thanthe initialattachment. Factors that will enhance retentionwouldinclude
attached cellsdryingontothe substratum,the presenceof organic material, and
the presenceof surface defectsand featuresin the substratum.
34.3 Hygienic surfaces
34.3.1 Stainless steel
The mostfrequentlyquotedexample of a hygienicsurfaceis stainlesssteel,
which willtherefore be used to address general principles. This material
demonstratesall the propertiesrequired for hygienic status.Highchromium
content (over10.5%) steel resists attack by food acids; engineered surface
finishesfacilitate cleaningin catering,foodprocessing and surgical applications.
All metalsreact withoxygenand waterin the atmosphere to forma surfacelayer
of oxide.On ordinarycarbonsteel,this is an hydratediron oxidewhich,being
porous, permitsfurtheroxidation,producingrust.By alloying the steelwith
chromium to produce stainlesssteel, the oxideformedis chromiumoxide.This
`passive'layera few atomsthick,protects the metalsurfacefrom further
reaction,and, whenscratched, has the ability to reformulate itself within
seconds. Aluminiumand titaniumare othernotablemetals in this context.
The austenitic stainlesssteels present goodcorrosionresistance and tough-
ness properties. Gradings, for example 304 and 316 (ASTMdesignation),are the
stainlesssteelsmost commonlyusedin the foodindustry,related to the different
proportionsof chromium, nickel and molybdenumin the formulations (only 316
contains molybdenum, which enhances corrosion performance). There are
several other designations,with European national differencesin terminologies
nowbeing superseded by the EN prefix. The JIS Japanese designationprovides
the thirdusedto definethe globalsteel market(www.outokumpu.com).
34.3.2 Surfacefinish
During production,coilsof stainlesssteelare first softenedand descaled, in a
process knownas annealing and pickling.This processusesgas-firedfurnaces,
cooling, shot blastingand a mixtureof hydrofluoricacid and nitric acid to clean
the surfaceof the material to a mattgreyappearance, priorto processingto a
given finish. Aftercold rolling,whichreduces the thickness of the steel,material
may be softened using an inertatmosphere whichpreventsoxidation and ensures
a brighthighlyreflective finish, called BrightAnnealed(BA,or 2R).Typical
applicationsare the productionof sinks and washingmachine drums. Alter-
natively, materials can be softened and processedfurtherfor enhancementof
surface finish. The surfaceproducedby this routeis described as 2B, and is the
most common`mill finish' witha smooth,greyappearance.Thusthe 2B and 2R
finishesare usedmostcommonly in the foodindustry. Grain boundariesare
apparent on steels witha 2B finish. These features serve to entrapmicro-
organisms,and are also less easyto clean, enabling marksdue to fingerprintsfor
560 Handbookof hygiene controlin the foodindustry