 1 % increase there is a 1.5% decrease in transmission rate. PET oriented 25-30 50-110 15-18 180-390 PET oriented and 1-2 9-15 20-30 coated PVdC

WVTR, water vapour transmission rate, RH, relative humidity.

systems plus the mouth closure required in this technique. The only format where the strength of the seal takes priority over the actual gas permeability is when 'perforated' films are used with respiring products such as salads.

As mentioned above so me films are laminated or coextruded. Figures 4.1 and 4.2 illustrate the fundamental difference in the techniques. Significantly, perforated and extruded pp films are the only films used singly in MAP, and it is normal to coextrude from two to five layers to provide suitable properties, with possibly two of the five in the coextrusion being bonding layers, for example, for forminglbarrier webs and sealing provisions.

Adhesive lamination can be simple with just a forming and sealing provision, but inclusion of a base web or the adhesive bonding of a coextrusion or coated film to a basic sealing web, making a multilayer product, elaborates the process and, of course, increases the cost. Further to this, barrier properties vary with the gauge (thickness) of most films. Table 4.7 provides a list of common specifications. Appendix 4.A shows how to calculate the probable values of alternative gauges and combinations of films.

The word 'probable' is carefully chosen, indicating a practical expectation

PACKAGING MATERIALS FOR MAP OF FOODS

Table 4.6 A simple guide to range of barrier properties of plastic films

Water

1 ()() ()()()

10 ()()()

1000

100

10

Polystyrene

MOST PERMEABLE

Carbon dioxide

EVA

LOPE

Polystyrene

OPP HOPE

Polyester

Nylon-6 Nylon-6.6

Polyacrylonitrile Nylon-6.6 PV dC copolymer EVA CoatedOPP

Polyester Coated polyester

LOPE Polyacrylonitrile

HOPE

OPP CoatedOPP PVdC copolymer

LEAST PERMEABLE

Oxygen

EVA

LOPE

Polystyrene

HOPE OPP

Polyester

Nylon-6.6

Nylon-6

Coated OPP

77

Polaacrylonitrile PV C copolymer

of some deviation from a theoretical value. As values vary with gauge, gauge can vary from film to film. Manufacturers' tolerances, while smalI, are a fact of life. While trade tolerances exist, they can vary slightly between producer, converter and end-use. However, tolerances vary with quantity purchased, gauge and film type. When part of a composite, a web


air tlow

000

adhesive unwind appUcation web

Adhesive Larninatlon

Figure 4.1 Adhesive lamination.

polyester example~~~thin prime coating

~_~_~,:lIo:,lo,;,:o.:,:o.;:, ~.::!I_ polyethylene

extrusion coating

Figure 4.2 Extrusion coatings.

combining nip

unwind web 2

can vary within its own specification and 'gross-up', or cancel OUt. Therefore, many converters prefer to quote a range of permeability values, those ascertained in their own laboratories. As a guide, gauge tolerances on.large tonnage may average out less than ± 5% from nominal values. Smaller tonnages (i.e. < 500 kg) may carry a 10-12% gauge tolerance range on nominal. It should be recognized that these values relate to 'yield' factors also, and the given number of packs per kilo vary to the same ratio.

The primary choice of materials and gauge basically controls the performance of the pack, but it is necessary to consider thinning of walls of thermoforms during the thermoforming process. To a lesser or greater degree, the thickness of the tray lip, the upper side, lower side, corners and base will all vary, the corners being thinnest. In poor processing, the corners could thin down to < 20 flm from a web that started at 500 flm


Table 4.7 Typical specifications

Thermoform Base web 200 11m UPVC170l1m LDPE 400 11m UPVC/1OO 11m LDPE 650 11m UPVC/100 11m LDPE 400 11m APET/lOO 11m LDPE 300 11m Barex/loo 11m LDPE In addition to these, some European markets favour specifications based on polystyrene/ EVOHILDPE of smaller percentages of APETIEVOHIPE

Tray lidding Preformed trays APET, CPET, UPVC or HDPE based specifications Horizontal form-fill-seal

Vertical form-fill-seal Bulk gas packs

Lidding laminate 15 11m polyester-PVdC/60 11m LDPE/antifog coating 12 11m polyester/coextruded polythene 15 11m oriented PA (nylon)/60 11m LDPE 21 11m coated coextruded pp/50 11m LDPE

The inclusion of EVOH in some lidding laminates is a developing area

Lidding lamination As above

1S 11m polyester-PVdC/60 ,um LDPE/antifog coating 12 11m polyester/38 11m LDPE 30 ,um coextruded PP 1S I1m/coextruded PP/30 11m LDPE 28 11m coated coextruded PP As horizontal form-fill-seal LDPEIPA (nylon)ILDPE PA (nylon)/EVHOHIMDPE LDPEIEVOHILDPE

Note: The potential list is extensive, but this is an indication of the range of specifications currently in use.

280

260

240

220

Z 200 .. ~ 180 -~ 160 ~ D 140 .. .. !§

120 GI "-

100

80

60 -40

20

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Gauge (mleron)

Figure 4.3 CAP. Change of nominal permeability with gauge of 300,um PVC/lOO ,um PE film to 100% oxygen under temperate conditions.


~~---------------------------------------------, 19

18 17

16

15 14

! 13

1 ,,~ ~ 10 :l5 : 9 ~ 8 :. 7

6

5 4

3

2 1 O~-L __ ~-L __ ~-L __ L--L __ ~-L __ ~-L __ L--L __ ~~~

~ 40 60 80 100 1~ 140 160 180 200 2~ 240 260 280 300

Gauge (mIcron)

Figure 4.4 CAP. Change of nominal permeability with gauge of 300 /-Im PVCIlOO /-Im PE film to moisture vapour under tropical conditions.

total thickness (400 Ilm UPVC/lOO Ilm PE). The properties will vary according to the gauge reduction. Corner permeability, stiffness and strength will all be significantly reduced.

Figures 4.3 and 4.4 provide a guide to the theoretical permeability of one laminate, a 300 Ilm PVC/WO Ilm PE. From this can be seen the merits of considering the most suitable gauge to provide the minimum acceptable corner thickness for a particular draw depth. The characteristics of thermoforming plastic films - while discussed in relation to machinery elsewhere - must also be considered. Machinery companies can utilize 'plug assist' systems or increase heating facilities to improve form. They also provide guidance for their particular machine as to the maximum depth a film can be drawn in relation to the surface area of the forming die top. One major company recommends that the following formula be used to ascertain the form depth limit.

Length of pack X Width of pack = Recommended maximum draw depth

Length of pack + Width of pack

When modifying dies internally to provide a transverse wall or side protrusion, the formula needs to be reviewed.This area does not necessarily meet the required ratio. Table 4.8 provides an indication of gauges in relation to draw depth, and ratio of reel usage, top to bottom web, with commonly marke ted polyesterlPE lidding or UPVCIPE base webs.


Table 4.8 Guide to usage of UPVC/PE base web and 15 firn polyester PV dC/60 firn PE lidding web

Base web

250 11m UPCV170 11m PE 300 11m UPCV 170 firn PE 400 11m UPVC170 11m PE 550 11m UPVC/lOO 11m PE 650 11m UPVC/loo 11m PE

4.3.2 Speci[ications

Draw depth (mm) Usage ratio base to lidding

15 5:1 30 6:1 60 8:1 90 11:1

110 13:1

There is a wide range of practical specifications that can be selected from the films discussed, some of them for special uses, others for the main European and North American MAP markets. In spite of the call for higher barrier specifications and 'environmental' calls for progressive elimination of all films with a chlorine content, even when it does not co me into contact with the product itself, UPVC/PE remains the most commonly used base web for MAP thermoforms with PET/PVdClPE as the main lidding material. With horizontal form-fill-seal MAP systems, again PETI PVdCIPE is the conventional material, with coextruded pp commonly used on vertical and horizontal form-fill-seal systems, perforated or otherwise.

BGP systems have different requirements, with a greater necessity for physical strength and puncture resistance at a range of low temperatures for weighty meat packs, where they must be capable of resisting punctures, splinters and cuts from bone edges. Orientated nylon suits this market, but the need to seal easily and strongly, and provision for some moisture protection for the hygroscopic nylon film, necessitates that a PE film be laminated or coextruded either side; an EVOH layer may be inc1uded where a higher degree of oxygen barrier is required.

4.4 Typical specifications for MAP use

While there is a very wide range of alternative films and laminates available for the various forms of MAP, and material suppliers will always promote their own developments, in practice the tonnage throughout the UK, Europe and North America is limited to a relatively short list specifications. It is probable that most materials offered not only perform weil, but asound technical reason could be made for using them. However, in the hard commercial world of food packaging, cost and availability exc1ude many films and laminates; additionally many packers are reluctant to purchase from a single source.


The base web. Some markets find a particular specification suitable in 'Iocal' supply, commercially and with a marketing preference.

In France, the major percentage of MAP base webs are of PS/EVOH/ PE with UPVC/PE making up the remainder.

In the UK, the main use is in UPVC/PE with development runs in APET/PE. Also foamed PP/polythene laminates are progressing.

Germany shows a preference for UPVC/PE with some use of APETIPE. Denmark, Italy and the Netherlands mainly use UPVC/PE, but market

shares are held by other combinations, e.g. APETIEVOH/PE and PPI EVOHIPP. Laminates incorporating a PVdC layer (which is undoubtedly an excellent barrier web) have so me environmental problems since so me countries exclude this from some markets. Consequently, developments are in hand to find more acceptable alternatives to PVdC.

In base web terms, APET is not always interchangeable with UPVC, the most tolerant of thermoforming films. Adjustments to temperatures, time and especially cutting techniques, are sometimes required, depending upon source or the particular machine in use.

The top web. The most commonly used lidding (top web) film is 15 {Lm PVdC coated polyester/50 or 60 {Lm PE with antifog properties, laminated or extrusion coated, sometimes printed, and most with adhesive label decoration. However, this is a high-barrier system, and it is in this area that major developments are progressing to provide a clear non-PVdC barrier lidding film. Biaxially oriented nylon/PE antifog coated is one possibility, unfortunately with a reduction in oxygen barrier, typically down to 30 to 40 cm3 m-2 per 24 h from 8 to 10 cm3 m-2 per 24 h on a 15 {Lm PVdCcoated PET/60 {Lm antifog PE laminate. This would be adequate for so me products. Polyester laminated to a multilayer coextrusion provides the greatest oxygen barrier potential, around 2 to 4 cm3 m-2 per 24 h, but there is a cost premium. As distinct from these mainline specifications, should the product carry the cost, there are a range of specially developed coextrusions available for both lidding and base webs. One commercially available material consists of a heat-resistant outer web, EVOH, and a base sealant polymer with two tie layers. This can also be used on vertical and horizontal form-fill-seal machinery but with a coextrusion the sandwich print facility is not available, although it is possible for surface print to be applied.

The top web version of these coextrusions can provide an oxygen barrier level of 4.5 cm3 m-2 per 24 h at 50 {Lm, the base web 1.5 cm3 under the same conditions at 130 {Lm.

Within the standard range of lidding webs, while a clear film is necessary to see the product, indeed relative clarity is commercially important, it is possible to provide white or metallized versions, or even a near complete printed design. Whatever print coverage is used, it is advisable to eliminate ink from the seal areas to maximize the seal strength potential.


4.4.1 Horizontal and vertical form-fill-seal systems

With both horizontal and vertical form-fill-seal systems, high-barrier or low-barrier (for respiring products) film can be utilized. Conventional rotary sealing systems used on 'horizontal' machinery are not recommended where totally valid seals are required. Tbe vertical form-fill-seal machinery normally used in salad or vegetable packaging is fitted with relatively wide reciprocating sealing jaws that allow adequate sealing time and pressure. Horizontal machinery supplied for this field normally utilizes reciprocating box action sealers, and with this in mind, films or laminates should be considered.

For simplicity, these machines can be described as flowpack machines in that the web runs from the reel through a former that creates a tube, the sealing system providing a linear back seal and a transverse crimped seal at the beginning and end of the pack.

For high-barrier purposes, gas flushed or not, the weak points are the transverse crimp seals. The primary film barrier properties are chosen initially - one cannot improve on that - and the back seal conventionally is a fin seal with a linear configuration and should be as good as the primary barrier. However, the end crimp seals carry any linear creasing and fold over on flattening of the tube and any variability in the number of layers included in the seal. It is in this area where 'piping' can occur: minute machine direction 'tunnels' within the crimp seal that allow varying degrees of permeation via capillary action. For this reason, as distinct from reasons to choose the laminate or film to provide the barrier properties, a polymer is used that flows to develop a totally valid seal.

To this end, variations on PVdC-coated polyesters/PE are most frequently used for high-barrier films, with coextrusions within the earlier defined top web (lidding) range. Films with various barrier levels are listed in Table 4.9. While in scientific terms there may be a need to calculate the

Table 4.9 Flowpaek films: barrier levels·

15 11m PVdC-coated polyester/60 11m PE 12 11m polyester/50 11m PE 15 11m biaxially orientated nylon/50 11m PE 15 11m orientated eoextruded PP/25 11m PE 30 11m coextruded PP Perforated 30 11m coextruded PP

MVTR (tropieal) (38°C, 90% RH) (g m-2 per 24 h)

5 6--7 8

10-12 5

Oxygen permeability (25°C, 0% RH)

(em3 m- per 24 h)

4-6 120

30-40 2200 1600

Permeabilities dependent upon perforations

MVTR, moisture vapour transmission rate; RH, relative humidity. ·Data provided by Lawson Mardon Paekaging.


theoretical barrier properties of a particular pack to relate to the envisaged product requirements, in practical terms shelf-life tests are essential. Permeabilities do vary according to the conditions prevailing at the time of testing; also in practice, with a thermoformed base, gauge variation will depend upon degree and evenness of draw. Figure 4.4 illustrates the degree of potentiallocalized variation. Table 4.10 illustrates values taken from a laboratory check and base material supplier values.

4.4.2 Bulk gas packaging

Bags for BGP are premade and supplied by manufacturers who specialize in bag or pouch fabrication. Each market calls for an agreed range of bag sizes, usually with a limited range of gauges.

Suitable barrier properties are essential with a specific requirement for a high physical strength, both to contain a substantial weight of product and to resist puncture (Table 4.11). Current materials include PEIPA (nylon)/ PE, PA (nylon)IEVOHIMDPE and PEIEVOHlPE. The principal performance parameters are:

• low gas permeability; • high puncture and impact resistance; • differential sealing between inner and outer surfaces; and • transparent construction for conte nt identification.

Table 4.10 Permeabilities of some commonly used specifications

(a) Tested values Standard top web 12/lm Shallow draw Deep draw

PVdC-coated base web base web Permeabilities polyester (300/100) (650/100)

Oxygen (25°C 0% RH) (em3 m2 per 24 h) 4-6 5-10 5-8 WVTR (temperate) (25°C 75% RH) 0.5-1 0.5 0.3

(g m-2 per 24 h) WVTR (tropieal) (38°C 90% RH) 5 0.7 (g m-2 per 24 h)

(b) Base material suppliers values

12/lm PVdC 300/lm PVC 650/lm PVC coated polyester

Nitrogen (25°C 0% RH) (em3 m-2 per 0.4-0.8 6-10 2-3 24 h)

Carbon dioxide (25°C 0% RH) (cm3 m-2) 32 30-40 15-20 per 24 h)

WVTR, water vapour transmission rate; RH, relative humidity.


Table 4.11 Technical and mechanical data of a possible barrier film format based on PEt nylon/PE

Gauge (jim) Yield Oxygen permeability (cm3 m-2 per day) WVTR (g m-2 per day at 38°C 90% RH) Ultimate tensile strength (kg cm-2) MD

TD Elongation (%) MD

TD Puncture resistance (g)

Impact Fso (g)

Nominal

80 (± 10% tolerance) See sizes

<70 <6

375-400 300-350 600-650 600-650

1200-1400

>600

MD, machine direction; TD, transverse direction.

4.4.3 Microwavable packs

Test method

ASTM D882-83 ASTM D882-83 ASTM D882--83 ASTM D882-83

3 mm diameter ball puncture

Davenport falling

The range of softening temperatures of polystyrenes and amorphous polyesters are not sufficiently above the temperatures produced within the food products packaged for microwave heating; this makes these and many films unsuitable for microwaving thermoformed trays as they soften and become distorted to varying degrees with he at. Until recently, the only truly suitable system for microwavable MAP packs was the use of filled and lidded premade CPET trays. However, there is one patented system utilizing a UPVC tray that does produce a truly microwavable MAP pack.

Freshcap MW (Figure 4.5) uses conventional PET-based lidding and UPVC-based trays, but with a heat-resistant inner web flexible coextrusion harnrnock that supports the product while cooking. The base tray remains cool to the touch allowing handling, and the lidding can be pierced if desired before cooking. Developed in conjunction with Multivac Sepp Hagenmueller KG, West Germany, Lawson Mardon Packaging-Flexible in the UK are the sole suppliers of the system.

4.4.4 Technical problems

One potential problem that can only be resolved by calculation or laboratory evaluation is the gas and water vapour transmission rate of a particular laminate. With the exception of coextruded PPs, films are combined and/or coextruded to provide the specific range of properties required for a particular product/machine/process mix.

Part of the problem is that the values supplied by film manufacturers only relate to the units used in their market, hence for any given film they may be quoted for a particular gauge, commonly 25 11m or 1, 2 or 3 mm.


Mlcrowavable, heat resistant inner barrier film (hammock) supports the food.

PRODUCT

Barrier lidding film, with anti-mist performance available. Can be produced plain or printed. ,

frozen or chilIed in a controlled atmosphere

Insulating air gap prevents heat transfer from food to base, Outer semi:rigid tray formed from the reel maklng tray cool to the touch and keeping food hot longer. ensuring efficlent high speed production.

Figure 4.5 How Freshcap MW works. Freshcap MW appears similar to other thermoformed packs except that it has a microwavable inner barrier film that suspends the food in the pack like a hammock. This creates an insulating air gap between food and base, keeping the tray

cool to the touch even when the food is boiling inside.

Units vary considerably and may, for example, be in cm3 (NTP) cm-2

S-1 cmHg-1 for gases or g m-2 25 .um-l atm-l per 24 h for water vapour, or indeed in several other units. Additionally, temperate, tropical or ambient conditions may be used in the test. Therefore, it is important to compare like with like.

Another point for consideration is the modification of pack performance, in barrier terms, caused by thinning during forming or any imperfection in seal areas, e.g. the 'piping' or minute localized areas of access possible in a coextruded pp crimp seal. Whatever theoretical barrier format is chosen, its performance can only be ascertained by properly conducted shelf-life tests on the packages as produced on the actual production line.

4.4.5 Testing MAP packs

In the design and production of all barrier packs the properties of the constituent materials have usually been carefully investigated. Encouraged by research and testing organizations, such as Pira International and C & C Research Association in the UK, many companies now measure the barrier properties of complete packages. This is particularly important where the packaging is a lidded thermoformed tray. Improved test procedures are now available to determine transmission rates of gases and water vapour at the specific conditions of temperatures and humidity anticipated in distribution. Since leakage caused by either pinholes in materials or poor sealing is more common than many packers realise, such measurements can give good control of the packaging line.

Pinholes and leaks can also develop through stresses in transport, and such handling can be simulated on modern vibration tables and again provide a means of controlling the quality of delivered packages.

With respect to laminates, the calculations given on p. 402 of Paine and Paine (1992) allow a calculation to be made for a laminate taking 'standard'


permeability values to arrive at a final value for a given laminate of chosen film gauges. The following equation can be used to estimate the barrier properties of combinations of film.

1 1 1 +

where P1am is the permeability of the combined laminate of thickness L; Xl

and X2 are the respective thicknesses and PI and P2 the respective unit permeabilities of the layers 1 and 2. When the actual transmission rates for each film are known, this reduces to:

1 1 1 +

Example. With two films laminated together, one having a MVTR (moisture vapour transmission rate) of 18 g m-2 per day and the other of 5 g m-2 per day under tropical conditions at 38°C and 90% RH (relative humidity), the nominal permeability of them together as a laminate would be:

1 1 1 18 + 5 --=-+-=--P1am 5 18 90

P1am = 90/23 = 4 g m-2 per day

4.5 Seal system and quality

There are five main forms of modified atmosphere packs, namely:

• thermoform and lidding; • premade trays and lidding; • bulk gas packaging; • vertical form-fill-seal; and • horizontal form-fill-seal.

Each form suits a particular market or range of products. While there is some overlapping, for example with bakery products or meats, the areas are well defined. The films used to make the packages need to be chosen carefully to suit product and machine, because within the five forms, five different sealing techniques may be used.

Whatever the primary barrier materials used, and whatever changes occur within the pack as a result of respiration of the product (and bearing in mi nd that temperature and its control infIuence the result enormously), the most fundamental factor in pack stability is the integrity of the sealed areas.


The sealing systems currently in use are:

• reciprocating heated die pressure against a resilient face; • reciprocating heated die pressure against a resilient face with facility for

snorkel withdrawal; • reciprocating sealer bar against a release coated pad; • reciprocating crimp patterned jaws; and • rotary heated crimp patterned wheel.

It will be appreciated that without an understanding of sealing characteristics, films and machines cannot be suitably matched. Indeed, failure in this area negates a11 efforts to create a valid MAP pack.

A seal is made at the interfaces of the films by the bonding or intermixing of polymer film and/or a coating. This occurs at a given temperature of that interface. To create the correct interface conditions, temperature, dweIl time and pressure must be correctly related; there is some tolerance or variation permitted on any one factor but this must be compensated by another. Therefore, it is possible to run with a higher or lower sealing temperature - within a given range - as long as pressure and time are adjusted to compensate.

This means that the main reasons for seal failures, once the correct interface temperatures have been achieved, are likely to be in those areas where positive contact has not been made owing to uneven sealing faces, contamination in the seal area or internallexternal pressure displacing the seal before it has set. Compensatory adjustments can be made to improve seal quality. Knowledge of both film and machine are essential, but the values will be very different for a thermoform and either vertical or horizontal form-fill-seal machines.

Subjectively ascertained seal strength can be assessed rapidly off the machine by manually pulling the seal apart (hopefully indicating a total surface disruption by virtue of the separation of welded interface surfaces). Objective comparison of seal quality should be based on a standard width of the seal area, usually 25 mm. Sampies made under standard sealing conditions, say 140°C, for 1 second at 20 psi pressure on 25 mm wide specimens, are tested by pulling the tails apart on universal tensile testing equipment. With narrower width seals, (such as a typical5 mm thermoform edge seal), testing on measuring equipment of this type, the results can be grossed up and compared with a standard 25 mm value.

Peelable seals do not weId, of course, but a value of 350 to 800 g per 25 mm width of seal would be a reasonable target. Additionally, with a laminate, the laminate bond strength will contribute to the achievable pack seal strength. However, in this instance, measuring a strip peel will not necessarily match what could be a much higher resistance to parting from within the pack, i.e. the angle of stress will be much less.

Laminate bond strength is very dependent upon the substrates and


would always release at the lowest potential level. A UPVCIPE laminate might provide a bond exceeding 1200 g 25 mm-1 while a PETIPE lidding laminate may provide between 400 to 500 g 25 mm-I . In practice, the particular specifications chosen should be discussed with the supplier, together with the print and metallization effects on peelability. All these, together with film gauges should be confirmed, before production specifications are agreed.

With coextruded pp films, conventionally a weId seal is achieved that fails by film tearing at or within the seal area.

4.5.1 Peelable seals

As discussed above, totally valid seals are essential and the limitations in certain areas have also been noted. In general, this means a hermetically locked up or completely bonded seal with a high resistance to opening, yet against this a peelable seal may be required as a marketing feature. While there are several reclosable systems available for vertical and horizontal form-fill-seal packaging, including cold seal adhesives or mechanical grip devices, it is recognized that they are inadequate for modified atmosphere packs, although they are often considered suitable to match the pack performance barrier level of a particular basic web or laminate, for instance a coextruded pp at around 25.um thickness. However, with thermoformed trays, in line or as a preformed container, there is a method to obtain a peelable seal that does not make it necessary to cut around the inside rim of the pack body to remove the lid. If totally compatible plastics (normally identical polymers) are used to provide the sealing medium of lid and tray, a strong total seal will be obtained. However, by selecting appropriate dissimilar polymers and using a conventional flat-heated pressure seal, a peelable seal can be obtained. The polymers have sufficient affinity to seal but not to totally interrnix, thus the lid and tray separate cleanly at the polymer interface.

It is necessary to design the tray, sealing faces and cutting punch of the machine to provide a tab on the tray corner to facilitate starting the peel, an extra cost to add to the price of the pack. Also it is strongly recommended that interlayer print is not allowed into the seal area to weaken the bond at this area of stress on opening. There are several peelable seal systems available for retort packs or other forms of lidding application, using prescored trays, profiled seals or alternative sealing media, but these should not be confused with the system commonly used for MAP.

Peelable seal strength is measured in grams for a standard seal width of 25 mm, and target values between 600 and 1200 g 25 mm-1 are generally suitable. The peel should provide a rubbery, even load on the measuring device, whether a tensile tester in the laboratory or a spring balance in the


'field'. It should be noted that a typical thermoform machine side seal would be nominally 5 mm wide; therefore, a longitudinal peel would provide only one-fifth of the primary target value.

4.5.2 Antifag (AF) praperties

In marketing terms, the fogging or misting of a modified atmosphere pack impairs the presentation of wh at should be as attractive a pack to the potential purchaser as possible.

Primarily confined to the dear lidding face, the 'mist' reduces darity and visibility of the product and is composed of the free moisture within the container condensing on the inner lidding face as a result of temperature differentials. The problem in its most serious form is seen in chili cabinets, the pack position within any given cabinet improving or worsening the degree of occurrence, depending upon the cabinet design. With a general temperature level in the range of 0-5°C, a defrosting cyde is needed for the cabinet to provide 24 h a day operation, without excessive frosting and a build-up of ice within the confines of the cabinet.

Defrosting cydes vary depending on design, but generally the cold air flow is interrupted for a given short period for relatively warm air to provide defrosting. Within each package, the product (e.g. meat or fish) lies dose to, but not actually touching, the dear lidding 'membrane', its bulk temperature providing a thermal he at sink not significantly affected by the relatively brief defrost eycle. However, the defrosting air flow is in contact with the lidding film, which, with a low thermal inertia, warms to the defrosting temperature. Thus, with the product bulk temperature and the lidding film temperature differing by several degrees (the actual amount depending upon cabinet design and the position of the individual package within the cabinet), misting develops on the film face dosest to the product bulk. When the film and product temperatures reach equilibrium again, the misting dears, only to return when the cyde recurs.

Antifog coatings are produced by film manufacturers or converters and create a high surface tension with hydrophilie properties that allows the condensed moisture to completely wet the undersurface of the lidding. Techniques and processes are confidential to the film manufacturers and converters and are based upon choice of polymers or polymer plus additional antifog coating.

However, it can be said that surfaces like those presented by conventional polyvinylidene coatings react satisfactorily on their own if the SUrface tension is in the right range. Conversely, in the choice of polymers to provide a MAP peelable seal, antifog coating is not an option.

Interpretation of the required antifogging performance varies between individual food suppliers. Because of variable storage conditions and handling, the moisture content of a particular product, and the surface


tension arrived at (so as not to impair other film or laminate characteristics) whenever comparisons are made of pack-to-pack, store-to-store, or filmto-film, it is important to ensure that evaluations are made objectively on exactly the same basis and at the same time. Details of practical test methods for antifog properties are contained in Appendix 4.B. Storage conditions for films with antifog properties are commented upon in Appendix 4.C.

4.5.3 Printing and labe/fing

There are a range of possibilities for printing or labelling modified atmosphere packs. Labels are more often used on shorter runs of thermoformed meat packs, while flexographic or gravure printing (surface or sandwich print) is used on all forms of packs other than bulk gas packs.

Labels can be applied on the thermoform machine itself or more often by aseparate labelling machine in-li ne with the process machinery, possibly after a weighing unit linked to the label applicator, hence the label carries product weight data. Lidding specifications are suitable for label adhesion; however, to maximize the efficiency of application, limited contact should be over the largest possible area. The significance of this statement relates to the practice of setting the gas-flushing unit to supply a marginal over pressure, perhaps 0.1 bar, to ensure a full stable product/gas fill after a pack setting period. This will provide a convex lidding surface to which the label is applied, the effect of which is to reduce the initial contact area.

One popular machine for this use blows the flat label from the head a short distance, about 10-12 mm, to the convex lidding surface, making the initial contact with an area less than 100%. The label usually does settle flat, however, and can be handled without disturbance. One suggestion is to use deep-freeze grade labels for MAP packs, the tack value of these compared with standard labels under cold factory and storage conditions providing a safety factor. Should labelling conditions warrant it, some companies provide a soft roller in-line with the labelling unit, to ensure 100% contact area.

A system of printing available to the packer is use of one of the pressure date co ding units commercially available. One of the thermoform machinery manufacturers supplies a sophisticated multilane print unit providing a limited choice of colours. Reciprocating date coding machinery can be easily utilized on vertical form-fill-seal machinery, but a continuously running rotary unit must be used for horizontal form-fill-seal equipment.

These units all have the advantage of printing the stationary web with the correct repeat distance. Preprinted reels in flexographic and gravure processes may need an automatic registration system, but not all machinery companies produce them, and indeed many thermoform


machines on the market are of mechanical-drive formats. Should a random repeat design be printed, no print-to-pack registration is needed. However, if a design is intended to lay in register with a tray or flow-packed product, a technique to create registration is needed on the machine.

Taking thermoforming first, conventionally the design is positioned correctly over the tray, and the lidding web sealed to the base tray. The pack is now cycled forward towards cut-off. With the print repeat length being printed marginally less than the machine cycle, this draws the thermoformed base web forward, the lid ding web pulling off the braked reel of printed laminate. Consequently, the printed design is stretched into registration for sealing and cut-off. A photoelectric cell triggers a web brake that traps the lidding in position.

The tolerance in packaging film terms is smalI, hence it is imperative that the MAP producer knows accurately the pull length of each of his machines - not only the nominal value, wh ich can be modified by wear of parts. The printer must then print to between the mutually agreed repeat length, theoretically the exact size with the web under unwind tension, down to the accepted tolerance range with which the packaging machine can cope. Plastic films are elastic; therefore, great care is necessary to control linear web movement during printing and/or laminating. The recognized trade print repeat tolerance standard is, +0 to -4 mm per 1000 mm for conventionallidding webs and -6 mm per 1000 mm for more extensible specifications. It is interesting to note that this reduces to +0 to -2 mrn per 1000 rnrn for paper lidding webs used in markets other than MAP. It should be recognized that any print-repeat dimension that exceeds the actual pulliength of the machine by even a very small amount will not hold register, the design creeping back by that small amount on each cycle. The print repeat length should be recorded under a light tension that matches the unwind load on the particular thermoform machine.

There remains one area of potential controversy between purchaser and supplier of printed webs for thermoform use. In the gravure printing process, the printing cylinder circumference is set by the print repeat requirement. When flexographic printing is specified, the process is different: the printing press drives the print cylinder through a ge ar train and the circumference must relate to the number of ge ar teeth. The effect is that standard progression for a flexographic printing press may be in 5 mm steps on the printing cylinder circumference, hence while the packaging film buyer has adefinite pull length for his thermoform machine, the printer must progress his repeat lengths in 5 mm steps. It is only if the two size requirements are close enough for the print manager to know that he could print that design between +0 to -4 mm tolerance that flexographic printing is suitable. Gravure printing is the alternative. With respect to horizontal and vertical form-fill-seal machinery, print registration


is undertaken in different ways. In all cases a ± 1 or 2 mm tolerance per repeat is generally satisfactory.

4.5.4 Supplier/packer relations

It is essential that specifications commonly negotiated between supplier and purchaser constitute a practical document derived with the best expectations, but while the top value is the target for production, a working average that will prove satisfactory must be achieved commercially. This average may be achieved by producing quantities of quite usable film but working from the lowest practical value, hence providing a tolerance range. Therefore, while targeting the highest value, and monitoring production to ensure the total performance is acceptable, the specification must allow for that lower figure that makes up the average value. It is incorrect to provide a specification showing average or above average values only, because this means that everything below average is out of specification. Setting the specification on minimum practicallevels, knowing that average and maximum values lie above this grade, creates a workable document.

A draft working specification containing adequate detail for practical purposes is illustrated in Appendix 4.D. Sometimes, however, more data are required for the record or comparative purposes. Then available data can be extended to cover standard sealing conditions, texture, extended range barrier properties, ultimate tensile strength, haze and gloss. However, all of these properties must be correctly interpreted in terms of the performance of the film or laminate on the machine and in relation to the product.

Film manufacturers supply tables of physical properties for their films covering, for example, ultimate tensile strength, elongation, puncture resistance, impact strengths and coefficient of friction. Bear in mind the range of specifications and combinations of films discussed in this section; only one example has been included. It is suggested that should this area be of major interest, the subject is discussed with a supplier of packaging materials.

4.6 Legislation and the environment

Much recent legislation in Europe and North America has been linked to environmental considerations. This has become an emotive subject and careful study of relevant documentation is necessary to comprehend fully the full significance to packaging and food-producing companies.

With respect to plastic films, the most significant areas are in the reduction of waste generation, reuse considerations, possible recycling and


food contact regulations. There are differences in current European and North American legislation, yet the eventual aims are similar. The food contact area relates to the migration of certain constituents of film that may pass into the foodstuff while in contact. The EC directive 90/128/EEC dated 23/02/1990 relates to materials and articles that are made of plastic and are intended for contact with foodstuffs. In the US, environmental impact of packaging is currently astate legislature issue, while indirect additives to food by way of packaging materials is considered under the Threshold of Regulation, meaning the migration of a substance is below the level of FDA action. This 'threshold' amount is a dietary concentration of 0.5 parts per billion or less.

When an EC directive is issued by the community, in order for it to become valid, each member country must adopt it into national law; the individual governments are expected to understand its effect on competitive markets and to satisfy the customers, consumers and critics. Table 4.12 briefly outlines the main points of the EU Packaging Directive.

All producing companies have to formulate company policies recognizing the need to study the environmental effects of their processes and

Table 4.12 Summary of the main EU Packaging Directive Requirements

Objectives Aim is to:

1. Reduce the overall impact of packaging on the environment by: (a) reduction of packaging material used at source, i.e. by good design; (b) reuse of refillable packages for multiple journeys; (c) recycle materials either mechanically or chemically; (d) energy recovery by efficient incineration; (e) utilize suitable materials in composting.

2. Ensure National Legislation does not create obstacles to trade or distortions of competition.

Recovery for reuse, refilling, recycling, energy recovery and composting Aim is that within 5 years of implementation into nationallaws:

1. between 50 and 65% of packaging by weight is recovered (i.e. some value is regained (the French call it 'valorisation');

2. between 25 and 45% of packaging by weight shall be recycled with a minimum of 15% of each material being recycled.

Note: There is so me leeway and discussion permitted on the starting percentages and targets will be revised after first and subsequent 5 year periods.

Marking A system is to be developed that will indicate the nature of materials used to facilitate recovery, etc.

Standards Committee European de Normalisation (CEN) is to draw up standards for 'essential requirements' such as minimization of weight and volume, suitability for use, recycling energy recovery, etc.


products, from gauge reduction and modification of processes to ensuring compliance with the applicable regulations. However, the value of packaging in its own right must be recognized, as must the increasing demand for plastic packaging, for example for precooked convenience foods, improved handling and longer shelf-life requirements, and the widening potential for MAP and increasing food-quality demands, all of which help to create an increasing environmental awareness.

One significant point often overlooked is the wastage incurred in not packaging efficiently. One re port refers to a Third World food spoilage rate of up to 50% against 2-3% in the sophisticated societies, owing to inadequate packaging, storage and distribution systems in less-developed countries (Testin and Vergano, 1990).

As mentioned above, when the EC issues a directive, individual countries issue their own legislation. Responsible companies recognize their overall responsibility within this framework. In 1991, the Lawson Mardon Packaging Group adopted a policy in support of 'the relationship between its business activities and the Global environment', with the following stated objectives:

• to ensure that the operations of all companies within the group are always fully in compliance with, or exceed, applicable environment regulations;

• whenever possible, to support the environmental improvement programmes of both customers and suppliers;

• to strive to reduce waste generation in our production processes by adherence to the principles of waste prevention, reduction, reuse and recycling;

• wherever technically, legally and economically possible, to maximize the use of postproduction and postconsumer recycled materials in consultation with our customers.

Additionally, bearing in mind legislation relating to materials in contact with food, when asked, Lawson Mardon would confirm that packaging materials supplied by the company comply with The Materials and Articles in Contact With Food Regulations (1987) (Statutory Instrument 1987 No. 1523). This of course would be adjusted to conform to any future implementation of EC directives in the UK.

In North America, the Canadian packaging industry has prepared a draft guideline for preferred packaging practices in response to a demand for a 50% reduction in waste packaging sent to landfill. The US Environmental Production Agency is taking a serious look at incineration and landfill policies. Generally, while much was te at present ends in landfill, incineration and recycling will play a much more significant part in the future. From aglobai perspective, the International Climate Treaty bears watching by all nations because in December, 1997 the treaty could


become a legally binding protocol to control greenhouse gas emissions; this will affect municipal solid waste.

Acknowledgement

T.D. Frey, Packaging Partners, Oakland, CA provided the comments on microperforation technology. Larson Murdon Packaging-Flexibles IS

acknowledged for their assistance with preparation of this chapter.

References

Frey, T.D. (1997) High gas permeability key to 'passive' CAP for produce. Packaging Technology and Engineering 6(3), 40-42, 45.

Paine, F.A. and Paine, H.Y. (1992) A Handbook 01 Food Packaging, 2nd edn., Blackie Academic & Professional, Glasgow.

Testin, R.F. and Vergano, P.J. (1990) Packaging in America in the 1990s. Institute of Packaging Professionals, Herndon, VA.

Appendix 4.A Film calculations

Figure 4.A.1 shows the dimensions of a roll of film. Any buyer/user of packaging film needs to be able to calculate film or laminate requirements and usage. The following dimensions are used:

L = roll length in metres R = repeat length in met res W = roll width in millimetres Y = yield in m2 kg-l

S = substance in g m-2

T = film gauge in micrometres D = roll diameter in millimetres C = core diameter in millimetres (O.D.) P = film price in f kg-1

K = a constant for calculating reellength = 0.7855

Yield (Y) is 1000/S. Substance (s) is 1000/Y.

Example 1: top web 15/60

Roll diameter 240 mm, core diameter 88 mm, film gauge 75 {lm, repeat length 0.160 m.


Figure 4.A.l Diagram of a roll of film.

1. Wh at is the length of film on the roll?

(D - C) X (D + C) X K (240 - 88) X (240 + 88) X 0.7855 L= = ------------------------

T 75

=522 m

2. How many impressions in the roll?

L 522 Impressions = - = --

R 0.160

= 3262.5

Example 2: base web 300170

Roll width 420 mm, print repeat length 0.2 m (three lanes), substance 480 g m-2 , film price f2 kg-1.

3. What weight of film is required to make 1000 packs?

Weight per 1000 cydes = W (m) X R (m) X S = 0.42 X 0.20 X 480 = 40.32 kg

Note: This is per cyde, therefore at three lanes across this would equate to 3000 packs, conversely 40.32 kg + 3 = 13.44 kg/1000.

4. How many packs per kilogram of film?

1000 Number of packs kg-1 =

Weight per 1000 packs

1000 =

13.44

= 74.4 packs kg-1


5. What is the cost per square metre of film?

SXP 480x2 Cost (pence m-2) = -- = --

10 10 =fO.96

6. What is the cost per impression or form?

Cost per impression = Cost (m-2) + Area of impression (m2)

= 96 P X (0.2 X 0.140) = 2.69 P

Appendix 4.B Antifogging properties

Test method I: rapid beakerlsteam test

11 A Beaker test.

Purpose To provide a rapid method of reel-by-reel quality assessment.

Equipment Parallel-sided ceramic container with capacity 500 ml and diameter between 75 and 100 mm.

Method Fill container to within 10 mm of top with water at 90°C and allow to cool to 85°C before commencing test. SampIe is placed on container in full contact with lip avoiding scraping on lip and left for 3 seconds.

Result Any sampIe found to be fog-free after 3 seconds is deemed to pass.

11 B Steam test

Purpose

Equipment Method

Result

To provide an alternative rapid method for reel-by-reel quality assessment. Standard household kettle without auto switch. Hold the antifog surface at right angles to steam jet at approximately 450 mm from spout of kettle for 3 seconds. Any sampIe found to be fog-free after 3 seconds is deemed to pass.

The above rapid test methods require an experienced operator; in case of doubt, material should be subjected to testing by alternative methods, to be negotiated as necessary.


Appendix 4.C Recommended storage conditions for MAP materials

To maintain the quality and performance of our antifog top web and thermoform base web the following 'ideal' storage conditions are recommended.

Relative humidity (RH) Temperature

30-50% 15-20°C

The reels should be stored on their ends (to prevent localized pressure bands) out of direct sunlight and the RH and temperature should be kept as constant as possible. Strict stock rotation should be maintained to ensure 'first in, first out'.

There are two possible problem areas specific to MAP materials that may come to light if reels are stored under excessively cold or damp conditions.

Top web. If a reel is brought from storage at temperatures approaching or below freezing and immediately mounted on the packing machine, the antifog properties will show a marked deterioration. Also the layer of condensation formed on the web could impair sealing.

However, this problem can be reduced by allowing the reel to thoroughly warm to anormal acceptable temperature. An acclimatization period in excess of 24 h is recommended.

Tests have shown that a reel treated in this way will retain 90% of its original antifog performance even after storage under deep freeze conditions.

Base web. Again problems may occur if a reel is taken from storage conditions approaching freezing and thermoformed before the material has warmed. As with top web, condensation may form, which will impair sealing; however, the major risk will be of shattering during thermoforming. Again, these problems can be overcome by allowing the material to acclimatize thoroughly. Unfortunately owing to the thickness and nature of the base web, this can take a considerable time probably in excess of 48 h dependent on reel size.

The above are problems that may occur if storage conditions are of the nature described, but obviously only the customer can determine if the storage conditions are causing problems on the packing line.

Appendix 4.D Draft specification for MAP reels

Scope. This specification refers to unprinted top and bottom webs designed for MAP use.


Top web

Materials. PVdC-coated polyester film, adhesive laminated on PVdC surface to LDPE with antifogging surface.

Bond strengths. Minimum bond strength between film plies to be 300 g per 25 mm width of strip when measured in both machine and transverse directions on a tensile testing machine with jaw separation rate of 200 m min-1.

Seal strengths. Top web to bottom web, 145°C 20 psi-1 S-1 on BCL flat bar sealer, minimum 500 g 25 mm-1•

Anti[ogging properties. Test method: rapid beakerlsteam test (Appendix 4.B); frequency of test, each slit ree!.

Retained solvent levels - top web. Maximum 20 mg m-2 ofwhich no more than 10 mg m-2 should be ethyl acetate and trace solvents. Maximum level of toluene 2 mg m-2 . Test method: FCA retained solvent determination method.

Bottom web

Materials. Unplasticized calendered PVC sheet adhesive laminated to LDPE.

Bond strength. Minimum bond strength between film plies to be 500 g 25 mm-1 width of strip when measured in both machine and transverse directions on a tensile testing machine with jaw separation rate of 200 mm min-1•

Retained solvent levels: bottom web. Maximum 20 mg m-2 of which no more than 10 mg m-2 should be ethyl acetate and trace solvents. Toluene should only be present as a trace solvent. Test method: FCA retained solvent determination method.

General

Width o[ stit reels. Top web: ±1 mm. Bottom web: width and tolerance to be specified by customer. Difference between limits should not be less than 1 mm.

Reeling direction. Sealing surface wound inside (unless otherwise instructed) .


Cores. 76 mm inside diameter.

Labels. Applied to outer wrap and inside surface of reel core. Will include material specification, weight, width, work reference number.

Wrapping. Reels to be sleeved in clear polythene secured in position with plastic bungs.

Palletization. Where appropriate to be palletized and labelled with specification and date of production. Pallet to be stretch-wrapped and will contain packing slip giving details of reel weights.

Material status. Materials used will be of food grade and comply with the appropriate FDA or BGA regulations.

Addendum to chapter

Specification development

There are several new specifications under evaluation, with one international company providing sophisticated answers to problems posed in several food product areas. The following combinations of materials are eminently suitable for the targeted markets:

1. Foamed lightweight polypropylene, SiOx-coated polyester and a high mineral-filled polypropylene are selectively married up to provide suitable features. The lidding is based on the SiOx-coated polyester, the base tray/film combination providing an adjustable format for modified atmosphere applications, for example, the red meat market.

2. Frozen food is catered for with a foamed pp tray that is microwavable up to 90°C.

3. Sliced meats are packaged with a mineral-filled base film with a paperlike quality allied to a high clarity lidding. Excellent barrier properties are offered provided by a combination of ultra-thin SiOx-coated lidding film with an EVOH barrier base film.

4. A flow wrapping grade, polyamide/SiOx/PE, provides high barrier properties including low oxygen transmission allied to high puncture resistance.

Poultry, meat and bread packaging can be served by this specification.

5 Quality assurance of MAP products I. ALLI and L.M. WEDDIG

5.1 Introduction

The assurance of safety and quality of MAP foods presents many special challenges to both the food technologist and the quality professional. In the preparation of these foods, both safety and quality issues must be addressed. Concerns to be addressed include raw materials and ingredients, packaging materials, the manufacturing and packaging processes, finished package evaluation, post-production storage and distribution and, finally, maintenance of these safety and quality attributes until consumption of the MAP product.

MAP products are able to meet consumers' multi-faceted demands for safe, fresh or fresh-Iooking nutritious products, conveniently packaged in terms of both package size and package form, and at an affordable cost. As more MAP foods become available, there is a tremendous requirement on quality ass uran ce (QA) of MAP to satisfy consistently these consumer expectations.

The variety and range of foods now available as MAP products make it difficult, if not impossible, to take a commodity approach to safety and quality of MAP foods. MAP foods include meats, poultry, fish, entrees, pasta, baked goods, fresh produce, egg products and sandwiches. These items may be in the raw, partially cooked or fully cooked state. The emergence of sous-vide products from processes involving only minimal thermal processing is viewed as an area of MAP in which safety and quality relationships are critical. Daniels (1991) describes these 'new refrigerated foods' as having the desired quality and convenience attributes but with high 'fragility'. Because of the wide range of MAP products, a principles approach is the most logical method for describing the application of current concepts and procedures to ensure the safety and quality of MAP foods. Traditional, but still extremely important, areas of testing and analysis, and methodologies associated with routine quality control and inspection, will be presented only briefly.

5.2 Safety and quality of MAP foods

Manufacturers of MAP foods, like all other processed foods, have both the legal and the moral obligation to ensure that their products do not present

QUALITY ASSURANCE OF MAP PRODUCTS 103

a public heaIth risk to the consumer. Monitoring pro grams of government regulatory agencies are designed specifically to determine compliance with regulations dealing with this public heaIth aspect. In addition, MAP foods that have presented a safety hazard in the past can expect to generate little confidence in the consumer. At the same time, food manufacturers recognize that quality characteristics of food (e.g. flavour, texture, color, nutritional value, etc.), are major factors that affect customers' expectations and, consequently, purchasing decisions. These desirable quality characteristics can serve to reinforce consumer 10yaIty and confidence in certain brand names.

It is clear that by their very nature MAP foods represent a category of products in which both safety and quality are of crucial concern. In response to arecent survey of large food processors, microbiological concerns as potential food hazards ranked at the top (Swintek, 1991). Surveys of consumers have shown consistently over the years (Penner and Kramer, 1985; Wolf, 1992) that microbiological problems associated with food spoilage are a major concern. In the USA, it has been established (Titus and Talbot, 1991) that microbiological safety of foods continues to be a perennial concern among federal regulatory agencies; and new packaging and processing technologies are cited to be among the emerging issues relating to food safety and food quality. Wolf (1992) singles out 'the use of improved films and trays with controlled or modified atmosphere conditions for fresh or minimally processed food' as an important and new food safety issue. The increasing appearances of refrigerated, increased shelf-life MAP foods are cited as examples that directly relate to food safety issues.

The above evidence points to the tremendous concern firstly for the safety assurance and, secondly, for the quality assurance of MAP foods. It is also clear, on the basis of contemporary safety ass uran ce and quality assurance philosophies, that traditional screening or inspection approaches for food safety and quality will not suffice to guarantee that MAP foods will meet the safety and quality requirements. The current hazard analysis and critical control point (HACCP) approach is seen as the desirable and appropriate me ans for addressing food safety, while a comprehensive and integrated total quality approach is seen as the means to achieve product quality. A detailed treatment of the application of these two approaches is provided in the following sections.

5.3 Application of HACCP to MAP foods

Much has been written and said concerning HACCP and its potential for use in food safety ass uran ce programs since the mid-1970s. Following its initial use by the Pillsbury Company, the point has now been reached


where formal recognition and endorsement of HACCP has been granted by both international and national organizations that address food safety issues, as weil as by national governmental agencies that routinely regulate food processing industries through development and enforcement of food safety regulations.

HACCP is a system that emphasizes a preventive approach to food safety. Significant hazards that contribute to the safety of the product are identified, and emphasis is placed on preventing, eliminating or reducing these hazards to an acceptable level at some stage in the manufacturing or production system. Minimal thermal processing is a desirable feature of MAP foods in order to retain certain quality attributes. Because the potential microbiological hazards associated with this minimal thermal processing are a major concern to manufacturers, consumers and regulatory agencies, the HACCP approach appears to be weil suited to ensure the safety of MAP foods. In fact, information on the use of HACCP for MAP foods has already appeared in the literature. Smith et al. (1990) proposed the use of HACCP to ensure the microbiological safety of a sous-vide producL A Canadian Code of Recommended Manufacturing Practices for PasteurizedlModified Atmosphere PackagedlRefrigerated Food (Agriculture Canada Working Group, 1990) endorsed the use of HACCP as a means to obtain microbiologically safe foods from these processes. These examples can serve as useful generic guidelines for development and implementation of HACCP for similar MAP products.

The recognition of HACCP by several national and international organizations that address microbiological safety of foods is evident by the World Health Organization's (WHO) International Commission on Microbiological Specification for Foods (ICMSF) recommended use of HACCP for the prevention and control of food-borne salmonellosis (Simonsen et al., 1987). In addition, Codex Alimentarius has an HACCP working group that is in the final stages of revising the Codex Guidelines for the Application of Hazard Analysis Critical Control Point System (Codex Alimenarius, 1996). In the USA, the National Advisory Committee on Microbiological Criteria for Foods (NACMCF) - an expert advisory panel to government agencies - has established guidelines for the application of HACCP (NACMCF, 1992). The document, Hazard Analysis and Critical Control Point System was adopted by NACMCF in 1992 and has served as the standard in the USA. Tbe NACMCF is currently redrafting their HACCP document to provide more guidance to food processors.

The activities of these recognized, scientific and technological organizations has led to actual procedures for implementation of HACCP. The International Association of Milk, Food and Environmental Sanitarians has published (IAMFES, 1991) procedures for developing and implementing the HACCP system; this publication also serves as an excellent


reference for someone who is faced with the challenge of developing a HACCP program. The USDAIFDA Foodborne Illness Education Information Center associated with the US National Agricultural Library now maintains a website with a database of HACCP training and resources (http://www.nal.usda.gov/fnic/foodborne/foodborn.htm).

Although most of the HACCP activities, particularly those relating to endorsement, recommendations or implementation, have been relatively recent - since the mid-1980s - it should be pointed out that the HACCP concept go es back to the late 1960s when HACCP was used as part of the food safety assurance program for the NASA space flights at that time. The Pillsbury Company, which was the food industry partner in this project, essentially served as the pioneers in formalizing HACCP into a workable program; many of the general and specific considerations that must be addressed for the use of HACCP as a safety assurance program have been documented (Pillsbury Company, 1973). Soon after, the US Food and Drug Administration (FDA) adopted an HACCP-based inspection system for processing plants producing low-acid canned foods (Kauffman, 1974).

At the present time, HACCP has become formalized to the point where a set of seven principles (the seven principles of HACCP) serves as the basis for developing a HACCP program. While the wording of the seven principles may vary depending on the authoring organization, the intent is essentially the same. The seven principles as defined in NACMCF's document (1992) are:

1. Conduct a hazard analysis. Prepare a list of steps in the process where significant hazards occur and describe the preventive measures.

2. Identify the critical control points (CCPs) in the process. 3. Establish critical limits for preventive measures associated with each

identified CCP. 4. Establish CCP monitoring requirements. Establish procedures for using

the results of monitoring to adjust the process and maintain control. 5. Establish corrective actions to be taken when monitoring indicates that

there is a deviation from an established critical limit. 6. Establish effective record-keeping procedures that document the

HACCP system. 7. Establish procedures for verification that the HACCP system is working

correctly.

Development of a HACCP program on the basis of these seven principles requires a clear understanding of hazards, hazard analysis and subsequent selection of critical control points since these represent the cornerstone of HACCP. A hazard has been defined as 'a biological, chemical or physical property that may cause a food to be unsafe for consumption' (NACMCF, 1992). The hazard analysis is a means by which


hazards are identified and the significance of the hazard to consumer safety is assessed or evaluated. A critical control point is a point, step or procedure that if not effectively controlled may cause, allow or contribute to a hazard in the final product.

The significance of an identified hazard is assessed based on the risk (likelihood of occurrence) and severity (seriousness) of the hazard. Every aspect of the manufacturing system, including raw materials and ingredients, processing, distribution, marketing and finally use by the consumer, must be considered. Examples of factors to address during the hazard analysis for MAP products include the following points.

1. Food intended for an at-risk population, (e.g. the very young, elderly, infirm, or immunocompromised).

2. The presence of a sensitive ingredient (a potential carrier of a hazard) in the prepared product.

3. Lack of a processing step to eliminate or control for the hazard (e.g. a heating step to eliminate a microbiological hazard or a device such as a metal detector to control for a physical hazard).

4. The potential for recontamination before packaging of the product. This is a concern for chilled ready-to-eat foods that are heat processed prior to packaging.

5. The potential for abuse of product during distribution, retailing or consumer handling. This is of particular concern in minimally processed foods that are packaged under modified atmospheres hut is of relevance to all MAP foods.

Identified hazards that are not considered significant enough with regards to risk and severity should not be addressed in a HACCP program. It should be pointed out that hazards which have not previously been associated with the ingredients, materials, process and product could be overlooked during a hazard analysis. Because of this, HACCP plans are to be periodically reviewed to confirm that all potential significant hazards are being addressed.

An examination of MAP foods in general clearly indicates that identification and classification of hazards associated with this type of product is a critical aspect of assuring the safety of MAP foods. The fact that numerous MAP foods are made from sensitive ingredients (e.g. meats, poultry, fish, milk) indicates that HACCP can playa tremendous role in safety assurance of MAP products since in many of these foods the processing step does not result in sterile products and the potential for abuse, particularly temperature abuse, is very high.

The application of the seven HACCP principles has now become a somewhat formalized process. This process can be used to develop implementation activities for use of HACCP in the manufacture of MAP foods. A detailed example of the application of these principles is provided


by Smith et al. (1990) for a sous-vide meat/pasta producL Day (1992) provides an example for freshly prepared mushrooms. The Microbiology and Food Safety Committee of the National Food Processors Association (NFPA) has developed a generic model for the implementation of HACCP for chilled foods (NFPA, 1993). A generalized set of consecutive step-bystep activities that can serve as the basis for developing a HACCP program for any type of MAP product folIows.

1. Assemble a multi-disciplinary team to develop the HACCP plan. Team members should have appropriate knowledge and expertise to address the product and process.

2. Describe the MAP product in terms of its end characteristics, method of distribution, intended use, expected consumers and any special features or special instructions associated with its use. Examples of these characteristics include potential for abuse, pH, water activity, preservatives, type of packaging, shelf-life stability at a specified temperature, labeling instructions and any special distribution controls.

3. List all ingredients and raw materials that are used for manufacture of the MAP product including all packaging materials.

4. Develop a step-by-step process flow diagram that includes every step of the process under direct control of the manufacturer, including packaging and storage. Post-production steps dealing with distribution, retailing and consumer handling mayaIso be included. Verify that the developed flow chart accurately reflects the actual operations that are used for preparation of the product. Examples of flow charts developed for HACCP are given by Smith et al. (1990), Curtis and Huskey (1974), and NFPA (1993).

5. Carry out a hazard analysis to identify the potential significant microbiological, chemical or physical hazards associated with raw materials, ingredients and the various processing, packaging and postmanufacturing steps. Particular attention should be given to the steps that control package integrity and product abuse. After the significant food-safety hazards have been identified, preventive measures necessary for the control of the hazards are identified.

6. Determine the point(s) in the production of MAP food where control can be applied to prevent, eliminate or reduce to acceptable levels the significant food-safety hazards. This represents the critica! contro! point determination. For MAP products where headspace atmospheres are critical to product safety, this step could lead to the identification of critical control points relating specifically to the packaging operation. A formalized 'HACCP decision tree' for critical control point determination has been developed by the NACMCF (1992) and Codex Alimentarius Commission (1993) and is shown in Figure 5.1. Campden & Chorleywood Food Research Association


Question 1. Could preventive measure(s) ...... __ ------Modify step, process exist for the identified hazard? or product

+ + YES NO

+ Is control at this step -----I~ .. YES necessary for safety?

NO ~ NOT A CCP --. STOP

Question 2. Is the step specifically designed to eliminate or reduce the likely occurrenc. of a hazard to an acceptable level? ~

NO Y~

+ Question 3. Could contamination with identified hazard(s) occur in excess of acceptable level(s) or could these increase to unacceptable level(s)?

+ + YES NO ----1~~NOT A CCP --'STOP

+ Question 4. Will a subsequent step eliminate identified hazards(s) or reduce the likely occurrence to an acceptable level?

+ + YES ----I~ .. NOT A CCP--. STOP NO ----.. CRITICAL

CONTROL POINT

Figure 5.1 Critical control point (CCP) decision tree. Each question is answered in sequence. If STOP is reached, proceed to the next step in the described process.

(1992) has published practical guidelines on HACCP and The Food Processors Institute (Stevenson and Bernard, 1995) has published a workshop manual on HACCP, both of which include such adecision tree. A detailed set of questions that must be addressed for determination of microbiological critical control points for canned foods was proposed by Ito (1974). A similar set of questions for each type of MAP food produced needs to be developed.

7. Determine how the established critical control points will be judged to be in control by establishing the actual specifications and associated tolerances (critical limits) that are required to ensure control of the safety of the product.

8. Determine the actual testing, measurements and observations that are required to demonstrate control at each critical control point. This monitoring may require the use of statistical quality-control


procedures. Corlett (1991) describes in detail the monitoring of a HACCP system.

9. Establish the procedures that are to be carried out in the event that the monitoring of critical control points demonstrate that deviations from the criticallimits exist; this represents the obligatory corrective action that must be implemented once the monitoring indicates that an outof-control event has occurred. This step of the HACCP procedure is equally as important as the hazard analysis and critical control point determination steps, since the capacity to identify and correct for any safety problems that have occurred is crucial to the success of the HACCP program.

10. Develop a system for documenting the complete HACCP program, incIuding the detailed description of procedures to be followed, cIear instructions for personnel to follow, along with a cIear indication of responsibilities, the recording and analysis of all qualitative and quantitative data that are obtained from the monitoring procedures, and any deviations and required corrective actions.

11. Establish the process that will demonstrate that the HACCP pro gram has been properly developed and implemented to ensure a safe MAP product. The verification process provides the proof and documentation that the critical control points, critical limits, monitoring techniques and corrective actions are adequate to control the significant hazards and that the HACCP pro gram is being followed as written. The overall HACCP program will be verified periodically over time to ensure that any changes in ingredient(s), process and packaging methods, or equipment do not negatively impact on the safety of the product. NFPA (1993) provides examples of verification activities for chilled foods.

The above procedures for implementation are considered to be a general set of procedures that can be applied to any type of food product incIuding a MAP product; the exact details of the HACCP program will be specific for a given MAP food product and will depend on the nature of product, incIuding the nature of the raw ingredients used for the product and the nature of the manufacturing and packaging processes.

It is cIear that HACCP does not address all aspects of the on-line operations that are required to maintain the integrity of manufacturing MAP products. In fact, the HACCP approach assurnes that certain minimum but key functions pre-exist at the time that a HACCP program is to be imp\emented. These can be considered as prerequisites for the implementation of HACCP. Examples of prerequisite programs incIude:

• pest control; • sanitation; • building and equipment design and maintenance;


• personnel training; • good manufacturing practices and personal hygiene control; • effective recall procedure; • receiving and storage; and • ingredient specifications.

5.3.1 Revisions 10 HACCP Guidelines

See Note added in proof following the Preface on page xiv.

5.4 Total quality management and quality of MAP foods

In traditional approaches to the quality of foods, there was little distinction between the safety and quality of foods; food quality routines also addressed food safety. Because of current emphasis on food safety, the distinction between food safety and food quality is being drawn. Wooden (1988) makes the point that although food safety and food quality assurance can be addressed using the 'same tools and techniques' , the two aspects cannot be combined entirely. The discussion presented above on HACCP describes the application of this technique to address only food safety issues of MAP foods; non-safety, quality characteristics are not addressed. HACCP allows the MAP food processor to focus and concentrate attention and resources for controlling and preventing the food safety hazards associated with the process and product. However most processors recognize that quality issues warrant attention as weIl.

Most MAP foods are fabricated to provide certain desirable quality characteristics (e.g. flavor, texture, freshness) and/or to retain nutrition al value. Use of HACCP for safety assurance of MAP food will require some additional techniques to address these non-safety aspects; total quality management (TQM) is being advocated as the me ans by which these quality characteristics can be assured.

Total quality management is a comprehensive, integrated approach for achieving quality targets during the manufacturing of a product; it is a system designed to provide a competitive advantage to the manufacturer by achieving quality targets consistently at optimum cost, thereby satisfying customer requirements all the time. This approach to quality assurance emphasizes a demonstrable commitment and involvement by the highest levels of management and is based on deliberate, conscious planning of all quality and quality-related activities and on continuous monitoring and evaluation of all programs and processes in the manufacturing operations. Shores (1989) describes total quality as a management philosophy that links quality to customer satisfaction.

Although total quality management has been in place in general industrial manufacturing processes (electronics, automotive, aerospace, etc.) for some years, the food industry has only recently begun to adopt


this approach. Pedraja (1988; 1986), Kepper (1985) and Sterling (1985) have discussed the role of the management function in attaining the quality objective in food manufacturing. The expectation is that this trend towards the food industry's adoption of total quality management, which embraces a quality system approach, will continue as it becomes more widely recognized as a strategy to gain competitive advantage in an increasingly competitive industry.

5.5 Combining hazard analysis with both critical control point and total quality control

Quality assurance has served as the tradition al means by which food manufacturers have assured both safety and quality in the past. The separation of the safety and quality functions of food manufacturing including MAP foods, has been advocated. HACCP and a quality system appear to provide a two-pronged approach that can address the safety concerns associated with raw materials and ingredients, the process, the product and the post-productionldistribution processes, as weil as the assurance of quality attributes in order to gain market share and, hence, competitive advantage for MAP foods. Bauman (1991) describes how HACCP can be incorporated into a food manufacturer's overall quality assurance program. NFPA (1992) discusses how HACCP and total quality management can be used in concert.

5.6 International Organization for Standardization and ISO 9000 Series as a quality management tool

Much of this chapter has been devoted to the important subject of food safety and in particular the HACCP approach applied to MAP foods. However, HACCP is not a quality management program, but compliance to the ISO 9000 standard series iso Actually, about 20% of the industries that manufacture 80% of food and drug in the UK are ISO certified. In fact, the UK dominates in world-wide ISO 9000 certification of all industries, with the latest figures showing 41.3% registered companies compared with 31.4% in the rest of Europe and 8% in North America (Symonds, 1996). In the USA, ISO has been slow to attract the food industry, but as of the end of 1996, there were approximately 60 registered food or food-related companies (approximately 25 of these achieved this in 1996). The benefit of having a quality management system that meets ISO requirements is the structure and discipline that it brings to the total process. When integrated into the quality-control function, ISO compliance provides the foundation, through documentation and supporting objective evidence (inspection and test), for the maintenance of the quality system. ISO provides a systematic approach; ISO compliance certifies the system, it


does not certify the product. MAP is an interactive packaging technology that requires disciplined management for maximized quality of product. Otherwise, the product might as weIl be processed using the more traditional technologies (e.g. canning or freezing), or, in the case of bakery products, be simply baked, packaged and sold immediately. So establishing an ISO 9000 compliance system is important to a food processor involved in MAP especially since a food safety program such as HACCP can be incorporated into its structure. In fact, it is very difficult to implement an effective HACCP program into a food manufacturing process without the existence of a structured, systematic quality program such as that existing with ISO 9000 compliance.

The ISO is an international organization, founded in 1946 and located in Switzerland. This organization is made up of approximately 90 countries, including the USA, that jointly establish international standards. These standards have been accepted in the USA by the American National Standards Institute (ANSI) and the American Society for Quality (ASQ). (Golomsk, 1994) and in the UK by the British Standards Institute. In 1983, as a result of growing international trade, the Technical Committee 176 was established to develop international standards that focused on quality assurance and management. In 1987, the efforts of this committee resulted in the publication of the ISO 9000 series, which was accepted nationally and internationally, and was adopted as European Standard EN29000:1987. (The term 'ISO' was coined to reflect the effort to equalize or standardize quality-assurance and management procedures among the European nations.) The ISO 9000 se ries was revised and reissued in 1994 to clarify further and define the requirements. As of the end of 1996, the ISO 9000 conformance standards have been adopted by approximately 120 different countries (Newslow, 1996) and the EU.

The ISO 9000 series consists of three conformance standards:

• ISO 9001: quality assurance in design, development, production, installation, and servicing;

• ISO 9002: quality assurance in production, installation, and servicing; and

• ISO 9003: quality assurance in final inspection and test.

The se ries also contains guidance standards to aid in understanding and implementing the conformance standards, such as:

• ISO 9000-1: quality management and quality assurance standards guidelines for selection and use; and

• ISO 9004-1: quality management and quality system elements guidelines.

ISO 9001 is the most comprehensive standard with 20 elements including the design function. ISO 9002 has identical requirements except for the


absence of the design clause. This standard is frequently applied in the food industry to manufacturing operations. ISO 9003 focuses on warehousing and distribution operations; however, owing to its limited scope, it is the least applied. World-wide statistics for all ISO-certified companies show that ISO 9002 dominates with 66.0% compared with 33.1 % for ISO 9001 and 0.9% for ISO 9003 (Symonds, 1996).

Basically, ISO compliance requires that you write down what you do, do what you say you do, document what you have done, audit to confirm compliance and act on the differences.

ISO 9001 can be divided into three categories of clauses: plan the business, control the process, and maintain the system (Newslow, 1996).

Plan the business

Management responsibility (Clause 4.1) and the Quality System (4.2), related to the quality policy and defined objectives to achieve them, apply to implementing policy, documenting the system and controlling all activities.

Control the process

The following clauses place the greatest emphasis on controls associated with providing the final product or service: Contract Review (Clause 4.3); Purchasing (Clause 4.6); Control of Customer Supplied Product (Clause 4.7); Product Identification and Traceability (Clause 4.8); Process Control (Clause 4.9); Inspection and Testing (Clause 4.10); Control ofInspection, Measuring and Test Equipment (Clause 4.11); Inspection and Test Status (Clause 4.12); Control of Non-conforming Product (Clause 4.13); Handling, Storage, Packaging, Preservation and Delivery (Clause 4.15); Servicing (Clause 4.19); and Statistical Techniques (Clause 4.20).

Maintain the system

Once the system is implemented, it is necessary to insure that it continues to operate effectively. The pertinent standard clauses are: Management Review (Clause 4.1.3); Document and Data Control (Clause 4.5); Corrective and Preventive Action (Clause 4.14); Control of Quality Records (Clause 4.16); Internal Quality Audits (Clause 4.17); and Training (Clause 4.18).

Compliance to the ISO 9000 conformance standards can provide a structured quality management system where the requirements for preparation and packaging of MAP foods can be integrated. This will provide the discipline to monitor the MAP process through the internal audit activities and for continuous improvement through corrective and


preventive actions. These not only evaluate existing problems through documented root cause analysis but also evaluate 'appropriate sources of information such as processes and work operations which affect product quality, concessions, audit results, quality records, service reports, and customer complaints to detect, analyze and eliminate potential causes of non-conformities [problems]' (ASQC, 1994).

5.7 Inspection and testing methods

The quality of the packed product has to be routinely checked. Testing, however, is expensive, slow and generally destructive, so that the extent of the testing program is a balance between cost and level of testing necessary to ensure reliable product quality. Hence the emphasis in this chapter is on the total quality approach rather than packed product testing, which may be minimal if the system is functioning effectively. The four main quality criteria that need monitoring are:

• film faults; • headspace gas composition; • seal strength; and • temperature.

5.7.1 Film faults

Types of fault that can occur are incorrect film structure or too thin a barrier layer, which affect permeability and pinholes, which give rise to leaks. Faults like these are not readily detected and require measurement of the gas transmission rates.

5.7.2 Headspace gas analysis

Equipment for the routine analysis of the gas mixtures injected into the packaging machinery is a common feature of the design, but random sampling of the headspace atmosphere in packs is still necessary.

A range of portable gas analysers are available that either are compact gas chromatographs, which measure the thermal conductivity of the gases using zirconia, or are paramagnetic detectors. It is not always necessary to test the levels of all the gases in the mixture as it is often the level of oxygen, when it is present, that is the critical factor.

5. 7.3 Seal strength

Seal integrity is one of the crucial factors in determining the shelf-life of MAP products. Poor seal integrity may be the result of contamination of


the sealing surfaces by the product, drip from the product and moisture or incorrect alignment of the sealing heads.

Packs coming off the li ne can be tested by exerting light pressure on the top web. Packs with poor seals will lose this positive pressure and should be repacked. Another simple test of seal integrity is to check the packs under reduced pressure in water observing whether bubbles emerge from the edges of the pack.

Destructive testing includes traditional dye testing. It is recommended that the package be thoroughly rinsed and dried before initiating the test. Methylene blue is frequently referenced in the literature, but it can be hard to see penetrating through a leak unless used in high concentration. The FDA (Gilchrist et al., 1989) has reported the use of 1 % fluorescein dye solution containing 1% wetting agent, such as Triton X-loO. A more aggressive dye solution for use in package testing is reported by the NFP A (1989). They suggest 0.5% rhodamine B in isopropanol. Recently reported for use in medical package testing is an aqueous solution of 0.5% Triton X-loO and 0.05% toluidine blue (Hackett, 1996). The author concedes that the dye solution may not be the best choice for all materials, though it did work well for TYVEK®. Hackett points out that the capillary wetting force is the primary mechanism for detecting seal defects, and this force is a function of surface tension, interaction between the dye penetrant and seal material, the effective contact angle and the effective radius of the capillary. Therefore, some exploratory work may be necessary to find the best dye solution for the packaging under test.

Another acceptable method of destructive testing is burst testing, though the device is more likely to be found in a research laboratory than a quality assurance laboratory (Figure 5.2). A hollow needle pierces the pack, and air inflates the package until the seal bursts. The internal pack pressure at which the seal bursts is recorded. Seals should withstand apressure of 0.4 to 0.5 bar depending on the internal volume of gas.

Over 70 types of package inspection system were listed in the April (1996) issue of Packaging Technology and Engineering (Noone, 1996). The list was not comprehensive but does give the reader a good account of what's available on the USA market for a range of packaging needs. Eight of the systems listed do test for seal strength or leak. Instruments for sensitive, non-destructive, mechanical testing of seal integrity fall into several categories based on the properties of the sensor:

• internal pressure testing; • extern al pressure/vacuum decay testing; • machine vision imaging; and • acoustic microscopy imaging.

No system is perfect for all applications, and the purchaser is advised to review at length with the supplier the capabilities of the system so that


Figure 5.2 The Skye 2000 burst tester (MOCON, Minneapolis, MN).

characteristics such as on-line speed, defect size detected, type of defect detected, percentage of false positives and negatives, etc. are clearly understood.

Internat pressure testing. In internal pressure testing, a mechanical pressure is applied to the package by a plunger. If the container is properly sealed, the plunger will ce ase to move forward when the back pressure from the package equals the thrust pressure. The distance the plunger must travel is monitored and electronically a comparative difference is calculated between a reference standard and the package under test. Such a tester is marke ted by TapTone®, North Falmouth, MA.

Externat pressure/vacuum decay testing. External decay testers either apply a positive pressure or pull a vacuum on the package. Lid deflection may be monitored (Nikka Densok Detectors, Lakewood, CO) as the package is pressurized or subjected to vacuum, or the vacuum chamber may be monitored for increasing pressure by the leaking container (Packaging Technologies & Inspection, Tuckahoe, NY) (Figure 5.3). Alternately, in the case of MAP, specific gases, such as carbon dioxide may be assessed.


Figure 5.3 A Wilcomet K-121F-200 (Packaging Technologies & Inspection, Tuckahoe, NY) seal integrity tester wh ich senses defects by monitoring for pressure changes in a package

subjected to a vacuum chamber.

Machine vision imaging. This system uses high resolution CCD cameras and specialized illumination techniques to acquire images of the seal as packages travel along a conveyor. The imaged area is analysed for a change in saturation of color indicative of a defect of so me type (Figure 5.4). Water as a defect is more difficult to detect, and better results can be achieved with color imaging over black and white. Multiple imaging units may be necessary to scan all critical package areas. Machine vision imaging is a mature technology that has somewhat recently found application in the food industry. Refinements in differentiating a range of flaws in the seal area, such as wrinkles in the material, contaminants in the seal, channel leaks, etc., are making the technology more applicable to food packaging. Many companies offer these systems, to name several: Allen-Bradley, Highland Heights, OH, Perceptics, Knoxville, TN, and Sonoscan, Bensenville, IL.

Acoustic microscopy imaging (AMI) and laser AMI. Microelectronics has long used AMI as a non-destructive method to image the bonding of a dye to its substrate, but the food industry is just beginning to apply AMI and laser AMI to seal defect detection (Sonoscan, Inc., Bensenville, IL). C-SAM, a form of AMI, is a reflection mode acoustic imaging system that produces a cumulative image based on return echoes from pulsed acoustic


Figure 5.4 Machine vision imaging of a seal flange showing an area of no seal (Ieft, insert) and product in the seal (right insert) . (Perceptics, Knoxville, TN)

energy sent into a sam pIe (Figure 5.5). In this technique, each layer of a laminate can be examined for defects . The second type of AMI system (Yalamanchili et al. , 1994) is a scanning laser acoustic microscope (SLAM). SLAM is a 'through transmission' system that gene rates realtime single images similar to those created with X-rays; however SLAM is more sensitive to material continuity and bonding between materials while X-ray only detects density variations (Figure 5.6). In SLAM, acoustic energy is introduced into the package from one side and detected on the


Figure S.S AC-SAM image of a 10 f.lm channel leak (arrows) created in a package. (Photograph provided by Sonoscan, Bensensville, IL.)

other side by a scanning laser. Internal defects are easily detected using SLAM, but defects will not be identified as to wh ich layer is invQlved, so C-SAM is a better choice for this type of monitoring. The disadvantage of C-SAM and SLAM is that a fluid coupling agent, typically deionized water is required to transfer the ultrasound into the seal, and it may not be acceptable to submerge, even partiaIly, the package. Sonoscan is working on an air coupling system.

5.7.4 Temperature checks

Temperatures should be monitored regularly. The temperature of the rooms in wh ich the product is prepared, held prior to packaging, packed and stored should be recorded and regulated as weIl as the product and packed product temperature.

Numerous electronic instruments are available for the monitoring and measurement of temperature at all stages of production, packaging, storage, distribution and retail display. SmaIl, printable color-changing time-temperature indicators (LifeLines Technology, Inc., Morris Plains, NJ) of food products exist to monitor temperature abuse during handling and storage or to indicate the end of the useful shelf-life of the product. Indicators can be manufactured to be active at all temperatures or only


'-, -~

. .. " ~~ -4't-_-=-~~--

Figure 5.6 A scanning laser acoustic micrograph of a seal with wrinkles (creases). Notice the contrast of images of a good seal area (a) versus a bad seal area (b). (Photographs provided by

Sonoscan , Bensenville, IL.)


above a specific threshold temperature. In addition, these two types of indicator can be combined to display a different time-temperature profile for temperatures above and below the threshold. Tests of the indicators have shown a high degree of accuracy and reliability, and surveys have shown good consumer acceptance.

5.8 Regulatory aspects of MAP foods

5.8.1 HACCP

Regulation of manufactured foods by government regulatory agencies has undergone considerable changes since the mid-1980s. One of the more recent of these changes is the promotion and adoption of HACCP by government regulatory agencies as part of their regulatory process; in fact, several North American agencies have now adopted HACCP while others are in the process of developing and implementing HACCP. An Institute of Food Technologists (1FT, 1992) scientific status summary Government Regulation of Food Safety makes note of enforcement and monitoring programs by a preventive approach, the approach emphasized by HACCP.

The first use of HACCP for regulatory purposes in the USA was the US FDA inspection of low-acid canned foods (Kauffman, 1974). Since this initial program, after detailed scrutiny of HACCP, the seafood industry in the USA and Canada is faced with a mandatory HACCP-based inspection system for fish, seafood and processed seafood products, including MAP foods containing seafood. All USA commercial processors and packagers of seafood and importers of seafood products into the USA are subject to HACCP-based regulation beginning December 18, 1997. In July of 1996, the US Department of Agriculture finalized a food safety regulation that will require all processors of meat and poultry products to implement HACCP in their establishment. The phased-in implementation process begins in January, 1998.

5.8.2 Labeling

The FDA prohibits misbranding of FDA-regulated products, i.e. food, drugs and cosmetics. 'Misbranding' of products means the label is 'false' or 'misleading' or violates other specific statutory or regulatory requirements. The FDA has issued separate regulations on its regulated products, the most extensive of which are on food, and include nutrition labeling on most food packages (Simmons, 1997). The labeling directive EU Directive 79/ 112 Labelling, Presentation, and Advertising of Foodstuffs for Sale to the Ultimate Consumer, adopted December 18,1979, does not require nutrient labeling for EU members. The closest equivalent is the directive on Food Particular Nutritional Uses (77/94), which is concerned with nutritionally


balanced foods such as slimming foods, infant and baby foods, and foods designed for a specific need, e.g. for diabetics.

EU date marking is based on the principle of minimum dur ability (White and Tice, 1997). This is the date until which a product retains its specific properties when properly stored. The directive specifies the marking, wh ich is usually 'best before' followed by the minimum durability date. For foods that are highly perishable and present a food safety risk, EU members can adopt the words 'use before' or its equivalent, which in the UK is 'use by (date)'. Foods such as fresh fruits and vegetables are not required to have open shelf-life dating.

Canada uses the term 'durable life' and terms this the time from the day of packaging for retail sale that the food will retain its normal wholesomeness, palatability and nutritional value when stored under proper conditions. A 'durable life date' is required on pre-packaged foods with a durable life of 90 days or less, with the exceptions of the following foods:

• pre-packaged fresh fruits and vegetables; • pre-packaged individual portions of food served by restaurants, airlines,

etc. with meals or snacks; • pre-packaged individual servings prepared by a commissary and sold in

automatic vending machines or mobile canteens; and • pre-packaged donuts.

In the USA federal regulations require open shelf-life dating ('use by') only on infant formula. If the product does not have an open shelf-life date, all properties of the product, nutritional and otherwise, must last indefinitely, or the product may be considered misbranded and be subject to seizure by regulatory officials. Enforcement is at the state level, and states may have their own regulations on shelf-life dating. For example, Nebraska has a new law to mark refrigerated, hazardous foods with a 'consume by' or 'use by' date. Under HACCP, but not under federal regulations, foods needing refrigeration to prevent microbiological hazards must be marked for refrigerated storage. The industry is conscientious about marking packages that must be refrigerated, but that has not always prevented consumers from storing these packages without refrigeration.

The EC has published a labeling directive on packaging gases, EU Directive 95/54 EC (effective January 1, 1997), which says that 'the declaration packaged in a protective atmosphere required by regulation 33 need only be given on the label of a pre-packaged food when that food's shelf life has been extended by the use of any packaging gas authorised by Council Directive 89/107/EEC on food additives.' This applies to MAP foods but does not apply to foods like soft drinks where the gas is used at the packaging stage to solve a technical problem of package deformation. The gases used need not be included in the ingredients list.


5.9 Summary

The current emphasis on food safety has led to the emergence of HACCP as a universal technique for assuring safety both in the food manufacturing and the food-service industries. With the momentum that HACCP and total quality management has generated since the mid-1980s, it is difficult to imagine that MAP foods will continue to be manufactured without the presence of HACCP and a total quality management program such as ISO to assure safety and quality.

Acknowledgements

Information on ISO was provided by D.L. Newslow, Lloyd's Register Quality Assurance Ltd, Hoboken, NJ, and the time-temperature indicator information was supplied by F. R. Grabiner and T. Prusik of LifeLines Technology, Inc., Morris Plains, NJ.

References

Agriculture Canada Working Group (1990) Canadian Code of Recommended Manufacturing Practices for PasteurizedlModified Atmosphere-PackagedlRefrigerated Foods. Agriculture Canada Safety Division, Ottawa, Canada.

ASQC (1994) 4.14.3 Preventive action, in American National Standard, Quality Systemsmodel for Quality Assurance in Design, Development. Production, Installation, and Servicing, ANSIIISOIASQC Q9001-1994. American Society for Quality Control, Milwaukee, WI.

Bauman, H. (1991) Fitting HACCP into the company QA system. Cereal Foods World, 36(1),42-43.

Campden & Chorleywood Food Research Association (1992) HACCP: A Practical Guide. Technical Manual No. 38, Chipping Campden, Gloucestershire, UK.

Codex Alimentarius Commission; Codex Committee on Food Hygiene (1996) Draft Guidelines for the Application of Hazard Analysis Critical Control Point System (Alinorm 97113, Appendix II1Annex). Food and Agriculture OrganizationlWorld Health Organization, Rome.

Codex Alimentarius Commission; Codex Committee on Food Hygiene (1993) Guidelines for the Application of Hazard Analysis Critical Control Point (HACCP) System (Alimorn 931 13A, Appendix B). Food and Agriculture OrganizationlWorld Health Organization, Rome.

Corlett, D.A. (1991) Monitoring a HACCP system. Cereal Foods World, 36(1), 33-40. Curtis, J.E. and Huskey, G.E. (1974) HACCP analysis quality assurance. Food Prod. Devel.,

8(3), 19, 24-25. Daniels, R.W. (1991) Applying HACCP to new generation foods at retail and beyond. Food

Technoi., 46(6), 122, 124. Day, B. (1992) Guidelines for the Good Manufacturing and Handling of Modified Atmosphere

Packed Food Products, Technical Manual No. 34. Campden & Chorleywood Food Research Association, Chipping Campden, Gloucestershire, UK.

Gilchrist, J.E., Shah, D.B., Radle, D.C. and Dickerson, R.W. Jr. (1989) Leak detection in flexible retort pouches. J. Food Protection, 52(6), 412-415.

Golomski, W.A. (1994) ISO 9000 - the global perspective. Food Technoi., 48(12), 57-59.


Hackett, E.T. (1996) Dye penetration effective for detecting package seal defects. Packaging Technol. Eng., 5(8), 49-52.

IAMFES (1991) Procedures to Implement the HACCP System. International Association of Milk Food and Environmental Sanitarians, Ames.

1FT (1992) Governments regulation of food safety: Interaction of scientific and societal forces. Food Technoi., 46(1), 73-80.

Ito, K. (1974) Microbiological critical control points in canned foods. Food Techno/. , 28(9), 46-48.

Kauffman, F.L. (1974). How FDA uses HACCP. Food Technoi., 28(9), 51, 84. Kepper, R.E. (1985) Quality motivation. Food Technoi., 39(9), 51-52. NACMCF (1992) Hazard analysis and critical control point system. Int J. Food Microbiol.,

16(1), 1-23. Newslow, D. (1996) ISO 9000, HACCP and GMPs: the family tie. Food Quality, 11(7), 17-18. NFPA (1989) Plastic packages with he at sealed lids for commercially sterile foods. Flexible

Package Integrity Bulletin 41-L. National Food Processors Association, Washington, DC, p.27.

NFPA, Microbiology and Food Safety Committee (1992) HACCP and total quality management - winning concepts for the 90's: a review. J. Food Protection, 55(6), 459-462.

NFPA, Microbiology and Food Safety Committee (1993) HACCP implementation: a generic model for chilled foods. J. Food Protection, 56(12), 1077-1084.

Noone, W.J. (1996) Packaging inspection systems making their presence feit. Packaging Techno/. & Eng., 5(4),27,28,30-35.

Pedraja, R.R. (1986) Quality assurance. Food Process, 47(2), 57-59. Pedraja, R.R. (1988). Role of quality assurance in the food industry: new concepts. Food

Technoi., 42(12), 92-93. Penner, K.P. and Kramer, C. (1985) Consumers' Food Safety Perceptions, Pub. MF-774.

Coop. Ext. Service, Kansas State University, Manhattan, KS. Pillsbury Company (1973) Food Safety Through the Hazard Analysis Critical Control Point

System. Published by the Pillsbury Co., Minneapolis, MN. Shores, D. (1989) TQC: science, not witchcraft. Qua/. Progress, 22(4), 42-45. Simmons, R.A. (1997) Laws and regulations, United States, in The Wiley Encyclopedia o[

Packaging Technology (ed. A.L. Brody and K.S. Marsh). Wiley, New York, pp. 552-558. Simonsen, B., Bryan, F.L., Christian, J.H.B., Roberts, T.A., Tompkin, R.B. and Silliker,

J.H. (1987) Prevention and control of food-borne salmonellosis through application of hazard analysis critical control point (HACCP). Int. J. Food Microbiol., 4, 227-247.

Smith, J.P., Toupin, c., Gagnon, B., Voyer, R., Fiset, P.P. and Simpson, M.V. (1990) A hazard analysis critical control point approach (HACCP) to ensure the microbiological safety. Food Microbiol., 7,177-198.

Sterling, R. (1985) Relationship between manufacturing and quality costs. Food Techno/. , 39(9), 54-55.

Stevenson, K.E. and Bernard, D. (eds) (1995) HACCPIEstablishing Hazard Analysis Critical Point ProgramslA Workshop Manual, The Food Processors Institute, Washington, DC.

Swintek, R.J. (1991). New products are king. Food Process., 52(8), 38-40. Symonds, J. (1996) The Mobil Survey. Mobil Europe Ltd, London. Titus, E.O. and Talbot, J.M. (1991) Emerging Issues in Food Safety and Quality for the Next

Decade. Life Sciences Research Office, Federation of American Societies for Experimental Biology, Washington DC.

White, R.M. and Tice, P.A. (1997) Laws and regulations, Europe, in The Wiley Encyclopedia of Packaging Technology (ed. A.L. Brody and K.S. Marsh). Wiley, New York, pp. 541-552.

Wolf, I.D. (1992) Critical issues in food safety, 1991-2000. Food Technoi., 46(1), 64-70. Wooden, R. (1988) HACCP approach to product safety. Bull. Int. Dairy Fed., 229, 35-40. Yalamanchili, P., Christou, A., Martell, S. and Rust, C. (1994) C-SAM sounds the warning

for IC packaging defects. IEEE Circuits and Devices Magazine, 10(4), 36-41.

6 Fresh-cut produce E.H. GARRETf

6.1 Introduction

More than 20 different kinds of fmits and vegetables are sold as 'minimally processed' or 'fresh-cut' produce. The International Fresh-cut Produce Association (IFPA) defines fresh-cut produce as 'any fresh fmit or vegetable or any combination thereof that has been physically altered from its original form, but remains in a fresh state'. Because of the importance of packaging to fresh-cuts and with the growth of this product category, it may be time to include packaging in that definition.

Some examples of typical fresh-cuts products are:

• packaged salads; • gourmet lettuce blends; • coleslaws; • lettuce (shredded, chopped, halved, cored); • cabbage (shredded); • spin ach (washed and trimmed); • celery (chopped and sticks); • carrots (shredded, sticks, peeled); • broccoli and cauliflower floret; • squash and zucchini (courgettes) (sliced, julienne); • onions (diced, sliced, whole peeled); • fmit salads (cubed); • pineapple (cored, sliced, cubed); • melons (cubed); • grapes (de-stemmed).

In the USA alone, IFP A estimates that 6-8 billion pounds of fresh-cut produce was sold in 1996, accounting for 8-10% of total produce sales. In the more mature European market, fresh-cuts represented about 12% of 1996 total produce sales (Buck, 1996). Many other countries have just introduced fresh-cut produce and anticipate considerable growth potential.

Fresh-cuts have been successful because of the consumer's desire for more convenient foods, improvements in processing technologies and introduction of new packaging materials. Actually, highly breathable


paekaging films other than polyethylene were not in use for produee before 1988. MAP has played an integral role in the development of fresh-euts because it allows produce to breathe and stay protected from spoilage. Packaging offers solutions to some of the challenges of delivering a highquality product to the consumer.

6.2 Produce respiration and MAP

MAP as applied to fresh-cut produce is a system that allows the cut produce to continue to respire and maintain its freshness. The United States Department of Agriculture (USDA) describes respiration as a process ' ... by which the oxygen of the air is combined with the carbon of the plant tissue, occurring chiefly in sugars, to form various decomposition products and, eventually, carbon dioxide and water' (Hardenburg et al., 1986). This chapter will foeus on how MAP interacts with the produce respiration process to maintain freshness and will describe current production standards in the North American market.

One of the unique qualities of produce is that it respires, that is, it continues to breathe after harvesting. In fact, produce is still fresh and 'alive' many days or weeks after harvest, depending on the variety of fmit or vegetable and the handling conditions. When cut and handled in production, the internal tissue of the produce becomes exposed to the environment and the plant responds to that trauma by increasing its respiration rate to survive and repair the damage. Increased respiration rates generally mean an increased spoilage rate as the produce uses up its stored nutrients and moisture to survive (Zagory, 1994a). Controlling the spoilage rate is of paramount importance. Even though the most effective way to control respiration may be by reducing the temperature, there are several steps that can reduce the respiration rate of cut produce:

• storage at refrigeration temperatures of 33--41°F (O.S-S°C); • reducing trauma in handling during production; and • proper packaging materials.

Because produce respires by taking in oxygen and giving off carbon dioxide, the packaging material must be permeable to these gases. It is also critical that the headspace atmosphere formed in the hermetically sealed package contains suitable oxygen and carbon dioxide concentrations for compatibility with the produce to preserve quality (Table 6.1).

An optimal MAP system creates an environment inside the package that slows the respiration rate of the produce but also protects its quality. Immediately after package sealing, the headspace gases will be -21% oxygen and < 1 % carbon dioxide, as found in air (Zagory, 1994b). The

FRESH-CUT PRODUCE 127

Table 6.1 Classification of fruits and vegetables according to their tolerance to low oxygen and high carbon dioxide concentrations:

Minimum oxygen Commodities tolerated (%)

0.5 Tree nuts, dried fruits and vegetables 1.0 So me cultivars of apples and pears, broccoli, mushrooms, garlic,

onion, most cut or sliced fruits and vegetables 2.0 Most cultivars of apples and pears, kiwifruit, apricot, cherry,

nectarine, pe ach plum, strawberry, papaya, pineapple, olive, cantaloupe, sweet corn, green bean, celery, lettuce, cabbage, cauliflower, Brussels sprouts

3.0 Avocado, persimmon, tomato, pepper, cucumber, artichoke 5.0 Citrus fruits, green pea, asparagus, potato, sweet potato

Maximum carbon Commodities dioxide tolerated (%)

2.0 Apple (Golden Delicious), Asian pear, European pear, apricot, grape, olive, tomato, sweet pepper, lettuce, endive, Chinese cabbage, celery, artichoke, sweet potato

5.0 Apple (most cultivars), peach, nectarine, plum, orange, avocado, banana, mango, papaya, kiwifruit, cranberry, pea, chili pepper, eggplant, cauliflower, cabbage, Brussels sprouts, radish, carrot

10.0 Grapefruit, lemon, lime, persimmon, pineapple, cucumber, summer squash, snap bean, okra, asparagus, broccoli, parsley, leek, green onion, dry onion, garlic, potato

15.0 Strawberry, raspberry, blackberry, blueberry, cherry, fig, cantaloupe, sweet corn, mushroom, spinach, kaie, Swiss chard

From Kader et al. (1989).

respiration of the produce will actively change that environment since oxygen levels decrease and carbon dioxide levels rise. These gases will then move across the film in response to the gradient formed. Eventually, the oxygen and the carbon dioxide gases establish an equilibrium within the headspace atmosphere (Exama et al., 1993).

The goal is to find the correct packaging materials to create the optimal atmosphere inside the package. Another way to approach this goal is to match the respiration rate of the produce and the oxygen permeability of the packaging (Figure 6.1). This is difficult because plant respiration rates change in response to many environmental factors, but the permeability of the packaging film may not adapt. The level of permeability that works at a fixed temperature, for instance, may not be successful at a higher or lower temperature for a given product. Data describing the respiration rates for whole fruits and vegetables exist (Hardenburg et al., 1986), but research needs to be done to establish respiration rates of cut fruits and vegetables.

128

25

20

0'" 15 ~ 0

*' 10

5

0 0

PRINCIPLES AND APPLICATIONS OF MAP OF FOODS

./_ .....

i.;:::~;::::~~~:: ............ . 10 20 30 40

. ' .' .' .' .' .' .' .'

.' .'

50 60 70 80 Time

90 100

Perm too high

%°2

....... :.~~.~?; ... Permtoo low

%°2 %C02 ....................

Equilibrium

%°2

Figure 6.1 Atmospheric changes that can occur within a package. If the permeability is too high, an excessive oxygen supply will continue to exist (the product is respiring at roughly its normal rate). If the permeability is too low, the oxygen level will fall below 1 % to 2% and the product will become anaerobic. The equilibrium state is a favourable atmosphere for extending shelf life; the 10% oxygen environment will somewhat inhibit respiration while avoiding anaerobisis. (Reproduced with permission (Frey, T.D. (1997) Packaging Technology and

Engineering, 6, 40-42, 45.)

6.3 Quality maintenance

The market for fresh-cuts has grown in large part because of improved technologies and improved raw material quality. Fresh-cut processing has matured into a food production system that rejects unmarketable raw materials. One sign of that maturity is the fact that processors are contracting large quantities of raw materials for their operations.

Growers have realized the value of contracting fields to processors, and many have committed more than 50% of their production because of the stable contract prices. With food safety an increasingly important issue in the USA, these relationships will become even more critical. For example, the processor needs to track ingredient sources for a produet reeall or the grower needs to deseribe safe harvesting praetiees to win a eontraet (Alexander, 1997).

Onee the produets arrive at the proeessing plants, the following produetion steps oeeur:

1. Raw produet reeeipt and storage: refrigeration 2. Trim/eore operation: removal of peeling, outer leaves, eores, dirt, ete. 3. Cutting: reduetion of whole produee to smaller pieees of varying sizes 4. Washing: eold, sanitized water used in washing, transporting and

refrigeration of the eut produee 5. Drying: removal of moisture from the surfaee of the produee via air or

eentrifugal foree


6. Packaging: permeable films used in form-fill-seal automatie machines or as pre-made bags that are hand-filled and weighed

7. Finished product storage: refrigeration 8. Distribution: refrigerated trucks 9. Restaurant/grocery/home storage: refrigeration

The typieal shelf life of fresh-cut produce ranges from 5 to 21 days, depending on the product. Packaging, refrigeration and produce quality can be considered as the three factors that maintain the quality of fresh-cut produce during its shelf life.

6.4 Safety of MAP produce

To address another concern of this industry, the IFPA has just completed the third edition of its Food Safety Guidelines for Fresh-cut Produce, which covers Good Manufacturing Practiees (GMP) for this industry (IFPA, 1996). Starting with a chapter on good harvesting practiees, the book follows the entire production process through stock recovery. Implementing HACCP, preventing cross-contamination, engineering proper temperature control, auditing equipment cleaning and sanitation programs and training employees in good hygiene are the cornerstones of safe fresh-cut production.

Even though packaging acts as a barrier to cross-contamination, the low oxygen levels in the headspace of MAP products may create a suitable environment for the growth of Clostridium botulinum (Hotchkiss and Banco, 1992). Recent research has shown that the risk is minimal for freshcut produce because the produce will spoil before toxin production (IFP A, 1993). Packaging can further protect the produce by maintaining an aerobic atmosphere to support the growth of competing, resident mieroflora.

Typieal headspace gas concentrations for maintenance of best quality may range from 2 to 10% oxygen and 10 to 20% carbon dioxide for cut produce (Marston, 1995). If the carbon dioxide level rises in the package and is left unchecked, the produce could spoil rapidly and the resident bacteria could shift to an anaerobie population. This could happen if the packaging films are not the correct permeability for the produce or if the package has become severely temperature abused. Since this is a suitable environment for some human pathogens, a very high carbon dioxide concentration should be avoided.

Two things will occur if the carbon dioxide levels get too high: first, the produce will start anaerobie respiration and spoil abruptly. Second, the resident mieroflora, whieh is present in much higher populations than any pathogen and, therefore, would have a considerable advantage, will also


contribute to the produce breakdown. This rapid spoilage can be visible to the consumer. Therefore, package designs for fresh-cut produce should have transparent sections for viewing the contents of the package.

Proper film permeability, a healthy resident microflora and good refrigeration storage temperatures work together to maintain the safety of fresh-cut produce.

6.5 Packaging materials

The importance of packaging to the sales of produce cannot be overstated. MAP provides protection from environmental contamination, controlled permeability to preserve freshness and a printed surface. Printed films introduced the concept of category management into the produce sections of grocery stores. Sales data from Universal Product Council (UPC) codes on the bags has opened up a world of information to produce buyers about consumer's shopping habits. Clearly, packaged produce has revolutionized produce marketing.

The following selection criteria should be considered when making choices of materials for fresh-cut packaging (Barmore, 1995):

• gas transmission rates (produce and film compatibility); • printability; • optics; • anti-fog characteristics; • machinability; • sealing properties (in the produce processing facility); • factory seal strength (of pre-made bags); • puncture and abrasion resistance; • unit price; • performance in the marketplace.

Permeability is just one factor to consider when looking for suitable materials for produce packaging. If the material is too expensive, the processors will not use it, and if the material is too fragile, it will not hold up during distribution. In addition, processors will reject film that does not operate efficiently on their packaging machinery. Films currently used in the produce industry fall within three general categories:

• mono-layer: produced by extruding a single polymer or ablend of polymers through a die, e.g. LDPE, LDPEIEV A

• laminate: a multi-Iayered structure created by putting two or more films together with adhesive, e.g. PP/adhesive/LDPE

• co-extruded films: a multi-Iayered film created by extruding each layer through a die as one structure, e.g. PPIEVAILDPE.


Some important film characteristics are listed in Table 6.2 (Day, 1993). The oxygen transmission rate (OTR) is a measurement of the oxygen that moves through a unit area of the film for a given amount of time at a given atmosphere and temperature. The OTR values in Table 6.2 are true for room temperature. OTR values calculated at refrigeration temperatures may provide more practical permeability rates for films used in refrigerated storage conditions (Day, 1993; Zagory, 1994a).

In Table 6.2, the OTR is expressed in terms of 1 m2 . If the film is marketed by a USA firm, OTR is expressed in terms of 100 in2 • To compare, divide the metric OTR factor by 15.5 for the corresponding OTR for 100 in2 (Barmore, 1995).

It is important to consider the amount and type of produce for the package, the surface area and thickness of the bag and the storage temperature of the product for optimum headspace development. Simply using a film with the appropriate OTR will not ensure a successful package.

Other characteristics to be considered include the kind of packaging machinery used, as poor seal integrity will compromise the hermetic seal needed for optimum headspace atmospheres. Conversely, a film that does not have good sealant properties will not be reliable. Also, a film that tears or punctures easily will not perform weH under rough handling in the marketplace. Those punctures will lead to loss of the optimum headspace atmosphere.

In addition to using films designed for produce, other technologies are employed in produce packaging. Vacuum sealing is not as popular as in the past because processors found that a hard vacuum resulted in bruised produce. Using a light vacuum before sealing lowers the oxygen content in the initial headspace.

Another technique used to lower the initial oxygen content is to backflush with nitrogen or other gas mixtures. This is particularly popular with bagged salads. The resulting package has a 'pillow' appearance and the cushioning effect protects the lettuces during handling and transport. Little benefit to shelf life has been observed, but headspace equilibrium is established more quickly.

A third current technology for produce packaging is that of film perforation. The goal is to take films with low permeability properties and create openings of various sizes and formations to increase the OTR. Perforations are especiaHy wen suited for high respiring produce such as spinach, but there is some concern about cross-contamination once the bags are in distribution. Control of the headspace atmosphere is also limited.

6.6 Future industry needs

Promising research for the future includes anti-microbials imbedded in films. Lasers have been shown to create amines on the surface of nylon

Tab

le 6

.2 O

xyge

n an

d w

ater

vap

our

tran

smis

sion

rat

es o

f sel

ecte

da p

acka

ging

mat

eria

ls f

or f

ruit

and

veg

etab

les.

Rep

rodu

ced

wit

h pe

rmis

sion

, Day

(199

3)

Pac

kagi

ng f

ilm

(25

ftm

)

Alu

min

ium

(A

I)

Eth

ylen

e vi

nyl

acet

ate

(EV

OH

) P

olyv

inyl

iden

e ch

lori

de (

PV

dC)

Mod

ifie

d ny

lon

(MX

DE

) P

olye

ster

(P

ET

) P

olya

mid

(ny

lon)

(P

A6)

M

odif

ied

poly

este

r (P

ET

G)

Met

alli

zed

orie

ntat

ed p

olyp

ropy

lene

(M

OP

P)

Unp

last

iciz

ed p

olyv

inyl

chl

orid

e (U

PV

C)

Pol

yvin

yl c

hlor

ide

(pla

stic

ized

) (P

VC

) O

rien

tate

d p

olyp

ropy

lene

(O

PP

) H

igh-

dens

ity

poly

ethy

lene

(H

DP

E)

Pol

ysty

rene

(P

S)

Ori

enta

ted

pol

ysty

rene

(O

PS

) P

olyp

ropy

lene

(P

P)

Pol

ycar

bona

te (

PC

) L

ow-d

ensi

ty p

olye

thyl

ene

(LD

PE

) P

olyv

inyl

chl

orid

e (h

ighl

y pl

asti

cize

d)

(PV

C)

Eth

ylen

e vi

nyl

acet

ate

(EV

A)

Mic

rope

rfor

ated

(M

P)

Mic

ropo

rous

(M

PO

R)

Oxy

gen

tran

smis

sion

rat

e (c

m3

m-2

da

y-I

at r

n-I)

(a

t 23

°C,

0% R

H)b

<O

.le

0.2-

1.6d

0.8-

9.2

2.4d

50-1

00

80d

100

100-

200

120-

160

2000

-500

0 20

0-25

00

2100

25

00-5

000

2500

-500

0 30

00-3

700

4300

71

00

5000

-10

oooe

12

000

> 15

OOO

f >

15 O

OOf

Rel

ativ

e pe

rmea

bili

ty

(at

23°C

, 0%

RH

)

Bar

rier

<

50

Sem

i-ba

rrie

r 50

-200

Med

ium

20

0-50

00

Hig

h 50

00-1

0 00

0

Hig

h 50

00-1

0 00

0 V

ery

high

10

000-

1500

0 E

xtre

mel

y hi

gh

>1

50

00

Wat

er v

apou

r tr

ansm

issi

on r

ate

(g m

-2 da

y-I)

(a

t 38

°C,

90%

RH

)b

<O

.le

24--

120d

0.3-

3.2

25

20-3

0 20

0 60

1.5-

3.0e

22-3

5 20

0e

7 6-8

110-

160

170

10-1

2 18

0 16

-24

200e

110-

160

Var

iabl

ef

Var

iabl

ef

alt

shou

ld b

e no

ted

that

mos

t pa

ckag

ing

film

s fo

r fr

esh

prod

uce

are

not

sing

le f

ilms

but

lam

inat

es a

nd c

o-ex

trus

ions

.

Rel

ativ

e w

ater

va

pour

tr

ansm

issi

on r

ate

(at

38°C

, 90

% R

H)

Bar

rier

, <

10

V

aria

ble

Bar

rier

, <

10

Sem

i-ba

rrie

r, 1

0-30

S

emi-

barr

ier,

10-

30

Ver

y hi

gh,

200-

300

Med

ium

, 30

-100

Bar

rier

, <

10

V

aria

ble

Ver

y hi

gh,

200-

300

Bar

rier

, <

10

Bar

rier

, <

10

H

igh,

100

-200

H

igh,

100

-200

S

emi-

barr

ier,

10-

30

Ver

y hi

gh,

100-

200

Sem

i-ba

rrie

r, 1

0-30

Ver

y hi

gh,

200-

300

Ver

y hi

gh,

100-

200

Ext

rem

ely

high

, >

30

0

Ext

rem

ely

high

, >

30

0

bIt

shou

ld b

e no

ted

that

con

diti

ons

of

oxyg

en a

nd w

ater

vap

our

tran

smis

sion

rat

e m

easu

rem

ents

are

not

at

real

isti

c ch

ili c

ondi

tion

s.

cDep

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nt o

n pi

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es.

dDep

ende

nt o

n m

oist

ure.

eD

epen

dent

on

moi

stur

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d le

vel

of p

last

iciz

er.

fDep

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nt o

n fil

m a

nd d

egre

e o

f m

icro

perf

orat

ion

or

mic

ropo

rosi

ty.


film. In this research, the amines had an anti-microbial affect on solutions of bacteria (Marston, 1996). There is also speculation that other anti-microbial compounds imbedded in films can slowly emit as vapours inside the package headspace to kill bacteria.

As fresh-cut fruit becomes a more mainstream product, ethylene gas may be of concern. More research is needed to study the effects of ethylene on the quality of cut produce. Does ethylene adequately permeate through packaging film to prevent deterioration? Does MAP effectively suppress ethylene production?

Consumers are also asking about easy-open packages. If potato chip bags can be pulled opened easily, then they expect the fresh-cut produce industry to provide easy-opening bags. Hermetic seals are very important and should not be compromised in this effort.

In this era of produce globalization and demands for more convenience, there will be no shortage of challenges for packaged produce. 'Smart' films that change permeability with changes in temperature are in production. Seed varieties are being developed specifically for processing characteristics. Can 'smart' produce be far behind?

The commitment to food safety must take priority in all phases of MAP development for produce. Fresh-cut produce is a safe, wholesome food when handled properly. It is the nature of produce overtly to spoil when optimum storage conditions are not met. Therefore, refrigeration throughout the distribution channels is paramount to the success of packaged produce. With a focus on maintaining the cold chain during distribution, the market will continue to boom in this category.

References

Alexander, G. (1997) Private label fuels rethinking of business, presented at the IO'h Annual Conference & Exhibition of the International Fresh-cut Produce Association, Alexandria, VA.

Barmore, C.R. (1995) Packaging materials for the fresh-cut produce industry, in Proceedings of the 1995 Annual Meeting of the Institute of Food Technologists, Chicago, IL.

Buck, B. (1996) Salad vegetable suppliers spice up international markets. American Agriculture and Food Exporter, 46-54.

Day, B.P.F. (1993) Fruit and vegetables, in Principles and Applications of Modified Atmosphere Packaging of Food (ed. R.T. Parry), Blackie Academic & Professional, London, pp. 115-133.

Exama, A. et al. (1993) Suitability of plastic films for modified atmosphere packaging of fruits and vegetables. Journal of Food Science, 58(6), 1365-1370.

Frey, T.D. (1997) High gas permeability key to 'passive' CAP for produce. Packaging Technology and Engineering, 6(3): 40-42, 45.

Hardenburg, R.E., Watada, A.E. and Wang, C.Y. (1986) The Commercial Storage of Fruits, Vegetables and Florist and Nursery Stocks, USDA Handbook 66. USDA, Washington, DC, pp. 9-13.

Hotchkiss, J.H. and Banco, M.J. (1992) Influence of new packaging technologies on the growth of microorganisms in produce. Journal of Food Protection, 55(10), 815-820.


IFPA (1993) Assessment 01 the Risk 01 Botulis'; Contributed by Modilied Atmosphere Packaging 01 Fresh-cut Produce. International Fresh-cut Produce Association, Alexandria, VA.

IFPA (1996) Food Salety Guidelines lor the Fresh-cut Produce lndustry, 3rd edn. International Fresh-cut Produce Association, Alexandria, VA.

Kader, A.A., Zagory, D. and Kerbel, E.L. (1989) Modified atmosphere packaging of fruits and vegetables. CRC Critical Reviews in Food Seien ce and Nutrition, 28(1), 1-30.

Marston, E.V. (1995) Suitability of films for MAP of fresh produce. Produce Technology Monitor, 5(7), 1-2.

Marston, E.V. (1996) Antimicrobial films produced with lasers. Produce Technology Monitor, 6(6), 3-4.

McDonald, R.E., and Risse, L.A. (1990) Bagging chopped lettuce in selected per me ability films. Hortscience, 25(6), 671-673.

Zagory, D. (1994a) Principles and practice of modified atmosphere packaging of horticul tural commodities, in Principles 01 Modified Atmosphere and Sous-vide Packaging (eds Farber, J.M. and Dodds, K.L.), Shotland Business Research, Inc., Princeton, NJ.

Zagory, D. (1994b) Fundamentals of reduced-oxygen packaging, in Modilied Atmosphere Food Packaging (ed. A.L. Brody). Institute of Packaging Professionals, Herndon, VA, pp. 9-17.

7 Bakery products D.A.L. SEILER

7.1 Introduction

A significant proportion of the average consumer's grocery bill is made up of bakery products. A look round a modern supers tore shows that a wide variety of such goods are now on sale. So me of these are listed in Table 7.1. The majority are sold fresh but there is a growing range of products that are retailed refrigerated or frozen.

7.1.1 Types of wrapper

Nowadays, almost all items are wrapped. In the case of some breads and the more perishable filled cakes, the wrapper acts purely as a hygienic protection and has little effect in preventing moisture loss. With other products, such as crumpets, pies and pastries, the wrapper is semipermeable and allows sufficient moisture loss to prevent the pastry from becoming soggy. For the majority of items a moisture-proof wrapper is used. Although referred to as moisture proof, such wrappers do permit a small moisture loss or gain during storage either through the material or through inadequate seals. In the case of longlife goods, such as biscuits and some cakes, special highly moisture-proof wrappers and improved sealing methods are employed. These steps are necessary since small changes in moisture will adversely affect product quality during storage. Similar methods and materials are required where products are preserved by MAP.

7.1.2 Reasons for improving shelf-life

Bakers are always looking for ways to improve the shelf-life of their products. The extra life can be used not only to reduce wastage but also to improve the productivity and profitability of the business. The latter aims can be achieved in a number of ways. First, longer life enables a larger range of goods to be delivered less regularly to shops and stores or to be distributed over a wider area. Since distribution costs make up a large part of the retail value of the product, this can be an important way of


Table 7.1 Range of bakery products that may be seen on the supermarket shelves

Type of product

Unsweetened goods

Sweets goods

Filled goods

Examples

Bread: sliced, crusty, part-baked, ethnic RolIs: soft, crusty Croissants English muffins Crumpets Pizza base Raw pastry"

Large cakes: plain, fruited Small cakes Doughnuts Pancakes Waffles American muffins Biscuits Cookies Wafers Buns

Tarts: with fruit, jam Pies: with fruit, meat" Sausage rolls" Pasties" Cakes: cream, custard Cheese-cake" Pizza" Quiche"

'Products that are normally sold refrigerated or frozen.

improving profits. Second, preservation methods resulting in extra shelflife can bring about savings in production costs by allowing the opportunity to modify recipes to give a cheaper product with a higher moisture content and thus more acceptable eating and keeping quality. Third, they can allow changes in production scheduling so that perishable lines are produced less frequently but in long runs, with the possibility of selling from stock rather than on demand. Of course, the financial advantage gained from such approach es will depend on the relative cost of the preservative measure used to obtain the extra shelf-life.

Before discussing methods of preservation and, in particular, MAP techniques, it is important to have an appreciation of the factors that limit the shelf-life of bakery products.

7.2 Factors governing shelf-Iife

The most common forms of deterioration are microbiological spoilage, staling and moisture loss or gain. Each of these are discussed briefly below.

BAKERY PRODUCTS 137

7.2.1 Microbiological spoilage

Of the three forms of deterioration, microbiological spoilage is the most important because not only can the baker be held legally responsible but also the adverse publicity that may arise from such an event can seriously affect sales. There are two groups of factors that influence the rate at which microorganisms grow in a food. First, there are those which relate to the food itself, known as intrinsic factors. These include water activity (aw ),

degree of acidity (pH), nutrient content and the presence of natural inhibitory substances. The second group of factors are those which concern the environment in which the food is stored, known as extrinsic factors. These include temperature and the relative humidity (RH) and the gaseous composition of the atmosphere surrounding the product.

Intrinsic Jactors. The main factor governing whether microorganisms will grow and the rate at which they will grow in a food is aw . Figure 7.1 shows the minimum aw value for the growth of different groups of spoilage microorganisms together with the aw values of selected baked goods. It is

PRODUCT aW MICROORGANISMS

1.00

bread: crumpe!s: { creameustard; meatfillings

0.90+-- normal bacteria

plain eakes; buns; +-- normal yeasts paneakes; waffles

fruited eakes; 0.80+-- normal moulds buttercream; jam fillings

0.70 eookies; tarts; dried fruit

+-- xerophilie moulds

0.60+-- osmophilie yeasts

Figure 7.1 Minimum aw values for the growth of spoilage microorganisms and aw values for selected bakery goods.


evident that bacteria are only likely to grow and cause a spoilage problem in moist products with an aw value above 0.9. The majority of yeasts also prefer conditions of high aw but there is an important group, known as osmophilic yeasts, that are capable of growing in foods with an aw value as low as 0.6, although they gene rally prefer products with a higher aw in the range 0.8-0.9. Because of their wide aw tolerance range, moulds are the most frequent cause of spoilage of bakery products.

Intrinsic factors other than aw are less important. The growth of so me bacteria is retarded in acid foods, but yeasts and moulds are largely unaffected by pR within the normal range found in baked goods. Natural inhibitors can occur in richly fruited items and shortage of nutrients can restrict microbial growth in some imitation cream fillings, but, these instances apart, such factors are of minor importance.

Extrinsic /actors. Storage temperature is the most important extrinsic factor influencing the rate at which microorganisms will grow in or on a food. Most of the bacteria, yeasts and moulds causing spoilage of bakery products are mesophilic, having a temperature growth range of between 10 and 40°C and an optimum temperature of 25-35°C.The importance of storage temperature on the rate at which microorganisms will grow and cause spoilage of food can be judged from the results shown in Figure 7.2.

The relative humidity of the storage atmosphere affects the rate of moisture loss from the producL The faster the surfaces dry, the less chance there is that microorganisms will grow on them, although, of course, they may still grow on internal cut surfaces, which are less subject to drying. Atmospheric relative humidity can be a significant factor in determining whether mould will occur on products that are unwrapped or enclosed in a moisture-permeable wrapper.

Unless deliberately altered, as in MAP, the gaseous composition of the atmosphere is unlikely to affect microbial growth in baked goods.

Types 0/ microorganism.

Bacteria. With bakery goods, the major cause of bacterial spoilage are members of the Bacillus subtilis group, which produce heat-resistant spores that can survive the baking times and temperatures. Such spores are commonly present in raw ingredients such as flour. In bread, the spoilage condition is known as 'rope', where the symptoms are the initial development of a characteristic fruity odour followed by a gradual decomposition of the crumb, which becomes sticky to the touch. B. subtilis spoilage, with or without the above symptoms, can also occur in other high aw products such as crumpets and the filling of meat pies or pasties. These type of problem usually only occur where the product is cooled slowly and stored under warm temperatures.

200

100

50

~ 0 :s ~

20 ~ -& Cl Cl

.t= 10 -6 "5 0

:::!

5 Key

• 27'C

· 21°C

· 16°C 2 + lOT

0.70

•

0.75

BAKERY PRODUCTS

"'''''''''' +

+

',~ + "'''''''''' ,

"':" . " ',fI +" ...

',+ ... '~,

."', ''''+, '~ ...

•

0.80 0.85 0.90

139

I

0.95

Figure 7.2 The effect of temperature on the shelf-life at varying aw values for products, based on storage tests for a variety of bakery products wrapped in moisture-proof film.

Occasionally, souring-type spoilage is encountered in high aw fillings of cream, custard or meat. Lactic acid bacteria and/or coliform bacteria are usually involved in such instances. The spoilage rate depends not only on the aw and pH of the food and storage temperature but also, most importantly, on the numbers of spoilage bacteria originally present.

Yeasts. Fermentation problems caused by osmophilic yeasts occur from time to time in the low aw , high sugar coatings and fillings used for flour confectionary items. The effect on the product can be twofold. First, the carbon dioxide gas produced during yeast growth may give an undesirable appearance. For example, cracking of royal icing on birthday cake or bubbling on fondant coatings. Second, a typical alcoholic odour and flavour is produced which can be the subject of complaint. When such fermentation problems arise it is often difficult to trace the source of the yeast contamination. Unclean plant and equipment contaminated with already spoiled material is often to blame (Tilbury, 1976). The rate at which a given high sugar content coating or filling will start to ferment depends on the aw of the substrate, the number and type of yeasts originally present and the storage temperature (Leg an and Voysey, 1991).


Moulds. Mould growth is the most frequent cause of microbiological spoilage of bakery products and is often the main factor governing shelflife. Moulds are destroyed by the baking temperatures. Hence, contamination occurs after the product leaves the oven from mould spores in the atmosphere and through contact with contaminated surfaces. The most important factors determining whether a mould spore will germinate and develop into a visible colony are the aw of the product and the storage temperature. The relationship established between these factors and mould-free shelf-life is shown in Figure 7.2.

The effect of initial level of contamination is less pronounced with moulds than with bacteria and yeasts, although it is still important. The type of mould present can have a marked effect on shelf-life. With goods with a high aw value, such as bread and moist cakes, a wide range of different types of mould can be encountered, of which the blue-green coloured Penicillium group are the most common. Occasionally, fast growing-species such as the orange coloured Chrysonilia (Neurospora) sitophila and members of the Mucoraceae cause serious problems with these moist items. For products of lower aw (below 0.85), the main spoilage moulds are members of the Eurotium glaucus group, which pro du ce pale green spreading colonies. When present, the xerophilic mould Wallemia sehi, which produces chocolate brown coloured colonies, can limit the shelf-life of these low aw goods.

7.2.2 Staling

Staling is the name given to physicallchemical changes that occur in a product resulting in a progressive firming of texture during storage, with an associated loss in flavour and mouthfeel. These changes take place in the absence of moisture loss (Kulp, 1979). Staling poses a serious problem with bread and other fermented products but is less crucial for items that contain high levels of sugar and fat, such as cake. The shelf-life of the latter could often be much longer if it were not for other forms of deterioration. Staling is prevented by freezing and can be reversed by reheating prior to consumption.

7.2.3 Moisture loss or gain

Deterioration in the eating quality of bakery products results from moisture loss or gain during storage. With most items, the problem is 'drying out', but with dry products, such as biscuits, the problem is moisture gain, which results in 'loss of crunch'. These defects can be overcome by the use of moisture-proof wrapping materials and effective sealing. It should be noted that, even in the absence of moisture loss, some redistribution of water occurs within many products, which can cause

BAKERY PRODUCTS 141

changes in sensory properties. These changes can be beneficial in some products, for instance fruited cakes, or they can be detrimental in others, for example some plain cakes (Guy, 1983). The losses, gains or changes in moisture within the product tend to occur more rapidly at higher storage temperatures. The position of a pack when stacked on the shelf of the shop or store wil\ also affect the amount of moisture loss or gain which occurs.

7.3 Methods of extending mould-free shelf-life

7.3.1 Hygiene considerations

Large numbers of mould spores are present in the atmosphere and on surfaces within the bakery no matter how good the standards of hygiene practised. From these sources they become attached to the surfaces of products after cooling and before wrapping. Although there is no doubt that reducing the initial microbial load by the instigation of a rigorous c1eaning and disinfection programme is helpful in maintaining and even increasing the inherent shelf-life, it should be appreciated that the presence of only a single mould spore has the potential to limit the life of the product. Therefore, so me positive preservative measure is usually required to obtain a worthwhile extension in mould-free shelf-life.

The approaches that can be considered for extending mould-free shelflife involve preventing contamination entirely, destroying any contamination present, or limiting the rate at wh ich moulds are able to grow on the product.

7.3.2 Preventive measures

A mould-free product can be achieved by packaging before baking, packaging directly after baking or by cooling and packaging in a sterile atmosphere. In general, such measures are limited to items that do not need to be sliced, coated or fi1led after baking.

There are a number of wrapping materials that are capable of withstanding the temperatures achieved during baking. Some will tolerate normal oven temperatures, others can only be used where the product is steamed or baked at a low temperature for a long time. A long shelf-life can be obtained by depositing doughs or batters into a vacuum-formed heat-resistant base which is lidded but left part sealed to avoid expansion of the pack during heating. The final sealing takes place shortly after leaving the oven or steamer. This technique has been used successfully with partia\1y baked fermented goods and for steamed puddings, where crust colour, condensation and staling are less important.


Another approach is to bake the product fully in a container that is closed before the surface has cooled to 80oe. This method is commonly used for canned cakes but can also be used to preserve items in plastic containers Although potentially a cheap and convenient method of achieving a mould-free product, it has the dis advantage that quality may be adversely affected by the condensation which occurs. Moreover, a second standby lidding machine is necessary in ca se of breakdown.

In theory, contamination can be prevented by cooling and wrapping in a sterile atmosphere, but in practice, it is difficult and expensive to achieve this aim under commercial conditions.

7.3.3 Destructive measures

Various methods are available to destroy mould spores present on the outside surfaces of oven-finished wrapped products. Some form of he at treatment, ultraviolet irradiation or gamma irradiation can be used.

Heating in an oven. A mould-free product can be achieved by returning the wrapped items to the oven and heating at a low temperature until all parts of the surface reach at least 80°e. This is a labour intensive and, therefore, expensive procedure unless the pasteurizing oven is placed online. Moreover, it can be unreliable if the oven conditions are not carefully controlled and the seals on the packs weil made. Instead of a conventional oven, a microwave oven can be used. The heating time in such an oven needs to be precise to prevent excessive expansion of the pack, which may' damage the seals. Microwave pasteurization has been employed successfully as a means of obtaining long-life fermented products.

Infrared irradiation. Irradiation by means of a tunnel of infrared projectors has been used to heat treat the surfaces of wrapped bread and cake and thereby increase mould-free shelf-life (Seiler, 1984). This' approach has the advantage that problems of pack expansion and condensation are reduced. However, a rather complicated installation is required to heat all parts of all surfaces of a many sided product. Infrared heat treatment is particularly suitable for items where only a single surface is at risk, such as puddings in basins.

Ultraviolet irradiation. In a similar fashion, the mould-free shelf-life of a range of oven-finished products wrapped in a clear film can be increased by surface treatment in a tunnel of ultraviolet lamps. To be effective, the surfaces of the product need to be smooth and with few cracks or crevices, which protect the mould spores from the light. The fact that the product is not heated by this procedure is a great advantage, but, like infrared, a complicated installation is required to treat a many sided product properly.

BAKERY PRODUCTS 143

Moreover, careful design is necessary to ensure that the eyes of workers are protected from the light.

Gamma irradiation. A brief treatment with gamma radiation can result in useful extensions in the mould-free shelf life of most bakery products. Grecz et al. (1985), in tests with Arabic bread, found that gamma irradiation successfully increased mould-free shelf life to over seven days without affecting its sensory properties. However, Seiler (1984), in experiments with a range of bakery items, showed that a dose having ä worthwhile effect will often adversely affect odour and flavour. In view of public disquiet regarding the safety of gamma irradiation, it is doubtful whether this method of preservation has much future.

7.3.4 Measures involving growth inhibition

Recipe adjustment. Whenever a longer shelf-life is desired, the first step is to assess wh ether there is any way in which the recipe can be altered to re du ce aw . Methods of adjusting recipes to give a lower aw and increased shelf-life are discussed by Cooper et al. (1968). With flour confectionery lines this can sometimes be done by the inclusion of humectants such as glycerol or sorbitol. In general, the approach has only limited application since steps to reduce aw tend to affect the eating quality of the prodflct adversely.

Preservative. The most common method of inhibiting the growth of moulds on bakery products involves the inclusion of a permitted preservative. Propionic acid and its salts are widely used in bread and other fermented goods, not only to delay the onset of mould growth but also to insure against the possibility of a rope spoilage problem. Sorbic acid and its salts are the most commonly used preservatives in flour confectionery items. The effectiveness of these substances is strongly linked with pH and aw . This is apparent from Table 7.2, which shows the concentration of sorbic acid caJculated to increase mould-free life by about 50% in cakes of differing pH and aw values, based on the results of a large number of storage tests (Seiler, 1984). The levels indicated in Table 7.2 are likely to

Table 7.2 Effect of aw and pH on the antimould activity of sorbic acid in cake

Water activity (aw )

0.90 0.85 0.80

Sorbic acid (% by product weight) estimated to give a 50% increase in mould-free shelf-life at pH va lues of

7.0

0.23 0.15 0.08

6.5

0.11 0.08 0.04

6.0

0.06 0.04 0.02

5.5

0.03 0.02 0.01


be the maximum that can be used without affecting the odour and flavour of the product. Similar effects can be expected with propionic acid in breads, although the pH variation is likely to be less in such products. While these materials have an important role to play in protecting bakery products from microbial spoilage, the extension in shelf-life that can be obtained is relatively short. Moreover, there is a consumer objection to the use of such additives in food.

The US Army has explored many antimicrobial/antifungal agents to extend shelf-life of both non-refrigerated and refrigerated ration components; one of these agents is allyl isothiocyanate (AlT). Several papers described the potential of this potent volatile antimicrobial compound extracted from natural sources such as mustard seeds, radish, and horseradish (Uda et al., 1993; Tokuoka and Isshiki, 1994; Delaquis and Mazza, 1995). One of the major sources of AlT is wasabi (Japanese mustard oil), and an extract system, WasaOuro (WO), a patented extraction method and application of a naturally existing compound from wasabi have been developed by the Green Cross Corp., Osaka, Japan. The WO system uses volatile gases to inhibit fungal and bacterial growth, from either outside or inside the food package. There is no direct contact with the food and no possibility of accidental ingestion, as can occur with other preserving packets enc\osed with the food. The duration of the effectiveness can be monitored by the unit concentration of the preservative and the size of the WO label/sheet. Several potential applications of WO system in ration systems were validated and suggested (Conca and Yang, 1996; Sikes el al., 1996; Yang and Conca, 1996; Worfel el al., 1997). One of the applications of the antifungal agent is in bakery products. Figure 7.3 shows two loaves of bread that were stored at room temperature with high humidity (simulating the household kitchen); the one with the WO label (non-permeable film - WO) attached on the outside of bag permeable to WO showed no trace of mold or yeast after one month while the control bread was moldy and mushy after one week. When a release-control WO label was used, the bread lasted more than three months. Another application is a combination of carbon dioxide, nitrogen and WO gas; its antibacterial effect is shown in Figure 7.4.

Allyl isothiocyanate had been in a category of 'generally recognized as safe' (GRAS) as a flavouring ingredient with regulated maximum use level (Hall and Oser, 1965). However, in order for it to be used as a food preservative or additive, a regulatory approval is required because the level required is higher than that allowed under GRAS status. Approval is currently being sought in the US and Europe. The agent is approved in a number of Pacific Rim countries (e.g. Japan, Taiwan).

Freezing and refrigeration. Staling andmicrobial growth can be prevented indefinitely by freezing, but, unless products are very weIl protected,

BAKERY PRODUCTS 145

Figure 7.3 The use of the WasaOuro (WO) system as an antifungal in bakery products. Two loaves of bread were stored at room temperature (68-70°F) and with high humidity (simulating the household kitchen) fOT 1 month. The loaf on the left had no WO label, the one on the right had a WO label on the outside of the bag. WO is applied to a non-permeable

film that is then affixed to a permeable packaging film.

moisture losses and other quality changes can occur with time. Although an excellent method of preservation, freezing tends to be too expensive for all but more profitable lines. Refrigeration is widely used for the more perishable filled bakery products but is not suitabIe for the large range of fermented goods, where staling occurs more rapidly at reduced temperatures.

7.4 MAP

Of more recent interest are methods whereby the atmosphere surrounding the product is modified so that mould growth is retarded. There are three


Bacterialg

108 ~~======~----------------------------D Control

D NIC02

NICOINO

106 f----------------i

105 f-----------------i

104 f-----------;

o 3 10

Storage (days)

14

Figure 7.4 Experimental data on the- effect of 1% WasaOuro on bacterial growth in a food product . The gas is introduced by compensated vacuum in a package of biaxially-oriented pp coated with PUdC. (Figure supplied by Midori Pharmacia Corp., New York, NY.)

approaches, namely, gas packaging, the use of oxygen scavengers and the use of ethyl a1cohol vapour. Each of these will be discussed in some detail in the following paragraphs.

7.4.1 Gas packaging

Type 01 gas. It has been known for many years that altering the composition of the atmosphere surrounding a food can result in useful increases in shelf-life. Some gases, such as nitrogen and argon, are inert and, therefore, show no inherent antimicrobial activity. They restrict the growth of aerobic organisms, like moulds, by reducing the amount of oxygen present. To be effective, at least 98% of the inert gas needs to be present in the headspace atmosphere, and this must be maintained during storage. In practice, it is very difficuIt to achieve these conditions since, even withan impermeable wrapping material and good seals, some ingress of oxygen is likely during storage. Also, with gas flushing systems, most bakery products contain signifiqmt amounts of entrained air, which diffuses out into the headspace after packaging. A number of workers (Smith et al., 1983; Ooraikul, 1991; Black et al., 1993) have shown that

BAKERY PRODUCTS 147

nitrogen packaging will only give a relatively small increase in mould-free shelf-life. Nitrogen is often the preferred gas in cases where the aim of gas packaging is to protect the food from undesirable oxidative changes.

A number of gases exert a positive antimicrobial activity. These include carbon dioxide, carbon monoxide, ozone, ethylene oxide, propylene oxide, nitrous oxide and sulphur dioxide. Of these, only carbon dioxide is suitable for food preservation on stability, toxicity, organoleptic and economic grounds. For these reasons, gas packaging usually involves the use of this gas.

It appears to be the practice these days to use a mixture of carbon dioxide and nitrogen rather than carbon dioxide on its own. The reason for this is the finding by a number of workers (Brummer et al., 1980; Ooraikul, 1982; Seiler, 1989) that packs can show shrinkage during storage, so that the food becomes damaged, when high concentrations of carbon dioxide are employed. The partial vacuum that is formed inside the pack is caused by the use of wrapping materials which are more permeable to carbon dioxide than to air. Mixtures of nitrogen and carbon dioxide serve to overcome such pack collapse problems. Laminated films based on nylon appear to be particularly prone to pack collapse using carbon dioxide. However, Seiler (1989) points out that there are a number of packaging films available that do not give rise to such problems when used with high levels of carbon dioxide (section 7.4.4).

Increasing mould-free shelf-life with carbon dioxide. Carbon dioxide has an inhibitory effect on the growth of most microorganisms, its effect increasing with concentration. According to Wilbrandt (1989), carbon dioxide interferes with the various enzymatic and biochemical pathways necessary for the metabolism and hence the growth of microorganisms. It has no lethai effect and growth will recommence after the gas is removed.

In the earliest known work on bakery products, Skovholt and Bailey (1933) showed that the mould-free shelf-life of bread was increased in an atmosphere of 17% carbon dioxide and doubled in 50% carbon dioxide. Aalund (1961) confirmed these findings and found that high concentrations of carbon dioxide delayed the onset of mould on rye bread far 16 days. In more detailed tests, Seiler (1989) compared the percentage increase in mould-free shelf-life at 21°C of a number of bakery products when packaged in atmospheres containing different amounts of carbon dioxide (Figure 7.5). It is apparent that the shelf-life extensions at a given carbon dioxide concentration vary with product but not with storage temperature. With a product such as cake, wh ich usually has an aw below 0.85, the mould-free shelf-life can be expected to be increased by at least five times when the carbon dioxide concentration is maintained at above 80% by volume. Much longer increases may be anticipated as the concentration approaches 100%, where shortage of oxygen will also restrict mould

148

g Q) ~ -Q) .J::. (J)

Q)

~ -6 :; 0 E .~ Q) (J)

0 ~ () .f;

PRINCIPLES AND APPLICATIONS OF MAP OF FOODS

500

400

300

200

100

o

Storage

Produet ~ tempo ( ('C)

+ Madiera eake 0.85 21

r-" Madiera eake 0.83 27 0 Crumpets 0.94 27 0 Part baked bread 0.91 21 A Fruit pies 0.95 27 ++

+ • Rye bread slices 0.92 21 +

• Part baked rolls 0.88 21 +

++ +

+ +

" " " " 00

,,+ + A 0

)(

+ )( 0 +

+ 0

0 0AO A

)(

)( • • • 0 0 A

I

o

+ )( • • • " OA •• • •

+ • • • • • • •• •

I I I I

20 40 60 80 Average earbon dioxide eoneentration

(% by volume)

I • I •

I

100

Figure 7.5 The increase in mould-free shelf-life at 21°C of various bakery products packaged in different amounts of carbon dioxide.

growth. Even at an average carbon dioxide of 50%, a threefold to fourfold increase in shelf-life should be obtained.

With short shelf-life items with aw values above 0.90, the results in Figure 7.5 suggest that the effects of MAP in carbon dioxide are less pronounced and more variable. At high concentrations, extensions in mould-free shelf-life of about 2.5 times were obtained in tests with crumpets, fruit pies and bread, whereas, with rye bread and roUs of similar aw value, the increase was only 1.5 times. The reason for this difference

BAKERY PRODUCTS 149

was considered to result from the nature of the mould population present rather than the effect of product aw . The predominant moulds on cake and other products with an aw value below 0.85 are members of the Eurotium glaucus group, which are more susceptible to the effects of carbon dioxide than the Penicillium group of moulds, which predominate in products of higher aw . Similarly, some species of Penicillium, such as P. roquefortii, are particularly resistant to the effects of carbon dioxide. The rye bread and rolls used for these tests were manufactured in German bakeries, where Spicher (1985) showed P. roquefortii to be common. Surveys carried out in English bake ries have shown that these carbon dioxide-resistant moulds are rarely found, wh ich explains the longer shelf-life with the crumpets, fruit pies and bread used in these tests, which were carried out in England.

Unfortunately, it is not easy to relate these results with those obtained by other workers. In the majority of literature references on the subject, the products are not examined frequently enough to obtain an accurate increase in mould-free shelf-life and the nature of the mould population is rarely mentioned. In tests with Arabic bread, Avital and Mannheim (1988) found that an atmosphere of 47% carbon dioxide in air increased mouldfree shelf-life by 1.3 times, which is of a similar order to that expected with bread in Figure 7.5. However, with mixtures of 40 and 73% carbon dioxide in nitrogen, the increases were 2.0 and 3.7 times, respectively, which is more in line with the cake relationship in Figure 7.5. Presumably, the extra shelf-life resulted from the an aerobic conditions created by such gas mixtures.

There is no reliable evidence to indicate that the antimould activity of carbon dioxide is affected by product aw other than by its indirect effect on the nature of the mould population, as discussed above. Nor would storage temperature appear to have any pronounced effect. It seems likely that the antimicrobial activity of carbon dioxide, like that of other preservatives, increases with a reduction in the pH of the product, but no research appears to have been undertaken to substantiate this hypothesis.

Other forms of microbial spoilage. Sometimes the shelf-life of MAP baked products is limited by the growth of bacteria and yeasts rather by moulds.

Bacteria. Preliminary investigations (D.A.L. Seiler, unpublished data) suggest that packaging in carbon dioxide will effectively prevent the growth in bread of the aerobic spore-forming bacteria belonging to the B. subtilis group, which cause the condition known as 'rope'. The inhibition may be as much to do with the acidic nature of the gas as its positive antimicrobial activity However, it is possible that B. licheniformis, a c10sely related but facultatively an aerobic species that is more acid tolerant, may be able to


grow and cause rope spoilage even in the presence of carbon dioxide. As an additional safeguard, permitted preservatives like acetic acid or propionates may need to be used in conjunction with MAP to ensure rope prevention.

There is also a potential hazard of spoilage by certain acid-producing, facultative anaerobic bacteria using MAP. Ooraikul (1982) describes problems of swelling of the pack and acidity caused by the growth of lactic acid bacteria in crumpets and other high aw products stored in a mixture of 66% carbon dioxide and 33% nitrogen. The inc1usion of permitted preservatives was shown to overcome this type of problem.

Spore-forming an aerobic bacteria belonging to the genus Clostridium, some of which can cause illness, might be able to multiply in some high aw

value bakery products in MAP. However, their growth is unlikely since these bacteria require conditions of very low oxygen tension to multiply, which are difficult to achieve in commercial MAP. Moreover, there are few bakery products that would satisfy the aw , pH and nutritional demands of these organisms.

Yeasts. Ooraikul (1991) found that yeast growth limited the shelf-life of items with high sugar and low pH such as cheese cake, apple pie, apple turnover and layer cake. Even with high levels of carbon dioxide, it proved difficult to prevent the yeast growth and thus obtain a worthwhile extension in the shelf-life of these products. It is not easy to understand why yeasts were such a problem with these items since they should have been destroyed during baking. Seiler (1989) in tests with carbon dioxidepacked bread and crumpets encountered occasional contamination with filamentous yeasts (chalk moulds). These organisms are recognized by a white, powdery spreading growth on the surfaces of the product. During growth they release additional carbon dioxide, which causes the pack to expand. Fortunately, post-baking contamination by these yeasts can be eliminated by appropriate hygienic measures.

Although it is important to be aware of these other types of microbial spoilage, it should be emphasized again that mould growth is the major factor limiting the shelf-life of MAP bakery products. Problems with bacteria and yeasts can usually be overcome by ensuring that the product is always fully baked and that post-baking contamination does not occur through poor hygiene.

Product quality. If a high concentration of carbon dioxide is employed, the product may have the bitter acidic flavour of carbonic acid when consumed directly on removal from the pack. However, the gas is lost very rapidlyon opening the pack so that any acidic flavour disappears within a few minutes. Nitrogen, the other commonly used gas, is inert and does not react with the food. There are no reports of any off-flavour using this gas.

BAKERY PRODUCTS 151

A number of workers have examined the effects of MAP on the rate of staling of bakery products. Knorr and Tomkins (1985) found that the rate of staling of white and wholemeal bread was significantly reduced when packaged in carbon dioxide compared with nitrogen or air. Brummer et al. (1980) with toast bread and rye bread and Avital et al. (1990) with Arabic bread, confirmed these findings. In contrast, Doerry (1985) and Larsen (1992) failed to find any difference in the staling rate or sensory appeal of bread or cake stored in carbon dioxide, nitrogen or air. Tests support the laUer findings (D.A.L. Seiler, unpublished data). Therefore, the evidence on whether carbon dioxide has an antistaling effect or not is conflicting. Further research is needed in this area.

The packaging materials and sealing methods needed to retain gas also retain moisture. Hence, sensory problems owing to moisture loss or gain are rarely encountered with MAP bakery goods.

7.4.2 Oxygen scavengers

A novel approach to MAP, which originated from Japan, is the inclusion within the food pack of a sachet containing a substance that absorbs the oxygen from the headspace by chemical reaction. The an aerobic conditions created retard the growth of aerobic microorganisms. In addition, they prevent oxidative changes that can cause a deterioration in the eating quality of food during storage. In Japan, such sachets containing oxygen scavengers are included in packs used for a wide range of different foods. A range of oxygen-scavenging substances are used. Some act rapidly, some slowly; some are used with moist foods, others with dry; and so me have a dual effect such as oxygen absorption and carbon dioxide production (Harima, 1990).

Storage tests with artificially mould-contaminated slices of bread and cake in packs of oxygen-impermeable film containing a rapid acting oxygen scavenger sachet are reported by Seiler (1989). No mould growth was encountered after prolonged storage at 27°C. With the bread, both rope and the growth of filamentous yeasts were considerably delayed. Interestingly, useful increases in mould-free shelf-life were still obtained when a leakage si te was deliberately included in the seals of the packs. In further tests with biscuits, the presence of an oxygen scavenger resulted in retained flavour, retarded rancidity and discoloration and an increased shelf-life by four to fivefold. Powers and Berkowitz (1990) with bread, Ooraikul (1991) with crusty rolls and Black et al. (1993) with pi ta bread confirm the very long extensions in shelf-life that can be achieved using oxygen scavengers.

The use of oxygen scavengers has the advantage over gas-packaging methods in that it is possible to include more scavenger than really required, to allow for any ingress of oxygen through faulty seals. Indicator


tablets, which change colour according to the amount of oxygen present, can also be introduced into the pack to show whether any leakage has occurred. The major disadvantage of this system is the cost and likely consumer objection to the presence of a sachet inside the pack. Even if this is separated from the food and firmly attached to prevent it from falling out and being eaten by a child or dog, its presence is likely to be noted.

7.4.3 Ethanol

Ethanol is weil known as a powerful bactericide that is widely used in hospitals and in food premises to sterilize instruments, utensils, working surfaces, etc. What has not been fully appreciated until comparatively recently is that ethanol is also an effective antifungal agent that can be used for food preservation purposes.

An early reference to the use of ethanol as a preservative for bakery products comes in a 1976 United States patent that describes tests with pizza bases where a fivefold increase in mould-free shelf-life was obtained by spraying the bases with 2% by weight of ethanol before packing. Subsequently, extensive tests were carried out by Seiler (1989) to evaluate the usefulness of 95% food-grade ethyl alcohol for preservation of a range of baked goods. Initial experiments showed that ethanol acted in the vapour phase, since much the same increase in mould-free shelf-life was obtained whether the same amount of alcohol was sprayed over the surfaces or placed in the pack out of contact with the product. The extensions in shelf-life with a given level of treatment with ethanol were found to vary according to the aw of the product, the tightness of wrapping, the gas permeability of the film and the seal integrity. The relationship established between the alcohol applied and the increase in mould-free shelf-life of bread is shown in Figure 7.6. Not only did the alcohol prove to be an effective antimicrobial agent but it also delayed the rate of staling of both bread and cake. The odour and flavour of ethanol could be detected in products treated at the 1.0% level by product weight but was not considered objectionable.

A development originating in Japan is a sachet containing food-grade ethanol adsorbed on to silicon dioxide powder. When placed in a food pack, the alcohol vapour is slowly released into the headspace. Flavouring agents can be included in the sachet to mask any alcoholic odours or flavours in the food. In their trade literature, the suppliers indicate that the concentration of ethanol required to give a worthwhile increase in shelf-life varies with the aw of the product. About 0.5% of ethanol by weight may be needed for preservation of products with an aw value below 0.90, whereas as much as 4.0% may be needed to protect very high aw level goods, especially if contaminated with yeasts. Studies by Ooraikul (1991) showed that ethanol atmospheres are more effective than gas packaging in

BAKERY PRODUcrS 153

250

~ 200 ~ -Q) .r::. CI>

(J) 150 ~ -I

"0 "5 0 E 100 .!; (J) CI> 0 ~ 50 u .!;

o o 0.5 1.0 1.5

% by weight of 95% ethyl alcohol

Figure 7.6 The effect of ethyl alcohol application on the mould-free she1f-life of bread.

controlling yeast fermentations in doughnuts, layer cake, apple turnover and cheese cake.

Modifying the headspace atmosphere with ethanol vapour is an excellent way of preserving bakery products. Not only will the growth of moulds, yeasts, lactobacilli and other microbial contaminants be retarded, but the rate of staling will also be reduced. Powdered aIcohol in sachets is Iikely to be duty free and hence should be economical to use. Like oxygen scavengers, discussed above, a major difficulty may be to persuade the consumer to accept the presence of a sachet in the pack. Also, despite the inclusion of flavouring agents, there may be off-odour/flavour problems with some products at an ethanol concentration that gives a useful increase in shelf-life.

7.4.4 MAP methods and materials

It is not proposed to give a detailed description of the methods that can be used to pack baked goods in an atmosphere of gas or the packaging materials involved, since these matters are covered in other chapters. The


intention is merely to point out certain aspects that may not have been given sufficient attention elsewhere.

Bulk gas packaging. Packaging of individual products or small groups of products in gas using either form-fill-seal or evacuation-flush machinery is now weIl established. What is less frequently adopted is a bulk packaging system whereby large numbers of units are enclosed in a gaseous atmosphere. This approach is particularly suited to situations where preservation is required during the early stage in the life of a product. For example, bulk gas packaging could prove worthwhile where the aim is to seIl from stock rather than on demand, deliver less frequently or widen the area of distribution, e.g. by exporting. It is possible to consider placing multiples of pre-packed units in fibreboard boxes fitted with agas impermeable liner, which are either evacuated and flushed with gas, or have suitably sized sachets of oxygen scavenger or powdered ethanol added prior to sealing. Alternatively, where goods are transported over long distances, bulk containers or box vehicles could be continually flushed with carbon dioxide gas. This is already done to slow down the respiration rate of certain fruits. For these systems to work, it is necessary for the individual packs to be permeable to the gas. This can be achieved by using agas-permeable wrapper or by deliberately leaving a sm all gap in the seals. Bulk gas packaging has the advantage of reliability since it should be easier to ensure good gas-tight seals on bulk packs and maintain the desired gas concentration in containers or vehicles.

Alternative wrapping materials. It is usually assumed that it is necessary to use more expensive, impermeable laminated films for gas packaging purposes. Certainly this is true where a consistently long increase in shelf-life is required. However, sometimes only a relatively short increase may be needed. For example, to help overcome a current mould problem or where it would be beneficial to make a sm all increase in product aw to improve eating quality. In such cases, consideration can be given to the use of a less expensive wrapping material in conjunction with gas packing in carbon dioxide, an oxygen scavenger or ethanol. Gas-permeable films that seal weIl, such as polyethylene, will hold carbon dioxide for long enough to extend mould-free shelf-life by several days. Longer increases can be expected using polypropylene, polyester or cellulose films coated with polyvinylidene chloride, which are sufficiently gas impermeable but need ca re with sealing to prevent leaks. Cold-seal films also appear to have potential for gas packaging purposes. Preliminary tests with cakes gas flushed in carbon dioxide on a form-fill-seal machine showed that the sealant was permeable to the gas but the leakage was uniform so that an increase in mould-free shelf-life of more than a week was achieved. An additional advantage with cold-seal films in terms of reliability is that

BAKERY PRODUCTS 155

heated sealing bars, wh ich can become distorted with use and do not then seal weil, are not used. It mayaiso be unnecessary to employ costly machinery to give a dweil time on the seals. Most of the comments on these less expensive wrapping materials are based on tests with carbon dioxide gas. Even better results may be anticipated using oxygen scavengers or ethanol.

Indicator systems. Mention should be made of developments in packaging technology (Day, 1991) whereby it may become possible to include indicator dyes within the laminations of a film that change colour according to the amount of carbon dioxide or oxygen present in the headspace of the pack. If such a film can be produced, it mayaiso prove feasible to include an oxygen scavenger or a powdered alcohol formulation between the layers of film. From a consumer point of view this would be better than adding a sachet into the food pack. Such developments mayaiso be beneficial for providing evidence of tampering.

7.5 Conclusions

Despite the proved efficacy of gas packaging procedures for preserving bakery products, it has been slow to become established. One of the main reasons for this is the low level of profitability of the baking industry, which makes it difficult to justify the cost of gas, oxygen scavengers or ethanol, plus the special machinery and laminated films which may be required. Some of the suggestions made regarding the use of less expensive wrapping materials may be applicable in some situations but further work is needed to establish their usefulness.

Doubts on the reliability of gas packaging methods is another important factor that has delayed the advancement of the process. Often difficuIties have been encountered in maintaining consistent gas-tight seals. Bulk packaging methods or the use of cold-seal films may provide the answer in some applications, but the most valuable all-round solution would be the development of so me system to detect and reject leaky packs at point of sale.

Acknowledgement

The editor thanks Dr Tom C.S. Yang, Senior Food Technologist, Department of the Army, Natick Research, Development and Engineering Center, Natick, MA for providing the comments on the antimicrobial compound allyl isothiocyanate.


References

Aalund, O. (1961) Mould problems in the production of sliced and packaged rye bread: effects of carbon dioxide. Nordisk Veterinaermedecin, 13,457-470.

Avital, Y. and Mannheim, C.H. (1988) Modified atmosphere packaging of pita (pocket) bread. Packaging Technology and Science, 1, 17-23.

Avital, Y., Mannheim, C.H. and Miltz, J. (1990) Effect of carbon dioxide atmospheres on staling and water relations in bread. Journal 01 Food Science, 55(2), 413-416, 461.

Black, R.G., Quail, K.J., Reyes, V. et al. (1993) Shelf life extension of pi ta bread by modified atmosphere packaging. Food Australia, 45(8), 387-391.

Brummer, J.M., Stephan, M. and Morgenstern, G. (1980) Mould prevention measures for bread. Part 2. Atmosphere exchange with carbon dioxide. Getreide Mehl und Brot, 34, 164-168.

Conca, K. and Yang, T.C.S. (1996) A unique allyl isothiocyanate preservative system, presented at the Annual Meeting 01 the Institute 01 Food Technologists, New Orleans, LA.

Cooper R.M., Knight, R.A., Robb, J. and Seiler, D.A.L. (1968) The equilibrium relative humidity of cakes. Food Trade Review, 38(5), 49-54, 56.

Day, B.P.F. (1991) Active packaging. Proceedings olthe Shell Lile '91 Conlerence, 14 June, The Packaging Group Inc., Milltown, NJ.

Delaquis, P.J. and Mazza, G. (1995) Antimicrobial properties of isothiocyanates in food preservation. Food Technology, 49(11): 73-84.

Doerry, W.T. (1985) Packaging bakery goods in controlled atmospheres. American Institute 01 Baking Research, Department Technical Bulletin, 7(4), 1-8.

Grecz, N., Bannon, R., Jaw, R. et al. (1985) Gamma processing of Arabic bread fOT immune compromised patients. Applied Environmental Microbiology, 50, 1531-1534.

Guy, R. (1983) Factors affecting the staling of Madiera cake. Journal 01 Food Science and Agriculture, 34, 477-491.

Hall, R.L. and Oser, B.L. (1965) Recent progress in the consideration of f1avoring ingredients under the Food Additives Amendment, III. GRAS substances. Food Technology, 19(1): 151-156.

Harima, Y. (1990) Free oxygen scavengers, in Food Packaging (ed. T. Kadoya). Academic Press, New York, pp. 477-491.

Knorr, D. and Tomkins, R.1. (1985) Effect of modified atmospheres on the compressibility of bakery products. Journal 01 Food Science. 50, 1172-1173, 1176.

Kulp, K. (1979) Staling of bread. American Institute 01 Baking, Department Technical Bulletin, 1(8), 1-7.

Larsen, H. (1992) Keeping quality of bakery products packaged in modified atmospheres. Tema lra Matlorsk, No. 8, 1-35.

Legan J.D. and Voysey, P.A. (1991) Yeast spoilage of bakery products and ingredients. Journal 01 Applied Bacteriology. 70(8), 361-371.

Ooraikul, B. (1982) Gas packing a bakery product. Canadian Institute 01 Food Science and Technology Journal, 15(4),313-315.

Ooraikul, B. (1991) Modified atmosphere packaging of bakery products, in Modilied Atmosphere Packaging 01 Foods. (eds B. Ooraikul, and M.E. Stiles). Ellis Harwood, Chichester, pp. 49-117.

Powers, E.M. and Berkowitz, D. (1990) Efficacy of an oxygen scavenger to modify the atmosphere and prevent mould growth on ready-to-eat pouched breads. Journal 01 Food Protection, 53(9), 767-771.

Seiler, D.A.L. (1984) Preservation of bakery products. Institute 01 Food Science and Technology Proceedings, 17,31-39.

Seiler, D.A.L. (1988) Microbiological problems associated with cereal based foods. Food Science and Technology Today, 2, 37-41.

Seiler, D.A.L. (1989) Modified atmosphere packaging of bakery products, in Controlledl Modified Atmosphere/Vacuum Packing 01 Foods. (ed. A.L. Brody). Food and Nutrition Press, Turnbull, CN, pp. 119-133.

Sikes, A., Yang, T.C.S. and Richardson, M. (1996) Antifungal activity of mustard oil extract, presented at the Annual Meeting 01 the Institute 01 Food Technologists, New Orleans, LA.

BAKERY PRODUCTS 157

Skovholt, O. and Bailey, C.H. (1933) Effect of carbon dioxide on mould growth on bread. Cereal Chemistry, 10,446-451.

Smith, J.P., Jackson, E.D. and Ooraikul, B. (1983) Storage study of agas packed bakery product. Journal of Food Science, 48(4), 1370-1371.

Spicher, G. (1985) Causes of mould growth on baked products. Deutche Lebensmittel Rundschau, 81, 16--21.

Tilbury, R.H. (1976) The microbial stability of intermediate moisture foods with respect to yeasts, in Intermediate Moisture Foods (eds R. Davies, G.G. Birch and K.J. Parker), Applied Science, London. pp. 138-165.

Tokuoka, K. and Isshiki, K. (1994) Possibility of application of allylisothiocyanate vapor for food preservation. Nippon Shokuhin Kogyo Gakkaishi 41(9), 595-599.

Uda, Y., Matsuka, H., Kumagami, H., Shima, H. and Maeda, Y. (1993) Stability and antimicrobial property of 4-methylthio-3-butenyl isothiocyanate - the pungent principle in radish. Nippon Shokuhin Kogyo Gakkaishi, 40(10), 743-746.

Wilbrandt, C.S. (1989) Checklist for MAP strategy. National Food Processors Association Convention Proceedings. Food Engineering, 6(4), 621-622.

Worfel, R.C., Schneider, K.S., and Yang, T.C.S. (1997) Suppressive effect of allyl isothiocyanate on populations of stored grain insect pest. J. Food Processing and Preservation, 21(1): 9-19.

Yang, T.C.S. and Conca, K. (1996) Potential applications of an antifungal/antibacterial agent (WasaOuro system) for military ration systems, presented at the Annual Meeting of the Institute of Food Technologists, New Orleans, LA.

8 Dairy foods, multi-component products, dried foods and beverages P.l. SUBRAMANIAM

8.1 Dairy products

MAP is used for a wide variety of products in the dairy sector, but it has mainly been applied to the packaging of cheese.

8.1.1 Cheeses

There are many different types of cheese, varying in composition and in shelf-life. Therefore, the packaging of each type of cheese needs to be considered separately. Another factor to consider is that some cheeses are ca rb on dioxide (C02) producers, while others are not. Furthermore, the age of the cheese may vary from three months to two years at the stage of packaging (MacDonald, 1985).

With cheese, the main factor limiting the shelf-life is mould growth, wh ich can be controlled by reducing or excluding oxygen (02) from the headspace of the packs. This can, of course, be achieved through vacuum packaging. Vacuum packaging has been found to be useful in packaging cheese for the retail market, giving an extended shelf-life. However, a disadvantage of this packaging method is that the packs are not 'userfriendly', because they cannot be opened easily. They also give the product a low-quality plastic image. The use of MAP has overcome these problems by making the packs easier to open and by extending the shelf-life. Furthermore, MAP can be used to package soft and the more crumbly textured cheeses without damaging them, which is not possible with vacuum packaging.

In Europe and, in particular, the UK, the application of MAP within the cheese sector has increased considerably, at the expense of vacuumpackaged products. The sale of MAP products has been growing at an annual rate of 11.2% (Rice, 1995). The volume of MAP products is thought to have exceeded that of aseptic and retort pouch and hot-fill packaging combined (Berne, 1994). The cheese sector of the MAP market has been particularly active over the past few years, introducing a number of important developments in the form of new products and package types. The developments have been driven by consumer demands for convenient

DAIRY FOODS, MULTI-COMPONENT PRODUCTS, DRIED FOOD AND BEVERAGES 159

products with extended shelf-life. The development of easy-open and resealable packs was an important breakthrough. The range of cheeses packaged under MAP has also expanded, bringing to the market many tradition al and difficult-to-package soft cheeses with extended shelf-life. For the sake of convenience, the wide range of cheeses covered here has been broadly categorized under either hard or soft cheese categories.

8.1.2 Hard cheeses

Hard cheeses such as Cheddar are being packed in 100% CO2 using horizontal form-fill-seal (FFS) pillow pack machines. However, there have been some recent developments in terms of gas mixtures used. Although, CO2/N2 (nitrogen) mixtures are commonly used for MAP of cheese, CO2

was said to develop a spongy texture in cheese and N2 to cause drying out. The optimum mixture to use for cheese has been claimed to be 75% CO2/

25% N2 (Berne, 1994). The usefulness of other noble gases such as argon has also recently been discussed, with claims that these gases preserve both the sensory and microbial quality of food products (Spencer, 1995). The packaging materials used include polyvinylidene chloride (PVdC)-coated cellophane or polyester/polyethylene (PE) (Damske, 1990), 15 11m oriented polyester (OPP)/50 11m low-density polyethylene (LDPE) with 4% ethylene vinyl acetate (Addington, 1991) and clear polypropylene (PP) (Hampton, 1982). MAP of cheese in PP film has a shelf-life of up to four weeks, compared with only 14--15 days when packaged under normal conditions (Hampton, 1982).

The form-fill-seal packaging operation involves forming the packaging material around the product, gas flushing, cross-sealing and cutting to give individual packs. Depending on the size of the blocks and other factors, the packaging speeds possible are 42 to 180 packs per minute (Damske, 1990). The packs are flushed with CO2 until air is displaced to give a residual O2

content of less than 2% and sometimes 1%. Soon after gas flushing, the packs have the appearance of pillow packs, but over the subsequent few hours the film collapses around the product because of the absorption of CO2 by the cheese (Figure 8.1). The package appearance, therefore, becomes similar to that of a vacuum-packed product. Calcium precipitation on the surface is a problem often found with cheeses. Although this problem can be prevented by the use of vacuum packaging, it cannot be overcome by the use of MAP (McDonald, 1985).

Sliced and grated cheeses are also packaged und er modified atmospheres. For these products it is not possible to use 100% CO2 as the absorption of the gas by the product causes the packaging to collapse, crushing the product and interfering with the ease of separation. Therefore, N2 is used as part of the gas mixture to stop the total collapse of the film around the product. The gas mixture typically used for these value-


a

b

DAIRY FOODS, MULTI-COMPONENT PRODUcrS, DRIED FOOD AND BEVERAGES 161

c Figure 8.1 Form-fill-seal packing of cheese in carbon dioxide. (a) Pillow appearance soon after flushing; (b) absorption of gas by the cheese causes the film to collapse; (c) the final

product looks similar to a vacuum pack.

added products is 70% N2/30% CO2 . Grated cheese is packaged using flexible films similar to those used for hard cheese blocks (Figure 8.2). However, the films mayaiso be metallized. Vertical form-fill-seal machines with packaging speeds of 45 to 75 bags per minute are used (Damske, 1990). Cheese slices are packaged in similar gas mixtures but interleaved with paper for easy separation of slices. The slices are often packaged in rigid punnets on thermoforming machines. A possible problem during MAP of grated cheese is that, on applying vacuum prior to gas flushing, the product can become crushed. The use of repeated low levels of vacuum prior to gas flushing is said to avoid this problem and give low levels of residual O2 (Russell, 1995).

The last few years have seen the introduction of new packaging to improve product quality and consumer-friendliness in MAP of hard cheeses. Packaging cheese in plastic packaging was claimed by one cheese manufacturer in the UK to lead to loss of the open text ure found in fresh cheeses. Hence, this company introduced traditional cheeses (8 oz and 10 oz wedges) with very distinct flavours into the UK market packaged in brown paper packs with a waxed finish. This type of packaging was said not


Figure 8.2 MAP of grated cheese.

only to overcome the texture problem but also to give the appearance of hand-made quality (Dunn, 1993).

Both vacuum- and modified atmosphere-packaged cheeses have recently come under considerable criticism from organizations campaigning on behalf of senior citizens and people suffering from arthritis, because of the difficulties involved in opening the packs. Another problem, in common with other food products, has been that of resealability. Hence, the introduction in the UK of MAP of cheese in resealable bags produced by a company named Supreme Plastics was hailed as a breakthrough within this sector. The first resealable pack of block cheese was introduced in the UK in 1992 using the Keyseal® system, where the reclosable male and female plastic profiles were extruded onto a laminate (Anon., 1992a). The packaging was supplied to the cheese manufacturer as a ree!, which was then fed into a horizontal form-fill-seal machine. The packs containing a 12 oz wedge of premium Cheddar had to be opened by removing a tamper evident device, which ran along the top of the pack, after which a grooved strip needed to be pulled apart. The strip could be resealed after forcing out air from the packs, thus keeping the cheese relative!y fresh. The packs have performed successfully with a system producing about 60 packs per minute, giving no significant increase in leakers during storage. Although a laminate film composed of oriented nylon and special PE has been


specified as giving the required shelf-life for cheese while accepting the Keyseal® extrusion, packaging can be tailor-made to suit the requirements of customers (Anon., 1992a; Supreme Plastics Ltd, technical brochure).

Grated cheese is less stable than block cheese owing to the exposure of the increased surface area to air. The introduction of gas-flushed resealable packs made a great impact on the grated cheese market. A range of grated cheeses was test marketed by another cheese manufacturer in the UK in the resealable Keyseal® bags in 1993 (Anon., 1993a). The introduction of this pack into the grated cheese sector had the effect of increasing the sales of the packaged product by 25%, forcing other cheese manufacturers also to consider using resealable packs. Gas-flushed, resealable systems are also being used for block and grated cheese in France (Figure 8.3) and for sliced cheese in the Netherlands. These packs, although new within the MAP cheese sector, have been widely used in the US for applications such as biscuits, nuts and pasta, where the consumers pay a premium for the convenience of being able to reseal the packs (Anon., 1992a).

Freshclose, a reclosable multi-Iayer pack developed by Kramer and Grebe and Dupont de Nemours, is another new development said to have an application for MAP of cheese. The pack has embossed male and fern ale elements on the base and lid to make it easy to open and allow resealing. The packs produced on a modified-atmosphere form-fill-seal machine, can be in the form of easy-pe el and easy-peel with reclosable lid. The packs are currently being used by a manufacture for MAP of cooked sliced meats in the UK but are expected to expand into other MAP product sectors in the near future (Anon., 1994a).

Another new packaging, developed by a French company called Hyperembal, with application for MAP of cheese uses expanded polystyrene (EPS) trays lined with a barrier film, which is c1aimed to give double the shelf-life of those products packaged in EPS and stretch film. This packaging system, launched in 1995, is said to be applicable also for fresh meats, offal and fish. The concept was said to have been made possible by a tray-lining technique developed jointly by Multivac and Wipak Gryspeert, both of France, in which certain thermal characteristics were built into the barrier film. The packs are produced by heating the barrier film and then blowing it over the tray by differential pressure. This material is then cut to give individual trays. After filling with product, a barrier lidding film is applied on vacuum- and modified-atmosphere machines producing rigid trays, which can be fitted with a cover (Anon. , 1995a).

A te ar tape system has recently been introduced into the market for hermeticaliy sealed packs as an alternative method of improving openability (Anon. , 1996a). The FDA-approved system consists of a tape manufactured from a high-strength film that is said to be able to tear through a range of laminates and can be applied to cheese packs.


Figure 8.3 Gas-flush resealable cheese packs in France.

Developments in cutting and packaging equipment have accompanied the developments in packaging materials. Grated cheeses are mixed with anti-caking agents to make them free-flowing to aid filling into packs. Both N and CO2/N2 mixes are being used for MAP of cheese, not only in single form but also as blends of more than one type of cheese (e.g. Cheddar and


Mozzarella, and Emmental and Gruyere). Such blends are said to be particularly popular with the catering establishments (Anon. , 1993a; RusselI, 1995). The production of selection packs containing individually wrapped cheeses of varying types within one MAP system is the next development expected in the cheese sector. Similar techniques may allow cheese and biscuits to be packaged together within the same pack (RusselI, 1995).

Packaging of bulk quantities under modified atmospheres is also becoming more common. Italian cheeses such as Parmesan, Romano, Asiago, Fontinella and Italian sharp are reported to be successfully packaged in 25 and 50 lb pouches for foodservice and industrial users (Rice, 1995).

8.1.3 Mould-ripened and soft cheeses

The soft cheese category consists of a wide range of products, including products that are internally and surface-ripened with moulds. Mould ripening can be a lengthy process and, therefore, the ripening process is often allowed to be completed after the packaging stage when the product is stored, before marketing.

The packaging of these products under modified atmosphere is more complicated than MAP of hard cheeses owing to the presence of live mould. The requirement for the packaging used for these products is that the mould growth is allowed to continue, but at a controlled rate, without causing the whole surface of the cheese to be covered with mould. The presence of air within the pack causes the mould to spread uncontrollably on the product. However, if the O2 is totally excluded from the pack, the mould dies off, making the product unacceptable. Therefore, a compromise is required in the packaging of these cheeses.

In the early 1990s, MAP was not thought to be a viable proposition for mould-ripened cheeses. Tightly packaging the cheese with a slightly permeable film to physically stop the spread of mould growth was thought to be a better approach to take (Addington, 1991). Hence, patents exist describing packaging materials with high permeabilities to both gas and moisture that are said to be suitable for packaging mould-ripened cheeses. Although MAP of mould-ripened cheeses appears to be a difficult proposition, products are available on the market packaged in this way.

Gas mixtures containing 20-40% CO2 are generally recommended for MAP of soft cheeses (Anon., 1995b). Owing to the soft texture of these products, they are not suitable for vacuum packaging. These products are more successfully packaged under MAP because of the cushioning effect of the gas. Gas flushing with CO2/N2 mixtures has been successful in doubling the shelf-life to 21 days. The products are packaged using horizontal formfill-seal flow-wrap machines (Addington, 1991). Half-fat soft cheeses


packaged in metallized films have also benefited from gas flushing of the headspace (Addington, 1991).

French soft cheeses such as Camembert and Brie continue to ripen, using up O2 and releasing CO2 and ammonia, which needs to be lost through the packaging. Moisture permeability of the packaging is also critical as the pack must give a relative humidity within the pack of ab out 73 % as the mould starts to develop and re ach a level of 85-90% at the stage of optimal growth. However, any further increases in the relative humidity has a negative effect, promoting the growth of harmful bacteria that compete with the mould (Sweetman, 1991). The most common forms of packaging for these products are cellophane and waxed paper laminate. Although these materials give the appearance of a traditional product, they can be considered as active materials because the permeability characteristics maintain the atmosphere and humidity at the required levels, giving the same effect as MAP.

Gorgonzola is another cheese that is difficult to package. Hence a new approach has been used to solve the packaging problems of this cheese. Vacuum packaging of Gorgonzola is not totally acceptable, because this soft cheese continues to respire after packaging, causing the packages to distort and inflate. In addition to this, the development of a strong odour limits its distribution. The blue mould that starts to grow on the product after a few days alters the flavour and colour of the cheese, and as growth progresses the mould turns brown, causing a further change in flavour. The quality and shelf-life of the cheese have been improved by the use of a specially vented pack in combination with gas flushing (Anon. , 1993b). The technique, based on that used for packaging freshly roasted cOffee, was developed by Multivac in conjunction with its labelling subsidiary MR Etikettiertechnik. The package is composed of a small transparent oneway valve, which allows respiration gases to leave the pack without allowing O2 to enter. A mixture of CO2 and N2 is said to be introduced into the pack through the valve. This modified atmosphere technique was said to give the product a shelf-life of 4 weeks and allowed normal distribution of the product.

Another product successfully packaged under MAP is Stilton. Stilton tends to be a seasonal product, 70% of the total sales occurring around Christmas. In order to achieve a steady production schedule, some producers were said to package and freeze the product until the market was ready. MAP has been found to be useful in delaying the maturation of the cheese by slowing down the growth of the culture present. MAP of Stilton was said to allow the cheese to be marketed as a fresh product, with premium quality and, therefore, at a higher price (RusselI, 1995).

Some recent research has also shown that MAP can be used to improve the shelf-life and quality of Italian mould-ripened cheeses. Taleggio cheese, a soft cheese with a mouldy and smeared rind, is traditionally

DAIRY FÖODS, MULTI-COMPONENT PRODUCTS, DRIED FOOD AND BEVERAGES 167

packaged in either PE- or aluminiumlPE-coated paper and has a shelf-life of seven to eight days at chili temperatures. Packaging the product under 10% CO2/90% N2 was found to retain the rind colour and delay overripening of the cheese after 21 days at 6 ± 2°C. However, there was no significant advantage in using MAP over the traditional packaging of LDPE-coated paper in terms of microbiological quality of the rind. Increasing the level of CO2 to 20 and 30% had an undesirable effect on the sensory characteristics of the cheese (Piergiovanni et al., 1993).

8.1.4 Unripened cheeses

Cottage cheese has a very short shelf-life of about 21-28 days under even proper refrigerated conditions. The primary cause of spoilage is the growth of psychrotrophs and Gram-negative bacteria, wh ich results in the production of undesirable flavours and turns the curd slimy. The use of MAP has become a viable proposition because the use of preservatives, such as sorbates, is known to affect flavour even at very low concentrations (Chen and Hotchkiss, 1991). Hence, a number of investigations have been carried out to study the effectiveness of CO2/N2 mixtures in retarding the growth of microorganisms and to evaluate the possible risks from the growth of food-poisoning organisms such as Listeria sp. and Clostridium botulinum.

In one study, packaging cottage cheese in plastic tubs under a mixture of 67.1 % CO2/26.3% N2/6.6% O2 achieved a complete inhibition of yeast and mould growth and pseudomonads both at the surface and bottom layers of the product after 21 days at 4°C (Rosenthai et al., 1991). At the surface of contral sampies packaged in air, the number of pseudomonads, yeasts and mould present exceeded that found at the bottom layers of the product. Similar benefits were seen by Moir et al., (1993) on creamed cottage cheese, indicating that 40% CO2/60% N2 inhibits growth of psychrotrophs inoculated into the product at storage temperatures of 5 and 15°C. An increase in the shelf-life of up to three weeks was possible for the producL The beneficial effect was greater at 5°C than at 15°C and at the surface than in the depth of the cheese. No flavour problems were noticed in the product stored under the modified atmosphere.

The effect of dissolved CO2 on growth of psychrotrophic organisms in creamed cottage cheese was studied by the use of challenge tests (Chen and Hotchkiss, 1991). In this study the cream was carbonated to cause CO2 to go into solution prior to mixing the cream with the curd. Although the level of dissolved CO2 was not quantified, the headspace was found to contain 35-45%. The CO2 was found to inhibit yeast and mould growth for 70 days at 4°C and 30 days at 7°C. The control samples packed without CO2 were found to have 104-fold higher counts after 10 days and 17 days at 7°C and 4°C, respectively.


Although packaging creamed cottage cheese under 100% CO2 gives beneficial effects in terms of microbiological quality, the effect on sensory quality is not clear. Sampies packaged in 100% CO2 have been perceived by so me as having a slight fizziness (Kosikowski and Brown, 1973). However, others (Maniar et al., 1994), report that the gas has no effect on flavour characteristics. In fact, in one study, CO2 levels greater than 75% were found to be best at maintaining the quality of direct set cottage cheese by inhibiting the growth of psychrotrophic and lactic acid bacteria without affecting the colour or imparting a sharp flavour to the product (Maniar et al., 1994).

Flushing of cottage cheese with CO2 is said to be used commercially in Germany (Anon. , 1987a). In this process, cottage cheese from the vat is pumped into an enclosed vessel that has been flushed with CO2 . This causes the air in the curd to be replaced with CO2 before the product is filled into cups. The cups are also flushed with CO2 before filling. At the end of filling, the headspace is again flushed with CO2 before closing. The tubs are sealed with aluminium foil and a cap is then placed over the foil (Anon. , 1987a). CO2 inhibits the growth of psychrotrophs, thus extending the shelf-life of the product for five days. The flavour of the product is also said to be protected. Although reservations have been expressed about the use of this process, in that CO2 may have an adverse effect on flavour and pH owing to the formation of carbonic acid, it is claimed that there is no such adverse effect in this case.

Cottage cheese has been shown to be a possible vehicle for listeriosis, but the use of MAP (50% CO2/50% N2 and 100% CO2) has been found to reduce the risk from this organism (Fedio et al., 1994). Another inoculation study found that L. monocytogenes and C. sporogenes did not to grow in low-fat cottage cheese in the presence of dissolved CO2 (Chen and Hotchkiss, 1993). However, yet another study showed that L. monocytogenes does not grow in cottage cheese packaged in air or 40% CO2 (Moir et al., 1993). The reasons for this effect were not clear.

The benefits of storing quarg under CO2 were investigated by Rosenthai et al. (1991). Quarg is an unripened cheese that spoils as a result of yeast and mould growth. The cheese (250 g) was packed in commercial plastic containers with resealab\e lids under air or under 67.1 % C02126.3% N2/

6.6% O2 and stored at 4°C. The product packaged in air spoi\ed after 42 days but the product packaged under the gas mixture remained acceptable after 67 days with no active mould or yeast growth, no change in pH and acceptable flavour and texture. When the product packaged under the gas mixture was stored at the high temperature of 32°C, the inhibitory effect on yeasts and mould was apparent but at a lower level. However, in this case a sour unacceptable taste was perceived caused by an increase in the acidity.

Sour cream has also been shown to have potential benefits from the use of MAP. Sampies of sour cream packaged in 100% CO2 , were said to have


an excellent flavour for 120 days at 4°C, but problems associated with the texture (heavy body, curdiness and free whey) were noticeable after only 25 days (Kosikowski and Brown, 1973).

8.1.5 Yoghurt

Owing to the low pH, bacterial growth in yoghurts is retarded. However, gas flushing of the headspace of yoghurts after filling into cups has been shown experimentally to be beneficial in extending the shelf-life (Blakistone, 1990). In this study, plain yoghurt was aseptically filled into high-barrier cups, which were then sealed with a foillidding material, with and without N2 flushing of the headspace, following which the cups were stored at 40°F (4°C). Flushing with N2 gave a residual O2 level in the headspace of 0.1-0.2% after eight weeks in storage. Under these packaging conditions, yeast and mbuld growth were measured. The NTflushed yoghurt was found to show no contamination after eight months, but the products filled under ambient conditions lasted for only 14 weeks.

8.1.6 Milk

There appears to be little work carried out on the preservation of pasteurized milk by the use of gases such as COz, probably because it poses potential problems in maintaining the sensory quality of the product. However, a great deal of research has been carried out on the potential use of CO2 to extend the shelf-life of unprocessed milk in terms of microbial quality.

Raw milk is normally stored in vats and transported under refrigerated conditions in tankers to the processing plants. Although the use of refrigeration has virtually eliminated problems of lactic acid bacteria, the growth of psychrotrophic bacteria is still a possibility (Roberts and Torrey, 1988). The psychrotrophs do not survive the he at treatment, but the enzymes liberated by them (proteases and lipases) can survive and cause problems during subsequent production stages. Hence retarding the growth of these organisms in raw milk is of importance to the dairy industry.

A number of researchers have shown that 10-30 mM CO2 can be used to suppress the growth of psychrotrophs in raw milk (King and Mabbitt, 1982; Shipe et af., 1982; Roberts and Torrey, 1988). Roberts and Torrey (1988) showed that in addition to the effect on psychrotrophs, the numbers of coliforms, and anaerobes were also reduced by the presence of CO2 .

Although the presence of CO2 reduced the pH of the milk from pH 6.6/6.7 to 6.2, this was not responsible for the bacteriostatic effect (King and Nagel, 1975; King and Mabbitt, 1982). The CO2 was found to prolong the


lag phase and not significantly affect the growth phase (King and Mabbitt, 1982). This finding is in common with the effect of the gas on other products.

Different mechanisms are suggested for the action of CO2 in raw milk. A decrease in the ionic permeability of the cell membrane by CO2 was thought to stress the system and depress the growth rate (Sears and Eisenberg, 1961). Others suggest that CO2 exerts a 'mass' action against decarboxylating enzymes (Gill and Tan, 1979; King and Nagel, 1967). AIthough the mechanism of CO2 action remains unclear, the usefulness of the technique has been demonstrated. Depending on the initial microbial quality of the milk, shelf-life extension of 3 to 11 days or longer is thought to be possible under refrigerated conditions.

The presence of CO2 is said not to affect the processability of the raw milk when concentrations below 30 mM are present (King and Mabbitt, 1982). However, the CO2 has to be removed from the milk by the use of vacuum or gentle agitation prior to heat processing. Although a potential problem associated with the use of CO2 is that the growth of anaerobes and facultative anaerobes will be encouraged, CO2 (20-30 mM) has not been found to stimulate the growth of Escherichia coli or other mesophilic spoilage bacteria when milk is stored at 7°C.

The survival of L. monocytogenes is possible in raw milk if poor hygienic practices are followed (Mann, 1995). However, the pathogen does not survive heat pasteurization. Even high levels of listeria were found not to survive HTST plate pasteurization (71.7°C/15 s) (Macdonald and Sutherland, 1993). The mechanism by which high levels of CO2 have a suppressive effect on the growth of Iisteria is thought to be similar to that found in cottage cheese (Fedio et al., 1994).

Modified atmospheres are not used to any significant degree in the retail packaging of milk, aIthough N2 flushing has been shown to be useful in preserving quality (Addington, 1991). Some feel that the shelf-life of products such as milk should not be extended as it portrays a negative image to the consumers, suggesting that the product is not very fresh. However, fresh milk with an extension of shelf-life from 10-12 days to 28 days is being sold in North America (Anon. , 1996b). Shelf-life extension is being achieved by a modification of the heat process rather than by the use of MAP. One such recent development in UK has been the production of single-serve cartons of fresh milk (125 ml) targeted at airlines and catering establishments that uses a patented heat-processing system to give a shelflife of 28 days (Anon., 1996c).

The use of N2 as the headspace gas in storage vats and tanks has been shown to have a significant effect on reducing the growth of Pseudomonas. N2 used as the headspace in UHT milk packs also has been shown to retain the flavour during the latter half of the storage life of the product, probably by reducing oxidative changes in the product (Addington, 1991). As far as


the author is aware, gas flushing is not used to any significant extent in the packaging of commercial milk products.

8.1. 7 Milk powders

Whole milk powder benefits from N2 flushing, which helps to re du ce oxidation of the fat. Milk powder in bulk packs, particularly for exporting to tropical countries, can be flushed with N2 to retain quality. Examples of the packaging used are bag-in-box with high-barrier lining and form-fillseal pouches (Addington, 1991).

8.2 Coffee

Coffee is said to be the second most widely traded commodity in the world, the first being oil. There are many varieties of coffee beans, grown in different parts of the world. Coffee beans are shipped from the countries of origin in the green state, the processing of the beans being carried out by companies that then convert them into final products such as roasted whole beans, roasted ground beans, instant coffee and coffee extracts.

The various types of raw bean are c1eaned, bIended and roasted by the application of heat to give the desired flavour and aroma characteristics to the product. Different varieties of beans may vary in their chemical composition. The roasting conditions will be selected according to the bean type and the flavour required. During roasting, moisture is lost from the beans and chemical reactions are initiated. These reactions produce aroma and flavour volatiles from non-volatile components of the beans, which break down and react together in very complex reactions. The result of such reactions is the production of CO2 .

8.2.1 Whole beans

During roasting CO2 is produced and becomes trapped within the beans. The amount of CO2 produced depends on many factors, including the bean variety and roasting conditions. If the roasted beans are left intact, the CO2

is released slowly from the bean. Various researchers have attempted to quantify the amount of CO2 released from roasted coffee beans. One figure quoted is 10 ml of CO2 g-l of roasted coffee (Barbera, 1967). If the beans are sealed in packages directIy after roasting, the CO2 reIeased from the be ans builds up within the pack, eventually causing them to explode. However, the roasted beans cannot be left unpackaged, as beans may absorb moisture and oxygen, wh ich leads to a deterioration of the flavour. Clarke (1987), studying such situations, stated that roasted whole beans exposed to air remained at a quality equivalent to that of the freshly


roasted product for only up to 10-12 days. After 40 days, flavour changes became apparent and after 70 days the beans were considered unacceptable.

Whole beans are degassed to remove CO2 before packaging. The degassing process usually involves the application of a vacuum. Packaging is an important consideration as the barrier properties of the materials significantly affect the shelf-life of the producL Whole beans are more stable to oxidation than their ground counterparts and, therefore, simple bags may be adequate if the product is to be sold within a short time-scale. However, dark roasted coffees can release oil and so the packaging of these products needs to be grease resistant (Clarke, 1987).

Vacuum packaging can be used to minimize O2 within the packs, which causes staling. This form of packaging can keep the beans in an acceptable condition for up to 18 months (Cros and Vincent, 1980). The alternative to using this method is gas flushing, which achieves a low residual level of O2

within the packs. In this case, an inert gas such as N2 is flushed into the packs.

MAP is being used commercially to replace vacuum packaging of whole beans. The beans are packaged under N2 in valve packs, which remove the need for the long storage period required after roasting to degas the beans. The one-way valve allows CO2 to escape from the be ans without allowing O2 to enter the packs. Ouring the filling process and before sealing, the packs are flushed with N2 giving a residual O2 level within the packs of less than 1 %, which results in a shelf-life of at least 12 months without the loss of flavour. The presence of peel seal characteristics and a tie on the pack allows easy opening and closing of the packs (Anon., 1994b).

8.2.2 Ground coffee

When roasted be ans are ground, one-third of the CO2 is said to be released instantly and another one-third is released during the 30-40 minute time period before the product is packaged (Jenkins and Harrington, 1991a). Therefore, the CO2 problem is minimal compared with the situation with whole beans. However, CO2 released from the ground coffee may carry with it so me aroma and flavour volatiles. The major problem with the ground product is its instability to oxidation and staling, which means that the product needs to be packaged in higher-barrier materials than for whole beans. If the product is not packaged hermetically it becomes stale after one month, judged by the standard of an average coffee drinker (Jenkins and Harrington, 1991a).

Studies have shown that coffee at about 4% moisture content sealed in a container with 0.5% residual O2 and stored at 21°C, will remain at high, medium and low quality for 6 months, 12-17 months and 20-25 months, respectively (Heiss and Radtke, 1977; Radtke-Granzer and Piringer,


1981). If the residual O2 increases to 1%, storage time to re ach the respective quality grades will reduce to 4 months, 9-17 months and 14-20 months, respectively. Therefore, it is important that contact with O2 is minimized at all stages of production. The exposure of the product to O2

during the lag time between grinding and packaging may be a significant factor to consider. During the package-filling operation, the product is often mixed with inert gas as it flows into the pack, in order to reduce O2

absorption by the coffee. Ground coffee is packaged in various materials, both rigid and flexible.

Whatever the material used, measures are always taken to minimize the residual O2 content of the packs by vacuumizing or gas flushing. If the product is packaged in metal cans, a high level of vacuum is applied to give about 1 % residual O2 level within the sealed pack. Inert gas flushing is used as an alternative to vacuumizing. In this case, the product is initially vacuumized (only a low level of vacuum is used) and then flushed with N2

to give the required residual O2 level. Subsequent to gas flushing a low level of vacuum may be applied to the can before cIosing, in order to prevent build-up of pressure through evolution of CO2 from the product (Clarke, 1987).

Ground coffee packaged in flexible pouches can also be vacuumized or gas flushed. The application of a vacuum produces hard blocks, which have the disadvantage that rough edges of coffee particles may cause pinholes in the packs. Inert gas flushing to produce pillow packs, therefore, is a useful alternative. Gas flushing is carried out during the vertical form-fill-seal packaging operation. The flexible packaging materials used are commonly laminates, often metallized (e.g. metallized polyester) to improve the barrier properties. The ground product still needs to be degassed before being packed into flexible pouches.

In order to reduce the degassing time and thus the time taken before the product is packaged, one-way valves are fitted to the flexible packs. These valves release CO2 from the pack without allowing O2 to enter. Coffee packaged in this way is said to be fresher as it is packed straight after roasting and grinding. Packs of coffee produced in this way are still modified atmosphere packs even without N2 flushing, as the headspace is modified by release of CO2 from the coffee. However, these ground coffee packs are commonly N2 flushed to reduce the residual O2 levels within packs. Ground coffee packaged in metallized pouches with one-way valves with N2 flushing contain 20 to 50% CO2 and less than 1 % O2 at the retailing stage.

Various patents exist on the construction of one-way valves for coffee. An alternative to one-way valves is said to be a PE pouch that is permeable to CO2 . This pouch contains a mixture of calcium oxide and activated charcoal as a CO2 absorber (Jenkins and Harrington, 1991a). The cost of the absorber was stated as being similar to that of a one-way va\ve.


Another method of improving shelf-life is by the use of oxygen scavengers. Both ferrous and non-ferrous O2 scavengers are said to be used for freshly roasted ground coffee in Japan (An on. , 1995c) and have been used in the USo

8.2.3 Instant coffee

The production of instantized coffee involves the extraction of watersoluble components from the product; these are then filtered and dried, commonly by spray drying or freeze drying. The oil from the pressed ground coffee is added back to the dried so lids to improve aroma (Clarke, 1987). The evolution of CO2 is not a problem to consider in packaging the instant product, but moisture absorption, oxidation and staling are the problems to overcome, particularly owing to the high surface area and porous structure of the particles. As with ground coffee, it is important that instant coffee packs have a low residual O2 level (Clarke, 1987). For this reason, gas flushing with inert gas is very important for instant products.

Instant products are commonly packaged in tins and glass jars, these packs being gas flushed before sealing. The jars are sealed with a diaphragm before the lid is attached. The diaphragm is composed ofwaxed paper or foil to improve the moisture- and gas-barrier properties. Arecent competitor to the glass jar is the plastic jar, which is seen as having the advantage of being lighter in weight.

Flushing with noble gases such as argon and xenon singly or in mixtures of more than one gas has been proved to be effective in reducing oxidation of coffee for over a year. The performance of these gases is said be significantly better than that of N2 and CO2 (Spencer, 1995).

8.3 Tea

MAP has been found to be useful in maintaining the fresh quality of tea. Tea accounts for nearly half of all drinks consumed in the UK and is said to be the most popular beverage in the country (Anon., 1996d). Research carried out on black tea showed that post-harvest processing pro duces most of the aroma volatiles in the product. However, changes in the aroma quality of the tea was found to occur during storage owing to oxidative deterioration, reducing the quality of the product (Springett et al., 1994). These changes occurred during the first 10 weeks of storage of the picked and fired tea.

Following much research, a UK company found a route to capturing the true freshly picked flavour and aroma of the tea by protecting the tea against oxidation from the plantation all the way to the consumer. The first stage of the operation involved vacuum packing the fresh tea in large


blocks in aluminium foil/polyester/polyethylene laminate at the tea estate. On arrival at the UK factory, the tea was blended and held in stainless steel tote bins, which were continuously flushed with N2 . The tea was then filled into tea bags and the bags were sealed inside foil pouches after N2 flushing (Darrington, 1991). The gas-flushed tea had a milder taste and astronger aroma than the conventional product and was targeted at tea drinkers who would be willing to pay the extra margin for the higher quality.

Although this technology was hailed as the biggest breakthrough since the introduction of the tea bag (Anon., 1989), it unfortunately did not succeed in the market place. The use of the MAP/vacuum-packaging technique was proved to improve the quality of tea, but consumers seem not to be willing to pay the extra margin for the product.

8.4 Snacks

The snack market has expanded considerably over the last few years, particularly in Europe. More money is said to be spent on snacks than on staple products such as tea and breakfast cereal (Anon. , 1992b). A wide range of high-quality snacks targeted at adults has been introduced into the market. The use of gas flushing for snack products has grown considerably over the last few years, owing to the consumer demand for higher-quality products. A very wide range of products is cIassified as snacks, and it would be very difficult to cover every group of product that might benefit from gas flushing. Although this section will concentrate on only the main categories of snacks, the underlying principles of using MAP for these products would also be applicable to other similar snack products.

8.4.1 Nuts

Nuts in general have a very high level of fat, although the level varies with the type of nut. In general, tree nuts have a higher fat content than ground nuts, for example, more than 60% in Brazil nuts, hazelnuts and walnuts compared with about 50% in peanuts. Owing to the high fat content, oxidative rancidity is the main problem to overcome. Nuts are commonly sold as roasted products. Peanuts are the most popular type of nut but are said to be least stable, with the shortest shelf-life of all the common nuts.

Peanuts are often blanched before being oil-roasted or fried in oil. Nuts can also be dry roasted, in which case they are sometimes glazed with 1.5 to 2% of a special oil and salt. Anti-oxidant may be added to the oil salt in order to improve the shelf-stability of the product (Woodroof, 1966). The frying technique improves flavour but leads to further increase in the fat. Although the limitation in shelf-life usually results from the oxidation of


the fat, moisture pick-up will also cause a problem by causing loss of texture. For these reasons it is important that nuts are packaged adequately.

Nuts are packaged in flexible materials such as plastic pouches and also in rigid materials such as glass jars and metal cans. Good barrier properties are important if shelf-life is to be increased. The headspace O2 content within filled packs can be reduced by applying a vacuum. However, gas flushing with an inert gas is commonly used when an improvement in shelflife is required. When product is packaged in flexible pouches, vertical form-fill-seal machines are used.

Many different flexible plastics can be used to package nuts, but the greater the barrier, the greater the shelf-life of the product. PVdC is often used as part of flexible film construction in order to improve the barrier properties. PVdC-coated polyester laminated to LDPE is commercially used to package nuts to improve shelf-life of small packs (8 oz). The larger packs (14 oz) have Surlyn® substituted for LDPE to give improved seal strength (Conger and Ellis, 1982). Metallized films are also popular when N2 flushing is to be used. N2 flushing is carried out to achieve a residual O2

level of less than 2% within the packs. The combination of increased barrier and gas flushing is said to give peanuts a shelf-life of four to five months.

Rigid containers for packaging nuts include plastics and metal, but composite cans are now used widely. Composite cans are said to have adequate barrier properties and to be cost effective compared with the metal cans (Anon., 1991a). The composite cans have a foil liner, an aluminium ring-pull tab and a plastic cap for resealing the product after opening. After filling, these canisters are flushed with N2 to give improved shelf-life.

Flushing with N2 is currently commonly used to re du ce residual O2

in packs containing cashews, pistachios, mixed nuts and dried fruit and nuts (Anon. , 1995d). The use of this technique in combination with the use of high-barrier films (02 permeability of 0.04 cc 100 in-2 24 h-1

(0.64 ml m-2 24 h-1)) has doubled or tripled the shelf-life of the products to give shelf-lives of 10--12 months. Gas flushing is being successfully used on raw, fried and roasted nuts. The shelf-life of shelled nuts also can be increased from 4 to 6 months by the use of N2 flushing (Anon. , 1992c).

An alternative to gas flushing is the use of O2 scavenger systems. The use of O2 scavengers is said to be more effective than gas flushing for reducing the residual O2 level within containers, thereby further increasing the shelf-life of packaged nuts. Sachets containing the scavenger are placed inside the packs and have been found to reduce the O2 level in air-tight containers to 0.01% or less (Anon., 1995e).


8.4.2 Crisps and other snacks

The extension of shelf-life that can be achieved through gas flushing is dependent on the composition of the product (Jenkins and Harrington, 1991 b). However, a reduction in the level of Oz within the pack in general gives the product an increased stability to oxidation. Snacks vary in the barrier properties required, but the strictest requirement is said to be for potato crisps. Three different requirements are said to be important when packaging snacks: moisture, Oz and light-barrier properties (Broomfield, 1979).

Crisps have a moisture content of 1 to 1.5% at the stage of being packaged. 1fthis level increases to 4-5% the product becomes unacceptable. Snacks with high fat content will be sensitive to Oz. Crisps sold without Nz flushing in clear plastic bags have a short shelf-life, owing to the high volume of air packed into the bags. Approaches such as the use of antioxidants in the cooking oil are often used to improve the shelf-life of these products. Flushing with Nz will improve shelf-life but until recently was not used for low-value products such as crisps as it was claimed not to be economical.

Oxidative rancidity is accelerated by light and, therefore, light-barrier properties are advantageous in increasing shelf-life. A potato snack product packaged in clear OPP film has a shelf-life (assessed by flavour and moisture changes) of only 8 to 10 weeks compared with over 26 weeks when packaged in metallized OPP film (D. Cann, personal communication). Other products have also benefited when gas-flushed with Nz. A product newly packaged in this way is popcorn. This product is packaged in a laminate of metallized PET/peelable PE (Anon., 1991b).

There are various ways in which Nz is introduced into the packs. One way is for flexible packaging to form a tube, through which gas flushing takes place. Another method is the use of liquid Nz, which is dispensed into the packs. The laUer method is said to be sometimes used for packaging mixed fmit and nuts in the UK (Anon., 1991b). The snack industry aims to reduce the residual level of Oz in the packs to less than 2% when Nz flushing is used, the levels achieved in practice often being less than 0.75% (Johnson, 1991).

Some recent research demonstrated the need for low residual Oz levels in obtaining significant benefits against oxidative deterioration of potato chips (Paik et al., 1994). In this study, commercially prepared potato chips were packaged in 20 .um OPP/12 .um metallized polyester/15 .um PE/7 .um ionomer, Nz flushed and then stored at 23°C and 40°C at 90% RH, in the dark and under fluorescent light. The residual Oz level in Nz-flushed packs increased from 0.015 atm to 0.02 atm after 80 days at 23°C, whereas in control packs without Nz flushing the level remained at 0.2 atm.


The results showed that the O2 consumption rate was lower than the level of O2 permeating through the packs, owing to oxidation. Although, there was little difference in the lipid oxidation rate, the sensory results showed a noticeable difference between Nrflushed packs and those packaged in air. It was concluded that O2 levels less than 0.01 atm were required to re du ce oxidation of potato chips significantly. The use of the metallized film was found to protect against acceleration of oxidation in potato chips.

Flushing of crisps with N2 without the use of any preservatives is said to give a shelf-life of 120 days compared with 55 to 65 days without N2

(Anon. , 1988). Another advantage of N2 flushing is that uniform pillow packs are produced, which prevent damage of the fragile snack products during handling and distribution.

Meat snacks such as beef jerky, which are popular in the US, have also benefited from the application of MAP. Beef jerky can be stored at ambient for up to 12 months before reaching the consumer. As the product ages, it becomes dark in colour, develops rancid flavours and sours as a result of microbial spoilage. The use of CO2/N2 mixtures has been found to reduce oxidation and microbial spoilage and increase the shelf-life of the products (Johnson, 1991).

The use of gas flushing has required improvements in both barrier properties of packaging and seal performance. Pouches made of metallized films such as metallized pp and metallized PET are now commonly used (Johnson, 1991). Metallized films are relatively expensive and, therefore, increase the cost of the products. Crisps packaged in metallized OPP were found to retain their freshness for a longer period in the presence of Nb thus allowing the product to be distributed more easily. The use of metallized films is expected to grow with the use of gas flushing (Anon. , 1992d).

8.S Delicatessen/multi-component products

The shelf-life of pre-cooked foods is usually limited by two factors: microbial growth and the O2 sensitivity of the product (Coulon and Louis, 1989). Therefore, the two main requirements when packaging cooked products under modified atmospheres is that O2 should be excluded and a fungistatic or bacteriostatic agent should be present. In order to meet these requirements, cooked products are packed in gas mixtures comprising N2

and CO2 . The CO2 acts to suppress microbial growth and the N2 is used as the filler gas when O2 is to be excluded from the pack. The residual O2

level within the packs needs to be significantly below 1 % (Girardon, 1986). If the product contains combinations of meat and pastry, the residual O2

level needs to be reduced further, to less than 0.5%. Products with higher


water activities are said to be able to tolerate higher Oz levels (1 %) (Coulon and Louis, 1989).

When packaging any cooked product, it is important that its Oz sensitivity is determined. The packaging equipment used needs to be chosen on the basis of the residual Oz level tolerable within the pack. There are two basic techniques for packaging products under modified atmospheres: gas flushing and gas packing. In the gas-flushing technique, the product is continually flushed with the gas mixture, which then displaces the air present within the pack. After the required period of flushing, pre-determined to give the required residual Oz level, the pack is sealed. When using the gas-flushing technique, commonly used for flexible packs, the residual Oz level can only be reduced to a practicallowest level of 1-2%.

The gas-packing technique refers to the process of applying a vacuum to a pack, commonly a tray, to remove the air present and then replacing the headspace with the gas mixture before sealing the pack. This technique allows the residual Oz level to be reduced further to 0.5% or less (Coulon and Louis, 1989). Both MAP techniques are used to package delicatessen products.

MAP is increasingly being used to extend the shelf-life of delicatessen products. It may be thought that if Oz is to be excluded and COz is to be included, the products can be simply packed in COz alone, However, in practice it is less simple, because the products may have a COz sensitivity and in some products increasing the level of COz beyond a certain value may not continue to increase the shelf-life of the product significantly. It appears that, in general, at least 20% COz is required before the atmosphere is seen to show benefit in terms of extending shelf-life of products (Girardon, 1986; Coulon and Louis, 1989). Inhibition of microbial growth has been found to increase linearly up to 50% COz. Above this level, growth inhibition may not be significant (Goodburn and Halligan, 1988).

Another point worth mentioning is that microorganisms differ in their sensitivity to COz. Although most moulds and yeasts can be inhibited by 5 to 50% COz (v/v), many yeasts can grow in the absence of Oz and are resistant to COz. Therefore, for products in which yeast growth limits the shelf-life, MAP may not be the appropriate solution in extending the shelfIife (Girardon, 1986; Goodburn and Halligan, 1988; Coulon and Louis, 1989) and yeast contamination should be avoided.

In many products, a high level of COz has been shown to have a negative effect by causing pack collapse owing to the absorption of COz by the product and by inducing unpleasant sensory changes. For these reasons it is important to establish the effect of a range of COz levels on both microbiological shelf-life and the sensory quality of the products, in order to determine the optimal atmosphere for the product.


Pre-cooked foods will normally be packed in a mixture of CO2 and N2 .

The ratio of CO2 to N2 to be used can be predicted to some extent by the relative humidity of the product. Relative humidity determines the rate of microbial growth, oxidation and enzyme activity (Girardon, 1986). The higher the water activity, the higher the level of CO2 used as the headspace for the product. However, as mentioned above, a high C02 1evei can cause its own problems in products. Different products, because of the differences in composition, may vary in their optimal modified atmospheres. Hence, each product needs to be considered separately.

Good temperature control is vital if products are to benefit from the use of MAP. The inhibition of bacterial growth by CO2 increases with decreasing temperature. The reason for this is said to be the solubility of CO2 , which increases with a decrease in temperature (Genigeorgis, 1985). A headspace to product ratio of 11: 1 is said to be required when using 100% CO2 in order to solubilize enough CO2 to completely inhibit microbial growth (Rönner, 1994). Therefore, additional barriers to microbial growth such as pH, water activity and temperature are essential to ensure microbial safety of products. The storage temperature of the MAP products should be controlled to a maximum of 4°C (Coulon and Louis, 1989). Above a temperature of 5°C, CO2 is said not to be effective (Watkins, 1984) and, therefore, may not give rise to any advantage in terms of shelf-life. Many of the major retailers in Europe using MAP for chilled products try to maintain the temperature throughout the storage and distribution chain at 2 ± 1°C.

Apart from the microbial problems experienced by any cooked product, the multi-component products experience additional physical problems of migration of components such as moisture from one component to another. Hence, MAP of each product has to be carefully considered, as some of the physical reactions may not be delayed by the use of MAP.

Packaging developments in this sector have included the development of domed modified atmosphere packages, which are used for bakery and delicatessen products. The packaging, consisting of a base tray and a domed lid, can be gas flushed and is reclosable (Anon., 1993c). The coextruded packaging (polyester, PVC or a general styrene, and EVOH and a tie layer) consists of an active film that repels moisture and eliminates the requirement for anti-fog coatings. The combination of the gas mixture and the active film forces the moisture to stay in the products and become evenly distributed to retain freshness. Although this packaging is suitable for frozen products, it is recommended for use on fresh products. Products such as croissants stay fresh for 45 days and the denser products such as filled pies for about 25 days when this packaging system is used.


8.5.1 Sandwiches

Pre-packaged sandwiches have grown in popularity over the last few years and their sale has extended from large retail outlets to snack bars and petrol stations. An increase in shelf-life would be of great advantage in allowing more flexibility in the distribution of these products. Gas-packing sandwiches in CO2 is said to give the sandwiches about 28 days refrigerated shelf-life (Fierheller, 1989) compared with 10 days when conventionally packaged.

Although the use of MAP for these products has mainly been introduced for distribution purposes, the sandwiches are also said to have a higher quality image. The packaging machinery used consists of horizontal packaging machines and thermoforming machines. The recommended level of CO2 is said to be 50--60%, the residual level of O2 in the headspace remaining relatively high at 8-10% if gas flushing is used. If a va cu um is pulled before the gas is introduced, much lower residual levels (less than 0.5%) are possible (Fierheller, 1989). This would be advantageous, particularly if the fillings used are sensitive to 02. The packaging films used in MAP of sandwiches are said to have high CO2 barrier properties, for example PVdC-coated polyester.

A survey of sandwiches sold in the UK at various outlets found that the overall hygiene of these products was very poor (Manwell, 1991). This survey detected the presence of Listeria spp. in some of the sampies and pointed out the importance of Good Manufacturing Practices. Sandwiches can contain uncooked items as part of the filling, and, therefore, it is vital that every effort is made to preserve the safety of these products. The use of good refrigeration is also vital not only for achieving an extension in shelf-life but also for assuring safety in MAP of sandwiches.

Turkey sandwiches in MAP have been found to be vehic1es for L. monocytogenes (Farber et al., 1990). Research was carried out on turkey roll slices inoculated with the microorganism to investigate the effect of 30/ 70, 50/50 and 70/30 mixtures of COzlN2 on its growth (Farber and Daley, 1994). Results showed that 30/70 and 50/50 CO2/N2 mixtures reduced the rate of growth of Listeria spp. relative to that found when product was packed in air at storage temperatures of 4°C and 10°C. The growth of the microorganism was completely inhibited in 70% CO2/30% N2 at the same storage temperatures. MAP of turkey sandwiches was said to have a shelflife of 30 days at refrigerated temperatures. Since gas mixtures such as 30% CO2/70% N2 or 5% CO2/95% N2 are often used for sandwiches (Farber et al., 1990), they may pose a potential hazard by allowing the growth of Listeria spp. even at 4°C. Therefore, a minimum of 70% CO2 was recommended for sandwiches if long shelf-lives were required (Farber and Daley, 1994).

Apart from microbial deterioration, other problems such as loss of


texture and flavour can also limit the shelf-life of products. Migration of moisture from the filling to the bread is a major problem in sandwiches. The presence of low levels of O2 is said to reduce the development of stale flavours. Gas packaging with 25% CO2/75% N2 is said to give a shelf-life of 90 days at chill conditions (Brody, 1995). However, the product safety may not be totally assured at such a low level of 02'

8.5.2 Dressed salads

Mayonnaise-based salads can contain a wide range of bacteria and yeasts, the microorganisms often being introduced as contaminants of the added solid components of the salad (Brocklehurst, 1989). Products containing high-protein ingredients such as meat and fish have a higher pH than those containing fruit and vegetables (Rose, 1984). Salads containing highprotein components will, therefore, be more susceptible to spoilage. To a certain extent, microbiological protection is given to these products by the presence of acetic acid in the mayonnaise. The composition and quantity of dressing used in a salad have a significant effect on the shelf-life of the product (Whitham, 1989).

MAP of dressed salads has been investigated. In one study mayonnaisebased vegetable salad spoilage was delayed by the use of 20% CO2/80% N2

(Buick and Damoglou, 1989). The vegetable components of the salad were boiled potato, carrot, peas, sweetcorn, runner beans and broad beans. The pR of the product was 4.1. The salad was filled ioto HDPE trays aod sealed with a top web of 75 flm film particularly suited to MAP. At 4°C, the salad packed in the modified atmosphere had a shelf-life of 54 days compared with 40 days without modified atmosphere, and at 15°C the MAP product had a 12 day shelf-life compared with only 5 days without MAP.

A similar study was carried out on potato salad (containing potato, mayonnaise, pickled cucumber, on ion and seasoning) by Ahvenainen (1989). It was found that the use of only N2 (to exclude O2) did not increase the shelf-life. Levels of CO2 greater than 20% were also found to give problems by producing off-odours and off-flavours in the potato salad. The study concluded that MAP was not particularly beneficial for this product. Another vegetable salad, additionally containing herring, did show some benefit when packaged under modified atmospheres (Ahvenainen, 1989). The optimal gas mixture was found to be 60% CO2/

40% N2 • Gas packaging delayed product deterioration caused through both microbiological and sensory changes. MAP was said to retard yeast growth and help to preserve the flavour of herring. The herring component was very susceptible to oxidation and residual O2 levels of 0.5 to 1.5% were not thought to be adequate for achieving long shelf-lives. The shelflife of this salad was extended for only two days by the use of MAP, owing to rancidity of the herring.

DAIRY FOODS, MULTI-COMPONENT PRODUcrS, DRIED FOOD AND BEVERAGES 183

The composition of a salad has a significant effect on whether MAP can be successfully used to extend the shelf-life. Products with very low acidity may not benefit greatly from the use of MAP, but oxidative changes can be minimized by the exclusion of 02. MAP may be more useful for products in which the microbiological factor causes the limitation in shelf-life. MAP of coleslaw containing shredded cabbage has been associated with cases of botulism (Hutchinson, 1992). Inoculation studies showed that the toxin could be present in the cabbage of the product within four days. Hence, the potential problems of the growth of food poisoning organisms should not be taken lightly. Storing the products at temperatures of 3.3°C or less is said to be the most important safety factor (Doyle, 1991).

8.5.3 Breaded and batter-coated products

The sales of value-added breaded and batter-coated products, particularly in the meat and fish sector , have expanded considerably since the mid-1980s. These industries consider these products as an effective way of extending the sale of meat and fish. When the vegetable-based products are also taken into consideration, the vastness of this product category can be fully realized. The shelf-life of these products is affected by the characteristics of the coating material and by the centre material. The spoilage pattern of these two components needs to be understood.

Generally , the breading and battering stages are followed by a frying stage. The products can be deep-fried to cook the product fully, or parfried to help the adhesion of the coating material to the surface of the product. The oil pick-up by the products varies depending on the extent of the frying process. Par-fried products pick up only sm all amounts of oil. The presence of oil leads to rancidity problems during the storage of the products. If rancidity of fat limits the shelf-life of a product, exclusion of O2 through the use of N2 flushing may be the answer. However, if microbiological spoilage causes the limitation in shelf-life, as is possible mainly with the meat and fish products, the use of CO2 will be necessary to achieve an extension in shelf-life. Very little published information is available on MAP of these products, but the MAP considerations for precooked foods would apply for breaded and batter-coated ready-to-eat products.

Mould growth may be a problem in the storage of breaded products. The experience of MAP applied to bakery products shows that mould growth can be delayed by the presence of CO2 • At least 20% CO2 would be required to have an effect on microbial growth in products. The presence of and the exclusion of O2 appear to be the important factors in applying MAP to this category of products. The level of CO2 to be used is determined based on the sensitivity of the products to this gas. Although, in general, increasing the level of CO2 decreases the growth of moulds and


bacteria, taints may be caused by the absorption of the gas by product, thus limiting the level of CO2 that can be used.

8.5.4 Pastry-based products

MAP is being applied widely to multi-component products such as bakery products with various fillings, particularly in Europe. As in the case of other pre-cooked foods, a combination of N2 and CO2 is used. The level of CO2 used is usually as high as possible before taints become noticeable. Some published information on work carried out in France gives useful indications of gas mixtures suited for this category of product.

Baked rolls stuffed with harn pie ces and Emmental cheese are said to have an optimum MAP composition of 50% CO2/50% N2 , giving a shelf of 15 days at 5°C compared with only five days when packed in air. A Danish pastry stuffed with chicken pieces in a bechamel sauce is said to benefit by being packaged in the same gas mixture, doubling the shelf-life to give 10 days at 5°C. The 50% CO2/50% N2 gas mixt ure has been found to be a compromise, delaying microbial spoilage without causing collapse of packs and off-flavour in these products (Louis, 1984; Coulon and Louis, 1989).

The shelf-life of pizzas and quiches has also been extended considerably by the use of similar MAP gas mixtures. The gas mixtures used need to be varied depending on the ingredients present, owing to various sensitivities to CO2 and conditions leading to pack collapse. Generally , the higher the water activity of the product the higher the level of CO2 required to be used (Coulon and Louis, 1989). However, the problems of pack collapse and taints also need to be considered.

The potential use of MAP to improve the shelf-life of a wide range of products was investigated in Canada (Ooraikul, 1991). In this study, a 3:2 ratio of CO2:N2 was applied to all the products and the products were stored at 25°C. Chocolate Danish, a pastry with chocolate-based filling, which developed mould after 14 days when packaged in air, was found to be virtually unchanged in quality after 28 days at 25°C when the MAP system was used. The same CO2/N2 mixture was tested on both raw and cooked pies containing various fillings. Butter tarts containing an eggbased filling did not show significant improvement when packed under the gas mixture. Apple turnover, however, showed considerable benefit under MAP, the shelf-life being extended from 7 days in air to 21 days in MAP at a storage temperature of 25°C. This product showed the potential of being able to be stored under ambient conditions if yeast growth could be retarded. Blueberry pie stored in air showed problems of moisture migration from filling to crust, causing the crust to develop mould growth. Under MAP, the same product was shelf-stable for at least three weeks, although moisture migration problems were still apparent, causing softening of the product. The migration problems can only be solved by


modifying the composition of the product. Although cooked apple pie benefited from the use of MAP, raw apple pie showed no benefit. Uncooked products were said not to be suitable for MAP because of high microbial counts. Although this research demonstrated the possibility of ambient storage of products normally frozen, through the use of MAP, the safety aspects still remain a major concern.

8.5.5 Safety cancerns

When packaged under modified atmosphere, ready-to-eat products can raise safety concerns. If the products were not processed adequately or have become contaminated, they may contain pathogens, which will grow under the anaerobic conditions of MAP. If the products are then not reheated adequately before consumption, the presence of the microbes or their toxins can cause food poisoning. It is important that products not heat-treated to achieve commercial sterility are stored under good chili conditions (lower than 3.3°C) to stop the growth of pathogens.

It has been stated that the growth of these anaerobic bacteria can be minimized by retaining a minimal level of 2% O2 within these packs (Hotchkiss, 1987). However, many products are sensitive to 02, and their sensory quality is reduced by the presence of the gas. If an aerobic gas atmospheres are to be used, the overall advantages need to be assessed versus the safety concerns, and steps taken to ensure that ready-to-eat products in MAP do not become a health hazard.

8.6 Fruit juices and other beverages

The presence of 02, temperature, exposure to light and storage time are the factors that affect quality loss in fruit juices (Solomon et al., 1995). Dissolved O2 is the primary problem in limiting the shelf-life of aseptically packaged products (Marshall etai., 1986; Mannheim etai., 1987), and bagin-box products (Rouhana et al., 1988), but O2 permeation is the problem when using flexible packs (Mannheim et al., 1987; Rouhana et al., 1988). The level of dissolved O2 normally present in aseptically packaged juice can cause a considerable loss of L-ascorbic acid when the product is stored at 37°C (Kennedy et al., 1992). Therefore, packaging materials can significantly affect the shelf-life of juices (Nagy and Rouseff, 1986).

The greater the permeability of the materials to 02, the poorer the vitamin and flavour stability of the products. Fruit juices pasteurized and cold-filled aseptically into cartons with an aluminium foillayer can have a shelf-life of 4 to 5 weeks at 6°C (Solberg et al., 1990). If the aluminium foil layer is removed, the product shelf-life can decrease to 7 to 10 days, owing mainly to the oxidation reaction. It is, therefore, very important that the


contact with air be minimized at all stages of processing. If juices are to be preserved for a longer period of time, it is vital that the headspace O2 level be reduced. Other 02-sensitive beverages include beers and wines.

The quality of juices has become an important issue with consumers over recent years, and this is being reflected in their change in preferences. In the USA, the trend has shifted away from the use of frozen concentrates to that of single-strength ready-to-drink products because they are perceived to be healthier, of a high er quality and more convenient. In Europe also there has been a growth in the proportion of fresh juice products being consumed.

Untreated juice is also available to consumers in certain countries through the use of in-store facilities (Sadler et al., 1992). Although such products have the best flavour profile, they are unstable owing to the activity of endogenous enzymes that cause loss of cloud, which affects the appearance and mouthfeel of the juices. The heat process used not only inactivates the enzymes but also destroys microorganisms that cause spoilage of the products. Recent research has concentrated on identifying minimal processes that result in stable products without significantly affecting quality. Light pasteurization (66°C/10 s) of orange juice was found to give a product that was stable for four weeks at 4°C (Sadler et al., 1992). This study showed that cloud loss is greater when the O2 barrier is reduced.

Although the shelf-life of Oz-sensitive products can be extended by the use of N2 flushing, the technique was not always seen as a viable proposition. It has been stated that if juices need a shelf-life of only four to five weeks at 6°C, then air in the headspace need not be a major consideration. Flushing with N2 to displace air before sealing of packs is said to be a useful precaution but is claimed not to be vital if Good Manufacturing Practices are adopted (Solberg et al., 1990). However, as consumers' tastes become more sophisticated, the use of N2 flushing of the fresh products is likely to increase, being used to maintain the quality of the fresh minimally processed products. Flushing with N2 is currently used to give added protection to fmit juices, particularly when they are packed in bulk units such as bag-in-box packs.

A new technology currently being applied to fresh juices is called Maptek Fresh, which is cIaimed to give the aroma and flavour of fmit ripened on the vine and show negligible loss of vitamin C for up to 8 weeks (Pacific Asia Technologies Inc., technical brochure; Wu and Powrie, 1995). Maptek Fresh®-treated and stored orange juice is said to have a long shelf-life after processing without the use of a heat treatment. The technology involves sanitizing the surface of the ripe fmit, extracting the juice and then gassing the juice with agas mixture containing 02, CO2 and possibly an inert gas (N2, helium and/or argon). The juice is then filled into high-barrier packs, leaving some headspace, hemetically sealed and then


rapidly cooled. The MAP system is c1aimed to re du ce enzyme activity and microbial growth in the juice and maintains the fresh quality and gives an extended shelf-life. The level of c10ud stabilization achieved in the products, although adequate, may be improved by additional process parameters not covered in the patent (W.D. Powrie, personal communication). The system controls the processing, packaging and storage conditions to give a shelf-life of 35 days for fresh orange juice and 42 days for fresh apple and grapefruit juice (Worral, 1994). The technique is not currently being used on commercial products but may be used in the near future.

In other recent research, the use of noble gases to improve the quality and shelf-life of juices has also been demonstrated (Spencer, 1995). Significant improvements were found in terms of preservation of colour, flavour and aroma by the use of these gases relative to the preservative effects of CO2 and N2 .

Liquid N2 injected into beer at the filling stage is said to stabilize flavour and preserve shelf-life. Many different systems exist for N2 flushing of beverages. One system is described as injecting liquid N2 into the bottom of aluminium cans for 4 s before the cans enter the filler. The N2

evaporates, flushing out O2 and thus reducing the absorption of O2 by beer during filling from 0.25 to 0.05 mg I-I (Anon., 1987b). The dose of N2

injected is determined by the volume of package the gas has to fill. Aluminium cans are usually only used to package carbonated beverages,

as the internal pressure exerted by the CO2 in the drinks is essential for providing rigidity and strength to the thin walls of the cans. Injection of liquid N2 allows non-carbonated beverages such as fruit juices and iced tea also to be packaged in aluminium cans. In one such system, N2 was said to be injected at volumes of 0.12 to 0.25 g into filled cans at a speed of 20 cans per second before lidding. The evaporation of N2 displaced the air in the headspace before seaming (Anon. , 1987b). An internal pressure of 1 to 2 bar was created by the presence of N2 to give the can rigidity. The low level of O2 in the headspace was said to increase flavour stability of products. The system was being used not only for fruit juices but also for wine and other products, where the presence of O2 can considerably degrade the quality of the product.

Other advantages of N2 flushing are said to be the release of an intense aroma from the product on pack opening, owing to the release of internal pressure. Can corrosion mayaiso be reduced by the use of N2 flushing (Goodburn and Halligan, 1988). The increase in rigidity attained through N2 flushing is said to be particularly useful for vending operations (Anon. , 1992e) and mayaiso allow light-weighting of metal containers, resulting in savings in both material and distribution costs.

The use of active packaging that reduces the O2 level in the package is also finding acceptance in the beverage sector . These packs are said to be more efficient than the use of gas flushing in reducing the residual levels of


O2 and thus increasing shelf-life. Materials absorbing O2 incorporated into the body and caps of the botdes have a potential application for fmit juices and other 02-sensitive products such as beer (Rysstad, 1994).

A more novel use of liquid N2 is in the packaging of beer in cans with a widget device, which were primarily developed for the UK market. The widget devices, made of a food-grade plastic, were developed to enable consumers to drink draught beer at horne. Various types of widgets have been developed and paten ted by beer manufacturers, but the functional principle of the different devices is the same. The plastic insert, consisting of a 15 ml chamber with a 0.3 mm diameter hole, is positioned in the base of the can. The can is then filled with the product and pressurized with the addition of liquid N2 prior to sealing (Sargeant, 1994). After processing, the widget contains a mixt ure of beer and gas and remains at apressure in equilibrium with the can contents. When the can is opened, the pressure is released, causing the drink to be forced through the hole in the widget, to produce a surge of bubbles, which then result in the formation of a creamy foam head.

8.7 Use of MAP in combination with other processes

MAP can be used in combination with other preservation treatments to extend the shelf-life further. MAP used in combination with heat processing gives shelf-stable products with improved quality in the case of products such as ready meals. MAP is now also being used to package products for frozen storage. The reasoning behind the use of MAP for ready-to-eat products is that they can be distributed frozen, then thawed and sold as chilled products but with an extended shelf-life (Morris, 1989). In this case, the packaging materials used would need to be suited for both the low-temperature storage and MAP applications.

MAP has been found to be a useful technique combined with irradiation. Particularly at high doses, irradiation can lead to the production of offodours and off-flavours in foods. During the irradiation process, the water molecules produce free radicals. The re action of these free radicals with other components in the food can cause a change in the organoleptic quality. Fats present in the food can become rancid after irradiation. The presence of O2 will accelerate this rancidity. Exclusion of O2 from the headspace by using N2, therefore, will help to reduce rancidity of the products. Meats can develop off-flavours after irradiation. The use of MAP to exc1ude O2 has been found to reduce the changes caused by irradiation. As food irradiation becomes more widely used, the benefits of MAP to extend the shelf-life of irradiated foods will be greatly appreciated.

Although MAP has removed the need for preservatives in certain


products, the use of low levels of preservatives has been useful in some cases to add a safety margin against the growth of anaerobic bacteria. Natural anti-microbials are seen to be a more favourable alternative from the point of view of the consumer. The combination of MAP with natural anti-microbial agents has also produced useful effects. Nisin, an antimicrobial agent produced by astrain of lactic acid-producing bacteria named Lactococcus lactis, is used in foods packaged under modified atmosphere and has given an advantage in terms of shelf-life. Although nisin has limitations in terms of application, cheese is one product that is thought to benefit (Wasik, 1992).

Gases have been used as process aids in the industry. Processing in the presence of gases has also been shown to have beneficial effects in terms of product quality and shelf-life. Pasteurization of food under CO2 may allow the pasteurization temperature to be reduced to lower than 60°C for fruit and vegetables (Varoquaux, 1993). However, the CO2 causes the green chlorophyll pigments to be converted to phaeophytin by a transitory acidification caused by the presence of dissolved CO2 . Therefore, the process will not be suitable for green vegetables. Although in the author's experience heat processing in the presence of CO2 has potential benefits in terms of improving microbiological quality, its usefulness is limited also by the negative effects on sensory quality. However, recent studies have highlighted the usefulness of noble gases in similar applications (Spencer, 1995). No adverse effects on sensory quality have been reported in this case.

MAP techniques are being applied to chilIed foods, dried foods, he atprocessed/shelf-stable products and frozen products, and in the future may even be applied to irradiated products. The potential of MAP will continue to be exploited and the technique applied in new applications in the future.

Although the benefits of MAP can be numerous, it is important that the microbiological safety considerations of using an aerobic conditions are understood. Anaerobic conditions can promote the growth of pathogens and, therefore, steps must be taken to assure the safety of products. Particular attention is required for MAP of ready-to-eat products with a high moisture content. Tbe use of low-temperature storage conditions is important for chilIed MAP products. A temperature of less than 3°C is required if C. botulinum growth is to be prevented. The anaerobic risks can be minimized by retaining a residual O2 level of at least 2% within the packs (Hotchkiss, 1987). Certain products, particularly those with a high level of fat, require residual O2 levels lower than 1 % in order to suppress oxidative changes. In these cases, it is important that microbiological safety is assured. If Good Manufacturing Practices are adhered to and the products packaged have a low initial microbial count, the use of MAP can lead to a considerable extension in shelf-life of products.


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9 Fish and shellfish H.K. DAVIS

9.1 Introduction

Most fish are caught from the wild in nets or with lines of baited hooks. Some die before being hauled from the water, some suffer physical damage and, farmed or wild, all are inevitably stressed before death. These, and other differences owing to biological condition, structure, composition and post-mortem change, present fish processors and distributors with a raw material that is very different from other food materials used in modern processing industries.

As with other raw meats, fish at retail sale requires some form of wrapping to protect the food from contamination, and the purchaser from soiling their hands and c1othing. Wrapping in greaseproof (kraft) paper or, later, polyethylene film has long satisfied the need in sm all shops and markets. Demand from higher volume stores for conveniently prepackaged products led to some 'in-store' production of cellophaneoverwrapped fish and shellfish products (Anon., 1956) and, eventually, centralized factory production of overwrapped, chi lied portions on trays (Almaker, 1965) and frozen portions in bags (Anon., 1967). Apart from some chilIed, high-value products such as smoked salmon, vacuum packaging was adopted more as a means of protecting frozen fish from dehydration (An on. , 1967). Whilst plastic films were appreciated as a necessary component in the growth of the market for frozen fish, Gibson and Soulsby (1970) found that the need for similarly convenient prepackaging of wet fish continued to be challenged through the 1960s. Limited shelf life was aggravated by problems of poor temperature control in storage, distribution and display systems. Instead, the products were often frozen for distribution and buffer storage and then thawed for retail sale. Centralized factory pre-packaging of chilled retail fish products continued to develop but injection of preservative gas mixtures did not appear on a significant commercial scale for another decade.

Interest in CO2 as an aid in the preservation of fish has a longer history than the plastic films which eventually made practical the MAP of retail products. Developments that started in the 1930s were summarized by Shewan (1950), who concluded that gas storage on board fishing vessels was only worthwhile for fish that would be more than 14 days in ice before

FISH AND SHELLFISH 195

landing at the quayside. Subsequent interest turned to the use of CO2 in modified refrigerated seawater systems, on board catching vessels and lorries, as an alternative to the use of iced storage for sm all fish and shellfish (Nelson and Barnet, 1971; Hiltz el al., 1976; Barnet el al., 1978; Bullard and Collins, 1978). loteTest in the technically more difficult task of transporting chilIed, whole fish in CO2 atmospheres then reappeared. With diminishing supplies and higher prices of fish, more recent studies of wholesale fish in MAP have included cod (Villemure el al., 1986; Einarsson and Valdimarsson, 1990; Wignall el al., 1990) and cod fillets (Leblanc and Leblanc, 1992), as weil as salmon (Sorensen el al., 1990) and some shellfish, including octopus and squid (Moral es el al., 1995), and Norway lobster (Moral el al., 1995). Salmon was of most concern in earlier studies conducted by Nelson and Tretsven (1977), which were followed, in 1977, by commercial trials with specially constructed containers (Barnett el

al., 1982). This was aperiod when all variables associated with manipulation of storage atmospheres were keenly examined. Reductions in rates of deterioration of several species of fish have been recorded in tests using pressure chambers under both hyperbaric (Charm el al., 1977) and hypobaric (Haard el al., 1979; Varga el al., 1980; Haard and Lee, 1982) conditions.

The earliest use of CO2 atmospheres in retail products exploited its high solubility to produce a 'snugging down' effect (Douglas, 1970), regarded as characteristic of CO2 packs. lronically, in view of the later developments in MAP, the 'vacuum appearance' and prevention of movement of the product was seen as being advantageous, but there was only a sm all amount of kipper fiBet being packed in this way, with no application to white fish or other fish products (Abbey , 1970). Earlier, vacuum packaging had been adopted more as a means of protecting frozen fish from dehydration (Anon. , 1967). For unfrozen material, flushing of Orsensitive products with N2 was recommended and applied to shrimps and prawns but was not recommended 'for meat and fish which are susceptible to spoilage by anaerobic bacteria' (Anon. , 1966).

At first, development and application of controlled gas mixtures for consumer packaging was concerned mostly with foods other than fish (Anon., 1970; 1977). Georgala and Davidson (1970) did, however, include fish, but the example given paid little regard to any need for different gas mixtures to be applied. According to Kimber (1984), the technology of gas packaging was first perfected and patented in 1963 by Böhme and Kalle Films but it took until 1977 to produce gas-flushed packs successfully. UK manufacturers were slow to adopt the process but appear to have been more willing to widen the range of products and include fish. The first UK application to fish products was in 1979 in Northern Ireland (Kimber, 1984), with a few speciality products. The technique became more widespread as manufacturers of vacuum-packaging equipment adapted


their products, and supermarkets sought alternative ways of presenting fish.

With equipment capable of making more precisely designed and practicable products, research workers returned their attentions to the demands of pre-packaging fish products and the use of gases, mainly CO2 .

The ways in which fish spoil, and the differences caused by their different composition, greatly affect the consequences from storing them in modified atmospheres, and the choice of gas mixtures to be applied. Trials were conducted on a large range of species and products, and many of the results have been published in a variety of symposia proceedings, recommendations and codes of practice (Martin, 1981; 1982; Cann, 1984; Anon., 1985) and in many individual research reports and theses. These studies have been conducted as much in the USA as in European countries but most commercial exploitation has taken place in the UK and France, where the numbers of modified atmosphere packages produced in 1994 has been estimated at 83 X 106 units of fish, -6% of the total market, and 197 X 106 units of shellfish, -15% of the total market; considerable further growth is predicted (Anon., 1994a, b).

9.2 Spoilage of fish

Like mammalian meats, fish spoil through the combined effects of chemical reaction, continuing activity of endogenous enzymes and bacterial growth. There are differences, however, in chemical composition, metabolism and environment of the live animal that subsequently reduce the storage life of most fish when compared with meat. Being poikilothermic, their metabolism and the commensal bacterial flora of their skin and intestines are adapted to lower temperatures than mammals. Hence, chilling results in a smaller temperature difference, especially for fish caught in temperate and polar regions. During what can often be a prolonged capture struggle, much of their carbohydrate reserves may already have been converted to lactic acid and, after death, remaining energy reserves decline as catabolic activity continues. The amount of glycogen present in the muscle of fin fish is much less than in mammals, and it is not all converted to lactic acid. Hence, muscle pH of fish does not fall to the same extent as in many other meats.

The muscle of live cod is usually close to neutral pH (McCallum et al., 1967), but values for the ultimate pH post-mortem can vary from about 6.0 to over 7.1 depending on seasonal and other factors. There is also variation between species; the muscle pH may fall to less than 6.0 in some fish, including halibut, tuna and mackerel (Buttkus and Tomiinson, 1966). Concomitantly, adenosine triphosphate (A TP) concentrations fall and, at a concentration that varies with temperature, enzymes that hold muscles in


the relaxed state can no longer function. Within hours, depending upon species, condition and temperature, the muscles began to contract and the fish stiffens in rigor mortis. Most processing must wait until rigor mortis is resolved, when the muscles have returned to a relaxed state. By this time (usually one to three days), the continuing activity of endogenous enzymes has degraded most of the adenosine nucleotides to inosine monophosphate (IMP), wh ich is at, or past, its maximum concentration (Kassemsarn et al., 1962; Dingle and Hines, 1971). As the degradation sequence continues, inosine (INO) and then hypoxanthine (Hx) is produced.

Accumulation of IMP is of particular significance because of the synergistic effects that it is known to have with many flavorous substances (Kuninaka et al., 1964; Yamaguchi, 1987) as well as some evidence of an inhibitory influence on bitter substances (Woskow, 1969). The initial sweet, meaty and species-characteristic flavours of fresh fish reflect the combination of IMP and free amino acids present in the flesh, as well as some sugars and sugar phosphates (Jones, 1969). Decreasing flavour intensity is largely a consequence of the loss of glucose, hexose phosphates and IMP (Jones, 1961). A major influence is the pH sensitivity of IMP phosphohydrolase. It has a pH optimum between 8.5 and 9.5 (Dingle and Hines, 1971), and IMP is more stable in the tissues of fish of lower pH (Dingle and Hines, 1971; Spinelli, 1971).

At any stage of spoilage, the synergistic influence of any remaining IMP will depend upon stability during cooking. Although IMP is not particularly stable to heating in aqueous systems, it is sufficiently so under some of the less severe temperature and time conditions encountered in domestic cooking. Most or all IMP may, however, be lost when longer periods of cooking are applied to fish that contain a more active enzyme. The influence of IMP on the flavour of fish has been demonstrated in laboratory experiments that have included the blocking of IMP dephosphorylation with ethylene diamine tetraacetic acid (EDT A) (Groninger and Spinelli, 1968), and the addition of IMP to sterile fish muscle after it had been stored until all naturally present IMP had degraded (Spinelli and Miyauchi, 1968). (In both cases, sensory testing showed the treated sampies to be preferred.) More recently, Fletcher et al. (1990) have examined the relationship between acceptability and naturally occurring levels of IMP and Hx for a range of fish species and treatments.

In some shellfish species, it is adenosine monophosphate (AMP) that accumulates, but this also has synergistic properties and is thought to contribute greatly to the taste of molluscs (Kawashima and Yamanaka, 1995).

This first stage of spoilage of fish muscle is dominated by endogenous catabolic reactions, whilst the bacterial flora of the gut cavity, gills and skin acclimatize and adapt to a changed environment. Although Hx has a bitter taste, autolytic reactions are associated more with loss of those


flavours characteristic of fresh fish; this phase can be considered more as a loss of freshness. Aseptically exised, sterile muscle remains !ittle changed after reaching a bland, tasteless stage (Herbert et al., 1971) when most non-microbial degradation of carbohydrates and nucleotides will have taken place. Beyond this stage, most odours, flavours and other signs of spoilage appear as a result of the activity of the microbial flora of outer surfaces and intestines. Skin surfaces of fish from cold and temperate regions generally have total viable counts of 103 to 105 cm-2 , and aerobic Gram-negative psychrophiles and psychrotrophs predominate in a microflora that is largely determined by the environment of the live fish (Shewan, 1977). Two main generic groups, PseudomonaslAlteromonasl Shewanella and MoraxellalAcinobacter, comprise 60% to over 80% of this initial flora (Hobbs, 1991). Similar numbers of more mesophilic, Grampositive organisms are found on fish from warmer waters (Shewan, 1977; Matches, 1982). Many such fish remain edible for longer than similar coldwater fish, though the pattern of differing storage lives is very complex (Lima dos Santos, 1981). Shewan (1977) suggested that one likely cause of different storage lives may be the absence from the initial flora of tropical fish of many of the 'active spoilers' found on fish from cooler waters.

Whatever the composition of the initial microflora, as numbers increase the same few genera predominate. While others may continue to grow in number, it is the two large groups dominant on the skin of live cold-water fish that thrive during storage in ice and are responsible for spoilage (Stenstrom and Molin, 1990). Of these, the Pseudomonas/Alteromonas/ Shewanella group eventually comprise over 80% as the fish approach inedibility. The odours and flavours of spoiling fish are caused by metabolic waste products of some, not all, of the successful organisms as they utilize water-soluble constituents of tissues. It is only in the later stages, when spoilage is quite advanced, that tissue proteins are broken down by bacterial proteinases, replenishing the pool of small peptides and free amino acids. The more evident products of bacterial activity include lower-chain fatty acids from carbohydrates, aldehydes and ketones from lipids, ammonia, amines and 'biogenic' polyamines from amino acids, and volatile sulphides from sulphur-containing amino acids.

Compositional differences between fish add to the complexity of spoilage patterns. The flesh of most marine fish and shellfish contain tri methylamine oxide (TMAO), which is thought to have an osmoregulatory function in the live anima!. After death, TMAO can serve as a terminal electron acceptor for some of the spoilage bacteria, enabling them to grow when oxygen levels are depleted (Easter et al., 1983). The ammoniacal substance TMA is released as a consequence. Many ofthe fish species used as the basis for these observations are referred to as white or lean fish. Their fillets, the main edible portion, have a total lipid content of up to 1 or 2%, with only a very sm all amount of dark muscle, which


contains more haem pigments and lipid than does the skeletal musde. Fatty fish, such as herring and mackerei, are more active swimmers and have a greater proportion of dark musde and lipid. Fillets from autumncaught mackerel sometimes contain more than 30% lipid, but the high degree of unsaturation, which gives fish oils their nutritional significance, makes them very vulnerable to oxidation. The rancid flavours of products of lipid oxidation and hydro lysis reduce the acceptability but do not normally dominate the odour or flavour of fatty fish spoiling at chili temperatures. Molluscan shellfish often contain glycogen at concentrations approaching those found in mammalian meats. eonsequently, ultimate pH values can be relatively low, mainly as a result of the production of octopine via a different glycolytic pathway from that occurring in finfish and meat (Paredi et al., 1995). Like crustacean shellfish, however, they contain greater amounts of free amino acids. This, combined with a generally sm aller size is thought to account for much of their greater susceptibility to bacterial spoilage and consequent short shelf life.

9.2.1 Effeets of temperature on fish spoilage

Handling and processing often causes fish to rise in temperature, and modern packaging and display systems present additional refrigeration problems. Rates of chemical and autolytic reactions increase with temperature, and fish spoilage bacteria are dose to their optimum growth rates at normal ambient temperatures for human activity. Spencer and Baines (1964) derived a linear relationship for the spoilage rates of white fish between -1°e and +25°e, but Olley and Ratkowsky (1973) found this model to be limited. Ratkowsky et al. (1982) showed that the grawth rate of a wide range of spoilage bacteria fitted a square law. Solving their equation for spoilage rates of fish stored at temperatures between ooe and 15°e, Storey (1986) obtained the relative rate function R = (0.1 T + 1)2, where T is temperature in oe, and confirmed its relevance to data fram sensory analyses. From this, one can see that fish spoil more than twice as fast at 5°e than at ooe, and four times as fast at 10°C.

9.2.2 Measurement of spoilage

Examination of the relative merits of different handling and storage techniques requires some means of measuring their effects, and sensory tests have been developed to measure the relative rates of spoilage, or loss of freshness. Taste panels can function in a highly objective manner but, no matter how weil trained and disinterested a panel may be, the risk of subjectivity creates a need, also, for non-sensory tests. The latter inevitably measure the changing amount, or physical effect, of some individual or li mi ted group of substances and consequently do not measure


the wider impact on a potential consumer. Correlation with some sensory measure needs to be established, but such relationships cannot be assumed to be valid for all circumstances.

Standard procedures for counting viable bacteria are frequently used as a means of comparing experimental treatments but, because of the additional problem of contamination, such measures are more an indication of overall bacteriological quality, affected by both spoilage and hygiene. Attempts have been made to differentiate 'active spoilers' because growth of organisms that produce few offensive metabolites will be of less significance. One example is the enumeration of hydrogen sulphide producers, though, like the various chemical methods, this has inherent restrictions because there are other 'active spoilers'.

Of the various chemical and physical measures of fish spoilage that have been developed, those most frequently used are variants of the analytical procedures for measurement ofTMA and nucleotide degradation products. TMA is released during spoilage, and concentrations in the muscle increase in the same exponential manner as bacterial growth. Specific detection techniques can be used but, more commonly, TMA is included within a general measure of related substances, the total volatile base, which includes ammonia and dimethylamine. Little change is detected in these tests until bacterial spoilage is evident. In contrast, measurements of nucleotide breakdown products detect deterioration in the earlier, autolytic, phase. Hypoxanthine alone has been used as a spoilage index but an alternative proposed by Saito et al. (1959), the 'K-value' (inosine plus hypoxanthine as a proportion of the total purine nucleotide pool), is more sensitive during the earlier stages. Many other chemical tests have been examined but the main ones attempt to assess a change in some specific sensory characteristic (e.g. peroxide value or thiobarbituric acid value for rancidity) or to assess so me potentially hazardous product by measuring an indicator substance (e.g. histamine for scombrotoxin).

Whatever such tests impart, the ultimate tests are the responses of consumers. Is the product liked and is it purchased? But this also is fraught with problems (lul, 1986). Sales figures alone are complicated in many ways, not least by the bias produced through not wanting to risk offering borderline or spoiled products. Therefore, sensory laboratory procedures are needed to provide quality control in development and manufacture of the pro du cL Hedonic assessment may often be appropriate but, because of the large differences between people in terms of chemical sensitivity and cultural variations, large numbers of panelists and large amounts of material are required. Alternatively, expert panels trained to assess 'freshness' can discriminate objectively between sampIes that may differ only slightly. One of the earliest descriptive schemes was developed by Shewan et al. (1953) and has since been used extensively in research and commerce. Scoresheets for assessment of cooked odours and flavours of


iced white fish are given in Table 9.1. After much training, practice and experience, a high degree of precision can be obtained, with variances of 0.5 or less for cooked flavour scores for the more familiar and frequently examined species such as cod (Baines, 1965).

Techniques for scoring 'freshness' must, however, be applied and interpreted with caution. There are important limitations that can affect their use in studies in MAP of fish. Such schemes are necessarily based on some readily achievable, reproducible conditions, usually whole fish stored under conditions of ideal icing practice. There can be considerable differences between sam pies of similar history , and the skill exercised by trained panelists is to fit a range of complex odour and flavour notes to the scoresheet descriptors. The panelists' task remains the same with sam pies taken from other storage environments. But different storage conditions can be expected to cause different reactions via, for example, a changed microbial flora flourishing at other temperatures, and differences may occur in the amounts of each reaction product contributing to the overall change. Consequently, it is possible that the relationship between panel scores and consumer acceptability, usually presumed to be a direct

Table 9.1 Odour and f1avour scales for cooked white fish

Characteristic Score

Odour Strong fresh 'seaweedy' odours 10 Some loss of fresh 'seaweediness' 9 Lack of odour, or neutral odours 8 Slight strengthening of the odour but no sour or stale odour; 'wood shavings', woodsap, vanillin or terpene-like odours; slight salt-fish or cold storage odour 7

'Condensed milk', caramel or toffee-like odours 6 'Milk jug', 'boiled-potato' or 'boiled clothes' , or metallic odours 5 Lactic acid, 'sour milk' or o-toluidine-like odours 4 Some lower fatty acid (e.g. acetic or butyric acids) 'grassy', 'soapy', 'turnipy' or 'tallowy' odours 3

Ammoniacal (trimethylamine and lower amines) odours 2 Strong ammoniacal (trimethylamine etc.) and so me sulphide odours 1 Strong putrid and faecal (ammonia, indole etc.) odours 0

Flavour Fresh, sweet f1avours characteristic of species 10 Some loss of sweetness 9 Slight sweetness and loss of the f1avour characteristic of the species 8 Neutral f1avour, definite loss of f1avour but no off-f1avours 7 Absolutely no f1avour, as if chewing cotton wool 6 Trace of off-f1avours, some sourness but no bitterness 5 So me off-f1avours, and so me bitterness 4 Strong bitter f1avours, some rubber-like and slight sulphide-like f1avours 3 Strong bitter f1avours, but not nauseating 1 Strong putrid f1avours (e.g. sulphides) tasted with difficulty 0


relationship, could change through 'atypical' odours and flavours affecting expert panelists and consumers in different ways.

9.2.3 Hazards associated with stored [ish

In addition to a simple increase in the rate at which fish become unpalatable, and the greater risk that any unhygienically handled food presents, some fish products may become hazardous if stored at too high a temperature. The only food poisoning organism of true marine origin, vibrio parahaemolyticus, can survive freezing better than chili temperatures. Above 10°C, it can grow rapidlyon fish, producing a heat-labile toxin that causes gastroenteritis and severe abdominal pain. It is mainly a problem in fish from warmer waters, particularly where cross-contamination of cooked products might occur.

Scombrotoxic poisoning is associated mainly with mackerel and tuna and is more likely to arise when the fish are inadequately chilled. Because such fish contain free histidine, which some organisms can decarboxylate to histamine, this continues to be regarded as the cause of scombroid fish poisoning (Morrow et al., 1991) despite evidence to the contrary. Measurements of histamine content provide an indication of the scombrotoxic potential (Bartholomew et al., 1987), but the absence of significant symptoms from volunteers who ate mackerel containing relatively high levels of histamine, suggests that this alone is unlikely to be the cause (Clifford ct al., 1989; Ijomah ct al., 1992).

Botulism, the neuroparalytic disease caused by toxins produced by Clostridium botulinum, is a potential hazard with some products that are eaten uncooked. Some non-proteolytic strains that produce toxin serotypes B, E and F may be found in high er numbers on fish, particularly some farmed fish, than are normally found on red meats. These mesophilic, an aerobic spore-forming bacteria can grow at temperatures below 5°C, and there have been several reported outbreaks associated with the consumption of fish products (Huss, 1981), mainly involving type E toxin.

9.3 Gaseous preservatives used in modified atmosphere storage of fish

9.3.1 Properties o[ the principal components

Various gases have been examined with a view to extending the shelf life of fish and fish products. Solution in the aqueous phase of fish tissues or bacterial cells is necessary in order to exert an effect, and studies have included the application of solutions of gases e.g. ozone (Haraguchi et al., 1969; Nelson, 1982). ,Ammonia, sulphurous acid and hydrogen peroxide have been examined as potential preservatives for industrial fish (Windsor


and Thoma, 1974). Gaseous ozone and ethylene oxide have been investigated (Silva, 1981), and ammonia vapour has been claimed to have been partially successful (Mandal and Mukherjee, 1974) as food fish preservatives. Fish and shellfish were included in a variety of packaged foods shown to benefit from inclusion of noble gases, which, despite their general inertness, appear able to interfere with enzymic and bacterial activities via active as weil as passive mechanisms. Data presented in support of a patent application by Schvester and Saunders (1989) indicated some inhibition of anaerobe spore germination. Subsequently, the results were attributed to differences in hygiene control during the preparation of experimental sampies (Spencer and Rojak 1993), hence removing this prospect of a specific anti-botulinogenic gas mixture.

The only modified atmosphere systems to have seen significant commercial development and sustained growth have been vacuum packaging (VP), in which any preservative effect may result from accumulating CO2 (Enfors et al., 1979), and the use of added gas mixtures with CO2 as the principal active ingredient. The latter is referred to as CAP or, in sealed packs, MAP, though only MAP is appropriate to retail products. The use of gas mixtures permits the addition of CO2 in sufficient amount to exert some inhibitory effect on spoilage, with minimal deleterious effects on appearance and exudate that may be seen when 100% CO2 is employed. The CO2 is usually diluted by inclusion of N2 and/or 02' Nitrogen has low solubility and appears to exert no direct influence on typical spoilage bacteria, even at concentrations approaching 100% (Coyne, 1932) but is used as an inert 'filler' in mixed gas systems. Oxygen has a similar low solubility and is used at high concentrations, along with CO2 , to preserve the red colour of haem pigments in some MAP meat products. Tbe reasons for inclusion of O2 in recommendations for gas mixtures in MAP fish products are less clear. Some beneficial influence on bacterial spoilage flora has been demonstrated, but adesire not to aggravate potential risks of botulism from products in sealed packs, although not often stated, is probably the main motive.

The inhibitory effect of CO2 on microorganisms has been recognized for more than a century (Valley and Rettger, 1927). The physical properties of CO2 greatly influence its potency, and Edsall (1969) noted that, unique amongst gases, it distributes in almost equal amounts per unit volume between air and water. Hydration to HCOJ and H2C03 , which can be monitored by determination of the accompanying change in pH, is relativeiy slow and most CO2 remains in solution in the unhydrated form. In foods, the pH falls to an extent that is dependent upon the amount of carbonic acid formed and the buffering capacity of the food and rises again when CO2 is released by exposure to air or on warming (Clark and Takacs, 1980). Carbon dioxide is much more soluble in water (-1700 ml 1-1 at O°C) than the other gaseous components of MAPs, and Gill (1988) found the


solubility in animal fats to be of the same order as that in whole musc\e tissue. Clearly, a large proportion of the CO2 in gas mixtures surrounding fish will dissolve in the tissues - both the aqueous and lipid phases.

Solubility in aqueous systems decreases as temperature increases; the rate of decrease is non-linear and changes most rapidly at lower temperatures. A slight rise from the 'ideal' temperature (O°e) will have a marked effect on the amount of CO2 dissolved and, consequently, on the efficacy of the system. Carbon dioxide also permeates much faster than O2

or N2 through the plastics used in MAP of fish products, though losses by dissolution into the plastics have a greater effect than those from diffusion properties (Sacks and Gore, 1987). The materials, usually in laminated combinations, are selected to minimize losses of CO2 , and these are insignificant over the limited shelf life of fish in packages. As Tiffney and Mills (1982) observed, precise control of the initial gas mix is not critical and, like many authors working with fish, they paid little attention to the transmission properties of their packaging materials beyond choosing those already widely used in industrial production of MAP foods.

The response of microorganisms to CO2 varies considerably; those most affected, in decreasing order of sensitivity, are moulds, Gram-negative bacteria and yeasts (Clark and Takacs, 1980), whilst some organisms are hardly affected, e.g. Clostridium perfringens and Lactobacillus spp. Generally , Gram-negative spoilers are markedly inhibited by CO2 , though a significant exception for fish is Shewanella putrefaciens, which is much more resistant than Pseudomonas spp. (Molin and Stenstrom, 1984). Microbial inhibition is demonstrated by an increase in the dura ti on of the lag phase, and in the generation time during exponential growth. At low concentrations of CO2 the effects are substantially reduced and can actually stimulate growth (Enfors and Molin, 1980) provided the organisms are growing in complex rather than minimal media (Gill and Tao, 1979).

Most food-borne pathogens grow slowly, or not at all, at the temperatures required for distribution of perishable foods, but where temperatures rise above growth minima, the limited evidence of their response to CO2

suggests that, in general, they survive (Clark and Takacs, 1980). There is evidence that germination of Clostridium spp. spores may be enhanced by CO2 at normal atmospheric pressures, but Doyle (1983) found that production of toxin by C. botulinum was inhibited. He suggested that CO2

perhaps stimulates germination but slows enzyme reactions critical for production of toxin, with a net decrease in botulinogenic potential.

9.3.2 Proportions of gases recommended for use in MAP

Many combinations of gas mixtures have been examined experimentally for their effects on stored fish. Some workers report success with relatively low concentrations of CO2 : 20%, (Brown et al., 1980), 11.5% (Nelson and


Tretsven, 1977),25% (Villemure et al., 1986)), while others have recommended 100% CO2 (Sacks and Gore, 1987; Weber and Laux, 1992). Most recommendations and practices have initial concentrations of CO2 between 30 and 60%. Therefore, in most instances, levels of CO2 higher than 25%, the maximal requirement indicated by Gill and Tan (1979), have been found to be necessary.

There are similarly divergent views on the need to include O2 along with N2 as the diluent gas. Tiffney and Mills (1982) found that O2 actually increased shelf life of white fish and, as did Lagoin (1985) and Sacks and Gore (1987), they recommended inclusion of 30% O2 for white fish but exclusion for fatty fish and cured products. Sacks and Gore (1987) also included 40% O2 with 60% CO2 as an alternative for white fish. At the other extreme, Kimber (1984) declared that fish require an inert mixture of CO2 and N2 , and that great care should be taken to remove any 02'

While there has been much experimentation with fish products stored under many different gas mixtures, most have ignored the concomitant matter of the amount of gas required. Cann (1984) assumed a gas-to-fish ratio of 3: 1 when recommending appropriate gas mixtures for different fish products. Work reported by RandeIl and Ahvenainen (1994), aimed at reducing the costs associated with storage and transport of such high volume packs, concluded that gas-to-fish ratios of 1:1 and 0.4:1 were at least as effective as vacuum packaging. To achieve maximal benefit, with cod fiBets packed in the same gas mix (40% CO2), a minimum gas-to-fish ratio of 2:1 has been found to be necessary (H. Davis, unpublished work).

9.3.3 Vacuum packaging

By virtue of the exclusion of O2 and retention of COb sealing of products within a gas-tight, flexible pouch after removal of air is also a form of modified atmosphere pack. Oxygen, which is necessary for the growth of some spoilage organisms and contributes to the development of rancidity, is removed and CO2 , generated through bacterial respiration, is trapped within the tissues. Mostly, it is this retained CO2 that is thought to exert some preservative effect (Eklund, 1982a), but most results of research with chiBed fish have proved of limited value when compared with me at. Shewan and Hobbs (1963), Pelroy and Seman (1969), Hansen (1972), Huss (1972), Jensen et al. (1980) and Matches (1982) all reported longer shelf lives for a variety of whole white fish and fiBets. Conversely, Lamprecht, et al. (1984) found vacuum packaging conferred no benefit at all to hake fillets. Although Huang et al. (1992) observed a statistically significant inhibition of psychotrophic bacteria in weakfish stored in vacuum packs, there was no corresponding improvement in sensory scores. Regenstein (1986) also judged most of the benefits to be marginal. Cavett (1967), commenting on the 'unusual' situation with fish, suggested that either the


experiments were being curtailed too early or that some Pseudomas spp. were growing at very low, almost an aerobic O2 tension. But as the work of Jensen et al. (1980) demonstrated, the cut-off quality had to be poor before a pronounced difference could be seen. They attributed the difference between meat and fish to the presence of TMAO providing, in fish, an alternative electron acceptor for bacterial metabolism.

Vacuum skin packaging (VSP) is similar to packing in vacuum-packaged pouch, but the final result is a glossy, wrinkle-free pack (Dalheimer, 1986). Materials of different barrier properties are employed, and any antimicrobial effect of vacuum skin packaging would be dependent upon the choice of barrier film. At best, it would be the same as a conventional vacuum pouch, but an 02-permeable film has been used for some shipments of salmon to the USA. Gormley and Zeuthen (1990) attributed resistance in UK markets to the higher costs and a too-shiny appearance. They also expressed the same concern that Lindsay (1981) had regarded CO2 packs: that VSP may confer some additional perception of security, causing consumers to put themselves at increased risk.

9.4 Changes occurring during storage of fish products in MAP

9.4.1 Composition o[ the headspace gas mixtures

As CO2 dissolves in fish tissues, the proportion remaining in the headspace falls. Consequently, proportions of the diluent components increase and concentrations of 02' often above atmospheric levels to start with, are increased even further. Later, as the CO2 solubilization rate is overtaken by the rate of release caused by bacterial respiration, the curves reverse. Davis (1990) calculated that at the CO2 minima, the amounts absorbed at three different storage temperatures were approximately 30% of the saturation solubilities. That observation referred to a specific combination of gases and fish, and relevance to results of other workers is limited because important details, particularly the gas-to-fish ratio, have often not been reported. Any chemical effects on the fish tissues will be affected by the amount of CO2 that dissolves but, as the contaminant flora is limited to the fish surfaces, much of the bacteriostatic effect is likely to be more influenced by the residual atmosphere they have to face. For any gas mixture, a high gas-to-fish ratio will present a very different chemical balance from a low ratio. Similarly, any given mass ratio of CO2 to product will not necessarily confer the same benefits (or harm) when applied via different gas mixtures. Thus, combined with the considerable variation that exists between fish products in terms of surface area-to-volume ratio, as well as differences in composition, there can be little surprise at the


extent of the variation in reports of the benefits conferred by MAP on fish products. Much of the published work has relied upon chemical and bacteriological data, but the summary given in Table 9.2 incJudes only those estimates of shelf life that are based on sensory criteria.

9.4.2 Effeel of MAP on lhe pR of fish produels

Apparent contradictions arise in alm ost every aspect of MAP of fish incJuding, inevitably, muscJe pH (see Table 9.2). Some authors (e.g. Fey and Regenstein, 1982) report little or no change while others have seen decreases during storage (Lannelonge el al., 1982a,b; Belleau and Simard, 1987), or reductions in rate of increase (Reddy el al., 1994; L6pez-Galvez el al., 1995) that were proportional to e02 concentration. Fish muscJe of relatively high post-rn orte m pH can be expected to be more affected by a given amount of e02 than muscJe of lower pH, but this will be complicated by other variations in the chemical composition that influence the net buffering capacity of the tissue. Additional variation in reported measurements may occur because of differences in the method of measurement, mainly because of gradients between the exposed surface of the product and deeper tissue (Tiffney and Mills, 1982). Initial dissolution of e02

(preceding formation of carbonic acid) and bacterial activity (which produces high pH waste products) are both surface phenomena, and it is here that the most rapid and extreme fluctuations in pH occur. Particularly high rates of increase in surface pH have been observed in MAP cod fillets stored at 5De and lODe (H. Davis, unpublished work, Figure 9.1). The general pattern seems to be that after any initial fall, surface pH rises steadily while changes in the pH of deeper tissues lag behind.

9.4.3 Baeleriologieal ehanges

As Stammen el al. (1990) observed, microbiological data on seafoods, al ready subject to much natural and methodological variation, become more complex when MAP is added to the variables. Selection pressures are changed and the dominant flora consists of those organisms best suited to growth under the changed environment. One such beneficiary, sometimes, is Broeothrix thermosphaeta. This organism is more usually associated with meat, particularly high pH meat (which Stenstrom (1985) likened in some ways to fish), and is regarded as an important spoiler owing to production of acetoin and lower fatty acids. However, much smaller amounts of these sensorily unpleasant metabolites were found when spoilage of tuna in MAP was dominated by B. thermosphaeta (L6pez-Galvez et al., 1995). Hence, growth of this organism in fish products might, perhaps, result in a slower development of adverse sensory impact compared with spoilage by the normal aerobic spoilage flora.

Tab

le 9

.2 S

tora

ge l

ives

of

fish

pro

duct

s in

MA

P a

s de

term

ined

by

sens

ory

test

s

Sen

sory

ass

essm

ent

Init

ial

gas

Gas

-to-

She

lf-I

ife

Low

est

Max

imum

S

tora

ge

Spe

cies

and

m

ix (

%)

prod

uct

Typ

e"

Scal

eb

End

-poi

nt

Shel

f lif

e ex

tens

ion

Ref

eren

ce

Init

ial

MA

P

tJ.p

H'

tem

per-

prod

uct

type

C

O,I

O,

rati

o cr

iter

ion

(d

ays)

(d

ays)

m

ater

ial

pH

p

H

atur

e C

C)

Ref

eren

ce

(a)

Mar

ine,

lea

n fi

sh

Cod

(G

ad

us

mo

rhu

a)

Sin

gle

20 c

m

'pie

ce'

40

/-E

,C

10-0

4

9 0.

5 V

P

2 A

F

illc

ts

40/2

0 3.

5 E

,C

10-0

6

14

7 O

verw

rap

6.11

0.

44

0 B

60

/20

3.5

E,C

10

-0

6 17

10

O

verw

rap

5.95

0.

62

0 B

40

/30

3.5

E,C

10

-0

6 10

2

Ove

rwra

p -6

.4

6.26

0.

41

0 B

50

/50

3.5

E,C

10

-0

6 14

.5

6.5

Ove

rwra

p -6

.4

6.18

0.

70

()

B

40/3

0 3.

5 E

,C

10-0

6

17

10

Ove

rwra

p 6.

52

6.52

0.

31

0 B

60

/20

3.5

E,C

10

-0

6 19

12

O

verw

rap

6.52

6.

52

0.45

0

B

40/3

0 3

E,C

10

-0

6 12

3

VP

6.

58

0.49

0

C

1001

-E

,R

1-5

4 40

2

4+

V

P (

8°C

) ?

4 0

40/3

0 3

E,C

10

--3

5 13

.5

4?

Air

pac

k 0

E

70

/-0.

25

E,R

13

4

Air

pac

k 3

F F

illc

ts:

5 po

und

pack

60

/10

E,R

1-

4 4

12

3 A

ir p

ack

? G

P

acif

ic g

rey

cod

(Ga

du

s m

acr

oce

ph

alu

s)

3 ri

llets

40

/10

? E

,R

10-0

6

11

7 A

ir p

ack

6.4

0.4

3 H

H

addo

ck (

Ga

du

s ae

glef

inus

) F

illc

ts

40/3

0 3.

5 E

,C

10-0

6

13.5

6.

5 O

verw

rap

6.27

6.

27

0.51

0

B

60/2

0 3.

5 E

,C

10-0

6

11

4 O

verw

rap

6.27

6.

27

0.54

0

B

Whi

ting

(M

erlu

cciu

s bi

line

aris

) F

illc

ts

100/

-E

,R

1-5

4 15

5

VP

(8°

C)

4 0

Pla

ice

(Ple

uron

ecte

s pl

ates

sa)

Fil

lets

40

/30

3.5

E,C

10

-0

6 7

0 O

verw

rap

6.66

6.

50

0.43

0

B

60/2

0 3.

5 E

,C

10-0

6

7 0

Ove

rwra

p 6.

66

6.36

0.

35

0 B

H

ake

Fil

lets

40

/30

3 E

,C

10-0

6

IOV

, 5.

5 V

P a

nd a

ir p

ack

1 6

0/-

3 E

,C

10-0

6

8 -3

V

P a

nd a

ir p

ack

1 S

teak

s 50

105

E,C

10

-0

5 10

'/,

3.5

Air

pac

k 2

Ste

aks:

pol

y-ph

osph

ated

SO

lOS

E,C

10

-0

5 9Y

, 2.

5 A

ir p

ack

2

Sn

app

er (

Chr

ysop

hrys

aur

atus

) F

ille

ts

(sev

eral

) 10

0/-

E,C

8-

1 4

8 0

VP

3 K

o

r 4

Air

pac

k 3

K

100/

-5

H,C

15

G-O

75

18

9

Air

pac

k -6

.5

-6.4

(su

rfac

e )

-0.1

3

-I

L 4

0/-

5 H

,C

15G

-O

75

9 -2

A

ir p

ack

-6.5

-6

.4 (

surf

ace)

-0

.13

-I

L

(b)

Fat

ty f

ish

Tu

na

(Th

un

nu

s al

alun

ga)

Ste

aks

40/6

0 N

on-E

, IG

-O

4 19

9

Air

pac

k 5.

8 6.

6 (h

omo-

1.1

2 M

co

lour

ge

nate

s)

Her

ring

(C

lupe

a ha

reng

us)

Fil

lets

40

/30

3 E

,C

5--D

3

8 -5

V

P 6.

36

0.31

0

C

40

/-1

E,R

&C

5-

-D

2 8

2+

V

P 2

N

Mac

kere

l (S

<'o

mbe

r sc

om

bru

s)

Fil

lets

40

/30

3.5

E,C

10

--2

6 5

1.5

Ove

rwra

p 6.

41

0.48

0

C

60

/-3.

5 E

,C

10--

2 6

6.5

3 O

verw

rap

6.33

0.

38

0 C

S

alm

on (

Salm

o sa

lar)

S

teak

s 6

0/-

3 E

,C

9-1

6 13

I

VP

6.34

6.

26

0.06

0

0 6

0/-

-I

E,C

D

cscr

ipti

ve

15

+

0 V

P 6.

2 0.

3 1.

8 P

100/

-0.

66

H,C

9-

1 20

10

O

verw

rap

6.19

6.

19

0.06

2

0

(c)

Fre

shw

ater

fis

h C

atfi

sh

Who

le

100/

-?,

C

1-5

3 10

2

VP

3.3

R

( dre

ssed

) C

atla

(C

atl"

cat

la)

Fil

lets

50

/50

H,C

10

--1

5 20

8

Air

pac

k o

to 4

S

80/2

0 H

,C

10--1

5

28

16

Air

pac

k o

to 4

S

Str

iped

red

bas

s (h

ybri

d M

oron

e sa

xiti

lis

X M

. ch

ryso

ps)

Fil1

et s

trip

s 60

/60

? E

,C

9-1

5 7

+

3+

A

ir p

ack

2 T

T

ilap

ia (

hybr

id T

ilap

ia a

urea

X T

. ni

loti

ca)

Fil

lets

7

5/-

? N

on

-E,R

nl

r O

ff o

do

ur

27

18

+

Air

pac

k 6.

22

6.00

4

U

50

/-N

on

-E,R

n/

r O

ff o

do

ur

20

11+

Air

pac

k 6.

22

4 U

Tab

le 9

.2 C

onti

nued

Sen

sory

ass

essm

ent

Init

ial

gas

Gas

-to

-S

helf

-lif

e L

owes

t M

axim

um

Sto

rage

S

peci

es a

nd

m

ix (

%)

pra

du

ct

Typ

e"

Sca

leb

End

-poi

nt

She

lf l

ife

exte

nsio

n R

efer

ence

In

itia

l M

AP

L

lpH

' te

mpe

r-p

rad

uct

typ

e C

O/0

2 ra

tio

cr

iter

ion

(day

s)

(day

s)

mat

eria

l p

H

pH

at

ure

('C

) R

efer

ence

Tra

ut

(Sal

mo

gair

dner

i, n

ow r

edes

igna

ted

as O

ncor

hync

hus

myk

iss)

W

hole

60

/-2

E,C

9-

4 6

8 -I

VP

6.

55

6.62

0.

01

0 0

(gut

ted)

10

0/-

E,C

9-

1 6

11

4 V

P

I V

40

/30

2 E

,C

9-4

5 4

3,10

O

verw

rap/

VP

0

0 F

ille

ts

80

/-H

,C

5-1

2 2

5+

13

+

Ove

rwra

p ?

2 W

60

/-3.

5 E

,C

10--2

7

,5Y

, 10

.5,2

1 4.

5, 1

3 O

verw

rap

2 B

40

/30

3.5

E,C

10

--2

7,5'

1,

8,1

7.5

2

,9.5

O

verw

rap

? ?

2 B

4

fill

ets

100/

-E

,R&

C

5-1

2 14

7

VP

?

Lit

tle

4 X

ch

ange

(d)

She

llfi

sh

Esc

allo

ps (

Pec

ten

ma

xim

us )

S

huck

ed

40/3

0 3

E,C

5-

D

3 7

3 A

ir p

ack

0 Y

m

eats

80

/20

H,C

n/

r S

our

tast

e 11

4

Air

pac

k 4

Z

Cra

b (

Can

cer

pagu

rus)

C

law

mea

ts

40/3

0 3

E,C

5-

1 3

10

4 A

ir p

ack

0 Y

S

cam

pi (

Nep

hrop

s no

rveg

icus

) S

hell

-on

tail

s 40

/30

3 E

,C

5-D

3

4.5

Air

pac

k 0

Y

Shr

imp

(Pan

dalu

s bo

real

is)

Who

le,

100/

--3

to

4 E

,C

5-D

2.

5 18

7/

9 V

P/a

ir p

ack

7.69

0.

2 0

a co

ok

ed

100/

--3

to

4

E,C

5-

D

2.5

11+

7+

17+

V

P/a

ir p

ack

7.69

3-

5 a

60/2

0 -3

to

4 E

,C

5-D

2.

5 14

3/

3 V

P/a

ir p

ack

7.69

3-

5 a

60

/--I

E,C

1-

7 21

14

Jc

ed s

hrim

p 3

b

(e)

Oth

er p

rodu

cts

Sm

ok

ed c

od (

G.

mo

rhu

a)

Col

d-sm

oked

40

/30

3.5

E,C

1O

-ü

6 20

11

O

verw

rap

6.45

0.

48

0 B

fi

llet

s 60

/20

3.5

E,C

1O

-ü

6 20

11

O

verw

rap

6.47

1.

04

0 B

S

mo

ked

blu

e co

d (P

arap

erci

s co

lias

) H

ot-

smo

ked

10

0/-

4 (e

stim

ate)

H

,C

9-1

5 49

35

/35

Ove

rwra

plV

P

3 fi

llet

s 10

0/-

4 (e

stim

ate)

H

/C

9-1

5 49

85

/77

Ove

rwra

p/V

P

-1.5

c

Sm

oked

mac

kere

l (5

. sc

ombr

us)

Hot

-sm

oked

40

/30

3.5

E,C

10

--0

6 19

2

Ove

rwra

p fi

IIet

s 60

1-3.

5 E

,C

10--0

6

22

5 O

verw

rap

Sm

oked

sal

mon

(5.

sal

ar)

Col

d-sm

oked

60

1-3

H d

escr

ipti

ve;

25

0 V

P

fille

ts:

slic

ed

'una

ccep

tabl

e' b

ased

14

-1

1

VP

on

dis

colo

rati

on

Sm

oked

tro

ut (

0.

myk

iss)

C

old-

smok

ed

40/3

0 3.

5 E

,C

10--2

6

14

3 O

verw

rap

fille

ts:

slic

ed

60/2

0 3.

5 E

,C

10--2

6

15

4 O

verw

rap

Raw

fis

h fo

r sa

shim

i T

rout

(0

. 40

1-?,

R

5-1

<4

4

2 S

tret

chw

rap

myk

iss)

fi

llet

pie

ces

Yel

low

tail

40

1-?,

R

5-1

<4

4

3 S

tret

chw

rap

fille

t pi

eces

M

arin

ated

raw

fis

h T

rout

(0

. 40

1-0.

5 E

,C

5-D

2

21

4 L

eaki

ng

myk

iss)

(=

air

pac

k)

fill

ets

Not

es:

HE o

r H

: ex

pert

or

hedo

nic

pane

ls;

C o

r R

: as

sess

men

ts o

n co

oked

or

raw

pro

duct

s.

bSen

sory

ran

ge:

scor

e fo

r fr

eshe

st o

r m

ost

acce

ptab

le p

rodu

ct t

o sc

ore

for

stal

e o

r le

ast

acce

ptab

le p

rodu

ct.

'llp

H:

max

imum

rec

orde

d di

ffer

ence

in

pH

mea

sure

men

ts:

refe

renc

e sa

mp\

e -

test

sam

pie.

6.32

0.

22

0 B

6.

22

0.40

0

B

6.17

6.

09

0.01

0

0 0

6.49

0.

24

0 B

0

B

d e

5.3

Ref

eren

ces:

A,

Jens

en e

l al

. (1

980)

; B

, T

iffn

ey a

nd M

ills

(198

2);

C,

Can

n el

al.

(198

3);

0,

Pos

t el

al.

(198

5);

E,

Ein

arss

on (

1992

); F

, S

iver

tsvi

k (1

995)

; G

, W

oyew

oda

elal

. (1

984)

; H

, S

tras

dine

el

al.

(198

2);

I, L

ampr

echt

el

al.

(198

4);

J, A

lvar

ez e

l al

. (1

996)

; K

, S

cott

el

al.

(198

4);

L.

Sco

tt e

t al

. (1

986)

; M

, L

6pez

-Gäl

vez

el a

l. (1

995)

; N

, R

ande

Il a

nd A

hven

aine

n (1

994)

; 0

, C

ann

el a

l. (1

984)

; P

, R

osne

s el

al.

(199

5);

Q,

Pas

tori

za e

t al.

(199

5);

R,

Bie

de e

t al

. (1

98

1/2

); S

, G

opal

et

al.

(199

0);

T,

Han

dum

rong

kul

and

Sil

va (

1994

); U

, R

eddy

el a

l. (1

994)

; V

, P

artm

ann

(198

1);

W,

Bar

net

t et a

l. (1

987)

; X

, B

aldr

atti

et a

l. (1

990)

; Y

, C

ann

el a

l. (1

985)

; Z

, S

trou

d an

d F

inbo

w (

1990

); a

, S

trou

d et

al.

(198

1);

b, S

iver

tsvi

k el

al.

(199

5);

c,

Pen

ney

el a

l. (1

994)

; d,

Yas

uda

el a

l. (1

992)

; e,

Yas

uda

et a

l. (1

989)

; f,

Ran

deIl

et

al.

(199

5).

n/r

: no

t re

port

ed.


8.0

7.8

I vp aCe c.. 7.6 (J) 0 ro MAP ace 't: ::J (/)

(J) 7.4 C> ro .... (J)

~ 7.2

7.0

2 4 6 8 10 12 14 Storage time (days)

Figure 9.1 Surface pH cf cod fillets in vacuum packs (VP) and MAP (40% CO2).

Stenstrom (1985) showed that inhibition of microbial growth on cod in MAP increased with the proportion of CO2 . Inclusion of 10-50% O2

reduced the overall inhibition slightly but appeared to play an important part in determining the dominant microflora. Lactobacillus spp. eventually formed 80% of the microflora under 100% CO2 , increasing even further when 10% of the CO2 was replaced by 02. In contrast, with N2 instead of 02, the proportion of Lactobacillus fell to 62%. While very high levels of CO2 are more effective against spoilage organisms, they are not usually acceptable for fish products, for other reasons discussed below. Although the proportion of Lactobacillus spp. fell considerably when only 50% CO2

was used, they were still the largest group provided that O2 was included as the diluent gas. Others have also reported significant proportions of Lactobacillus spp. under high concentrations of CO2 (Banks et al., 1980; Finne, 1981; Molin et al., 1983; Layrisse and Matches, 1984; Wang and Ogrydziak, 1986; Gopal et al., 1990; Weber and Laux 1992) but Oberlender et al. (1983) and Lannelongue et al. (1982b) observed some contradictory influence of O2 in this regard. In the last study, this may have


been influenced considerably by the use of poor-quality fish when first packed, but another important source of variation was the use of sealed MAP rather than the CAP experiments in wh ich gas mixtures are replenished.

Wilhelm (1982) reported that growth of Lactobacillus spp. may cause souring, and Kim et al. (1995) reported an adverse response by trained panelists to fish treated with a culture of lactic acid bacteria and/or immersion in a 3% solution of lactic acid. Ingham (1989), however, found that after dipping fillets in solutions of lactic acid of up to 2.5% for 10 minutes, untrained panelists could not detect any significant differences from control sampies. Williams et al. (1995) obtained an increased shelf life for catfish fillets treated with sodium lactate, and where the lactate itselfwas detected, most panelists reported favourably. Dominance of the bacterial flora by lactic acid bacteria is preferred because of their less dramatic effect on sensory properties. They might also be particularly effective competitors through the release of anti-microbial products that are active against foodborne pathogens (Amerio et al., 1995).

After the initial period of absorption of CO2 , and consequent increased tension of diluent gases, O 2 is a rapidly declining component of MAP gas mixtures and, whatever the initial effects, residual influences are likely to be highly variable. Where studies have attempted to mimic retail production packs, the main effect has been significant slowing of the growth of spoilage organisms, but with the composition of the microflora little different from air pack or vacuum pack controls (Strasdine et al., 1982; Cann et al., 1983; 1984; 1985; Lamprecht et al., 1984). Prawns (Matches, 1982) and shucked meats of escallops (Cann et al., 1985) were significant exceptions; the prawns had a largely Gram-positive initial flora that changed to become composed predominantly of Lactobacillus spp. in both MAP and control packs, and bivalve molluscs such as escallops often have relatively high glycogen contents with the potential for lower ultimate pH. Packing escallops in CO2/02/N2 (40/30/30) led to maintenance of low pH (~6.3) for a longer period than in the controls. Total viable counts on those sampies packed in MAP or in air were not significantly different, but lactic acid bacteria were seen to increase more in MAP than in air packs.

9.4.4 Effects of MAP on microbiological hazards

Historically, fish has proved to be a generally safe food (Liston, 1982), but inhibition of competing psychrotrophic spoilage bacteria presents the danger that some products might be more vulnerable to growth of food poisoning organisms when packed under CO2 than is otherwise the case. The primary concern with fish products in sealed packs is the anaerobe C. botulinum, particularly toxin type E strains, for which fish meat is an excellent, if variable, growth medium (Eklund, 1982a). Since these strains


are non-proteolytic and able to grow at temperatures down to 3.3°C, inhibition of spoilers increases the risk of products becoming toxic before normal signs of decomposition can provide a reasonable safeguard. Such concerns led the US National Academy of Sciences to recommend that until the safety of the system was established fish should not be packed in MAP (LaBelI, 1986). While there is no nationwide ban on MAP or vacuum packaging of seafoods in the USA by the Food and Drug Administration, they are rigidly controlled when inspected and certified by the US Department of Commerce Voluntary Seafood Inspection Program. All US commercial processors and packagers of seafood are subject to HACCPbased regulation beginning 18 December 1997 (Anon. , 1995).

Much of the discussion on risks of botulism from fish products appears to ignore the evidence that botulinogenic conditions are not created only by depletion of O2 in sealed packages. Clostridia are less sensitive to O2 than other 'strict' anaerobes (Jones, 1989), and conditions for growth of C. botulinum occur only a few millimetres below the surface of fish flesh (Eklund 1982a), i.e. where spores could occur via the gaping that often appears between muscle blocks, and through knife cuts and other punctures. Several reports have found that fish need not be vacuum packaged for botulism to develop, and there can be a reduction in lag phase where the organism is present on fish surfaces.

Though not an entirely satisfactory criterion, the question that has been repeatedly addressed is whether or not packaged products that have suffered temperature abuse do, in practice, become toxie before rejeetion on sensory grounds. This is particularly important with products such as smoked salmon and hot-smoked fish that are not normally cooked prior to eating and may be packed under CO2 with further consequent inhibition of spoilage indicators. Challenge testing with inoculated sampies has produced mixed results but, clearly, there is a potential threat that cannot be dispelled by inclusion of O2 in the gas mixture (Eklund, 1982a). Much of the variability in the results from challenge tests has been attributed (Statham, 1984; Stammen et al., 1990) to variability of the raw material and to methodology, but one notable cause of variation appears to be the freshness of fish at packing. Unfortunately, in view of the importance placed on the use of fresh fish to alm ost every other aspect of MAP technology, Eklund (1982a) found that sensory rejection of MAP salmon occurred before toxicity only if the fish had been in ice for several days before packing. When fish were packed within 18 hours of catching, bacterial inhibition began before spoilage organisms reached significant proportions. Consequently, where sam pies suffered temperature abuse, overt spoilage characteristics were delayed beyond toxigenesis. Eklund (1982a,b) also detected some inhibition of C. botulinum both with salmon stored at 2°C under 60% and 90% CO2 , and in sampies that had been removed after nine days and stored, in air, at 10°C for another seven days.


Such effects give additional credence to the system recommended by Lindsay et al. (1985) that permeable films should be used for consumer packages so that, after removal from MAP master cartons, aerobic microbial spoilage would become evident if there was consumer abuse of the product.

The risks from botulism in MAP fish have been widely reviewed (Eyles and Warth, 1981; Statham, 1984; Genigeorgis, 1985; Hintlian and Hotchkiss, 1986; Baker and Genigeorgis, 1990; Stammen et al., 1990; Reddy et al., 1992). Although little MAP fish is sold through retailers in the USA because of the potential hazards, in Australia and much of Europe the risks are generally considered to be remote (Eyles and Warth, 1981; G. Hobbs, unpublished report 1989). Unfortunately, processors face continuous pressure from some parts of the market for excessively long shelf-life markings (mainly affecting vacuum-packaged smoked salmon). At present, there are no specific restrictions in the UK regarding fish products in sealed packages but the potential risks are recognized. Guidance provided by various advisory bodies, together with general statutory requirements for food hygiene, has served progressively to tighten recommendations to processors and retailers. For many years the concern lay almost entirely with vacuum-packed, smoke-cured salmonid products that are eaten cold. These and similar products have been assigned a high priority for attention by UK industry and enforcement agencies (Anon., 1994c). Control of the hazards requires products to be kept chilIed and to be given additional protection against modest temperature excursions by manipulation of the water activity with salt. Minimum concentrations of salt (i.e. in the water phase) of 3.5% (Anon., 1979; 1991a) and 5% (Anon., 1969) have been recommended.

The growing market for all vacuum-packaged and MAP products has renewed the fears of a botulism hazard in stored foods, and recent advisory committee reports have introduced stricter recommendations (Anon. , 1992; 1994c). While acknowledging that there was no evidence of illness caused by these products, the possibility could not be ignored. For vacuum-packaged and MAP foods that have chilled storage as the only controlling factor, storage at temperatures between 5 and 10°C should be limited to five days; shelf lives of up to 10 days could be assigned if the storage temperature is to be at, or below, 5°C (Anon., 1994c). Whatever such advice suggests, the onus remains with the processor to ensure that products are safe, and an appropriate series of challenge tests is advisable if a shelf life of more than five days is to be assigned.

The debate on containment of potential hazards continues with the European ChilIed Foods Association pressing for a maximum of five days' storage to be allowed for all vacuum-packaged and MAP foods, while UK industry wishes to retain a 10 day limit. A major factor that will play some part in determining the extent of any restrictions is the prevailing industrial


culture. While there exists a wide range of manufacturing and distribution practice in the UK, some sections of the MAP industry have raised their standards of hygiene and temperature control through all stages of production, storage, distribution and retail sale. One major retailer has reported an improvement in average temperatures of display cabinets from 6Se in 1988 to 3.7°e in 1991 (Anon., 1994c). Much of the greater fear in the USA arises from a history of many more deaths from botulism. Although only a sm all proportion of these incidents has been caused by consumption of fish products, it is inevitable that regulatory bodies will be adversely influenced by re ports of average cabinet temperatures varying between 7°e and lOoe (Rose and Hunt, 1992).

Studies on other pathogens have shown several to be capable of growth on packaged fish meats (Anon., 1991b) but, under MAP conditions, coliforms (Reddy et al., 1994), Listeria monocytogenes, Salmonella spp., Aeromonas spp. and Yersinia enterocolitica (Huang et al., 1992; Slade and Davies, 1995) appear to be inhibited or no more successful than under aerobic conditions. Some doubt remains about Campylobacter spp., a group that is both microaerophilic and capnophilic, requiring -10% e02

for optimal growth (Fain, 1986; Farber, 1991). Relatively small numbers are needed to cause illness, but they are difficult to grow in laboratory culture and their ability to grow on MAP fish has yet to be established. The observation by Taylor (1988) that some Lactobacillus spp. can decarboxylate histidine suggests that poor temperature control could lead to accelerated development of histamine, and, perhaps, a greater risk of scombrotoxicity in susceptible species. MAP appears more likely, however, to be inhibitory. Suzuki et al. (1990) found growth of a marine (but not a terrestrial) strain of a polyamine-producing Alteromonas sp. to be inhibited by eo2 , and rates of production of histamine and other potentially toxic amines were found to be little different, or slower, in MAP (Watts and Brown, 1982; eann et al., 1983; Oka, et al., 1993).

9.4.5 Sensory properties

Acceptance by consumers requires the appearance of a food product that inspires confidence that it will prove satisfactory when purchased and eaten. Therefore, the important sensory properties fall into two categories: physical effects (of both the package and the fish product) that can be assessed visually before purchase, including pack collapse, discoloration and production of exudate; and the organoleptic properties of odour, flavour and texture.

Pack collapse. Pack collapse occurs as e02 is absorbed by fish products and the internal pressure of MAP falls. There is a consequent reduction in volume both of pillow packs and the semi-rigid rectangular packs that are


commonly used with fish products. In extreme examples, side walls may buckle, causing serious distortion, and this is one reason for the proportion of CO2 in the initial gas mix to be reduced (Jensen et al., 1980; Mills and Tiffney, 1982). Even so, some contraction still occurs; the top forms a dished, concave surface, wh ich is considered detrimental to the overall visual appeal if the film makes contract with the product. Increasing the depth of the pack has considerable economic implications, and so me producers prefer to inject a small excess of gas. Initially, such packs have a convex surface that makes them temporarily unstable if stacked without support.

Exudate. Often, another consequence of treating fish with CO2 is a greater production of exudate, or drip. Normally, the small amount released from raw fillets is not a great problem, but it becomes a limiting factor for some products in MAP. The problem may be contained by limiting the amount of CO2 and by placing the fish on absorbent pads within the packs (Tiffney and Mills, 1982; Cann, 1984). The lower water contents of smoked products and fatty fish leaves them less vulnerable to losses when stored under 60% CO2 than is raw white fish, for which Tiffney and Mills (1982) had to reduce the initial level of CO2 to 40%. Some success in further reducing drip by inclusion of O2 was implied, but a re-examination of the raw data found no support for that conclusion (H. Davis, unpublished). In most reports detailing release of drip and pack collapse, the problems are aggravated at the lower storage temperatures, a reflection, perhaps, of the effect on solubility of CO2 . Nevertheless, there are unexplained variations (Tiffney and Mills, 1982; Cann et al., 1983, 1984, 1985).

Discoloration. Discoloration of fish in MAP can occur via a bleaching action of cut surfaces (Cann, 1984), probably caused by low pH precipitation of sarcoplasmic pro teins (Statham and Bremner, 1989). On whole fish, eyes become opaque and skin pigments fade (Coyne, 1933; Stansby and Griffiths, 1935). There is also risk of disoloration of haem pigments (Przybylski et al., 1989), but a green discoloration ofred snapper was slightly delayed, though not entirely prevented, by storage under 50% CO2 (Gerdes and Valdez, 1991). Less important as a visual feature in white fish, which has much less haem pigment than in meat, such discoloration is not usually a problem, but Basauri and Davis (1984) observed a brown discoloration in fish minces in MAP that was linked to elevated levels of 02. Conversely, the colour of intact muscle from tuna, and inhibition of metmyoglobin formation, were better when 60% 02, rather than 60% air, was included in CO2 MAP packs (L6pez-Gälvez et al., 1995). Brown et al. (1980) demonstrated that inclusion of 1 % carbon monoxide in the gas mix could help retain a red blood colour, unlike Fey and Regenstein (1982) who


observed a negative effect. Inclusion of O2 as a means of maintaining the more attractive colour of oxyhaemoglobin is commonly practised with red meats, but high concentrations (~80%) are necessary. Oka (1989) found that packing fish under N2 or CO2 minimized the oxidation of haem pigments to metmyoglobin, and Tiffney and Mills (1982) found that the fresh appearance of fatty fish was retained for longer in 02-free packs. Weber and Laux (1992) found that the gut incision and abdominal cavity of CO2-packed trout suffered discoloration that increased with the amount of O2 included in the gas mixture.

Amongst the beneficial effects claimed for noble gases is inhibition of oxidative deterioration both of haem and carotenoid pigments in fish (Spencer and Rojak, 1993). Astaxanthin and canthaxanthin are important carotenoid pigments of crustaceans and salmonids and are susceptible to oxidation. Fading can occur in frozen-stored material but, apart from an imprecise comment by Fey and Regenstein (1982) that might have been referring to blood, none of the reports on 'fresh' salmon have referred to problems with carotenoid pigments. Longard and Regier (1974) reported that inclusion of CO2 in refrigerated sea water actually enhanced retention of carotenoid skin colours of ocean perch. Problems can arise when smoke curing is combined with MAP. Cann (1984) found that sliced smoked salmon packed in 60% CO2 developed a bleached and/or green-brown discoloration, which preceded other acceptability limits and reduced the shelf life relative to vacuum-packaged controls (Table 9.2). A similar effect can, however, occur also in vacuum package(s).

Organoleptic responses. Organoleptic responses to fish stored under very high concentrations of CO2 are not all favourable. Longer sheif lives are obtained when measured on a basis of microbiological growth and the direct spoilage consequences, but other, adverse, changes occur. Reduction of bound water, which leads to excessive exudate, is accompanied by a coarsening of the texture, described by taste panelists as 'slight increases in toughness and dryness' (Tiffney and Mills, 1982), 'grainy' (Wang and Brown, 1983) and 'powdery' (Haard and Lee, 1982). Conversely, Weber and Laux (1992), observing only slight increases in weight lass from trout in MAP reported a loss of firmness as the most significant disadvantge of high concentrations of CO2.

When packs of fish are first opened, odours that otherwise would have slowly dissipated are suddenly released. They are generally inoffensive if the product has been properly stored but less so if products have been abused (Cohen, 1981). With packs handled correctly, there are subtle changes in odour, wh ich have been analysed by Lindsay et al. (1987). Acidic (Stier et al., 1981; Tomiins et al., 1981) or effervescent sensations (Tiffney and Mills, 1982) may be detected. Effervescence can be detected in the taste of products such as crabmeat (Cann, 1984), which are not


usually reheated, but also appears sometimes in the flavour of products that are cooked after removal from MAP (Haard and Lee, 1982; Jensen et al., 1982; Stroud and Finbow, 1990). Tiffney and Mills (1982) reported another flavour effect associated with cod under high levels of CO2 •

Described by panelists as 'cold store flavour', it is very like the taste that develops in lean fish during frozen storage at high temperatures.

The shifts in overall patterns of chemical change, which MAP gas mixtures must induce if they are to be effective, appear not to cause expert panelists any significant additional problems. Published results make little mention of unusual effects other than rancidity in fatty fish, which led to a shorter shelf life for herring in MAP (when O2 was included) than in vacuum packs. Tiffney and Mills (1982) did not describe the flavour consequences of packing mackerel and trout products in MAP but found that exclusion of O2 extended the times taken to re ach specified flavour scores. Some change in the balance of oxidative rancidity and microbiological spoilage flavours might be expected by packing under enhanced levels of 02, and Davis (1990) showed that, in minced fish, oxidation was, indeed, promoted by the inclusion of 30% 02.

Detailed patterns of change in sensory scores of MAP fish vary. In a comparison of fish packed fresh and not-so-fresh, both had a slightly slower, fairly uniform rate of spoilage over the main edible period (Tiffney and Mills, 1982). For the fresh fish, there followed a long plateau that markedly delayed the onset of gross spoilage (Figure 9.2); this contrasts with the inhibition seen in the earlier stages by Cann et al. (1983).

10

9

8 CI> Ci 7 u !J)

:s 6 0 > R!

5 :;: "Cl

CI> :.:. 4 0 0

0 3

2

o 5 10 15 20 25

Storage time (days)

Figure 9.2 The effect of initial freshness and use of a 40% COi30% Ni30% O2 gas mix on the storage life of cod fiBets. (From Tiffney and MiIIs, 1982.)


Table 9.2 summarizes the shelf lives and extensions reported for those fish and shellfish products in chilled MAP that have been examined by sensory techniques. They reveal the great variability that is to be expected from the differences within and between species, batches, treatments, experimental methods, attributes evaluated and, especiaIly, the end-point criteria employed. A major source of shelf-life variation between batches is the history of the fish between catching and packing, and there is broad agreement that the use of MAP is only worthwhile when applied to fish that is fresh. An exception was the suggestion by Regenstein and Regenstein (1981) that treatment with CO2 and/or sorbate may be more effective once spoilage organisms have reached the early logarithmic growth phase. Aside from some discussion of the relevance to inhibition of botulism (Lindsay et al., 1987), this 'delayed-pack hypothesis' has gained little credence. Nevertheless, some authors have acknowledged the use of relatively stale fish in their storage experiments, though many have not known, or have failed to define, the initial quality of their raw material.

9.4.6 Reference materials

Another important feature to note is the choice of reference material. A fundamental principle of objective study is to compare 'like with like'. Many studies of fish products in MAP appear to have confused the application of scientific probing of the effects of MAP gases on microorganisms and food products with the technological examination of practical alternative products. The choice of alternatives to gas-filled MAP must come from the range already in use, such as vacuum or overwrap packs, or others developing at about the same time, such as vacuum skin packaging. Although some products are packed with air simply to exploit the visual appeal of sealed MAP packs, in most instances an air-injected pack is an inappropriate alternative product; apart from bearing the extra costs of packaging, distribution and storage, an accelerated spoilage rate may occur. This is to be expected because, as weIl as exposing a high surface area of tissue to atmospheric O2 without the beneficial influence of CO2 , the fish is insulated from the cooling medium outside the pack. Consequently, comparisons with air packs risk exaggerating the benefits of MAP. They are, however, frequently used as reference material, and Regenstein (1986), critical of much of the methodology employed in such studies, was rightly sceptical of some of the claims made for extended shelf lives of MAP products. Nevertheless, there is a corollary to be noted. In cases where packing under gas mixtures containing CO2 appears to have produced no extension of storage life compared with vacuum or overwrap packed fish, the gas mix has at least succeeded in counteracting potential for accelerated spoilage and permitted the use of an attractive marketing device.


9.4.7 Ellects 01 temperature rises on MAP products

The need for maintenance of low temperatures, a common cause for all fish technologists see king improvements in handling and distribution, becomes, if possible, of even greater importance with fish in MAP. If the considerable extra costs of gas packaging are to be justified, the loss of benefit that occurs when storage temperatures rise by just a few degrees has to be minimized. Davis (1990), calculating the relative rates of spoilage of cod between 0 and lOoC, found both MAP and vacuum packaging to fit remarkably dose to values derived for other fish products (Table 9.3), but the data referred to are appropriate only for constant storage temperatures. Therefore, although temperature appears to have the same importance for MAP of fish as for other fish products, the insulating properties of the pack will delay the effects of extern al cooling media and make the consequences of temperature excursions so much worse.

9.4.8 MAP and chemical indices ollish spoilage

Changes in the amounts of volatile amines and hypoxanthine in stored packs have been interpreted in different ways. Although production of volatile nitrogen compounds has been seen to be inhibited to a greater extent than microbial growth (Brown et al., 1980; Weber and Laux 1992), others have suggested that volatile bases may still be useful as spoilage indices (Parkin et al., 1981; Lindsay et al., 1987). At first glance, the production curves might appear to be roughly the inverse of those showing reductions in flavour score, but there is an extended lag phase at

Table 9.3 Rates of change in cod during storage at 10 and 5°C relative to that at O°C

Estimated storage time Relative rates ( da ys to reach cri terion) (rate at O°C = I)

Pack Criterion type O°C SOC 10°C SOC 10°C

O 2 decrease of 100 .ul g-I fish VP MAP 14.3 5.0 3.1 2.86 4.61

Flavour score VP 9.0 4.0 2.3 2.25 3.91 = 6.0 MAP 12.3 5.5 3.1 2.24 3.96 TMA concentration VP 7.3 2.6 1.7 2.81 4.29 = 10 mg IOOg-1 fish MAP 10.8 3.0 1.8 3.60 6.00 Inosine concentration VP 9.0 3.5 1.9 2.57 4.74 = 2 .uM g-I fish MAP 13.3 3.9 2.3 3.41 5.78 Hypoxanthine concentration VP 8.3 5.0 2.3 1.86 4.04 = 4 .uM g-I fish MAP 13.0 3.8 2.6 3.42 5.00 From Storey (1986) Relative rate of spoilage =

(O.IT+ 1)2 2.25 4.00

From Davis (1990). VP, vacuum packaging.


temperatures close to ODC that is not seen at higher temperatures. Davis (1990) showed that for the main component (TMA), the relative rate was appreciably higher for fish in MAP (Table 9.3). Consequently, the relationships between TMA and flavour scores deviated significantly from each other. For most values of TMA, scores for MAP fish stored at low temperature were higher than for vacuum-packaged fish.

The prolonged delay in production of TMA at ODC might be expected to reflect an extended acclimation phase of TMAO-reducing organisms as a direct consequence of storage under CO2 . Parkin and Brown (1983) and Davis (1990) suggested that there mayaiso be an indirect effect resulting from a decline in the activity of bacterial TMAO reductase as the pH falls to -6 (Easter, 1982). Inclusion of O2 might also playa significant part, especially with the initial increase in O2 tension as CO2 dissolves. Oxygen exerts an inhibitory effect on TMAO reductase activity (Easter, 1982) and on the demethylation ofTMAO to dimethylamine (Lundstrom et al., 1982).

In contrast to the supposition made by Regenstein (1982) that MAP treatments have no effect on the non-microbial biochemistry of fish tissues, several authors have shown that MAP may delay the reate of nucleotide degradation (Lindsay et al., 1987; Yasuda et al., 1989; 1992; Davis, 1990; Handumrongkul and Silva, 1994; L6pez-Galvez et al., 1995) (Figure 9.3). Again, Davis (1990) found both the temperature function and the relationshiup between hypoxanthine and flavour score to be affected in much the same way as production of TMA. In this case, however, the inhibition was attributed entirely to the influence of CO2 on tissue pH because of the disproportionate effect that temperature can have on IMP degradation in MAP, and the greater persistence of IMP in fish of intrinsically low pH. If valid, there is particular significance in this thesis because of the rapidity of the early stages of A TP degradation. Only very fresh fish will retain enough IMP on which CO2 can exert such an influence and benefit from the synergistic effects mentioned earlier. Further , following the law of mass action, a reduced effect of a given mass of CO2

on the pH of fish of lower intrinsic pH may account for some of the variation in the effects of MAP. Conversely, fish of low pH are more likeh to retain IMP through commercial operations prior to packaging.

Brown et al. (1980) and Lanier and Korhonen (1981) observed only slight increases in the thiobarbituric acid reactant measure of lipid oxidation in MAP fish. More substantial increases were reported by Davis (1990) under the more aggressive conditions of minced fish in MAP, but there is !ittle indication of their relationship with sensory properties.

9.4.9 Fish products in bulk MAP

Developments in the use of MAP for individual products for retail sale has been accompanied by adesire to extend the process to bulk systems for


80

70 .--------.. "

Ii) Q) 60 c .;::

0 :::J a.

\ '+- 50 \ 0 \ ";:R ~

\ \

a.. 40 \

\ :::2: ß \

\ "C \ C .. C1l 30 Q) , c ,

'00 \ VP 0

20 ,

c , , , • " 10 " .... .... ...... 0

0 2 4 6 8 10 12 14 Storage time (days)

Figure 9.3 Retention of INO (e) and IMP (.) in MAP cod stored at O°c. Open symbols denote situation before packing.

transport and storage of fish in larger amounts. The scope is limited: although a system currently in use in the UK for small whole fish refers to the product as a bulk pack (Guise, 1993), it is necessarily limited to a single layer of fish on larger trays in a wider MAP pack. Success has been claimed (Christie, 1994) in trials with cod fillets but, using what appears to be a poor-quality end-point, the inverse relationship obtained between the weight of fillets per pack (between 5 kg and 20 kg) and extension of storage life, demonstrated the limitations of bulk systems. In the usual sense of the term, bulk packaging is most unlikely to prove beneficial. Apart from the arithmetic, which would require a proportionate increase in the amount of gas to affect the greater mass of fish, large packs of fillets afford inadequate exposure to CO2 (Woyewoda et al., 1984). This difficulty must also affect relatively small packs and, indeed, it is the reason for the dimpled base of most MAP containers. Even for just two fillets, access


for CO2 to overlapping areas will, inevitably, be limited and Lamprecht et al. (1984) found higher bacterial counts on MAP fillets stored in pairs than similar material packed singly. The failure of Boyle et al. (1991) to detect any advantage in terms of nucleotide degradation with whole trout packed very soon after slaughter may weil have been because of a low gas-to-fish ratio and lack of separation between whole fish. There is so me theoretical hope, and some success has been claimed, for the use of packaging film permeability in what has been termed the 'master pack' (Lindsay, 1981) or the 'mother bag' (Sacks and Gore, 1987) concept. Here, CO2 is extern al to the individual packaged units, to attenuate the adverse effects of the gas at high concentrations (Bell, 1982). Application of this concept has now turned full circle, with some individual retail packs employing a permeable membrane between the fish product and an outer barrier film (Anon. 1994b), though the main purpose intended appears to be to hold the product in place when packs are stored vertically.

9.4.10 Residual effects

Increased shelf lives were obtained when three species of fish were stored in a 60% CO2 controlled atmosphere for 7 and 12 days before transfer to conventional iced storage (Ruiz-Capillas et al., 1995). But differences resulting from the two treatments suggest that there was little or no continuing effect onee the fish had been removed from the CO2

atmosphere. Similarly, after removing fish from stored MAP, Banks et al. (1980) and Parkin and Brown (1983) found that as the microbial flora reverted to the aerobic pattern, bacterial growth rates quickly recovered to match those of fish that had not been treated. Wang and Ogrydziak (1986) measured the concentrations of CO2 in fillets exposed to high CO2

concentrations (80%), and found that some batches of fish did show a residual effect, and observed some delay before rates of bacterial growth equalled those of aerobically packed fish. With rapid losses of CO2 from the fillets, the authors concluded that the residual effect was not a direct result of exposure to CO2 but resulted from a modified bacterial flora, and to the influence of increased numbers of Lactobacillus spp. whieh delayed reassertion of the normally dominant Pseudomonas spp.

9.5 Adjuvant treatments

9.5.1 Chemical additives

Carbon dioxide dissolved in water has been examined not only as a pretreatment for fillets (Daniels et al., 1986) but patented as an appropriate


means of pre-treatment and killing of farmed fish prior to packing in MAP (Anon., 1980). Azam et al. (1989) and Eifert et al. (1992) found that relatively brief slaughter treatments using CO2 alone had only a slight effect on muscle pH, and although the levels of IMP at the time of slaughter were higher than in fish killed by a blow to the head, subsequent degradation was more rapid (Azam et al., 1989). With a prolonged (5 hours) anaesthetic exposure using CO2 and 02, Itazawa and Takeda (1982) found that the partial pressure of CO2 in carp blood increased more than 30-fold, with a concomitant fall in pH from 7.9 to less than 6.8. Being a study of fish anaesthetics and not food preservation, further data relevant to MAP were not obtained, but such changes should be sufficient to effect some beneficial post-slaughter influence though not necessarily without some harmful aspects, e.g. texture changes. Other chemical treatments of fish products have sought to bring about various effects: to pre-acidify by dipping products in a solution of carbonic acid (Daniels et al., 1986), acetic acid (Madden and Bolton, 1990) or glucono-ö-lactose plus lactic acid (Baldratti et al., 1990); to slow down bacterial spoilage in vacuum packs (successfully) using an EDTA dip (Pelroy and Seman, 1969) or an EDTAJ chlortetracycline (CTC) combination (Miller and Brown, 1984), though EDTA proved less effective with fatty fish (Varga et al., 1980); to reduce the amount of exudate released in MAP with conventional polyphosphate dips (unsuccessfully Tiffney and Mills, 1982), with qualified success (Alvarez et al., 1996); and to inhibit the outgrowth of Clostridium spp. (successfully) using nisin (Taylor et al., 1990).

Potassium sorbate, though not permitted for use with chilled fish in the UK and !ittle used in the USA, has been studied extensively as an inhibitor of bacterial spoilage and outgrowth of Clostridia, with mixed results. Increases in shelf life have been observed in most studies of sorbates with fish in MAP, but although Barnett et al. (1987) detected some inhibition of microbial growth, there was no corresponding increase in shelf life as judged sensorily. Where such shelf-life extensions have been seen (Licciardello et al., 1984; Sharp et al., 1986), improvements only benefited the later stages. Untreated fish were preferred in the earlier stages, with extensions of shelf-life depending on a prolonging of intermediate quality and, as Regenstein (1982) emphasized, the need for storage at ODC remains. Although Statham and Bremner (1989) found pre-treatments with potassium sorbate to be effective with air and vacuum-packed scallops, it was probably the higher pH of fin fish that led Fey and Regenstein (1982) to conclude that sorbates are only effective in conjunction with storage under CO2 . They also found that the combination significantly increased the amount of drip produced. Lindsay (1982) suspected some link between this and the mixed results obtained when examining the anti-botulinal property of sorbate in fish. Attempts to enhance uptake and retention of sorbate from dip solutions by combining


with sodium tripolyphosphate (STP) led to increased efficacy in delaying toxigenesis. Statham et al. (1985) subsequently found that sorbate uptake was not increased by combination with polyphosphate, and an alternative explanation may be found in the work of Hobbs (1976), who reported that STP itself had an inhibitory effect on germination of type E spores.

9.5.2 Physical treatments

Packing foods in MAP increases their exposure to higher temperatures, after which the surrounding gas presents an additional barrier to cooling. Tiffney and Mills (1982) sought to compensate by pre-cooling fillets to - 3°C but found no consistent benefit and an undesirable increase in drip volume. With the entire product frozen, Partmann (1981) reported that thiobarbituric acid values were lower for trout packed under CO2 than for similar air-packed fish, but there was little difference in sensory scores.

Spoilage of fish products that are cooked before sale is largely dependent upon the degree of recontamination after cooking. Relatively large extensions of shelf life obtained with some cooked shellfish and hotsmoked fish products is the result of an abrupt halt to all autolytic and bacterial deterioration. The flavour profile of wh at is effectively, in some cases, pasteurized products is 'fixed' until active spoilers in any crosscontaminating bacterial flora adapt to the pack atmosphere. Similariy, irradiated pasteurized products need to be enclosed in sealed cans or packaging films to prevent rapid recontamination. Increased storage lives with irradiation doses up to 400 krad have been demonstrated, with the predominant microflora changing (in vacuum packs) from Pseudomonas spp. to Lactobacillus spp. (Pelroy and Eklund, 1966; Licciardello et al., 1967), though the flavour also suffers increasingly, particularly in unfrozen fish (Shewan, 1962). Legal restrictions in the USA and most European countries have limited the commercial applications with food, but there has been some relaxation and more recent research on MAP fish has included irradiation. Licciardello et al. (1984) reported a prolonged shelf life for irradiated MAP cod, but the absence of a non-irradiated, CO2-treated control hinders assessment of any additive effect. Przybylski et al. (1989) did observe some additional inhibition of spoilage organisms in catfish, but this was accompanied by an increase in lipid oxidation, suggesting that the sampies, had they been tasted, may have been rancid. Of much greater importance would be irradiation to sterilize bacterial spores but the very high doses required have unacceptable effects on the palatability of fish (Hobbs, 1976). At the lower radiation doses used for pasteurizing fish products, it is not surprising that Eklund (1982b) should report an increase in the risk of toxigenesis preceding spoilage.


9.6 Conclusion

Vacuum packaging is now a weil established technique for protection and presentation of some frozen fish products. In Europe and some other parts of the world, both vacuum packaging and MAP are similarly weil established for various chi lied fish products. To what extent their success results from inherent advantages or, more simply, from demand for a new, alternative marketing device is difficult to say. The skeletal muscles of fish and other an im als are similar in many ways, but there are differences that cause fish to spoil faster and limit the benefits from storage processes such as MAP. While the desirable characteristics of fresh meat can persist for several weeks if microbiological spoilage is inhibited, autolytic deterioration of fish is much faster, and the sweet flavours characteristic of fresh fish last for less than one week even under good refrigeration. With relatively high muscle pH and no replenishment of the pack atmospheres, most fish products in MAP are unlikely to develop a significant lactic acid bacterial flora. As inhibition of the normal spoilage bacteria of fish is limited, extensions of shelf life are not usually as dramatic as can be achieved with meat, and the need for maintenance of low temperatures throughout processing, storage, distribution and display is not diminished. If the effort that is widely applied to controlling temperatures and turnover during production, distribution and storage of MAP had been applied to earlier packaging formats, MAP of fish using CO2 might have lacked sufficient advantage. The need for such control was, however, recognized. Large parts of the industry responded and cooperation between some processors and retailers has made MAP a success. There have been few re ports of illness from vacuum packaged fish products and none in recent years, nor any at all arising from the more recent development of COz-MAP. Rapid turnover and improvements in control of production and distribution will all have contributed. Earlier, there may have been some risk that the wh oie market for MAP fish might suffer a setback if any single supplier were to have attracted adverse publicity. The market has matured and, though based only on a few personal observations, within any one store the quality of MAP fish appears to be similar to the general standard of fresh produce; poor-quality MAP fish seems now to reflect mainly on the individual retailer.

Being so weil established, and with an extensive background of scientific and technological research on MAP fish, prospects for further worthwhile investigation on the process are li mi ted - especially if marketing trends moved on to make fashionable so me other product. Most aspects of MAP appear to have been examined to some degree and further efforts are likely to suffer diminishing returns. So far, the benefits of packaging fish under CO2 appear to have been the presentation of chilled products that are clean and dry and which, if not quickly sold, provide some safeguard in


that they will hold for a longer period at an 'intermediate' quality; dull rather than positively unpleasant. If, however, some further boost to the technology were to be sought, the aim should be to explore opportunities for better retention of, and association of the product with, excellent quality. The key, here, is to start with very fresh fish and to optimize the retention of those attributes that are attractive to consumers. There are problems, however, affecting the supplies of fresh fish. Difficulties of maintaining reliable supplies leads to continuing interest in the use of frozen fish for packing in MAP despite the mixing of marketing concepts. Trials have been conducted using frozen fish, thawed before packing, but there is little published work other than that of Lanier and Korhonen (1981), who reported some success. Retention of IMP may be an important factor, but studies of the stability of nucleotides to freezing, frozen storage and thawing provide conflicting evidence (Jones, 1963; EI Okki et al., 1988). Nevertheless, there is no doubt that this approach would not improve retention of IMP without the fish being very fresh when frozen (i.e. frozen pre- or immediately post-rigor) and stored for only short periods before use. Pre-rigor freezing of fillets presents the additional problem of needing to balance maximal retention of IMP with the need to ensure that rigor has fully resolved during frozen storage to avoid shrinking during thawing (McDonald and Jones, 1976).

Although the mode of action may not have been fully resolved, the microbiological consequences of packing fish in MAP are weil understood, and future research should concentrate on the earlier, autolytic phase of quality loss. Improvements here might be achieved through further investigation of 'master-pack' techniques as a means of controlling the rates of access by very high concentrations of CO2 ; the effects of using anaesthetic C02/02 treatments as a pre-slaughter treatment of farmed fish; the effects of packing whole fish, such as small farmed trout, at carefully monitored post-slaughter intervals; packaging of frozen fillets from preand post-rigor processed fish; and the use of noble gases for, particularly, protection of pigments in some high-value products.

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Note

Pastoriza et al. (1995). This work has been re-presented by the same authors with a modified title: Effect of carbon-dioxide atmosphere on microbial growth and quality of salmon slices. Journal o[ the Science o[ Food and Agriculture (1996) 72(3): 348-352.

10 Meats and poultry

B.A. BLAKISTONE

10.1 Introduction

The sale of packaged fresh meat has become a universally accepted form of retail marketing and is the preferred method for many busy shoppers who look for convenience and variety in the choice of cuts they can buy. While the system has failed to live up to earlier predictions that traditional forms of butchery would be replaced completely by central boning operations based on different packaging systems, the packaging of fresh meat continues to play an increasingly important marketing role, particularly in self-service outlets. Nowadays, both packaged meat and tradition al butchery systems often exist side by side, fulfilling different consumer requirements and expectations. Newer forms of MAP have increased the possibilities for improving presentation and extending shelf-life and have added to the number of options available to the retailer.

Wholesale marketing has also benefited from the new technologies and from improved understanding of the principal factors required to achieve maximum shelf-life. Moreover, there is a better appreciation of the need to produce an attractive end-product for the retail market. The tradition al means of distribution between abattoirs and retail outlets for beef quarters and lamb carcasses has been largely replaced by vacuum-packaged boxed meat, both bone-in and boneless, produced in specially designed packing plants. The partly prepared cuts offer considerable economic advantages in terms of weight reduction and space saving during distribution, and reduced labour costs in meat preparation at the point of sale (Hood, 1975). Vacuum-packaged cuts also offer other advantages, including longer storage life and elimination of evaporative weight loss.

The evolution of MAP for fresh meat and poultry has occurred in response to the need for more exact packaging environments depending on the properties of meat from different species and the need to satisfy specific marketing requirements. New developments have occurred as !imitations in conventional methods of packaging have become apparent. Refinements continue to be made to meet the exigencies of particular applications. This chapter considers the relative merits of the various systems available in marketing meat from the main meat species. Red meat and poultry will be considered separately. The same basic principles apply

MEATS AND POULTRY 241

in both cases, with the exception that colour is of primary concern in red meat but not so in poultry. The tendency of fresh meat to discolour under various packaging conditions is not a real problem in the much paler meat from poultry. While colour is frequently the determining factor in choosing a packaging system for red meat, it is much less important in poultry meat, and packaging requirements are consequently less demanding in the latter.

10.2 MAP of red meats

10.2.1 Microbiology of red meat

Fresh meat bacteria occur almost exclusively on the surface of the meat, the deep tissues remaining virtually sterile. Each stage of slaughter, carcass I dressing, cutting and packaging can be a source of bacterial contamination. The degree of surface contamination on a large peice of meat, such as a lamb carcass or a quarter of beef, has a major influence on the bacterial level on the cut meat prepared from it. In large pieces of meat, the surface area is small in relation to total volume, and surface effects are relatively unimportant, but with cutting, the situation changes dramatically, with bacterial contamination spread over a vastly increased surface area. After carcass dressing, the surface of a beef carcass can carry up to 104 organisms cm-2 (Brooks, 1938; Hood, 1971). Moreover, meat surfaces newly exposed by cutting provide a moist and nutritious medium that is ideal for rapid bacterial growth. After butchery, joints and pieces of meat for packing are likely to carry considerably higher numbers of organisms (Taylor, 1985).

The principal microbiological considerations in meat packaging have been reviewed by Egan et al. (1991). The spoilage of fresh meat stored aerobically under chilI conditions is mainly caused by growth and metabolism of the dominating Pseudomonas spp., including P. fluorescens, P. putida and P. fragi (Shaw and Latty, 1984), and Moraxella and Acinetobacter spp. (Stiles, 1991). These organisms at levels above 106 cm-2

give rise to the putrid odour commonly associated with spoiled meat and eventually cause slime formation when populations re ach 108 cm-2 . Under favourable conditions, spoilage can occur in about 10 days at O°C or five days at 5°C. Pseudomonads require O2 for growth but can multiply even at concentrations around 1 %, although they are inhibited in atmospheres enriched with CO2 .

Other bacteria that can playa role in the spoilage of meat include coldtolerant Enterobacteriaceae (these may dominate when temperatures are around 10°C) and Brocothrix thermosphacta, which causes souring in the absence of air but is inhibited in high concentrations of CO2 , especially when O2 is low (Campbell et al., 1979). Such conditions favour the growth


of lactic acid bacteria (Carnobacterium, Lactobacillus, Leuconostoc and Pediococcus), which produce a typicallactic souring in the meat.

Under aerobic conditions, the pseudomonads rapidly outgrow all the other spoilage bacteria and are unaffected by the pH range associated with red meats (5.5~.5). However, there is one type of me at that spoils much more rapidly than usual and is described as dark, firm and dry (DFD)./This type of meat has a pH value of 6.0 or above and results from animals that are subjected to pre-slaughter stress (Hendrick, 1980). In consequence, muscle glycogen is low, relatively little lactic acid is formed during postmortem glycolysis and the ultimate pH is, therefore, high. Tbere is also a reduced level of glucose in the muscle. As discussed by Gill (1982), pseudomonads grow preferentially on glucose and only produce malodorous metabolites when using certain amino acids, once the glucose supply has been exhausted. Under aerobic conditions, DFD me at spoils more rapidly because pseudomonads utilize the amino acids at an earlier stage.

With conditions of low O2 and high e02 , spoilage of DFD me at is also faster. In this case, however, the high pH allows growth of organisms such as Shewanella (Alteromonas) putrefaciens and Aeromonas spp., which produce abundant odiferous hydrogen sulphide and greening of the me at through sulphmyoglobin. For this reason, packaging of meat to extend shelf-life is not advised for DFD meat.

The shelf-life of fresh me at is greatly influenced by pH, which may vary in meat from 5.5 to 6.5 or more depending on species and muscle and the degree of pre-slaughter stress of the animal (Davey, 1984). Table 10.1 shows the comparative storage keepability for three meat species at ooe over a range of pH.

These microbiological considerations are important in choosing a suitable packaging system for fresh me at. Egan et al. (1991) listed the major responses of the main groups of bacteria to a combination of two environmental factors: (i) the availability of 02; and (ii) the pH of the muscle tissue (Table 10.2).

It is pointed out by Egan et al. (1991) that the creation of conditions where lactic acid bacteria predominate should be the target for all meatpackaging systems. These organisms grow relatively slowly and produce minimal spoilage changes.

Table 10.1 Storage life of vacuum-packaged fresh meat

Species pH Storage life (weeks) Spoilage defect

Beef 5.5-5.8 10-12 Souring Pork 5.5-5.8 6 Flavour

6.0-6.3 4-6 Colour (greening) Lamb Variable 6--8 Colour, fat appearance

After Shay and Egan (1986).


Table 10.2 Effect of oxygen availability and pH on the growth of the major me at spoilage bacteria

Pseudomonas spp. Enterobacteriaceae Bronchothrix thermosphacta Lactic acid bacteria Aeromonas Shewanella (Alteromonas) putrefaciens

After Egan et al. (1991). +, growth; -, no growth.

pH 5.5-5.7

Oxygen

+ + + +

No oxygen

+

pH 6.0 or higher

Oxygen

+ + + + +

+

No oxygen

+ + + +

+

Myoglobin (purpie) ::!"!I:::::============I~~ Oxymyoglobin (bright red)

Oxydation (slow)

Metmyoglobin (brown)

Figure 10.1 Important myoglobin pigments and colour reactions in fresh mea!.

10.2.2 Colour of red meat

The principal pigment of fresh meat is myoglobin, which can exist in three forms or derivatives depending on the O2 status of the environment surrounding the me at. These are reduced myoglobin (Mb), oxymyoglobin (Mb02) and metmyoglobin (Mb+). Reduced myoglobin is purpIe and is responsible for the colour of meat immediately after it is cut, or for the colour of meat held in the absence of air, e.g. in a vacuum package. Oxymyoglobin is bright red, the typical attractive colour of fully oxygenated meat. Metmyoglobin is brown and is formed by oxidation of the pigment to the ferric form. The actual colour of fresh meat depends on the relative amounts of these three derivatives present at the surface (Figure 10.1).


The depth of O2 penetration d into meat depends on the partial pressure Co of O2 at the surface, the rate of O2 consumption (Ao) by the musde tissue and the diffusion constant (D), according to the following equation (Brooks, 1938):

d = V2CoD/Ao

Pre-rigor meat has a very high rate of O2 consumption, resulting in a minimum penetration into the surface of the meat for several hours postmortem. After a couple of days, however, meat exposed for several hours to the air becomes red and the penetration depth of O2 may be 6--7 mm (Taylor, 1985). Particularly in a plentiful supply of 02, myoglobin is oxygenated to oxymyoglobin, the bright-red ferrous form of the pigment. A low partial pressure of 02. however, favours oxidation of the haem pigment and formation of the brown metmyoglobin derivative. The optimum partial pressure of O2 for oxidation is 4 mmHg (Brooks, 1938). Both of these reactions, oxygenation and oxidation, take pi ace at the surface of a freshly cut me at surface. Where O2 is freely available, the red oxymyoglobin is formed but as O2 penetration extends inwards its partial pressure is decreased owing to O2 consumption. Close to the limit of O2

penetration, optimum conditions exist for metmyoglobin formation to occur (i.e. a partial pressure of approximately 4 mmHg) and the brown form of the pigment predominates. Beyond the limit of O2 penetration where conditions are anaerobic, the purpie reduced form of the pigment, myoglobin, remains intact. Under practical conditions, aB three pigments may exist together at the surface of cut meat. Oxygenation occurs rapidly so that the meat turns red within half an hour at 5°C. Oxidation to metmyoglobin, however, occurs much more slowly, first appearing dose to the limit of O2 penetration, as a fine brown layer, and gradually becoming thicker and extending outward towards the surface. The meat becomes gradually darker over the next several days by diffusion and gradual accumulation of the metmyoglobin pigment throughout the translucent surface layer.

Autoxidation to metmyoglobin is also highly temperature dependent. Brown and Mebine (1969) calculated a QJO value of 5 for the reaction. The reaction is accelerated at lower pH values, wh ich have been shown to decrease the stability of the haem-globin linkage (Fronticelli and Bucci, 1963). Metal ions also stimulate the rate of oxidation of oxymyoglobin; Snyder and Skrdlant (1966) found copper to be most active in this respect, while iron, aluminium and zinc were less so. The coefficient of diffusion decreases less than does respiratory activity for the given fall in temperature so that the depth of the bright-red layer of oxymyoglobin will be greater at O°C than at, say, 20°C, hence the tendency for meat surface colour to be brighter the lower the temperature.

Optimum conditions required to avoid autoxidation of myoglobin


pigments may be summarized. First, low O2 partial pressure must be avoided, either by placing the meat under completely an aerobic conditions or by exposure to a high level of 02. Storage temperatures should be kept as close to O°C as possible and contact with metal ions, especially copper, should be avoided. Bacterial contamination, with a tendency to restrict the O2 available to muscle tissue pigments, should also be avoided. The high pH of dark-cutting beef might be an advantage in this particular respect, but other more important factors are also at work, especially the greatly increased growth of spoilage bacteria.

10.3 Packaging of meats and poultry

In common with other foods, meat was originally packaged to provide a convenient container, to avoid gross contamination, both of and by the meat, and possibly to reduce evaporative weight loss. With the development of new packaging materials specifically designed for meat, further attributes became possible, including improved storage life, a better means of presentation and making meat more attractive to the retail customer.

Meat is highly perishable because it is easily contaminated with spoilage bacteria, wh ich are immediately presented with a moist surface and a plentiful supply of nutrients for growth, providing an ideal environment for rapid proliferation. Chemical deterioration such as fat oxidation and pigment autoxidation mayaiso playa role in spoilage. In fact meat will normally discolour through to intrinsic biochemical reactions long before bacterial effects become important (Hood, 1984).

Packaging systems offer various possibilities depending on the particular conditions that are chosen, but all depend on changing the environment of the meat and especially the gaseous atmosphere within the pack. The appropriate method will depend on many factors, including the type of meat to be packaged (beef, lamb, pork, veal, etc.), the expected duration of storage or extent of market distribution and whether the meat is intended for wholesale or retail sale, etc. The composition of the gaseous atmosphere determines the colour of the meat and the rate at wh ich it discolours, as weIl as the likely pattern of microbiological spoilage. Within the pack, the atmosphere is a dynamic equilibrium of gaseous exchanges, possibly also occurring with the external atmosphere depending on the degree of gas permeability of the packaging film. Therefore, gases will dissolve in meat fluids at different rates according to solubility constants and the partial pressure in the atmosphere. Carbon dioxide is very soluble in both muscle and fat tissue (Gill and Penney, 1988). It will also be produced by respiration of the muscle tissue and of microorganisms within the pack (Ingram, 1962).

Carbon dioxide and O2 are the two gases of primary importance.


Oxygen will produce a bright-red colour, especially at higher partial pressures. The O2 concentration will te nd to decrease with storage because of respiration of musde tissues and the action of the enzymes of aerobic bacteria. It may be completely absent from the system, e.g. in vacuum packaging. Commercial packaging systems differ principally in O2 status and in the level of CO2 within the pack. The changes that occur and the effects which these produce on the quality attributes of colour and bacteriological storage life are the principal factors for consideration in MAP of meat.

10.4 Vacuum packaging

The simplest form of MAP is to remove air from the system and hold the me at in a vacuum pack. Vacuum packaging is the most frequently used packaging method for the storage and distribution of chilled prim als or wholesale cuts of beef. Particularly in the USA, the use of vacuum packaging for wholesale distribution of large pie ces of meat has practically replaced the movement of carcasses, and the concept of boxed beef is recognized as an intrinsic link in the meat marketing chain (Breidenstein, 1982). The system offers several advantages compared with handling carcasses or quarters of beef. Vacuum-packaged cuts are easy to handle and, provided adequate ca re is taken, the method is relatively trouble free. There are substantial economic savings in transporting and storing beef as boneless joints rather than as quarters or sides. Only about two-thirds of the carcass is useable meat and there is a huge saving in refrigerated space by using boxes during transport and storage. Moreover, there are also economies of scale in retaining low-value trimmings and bone at the processing plant. Vacuum packaging also allows individual cuts to be aged without weight loss, during which meat tenderness and eating quality characteristics are improved. There are further advantages to the retailer in providing greater flexibility in marketing, better stock control and reduced butchery costs in the preparation of retail packaged cuts dose to the point of sale.

Successful vacuum packaging depends on the physical properties of the film, which should have good mechanical strength, be puncture resistant and easily sealable and have low water vapour transmission rate and low O2 permeability. An O2 conte nt of <2% (v/v) is necessary to minimize oxidative rancidity (White and Roberts, 1992). As the residual volume of O2 is consumed by respiring meat tissue and microorganisms, CO2 levels rise. The plastic bags used for vacuum packaging have a low permeability to gases, especially O2 water vapour and CO2 . It is essential to exdude O2

as far as possible during cutting and preparation to produce the best colour effect and achieve the longest storage life. Protection of bone-in cuts may


be achieved by covering the bone with a reinforced plastic material before vacuum packaging.

Vacuum packing may produce a significant amount of drip that is unsightly and detracts from the appearance of the final product. This can be partly overcome by vacuum skin packaging using a film that fits very tightly to the meat surface, leaving little space for the accumulation of any fluid exudate. The technique uses an ionomer film, Surlyn®, which softens on heating making it pliable so that it can be draped over sharp objects such as the cut surface of bone without puncturing and leaving very little space for the accumulation of drip. The technique is especially suited to frozen meat, where the main requirement is to prevent moisture loss or transfer within the package. The accumulation of frost can otherwise have a very deleterious effect on appearance. Taylor (1985) points out that colour is the most important feature of frozen meat and if a bright red colour is required, it must be produced by oxygenation of the meat before freezing.

The colour of vacuum-packaged fresh meat is purple, but this is not regarded as a significant disadvantage in the wholesale market where people are aware that it is temporary effect which will be reversed when the meat is re-exposed to normal atmospheric conditions. Taylor (1985) outlines the principal changes that occur following vacuum packaging. The preliminary effect, when meat is first vacuum packaged, is that any residual O2 remaining in the pack is quickly consumed by meat pigments and muscle enzymes. If the meat is red at the time of packaging, the colour is quickly reduced to the purpie form of the pigment. A parallel effect is the production of CO2 , which occurs as a result of respiration; the concentration of this gas increases to a maximum of about 20%. The final partial press ure of O2 at the surface of the meat falls to less than 10 mmHg within two days of packaging. The remainder of the gaseous atmosphere in vacuum packaging is N2 . The actual gas headspace is tiny after vacuum packaging and its practical effect on the meat is slight within the pack. Any oxidized pigment that occurs as a resuIt of the low partial pressure of O2 appears as a monomolecular layer at the surface and the purpie colour of the underlying unoxygenated tissue is unaffected. The surface layer of metmyoglobin may be as thin as 1 mm with very good barrier materials but as much as 2-3 mm when a film with permeability approaching 200 is used. Beef stored under vacuum in low-permeability films should remain purple throughout storage. The development of a brown colour as a result of metmyoglobin formation during storage indicates the presence of O2 which has gained access to the pack either by the use of a film with inadequate impermeability to O2 or because the pack has leaked during packaging or subsequent storage.

The principal advantage of vacuum-packaged meat is its long shelflife. Provided it is stored at a low temperature, meat will remain in an


acceptable fresh condition for many weeks after packaging. Tbere are, however, SO me preliminary, important precautionary measures which must be taken to guarantee success. Only me at of good microbiological quality should be vacuum packaged. pH is of equal and perhaps even greater importance. Meat of pH 6.0 and above must not be vacuum packaged. Tbe combined effects of high bacterial contamination and high pH will seriously curtail the shelflife of the meaL Temperature is also a limiting factor and to achieve the best results the meat must be kept close to the freezing point (Hood, 1984).

Vacuum packaged beef with normal pH can be stored for periods of up to 14 weeks at O°C. The storage life of beef of high ultimate pH (> 6.0) will be much shorter than this at a similar temperature. Gill and Penney (1986) give a storage limit of eight weeks and they also apply this to most lamb cuts, which usually have so me muscle tissue of high ultimate pH as well as a non-respiring fat cover of neutral pH. Sheridan et al. (1997) reported a storage life of six weeks for lamb shoulder cuts when packaged under vacuum, 80/20 or 50/50 Oz/COz or 100% COz. Taylor and Shaw (1977) also showed that the storage stabilities of vacuum-packed pork and lamb are less than that of beef. Lamb loins, shoulders and legs remained unspoiled in vacuum packs held at 1°C but deteriorated rapidly during subsequent retail display at 5°C. Beef muscle has a lower pH than that of lamb, and beef is, therefore, less conducive to microbiological growth (Gill, 1989). Lamb requires a higher concentration of COz than generated within a vacuum pack for the bacteriostatic effect. Pork, with greater unsaturated fat, is more prone to oxidative rancidity (Brody, 1989). Commercial shipments of vacuum-packed lamb, both as joints and as whole carcasses cut and telescoped to facilitate packing, have been successfully shipped to Britain from New Zealand (Bruce and Roberts, 1982). Strict attention to high standards of hygiene during processing and packaging and rigorous control of temperature at -1°C gave best results in terms of storage stability and subsequent retail shelf-life. Taylor (1985) reports that the useful storage life for vacuum-packaged pork joints is little more than two weeks at a storage temperature 1°e. Sorheim et al. (1996) stored pork loin in gas-flushed bags at 1°C for 22 days under vacuum, two conditions of COz and Nz and 25% COz/65% Nz/lO% Oz. Retail chops prepared from the Ozcontaining bags of loins showed more surface greying and greening, a slight off-odour, caused, presumably, by aerobic bacteria, and a 1 10glO higher count than the loins stored under other conditions. Vacuum-held loins had colour changes similar or lower than the COz/Nz treatments, and vacuum packaging reduced drip loss compared with that in MAP packs. The extent of control of vacuum packaging reflects the type of meat and the degree of vacuum.

Baltzer (1969) summarized the microbiology of vacuum packaging in terms of replacing pseudomonads with lactic acid bacteria and further


noted that vacuum-packaged beef has a slower increase in total counts and shows a souring type of spoilage rather than putrefaction and sliming. The strains of lactic acid bacteria present in vacuum-packaged meat have proved difficult to iden tify. Shaw and Harding (1984) demonstrated that 90% of strains belonged to two groups of streptobacteria, while a third group consisted of Leuconostoc spp. These authors classified 20 atypical strains of lactic acid bacteria and described a new species Lactobacillus carnis, as weil as L. divergens, which appear to be important constituents of the vacuum packaged flora. Ahn and Stiles (1990) reported specific antibacterial activity of lactic acid bacteria isolated from vacuum-packaged meat. When stored at 4°C these organisms produce antagonistic substances active against closely related bacteria. Studies using meat that carried only a very small proportion of bacteria (less than 100 cm-2) have shown that an off-flavour develops and becomes significant after 14-16 weeks at O°C. This flavour is also described as bitter and liver-like and probably results from chemical changes in the meat caused by enzymic activity. When meat pH is about 6.0 or higher, a number of other species may re ach populations high enough to cause spoilage. In particular, the growth of Alteromonas putrefaciens, Aeromonas spp. or so me types of Enterobacteriaceae may cause spoilage through greening discoloration. These organisms produce hydrogen sulphide, wh ich reacts with myoglobin to form the green pigment sulphmyoglobin. The defect is more noticeable with beef, because the higher concentration of myoglobin produces a more intense green colour. It can be avoided by not packaging meat of high pH (Egan et al., 1991). Vacuum packaging has been reported as superior to modified atmosphere in the packaging of beef steaks (Fu et al., 1992; Canganella et al., 1993). Both groups reported lower microbial growth on the steaks under vacuum packaging, though Fu et al. noted that MAP was required to inhibit Enterobacteriaceae.

The storage life of vacuum-packaged primals may be extended by improving the bacteriological quality of carcasses from which the me at is derived. The objective is to decontaminate the surface of carcasses on the slaughter line by spraying with hot water. Dilute solutions of acetic or lactic acid mayaiso be used to extend the storage life of meat of higher pH values. Egan et al. (1991) reported that lamb carcasses may be treated online prior to chilling. If such unchilled carcasses are immersed in a 1.5% solution of acetic acid at 55°C for 10 s, there is a reduction of 95-99% in the population of bacteria on the meat. The acid treatment not only reduces the number ofbacteria present but also has a residual bacteriostatic effect, delaying the proliferation of putrefactive bacteria and resulting in an extension of the storage life to 10-12 weeks. Success of lactic acid treatment mayaiso be a function of packaging. Stiles (1991) commented that lamb packaged in foil-laminate remained unspoiled several weeks longer than that packaged in plastic film. Greer and lones (1991) are more


cautious, however. Beef carcasses in a research abattoir were treated with either a lactic acid spray or a water spray. Results suggest that the marginal reduction in carcass contamination produced by the lactic acid spray does not significantly improve the bacterial quality of subprimals, nor the aerobic spoilage of steaks. Stiles (1991) contends that results using lactic acid bacteria to extend the shelf-life of meats are equivocal, perhaps because inappropriate cultures are used. When dairy starter cultures have been used, a large inocula is required to achieve the desired effect. Beef steaks inoculated with lactic acid bacteria from vacuum-packaged meat failed to extend the shelf-life at 1 to 3°C, and the steaks were subject to offodours, surface discoloration and poor flavour ratings.

10.5 Gas atmospheres

As long aga as the early 1930s, chilled carcass meat was successfully shipped from New Zealand and Australia to Britain to satisfy consumer demand for fresh rather than frozen meat. This system, which demands strict control of temperature and atmospheric conditions, may be accurately described as controlled atmosphere storage. Gases surrounding the meat are continuously monitored and adjusted throughout the complete storage period. In the early days, a CO2 concentration of 10-20 was used. Higher concentrations of CO2 were found to be effective in preventing bacterial growth but also produced browning of the surface through formation of metmyoglobin. Temperature was also continuously monitored and rigorously maintained at 1°C (Haines, 1933). Controlled atmosphere storage of me at is confined to large-scale shipping or warehousing operations of this nature.

Since the introduction of vacuum packaging, high er concentrations of CO2 have also been used in package atmospheres to extend storage life. The use of CO2 in the atmosphere of packaged beef strip-loins is effective in limiting the growth of spoilage bacteria, even with beef of high pH (Gill and Penney, 1986). A system has been developed in New Zealand to ship whole lamb carcasses in CO2 in a foil-laminate master pack. Known as telescoping, the process consists of folding the hind legs of the carcass into the thoracic cavity, giving a significant saving in volume and space requirement during shipping. The package atmosphere can be maintained, irrespective of the physical shape or size of the cut of meat (Gill, 1987). Carbon dioxide is very soluble in both fatty and muscle tissue (Gill and Penney, 1988), and for gas efficacy in shelf-life extension, an amount greater than that which dissolves into the product is required (Gill and Penney, 1988). A reduction in partial pressure within a package may result from the gas moving into solution unless a sufficient excess is included to counteract the effect. Shay and Egan (1986) used high concentrations of CO2 to extend the storage life of packaged lamb and pork. The carcasses


may be vacuum packaged, but it is difficult to remove all the air from the system. Flushing with CO2 removes residual O2 from the system. Recently, Garout et al. (1989) reported an increased storage life for lamb loins and carcasses packed in CO2 compared with similar vacuum-packaged meat in consignments transported by air from New Zealand to Saudi Arabia. Given similar chilling conditions, the storage life of CO2-packed lamb was about 40 days longer than that of vacuum-packed lamb. Storage life was limited by the development of putrid spoilage, principally caused by psychrotrophic enterobacteria.

The gases of interest in aerobic MAP of meat are 02, CO2 and N2 ,

especially the first two. Nitrogen is inert and provides no bactericidal or bacteriostatic function, but it may be included to help prevent the collapse of the package as CO2 dissolves in the meat fluids. To ac hieve the best effect, there must be excess gas present (approximately 1.5-2 times the volume of the meat). Headspace-to-meat volume ratio has been shown to be an important packaging parameter for beef (Zhao et al., 1995). A deepdraw impermeable plastic pack is used with a dimpled base, which allows access of the gas to the lower surface of the meat.

The use of a high concentration of O2 produces a deep layer of brightred oxymyoglobin and ensures that the formation of metmyoglobin occurs at a maximum distance from the surface. The brown discoloration of the pigment is thus retarded, giving an extension to colour shelf-life. For a successful centralized packaged operation, it is necessary to maintain the red colour for at least a week. This can be achieved by modifying the packaging technique to delay aerobic deterioration owing to the proliferation of spoilage bacteria. The incorporation of CO2 and storage at low refrigeration temperature are essential for this purpose. MAP will not compensate for poor bacteriological quality or poor temperature control, but it will extend shelf-life provided both these prerequisites are met. The earlier the product is gas packaged, the more effective the CO2 is in shelflife extension (Brody, 1989).

Mixtures of O2 and CO2 have been used commercially for a considerable time (Brody, 1970). In a patent published in 1970, Georgala and Davidson specified a range of O2 and CO2 concentrations suitable for MAP of beef. They dealt with O2 concentrations above 70% combined with at least 10% CO2 . Results demonstrated that at least 60% O2 is required to achieve a colour shelf-life of ni ne days and the patent claims that a mixture of 80% O2 plus 20% CO2 keeps meat red for up to 15 days at 4°C. While this is too ambitious under commercial conditions, their recommended gas mixtures are now widely used in MAP systems. Atmospheric mixtures of 60-80% O2 and 20-40% CO2 are commonly used. An excellent summary of recommendations for MAP of meats and offal appears in Table 10.3 (Shaw, 1995). Provided a low storage temperature is also achieved, meat colour can be maintained for a week using this mixture of gases. Shay and Egan (1990) report that beef stored at 5°C in MAP (80% N2 and 20% CO2)


Table 10.3 Information and recommendations for the MAP of raw meats and offal

Major types of raw meat and offal

Principal spoilage mechanisms

Possible food poisoning hazards

Recommended storage temperature range (OC)

Achievable shelf-lives (days) In air In modified atmospheres

Recommended gas mixtures Retail red meats Retail offal Bulk red meats Primals

Typical MAP machines Retail Bulk Primals

Examples of typical MAP materials Lidding film (top web)

Tray (base web)

Bag-in-box and master pack

Note: lTFFS - thermoform/fill/seal

Beef, pork, lamb, veal, venison and offal (liver, kidneys, heart and brains), goat, wild boar, rabbit and hare, oxtail, marrowbone, foie gras, giblets, neck, feet, tongue, sweetbread, tri pe and trotters

1. Colour change (red to brown) 2. Microbial, e.g. Pseudomonas, Acinetobacterl

Moraxella and Brochothrix spp., lactobacilli, micrococci, Enterobacteriaceae, yeasts and moulds

Clostridium spp., Salmonella spp., Staphylococcus aureus, Bacillus spp., Listeria monocytogenes and Escherichia coli

-1 to +2

2-4 5-8

20-40% COi60-80% O2

0-20% COi60-80% 0 2/0-20% N2

20-40% C02/60-80% O2

20-40% COi40-80% N2

TFFS 1 and PTLF2 Snorkle-type and vacuum chamber CapTech, Cryovac

PETIPVDCILDPE P A/PVDCILDPE PCIEVOHIEV A UPVCILDPE HDPE EPSIEVOHILDPE PAILDPE PAIEVOHILDPE

2pTLF - preformed tray with lidding film Reprinted with permission (Shaw, 1995).

has a shelf-life more than three times that of similar beef and lamb meat stored in conventional overwrap trays. Storage life under retail display is dependent on muscle type, species and the length of storage in vacuum pack before retail packaging. Because of the high O2 content, the colour remains bright red during the period of retail display. The colour display life also depends on the previous storage history of the meat. Meat that has


been stored in vacuum packs prior to retail display will have a reduced retail display life. For example, the colour shelf-life of meat stored at 5°C for longer than six weeks before retail display at ooe is less than half that of fresh meat. This results from a combination of factors, which includes an increased bacterial load on the surface of the me at and decreased metmyoglobin-reducing activity in muscle enzyme systems with the age of the meat. Patterson (1990) has found that microbiological and sensory quality of pork can be improved by the combined effects of MAP (25% e02 , 75% N2) and irradiation treatment at 1.75 kGy.

Egan et at: (1991) report that consumer portions packaged in conventional overwrapped trays may be placed in a master-pack which consists of a large impermeable bag, evacuated and filled with agas mixture of 20% O2 and 80% e02 . The master-pack is stored at a low temperature until packs are required for retail display. Storage temperature is again extremely important; it should preferably be as low as possible without actually freezing, i.e. 0 or 1°e. The combined effects of O2 to produce an attractive red colour and the bacteriostatic effect of e02 are exploited in this system. The master-packs may be opened and the individually overwrapped trays removed as required for refrigerated retail counter display. Meat stored for up to nine days in the master-pack at -1 to ooe has a retail display life of three further days. With longer periods of storage in the master-pack, the retail display life is shorter, i.e. less than is obtained with fresh me at in overwrapped trays. Master-packs of this type are suitable for centralized pre-packing operations. The use of master-packs has some advantages compared with gas packs; volume is reduced and the system is consequently less costly to operate.

Pork under MAP retains better quality when O2 is not included in the gas formulation. As noted previously, Sorheim et al. (1995) reported offodours as being slightly greater when pork loins were packaged under 10% O2 or vacuum. Gill and Jones (1996) reported that pork chops formed odours at retail display after 12 days of storage under O2 versus 21 days when no O2 was present. Boned pork loins stored in Oz-containing atmospheres developed grey surface discolouration proportional to the O2

concentration present (Sorheim et al., 1995). When oxygen absorbers were used, the pink colour was maintained. An exception to the detrimental effects of oxygen on pork has been reported by Buys et al. (1994) who found that a 25% 02, 25% e02 and 50% N2 mixture was the most successful bulk packaging technique, giving at least three days retail shelflife with consumer acceptability and good colour scores.

10.6 MAP storage of poultry

Demand in Europe for chilled poultry has increased in recent years and there is now an extensive market for both carcasses and cut portions in this


form. In the USA it has been the custom to seil chicken in the fresh state, although carcasses are gene rally chilIed by water immersion rather than in cold air as they are in Europe. Whatever the method of chilIing, raw poultry is a relatively perishable commodity, especially when stored in air; like other meats, it is susceptible to spoilage from the growth and metabolie activities of certain species of bacteria that flourish at chilI temperatures. Under some conditions, the keepability of poultry meat is partly determined by its pH value, which may vary from 5.6 to 6.4 depending on bird species, type of muscle (i.e. breast or leg) and postmortem biochemical changes (Jones and Grey, 1989).

As with other flesh foods, the spoilage of chilIed poultry stored aerobically is frequently caused by growth of Achromobacter and Pseudomonas spp., especially P. fluorescens, P. putida, P. fragi and related strains. These organisms reach populations of about 108 cm-2 at the time off-odours are first detectable and are usually accompanied by lower numbers of other Gram-negative bacteria, especially Acinetobacterl Moraxella spp., some strains of which are now included with Psychrobacter immobilis (Juni and Heym, 1986). Spoilage microorganisms can grow weil and produce their characteristic odours on all cut muscle surfaces but, in the case of whole carcasses, the neck flap is one of the first sites at which spoilage is detectable (Patterson and McMeekin, 1981). Some producers remove a large part of the neck skin to gain a further day of shelf-life.

Treatments used to extend shelf-life usually do so by reducing levels of pseudomonads or by inhibiting their growth on the producL In consequence, a slower-growing microflora develops and this produces spoilage odours that are often described as 'sour' or 'cheesy' and are entirely different from those associated with Pseudomonas spp. The main species under these conditions are Carnobacterium spp., Lactobacillus spp., B. thermosphacta and sometimes S. putrefaciens (Mead, 1989).

In the UK, most chilIed poultry products are sold pre-wrapped in Ozpermeable film, which prevents moisture loss and the spread of contaminating microorganisms. Relatively little use is made of MAP for individual birds or sets of portions because of cost and the lack of any marked advantage in presentation, but, where Oz-impermeable barrier films are used, mainly for turkey and duck, there are clear benefits in extending shelf-life. The same is true for bulk packaging of poultry, wh ich has been used for many years in the USA and is now seen in Europe.

Vacuum packaging is not widely used for poultry of any kind, except for some cooked products, although oven-ready ducks have been marketed in this form in the UK. It was shown by Barnes et al. (1979) that in Ozpermeable film, carcasses spoiled in about 10 days at 2°e or 19 days at -1°e. In vacuum packs, however, shelf-life was extended by more than 50% at either temperature. With cut portions of turkey stored at 1°e, vacuum packaging delayed off-odour development from 14 to 20 days for


drumsticks and from 16 to 25 days for breast fillets (Jones et al., 1982). In each case, however, detection of definite spoilage odours was preceded by changes in me at flavour, as judged by a trained taste panel. A comparable extension of shelf-life and difference between breast and leg portions were noted for vacuum-packed chicken by Patterson et al. (1984), although no sensory tests were made in this study.

The pioneering work of Haines (1933) showed that the inhibitory effect of COz on an aerobic spoilage bacterium was significantly reduced when ihe storage temperature was raised from 0 to 4°C. A similar situation arose in a comparison of turkeys vacuum packed in barrier film or wrapped in Oz-permeable material (G.c. Mead, unpublished data). At 1°C, spoilage odours developed within 13 days in the Oz-permeable film, but not until16 days in vacuum packs. When the birds were held at 4°C, these times were reduced to seven and eight days, respectively.

Chill storage of poultry in vacuum packs leads to the development of a mainly lactic flora, sometimes accompanied by, for example, coldtolerant coliforms (Barnes et al., 1979) or S. putrefaciens. At spoilage, levels of Pseudomonas spp. will be up to lO00-fold lower than those of the predominating lactic acid bacteria, depending on storage temperature (Table 10.4).

Although MAP has only limited application to poultry meat at the present time, its use for bulk packs in the USA was described by Timmons (1976). Each pack, containing about 30 kg product, was first evacuated and then backflushed with COz before being heat-sealed. At -2 to +l°C, the expected shelf-life was 18-21 days. The basis for using atmospheres enriched with COz for chill storage of poultry derives from the classical work of Ogilvy and Ayres (1951). In this study, various cut portions of chicken were stored in air or in air containing different concentrations of COz. Within the range 0-25% COz, the ratio of shelf-life in COz to that in air was found to be a linear function of COz concentration, although the

Table 10.4 Effect of vacuum packaging on microbial counts and development of spoilage odours in dusk carcasses stored at 2°e or -l°e

Storage temperature COC)

2 2

-1 -1

Type of packaging

02-permeable Vacuum pack 02-permeable Vacuum pack

"Mean loglO colony forming units per cm2. bpreceded by flavour changes. From Barnes et al. (1979).

Total viable Pseudomonads" count"

(at 1°C)

8.1 8.2 7.0 5.2 8.5 8.5 7.3 4.2

Time to off-odour

(days)

10 16 19 41b


storage temperature was 4.4°C, which is above the optimum for CO2

inhibition of spoilage bacteria (Haines, 1933). The presence of CO2

affected both the lag phase and the doubling time of the bacteria present. The maximum usable concentration of CO2 was considered to be 25%, because higher concentrations were said to discolour the meat; even at 15%, Ogilvy and Ayres (1951) sometimes observed a loss of 'bloom'. Current thinking at the Campden and Chorleywood Food Research Association (CCFRA) is that significant extension of shelf-life requires levels of CO2 in excess of 20% (Shaw, 1995). For retail, MAP gases should be limited to 35% CO2 to minimize pack collapse and excessive drip. For bulk modified-atmosphere master-packs, collapse is not at issue, and, therefore, gas mixtures of 80-100% CO2 are recommended by CCFRA (Table 10.5).

Despite the suggested limit for CO2 concentration, much of the subsequent work on modified atmosphere storage of poultry has involved levels weil above 25% (Mead et al., 1983). Hotchkiss et al. (1985) stored chicken breast and leg portions at 2°C, either tray-wrapped in permeable film in barrier bags containing 80% CO2 in air or held in glass jars containing 0, 60, 70 or 80% CO2 in air. Differences in microbial levels (total viable count, 22°C) between storage in air and 80% CO2 in bulk packs reached 10 OOO-fold at 14 days of storage. Also, COTstored sampies had higher sensory-panel scores for raw meat odour and overall acceptability, with little effect on eating quality of the cooked meat up to 35 days. Moreover, on removal of portions from jars containing 80% CO2 , there was a residual inhibitory effect on microbial growth that was not observed with 60% CO2 . The residual effect was also lacking when portions were removed from bulk packs (80% CO2). This was attributed to some diffusion of CO2 from the packs during storage. Sawaya et al. (1995) compared 70% CO2 with 30% CO2 atmospheres on the shelf-life of fresh chicken carcasses (the remaining gas being N2). The high er CO2 level extended the shelf-life four to five days at temperatures of 2-7°C but had no effect at 9°C. Sarantopoulos et al. (1996) found that giblet-free chicken carcasses in master packs had an increased shelf-life of nine days over those packed in air.

Although a bulk storage atmosphere of 100% CO2 is used routinely by some companies for chicken and turkey, it is not known whether the same conditions would be suitable for duck. Mead et al. (1986) compared four packaging treatments for effects on microflora and keeping quality of duck portions stored at 1°C and reported an unacceptable waxen or milky appearance of the skin when portions were packed in 20% or 80% CO2 in N2• The effect was not observed with 20% CO2 in air but did occur in N2

alone, as had been found previously for beef (Smith et al., 1977), affecting the subcutaneous fat. With packs containing 20% CO2 in air, changes in both odour and flavour of the duck portions were apparent by 21 days,


Table 10.5 Information and recommendations for the MAP of poultry products

Major types of poultry products



Recommended storage temperature range CC)

Achievable shelf-lives (days) In air In modified atmospheres

Recommended gas mixtures Retail Bulk

Typical MAP machines Retail Bulk

Typical types of package Retail (pre-pack) Bulk


Tray (base web)

Bag-in-box master pack

Chicken, turkey, duck, pheasant, quail, goose and poussin, pigeon, partridge, Guinea hen, capon, poussin/Cornish hen, grouse

Microbial, e.g. Pseudomonas, Achromobacter, Flavourbacterium, Acinetobacterl M oraxella,

Alcaligenes, Aeromonas, Alteromonans and Brochothrix spp., lactobaciJIi and yeasts

Salmonella, Clostridium and Campylobacter spp., Staphylococcus aureus, Listeria monocytogenes

-1 to +2

4-7 10-21

25-35% C02/65-75% N2

80-100% C02/0-20% N2

TFFS and PTLF Snorkle-type and vacuum chamber

Tray and lidding film Bag-in-box and master-pack

PETIPVDCILDPE P AIPVDCILDPE PCIEVOH/EV A UPVC/LDPE HDPE EPS/EVOHILDPE PAILDPE PAlEVOHILDPE

Reprinted with permission (Shaw, 1995).

compared with off-odours at 14 days for portions traywrapped in an 02-permeable film.

Modified atmosphere retailing of red meat usually involves gas mixtures containing a high concentration of O2 to maintain and enhance the desirable red colour of oxymyoglobin. Generally , such treatment is unnecessary for poultry because of its natural whitish appearance, when the skin is kept intacL In the case of skinless portions, however, there is the possibility of enhancing me at colour by inclusion of O2 in the preservative gas mixture. When Mead et al. (1983) evaluated different gas mixtures for


extending the shelf-life of chilled turkey breast fillets, O2 levels of 10 to 20% were used. Meat colour enhancement occurred only at the higher concentration of 02, when a salmon pink colour was observed. However, this varied in intensity from one muscIe to another and in some cases patches of pink colour persisted after cooking, giving those parts of the meat a 'raw' appearance. Sante et al. (1994) found that turkey breast meat kept under vacuum or in 100% CO2 with an O2 scavenger showed the best colour stability over a 21 day storage period and microbiological contamination was the lowest under C02/02 scavenger conditions.

Hotchkiss (1989) has discussed a shelf-life study on MAP of raw chicken quarters. Panelists rated the chicken for odour, feel and overall appearance on a scale of 1-9,9 being the best score. He reported that fresh refrigerated poultry spoils in less than 14 days, but at day 14 the MAP product scored 7.1 for odour compared with 3.5 for an air-stored sampie. Scores for feel were similar. Overall acceptability was 4.6 for MAP product versus 3.0 for air-stored product. Had the storage temperature been reduced to 31 instead of 36°F (-OSC instead of 2.2°C), Hotchkiss predicted even higher scoring of the MAP product. Even at day 35, the MAP product was still rated higher than the product at day 14 of air storage. Consumers would accept chicken stored under MAP for six to eight weeks, though commercial processors might not get the shelf-life Hotchkiss achieved under controlled laboratory conditions.

10.7 Meat products

10.7.1 1mportant considerations

Owing to the wide range of meat products available, different formulations, manufacturing procedures, legislative compositional requirements and food chain distribution temperatures, gas compositions that are reported in one country to increase product shelf-life cannot be assumed to give similar resuIts in another country. Careful consideration needs to be given to the intrinsic and extrinsic properties (Table 10.6) of the product in question before deciding on an appropriate gas composition and packaging method. Similar extensions in meat product shelf-life cannot be assumed on the basis of gas composition alone. Shelf-life evaluations should be carried out to determine whether the optimum gas composition for a particular product has been selected.

Examples of gas mixtures used to package meat products, compiled from reported data, are given, for guidance only, in Table 10.7. This table shows that conflicting gas compositions are recommended for wh at appear to be identical products. The effectiveness of any gas composition in extending shelf-life is dependent on the factors listed in Tables 10.6 and 10.8. Careful

MEATS AND POULTRY

Table 10.6 Intrinsic and extrinsic factors affecting mean product shelf-life

Intrinsic

pH aw Initial microflora Developing microflora Available nutrients (e.g. glucose concentration, non-meat ingredients, e.g. rusk)

Concentration and type of preservative (e.g. sodium chloride, sodium nitrite, sodium metabisulfite )

Redox potential (Eh) Natural inhibiting substances Presence of microbial spores

Extrinsic

Temperature Gaseous environment Packaging/packaging operation Relative humidity Product process (e.g. he at treatment, cooking method, method of curing, e.g. dry cure, injection)

Light

259

consideration should be given to any potential safety risks. Hazard analysis of critical control points (HACCP) evaluation should also be carried out.

Reported shelf-lives for products listed in Table 10.7 have not been given, since the shelf-life of any MAP meat product will be influenced by factors listed in Tables 10.6 and 10.8, which are often not taken into consideration in reported data. Further , data are commonly obtained under strictly controlled laboratory conditions, e.g. process/temperature, that may be very different from the conditions to which the product is exposed commercially.

Particular ca re should be taken when relying on reported data. For example, one article (Anon., 1991a) recommended that O2 should be included in gas compositions for packaging bacon 'to preserve bacon's red colour'. Far from preserving bacon colour, if incorporated, O2 is likely to cause rapid greying.

In laboratory trials, the reported microbial shelf-life of meat products can be influenced by differences in a number of factors:

• inoculation growth temperature/inoculum atmospheric conditions; • method of inoculation; • inoculum concentration/cocktail inoculum; • pack permeability; • training of sensory panellists; • physicochemical measurements; • microbial sampling methods, e.g. surface, core; • method of recovery (stressed or injured microorganisms may not recover

because of method of recovery, e.g. warm poUf plates may kill stressed or injured microorganisms, but these microorganisms may be capable of recovery if product is temperature abused);


Table 10.7 Reported gas compositions: meat products

Gas (%)

Product O2 CO2

Bacon, cured <0.5 COzfN2

Bacon, sliced 20-35 Barbecue ribs 20--40 Beef, sliced cooked 10 75 Bouchee" 50 Chicken, cooked <0.2 30 Chicken thighs, breaded, baked 30 Chicken, breaded, flash fried Cooked meat 20-25 Cooked meat 25-30 Cooked meat 20--40 Cooked meat, sliced 80 Cooked minced meat products 20 Corned beef <0.3 60 Cured meat 50 Cured meat 20 Cured meat 40 Cured meat, bulk 35 Cured meat, retail 20 Frankfurters Frankfurters 100 Harn 20-35 Harn, Italian, sliced 20 Harn, sliced <0.3 60 Karelian pie 50 Lasagne 70 Luncheon me at 100 Meat pie 50 Meat pies 25-50 Pasta stuffed with meat «30% moisture ) 50 Pasta stuffed with meat 80 Pizza (dependent on topping) 30-60 Pizza, harn 60 Poultry products 25 Ravioli 20 Roast beef, sliced, cooked 10 75 Roast pork, sliced <0.3 60 Roule au fromageb 50 Salami 20 Salami 20-35 Sausage, British fresh (raw, uncured) <0.5 COzfN2

Sausage, British fresh (raw, uncured) 50 50 Sausage, sliced 20-30 Sausage, smoked 30 Sausage, summer Sausage, uncured 40 60 Sausage, Vienna 20 Sau sage in pastry 80 Turkey, cooked <0.2 30 Wieners, natural casings <004 30

"A Danish pastry stuffed with chicken pieces and bechamel sauce (vol-au-vent). bA bakery product stuffed with harn pieces and grated Emmenthal.

N2

Flushed 65-80 60-80

15 50 70 70 100

75-80 70-75 60-80

20 80 40 50 80 60 65 80 100

65-80 80 40 50 30

50 50-75

50 20

40-70 40 75 80 15 40 50 80

65-80 Flushed

70-80 70 100

80 20 70 70


Table 10.8 Factars affecting MAP meat product shelf-life

• Temperature control: during processing, packaging, storage, transportation, display, and consumer handling

• Hygiene control: adherence to good manufacturing practices during product manufacture and packaging operations; implementation of HACCP procedures

• Raw material quality: initial microflora (phase of growth) and chemical condition, e.g. peroxide value (PV), thiobarbituric acid (TBA) value, pH

• Finished product: other product ingredients • Time prior to packaging • Gas-to-product ratio • Gas composition and residual gas composition (dependent on MAP technique used) • Gas purity • Packaging permeability: gas/moisture in good and abuse conditions far top web, base web/

tray • Pack design, circulation of gases to product surfaces • CO2/0 21N2 permeability ratio

• recovery incubation temperature (since microorganisms growing on products are mainly psychrotrophic microflora, incubation at higher temperatures may mean that organisms growing on the product are not recovered as they may be incapable of growth at incubation temperatures used).

The organoleptic properties are also an important quality parameter. The semisubjective nature of organoleptic assessments can make it difficult to obtain accurate results. Many studies looking at the effects of MAP on me at products have not incorporated such assessments or they have used inappropriate sensory evaluation or cooking techniques. So me studies have defined acceptability by arbitrary panel scores, rnaking it difficult to make cornparisons between studies. Other problems include insufficient training of panellists. The ultimate measure of shelf-life extension treatment is product safety allied with consumer satisfaction: 'Will the consumer buy the product again?' Only the consumer, e.g. sensory panel, can answer this question .

10.7.2 Cured colour stability

In raw cured meat products (e.g. bacon, Parma harn, salami) and cooked cured meats (e.g. harn, corned beef, lunche on meat), the pink pigment is caused by the formation of nitrosomyoglobin. During heating, nitrosomyoglobin is converted to pink denatured nitrosomyoglobin. The presence of air inhibits the formation of both raw and cooked colour. Both raw and cooked colours are unstable in air. The removal of air, e.g. by vacuum packaging, or replacement of air, e.g. MAP, can help increase the colour shelf-life of such products, helping to prevent greying and fading. These pigments are unaffected by high levels of CO2 (Ahvenainen, 1989).


Nitrosomyoglobin can be oxidized by O2 to green, yellow, or colourless oxidized porphyrins, with concomitant dissociation of nitric oxide, by a variety of chemical or microbially produced compounds. The re action can be further catalysed by light, and results in fading of the colour (greying/ browning), which causes the product to become unacceptable.

Low humidity and high storage temperatures may result in a browning discoloration in cured products (blackening in fresh meat), caused by modification of meat pigments at the surface and dehydration. Packaging films with low O2 and water vapour permeability and storing at low temperatures help to minimize this problem, as can incorporation of antioxidants. It is common for vacuum packs and MAP packs to be stored in the dark for one to two days so that residual O2 in the pack is consumed by microbial growth before display (Ranken, 1984). This helps with colour retention, and subsequent exposure to light is not as critica!. Alternatively, metallized films have been used, but product visibility is lost. Flushing of vacuum packs with CO2 or N2 is often employed with thinly sliced meats to remove residual O2 and prevent slice adhesion. A simplified version of cured pigment interactions is given in Figure 10.2.

10.7.3 Water activity, pR and microbial spoilage

For most microorganisms of concern in meat products, water activity (aw )

levels above 0.98 are optimal for growth (fresh meat, at 0.999). The addition of sodium chloride (about 2% rn/rn) to meat when stored aerobically is sufficient to retard the growth of pseudomonads and encourage the growth of lactic acid bacteria, similar to the microflora that develops on vacuum-packed unsalted meat.

The inhibitory effects of CO2 have been found to increase as product aw

decreases. The interactions between the factors listed in Table 10.6 are complex. Storage temperature, product pH, aw and sodium nitrite concentrations exert the strongest selective pressures. If the aw and pH of a product are known, it is possible to predict product stability for a given temperature. The interactions of other factors, such as competitive microflora, should also be considered. Storage categories of meat products based on product aw and pH are given in Table 10.9. The data given in the table are based on the assumption that the product is correctly processed each time and do not consider spoilage caused by growth of yeasts or moulds, as they generally grow slowly and are poor competitors with other microorganisms.

Under laboratory conditions, pH and aw can be controlled very precisely. Under manufacturing conditions, it can be more difficult to exert such contro!. Consideration should be given to likely process variability and the adoption of manufacturing procedures to prevent faulty processing and faulty products from reaching consumers.


Addition NaCI (e.g. > I.S"7o)

High 0 , Low 0 , (O.S-t"7o) OXYMYOGLOBIN' ~ . ~ MYOGLOBIN' "!:~=========::;~ METMYOGLOBINJ

No O2 Meat enzymes

+- Curing (nitrite)

METMYOGLOBIN

I NITROSOMYOGLOBIN4

DARK RED/PINK

+- HEAT (cooking)

I PURPLE I

DENATURED NITROSOHAEMOCHROME'

·Colour problem

I GREY I e.g. catalysts (oxygen, light, metal ions); insufficient residual nitrite, PSE/DFD meat, no antioxidant, rancid fat

I BROWN I

·Colour problem

I BLACK I +-------i e.g. dehydration

·Colour problem

I GREEN I +-------i e.g. sulfmyoglobin, choleglobin, microbial growth

·Colour problem

I PINK RED 1+-------;

_HEAT (cooking)

e.g. nitrite contamination, gas combustion products (nitric oxide, carbon monoxide), partially denatured myoglobin (undercooked or raised myoglobin denaturation temperature, e.g. cytochrome C, nicotinamide)

DENATURED METMYOGLOBIN6

·Colour problem OXIDISED PORPHYRINS

I GREY /BROWN I

I GREEN/YELLOW /COLOURLESS I e.g. microbial growth, oxygen

Figure 10,2 The farms of myoglobin and their colours and the types of meat and me at product of wh ich these colours are characteristic.

1. vacuum-packed beed; 2. raw beef stored in air or MAP beef (80% 02, 20% CO2);

3. 'stale' fresh meats stored in air; 4. uncooked cured meats; 5. cooked cured meats; 6. cooked meats. Modified from Egan et al. (1988) 'For other causes of colour problems, refer to Trout (1991).


Table 10.9 Storage categories of meat products based on aw

Category

Storable

Perishable Easily perishable

Source: Leistner (1978).

Criteria Temperature

aw ~ 0.95 and pH ~ 5.2, or No refrigeration required aw ~ 0.91 or pH ~ 5.0 aw ~ 0.95 or pH ~ 5.2 ~ +lO°C aw > 0.95 and pH > 5.2 ~ +5°C

Lactic acid bacteria, in particular the heterofermentative species of Lactobacillus, Streptococcus and Leuconostoc, are a major cause of bacterial greening in cured meats. L. viridescens is the most common cause. Green discoloration usually becomes apparent when the pack is opened by the consumer and exposed to air. Oxygen becomes a hydrogen acceptor and hydrogen peroxide is formed; this reacts to form bile pigments etc. by oxidation of porphyrins. Adequate heat processing and prevention of contamination during slicing can prevent problems from occurring.

While conditions may halt microbial growth, the length of time that microorganisms are able to remain viable is also important in assessing their potential contribution to food spoilage and safety. If conditions allow microorganisms to transport sufficient nutrients for maintenance requirements, they may remain viable, even if they cannot grow. If conditions (e.g. temperature abuse) later become favourable for growth, these organisms may begin to grow.

Colour defects (e.g. those listed in Table 10.10) may be wrongly attributed to the packaging system. It can be argued that certain defects can be caused by the selective pressures of the packaging system on the developing microflora, particularly if the product is inadequately processed. Whiteley and D'Souza (1989) reported the appearance of small yellow spots three to four weeks after vacuum packaging of lunche on meats. This was caused by growth of Streptococcus faecium, subspecies casseliflavus. The problem did not recur when the heat process was increased from 71.1°C (20 min) to 71.1°C (30 min). CCFRA recommendations (Shaw, 1995) for the MAP of cured and/or cooked meat products are presented in Table 10.11.

10.8 EtTects of MAP on selected meat products

Owing to differences in meat product processing methods between countries, the effects of MAP on selected meat products only are reported in this section. This subject has also been reviewed by Goodburn and

Tab

le 1

0.10

Spo

ilag

e o

f cu

red

mea

ts b

y la

ctic

aci

d ba

cter

ia

Pro

blem

Sou

ring

Sli

me

prod

ucti

on

Gre

enin

g

Kha

ki/b

row

n pa

tche

s

Bro

wn/

swol

len

pack

Mod

ifie

d fr

om E

gan

et a

l. (1

988)

.

Des

crip

tion

and

cau

se

Gen

eral

mec

hani

sm o

f sp

oila

ge:

exce

ss a

cid

prod

ucti

on o

win

g to

he

avy

bact

eria

lloa

d, e

spec

iall

y in

pa

cked

slic

ed m

eats

Suc

rose

in

eure

may

be

conv

erte

d to

sli

me,

e.g

. by

lac

toba

cilli

Pro

duct

ion

of h

ydro

gen

pero

xide

, w

hich

rea

cts

with

mea

t pi

gmen

t to

for

m a

gre

en p

igm

ent,

eh

ole

glob

in (

unco

oked

),

verd

ohae

moc

hrom

e (c

ooke

d).

Pro

blem

par

ticu

larl

y in

som

e ty

pes

of f

rank

furt

er,

harn

, bo

logn

a

Exc

essi

ve n

itri

te a

ddit

ion

Exc

ess

CO

2 pr

oduc

tion

, cau

sed

by

grow

th o

f pa

rtic

ular

typ

es o

f la

ctic

aci

d ba

cter

ia a

nd o

f ye

ast;

m

ay c

ause

sw

ellin

g or

blo

win

g of

pa

cket

mea

ts

Con

trol

labl

e by

pac

kagi

ng

No

No

Yes

, P

ack

in g

as-i

mpe

rmea

ble

film

s. G

reen

col

our

may

app

ear

du ri

ng s

licin

g w

hen

expo

sed

to

02

' P

robl

em o

ccur

s ty

pica

lly 5

-10

day

s af

ter

proc

essi

ng.

May

or

may

not

be

acco

mpa

nied

by

slim

e pr

oduc

tion

No

Yes

. U

se p

acki

ng f

ilm w

ith h

igh

perm

eabi

lity

to

CO

2

Oth

er p

ossi

ble

rem

edie

s

Rea

sses

s an

d im

prov

e m

anuf

actu

ring

pr

acti

ces.

Red

uce

(a)

stor

age

peri

od;

(b)

Sto

rage

tem

pera

ture

; (c

) am

ount

of

add

ed c

arbo

hydr

ate

Eli

min

ate

sucr

ose

from

eur

e, b

ut t

his

may

cre

ate

mic

robi

al s

afet

y ri

sk

If c

ause

d by

pos

t-co

ok c

onta

min

atio

n,

reas

sess

pro

duct

ion

sequ

ence

and

m

odif

y. I

ffer

men

ted,

use

mic

roco

cci

to h

elp

dest

roy

pero

xide

s

Con

trol

of

brin

e fo

rmul

atio

n an

d in

ject

ion

Red

uce

amou

nt o

f ca

rboh

ydra

te

avai

labl

e fo

r gr

owth

. If

fer

men

ted

prod

uct,

use

alt

erna

tive

sta

rter

cu

ltur

es.

May

cre

ate

mic

robi

al s

afet

y ri

sk


Table 10.11 Information and recommendations for MAP of cured and/or cooked meat products

Major types of cooked, cured and processed meat product



Recommended storage temperature range (OC)

Achievable shelf-lives Cooked and cured meats

In air In modified atmosphere

Salami, pepperoni, etc. In air In modified atmosphere

Recommended gas mixtures Retail Bulk

Typical MAP machines Retail Bulk

Typical types of package Retail (pre-pack) Bulk


Tray (base web)

Bag-in-box and master-pack

Bacons, harns, chopped pork and harn, luncheon meat, cooked, cured processed meat meat slices, ox tongue, corned beef, mortadella sausage, frankfurters, salami, pastrami and pepperoni products, pätes, terrines, potted meats, rillettes

1. Microbial e.g. Brochothrix spp., Acinetobacterl moraxella spp., lactobacilli, Enterobacteriaceae, yeasts and moulds

2. Colour change for cured meats (red/pink to brownl grey/green)

3. Oxidative rancidity

Staphylococcus aureus, Salmonella spp., Listeria monocytogenes and Escherichia coli

o to +3 (NB. Salami, pepperoni, etc. may be ambient stable depending on formulation)

1-3 days 3-7 days

3--6 months 4-8 months

20--35% COzI65--80% N 2

50--75% C02/25-50% N 2

TFFS, HFFS' and PTLF Snorkle-type and vacuum chamber

Tray and lidding film and pillow pack Bag-in-box and master-pack

PETIPVDCILDPE P A/PVDC/LDPE PCIEVOH/EV A UPVClLDPE HDPE EPS/EVOHILDPE PAILDPE PAIEVOHILDPE

Note: 'HFFS - horizontal formIfilIIseal Reprinted with permission (Shaw, 1995).


Halligan (1988), lones (1989), Brody (1989), Ahvenainen (1989) and Ooraikul and Stiles (1991). Reviews of the different me at products and manufacturing methods available include Ranken (1984) and Cross and Overby (1988). The effects on developing microflora have been reviewed by the International Commis si on on Microbiological Specifications for Foods (ICMSF, 1980a,b), Brown (1982), Norris and Pettipher (1987) and Farber (1991).

10.8.1 Bacon

The sodium chloride concentrations used in raw cured meats (c. 2-3% rn/rn) are bacteriostatic to certain microorganisms. Salt-tolerant microorganisms are able to grow under these conditions and include lactobacilli, Sarcina, some Spirillae and Vibrio spp., flavobacteria and micrococci. The growth of enteric pathogens should be controlled by the restrictive effects of sodium chloride, sodium nitrite, pH, competitive microflora and temperature. If the product is unrefrigerated, staphylococci may grow, especially where competitive microflora have been reduced by cooking.

Bacon, whether smoked (generally longer shelf-life) or green (unsmoked), is widely available in vacuum packs or MAP packs. In general, there is no difference in shelf-life obtained between the two systems. The sodium chloride-tolerant microflora that develops in cured meats helps to suppress the growth of Gram-negative proteolytic microorganisms and some food poisoning microorganisms, e.g. Salmonella spp. Others, e.g. Staphylococci, may be capable of growth and toxin production, depending on storage temperature, gaseous environment and product characteristics. Typically, lactic acid flora develop in the lean portions of the bacon and micrococci on the fat (ICMSF, 1980b).

Marginally increased colour shelf-life, which is dependent on product composition (e.g. residual nitrite concentration), O2 permeability of films and residual O2 in the pack, has been found in MAP packs by so me researchers. The main advantage is the easier slice separation obtained in MAP packs. Compositions used in MAP of bacon include 100% N2 , and blends of CO2/N2, typically 30% CO2 with 70% N2 . It has been reported (Anon. , 1984) that Danepak ensures that O2 levels are less than 0.2%, and CO2 levels are ±2% of target levels for MAP of bacon.

The shelf-life of smoked bacon mayaiso be affected by the method of smoking, which is mainly influenced by the level of phenolic compounds (these have antioxidant and antimicrobial effects) generated in the smoke (Toth and Potthast, 1984).

Spencer (1967) reported that growth of Staphylococcus aureus on bacon was retarded by vacuum packaging. He attributed the inhibition to CO2 ,

which rose to >50% during storage, and reported greater CO2 production in unsmoked bacon. A study of the microbial flora showed that the levels of


CO2 could not have been caused by microbial metabolism alone; the rest was attributed to pork muscle metabolism.

10.8.2 Beef jerky

It has been reported that the shelf-life of smoked beef jerky could be doubled by packing it in aluminium cans, pressurized with N2 (Anon. , 1988). Liquid N2 was injected into the headspace of the filled can prior to lidding. Vaporization of the N2 helped to pressurize the can, allowing the use of thinner side walls, and removed any O2 in the headspace. The process was also found to inhibit can corrosion during storage.

Jerky dried to aw > 0.80 is considered to be more organoleptically acceptable. However, such an aw value may favour mould growth, and O2

scavengers may be required. Without O2 scavengers, Aspergillus glaucus typically spoils biltong, unless the aw is lower than 0.70.

10.8.3 Cooked beef roasts

McDaniel et al. (1984) examined the effects of packaging beef roasts (1-1.5 kg; cooked to a centre temperature of 60°C) in vacuum packs, 100% CO2 and 15% CO2/30% O2/55% N2 • Roasts were held at +4°C for up to 21 days. Mesophilic and psychrotrophic counts were similar for all packs after seven day.s. After 21 days, beef packed in vacuum packs had significantly (p < 0.05) greater numbers of mesophilic and psychrotrophic counts than beef packed in 100% CO2 . Microbial counts on beef packed in 15% CO2/

30% O2/55% N2 were higher than those on beef packed in 100% CO2 , but lower than those for beef packed in vacuum packs.

However, both gas-packed steaks were considered to be organoleptically unacceptable after 14 days of storage, whereas vacuum-packed steaks were still considered to be acceptable after 21 days of storage. Sensory evaluation was carried out on steaks that had been held overnight aerobically, reheated in a microwave for 1 min and kept warm under a heat lamp until evaluation. It is likely that such evaluation procedure influenced the results reported. This highlights the importance of standardizing and using realistic procedures when the sensory effects of treatments are being assessed.

Hintlian and Hotchkiss (1987) examined the effects of MAP of cooked, sliced roast beef in the following atmospheres: 75% CO2/25% N2; 75% CO2/2% O2/23% N2 ; 75% CO2/5% O2/20% N2 ; 75% CO2/1O% O2/15% N2 ; and 75% CO2/25% 02' They found that, of the atmospheres examined, 75% CO2/1O% O2/15% N2 was the most effective in inhibiting the growth of P. fragi, Salmonella typhimurium, S. aureus, and C. perfringens. At abuse temperatures of26.7°C, growth of C. perfringens was no longer inhibited in any of the atmospheres examined. At 12.8°C, growth


of C. perjringens was inhibited only in atmospheres containing moderate O2 concentrations. C. perjringens also grew in air. Oxygen consumption by P. jragi helped to create an aerobic conditions; O2 levels had fallen to <1 % on day 13 in air packs.

No an aerobic microorganisms were recovered from uninoculated beef. Counts of moulds in airpacked cooked beef reached 106 colony forming units (cfu) per g after 42 days but none were recovered from MAP sampies. These results support previous findings that high levels of CO2 can inhibit the growth of moulds. Counts of total aerobes correlated weIl with mould counts. The moulds that were recovered probably resulted from the contamination of the product by spores after cooking and before packing.

Sensory analysis data suggested that MAP cooked beef was inferior to fresh roast beef. Flavour changes in MAP beef were probably caused by oxidative rancidity (e.g. warmed-over flavour) rat her than microbiological changes.

These results emphasize the fact that, generally , MAP cannot be used effectively at room temperatures and cannot be used as a substitute for refrigeration. MAP can only be used effectively at room temperature when the meat product can already be stored safely at ambient temperatures.

Carr and Marchello (1986) also investigated the use of MAP (15% CO2/ 40% O2/45% N2) and vacuum packaging for cooked sliced beef. At 6°C and 10°C, psychrotrophic bacteria (total count on plate count ag ar (PCA) at 4°C for 12 days) were inhibited in vacuum packs but not in MAP. At 2°C, the converse occurred, highlighting the increased effect of CO2 at lower temperatures. Growth of mesophiles, thermophiles and moulds was unaffected by the type of packing. However, cooked beef slices stored under the gas mixture had off-colours after only four days, whereas the vacuum-packed slices remained purple-pink in colour for up to 12 days before slight fading occurred. Off-odours were detected after seven days in the modified atmosphere packs at 10°C, while the vacuum-packed products had only slight odours after 21 days. It seems likely that levels above 15% CO2 in the gas mixture are needed to extend the shelflife of cooked beef effectively.

In a later study, Carr and Marchello (1987) compared other combinations of CO2, N2 and O2 (15% CO2/40% O2/45% N2; 15% CO2/20% O2/65% N2; 15% CO2/10% O2/75% N2) with vacuum packaging. Growth of psychrotrophic aerobes was not detected after 6 days of storage at 4 ± 0.5°C. After nine days, growth occurred on slices stored under 02-containing atmospheres. Rapid growth occurred on slices stored under 15% COz/20% O2/65% N2, indicating that the atmospheric levels of O2 were more conducive to growth than 10% or 40% 02' Growth of the psychrotrophic flora occurred in vacuum packs after 12 days. As psychrotrophs should not survive the cooking process of this product, the growth observed must have resulted from recontamination of the product,


again indicating the importance of good control during repacking operations. Guidelines on good manufacturing and handling of MAP food products are available (Day, 1992).

Sensory analysis data indicated that off-odours developed in MAP cooked beef after 15 days of storage, and after 12 days the beef surface became dry. Meat colour did not vary significantly between packaging treatments, although there was some fading of the red colour.

10.8.4 Ground beef patties

Bentley et al. (1989) examined the shelf-life of ground beef patties that were either vacuum packed or gas flushed with 100% N2 or 100% e02. The 100% e02-packed patties were perceived as being better by sensory panellists. Higher drip losses were found in vacuum-packed sampies (c. 4.8% (rn/rn), compared with 3.5% (rn/rn) for 100% e02 and 2.9% (mim) for 100% N2). All sampies were considered to be unacceptable after 21 days of storage at +2°e, and plate counts greater than 108 cfu g-I were reported. On the basis of sensory results and appearance (improved colour maintenance during storage), flushing with 100% e02 was considered to be the best treatment. When flushed with e02, patties were still considered to be acceptable for up to seven days at temperatures below +2°e (based on sensory and microbiological results). Residual O2 levels were not reported.

10.8.5 British fresh sausages

The addition of sulphite or metabisulphite to give 450 ppm sulphur dioxide (S02) in British fresh sausages inhibits Gram-negative bacteria by a combination of S02 (e.g. unbound bisulphite and sulphite), low temperature and low O2 tension in the meat emulsion, thus helping to extend the shelf-life of sausages.

The S02 tolerance of the components of the microflora varies, with yeasts being most tolerant, followed by B. thermosphacta and then lactobacilli. The essential requirement for addition of sulphite to sausage mixes is illustrated by Stannard et al. (1988), who found that 200 ppm in pork sausages delayed spoilage at 2°C by 14-16 days. This effect was much less marked at high er temperatures (5°C, 10°C) and also in beef sausages. The concentration of S02 in sausages has a marked effect on the microbial association and final spoilage flora (Stannard et al., 1988). At low levels of sulphite, B. thermosphacta is the major spoilage organism, but at high levels of sulphite, yeasts and lactic acid bacteria predominate. Banks and Board (1982) clearly illustrated the preservative role of sulphite in controlling the growth of Enterobacteriaceae and Salmonella spp.

Sulphite is microbicidal or microbistatic, having the greatest effect at pH


values below 4 (Le. as free S02) and is most effective against Gram-negative bacteria at the pH values of meat. The mode of action of sulphite is probably through attack on a number of components of the cell, including the cell membrane fatty acids, enzymes and nucleic acids. Gram-negative bacteria are the most susceptible, followed by Gram-positive organisms and yeasts. Therefore, although Gram-negative oxidative and fermentative bacteria are frequently the major components of the flora initially, in the majority of cases, shortly after manufacture of sausages, Gram-positive organisms predominate and develop into the characteristic flora dominated by B. thermosphacta. In some cases, other characteristic flora develop (Dowdell and Board, 1971), e.g. Gram-positive organisms other than B. thermosphacta, or yeasts or Gram-negative bacteria predominating.

It has been suggested that because the flora of British fresh sausages is predominantly facultative Gram-positive bacteria and yeasts, vacuum packing would not offer an extension in shelf-life. However, Adams et al. (1987) have shown that this is not the case. In their study, British fresh sausages were vacuum-packed with low Oz-permeability film or were clingwrapped. Cling-wrapped sausages showed a rapid increase in total count to 108 cfu g-l within five days at 6°C. The major spoilage organism was found to be B. thermosphacta. After nine days, yeasty odours were detected, visible surface slime was observed and the total yeast count was 107 cfu g-l. In contrast, the vacuum-packed sausages retained a good odour and appearance for at least 20 days. Total counts, B. thermosphacta and yeast counts after nine days were 1.5-2.0 10glO cycles lower than those in aerobically stored sampies. Lactic acid bacteria became the dominant spoilage organisms, with the number of yeasts remaining at about 103 cfu g-l throughout spoilage. After 30 days' storage, sour odours were detected. Counts of S. aureus and Enterobacteriaceae remained at low levels (102 cfu g-l) throughout storage under both conditions; clostridia and pseudomonads were not detected.

The fate of the preservative sulphite was monitored during storage. The levels of free sulphite dropped rapidly in aerobically stored sausages and it was undetectable after seven days, just prior to spoilage. In vacuumpacked sausages, higher free sulphite levels were maintained for longer; 40 ppm was detected at 20 days. After 27 days, when spoilage occurred, free sulphite was not detected. Such losses in free sulphite were attributed to the action of sulphite-binding yeasts and as the numbers of such yeasts in vacuum packs remained low, sulphite levels were maintained for Ion ger .

Kenny and Corcoran (1986) reported that the organoleptically acceptable shelf-life of pork sausages containing 400 ppm S02 stored at +5°C could be increased from eight days (aerobic) to 10 days by packaging in 50% CO2 + 50% 02. If S02 was removed, the sausages remained acceptable for three days (aerobic) or six days if gas packed. No microbiological or gaspermeability data were reported.


Legarreta et al. (1988) examined the effects of 15 combinations of CO2/

02/N2 on Canadian breakfast sausage. They concluded that packing in 60-80% CO2 plus 20-40% N2 resulted in the greatest improvement in sheIf-life. Increased colour deterioration was found when sausages were packed in atmospheres containing more than 40% 02.

10.8.6 Cooked meat loaves

Ahvenainen et al. (1986) examined the effect of gas composition (100% N2

and 20% CO2/80% N2) and aerobic packaging on the shelflife of meat loaves. At 5°C, the growth rate of the aerobic mesophilic flora and of lactobacilIi was greatest in me at loaves packed under 100% N2 (106 cfu g-l after 15 days) and least in those under 20% O2/80% N2 (104 du g-l after 15 days). LactobacilIi were the predominant spoilage organisms. Yeasts and moulds were not detected in gas packs but were observed in aerobic packs (surface moulds and yeasts (103 cfu g-I after 14 days». The air-packed meat loaves were organoieptically unacceptable after 14 days, owing to an 'old-spoilt' taste, whereas the gas-packed product was stilI considered to be acceptable after 35 days' storage.

10.8.7 Frankfurters

Simard et al. (1983a) reported that the shelf-life of vacuum-packed frankfurters could be increased by gas flushing with 100% N2 . No headspace volumes were given. Canadian frankfurters were either vacuumpacked or back-flushed with 100% N2 in laminated pouches (02 permeability 8 cm-3 m-2 per day at 4°C and 100% RH; CO2 permeability 124 cm3 m-2

per day at 25°C and water permeability 18.6 cm3 m-2 per day at 37°C and 100% RH). Vacuum-packed frankfurters were no longer considered to be acceptable after seven days at 7°C or 21 days at -4°C because of adverse colour changes, i.e. green and brown discoloration. Frankfurters flushed with N2 were stilI considered to be organoleptically acceptable after 35 days at 7°C. No statistically significant differences were found among the numbers of lactobaciIli, psychrotrophic and an aerobic bacteria in vacuumor N2-packed frankfurters and none between packs stored in the light and dark (Simard etal., 1983b). Some workers have found that light appears to affect microbial growth rates; Marriot et al. (1967) found higher microbial counts when meat was displayed in the light at -1°C than in dark storage. However, this is probably because of the greenhouse lighting effect caused by radiation he at from the light. Marriott et al. (1967) did not report surface temperatures of the meat. Flushing with N2 was found to be particularly effective in inhibiting mould and yeast growth, which conflicts with the findings of other workers.

Lack of agreement on the apparent indirect inhibitory effects of N2 on


microbial growth in me at products has been attributed by Christopher et al. (1979) to:

• the nature of the meat products (e.g. processing conditions, product composition);

• the composition of media and conditions of enumeration; • the sampling methods; and • the purity of gases.

Some differences are probably also caused by gas permeability of packaging films, and initial microflora and growth phase at time of packaging (Gill and Tan, 1980). However, residual O2 levels in the pack after packaging should also be considered.

High O2 levels were found in the vacuum packs (c. 13-19% (rn/rn) one day after packing); this probably had a significant effect on the results. The O2 levels in Nz-flushed packs were c. 0--6% on the day of packaging and in vacuum packs examined after 49 days they were c. 13-19%. Levels of c. 12-23% O2 were found in N2 packs at the end of storage; this also conflicts with the findings of other research workers. No CO2 was detected in vacuum packs at the end of storage. CO2 levels of c. 5-8% were found in Nz-flushed packs stored at 7°C.

Egan and Roberts (1987) reported that gas flushing of frankfurters with 100% CO2 could prevent the appearance of milky fluids caused by yeast growth, as weil as preventing product distortion caused by vacuum packaging. Rawlinson (1971) reported that 100% CO2 had an inhibitory effect compared with 100% N2 on growth of bacteria in cooked cured meats stored at 4°C. No inhibitory effect was observed when packs were stored at room temperature, again emphasizing the importance of good temperature contro!.

10.8.8 Harn

In recent years, as a result of consumer demand and health recommendations, the level of sodium chloride in a variety of meat products has been reduced by about 1 % (rn/rn), from about 3% in the mid-1980s to 2% or less (Sofos, 1985). Consequently, lower salt levels have been used in products such as harn. There has been limited research into the microbiological implications of reducing salt levels. A survey of sodium chloride levels in retail cooked harns in the UK market showed a range of 1.7% to 3.9% (rn/rn) (aw 0.975-0.965); harns with reduced sodium chloride contents were not analysed (Knight and Wood, 1990).

Sodium chloride is added to meat products for several reasons:

• to act as a preservative, lowering aw and helping to retard growth of spoilage organisms, and to inhibit C. botulinurn toxin production when used in conjunction with nitrite, heat and acidity;


• to disperse myofibrillar proteins, mainly myosin and actin, which, when heatset, act as an adhesive between meat pieces, controlling cooking loss; and

• to add a salt taste.

Knight and Wood (1990) found wide variation within individual slices of harn, indicating that, if average salt levels are reduced too far, harn containing insufficient salt for microbial safety may be produced. Other cured products have also been demonstrated to have variable salt levels. Barke et al. (1985) reported that sodium chloride levels in individual slices of bacon from the same loin ranged from 1.1 to 5.2% (mim). However, Knight and Wood (1990) reported that actual product salt levels were generally higher than calculated levels, i.e. where the sodium chloride concentration in the final product was estimated from the amount of brine injected. Variation in slices was attributed to:

• variation in injection levels within the process; and • natural variation in the ability of muscles from the same and different

animals to retain salt and water; and • insufficient allowance for salt and water losses during cooking and for

water losses during slicing prior to packaging.

Barker and Woods (1990) concluded that reduction in sodium chloride contents in vacuum-packed harn led to an increased growth potential for food poisoning organisms. This occurred either by increasing the growth rate at a given temperature or by allowing growth at a lower temperature. Decreasing the sodium chloride content of vacuum-packed harns from 3% (rn/rn) to 1 % (mim) resulted in an increase in the growth rate of the foodpoisoning organisms S. typhimurium, Yersinia enterocolitica and L. monocytogenes. They recommended that food manufacturers should carefully consider the consequences of changes in recipes of their products, particularly if that change arose from consumer or media pressure to reduce the level of food additives known to have antimicrobial properties.

Stegeman et al. (1988) examined the thermal resistance of L. monocytogenes inoculated into harn. Their results indicated that standard thermal treatments were more than sufficient to produce listeria-free harn. However, once removed from the protective packaging, all cooked readyto-eat meats can become contaminated during slicing and repackaging operations. Glass and Doyle (1989) found that L. monocytogenes attained populations of 105_106 cfu g-l on organoleptically acceptable harn (pH 6.3-6.5) after 4 weeks of storage at 4.4°C, thus indicating that manufacturers cannot rely on the combination of vacuum packaging and refrigeration to prevent growth of Listeria spp. in harn. Flushing with CO2 may provide a safer alternative (Silliker, 1981); however, this does not appear to have been evaluated yet. Lower product pH would also be expected to help prevent growth of L. monocytogenes. The importance of adhering to good


'common sense' manufacturing practices during slicing and packaging operations of cooked products cannot be over-emphasized.

Anjaneyulu and Smidt (1986) examined the effects of gas flushing with 02, CO2 and nitrous oxide (N20) and vacuum-packaging harn using gas compositions of 35% CO2/65% N2; 35% CO2/65% N20; 100% CO2; 100% N20, and 95% CO2/5% N20. Gas flushing with 100% CO2 was found to be the most effective procedure examined; harn slices were still considered to be acceptable on day 30 when stored at 3--5°C. Anjaneyulu and Smidt found no increase in microbial numbers in the harn stored in 100% CO2 ; this conflicts with the findings of other workers (Ahvenainen, 1989). Composition of the harn, details of the heat process and organoleptic data were not reported. Flushing with 100% N20 did not inhibit microbial growth and it adversely affected the colour of the harn. Andersen et al. (1990) found that CO2 flushing and packaging und er a slight overpressure prevented discoloration of vacuum-packed sliced harn, which was caused by photo-oxidation of nitrosomyoglobin pigments during the first 24 hours of display in illuminated cabinets. Colour stability during storage was also improved. Photo-oxidation was not prevented when the harn was either vacuum-packed or CO2 flushed and vacuum-packed without overpressure, unless the packs were stored in the dark for four days prior to illuminated display. This was despite the presence of 200 ppm sodium nitrite and 350 ppm sodium ascorbate in the packed harn. Ranken (1984) reported that 15-20 ppm residual sodium nitrite was sufficient to ensure colour stability during storage of cooked harn (40-70 ppm for bacon). However, variability in cure distribution, brine composition (e.g. addition of ascorbate), residual 02, and O2 permeability of the film must also be taken into account when considering colour stability.

Andersen et al. (1990) stated that the efficiency of O2 removal during packing was thought to be critical to the colour stability of the product. Alternative methods for prevention of photo-oxidation include packing in metallized films or incorporating UV filters into the laminate. However, if UV filters are incorporated, it is important to ensure that they do not adversely affect the O2 permeability of the film. Some manufacturers have also investigated the incorporation of O2 scavengers.

Andersen and Rasmussen (1992) reported that photo-oxidation of nitric oxide pigments in sliced, pasteurized harn could be prevented by packaging the harn in pouches (02 transmission rate of 2 cm3 m-2 atm-1 per day) containing manually inserted O2 absorbers (Ageless SS-50). Vacuum packaging in similar films (99% vacuum) was found to be equally effective.

10.8.9 Meat pies

Only a limited amount of research on MAP of meat pies has been reported. In addition to the problem of the development of microorganisms


such as bacteria, yeasts and moulds, the shelf-life of this dass of product is limited by moisture migration into the pastry, the development of rancidity from fat oxidation and, in starchy products, drying caused by retrogradation (restructuring of starch molecules). Fat oxidation can be minimized by omitting O2 from the gas mixture, while retrogradation can be masked by adding shortenings. If the cooked food is encased with pastry, e.g. sausage rolls, it is recommended that residual O2 levels be below 0.5% to prevent pastry staling (Goodburn and Halligan, 1988).

10.8.10 Pastrami

Laleye et al. (1984a,b) reported that there were no beneficial effects when pastrami (pH 6.5) was flushed with 100% N2 , in comparison with vacuumpacked pastrami. Storage in the dark was also found to have no significant influence on physicochemical and sensory changes that occurred. They conduded that there was little justification in recommending N2 flushing as an alternative to vacuum-packaging.

10.8.11 Wieners in natural casings

Lawlis and Fuller (1990) reported a 30 day shelf-life when wieners in natural casings were flushed with 70% N2/30% CO2 if residual O2 levels were maintained below 0.4%.

10.8.12 Poultry products

While the effects of vacuum and MAP packaging on poultry (mainly chicken) and poultry portions has been extensively investigated (Coyne, 1933; Haines, 1933; Ogilvy and Ayres, 1951; Shrimpton and Barnes, 1960; Wabeck et al., 1968; Thomson and Risse, 1971; Sander and Soo, 1978; Igbinedion et al., 1981; Jones et al., 1982; Mead et al., 1983; Gray et al., 1984; Humphreys, 1985; Hotchkiss et al., 1985), the effects on poultry products have not.

Young et al. (1987) packed cooked Chicken a la King and fried chicken drumsticks in ding film vacuum packs and 70% CO2/30% N2 packs. They found that the total microbial count remained acceptable in vacuum and MAP packs (below 105 cfu g-l) for up to 15 days at 0 ± 1°C for both products. When packed in ding film, the drumsticks spoiled within 15 days and the Chicken a la King within seven days. The authors suggested that vacuum or MAP packaging could be used to extend the shelf-life of cookchill products and to ensure safety and quality during chilled storage. However, the organoleptic quality of these products was not examined, and no inoculation studies with food pathogens were carried out.

In a subsequent study, Young et al. (1988) examined the effects of the


above gas compositions on sensory qualities and found that storage had a significantly (p < 0.05) deleterious effect on all the sensory attributes of chicken drumsticks. The appearance of the Chicken a la King dish was also significantly affected. Similar effects on sensory attributes were obtained for vacuum and MAP packs. Vacuum and MAP packs maintained acceptable sensory attributes for 11-14 days at 0-3°C. The sensory characteristics of the products packed in cling film were no longer considered to be acceptable after four days. Further, MAP was considered to increase the rate of certain degradative changes. The colour of peas (in Chicken a la King) was found to change from pea-green to olive/khaki at a significantly (p < 0.05) greater rate in MAP compared with vacuum packs. The appearance of the Chicken a la King also appeared to be significantly more 'broken up' when compared with vacuum-packed or cling-wrapped sampies. The authors concluded that the use of MAP could be limited in certain multi-component foods because of adverse effects on certain product components, such as the colour changes observed with peas. Such problems could be overcome by either excluding the shelf-life-limiting component or packing it separately in the pack for mixing by the consumer on opening.

e. perfringens occurs commonly in poultry meat and is a common cause of food poisoning. Since MAP provides more favourable conditions for the growth of this organism, the safety of such products is dependent (as is the case with most other MAP products) on correct temperature contro!. In raw poultry, the growth of other organisms should result in very rapid and obvious spoilage if temperature abuse does occur. However, in cooked products, where other microorganisms have been significantly reduced or destroyed during processing, surviving spores of e. perfringens may grow if there is inadequate temperature control, sometimes with no overt sign of spoilage to warn the consumer.

The microbiological and organoleptic effects of packaging flashfried breaded poultry in 100% CO2 have been investigated at Leatherhead Food Research Association (UK) (unpublished data).

10.8.13 Cook-chill systems

Two systems developed as complete, cooking, packing and chilling systems are the Capkold system (Groen) and the conduction chiller (Williams Refrigeration). The Capkold system consists of a steam-jacketed kettle for cooking batches of food at pasteurization temperatures and a pump for passing the cooked food into Cryovac casings, which are sealed under vacuum and then cooled in a water-based tumbier chiller. Joints of meat and poultry mayaiso be cooked using this system but are first placed in the Cryovac casings and then slow-cooked in hot water prior to chilling in iced water. It has been claimed that shelf-lives of at least 45 days can be


achieved using this system (Anon., 1986); more commonly, product shelflives of 21 days are given.

10.8.14 Sous-vide cook-chill system

The system of sous-vide cooking was originally developed in France in the 1970s. The process involves the pre-packing of raw or par-cooked food in plastic bags or pouches, which are then sealed under vacuum. Some packs are gas flushed with a mixture of CO2 (70%) and N2 (30%) (Perron, 1988) prior to vacuum packing to prevent delicate items such as seafood from becoming crushed. The sealed food is cooked to pasteurization temperatures, and rapidly chilled to between 0 and 3°C.

The food is then stored at 0-3°C before being reheated to at least 70°C for 2 min prior to consumption. Under Department of Health (1989) guidelines for cook-chill, the maximum recommended shelf-life for such products is five days. However, these guidelines only refer to cook-chill products intended for the catering market, and the possibility exists for such products to have longer permitted shelf-lives if packaged under vacuum (sous-vide) or MAP by food manufacturers for the retail market (Day, 1992).

Concern has been expressed about the safety of the process. The pasteurization process given to foods in sous-vide cooking should destroy vegetative cells (bacteria, moulds and yeasts) but bacterial spores will survive this process. Therefore, in this system (and in any other cook-chill system) it is imperative that the food is cooked at the right temperature for an appropriate length of time to achieve pasteurization at the centre of the food, and thus to ensure product safety with respect to vegetative forms of pathogenic microorganisms. Of particular concern are the spore-forming bacteria C. botulinum type E and proteolytic B strains, which are anaerobes and able to grow at chill temperatures. Therefore, very high quality raw ingredients must be used for sous-vide cooking and very strict hygiene must be enforced throughout preparation of the food to minimize product contamination before cooking; rapid chilling and chilled storage (I-3°C) are critical to prevent germination of these spores and growth of other bacteria, if present, in the package. Several guideline documents concerning food manufacturing practices for the preparation of sous-vide products are available (Leadbetter, 1989).

10.9 Safety aspects of MAP

With increasing awareness of food safety issues, there has been concern over the suitability of MAP for meats because of the possibility that C. botulinum would be able to grow and produce toxin. Being an obligate


anaerobe, MAP conditions may favour this organism and non-proteolytic strains are known to produce toxin at temperatures as low as 3.3°C (Schmidt et al., 1961). Furthermore, it might be assumed that any temperature abuse would te nd to increase the risk, while germination of C. botulinum spores may be stimulated by CO2 in the pack (Wynne and Foster, 1948). The main question, however, is whether CO2 inhibition of the normal aerobic spoilage organisms and development of a slowergrowing microflora could leave the consumer without any reliable indication that the meat had become unsafe to eat.

In comparison with pre-cooked products that require only minimal reheating e.g. breaded and fried chicken, use of MAP for raw meats is seen as less hazardous (Hotchkiss, 1988) because of the need for proper cooking before consumption, and hence destruction of any botulinum toxin (Licciardello et al., 1967). The possibility of improving safety by incorporating low levels of O2 in the pack to prevent growth of anaerobes does not appear to be feasible for raw meats. Small amounts of O2 would soon be taken up by residual respiration in the meat and, in any case, would not necessarily prevent toxin formation by C. botulinum. Psychotrophic strains of the organism can grow and form toxin in the presence of up to 10% 02' when other conditions are ideal (Miller, 1988; cited by Connor et al., 1989). The other major food poisoning anaerobe, C. perfringens, is more aerotolerant than C. botulinum and growth can occur in foods that are not appreciably an aerobic if temperature abuse occurs (Genigeorgis, 1985). The effects of modified atmosphere conditions on growth of spoilage bacteria diminishes outside the temperature range used for proper chill storage (0-40 C) and, with meats held at ambient temperature, toxin formation by C. botulinum is little affected by gaseous environment. For example, Silliker and Wolfe (1980) showed that high CO2 atmospheres had no significant effect on growth of C. botulinum at 27°C on pork. More important is the relationship between the product becoming toxic and the onset of spoilage. Vacuum-packed raw beef, inoculated with spores of types A and B strains and stored at 25°C, was toxic after six days but, by this time, the meat had already spoiled (Hauschild et al., 1985). At lower temperatures, 100% CO2 has been found to delay growth of both C. botulinum and C. perfringens (Doyle, 1983; G.C. Mead, unpublished data). Also, CO2 concentrations of 45-75% delayed toxin production by C. botulinum in pork stored at 15°C (Lambert et al., 1991a). As with clostridia, experience involving other food-borne pathogens suggests that any hazard is unlikely to be increased by modified atmosphere storage of meats. Most strains of Campylobacter spp. are unable to multiply at temperatures below 30°C, but survival of this microaerophilic organism under modified atmosphere conditions could be favoured by the virtual absence of 02. However, Wesley and Stadelman (1985) studied the behaviour of C. jejuni at 4°C on broilers in 02-


permeable packs and in packs containing 100% CO2 • The CO2-enriched atmosphere had no detectable effect on the organism.

In ground beef inoculated with six strains of Salmonella and held at 10°C for seven days, Silliker and Wolfe (1980) observed a decline in viability under 60% CO2/25% O2/15% N2 , but not in air. After 10 days, there was a 1000-fold difference in count between the two storage atmospheres. Another food-borne pathogen that is sensitive to high concentrations of CO2 is S. aureus. In this case, there appears to be a synergism between CO2 concentration and temperature in controlling growth of the organism in MAP beef and chicken products (Hintlian and Hotchkiss, 1986). Salmonella spp., too, showed evidence of inhibition under CO2 .

Recently, attention has centred on the more psychrotrophic pathogens, particularly L. monocytogenes and Y. enterocolitica, both of which are capable of growth at 0-3°C. While Gill and Reichel (1989) showed that L. monocytogenes can grow between 0 and 10°C in beef of high pH under CO2 , or in vacuum packs, other studies have shown that some conditions are inhibitory. The growth potential of L. monocytogenes on meats appears to be influenced both by the type of meat and by its pH value, so that growth would occur more readily and possibly at a lower temperature on meat with high pH values. Bolder et al. (1991) observed better growth of L. monocytogenes at 0-4°C on chicken thigh portions with skin than on breast portions without skin. In both cases, however, growth was markedly inhibited in MAP containing 100% CO2 , while 20% CO2 or vacuum packaging had much less effect (Table 10.12). With skinless breast meat (pH 5.8), Hart et al. (1991) found that L. monocytogenes failed to grow at 1°C, even without MAP. At 6°C, some growth occurred under aerobic conditions before spoilage was evident, but not under 100% CO2 .

Cooking chicken to an internal temperature of 70°C for 2 min should significantly reduce the population of L. monocytogenes (Church and Parsons, 1995). Therefore, cooking with storage around 3°C and a

Table 10.12 Growth of Listeria monocytogenes on chicken portionsa stored at 4°C for seven days in packs containing different gaseous atmospheres

Packaging treatment

O2 permeable Vacuum pack 100% CO2

20% CO2 + N2

aInoculum 3.9 in all cases. From Bolder et al. (1991).

Counts of L. monocytogenes (loglO cfu g-l)

Breast portions

5.1 4.7 3.9 4.4

Leg portions

6.2 6.2 4.7 6.0


relatively short shelf-life should reduce the risk of listeriosis to an acceptable level.

Concern over possible growth of Y. enterocolitica in MAP meats appears to have originated from the study of Hanna et al. (1976), which showed that organisms resembling this species reached high levels during chill storage of vacuum-packed beef and lamb. Y. enterocolitica is relatively common on meats of all kinds, including poultry, but the pig is regarded as the most important source of serotypes that are pathogenic to humans, and strains isolated from meats other than pork are usually of the 'environmental' (i.e. non-pathogenic) type. Although yersinias grow readily in vacuum packs and in packs containing about 20% CO2 , higher concentrations inhibit their growth on chill-stored pork (Enfors et al., 1979). In an unpublished study, J.N. Karitu and G.C. Mead found that storage ofpork at 1°C under 50% or 100% CO2 prevented any marked growth of Y. enterocolitica, although the inhibitory effect was much less evident when the meat was held at 10°C.

In conclusion, use of MAP for raw meats does not appear to increase the hazard from food-borne pathogens, especially when packs are held under continuous and effective chilI conditions and use-by dates are followed. On the contrary, evidence suggests that CO2 alone, or at high concentration in combination with other gases, will significantly retard growth of foodborne pathogens that would otherwise multiply during chill storage. However, Genigeorgis (1985) suggests that packaging of muscle foods under low partial pressure of O2 should not extend to retail sale. This is because of the potential hazard from C. botulinum and the risk of temperature abuse by consumers. Nevertheless, the real risk of botulism from such MAP products remains largely unresolved.

10.10 The future

Some exciting opportunities and challenges face the meat product industry. Future developments are likely to be aimed at developing Orfree packaging. Success will be dependent on successful O2 removal during packaging, and maintenance of an Orfree environment around the product during storage. Caution must, however, be exercised for perishable products, since the potential growth of pathogens under these conditions has not been thoroughly researched.

Consumer resistance to excessive and 'environmentally unfriendly' packaging and demand for biodegradable packaging is likely to increase. Education of consumers by the food industry regarding the role and benefits of packaging will be vital in order to prevent legislative pressures being forced upon them that, while environmentally friendly, may have dire consequences for product safety and shelf-life. The correct balance between safety, shelf-life and the environment must be achieved.


It is evident from recent literature that, in the future, MAP methods such as the Captech system are likely to be increasingly used for meat products. Indeed, this system is already in commercial use for the shipment of lamb and for shelf-ready lamb and beef cuts. Tray systems using the same principle, such as the Dupont Survac system, are likely to be used at the retail store level. Improvements in the plastic films used in these systems, particularly in reducing COz/02 permeability, mayaiso further extend the shelf-life of products packaged in this way.

Preliminary commercial trials with Captech for me at products have demonstrated that shelf-lives of 8--15 tim es greater than those of meat products stored in air"can be obtained. Roast beef (whoie roast) was still considered to be acceptable up to 24 weeks stored at -1°C or 14 weeks at +2°C. A similar shelf-life was obtained with sliced stuffed lamb. Cooked chilled meals (meat, sauce, green vegetables and potatoes) were still considered to be acceptable after 12 weeks at both temperatures (Gill, 1990).

Commercial availability of so-called 'smart films', which are capable of major changes in permeability under abuse conditions to favour growth of spoilage microorganisms, and films capable of absorbing gases and/or aromas is also likely to expand the range of meat products currently packed under MAP. Increased interest in the application of edible films is also likely to occur.

In the Verfrais system, the CO2 is released from a sachet, but enzyme systems that produce CO2 and scavenge O2 are also a possibility. They may be present in the pack itself or they may be incorporated into the packaging film. Oxygen scavengers (e.g. 'Ageless') are already in wide commercial use in some countries, e.g. Japan. Novel approaches to MAP that may develop in the future are the active packaging technologies, which include the controlled release of ethanol or other preservatives, e.g. chlorine dioxide and zeolite, to inhibit microbial growth on the product surface, and moisture absorbers, e.g. Pitichit sheet (Showa denko), Aqua catch (Japan Catalytic) .

A potential microbial indicator of time-temperature abuse could arise from current work at the Leatherhead Food Research Association (UK) based on lactic acid bacteria. These organisms are GRAS (Generally Regarded as Safe) and many have growth-temperature characteristics that would give rise to acidic/sour odours and flavours in vacuum packs or MAP on abuse. Additionally, by judicious choice of strain, lactic acid bacteria applied to foods before packaging could give an increased protection against other types of food spoilage and food poisoning risk. Tanaka et al. (1985) reported an antibotulinal effect when bacon with reduced sodium nitrite levels and containing sucrose (0.7%) (temperature abused at about 27°C) was inoculated with lactic acid bacteria.

C. per/ringens and other pathogens have been shown to be inhibited by


bacteriocins (antibacterial compounds) produced by lactic acid bacteria. Lactic acid bacteria isolated from vacuum-packed meat products have been shown to inhibit L. monocytogenes and some Enterobacteriaceae. Coinoculation with suitable lactic acid bacteria and a fermentable carbohydrate source may provide a means of ensuring me at product safety without adversely affecting organoleptic quality. Results to date using lactic acid bacteria have, however, been mixed and problems of excessive acid production, greening, and slime formation have been encountered (Lambert et al., 1991b).

What is considered safe today may be considered to be unsafe tomorrow. Improved methods of microbial isolation, identification, reporting and tracing of food poisoning have resulted in the realization that an increasingly large number of bacteria are capable of surviving and/or growing in chilled products and of causing food poisoning.

It must be stressed that to date MAP has an excellent safety record and, with a proper understanding and control of the technology involved, will continue to do so. Whenever new applications are proposed, they must be carefully considered and properly researched before commercial introduction. Consumer education concerning product perishability and prompt refrigeration of purchased product is also vital to the continuing expansion and success of this technology.

Acknowledgement

The editor acknowledges her liberal use of the meat products and meat and poultry chapters from the 1993 edition ably written by P.N. Church, D.E. Hood and G.c. Mead. The two chapters were condensed into one for this edition and updated.

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Index

Page numbers appearing in bold refer to figures and page numbers appearing in italic refer to tables.

Absorbents carbon dioxide 5, 173 ethylene 6 oxygen 4, 151, 176, 187, 258

AcinetobacterlMoraxella 10, 198,241,254 Aeromonas hydrophilia 11 allyl-isothiocyanate (AlT) 144 Alteromonas putrefaciens, see Shewanella Aspergillus glaucus 268 Atmosphere

controlled 1 modified 1 passive 4

Bacillus cereus 11 Bacillus licheniformis 149 Bacillus subtilis 138, 149 Bacon 267 Bakery goods

history 17 spoilage of 137, 149

Beef jerky 268 Beef patties 270 Beef roasts 268 Beverages 185 Breaded and batter-coated products 183 British fresh sausages 270 Brocothrix thermosphacta 207, 241, 243,

254, 270

Cheese hard 159 mould-ripened and soft 165 unripened 167

Clostridium botulinum 8, 11,30, 129, 167, 189, 202, 204, 213, 273, 278, 279, 281

Clostridium perfringens 204, 268, 277, 279, 282

Clostridium sporogenes 168 Coatings, anti-fog 90, 98 Coffee 171

carbon dioxide production in 171 Controlled atmosphere packaging (CAP) 1 Controlled atmosphere storage (CAS) 2 Cottage cheese, effect of carbon dioxide

167

Critical control point 106 Cryovac 15, 277

Decision tree 107 Delicatessen products 178 Department of Commerce Voluntary

Seafood Inspection Program 214 Dressed salads 182 Drip

fish 217 me at 247

Escherichia coli 11 Ethylene-diamine-tetraacetic acid (EDT A),

as adjuvant 225 Eurotium glaucus 149

Films calculations 96 ethylene vinyl acetate 71 ethylene vinyl alcohol 73 faults in 114 high density polyethylene 69 ionomers 71, 247 linear low density polyethylene 68 low density polyethylene 67 performance characteristics of 75 polyamides 73 polyesters 74 polyvinyl chloride 72 polyvinylidene chloride 72 styrene polymer 73

Frankfurters 272 Fruit juices 185

Gas anti-microbial activity of 147 carbon dioxide 8, 40, 147 carbon monoxide 8 cocktail 2, 40 flushing 3, 178, 179, 187 headspace analysis 114 mixtures for modified atmosphere

packaging 7 nitrogen 8

292 INDEX

Gas cont'd oxygen 7 ozone 202 sulphur dioxide 9 tolerance of fruits and vegetables to 127

Generator, ethanol vapour 6, 152

Harn 273 Hazard analysis critical control point

(HACCP) 9, 103, 214, 259 Histamine 200, 216 History, CAP, MAP, vacuum packaging 15 Hypoxanthine 197, 200, 221

Imaging acoustic microscopy 117 machine vision 117

Indicator systems 155, see Timetemperature indicators

Inosine monophosphate 197 International Organization for

Standardization 111 Irradiation 142, 188, 226, 253 ISO 9000 9, 111

Labelling 91 Lactic souring

fish 212 meat 242, 265, 282

Lactococcus carnis 249 Lactococcus divergens 249 Lactococcus lactis 189 Lactococcus viridescens 264 Lamination 75 Legislation 93, see Regulations Listeria monocytogenes 11, 168, 216, 274,

280,283

Machinery chamber/pre-formed container 45 chamber/thermoforming 41 flexible foam-fill-seal 48 packaging systems for MAP 61

Markets by country Europe 19 France 21 Germany 23 Italy 23 United Kingdom 20 USA and Canada 25

Markets by food delicatessen salads 36 fruits and vegetables 30 pork 29 poultry 29 prepared foods 34 red meat 25 soft bakery goods 32

Material specifications for MAP 81 Meat

loaves 272 pies 275

Meats fresh 24 processed l7 red 17

Microorganisms growth characteristics 11 oxygen requirements 10

Microperforation technology 70 Milk 169 Moulds 140, 142, 158, 179, 183,272

National Advisory Committee on Microbiological Criteria for Foods 104

Nisin 189, 225

Packaging active 5 bulk gas (BGP) 84, 154, 222 chub 26,28 controlled atmosphere 1 forms of MAP 87 function of 64 gas 2,3 meat and poultry 245 microwavable 85 modified atmosphere 1 packaging materials 130 produce production 128 safety 129 vacuum (skin) 2, 3, 26, l79, 203, 205,

218,227,246,254,271,276 Pasta 18 Pasteurization, microwave 19 Pastrami 276 Pastry-based products 184 Penicillium roquetortii 149 Permeability of polymers 65, 76,84, 132 pH, effect of MAP on fish 207 Pigments

carotenoid 218 haem 218 myoglobin 243, 251, 257, 261, 275

Plastic materials, see Films Potassium sorbate 225 Poultry products 257, 276 Preservatives 143, 202 Printing 91 Pseudomonas/ Alteromonas/ Shewanella

198,204 Pseudomonas fluorescens 241, 254 Pseudomonas tragi 241,268 Pseudomonas putida 241 Psychrobacter immobilis 254

Regulations RACCP 121 labelling 121 packaging gases 122 shelf-life dating 122

Respiration, produce 126

Salmonella typhimurium 268, 274 Sandwiches 181 Scavengers, see Absorbents Seals/sealing

peelable 89 strength 88, 89

Shelf life factors governing 136, 259 regulations 122

Shewanella (Alteromonas) putrefaciens 204, 242, 243, 249, 254, 255

Snacks crisps 177 nuts 175 systems 88

Sodium chloride 273 Souring, see Lactic sou ring Sous vide 18, 22, 102, 278 Staling 140, 144 Staphylococcus aureus 11, 267, 268, 271 Storage

conditions for materials 99

INDEX

controlled atmosphere 2 hypobaric 3 poultry in MAP 253, 257

Streptococcus faecium 264 Sulphite 270

Tea 174 Tectrol 15 Temperature checks 119, 180 Testing, packages 57, 86, 114 Time-temperature indicators (TII) 119,

282 Tom AR Toes 32

293

Total quality management 9, 110 Trimethylamine oxide (TMAOITMA) 198,

200, 221, 222

Vacuum, compensated 3 Vacuum packaging (VP or VSP), see

Packaging Vapour, ethyl alcohol 152 Vibrio parahaemolyticus 12, 202

WasaOuro (WO) 144 Wieners 276

Yeasts 139, 150, 179,272 Yersinia enterocolitica 11,216,274,281 Yoghurt 169

Documents

Principles and Applications of Modified Atmosphere Packaging of Foods