et al., 2001a).Surface cleanability has beenrelatedto other descriptors suchas
the depthof inwardfacingdefects (Mettler and Carpentier,1999),and the area
of indentation(Walkeret al., 2000),but despitethe difficultiesencounteredin
relatingRato surfacecleanability, it is still the descriptormost commonly used
to describea hygienic surface.
While a surface is usedin a hygienic setting,wearwill inevitablyoccur.Even
hygienic surfaces are susceptible to some degree of wear,perhapsas the peaks
of brushedsteelsare damaged,or as scratchesare introduced (Verranet al.,
2000;Boyd and Verran, 2001).The effectof this wearon surface cleanabilityis
beginningto be explored.
As more sophisticated profilometers have been developed, information
describingsurfacetopographyhas increasedin amount and complexity. Probes
of decreasingsizes,fromsolidthroughlasersto the nanometre dimension probes
of the atomicforce microscope(AFM)generatedifferentRavalues, usually
describingfeatures within smallerand smallerareasof a surface. TheAFM
providesRadataon the nanometre scale,accompanyingan imageof the area
scanned. Interestingdifferencesbetween measurementsand the appearance of
the surfaceemerge.Wearof brushed stainlesssteelis apparentas small tags
occuron the peaks. However, thereis littlechangein theRavalue (Verranet al.,
2000).The effectof suchwearon surfacecleanability in termsof removalof
microorganismsis minimal(Fig. 34.2).Studies on a rangeof stainlesssteels,all
withRavaluesbelow0.8m showedthat bacteriawereremoved (by wiping
withdetergent)in comparablenumbers fromall substrata. Usingepifluorescence
microscopy,it wasapparentthatthe features wereretainingcells, thusthe
pattern of topographicfeatureswas affecting the distributionof cells, but not
theirnumbers, or the strength of retention (Verranet al., 2001b,c; Hilbertet al.,
2003).UsingAFM, cellscouldbe visualized withinfeatures, and on top of
features whose dimensionsweresmaller than cells.By increasingthe strength of
the AFMprobescanacrossthe surface, the relative strengthof retention could
Fig. 34.1 Profilesof two stainlesssteelsurfaceswiththe sameRavalue(0.8m)
presentedalonga 0.8 mm length,and at the sameverticalscale.The profilesdiffer
considerably;the surfacepresentedby the lowerprofilewouldbe easierto cleanbecause
the featuresare larger.
562 Handbookof hygiene controlin the foodindustry