the baked-on sample,as wouldbe expected, as that has beencookedontothe
surface and wouldbe expectedto bondmorestrongly.
The forcemeasuredby the micromanipulation probeis a composite of the
cohesive forcesbetween depositelements and the adhesion to the surface. This
is shownexplicitly by Fig. 29.4(b), whichshowsdata for differentthicknesses of
unbaked paste:
∑ As the thickness increases,the totalforcerequiredto removethe deposit
increases. Theincrease reflects the needto overcome the cohesive forces
between elementsof the depositand to forcethe depositto break and flow
withthe probeawayfromthe samplesurface; as the thickness increases, so
doesthis force.
∑ A similar minimumto that foundin Fig. 29.4(a),in similarregionsof surface
energy. However, the minimum becomes moredifficult to identify as the
thickness of depositincreases, reflectingthe decreasedcontributionof the
surface forcesto the whole.At the highest thicknessesused,thereis no
measurableminimum;the curvesimply flattensout at the lowestsurface
energies.
These resultsdemonstratethatmodifying the surface energycan affectthe
energy required to clean the surface. Theproblem in practice willbe in
producing surfaces that are cheap(andsafe)enoughfor use in the foodindustry,
and ensuringthat theydo not get broken downin operation;the surfaces must
stay effectiveoverthe lifetimeof a plant. It mightbe that a moreeffective
solution wouldbe the addition of a chemical duringthe last stages of cleaning
whichchanges the surfaceproperties to resistfoulingand/oraid subsequent
cleaning.
29.5 Conclusions
The problemof fouling, and the associated problemsof cleaning, havebeen
reviewed, withparticular reference to milkfluids, as theyhavebeenmost
thoroughlystudied. Foulingfrommilkresultsfromprotein and mineral deposi-
tion,eachof whichresults in different problems for cleaning.Cleaning timeis a
function of a numberof variables; bothchemical (such as the cleaning chemical
type and concentration,and the temperature)and physical(suchas the flowrate,
whichaffectsthe fluidshearon the surfaceof the deposit). In milkcleaning,
protein depositis firstswollen by action of hydroxide and thenremovedby
shear. The cleaning rate increases withincreasingtemperatureand surfaceshear
stress, but therecan be an optimalconcentrationof hydroxide, abovewhichthe
deposit becomes difficult to remove.A numberof methodshavebeentriedto
increase the cleaningrate,including pulsed flows,enzyme cleanersand ice pigs.
Surfacemodification has beentriedby a number of workers.
It is possible to separate adhesiveand cohesiveeffects in cleaning: some
deposits (suchas tomatopaste)are essentiallycohesive, while proteindeposits
490 Handbookof hygiene controlin the foodindustry