earlystageof foulingfromwheyprotein solutionson stainlesssteel pipesat
pasteurisation temperatures.Fromscanning electronmicroscopy(SEM)images
theydeterminedthat the metalsurface firstbecame coveredwitha layerof
material (120seconds),to whichaggregatesadhereor grow.The results (from
X-ray elemental mapping, X-rayphotoelectronspectroscopy(XPS)) for deposits
formedover60 minutesshow:
∑ the initialfilm formedhas a compositioncloseto the theoreticalcomposition
for pure -lg;
∑ after150 seconds: calciumand sulphur but not phosphorus are detected ± the
deposit has a non-uniformnature;
∑ after60 minutes:bothcalciumand phosphorus are detected ± a highconcen-
trationof these minerals was foundat the deposit±stainless steelinterface.
These results suggesta changein deposit structure withtime;withproteinbeing
deposited first,thena minerallayer forming at the surface.Visseret al.(1997)
proposed threefouling mechanisms for solutions suchas milkor whey protein
solutionswithmilksalts:
- Denatured wheyproteinsaggregate withthemselves or other proteins and
adhereto the surface. - Inverselysolublecalciumphosphateand calciumcitrate precipitate upon
heating. Crystallisationat the surface can occurand forma mineralscale. - Inclusionof productfluidwithinthe growing foulinglayersallows the
formation of differentlayers,which mayhavedifferentcompositions.This
mechanismis dependent on the processand the product conditions.
De Jonget al.(2002)describes the use of models basedon -lg kineticsto
predict fouling; this typeof model has beenusedby authorssuchas Grijspeerdt
et al.(2004)and Sahooet al.(2005)to study fouling in a varietyof situations.
The simulations suggest that holdingtimesat intermediate temperaturesreduce
fouling throughreactions takingplacein the bulkratherthan on the surface. This
typeof holdingsection has beenusedempiricallyfor manyyears;the models
allowrational selection of processtimesand conditions.
29.2.2 Factorsaffectingfouling
The factors affectingfouling havebeensummarisedby a number of authors
(such as Changani,2000;Tuladhar,2001)and includethe following:
∑ Natural variation:
± compositionand mineral content includingseasonalvariationand cattle
diet (Burton,1966,1968;Fryer,1985);
± pH (Skudderet al., 1986;Hegeand Kessler, 1986a);
± air content(Burton, 1968;Fryer, 1985).
∑ Engineeringfactors:
± age or holding, i.e. keeping milk prior to treatment (Burton, 1968;
Jeurnink,1991);
474 Handbookof hygiene controlin the foodindustry