showingthat cohesiveforcesbetweenthe depositsexceedthoseof adhesion
betweensurfaceand deposit,the gap betweenthe probeand substrateis 10m
for totalremovaland 50m for partialremoval.For milk(Fig.29.3b),the
initialthicknessof the depositlayerwas around 1300 m. Forcemeasurements
weretakenafterleavingthe gap betweenthe probeand substrateto 900,600,
100 m (partialremoval)and 20m (totalremoval)respectively.Here,the
cohesiveforcesbetweenelementsof the depositare weakerthan thoseof
adhesion.Thisis oppositeto the behaviourof tomatostarch,in whichit is
easierto removethe wholeof the depositthanit is to removea surfacelayer.
For breaddough(Fig.29.3c),the cohesiveforcewas measuredby leavinga
800 m gap betweenthe probeand the substrateduringremoval.The result
showsthat the adhesivestrengthis the sameas the cohesivestrengthat the
first 5 min of dehydration,but, beyondthis time,the adhesivestrengthexceeds
the cohesivestrengthand the differencebetweenthe adhesiveand cohesive
strengths are virtually constantwhile the dehydration time increases. For
albumin(Fig.29.3d),the cohesivestrengthwas measuredby leavinga 800m
gap betweenthe probeand the substrateduringremoval.The resultshowsthat
the cohesive strength is greater than the adhesive strength. These
measurementsshow clearlythat differentmaterials havedifferentbalances
betweendeposition and removal. This may enable us to select different
cleaningprotocolsfor differentdeposits.
29.4 Waysof improving cleaning
Conventional cleaning methodssimply circulate chemicals.A numberof non-
standardmethods have been tried at laboratory or largerscales, and are
discussedhere.
29.4.1 Non-uniformflow
The effectof flow rate has beendiscussed above.Non-uniformflowshave also
beeninvestigatedto determinewhetherthey couldhaveincreased cleaning
behaviour.Cleaningof milksoilsusing entrainedair gave limitedimprovements
(Tra»ga^ rdh,1981).Compressionwavesincreasedthe removalof brittleboiler
scales(Hanjalicand Smajevic,1994),whereaswater hammergavelittleimprove-
ment(Hankinson and Carver,1968)in removal of driedmilkdeposits.Cleaningof
milkingsystems has typicallybeencarriedout by singlephaseflowof the cleaning
chemicalthroughthe equipment.Usingtwo phase flow(air/water)reducesthe
water requiredfor circulation,increasesflowvelocitiesand enhancesmechanical
cleaningaction(Reinemann,1996).The use of ultrasonicshas been investigated
(Grasshoff,1997;Mottet al., 1995)to improveremoval;the penetrationdepthof
ultrasonicwaveslimitsapplicationto real dairyplants.
Pulsingof fluidflowat low frequency and highamplitude enhances heat and
mass transfer (Keil and Baird, 1971).Thereis someconflictin the literature: the
486 Handbookof hygiene controlin the foodindustry