example, to remain (Freemanet al., 2002a). Thus this material tendsnot to be
useddirectlyin foodapplications.Specialfinishes are applied to 316 2B, and to
othersteels, thusmasking the original grainboundary features. Thesefinishes
include ground, brushed or polished surfaces and pattern rolled surfaces.
Specially polished `hygienic' finishes are also available. Interestingly,the
surfacefinishis usedto describe theprocessrather thantheproduct, thus there
may wellbe considerable variability within the propertiesof steel witha
specifiedfinish.
34.3.3 Surface roughness
In the food,beverage and pharmaceuticalindustries,highlyexacting hygienic
demandsare imposed on the surfacesthat comeinto contact withthe product
beingmanufactured(Boulange-Petermann,1996).These demandsare specified
in the appropriate foodstandards,for example, DIN 11850-10/99specifies a
surfaceroughnessRamax0.8m for tubing,the implication being that surfaces
withRavaluesbelow this levelare hygienic'. The maximumfor the weldarea is 1.6m. TheRavalue describes the averagedeparture of the surfaceprofile from a calculatedcentre line'. A probe is scanned across a surface,
perpendicularto the lay of the surface features,and a `centre line'is plotted
so that the areasof the profile aboveand belowthis line are equivalent.TheRa
valueis a figurerepresentingthe average departureof the profilefromthis
centreline.TheRavaluegives an indication of the size (height) of peaksand
valleys, but says nothing about the character of the surface in different
directions, the occurrence of longwaviness or amplitude distributions(Fig.
34.1).Unlessotherwise specified the referencelengthoverwhichthe surfaceis
scanned is 0.8 mm.Otherparameters havebeen used to describe surface
features, and are definedwithinthe variousstandards (Anon,1990),but theRa
valueis the mostgenerally accepted.TheRais deemedof importance becauseit
is assumed, and has been demonstrated,that an increase in surfaceroughness
will causemicroorganismsto be moreeasilyentrapped withinsurfacefeatures,
hence reducing the cleanability of the surface (Verran and Boyd, 2001).
However, this relationshipis only demonstrablewithincertain limits ofRa
values.Whenfeaturesare significantlylarger thanthe microbial cells,thenthe
cellsare relativelyeasilyremoved fromthe surface.
Various workers havesoughtto definea minimumRavaluebelowwhich
thereis no relationshipbetweenRaand retention. Furthermore,sincetheRa
valueis derivedfrom a lineartrace, informationon the three-dimensional
character of surfacefeatures is lacking.Thefinish mayor maynot affect
cleanability (Freemanet al., 2002b;Leclercq-Perlat and Lalande,1994;Steiner
et al., 2000).Linear surface features suchas those on a brushed finishwill be
moreeasilycleanedalongthe lay of the features thanacrossthe features, and
presumably alsomoreeasilythansurfaceswherelinearfeatures (scratches)
occurrandomlyacrossthe surface. Pittedsurfaces presentyet anothervariable,
and representthe type of wearmorecommonlyseenin ceramicsurfaces (Verran
Testing surfacecleanability in foodprocessing 561