pathogens, rather than to chemicalor physical contaminants. As theyare
generally undetectableby the unaidedhuman senses (i.e. theydo not usually
causecolourchangesor produce off'-flavoursor taintsin the food)and they are capable of rapidgrowthunder favourablestorageconditions, muchtimeand effortare spentin controllingand/oreliminating them.Evenif the microbes in a foodare ultimately destroyedby cooking, theymayhavealreadyproduced toxins,so it is vital to prevent contamination through the use of hygienic practices. Likemicrobial pathogens, spoilageorganismscan eitherbe present naturallyor gainaccess to food.Althoughnot a foodsafetyconcern,increased levelsof spoilage organismswill usuallymeana reductionin the lengthof time that the foodremainsfit to eat. Thiscan affectproductqualityand thus influence the consumer's perception of the product. Growth of microorganisms depends on a number of factors, such as temperature, humidity/water activity (aW), pH, availability of nutrients, presenceor absenceof oxygenand inhibitory compoundssuchas preservatives. Different organismsrequiredifferentconditionsfor optimal growth(e.g.some grow only in the absence of oxygen, others prefer either warm or cool conditions). Bacterial growthis by the simple divisionof one cell intotwo (binaryfission), and theirnumberwill increase exponentially underfavourable conditions. The influence of factorssuchas temperature, oxygen,pH andaWon microbial activitymaybe interdependent. Microbesgenerally become more sensitive to oxygen, pH and aW at temperatures near growth minima or maxima.Often, bacteriagrowat higher pH andaW, and at a lowertemperature underanaerobic conditionsthantheydo aerobically.Organisms that growat lowertemperaturesare usuallyaerobicand generally havea highaWrequire- ment.LoweringaWby addingsalt or excludingoxygenfromfoods(suchas meat)that are heldat a chilltemperaturedramaticallyreducesthe growthrate of spoilage microbes. Normally, somemicrobialgrowth occurs whenany one of the factors that controls the growthrate is at a limitinglevel.If morethanone factorbecomes limiting, microbial growthis drasticallycurtailed or evencompletelyprevented. Effectivecontrol of pathogenicand spoilagebacteriathus depends on a thorough understandingof the growth conditionsfavouringparticularorganisms.This understanding can be used to minimise contamination of incoming raw materials, to inactivate bacteriaduringprocessingand prevent decontaminated foodfrombecoming recontaminated. It is alsoimportantto know where and how microorganisms can become established, if growth conditions are favourable.Theyare particularly attractedto surfaces thatprovide a stable environment for survival and growth. Surfaces exposedto the air are always vulnerable unless frequentlyand effectivelycleanedand disinfected.However, surfaces withinclosed equipment mayalsobe vulnerable. Thereare usually placesin processinglines,evenwhen correctlydesigned,where someproduct residues remain longer than is desirable. Evenifdead' areas have been
`designed out',some product willattach to equipmentsurfaces,despitethe
possibility of fast-movingliquids.Microbesmayresideon suchsurfaces long
Introduction 15