industries (Frankand Chmielewski, 1997;Fonnesbech Vogelet al., 2001);
Salmonellaspp.in the poultryindustry (Goughand Dodds, 1998);Bacillus
sporesand thermophilic streptococci in the dairyindustry (Faille and Benezech,
1999; Flint et al., 2000); Pseudomonas spp. or Escherichia coli and
Staphylococcus aureus in many general testing proceduresas examples of
typical Gram-positive and Gram-negative spoilage organisms and hygiene
indicators (Wirtanen and Matilla-Sandholm,1994).The susceptibility of these
differentorganismsto givenenvironments and antimicrobialagents suchas
detergents and biocides,and theirinteractionswitha given foodsoil in orderto
facilitate attachment,survivaland evenmultiplicationon a givensurfacewill
determine the hygienicstatusof the appropriatesurface.
34.2.3 Attachment or retention: activeor passive
The literature on factors affectingthe attachmentof microorganismsto surfaces
is significant. Surfacecharge effects and hydrophobicpropertiesare usedto
describe and predict(notalwayssuccessfully)the wayin whichapproaching
microorganismsand surfacewill interact. A recentreviewby Strevettand Chen
(2003)outlinesthe general principlesin more detail.Modificationsmaybe
madeto substratawiththe aim of reducingthis initial stage of colonization,for
example by alteringthe surfacehydrophobicity, or topography.Somemention
will be made of modificationslaterin this chapter; moredetailedexplorations
are made elsewherein this volume.Afterthis `non-specific'interaction,bio-
logicalpropertiesof the surfaces assumeimportance, and a more firmattach-
ment is established. Here, more overt antimicrobial strategies might be
implemented,for example by incorporationof antimicrobialagents into surface
coatings. These reversible-to-irreversible attachmentprocesses are morelikely
to occurin a reproducible manner at a solid±liquidinterface, where the presence
of the liquidenablesthe surfaces(celland substratum)to approach one another.
Cellswill attach at a solid±liquid±air interface, and remainin situ, for example
as the liquiddries.
Wherethereis a solid±air interface, the mechanismwill be different, with
cellsbroughtto the surfaceeithervia air, or by transfer fromother surfaces. In
foodprocessing,theremaywellbe a moreforceful and less activeencounter
between microorganism and substratum,where transfer of cells fromfoodstuffs
to equipment will occuras the foodis passedthrough the plant. Thispassive
transfer may also includefoodmaterial suchas proteins,fats, oils, carbohydrates
fromthe varioushot or coldmeator vegetable-derivedmaterials beingpassed
througha particular processingcycle.
Thus,viable microorganismsmay be presenton a surface, accompanied
potentially by a considerable amount of nutrient.Hydrophobicand surface
chargeeffects will havehad littleimpacton this phenomenon, althoughthey
mayaffectthe easeof removalof the material fromthe surface. Indeed,it is the
cellsremainingon the surface, i.e. theirretention aftera cleaningand sanitiza-
tion event,whichis of prime importance in the study of hygienicstatus, rather
Testing surfacecleanability in foodprocessing 559