to growin biofilms (vander Wendeet al., 1989;van der Wendeand Characklis,
1990).Thermophilic bacteria(suchasStreptococcus thermophilus) can forma
biofilmin the coolingsection of a milkpasteuriser,sometimeswithin5 hours,
resultingin massivecontaminationof the pasteurizedproduct (up to 10^6 cells
per ml) (Driessen and Bauman, 1979; Langeveld et al., 1995).On metal
(includingstainlesssteel)surfaces, biofilms may also enhancecorrosion, leading
to the developmentof microscopicholes.Suchpinholesallowthe passageof
microbes and thus may causecontaminationof the product. Likeother causesof
fouling, biofilms will also affect heat transfer in heat exchangers. On
temperature probes, biofilms mayseriously affect heattransferand thereby
the accuracy of the measurement.Reducing the effectivenessof heattreatment
mayitselfhelpto stimulate furtherbacterialgrowth. On conveyor beltsand on
the surfacesof blanching equipment, for example, biofilms maycontaminate
cookedor washed products, whichare assumedto havebeen madepathogen-
free by the temperaturetreatmentreceived.
Biofilmsmaybe much moredifficult to removethanordinary soil.If the
cleaning procedureusedis not capable of removing the biofilmcompletely,
decontaminationof the surfaceby eitherheator chemicals mayfail, sincea
biofilmdramaticallyincreases the resistance of the embeddedorganisms(IFT,
1994).It is therefore imperativethat product contact-surfacesare wellcleaned
beforedisinfection.Krysinskiet al.(1992)studiedthe effectsof a variety of
cleaning and sanitisingcompoundsonL. monocytogenes, whichwas allowedto
attach to stainlesssteel and plasticmaterialusedin conveyor beltsovera period
of 24 hours. They foundthat sanitisersalone had littleeffecton the attached
organisms,evenwhen the exposure timewas increased to 10 mins. Unattached
cells,on the otherhand,showeda 5-logreduction in numbers within30 seconds.
In general, acidic quaternaryammonium compounds,chlorine dioxideand
peracetic acidwerethe mosteffectivesanitisersfor eliminating attachedcells.
Leasteffective werechlorine, iodophors and neutral quaternary ammonium
compounds. Whenthe attachedorganismswere exposed to cleaningcompounds
priorto treatment withsanitisers, the bacteriawerereadily inactivated.
1.4 Hygienecontrolmeasuresin foodprocessing
Hygienein foodprocessingstarted withthe introductionof general measures,
including cleaning and disinfection, prevention of recontamination and
treatment of food products to kill any microbial pathogenspresent. Heat
treatment was introduced into foodprocessingevenbeforethe underlying causes
of foodborne illnesswereknown. It was Nicholas Appertin France and Peter
Durandin Englandwhointroduced canningof foodand the use of thermal
processing around 1800.However, neitherAppertnor Durandunderstoodwhy
thermally processed foodsdid not spoiland remained safeto eat (Hartman,
1997).Then,LouisPasteurshowed that certainbacteria wereeitherassociated
withfoodspoilageor causedspecificdiseases.Basedon Pasteur'sfindings,
Introduction 17