34.4.2 Repeated soiling and cleaning
In orderto monitorthe behaviour of organic soil and microorganismson
surfaces subjected to repeated soiling and cleaning, a combination of
microbiological culture techniquesand epifluorescencemicroscopywas used
to evaluate the effect. Up to 20 soiling (microorganisms plus milkpowder) and
cleaning(high-pressurespray) procedureswerecarriedout. Halfof the areaof
eachtest substratumwas swabbedto removecellsfor culture;the remainderwas
stained with acridine orange and a percentage coverage measurement of
substratumby material(soiland cells)was made.
Viable microbial cells(measureby colony count) wereremovedby cleaning,
and accumulationdid not occur, but therewas a gradualincrease in stained
material on the surface,indicating an accumulation of organicsoil.Thiswork
revealed different behaviours of soil and cellson surfaces (Verranet al., 2001a),
but wouldbe verylabourintensiveif implementedin routine procedures.
34.4.3 Soil,cells and wornsurfaces
Stainless steel substrata presenting differenttypesof roughness, all withRa
values of less than0.8m provedcomparable in termsof removingretained
bacteria fromthe surfaces (Verranet al., 2001b;Hilbertet al., 2003).However,
organic soil was retainedin different amounts(measuredby percentagearea
coverage of fluorescein stainedmaterial) relatedto the area of surfacepresenting
defects (not to theRavalue) (Walkeret al., 2000).In addition, soil was retained
more in deeper, sharper features(Boydet al., 2001a,b).
In orderto characterizethe distribution of material across a surface, surface-
sensitive analyticalmethodshavebeenusedto provideinformationaboutthe
elementsor molecular groupspresent.Limitationsincludethe lackof depth
Fig. 34.3 Filmsof 1% starchplus 0.0002%fluoresceinweredriedontoabraded
stainlesssteelsurfacesand heatedat 70ÎCfor 12 h. Afterlightmanualbrushingusinga
toothbrushwettedwithdistilledwater,surfaceswereviewedusingepifluorescence
microscopy.Stainedorganicmaterialis visiblewithinscratcheson the surface.(Image
fromRob Boyd.)
566 Handbookof hygiene controlin the foodindustry