An in-usesurfacewillbe soiledwithorganic material as wellas micro-
organisms:the removalof one component(organicsoil)wouldbe a particular
focusin termsof cleaning, the other(microorganism)for bothcleaningand
sanitizing, because of impact on hygiene,spoilage and disease.The importance
of a good cleaningregime in the removalof bothsoil and microorganisms
cannotbe underestimated.Disinfection/sanitization shouldbe appliedto the few
cellsremaining duringor afteran effectivecleaning process.Thusthe combined
presenceof soil and microorganismsprovides a challengingand realistictarget
for hygienetestingand surface cleanability strategies.
Somefoulingand cleanability studies use a soil±microorganism mixture, but
thenonlymonitorthe microorganisms.Thereare severalrecipesfor various
soilswhichcan be appliedto surfaces and theirpresencemonitored(Verranet
al., 2002).However, most of these methodstend to lie in the domainof chemists
and engineers rather than microbiologists. Fouling and cleaning in food
processing maybe additionallyconfoundedby the application of heat:for
example, both protein and minerals are deposited on the surfacesin milk
processing (Changaniet al., 1997):thermophilic bacteriaand sporeformersare
the microorganismsof particular concern (Flintet al., 2000).
34.4.1 Soil and microorganisms
Whenattempting to developan appropriate cleaning/sanitizing regime,it should
be deemedessentialto includesomeorganic soil in additionto microorganisms
as the surface inoculum,since the presenceof one componentmayaffectthe
behaviourof another(Barneset al., 1999;Boydet al., 2000).It is not easyto
comparethe relative removalof thesetwo componentsfroma surface,since
methodsof detectionare inevitably not comparable:usually, the numberof
microorganismsand amountof organic material. Microscopymethodsprovide
potential means for discriminating the components, by employing some
differential staining.Image analysiscan provide informationon the percentage
of an areacovered by cells,or amorphous material, and comparethe relative
changes in the two percentagevalues,but few studies havebeencarriedout.
Problems are encounteredwithintensity of staining,selectionof appropriate
stainsand theirdifferentiation, combinedwiththe presenceof an amorphous
material (Fig.34.3).The complexityof the soil is a furtherconfounding issue. If
a simple soilingmaterialis used,suchas starchor protein, thenthe selectionof
an appropriatestain is simplified.Differentsoilswill be encounteredin different
processes, and will be susceptible to differentcleaning regimes and mechanisms,
thus the modelusedneedto be selectedfor a given situation. Complexsoilssuch
as the Campden soil, whichcomprisesa mixture of oil, milkand starch,provide
a more realisticsystem, but differential staining in this context is almost
impossible,sincethe differentcomponents of the soil will behavedifferently,in
addition to the difficulty of visualizing microorganisms remaining on the
substratum.
Testing surfacecleanability in foodprocessing 565