Food Microbiology Third_Edition_1_-pages-15-33-chater-1-2

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.

2 The Scope of Food Microbiology

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

Chapter 1 3

exponential growth in population and the arithmetic growth in agricul-

tural productivity would be over-population and mass starvation. This in

fact proved not to be the case as the 19th century saw the development of

substantial food preservation industries based around the use of chilling,

canning and freezing and the first large scale importation of foods from

distant producers.

To this day, we are not free from concerns about over-population.

Globally there is sufficient food to feed the world’s current population,

estimated to be 6600 million in 2006. World grain production has more

than managed to keep pace with the increasing population in recent years

and the World Health Organization’s Food and Agriculture Panel

consider that current and emerging capabilities for the production and

preservation of food should ensure an adequate supply of safe and

nutritious food up to and beyond the year 2010 when the world’s

population is projected to rise to more than 7 billion.

There is however little room for complacency. Despite overall suffi-

ciency, it is recognized that a large proportion of the population is

malnourished and that 840 million people suffer chronic hunger. The

principal cause of this is not insufficiency however, but poverty which

leaves an estimated one-fifth of the world’s population without the

means to meet their daily needs. Any long-term solution to this must

lie in improving the economic status of those in the poorest countries and

this, in its train, is likely to bring a decrease in population growth rate

similar to that seen in recent years in more affluent countries.

In any event, the world’s food supply will need to increase to keep

pace with population growth and this has its own environmental and

social costs in terms of the more intensive exploitation of land and sea

resources. One way of mitigating this is to reduce the substantial pre- and

post-harvest losses which occur, particularly in developing countries

where the problems of food supply are often most acute. It has been

estimated that the average losses in cereals and legumes exceed 10%

whereas with more perishable products such as starchy staples and

vegetables the figure is more than 20% – increasing to an estimated

25% for highly perishable products such as fish. In absolute terms, the

US National Academy of Sciences has estimated the losses in cereals and

legumes in developing countries as 100 million tonnes, enough to feed

300 million people.

Clearly reduction in such losses can make an important contribution

to feeding the world’s population. While it is unrealistic to claim that

food microbiology offers all the answers, the expertise of the food

microbiologist can make an important contribution. In part, this will

lie in helping to extend the application of current knowledge and tech-

niques but there is also a recognized need for simple, low-cost, effective

methods for improving food storage and preservation in developing

4 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.

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 1C

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

6 Micro-organisms and Food Materials

Figure 2.1 Micro-organisms and the carbon, nitrogen and sulfur cycles

micro-organisms as agents of food spoilage or food poisoning, it will be

necessary to examine the natural flora of the food materials themselves,

the flora introduced by processing and handling, and the possibility of

chance contamination from the atmosphere, soil or water.

2.2 MICRO-ORGANISMS IN THE ATMOSPHERE

Perhaps one of the most hostile environments for many micro-organisms

is the atmosphere. Suspended in the air, the tiny microbial propagule may

be subjected to desiccation, to the damaging effects of radiant energy

from the sun, and the chemical activity of elemental gaseous oxygen (O2)

to which it will be intimately exposed. Many micro-organisms, especially

Gram-negative bacteria, do indeed die very rapidly when suspended in air

and yet, although none is able to grow and multiply in the atmosphere, a

significant number of microbes are able to survive and use the turbulence

of the air as a means of dispersal.

Chapter 2 7

2.2.1 Airborne Bacteria

The quantitative determination of the numbers of viable microbial

propagules in the atmosphere is not a simple job, requiring specialized

sampling equipment, but a qualitative estimate can be obtained by

simply exposing a Petri dish of an appropriate medium solidified with

agar to the air for a measured period of time. Such air exposure plates

frequently show a diverse range of colonies including a significant

number which are pigmented (Figure 2.2).

The bacterial flora can be shown to be dominated by Gram-positive

rods and cocci unless there has been a very recent contamination of the

air by an aerosol generated from an animal or human source, or from

water. The pigmented colonies will often be of micrococci or corynebac-

teria and the large white-to-cream coloured colonies will frequently be of

aerobic sporeforming rods of the genus Bacillus. There may also be small

raised, tough colonies of the filamentous bacteria belonging to Strepto-

myces or a related genus of actinomycetes. The possession of pigments

may protect micro-organisms from damage by both visible and ultravi-

olet radiation of sunlight and the relatively simple, thick cell walls of

Gram-positive bacteria may afford protection from desiccation. The

endospores of Bacillus and the conidiospores of Streptomyces are espe-

cially resistant to the potentially damaging effects of suspension in the air.

The effects of radiation and desiccation are enhanced by another

phenomenon, the ‘open air factor’ which causes even more rapid death

Figure 2.2 Exposure plate showing air flora

8 Micro-organisms and Food Materials

rates of sensitive Gram-negative organisms such as Escherichia coli. It

can be shown that these organisms may die more rapidly in outdoor air

at night time than they do during the day, in spite of reduced light

damage to the cells. It is possible that light may destroy this ‘open-air

factor’, or that other more complex interactions may occur. Phenomena

such as this, alert us to the possibility that it can be very difficult to

predict how long micro-organisms survive in the air and routine

monitoring of air quality may be desirable within a food factory, or

storage area, where measures to reduce airborne microbial contamina-

tion can have a marked effect on food quality and shelf-life. This would

be particularly true for those food products such as bakery goods that

are subject to spoilage by organisms that survive well in the air.

Bacteria have no active mechanisms for becoming airborne. They are

dispersed on dust particles disturbed by physical agencies, in minute

droplets of water generated by any process which leads to the formation

of an aerosol, and on minute rafts of skin continuously shed by many

animals including humans. The most obvious mechanisms for generating

aerosols are coughing and sneezing but many other processes generate

minute droplets of water. The bursting of bubbles, the impaction of a

stream of liquid onto a surface, or taking a wet stopper out of a bottle are

among the many activities that can generate aerosols, the droplets of

which may carry viable micro-organisms for a while.

One group of bacteria has become particularly well adapted for air

dispersal. Many actinomycetes, especially those in the genus Strepto-

myces, produce minute dry spores which survive well in the atmosphere.

Although they do not have any mechanisms for active air dispersal, the

spores are produced in chains on the end of a specialized aerial structure

so that any physical disturbance dislodges them into the turbulent layers

of the atmosphere. The air of farmyard barns may contain many millions

of spores of actinomycetes per cubic metre and some species, such as

Thermoactinomyces vulgaris and Micropolyspora faeni, can cause the

disabling disease known as farmer’s lung where individuals have become

allergic to the spores. Actinomycetes are rarely implicated in food

spoilage but geosmin-producing strains of Streptomyces may be respon-

sible for earthy odours and off-flavours in potable water, and geosmin

(Figure 2.3) may impart earthy taints to such foods as shellfish.

2.2.2 Airborne Fungi

It is possible to regard the evolution of many of the terrestrial filament-

ous fungi (the moulds) as the development of increasingly sophisticated

mechanisms for the air dispersal of their reproductive propagules. Some

of the most important moulds in food microbiology do not have active

spore dispersal mechanisms but produce large numbers of small

Chapter 2 9

Figure 2.3 Geosmin

Figure 2.4 (a) Pencillium expansum and (b) Aspergillus flavus

unwettable spores which are resistant to desiccation and light damage.

They become airborne in the same way as fine dry dust particles by

physical disturbance and wind. Spores of Penicillium and Aspergillus

(Figure 2.4) seem to get everywhere in this passive manner and species of

these two genera are responsible for a great deal of food spoilage. The

individual spores of Penicillium are only 2–3 mm in diameter, spherical to

sub-globose (i.e. oval), and so are small and light enough to be efficiently

dispersed in turbulent air.

Some fungi, such as Fusarium (Figure 2.5), produce easily wettable

spores which are dispersed into the atmosphere in the tiny droplets of

water which splash away from the point of impact of a rain drop and so

may become very widely distributed in field crops during wet weather.

10 Micro-organisms and Food Materials

Figure 2.5 Fusarium graminearum

Such spores rarely become an established part of the long-term air spora

and this mechanism has evolved as an effective means for the short-term

dispersal of plant pathogens.

As the relative humidity of the atmosphere decreases with the change

from night to day, the sporophores of fungi such as Cladosporium (Figure

2.6) react by twisting and collapsing, throwing their easily detached

spores into the atmosphere. At some times of the year, especially during

the middle of the day, the spores of Cladosporium may be the most

common spores in the air spora. Species such as Cladosporium herbarum

grow well at refrigeration temperatures and may form unsightly black

colonies on the surface of commodities such as chilled meat.

Many fungi have evolved mechanisms for actively firing their spores

into the atmosphere (Figure 2.7), a process which usually requires a high

relative humidity. Thus the ballistospores of the mirror yeasts, which are

frequently a part of the normal microbial flora of the leaf surfaces of

plants, are usually present in highest numbers in the atmosphere in the

middle of the night when the relative humidity is at its highest.

The evolutionary pressure to produce macroscopic fruiting bodies,

which is seen in the mushrooms and toadstools, has produced a structure

which provides its own microclimate of high relative humidity so that

these fungi can go on firing their spores into the air even in the middle of

a dry day.

In our everyday lives we are perhaps less aware of the presence of

micro-organisms in the atmosphere than anywhere else, unless we

Chapter 2 11

Figure 2.6 Cladosporium cladosporioides

happen to suffer from an allergy to the spores of moulds or act-

inomycetes, but, although they cannot grow in it, the atmosphere forms

an important vehicle for the spread of many micro-organisms, and the

subsequent contamination of foods.

2.3 MICRO-ORGANISMS OF SOIL

The soil environment is extremely complex and different soils have their

own diverse flora of bacteria, fungi, protozoa and algae. The soil is such

a rich reservoir of micro-organisms (Figure 2.8) that it has provided

many of the strains used for the industrial production of antibiotics,

enzymes, amino acids, vitamins and other products used in both the

12 Micro-organisms and Food Materials

Figure 2.7 Mechanisms for active dispersal of fungal propagules

pharmaceutical and food industries. Soil micro-organisms participate in

the recycling of organic and nitrogenous compounds which is essential if

the soil is to support the active growth of plants, but this ability to

degrade complex organic materials makes these same micro-organisms

potent spoilage organisms if they are present on foods. Thus the com-

monly accepted practice of protecting food from ‘dirt’ is justified in

reducing the likelihood of inoculating the food with potential spoilage

organisms.

The soil is also a very competitive environment and one in which the

physico-chemical parameters can change very rapidly. In response to

this, many soil bacteria and fungi produce resistant structures, such as

the endospores of Bacillus and Clostridium, and chlamydospores and

sclerotia of many fungi, which can withstand desiccation and a wide

range of temperature fluctuations. Bacterial endospores are especially

resistant to elevated temperatures, indeed their subsequent germination

is frequently triggered by exposure to such temperatures, and their

Chapter 2 13

Figure 2.8 Electron micrograph of micro-organisms associated with soil particles

common occurrence in soil makes this a potent source of spoilage and

food poisoning bacilli and clostridia.

2.4 MICRO-ORGANISMS OF WATER

The aquatic environment represents in area and volume the largest part

of the biosphere and both fresh water and the sea contain many species

of micro-organisms adapted to these particular habitats. The bacteria

isolated from the waters of the open oceans often have a physiological

requirement for salt, grow best at the relatively low temperatures of the

oceans and are nutritionally adapted to the relatively low concentrations

of available organic and nitrogenous compounds. Thus, from the point

of view of a laboratory routinely handling bacteria from environments

directly associated with man, marine bacteria are usually described as

oligotrophic psychrophiles with a requirement for sodium chloride for

optimum growth.

The surfaces of fish caught from cold water in the open sea will have a

bacterial flora which reflects their environment and will thus contain

predominantly psychrophilic and psychrotrophic species. Many of these

organisms can break down macromolecules, such as proteins, poly-

saccharides and lipids, and they may have doubling times as short as

ten hours at refrigeration temperatures of 0–7 1C. Thus, in ten days, i.e.

240 hours, one organism could have become 224or between 107and 108

under such conditions. Once a flora has reached these numbers it could

be responsible for the production of off-odours and hence spoilage. Of

14 Micro-organisms and Food Materials

course, during the handling of a commodity such as fish, the natural flora

of the environment will be contaminated with organisms associated with

man, such as members of the Enterobacteriaceae and Staphylococcus,

which can grow well at 30–37 1C. It is readily possible to distinguish the

environmental flora from the ‘handling’ flora by comparing the numbers

of colonies obtained by plating-out samples on nutrient agar and incu-

bating at 37 1C with those from plates of sea water agar, containing a

lower concentration of organic nutrients, and incubated at 20 1C.

The seas around the coasts are influenced by inputs of terrestrial and

freshwater micro-organisms and, perhaps more importantly, by human

activities. The sea has become a convenient dump for sewage and other

waste products and, although it is true that the seas have an enormous

capacity to disperse such materials and render them harmless, the scale

of human activity has had a detrimental effect on coastal waters. Many

shellfish used for food grow in these polluted coastal waters and the

majority feed by filtering out particles from large volumes of sea water. If

these waters have been contaminated with sewage there is always the risk

that enteric organisms from infected individuals may be present and will

be concentrated by the filter feeding activities of shellfish. Severe diseases

such as hepatitis or typhoid fever, and milder illnesses such as gastroen-

teritis have been caused by eating contaminated oysters and mussels

which seem to be perfectly normal in taste and appearance. In warmer

seas even unpolluted water may contain significant numbers of Vibrio

parahaemolyticus and these may also be concentrated by filter-feeding

shellfish, indeed they may form a stable part of the natural enteric flora

of some shellfish. This organism may be responsible for outbreaks of

food poisoning especially associated with sea foods.

The fresh waters of rivers and lakes also have a complex flora of micro-

organisms which will include genuinely aquatic species as well as com-

ponents introduced from terrestrial, animal and plant sources. As with

the coastal waters of the seas, fresh water may also act as a vehicle for

bacteria, protozoa and viruses causing disease through contamination

with sewage effluent containing human faecal material. These organisms

do not usually multiply in river and lake water and may be present in very

low, but nonetheless significant, numbers making it difficult to demon-

strate their presence by direct methods. It is usual to infer the possibility

of the presence of such organisms by actually looking for a species of

bacterium which is always present in large numbers in human faeces, is

unlikely to grow in fresh water, but will survive at least as long as any

pathogen. Such an organism is known as an ‘indicator organism’ and the

species usually chosen in temperate climates is Escherichia coli.

Fungi are also present in both marine and fresh waters but they do

not have the same level of significance in food microbiology as other

micro-organisms. There are groups of truly aquatic fungi including some

Chapter 2 15

which are serious pathogens of molluscs and fish. There are fungi which

have certainly evolved from terrestrial forms but have become morpho-

logically and physiologically well adapted to fresh water or marine

habitats. They include members of all the major groups of terrestrial

fungi, the ascomycetes, basidiomycetes, zygomycetes and deutero-

mycetes and there is the possibility that some species from this diverse

flora could be responsible for spoilage of a specialized food commodity

associated with water such as a salad crop cultivated with overhead

irrigation from a river or lake, but this is speculation.

Of the aquatic photosynthetic micro-organisms, the cyanobacteria, or

blue-green algae, amongst the prokaryotes and the dinoflagellates

amongst the eukaryotes, have certainly had an impact on food quality

and safety. Both these groups of micro-organisms can produce very toxic

metabolites which may become concentrated in shellfish without appar-

ently causing them any harm. When consumed by humans, however,

they can cause a very nasty illness such as paralytic shellfish poisoning

(see Chapter 8).

2.5 MICRO-ORGANISMS OF PLANTS

All plant surfaces have a natural flora of micro-organisms which may be

sufficiently specialized to be referred to as the phylloplane flora, for that

of the leaf surface, and the rhizoplane flora for the surface of the roots.

The numbers of organisms on the surfaces of healthy, young plant leaves

may be quite low but the species which do occur are well adapted for this

highly specialized environment. Moulds such as Cladosporium and the

so-called black yeast, Aureobasidium pullulans, are frequently present.

Indeed, if the plant is secreting a sugary exudate, these moulds may be

present in such large numbers that they cover the leaf surface with a

black sooty deposit. In the late summer, the leaves of such trees as oak

and lime may look as though they are suffering from some form

of industrial pollution, so thick is the covering of black moulds.

Aureobasidium behaves like a yeast in laboratory culture but develops

into a filamentous mould-like organism as the culture matures.

There are frequently true yeasts of the genera Sporobolomyces and

Bullera on plant leaf surfaces. These two genera are referred to as mirror

yeasts because, if a leaf is attached to the inner surface of the lid of a Petri

dish containing malt extract agar, the yeasts produce spores which they

actively fire away from the leaf surface. These ballistospores hit the agar

surface and germinate to eventually produce visible colonies in a pattern

which forms a mirror image of the leaf. An even richer yeast flora is

found in association with the nectaries of flowers and the surfaces of

fruits and the presence of some of these is important in the spontaneous

fermentation of fruit juices, such as that of the grape in the production of

16 Micro-organisms and Food Materials

wine. The bacterial flora of aerial plant surfaces which is most readily

detected is made up predominantly of Gram-negative rods, such as

Pectobacterium, Erwinia, Pseudomonas and Xanthomonas but there is

usually also present a numerically smaller flora of fermentative Gram-

positive bacteria such as Lactobacillus and Leuconostoc which may

become important in the production of such fermented vegetable prod-

ucts as sauerkraut (see Chapter 9).

The specialized moulds, yeasts and bacteria living as harmless com-

mensals on healthy, young plant surfaces are not usually any problem in

the spoilage of plant products after harvest. But, as the plant matures,

both the bacterial and fungal floras change. The numbers of pectinolytic

bacteria increase as the vegetable tissues mature and a large number of

mould species are able to colonize senescent plant material. In the

natural cycling of organic matter these organisms would help to break

down the complex plant materials and so bring about the return of

carbon, nitrogen and other elements as nutrients for the next round of

plant growth. But, when humans break into this cycle and harvest plant

products such as fruits, vegetables, cereals, pulses, oilseeds and root

crops, these same organisms may cause spoilage problems during pro-

longed periods of storage and transport.

Plants have evolved several mechanisms for resisting infection by

micro-organisms but there are many species of fungi and bacteria which

overcome this resistance and cause disease in plants and some of these

may also cause spoilage problems after harvesting and storage. Amongst

the bacteria, Pectobacterium caratovorum subsp. caratovorum (previously

known as Erwinia carotovora var. atroseptica) is a pathogen of the potato

plant causing blackleg disease of the developing plant. The organism can

survive in the soil when the haulms of diseased plants fall to the ground

and, under the right conditions of soil moisture and temperature, it may

then infect healthy potato tubers causing a severe soft rot during storage.

One of the conditions required for such infection is a film of moisture on

the tuber for this species can only infect the mature tuber through a

wound or via a lenticel in the skin of the potato. This process may be

unwittingly aided by washing potatoes and marketing them in plastic

bags so that, the combination of minor damage and moisture trapped in

the bag, favours the development of Pectobacterium soft rot.

Amongst the fungi, Botrytis cinerea (Figure 2.9) is a relatively weak

pathogen of plants such as the strawberry plant where it may infect the

flower. However, this low pathogenicity is often followed by a change to

an aggressive invasion of the harvested fruit, usually through the calyx

into the fruit tissue. Once this ‘grey mould’ has developed on one fruit,

which may have been damaged and infected during growth before

harvest, the large mass of spores and actively growing mould readily

infects neighbouring fruit even though they may be completely sound.

Chapter 2 17

Figure 2.9 Botrytis cinerea

The cereals are a group of plant commodities in which there is a

pronounced and significant change in the microbial flora following

harvesting. In the field the senescent plant structures carrying the cereal

grain may become infected by a group of fungi, referred to as the field

fungi, which includes such genera as Cladosporium, Alternaria, Helm-

inthosporium and Chaetomium. After harvest and reduction of the mois-

ture content of the grain, the components of the field flora decrease in

numbers and are replaced by a storage flora which characteristically

includes species of the genera Penicillium and Aspergillus. Some genera of

fungi, such as Fusarium, contain a spectrum of species, some of which are

specialized plant pathogens, others saprophytic field fungi and others

capable of growth during the initial stages of storage. Indeed, the more

that is learnt about the detailed ecology of individual species, the more it

is realized that it may be misleading to try to pigeon hole them into

simple categories such as field fungi and storage fungi. Thus it is now

18 Micro-organisms and Food Materials

known that Aspergillus flavus, a very important species because of its

ability to produce the carcinogenic metabolite known as aflatoxin, is not

just a storage mould as was once believed, but may infect the growing

plant in the field and produce its toxic metabolites before harvesting and

storage (see Chapter 8). Indeed, it is now recognized that many plants

carry fungal endophytes in their naturally healthy state.

2.6 MICRO-ORGANISMS OF ANIMAL ORIGIN

All healthy animals carry a complex microbial flora, part of which may

be very specialized and adapted to growth and survival on its host, and

part of which may be transient, reflecting the immediate interactions of

the animal with its environment. From a topological point of view, the

gut is also part of the external surface of an animal but it offers a very

specialized environment and the importance of the human gut flora will

be dealt with in Chapter 6.

2.6.1 The Skin

The surfaces of humans and other animals are exposed to air, soil and

water and there will always be the possibility of contamination of foods

and food handling equipment and surfaces with these environmental

microbes by direct contact with the animal surface. However, the surface

of the skin is not a favourable place for most micro-organisms since it is

usually dry and has a low pH due to the presence of organic acids

secreted from some of the pores of the skin. This unfavourable environ-

ment ensures that most micro-organisms reaching the skin do not

multiply and frequently die quite quickly. Such organisms are only

‘transients’ and would not be regularly isolated from the cleaned skin

surface.

Nevertheless, the micro-environments of the hair follicles, sebaceous

glands and the skin surface have selected a specialized flora exquisitely

adapted to each environment. The bacteria and yeasts making up this

‘normal’ flora are rarely found in other habitats and are acquired by the

host when very young, usually from the mother. The micro-organisms

are characteristic for each species of animal and, in humans, the normal

skin flora is dominated by Gram-positive bacteria from the genera

Staphylococcus, Corynebacterium and Propionibacterium. For animals

which are killed for meat, the hide may be one of the most important

sources of spoilage organisms while, in poultry, the micro-organisms

associated with feathers and the exposed follicles, once feathers are

removed, may affect the microbial quality and potential shelf-life of

the carcass.

Chapter 2 19

2.6.2 The Nose and Throat

The nose and throat with the mucous membranes which line them

represent even more specialized environments and are colonized by a

different group of micro-organisms. They are usually harmless but may

have the potential to cause disease, especially following extremes of

temperature, starvation, overcrowding or other stresses which lower the

resistance of the host and make the spread of disease more likely in both

humans and other animals. Staphylococcus aureus is carried on the

mucous membranes of the nose by a significant percentage of the human

population and some strains of this species can produce a powerful toxin

capable of eliciting a vomiting response. The food poisoning caused by

this organism will be dealt with in Chapter 7.

2.7 CONCLUSIONS

In this chapter we have described some of the major sources of micro-

organisms which may contaminate food and cause problems of spoilage

or create health risks when the food is consumed. It can be seen that most

foods cannot be sterile but have a natural flora and acquire a transient

flora derived from their environment. To ensure that food is safe and can

be stored in a satisfactory state, it is necessary to either destroy the

micro-organisms present, or manipulate the food so that growth is

prevented or hindered. The manner in which environmental and nutri-

tional factors influence the growth and survival of micro-organisms will

be considered in the next chapter. The way in which this knowledge can

be used to control microbial activity in foods will be considered in

Chapter 4.