Handbook of Hygiene Control in the Food Industry

logÖNÜàlogÖN 0 Üá

k 1 t

lnÖ 10 Ü

á

k 2 t

lnÖ 10 Ü

á

k 3 t

lnÖ 10 Ü

á

k 4 t

lnÖ 10 Ü

àH 0 áSC 1 áSC 2 áSC 3 áSC 4 Ö 2 : 4 Ü

If, for example, SC 2 is an inactivation, and the other three growth,
GàSC 1 áSC 3 áSC 4 and RàSC 2. In principle, the outcome willbe
equalif process stepsare interchanged. It doesnot matterif first a 4 log growth
and thena 6 log reduction takes place,or first a 6 log reduction and then4 log
growth: in bothcases the result will be an overall2 log reduction.Thiscan also
be seenfromthe fact that in eqn 2.2 the effectis dependent onlyonkand not on
the actuallevel.
Thereare threeexceptions:

1. If withingrowththe stationary phaseis reached,but this is generally not the
   casefor pathogens(andshould not be).


2. If the number of organismsin a product unit becomessmallerthan1. Even
   in that casefor largenumbersof productunitsand proportional dose±
   responserelations without threshold, this doesnot havean overalleffecton
   the outcomeof the risk estimate.


3. History effects maymakestagesinterdependent.

In orderto incorporatecontamination in the calculations,one can use the
second characteristicnumber, the contaminationcharacteristic(CC):

CCàlog

NináRc

Nin

 

for (re)contaminationÖCÜ Ö 2 : 5 Ü

in whichNinis the numbers entering the stageandRcis the (re)contamination
rate (in colony-forming units/g).
CCis not onlyconditiondependent but also state dependent, depending on
the number of entering microorganisms.Contaminationis `additive' on a linear
scaleand not on a logarithmicscale.
For a casewherein all stagesof the processbothgrowthor inactivation and
contaminationcan takeplace,one gets:

NàfââÖN 0 áRc 1 ÜexpÖk 1 tÜáRc 2 äexpÖk 2 tÜáRc 3 äexpÖk 3 tÜáRc 4 gexpÖk 4 tÜ...

Ö 2 : 6 Ü

In this casethe finaleffectcan be totallydifferentif contamination occursat
stage1, 2, 3 or 4 (for example beforeor afterpasteurisation).This can also be
seenfromeqn 2.5 wherethe characteristic number depends on the recon-
tamination level(Rc) and on the actualstate(Nin). A recontamination with 10
cellsper gramis much more importantif the actual concentration is 1 cfu/g than
if it is already100 cfu/g.Thisis illustrated in the following example(Fig.2.7).
An imaginary productionprocess is chosen and Staphylococcus aureusis
selectedas the pathogenthat can be present in the product. Duringthe first
production step (mixing) growthis an importantfactorcausing an increaseof
morethan1 log cfu/g (GCà 1 :36). Whencontaminationtakesplacein the next

The rangeof microbialrisksin foodprocessing 41