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Food Microbiology

Third Edition

Food Microbiology

Third Edition

Martin R. Adams and Maurice O. Moss

University of Surrey, Guildford, UK

ISBN 978-0-85404-284-

A catalogue record for this book is available from the British Library.

r The Royal Society of Chemistry 2008

All rights reserved

Apart from any fair dealing for the purpose of research or private study, or criticism or review as permitted under the terms of the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored or transmitted, in any form or by any means without the prior permission in writing of The Royal Society of Chemistry, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organisation outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page.

Published by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 0WF, UK

Registered Charity No. 207890

For further information see our web site at www.rsc.org

Preface to the Second Edition

The very positive response Food Microbiology has had since it was first

published has been extremely gratifying. It has reconfirmed our belief in

the value of the original project and has also helped motivate us to

produce this second edition. We have taken the opportunity to correct

minor errors, improve some of the diagrams and update the text to

incorporate new knowledge, recent developments and legislative

changes. Much of this has meant numerous small changes and additions

spread throughout the book, though perhaps we should point out (for

the benefit of reviewers) new sections on stress response, Mycobacterium

spp. and risk analysis, and updated discussions of predictive microbio-

logy, the pathogenesis of some foodborne illnesses, BSE/vCJD and

HACCP.

A number of colleagues have provided advice and information and

among these we are particularly indebted to Mike Carter, Paul Cook,

Chris Little, Johnjoe McFadden, Bob Mitchell, Yasmine Motarjemi and

Simon Park. It is customary for authors to absolve those acknowledged

from all responsibility for any errors in the final book. We are happy to

follow that convention in the unspoken belief that if any errors have

crept through we can always blame each other.

Preface to the Third Edition

In this third edition we have taken the opportunity to update and clarify

the text in a number of places, removing a few incipient cobwebs along

the way. Mostly this has entailed small changes within the existing text

though there are new sections dealing with natamycin, subtyping, emerg-

ing pathogens and Enterobacter sakazakii.

In addition to all those colleagues who have helped with previous

editions we are pleased to acknowledge Janet Corry and Marcel Zwietering

whose diligent reading of the second edition revealed the need for some

corrections that had previously eluded us. We have also rationalised the

index which we decided was excessive and contained too many esoteric or

trivial entries. As a consequence, terms such as ‘‘trub’’ have been deleted.

Those seeking knowledge on this topic will now have to read the book in

its entirety.

vi

Contents

Chapter 3 Factors Affecting the Growth and Survival of

CHAPTER 1

The Scope of Food Microbiology

Microbiology is the science which includes the study of the occurrence

and significance of bacteria, fungi, protozoa and algae which are the

beginning and ending of intricate food chains upon which all life

depends. Most food chains begin wherever photosynthetic organisms

can trap light energy and use it to synthesize large molecules from carbon

dioxide, water and mineral salts forming the proteins, fats and carbo-

hydrates which all other living creatures use for food.

Within and on the bodies of all living creatures, as well as in soil and

water, micro-organisms build up and change molecules, extracting en-

ergy and growth substances. They also help to control population levels

of higher animals and plants by parasitism and pathogenicity.

When plants and animals die, their protective antimicrobial systems

cease to function so that, sooner or later, decay begins liberating the

smaller molecules for re-use by plants. Without human intervention,

growth, death, decay and regrowth would form an intricate web of

plants, animals and micro-organisms, varying with changes in climate

and often showing apparently chaotic fluctuations in populations of

individual species, but inherently balanced in numbers between produc-

ing, consuming and recycling groups.

In the distant past, these cycles of growth and decay would have been

little influenced by the small human population that could be supported

by the hunting and gathering of food. From around 10 000 BC however,

the deliberate cultivation of plants and herding of animals started in

some areas of the world. The increased productivity of the land and the

improved nutrition that resulted led to population growth and a prob-

able increase in the average lifespan. The availability of food surpluses

also liberated some from daily toil in the fields and stimulated the

development of specialized crafts, urban centres, and trade – in short,

civilization.

1.1 MICRO-ORGANISMS AND FOOD

The foods that we eat are rarely if ever sterile, they carry microbial

associations whose composition depends upon which organisms gain

access and how they grow, survive and interact in the food over time. The

micro-organisms present will originate from the natural micro-flora of

the raw material and those organisms introduced in the course of

harvesting/slaughter, processing, storage and distribution (see Chapters

2 and 5). The numerical balance between the various types will be

determined by the properties of the food, its storage environment,

properties of the organisms themselves and the effects of processing.

These factors are discussed in more detail in Chapters 3 and 4.

In most cases this microflora has no discernible effect and the food is

consumed without objection and with no adverse consequences. In some

instances though, micro-organisms manifest their presence in one of

several ways:

(i) they can cause spoilage;

(ii) they can cause foodborne illness;

(iii) they can transform a food’s properties in a beneficial way – food

fermentation.

1.1.1 Food Spoilage/Preservation

From the earliest times, storage of stable nuts and grains for winter

provision is likely to have been a feature shared with many other animals

but, with the advent of agriculture, the safe storage of surplus production

assumed greater importance if seasonal growth patterns were to be used

most effectively. Food preservation techniques based on sound, if then

unknown, microbiological principles were developed empirically to ar-

rest or retard the natural processes of decay. The staple foods for most

parts of the world were the seeds – rice, wheat, sorghum, millet, maize,

oats and barley – which would keep for one or two seasons if adequately

dried, and it seems probable that most early methods of food preserva-

tion depended largely on water activity reduction in the form of solar

drying, salting, storing in concentrated sugar solutions or smoking over

a fire.

The industrial revolution which started in Britain in the late 18th

century provided a new impetus to the development of food preservation

techniques. It produced a massive growth of population in the new

industrial centres which had somehow to be fed; a problem which many

thought would never be solved satisfactorily. Such views were often

based upon the work of the English cleric Thomas Malthus who in his

‘Essay on Population’ observed that the inevitable consequence of the

2 The Scope of Food Microbiology

countries. Problems for the food microbiologist will not however disap-

pear as a result of successful development programmes. Increasing

wealth will lead to changes in patterns of food consumption and chang-

ing demands on the food industry. Income increases among the poor

have been shown to lead to increased demand for the basic food staples

while in the better-off it leads to increased demand for more perishable

animal products. To supply an increasingly affluent and expanding urban

population will require massive extension of a safe distribution network

and will place great demands on the food microbiologist.

1.1.2 Food Safety

In addition to its undoubted value, food has a long association with the

transmission of disease. Regulations governing food hygiene can be

found in numerous early sources such as the Old Testament, and the

writings of Confucius, Hinduism and Islam. Such early writers had at

best only a vague conception of the true causes of foodborne illness and

many of their prescriptions probably had only a slight effect on its

incidence. Even today, despite our increased knowledge, ‘Foodborne

disease is perhaps the most widespread health problem in the contem-

porary world and an important cause of reduced economic productivity.’

(WHO 1992.) The available evidence clearly indicates that biological

contaminants are the major cause. The various ways in which foods can

transmit illness, the extent of the problem and the principal causative

agents are described in more detail in Chapters 6, 7 and 8.

1.1.3 Fermentation

Microbes can however play a positive role in food. They can be con-

sumed as foods in themselves as in the edible fungi, mycoprotein and

algae. They can also effect desirable transformations in a food, changing

its properties in a way that is beneficial. The different aspects of this

and examples of important fermented food products are discussed in

Chapter 9.

1.2 MICROBIOLOGICAL QUALITY ASSURANCE

Food microbiology is unashamedly an applied science and the food

microbiologist’s principal function is to help assure a supply of whole-

some and safe food to the consumer. To do this requires the synthesis

and systematic application of our knowledge of the microbial ecology of

foods and the effects of processing to the practical problem of producing,

economically and consistently, foods which have good keeping qualities

and are safe to eat. How we attempt to do this is described in Chapter 11.

4 The Scope of Food Microbiology

CHAPTER 2

Micro-organisms and Food Materials

Foods, by their very nature, need to be nutritious and metabolizable and

it should be expected that they will offer suitable substrates for the

growth and metabolism of micro-organisms. Before dealing with the

details of the factors influencing this microbial activity, and their sig-

nificance in the safe handling of foods, it is useful to examine the possible

sources of micro-organisms in order to understand the ecology of

contamination.

2.1 DIVERSITY OF HABITAT

Viable micro-organisms may be found in a very wide range of habitats,

from the coldest of brine ponds in the frozen wastes of polar regions, to

the almost boiling water of hot springs. Indeed, it is now realized that

actively growing bacteria may occur at temperatures in excess of 100 1 C

in the thermal volcanic vents, at the bottom of the deeper parts of the

oceans, where boiling is prevented by the very high hydrostatic pressure

(see Section 3.2.5). Micro-organisms may occur in the acidic wastes

draining away from mine workings or the alkaline waters of soda lakes.

They can be isolated from the black anaerobic silts of estuarine muds or

the purest waters of biologically unproductive, or oligotrophic, lakes. In

all these, and many other, habitats microbes play an important part in

the recycling of organic and inorganic materials through their roles in the

carbon, nitrogen and sulfur cycles (Figure 2.1). They thus play an

important part in the maintenance of the stability of the biosphere.

The surfaces of plant structures such as leaves, flowers, fruits and

especially the roots, as well as the surfaces and the guts of animals all

have a rich microflora of bacteria, yeasts and filamentous fungi. This

natural, or normal flora may affect the original quality of the raw

ingredients used in the manufacture of foods, the kinds of contamination

which may occur during processing, and the possibility of food spoilage

or food associated illness. Thus, in considering the possible sources of