surface,reversible and irreversibleadsorption to the surface,cell replication,
transport of nutrientsand metabolism,productionof extracellularpolymers and,
finally,detachment.
3.1.3 Samplingand detectionof biofilm formationin foodprocessingsites
Methodsfor studyingbiofilm formation includemicrobiological, chemical,
microscopicaland molecular biologicalmethods(Wirtanen, 1995; Holah&
Timperley,1999;Wirtanenet al., 2000a,b; Saloet al., 2000,2002;Maukonenet
al., 2003). Practical methods for assessing microbes and organic soil on
processingsurfaces are neededto establish the optimalcleaningfrequency of the
equipment.Hygienemonitoring is currently basedon conventionalcultivation
using swabbing, rinsing or contact plates.Surface samplingcan be improvedby
wetting the surfacein advance. In methodsthat use swabs,sponges or something
similar, the detachment of surface-boundmicrobesis a limitingfactor.In the
cultivationof biofilmmicrobes, it is importantfor the sampleto be detachedand
mixedproperly. Agitation usedtoo forcefullyin the detachmentof the biofilm
fromthe surfacemayharm the cells, makingthemunableto growon the agar
plates, whereas insufficientmixing mayresultin clumpsand inaccurateresults.
Ultrasonicsdetaches aboutten times the numberof cells fromthe surface
comparedwithswabbing (Wirtanenet al., 2000b).In biofilm detection the
planktoniccell countsof processing fluidsshould be interpreted with caution
because theyare not alwaysrepresentative of the sessileorganismsfoundon
surfaces,especially in badly designedequipmentand processlines.Organisms
fromextreme environmentsare difficult to cultureand thereforestandardplate
counts do not giveaccurate estimates. The choice of agarand incubation
conditions during the cultivationis governed by the characteristics of the
microbes that are considered to be the mostimportant.
Conventional culturingtechniques are usedto measurethe number of viable
cells ableto growon the chosen agarat givencircumstances.The platesand
slidesare usuallyincubatedat 25±30ÎC for 2±3 days.Theagars are either
nutrient agars, whichmaycontaintryptose,yeast,glucoseand agar-agar, or
selectiveagars basedon growthinhibitors,e.g. nutritional,antibioticor acidic
compounds. The international standard methods for the detection and
enumeration of spoilage and pathogenicmicrobes are based on culturing
techniques(vanNetten& Kramer,1992;Saloet al., 2000).
Impedancetechniques can be usedto enumeratemicroorganismsdirectlyon
surfacesas the increase in conductance and capacitancedue to the metabolic
activityof the microbesin the sample leadsto a decreasein the impedance.The
measurement of the change in impedance value at suitable timeintervals
provides an impedance curveand thus the detectiontimeof microbial growthin
the sample (Firstenberg-Eden, 1986). The detection time depends on the
number of microbesin the sample.Results are achievedmore rapidly with
impedance measurements than withcultivation. Impedancemeasurement is
usedin the foodindustry to control product quality and to assess the effect of
48 Handbookof hygiene controlin the foodindustry