gelledbiopolymers).Understandingof the interactionbetweendeposits and
surfacesis clearly criticalin cleaning.On a nanoscale, atomic force microscopy
(AFM)has beenusedto characterisesurfaces and fouling(suchas Parbhuet al.,
2002;Weisset al., 2002).Low-adhesion coatings (Mu»ller-Steinhagen and Zhao,
1997)reduce foulingin some situations suchas mineral scalesand workis
underway to studythe application of modifiedsurfaces in foodcleaning(Santos
et al., 2004,for example).
Zhaoet al.(2004)demonstrate that biofouling can be reducedby changing
surface energy, and link this to adhesiveenergybetween surface and deposits;
thereis evidencefor the minimum in termsof foulantattachmentwhenthe
surface freeenergies coverthe range20±40 (mN/m). Theorygives thata
minimumadhesionenergybetweendepositand surfaceexists, givenby:
ÅÅÅÅÅÅÅÅÅÅÅÅÅ
LWsurface
q
à
1
2 ÅÅÅÅÅÅÅÅÅÅÅÅÅ(^) foulaLWnt
q
á
ÅÅÅÅÅÅÅÅÅÅ
(^) fluidLW
q
Ö 29 : 5 Ü
where LWsurface, (^) foulaLWntand LWfluidare the Lifshitz-vander Waals(LW)surfacefree
energy of the surface,foulant and fluid(e.g.water),and can be determined
experimentally fromcontact angle measurements(Zhaoet al., 2004). In general,
fouling formedon processing equipmentmay consist of various types of
foulants (suchas the mixturesof proteins and minerals deposited frommilk)and
LWfoulantin equation (29.5)is the average LWsurface energyof the fouling
deposit, whichcan be determinedby measuringcontact angles.If the surface
energy of the stainlesssteelsurfaceis reduced to the fouling-resistant value,
foulant adhesion force to the surfacecouldbe decreasedsignificantly,and the
fouling deposit could be removed moreeasily. The effectiveness of this
approach has beendemonstratedby Zhaoet al.(2004)for microbial adhesion.
Liuet al.(2005)discussadhesion to a rangeof stainless steeldisksmodified
by Ni±P±PTFE composite coatings. Contact angles of the Ni±P±PTFE
compositecoatings and tomato paste(baked and unbaked)wereobtainedand
surface energies calculated(vanOsset al., 1986)as
∑ LWsurfaceà26 mN/m,for whichthe adhesion of bakedtomato paste is minimal;
∑ LWsurfaceà25 mN/m,for whichthe adhesion of unbaked tomato pasteis
minimal.
The effectof surfacetreatmentcan be quantified by the probe. Experiments have
studied the removalof tomatopastefromsurfaces withsurfaceenergies ranging
from15 to 40 mN/m.Detailsof the surfaces are given in Zhaoet al.(2002,
2004):the topographyand roughnessescan be significantlydifferent.
Figure 29.4(a)showsdata for bakedand unbaked pastes;in bothcasesthere
is a minimumadhesivestrength betweenthe surfaceenergies of 20 and 25 mN/
m, withan increase in the adhesive strength on eitherside.Thedataare
scattered, but the minimumis clearin bothcases; it is moreobvious for the
unbaked material in whichthe changein adhesivestrengthwithfree energy is
smoother.The minimumis in the regionpredicted by the theory (see above);at
eitherside of this minimum the forceincreases,an effectespeciallymarkedfor
488 Handbookof hygiene controlin the foodindustry