29.1.2 Cleaning
Cleaning can be describedas the removalof foreignbodies to returna systemto
its original state beforefouling occurred. Theneed to cleanand the con-
sequencesof an ineffectivecleaningregimehavebeendiscussed above.The
cleaningrequirementof a systemvarieson the process. Differentclassifications
for the extent of cleaning are possible:
∑ atomically: cleanon a nano-scale;
∑ physically: no physicalmeasurement of the depositis possible and nonecan
be optically detected;
∑ chemically: absenceof substancesthat may interfere withproduct processing;
∑ biologically(or sterile):free of microorganisms.
Mostcleaningresearchin foodprocessing has studiedmilkbecause of the
ubiquity of the problem. Three phases are involved in the removal of
proteinaceous milkfoul:the heattransfer surface,the depositand the alkali
cleaningsolution.Any of the following reactionsmay be involved in cleaningof
fooddeposits (Plett, 1985):melting,mechanical break-up,wetting,swelling,
desorption,emulsification,hydrolysis,saponificationand dispersion.Removal
may be governed by a combination of masstransfer,diffusionand reaction(Bird
and Fryer,1991),any of whichmaybe the controllingfactor. Jeurninkand
Brinkman (1994)concludedthat the processcannotbe diffusioncontrolledsince
the diffusion coefficientis smalland cleaningwouldtake hoursby diffusion;it
takes onlya matterof minutes,however.
The extentof foulingand cleaning is oftenmonitoredby pressuredropand
heat transferchangesthroughoutthe plant. Historically,process equipmentwas
openedand cleanedindividually.Large-scale productionled to the needfor
cleaning-in-place (CIP)systems, in whichchemicals are circulatedto remove
the deposit; alternativelysurfaces can be subjectedto sprays or jets of the
cleaningchemicals. Rinsingstages,where wateris used,are ofteninvolved.
These processes havebecome highlydevelopedand automated, but are rarely, if
ever, optimised.Cleaning regimesgenerally involvea numberof cycles:
∑ Pre-rinse± circulationof waterto removeloosely boundsubstances fromthe
surface.
∑ Detergentcycle± actionof the cleaning chemical (acidor alkali) to release
the depositfromthe surface.The resultingcomponentsare heldin solution
and removedwiththe fluidflow. The majorityof cleaningtakesplaceduring
this cycle.
∑ Post-rinse± all tracesof depositand cleaningchemical are removedfromthe
system by circulationof water.
∑ Sanitisation± disinfectionand surfaceconditioning.
∑ Finalrinse± circulationof waterpriorto productprocessing.
Currently processes are carriedout underempirical conditions,eitherof fixed
timeor cleaningsolutionvolumeknown to giverepeatablecleanresults.Much
research has beencarried out to optimise the operationof milk processingplants
470 Handbookof hygiene controlin the foodindustry