step(homogenising)with1 cfu/g,this doesnot givea highincreasein total
number ofStaph. aureuscells,sincethe concentrationafterthe mixing step was
already1.36 log cfu/g(or 23 cfu/g).At the heatingstep,thereis a largereduction
of cells(RCàˇ 6 :49 log units).Whencontamination takesplaceafterthis
heating step(packaging) with 1 cfu/g, this becomes a very important step
(CCà5 log units). Becauseof the heatingstep,almostallStaph.aureuscells
are inactivated and additionalcontamination, althoughat a lowlevel, thus
causesa highincreasein concentration.Whengrowth is possible duringstorage,
the productcan end up with a highnumber of bacteria(GCà7 log units). This
example showsthat the relevance of recontaminationstronglydepends on the
number of microorganismsalready presenton the productand thuson the
processstage.
In the whole process H 0 à0 (or 1 cfu/g), Gà 1 : 36 á 0 : 08 á 0 : 08 á
0 : 22 á 6 : 96 à 8 :7, Ràˇ 6 :49, Cà 0 : 02 á 5 : 04 à 5 :06, resulting in an
exposure of 0á 8 : 7 ˇ 6 : 49 á 5 : 06 à 7 :27 log cfu/g (seelogNin the storage
step).
2.3.3 Quantification of recontamination
Growthand inactivation can be modelledusingvariouspredictive models, such
as the firstordermodels as presentedpreviously. Contamination, however, is
moredifficultto quantify.Nevertheless, attempts should be made to incorporate
this factorin the FSOequation so that the relevanceof contamination can be
compared withgrowthand inactivation. Recontaminationcan takeplaceat
severalstagesin a productionprocess. Examplesare throughbiofilmformation
in process lines,contaminatedequipment via air, or at consumerlevelwhere
cross-contamination can occur. A wayto obtaincross-contamination in the
kitchen is the use of the same cutting boardto cut chicken followed by
preparation of a salad. Cross-contaminationthen depends on the transferratesof
microorganismsfromone surfaceto the next. Transfer ratesfromchickento
stainlesssteelvarybetween 0 and 10%witha meanof 1.6%forSalmonellaand
2.4%forCampylobacter. Transfer fromstainlesssteelto cucumber has a larger
variation(between 0 and 100%)witha meanof 34.8%forSalmonellaand
42.5%forCampylobacter(Kusumaningrumet al., 2004).Thismeansthat when
a chickenis contaminatedwithCampylobacterat a concentrationof 4 log cfu/
cm^2 (or 10^4 cfu/cm^2 ), the meannumberof microorganismson the stainlesssteel
surfacewill be 2.4 log cfu/cm^2 (or 240 cfu/cm^2 à 2.4%of 10 000). The
cucumbersaladwill thenbe contaminatedwith2 log cfu/cm^2 (or 102 cfu/cm^2
since42.5%of 240à102).Thismeansthat when around 50%is transferred,on
a log scale this means that bothsurfaces end up witharound the same
concentration(initially2.4 log (240cfu/cm^2 ), aftertransfer 2.13log (140cfu/
cm^2 ) left on the surface(57.5% of 240 cfu/cm^2 ) and 2 log on the cucumber
(42.5%of 240 cfu/cm^2 )).
Thereare several modelsavailableto quantify the variousrecontamination
routesin the productionprocess (denAantrekkeret al., 2002).A relatively
The rangeof microbialrisksin foodprocessing 43