WHO Chapter 3

7/28/2019 WHO Chapter 3

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Basic Food Safety for H ealth W orkers

Chapter 3

Factors leading

to microbialfoodborne illness

Three key factors generally contribute tooutbreaks of microbial foodborne illness:

n contamination - pathogens must bepresent in the food;

n growth- in some cases they must alsohave the opportunity to multiply in thefood in order to produce an infectious

dose or sufficient toxin to cause ill-ness;

n survival - when present at a danger-ous level they must be able to survivein the food during its storage andprocessing.

Contamination: how

do microorganismsget into food?Microorganisms, particularly bacteria, canbe found almost everywhere. They arepresent in the air, water and soil; they cangrow wherever higher organisms cangrow, and can be found on the surfacesof plants and animals as well as in themouth, nose and intestines of animals,including humans. They also occur in

places that are far too inhospitable forhigher life forms, such as in hot sulfursprings. As a result, foods are hardly eversterile, that is to say completely free fromviable microorganisms. Foods carry amixed population of microorganismsderived from the natural microflora ofthe original plant or animal, those picked

up from its environment and thoseintroduced during harvest/ slaughter andsubsequent handling, processing andstorage.

Most of the microorganisms in ourenvironment cause us no harm. In factthey play very useful roles in making soilfertile and decomposing and recyclingorganic and inorganic materials thatwould otherwise accumulate. When they

occur in foods, many of these organismshave no evident effect on the food or theperson consuming it. In some cases,microorganisms may actually producebeneficial changes in the food and this isthe basis of the large range of fermentedfoods such as cheese, yoghurt andfermented meats. Others, however, willspoil the product making it unfit forconsumption and some can be harmfulto humans causing illness when they orthe toxins they produce are ingested.


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Factors leading to microbial foodborne i llness

It is possible to control and minimize thenumbers of organisms present in food byusing good hygienic practices in itspreparation and handling or by process-

ing the food in some way. If pathogensall came from the same source then thetask of controlling them would be muchsimpler. Unfortunately they can get intofoods from several different sources (Fig-ure 3.1), known as reservoirs of infec-tion, and by a number of routes.

Microorganisms that occur

naturally in foods (indigenous

microflora)Food materials, plant and animal, willcarry their own microflora during life andthis can persist into the food product. Interms of food safety, the naturalmicroflora is of greatest importance inanimal products. The muscle of healthyanimals and poultry is usually almostcompletely free from microorganisms butthe intestines in particular carry a very

large and diverse microflora that can in-clude human pathogens such asCampylobacter, Salmonellaand certainstrains ofEscherichia coli. In the processof slaughter and butchering a carcass,these organisms may be spread to othermeat surfaces. As a result, eviscerationand dressing are regarded as key stepsthat need to be hygienically performedto minimize meat contamination.

In most cases the animal or bird will carrythese organisms without showing signs ofill-health, and pathological lesions will notbe visible during meat inspection. Oth-

ers, such as Bacillus anthracis, the causa-tive agent of anthrax, can cause an ill-ness in the animal and visible lesions.Since this and other animal diseases canbe transmitted to humans, it is clear thatmeat from obviously sick animals mustnot be used as human food. Other dis-eases, such as bovine tuberculosis, bru-cellosis or the presence of parasites suchas the beef and pork tapeworms and theroundwormTrichinella spirali s, may also

be diagnosed during post mortem meatinspection. Thus ante and post mortemexamination by a trained inspector is anessential protection measure.

Natural inhabitants

of the environment

Many pathogens can be found as naturalinhabitants of the environment the

soil, air and water where the food is pro-duced and can, as a result, contami-nate the product. For example, V ibrioparahaemolyticusis a naturally occurringmarine organism in warm coastal watersand can contaminate fish. Some strainsare pathogenic and this can be a very im-portant cause of foodborne illness wherefish is a major item in the diet. Clostrid-ium botulinumandClostridium perfringensare

Figure 3.1 Sources of food contamination

Dirty pots &cooking utensils

Polluted water(e.g- wastewater,

irrigation andhousehold water)Food

(Raw/Cooked)

Domestic animals

Indigenousmicroflora

Infected food animals

Cross.contaminationduring foodpreparation

Flies &pests

Food handlers(e.g. soiled hands)

Human & animalexcreta


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found in soil and mud. Bacillus cereusspores can be isolated from soil and air,and L isteria monocytogenesis a relativelycommon environmental organism found

in unpolluted water, mud and numerousother sources.

Polluted environment:

insanitary practices in

agriculture and aquaculture

Pollution of the environment with ani-mal or human wastes such as sewage canbe a serious threat to food safety. Hu-man excrement can contain a wide rangeof pathogens transmitted by the faecal-oral route including bacteria such asV i-brio cholerae,SalmonellaTyphi, viruses suchas Hepatitis A, and parasites. These canbe transferred to foods if raw sewage isused to fertilize fields or if the water usedto irrigate, wash, cool or transport foodis contaminated with sewage.

Filter-feeding shellfish will filter largevolumes of water to extract nutrients. Ifthis water is polluted with sewage theywill also concentrate pathogenic bacteria

and viruses in their tissues. Polluted waterused in aquaculture can also lead to thecarriage of pathogens such as V ibriocholeraeby farmed fish and shellfish.

Animal excrement poses equally seriousproblems. For example, a large outbreakof listeriosis was caused by the contami-nation of cabbages with sheep manure.Chicken faeces adhering to the outsideof egg shells can contaminate the con-tents when the egg is broken and this hasbeen the cause of numerous outbreaksof salmonellosis. Sometimes the link be-tween the food and faecal contaminationcan be quite complex, as was illustratedby an outbreak of yersiniosis (Figure3.2). In this case, crates used to trans-port waste milk to a farm where it wasused as animal feed were contaminatedwith pig excrement. Back at the dairy, the

Figure 3.2 Faecal contamination leading to an outbreak of yersiniosis


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crates were insufficiently washed and dis-infected before being used to transportretail milk to the shops. During this proc-ess the outside of the milk cartons were

contaminated with Yersinia enterocoliticawhich was, in turn, transferred to the milkwhen the cartons were opened and themilk poured.

Water

Contaminated water is simply one aspectof a polluted environment, but in viewof its importance in foods and foodprocessing and its role as a major source

of diarrhoeal disease in developing coun-tries it merits special mention. Contami-nation of water with faeces can introducea wide variety of pathogenic bacteria,viruses, protozoa and helminths whichcan be transmitted to people when thewater is used for drinking or in foodpreparation. Since individual watersources tend to serve large numbers ofpeople, disease outbreaks where water isthe primary source of infection can be

very large. This linkage between waterand the spread of disease has beenknown for a long time and is reputed tohave been dramatically demonstrated in1854 by John Snow when he removedthe handle from a water pump in BroadStreet, London, to bring a local choleraoutbreak to an end.

Although pathogen-contaminated wateris clearly a prime source of infection, it

is also true that a simple insufficiency ofwater will hamper efforts to practise goodpersonal and food hygiene and contrib-ute to the transmission of disease.

Recognition of the pressing need to pro-vide safe drinking water led to the pe-riod 19811990 being declared the In-ternational Drinking Water Supply andSanitation Decade. This resulted in theWHO Guidelines for Drinking Water

Quality. These guidelines place their pri-mary emphasis on microbiological safety,

though chemical contaminants can alsobe a problem, since more than half ofthe worlds population is still exposed towater contaminated with pathogens.

Pests and pets

It is not just food animals that frequentlycarry pathogenic organisms in theirgastrointestinal tract. Surveys haveshown that up to 15% of pet dogs ex-crete salmonellae. Rats and mice cantransmit illness by contaminating foodwith organisms picked up from sewers,garbage and other sources via their fur,

urine, faeces or saliva.Wild birds can often find their way intofood processing areas, particularly in hotclimates where buildings are relativelyopen, and these may excrete pathogenssuch asSalmonellaandCampylobacter.

Flies, cockroaches, ants and other insectpests can transfer organisms from sourcescontaminated with pathogens to foods.Flies are particularly important in this

respect as they are associated with bothfood handling areas and contaminatedareas such as toilets and refuse heaps.

They also have the unfortunate habit offeeding by regurgitating their previousmeals on to foods to help liquefy them.

Spiders and wasps are rather less of ahazard because they tend not to breed incontaminated areas but nonetheless theyhave the potential to transfer pathogens

to food.

The food handler

Handling of food can introduce andspread pathogenic microorganisms. Foodhandlers may carry pathogens withoutexperiencing any serious ill-effects them-selves. Staphylococcus aureusis commonlyassociated with the skin, nose, throat andinfected skin lesions, particularly in higher

primates such as humans where 2050%of healthy individuals can carry the or-


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ganism. The organism is difficult to re-move from the skin where it hides inpores and hair follicles. If the hands aredamp it can be drawn to the surface and

transferred to foods. It is possible to iden-tify carriers by microbiological testing andthis has been done on a number of occa-sions. In one recent example, restaurantworkers in Kuwait city were tested andin a sample of 500 people 26.6% werefound to carry the organism (15). It is notusually feasible to do this routinely toidentifyStaphylococcus aureuscarriers so, asa general precautionary measure, peopleshould avoid handling foods with bare

hands as much as possible, particularlythose foods that support the growth ofS. aureus.

Organisms that reside in the gut can betransferred to food if food handlers failto wash their hands thoroughly after us-ing the toilet. Gut organisms adhere lessstrongly to the skin and should be read-ily removed by washing with soap andwater. Thorough hand-washing is essen-tial after using the toilet, not just afterdefecation, since pathogens can also bepicked up from previous users of the toi-let via door handles, taps and drying tow-els.

The risk is very much greater if the foodhandler is suffering from a gut infection.In many cases, however, infected foodhandlers may not know that they are car-

rying the pathogen in their gut as theymay not feel unwell and may exhibit nosymptoms. This could be because theyare in what is known as an acute carrierstate where they are infected, can spreadthe organism, but have not yet begun todisplay symptoms. Alternatively, theymay be chronic carriers who are infectedbut will not develop symptoms, yet willexcrete the pathogen over a long period.

This latter state has been most famously

associated with typhoid fever, particularlythe notorious case of Typhoid Mary, a

food handler who was also a chroniccarrier of the illness. In the early years ofthe 20th century the unfortunate combi-nation of her medical condition and her

chosen profession, a cook, is estimatedto have resulted in about 1300 cases oftyphoid fever in the USA. Infected foodhandlers are also a common source offoodborne viruses such as the HepatitisA virus and the diarrhoea-causing, smallround-structured viruses which are ex-creted in large numbers (108-1010 g-1 fae-ces) by infected individuals. Many casesof foodborne virus infection have beenassociated with catering.

Equipment, utensils and

kitchen practices

The equipment and utensils used in thepreparation of food can also act assources of contamination. For instance,knives or chopping boards used with un-cooked products such as raw meat orpoultry can become contaminated with

pathogens. If they are used again with-out being adequately cleaned, particularlyif they are then used with a cooked orready-to-eat product, the pathogens canbe transferred, posing a very seriousthreat to food safety. This can also hap-pen if food handlers who work with rawfood fail to wash their hands before han-dling ready-to-eat food. This process,whether mediated by hands or equip-ment, is known as cross-contamination.

Raw foods can also contaminate cookedor ready-to-eat foods if they are storedtogether improperly. For example, if rawmeat is stored above cooked foods in arefrigerator, liquid drip from the meat cancontaminate the foods stored below.

Dishcloths left wet can also act as animportant reservoir of contaminatingorganisms that can be spread around

foods and food contact surfaces as thecloth is used.


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Growth

We have already noted that an important

feature of the transmission of foodborneillness is that bacterial pathogens cangrow in some foods. In a relatively shortperiod, they can multiply from a low,possibly harmless level to numbers suf-ficient to cause illness. This does not hap-pen with other foodborne pathogens suchas parasites and viruses and it is there-fore important to understand the factorsthat influence whether and how quicklybacteria can grow in food.

Bacteria multiply by a process of dou-bling; each cell splits into two identicaldaughter cells which go on to repeat theprocess, giving four cells, which then pro-duce eight cells, and so on. The periodbetween cell divisions is known as thegeneration or doubling time. This can bequite short, typically around 20 minutesor so, and sometimes even shorter. A fig-ure of just over seven minutes at 41oChas been reported for the pathogenClostridium perfringens. This means that,given the right conditions, a bacteriumcan multiply very rapidly from extremely

low levels to numbers well in excess ofthe infectious doses quoted in Table 1.6.In eight hours a single organism with ageneration time of 20 minutes can reach

a population of more than 16 million(Figure 3.3). The ability of bacterialpathogens to multiply in foods explainswhy, in some circumstances, foods canbe more likely to provide the minimuminfective dose of a pathogen than con-taminated water.

Moulds and yeasts also have the capac-ity to grow in foods but, with the notableexception of mycotoxin production by

some moulds, this normally results inspoilage rather than health problems.Moulds and yeasts grow by differentmechanisms than bacteria: most yeaststend to multiply by budding to produce adaughter cell which separates from itsparent, while moulds tend to grow as finethread-like structures called hyphaewhich extend and branch to produce atangled mass known as a mycelium.

For microorganisms to grow at their fast-est, conditions have to be just right. Op-timum growth conditions differ slightlyfor bacteria, yeasts and moulds and thiscan determine the type of organism that

Figure 3.3 Exponential growth of bacteria

>16 million

One bacterial cell


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predominates when foods spoil. Mouldsand yeasts generally grow more slowlythan bacteria and are often outgrown bythem unless conditions in the food are

sufficiently inhibitory to bacterial growthto give the moulds and yeast a competi-tive advantage. The most important ofthe factors that affect microbial growthare:

n availability of nutrientsn temperaturen acidity/ pHn available water (water activity, a

w)

n oxygen (air)n

antimicrobial agents.n time.

Availability of nutrients

When microorganisms grow on foods,they use them exactly as we do as asource of nutrients and energy. Foodsgenerally contain a variety of chemicalsthat serve these purposes very well andlevels of nutrients are not usually limiting

factors on microbial growth in foods.This means that food handling equipmentmust be very thoroughly cleaned after useto remove all traces of food since bacte-ria will grow on the tiniest remnant andthis can contaminate subsequent batches.

A raw food will contain a diverse micro-bial population with many different or-ganisms competing for the nutrientsavailable. If a pathogen is present it may

not do well in this competition; its sup-

ply of nutrients will be limited and it willgrow only slowly or perhaps not at all. If,however, the pathogen is introduced afterthe food has been cooked and the natural

microflora has been eliminated orseverely reduced there will be little or nocompetition for growth and the patho-gens will grow more quickly.

Pathogen growth is much less likely tooccur in water where the nutrient supplyis more limited.

Temperature

Microorganisms can be found growing attemperatures ranging from about -10oCup to more than 100oC. The mostimportant consideration is that watershould be present in its liquid state. I f itis present either entirely in the solid state,as ice, or as water vapour, then bacteriacannot grow even if they can cope withthe extreme temperature under thoseconditions.

Individual microorganisms will not grow

over such a wide temperature span andare normally restricted to a range of about35oC. They have a minimumtemperature below which they cannotgrow, a maximum temperature abovewhich they cannot grow and in-betweenan optimum at which they grow best.

These three temperatures, known as anorganisms cardinal temperatures, areused to separate microorganisms into dif-

ferent classes (Table 3.1).

Table 3.1 Cardinal temperatures for microbial growth

Temperature (C)

Group Minimum Optimum Maximum

Thermophiles 4045 5575 6090

Mesophiles 515 3045 3547

Psychrophiles

(obligate psychrophiles) -5+5 1215 1520

Psychrotrophs

(facultative psychrophiles) -5+5 2530 3035

Adapted f rom ICMSF, Microbial Ecology o f Foods Volume 1, New York Academic Press 1980


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Most foodborne pathogens aremesophiles with an optimum growthtemperature around body temperature of37oC. This makes sense since for many

of them the preferred habitat is thehuman or animal body. I n tropicaldeveloping countries foods may be storedat temperatures as high as 37oC.Mesophiles will grow rapidly at this tem-perature, though they can also grow quitewell down to below 20oC. Their mini-mum growth temperature is generallyaround 8oC, so if a food is stored below10C then mesophiles will either growvery slowly or not at all (although they

maysurvive, see Figure 3.4 and below).

Because of the great use of refrigerationin modern food processing, handling anddistribution, there has been a lot of con-cern about pathogens that are capable ofgrowing at chill temperatures (


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Most foods are slightly acid althoughsome, such as citrus fruits, pickles andsauces, are much more acidic. Pathogenscannot grow in the more acidic foods but

are relatively unaffected by the pH foundin most food materials. Acids differ intheir ability to prevent microbial growth;acetic (ethanoic) acid is generally moreeffective than lactic acid which in turn ismore effective than citric acid. Usuallyif a food has a pH below 4.5 it isconsidered safe from bacterial pathogens.Despite this, there have been occasionalwell-documented outbreaks of foodborneillness associated with acidic foods such

as yoghurt.

Water activity (aw)

All microorganisms require liquid waterto enable them to grow. If there is littlewater present, or the water that is presentis not available to the microbe, then itsgrowth is slowed or even prevented. Itmay be the case that there is simply notenough water present, as in some dried

foods. Alternatively, water may bepresent but unavailable, as in foods thatcontain high levels of salt or sugar and

where much of the water may be occu-pied with keeping the salt or sugar insolution, or in frozen foods where thewater is present as ice.

Water availability is often measured orexpressed in terms of water activity (a

w).

This is a scale from 0 to 1; pure water withmaximum water availability has an a

wof

1, in the complete absence of water thea

wwould be 0. Dried milk powder has a

water activity of 0.2. Bacteria normallyrequire a very high water availability to beable to grow at their fastest, but will oftenbe able to grow somewhat more slowly in

salty or partially dried foods.Salmonellawillgrow in the presence of 6% salt, L isteriamonocytogenesin 10% salt, and some strainsofStaphylococcus aureusin 20% salt. To becertain that pathogen growth is prevented,it is necessary to dry foods down to verylow moisture content or add very highlevels of salt or sugar (Figure 3.5). Totalinhibition of growth, however, is notalways necessary; Staph. aureusis thepathogen that is most tolerant of low water

availability and will grow down to an awof 0.83 but it will not produce toxin if thea

wis 0.86 or below.

Water Activit y(aw) Food Microorganisms & minimum a

wfor growth

High moisture 1.000.98 Meat, fruit, milk, vegetables

0.980.95 Yogurt, evaporated milk,

tomato paste0.950.94 Clostridium botulinum0.93 Baked goods Salmonella

0.90 Most bacteria0.900.880.85 Jam, old cheese Most yeasts, Staphylococus aureus

0.800.800.75 Figs, dried dates, molasses Most moulds, halophilic

0.70 bacteria (salt loving)0.70

Parmesan cheese, dried fruits Osmophilic yeasts,

0.61 xerophilic moulds0.60 Chocolate confectionary,0.50 honey, cocoa

0.40 Potato flakes, crisps

0.30 Crackers, cake mixDry 0.20 Dried milk, dried vegetables

Figure 3.5 Water activity, foods and microbial growth


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As awis decreased, bacteria are inhibited

first (Figure 3.5), followed by moulds andyeasts. Some organisms are particularlyadapted to growing at low a

wvalues

such as the halophilic bacteria, and somemoulds and yeasts but these areassociated mainly with spoilageproblems. No microbial growth at alloccurs when the a

wis below 0.6.

The aw

of a food is also related to therelative humidity of its storageenvironment. If a food is stored in aclosed container and allowed toequilibrate with the atmosphere thatsurrounds it, the relative humidity of the

atmosphere will become equal to the aw

of the food. Thus, for example, driedfruit with an a

wof 0.72 would be in

equilibrium with an atmospheric relativehumidity of 72% (0.72 of saturation).

This has important implications for thestorage of dried foods where, if therelative humidity of the environment ishigher than the a

w, water will condense

on the food. This will increase the foods

aw, perhaps bringing it into a range wheremould growth can occur.

Oxygen (air)

Air comprises about 20% oxygen. Mostmicroorganisms grow much faster ifoxygen is present at this concentration andare known as aerobes. Some, such asmoulds, are obligate aerobes which meansthat they cannot grow at all if air (oxygen)is absent. However, for some bacteria

the obligate anaerobes the presence ofoxygen is actually toxic. Restricting thepresence of oxygen and increasing the levelof other gases such as carbon dioxide is auseful way of preserving some foods sincemany of the normal spoilage organismswill not grow under these conditions.Pathogenic bacteria, however, are largelyunaffected. Most are what are known asfacultative anaerobes which can grow in

the presence or absence of air, and some,such as Clostridium botulinumandClostridium perfr ingens, are obligate

anaerobes and positively require oxygen-free conditions.

Antimicrobial agents

Foods were all once living organismswhich possessed systems to protect themfrom microbial infections that mightdamage them. Some of these systems canpersist into the food product and helpinhibit microbial growth. They are mainlyassociated with plant foods, though thereare a number of antimicrobial systems infoods such as eggs and milk thatcontribute to their stability. Someantimicrobial plant components such asbenzoic acid have also been deliberatelyadded to foods as preservatives.Generally their effect is limited andshould not be overestimated. Garlic whencrushed produces the antimicrobialcomponent, allicin, but crushed garlic inoil has nevertheless been the cause of abotulism outbreak (16).

Antimicrobials are added to foodsprincipally to inhibit spoilage organisms.

One notable exception to this is theartificial preservative nitrite which is ofmajor importance in cured meats as aninhibitor of Clostridium botulinum.Outbreaks of botulism caused by curedmeats are often the result of failures inthe curing process which have resultedin insufficient nitrite being present.

Time

The final, and in many ways the mostimportant factor of all, is time. Bacteriacan grow to dangerous levels if they havethe right conditions for growth, but onlyif they have sufficient time to do so.Microorganisms will grow fastest in foodswith no inhibitory factors and thesetherefore have the shortest safe shelf-life.However, a food may have a wateravailability and pH that slows the growth

of a pathogen but this will be of littlehelp if the food is left for long periodsallowing sufficient growth to occur.


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Survival

It is also important not to forget the pos-sibility of microbial survival. I f bacteriaare present in sufficient numbers in a food,it may not be necessary for them to growin it to produce illness; all they have todo is surviveto maintain those numbers.

This is well illustrated by the bacterialpathogenCampylobacter jejuniwhich doesnot normally grow in foods but has a lowinfectious dose and can survive to cause

illness. Viruses will multiply only in thecells of the infected individual but cansurvive in, and be transmitted by, foods.

The question of bacterial survival is par-ticularly important since, in many cases,conditions that prevent growth enhancesurvival. For example, microorganismscannot grow in dried foods and frozenfoods, but they can survive in these foodsfor very long periods with only a slight

decrease in numbers. Foods which willnot support microbial growth can still,therefore, contain pathogenic bacteria in

numbers sufficient to cause illness. Evenwhere the numbers are low, the bacteria

present can resume growth and multiplyvery rapidly to high levels if conditionsare changed. This could happen if, for ex-ample, a dehydrated product is mixedwith water and left to stand, or a frozenfood is left too long defrosting at tem-peratures suitable for pathogen growth.

Some adverse conditions do affect bothmicrobial growth and survival. I f the pHis too low for a microorganism to grow, it

will die slowly during storage. It may,however, take some time for the food tobecome safe. Little is currently knownabout the survival of non-bacterialpathogens such as viruses and parasitesat the pH levels normally found in foods.

The most effective and accessible wayof killing microorganisms is by heating.Above the maximum temperature whichsupports their growth, microorganisms

will die. The rate at which they die willincrease as the temperature increases. Theconventional way in which we describe

Figure 3.6 Effect of heat resistance (D value) and initial numbers on the survival of

bacteria during heating

0 minutes 1 minute 2 minutes

A

(D=1/2min)

B

(D=1min)

Heating time u


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this rate is by what is known as a D-value.The D-value for an organism is the timethat it takes at a particular temperatureto kill 90% of a population of that or-ganism, i.e. to reduce the number of vi-able cells by a factor of 10. The smallerthe D-value, the faster the death rate atthat temperature.

Measured D-values are quite variable and

depend on the type of food material andthe particular strain of the organism con-cerned, but for a high moisture product atypical bacterial D-value at 65oC can be0.10.5 minutes (630 seconds). If thetemperature is increased by about 5oC thenthe death rate will increase about 10-fold.

How long a food needs to be heated tomake it safe also depends on how manyorganisms are present initially (Figure 3.6).

High numbers will take longer to kill. Topredict accurately how long a food shouldbe heated and at what temperature requiresquite a detailed knowledge of the food andthe pathogens present but, as a generalrule, a food should be cooked so that allof it reaches at least 70oC.

Cooking is not just a useful procedure toeliminate foodborne bacteria. Foodborneviruses and parasites can also be killed,

although in these cases we do not havesuch detailed knowledge of their thermal

stability. Foodborne toxins, however, maybe unaffected by heating. Mostmycotoxins are stable to normal cookingprocedures, as are some bacterial toxinssuch as those produced byStaphylococcusaureusandBacillus cereus.

Some pathogenic bacteria such asClostrid-ium perfringens, Clostridium botulinumandBacillus cereusproduce heat-resistant

spores. These will not be killed by con-ventional cooking procedures and couldresume growth after cooking if the foodis stored for too long at an inappropriatetemperature.

The ability of cooking to solve foodsafety problems is not therefore unlim-ited and wherever possible prevention ofdangerous levels of contamination in thefirst place is preferable.

Foods processed commercially whichhave received a moderate heat treatmentspecifically designed to eliminate non-spore forming pathogens and/or spoilagebacteria are described as beingpasteurized. To eliminate sporeformingpathogens and spoilage organisms heatprocesses far in excess of normal cooking,sometimes known as appertization, arenecessary. This is the treatment given to

canned foods to ensure that they can bestored safely for long periods without re-

Figure 3.7 Factors leading to foodborne illness


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Table 3.3 Factors contributing to outbreaks of foodborne illness

England and Wales1 U.S.A.2

Preparation too far in advance 57 29

Storage at ambient temperature 38 63Inadequate cooling 32

Contaminated processed food 17 n.i.

Undercooking 15 5

Contaminated canned food 7 n.i.

Inadequate thawing 6 n.i.

Cross contamination 6 15

Food consumed raw 6 n.i.

Improper warm handling 5 27

Infected food handlers 4 26

Use of left overs 4 7

Extra large quantities prepared 3 n.i.

1. 1320 outbreaks between 1970 and 1982 from Roberts 1985.

2. Outbreaks occurring between 1973 and 1976 from Bryan 1978.

n.i. category not included in analysis.

frigeration. Further reference to these twoprocesses is made in Chapter 5.

Treatment of food with ionizing radia-tion is similar to heat in its effect on mi-croorganisms. Non-sporeforming bacte-ria and parasites will be killed by quitelow doses (less than 10 kGy). This canbe considered the radiation equivalent ofpasteurization and is termed radicidation.Bacterial spores and viruses are more re-sistant and require much higher doses toensure their elimination. Such doses can

often produce unacceptable flavourchanges in the product and are less likelyto be used in normal commercial prac-tice.

Major factors leading to

foodborne illness

We have looked at the three factors thatcontribute to the presence of dangerousnumbers of microorganisms in a food

contamination, growth, and survival(Figure 3.7).

Hygienic food handling aims to controlthe presence of pathogens in foods by

controlling each of these contributoryfactors. When outbreaks of foodborneillness occur it is because there has beena loss of control over one of these fac-tors.

WHO data indicate that only a smallnumber of factors related to food han-dling are responsible for a large propor-tion of foodborne disease episodes eve-

rywhere. Common errors include:

n preparation of food several hours priorto consumption, combined with itsstorage at temperatures which favourthe growth of pathogenic bacteria orthe formation of toxins;

n insufficient cooking or reheating of

food to reduce or eliminate patho-gens;


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KEY POINTS

l Microorganisms are everywhere.

l Microorganisms can cause illness, they can spoil food, and some

can ferment it into desired products.

l Pathogenic microorganisms can be part of a foods naturalmicroflora, or may be contaminants.

l Contaminants can come from the normal environment, a polluted

environment, pests and pets, the food handler and/or equipment.

l Bacteria and moulds are able to grow in foods, increasing the risks

that they pose.

l The growth of bacteria and moulds can be extremely rapid.

l This growth is affected by the composition of the food and its

storage environment.

l The possibilities of both growth and survivalof bacteria must be

considered when assessing safety.

l Heating is the most effective single method for improving foodsafety.

l Ionizing radiation can also help make food safe.

l A number of different factors contribute to outbreaks of foodborneillness, but most include a failure to control temperature/time.

n cross-contamination;

n people with poor personal hygienehandling the food.

There have been several studies of out-breaks of foodborne illness which at-tempt to identify where the failure hasoccurred. The results of two such surveysare shown in Table 3.3.

Two points should be apparent from thistable. Firstly, the percentages in eachcolumn do not add up to 100%. This re-flects the fact that in many outbreaksseveral failures of good hygienic practiceswere identified. This is perhaps not so

surprising since, if people are not famil-iar with what good practices are, they are

likely to make more than just one error.It may also reflect the fact that to achievean infective dose of a pathogen can re-quire a combination of several errors:

initial contamination with the pathogen,allowing it to multiply, and then failingto eliminate it by adequate cooking. Thesecond significant point is that the mostcommonly identified causes are failure tocontrol temperature and time failureto cool foods correctly and store them attemperatures that prevent microbialgrowth, failure to heat them sufficientlyto kill microorganisms, or prolonged stor-age giving microorganisms time to mul-

tiply to dangerous levels.

WHO Chapter 3

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