spoilage of pasteurised milkcan occurby eitherof the bacterialgroupsaftera
storagetimeof about9 days.If the contaminationlevel of one of the two
bacterialgroups is increased during processing, that willprobably be the
limiting factorthat reduces shelf-life. As statedabove,during the 9-dayshelf-
life of the milk,the levelofB. cereusincreases by a factorof 1000(fromabout
100 to 10^5 ), whereas the levelof GNPsincreases by a factorof a million(from 1
to 10^6 ). An increase in initial contamination level by a factorof two will thus
havemore impactforB. cereusthanfor GNPs(twoout of a thousandis more
thantwo out of a million, relativelyspeaking).It can be calculatedthat an
increase in initialB. cereuslevelby a factorof two will reducethe shelf-life at
7 ÎC by 10 hours;the sameincrease in initialGNPcontaminationwill reduce
shelf-life by only5 hours(Eneroth, 1999).
Table20.2showsan overview of contaminationlevelsattributableto the
differentsources. It is clearthatthe recontamination withB. cereusin the
pasteuriser is the largestrecontaminationsource in this plant. It is, however, not
necessarily true that this is also the highest risk.To evaluate the risk,a link is
made with shelf-life, as done by Notermans et al. (1997). The extra
contaminationin the pasteuriser increasesB. cereuslevels by a factorof two
and thusreduces shelf-lifeby 10 hours.All the othercontaminationsources
increase the levelof GNPsby amountsadding up to 1 cfu/L.Assumingthat
1 cfu/Lis a normalcontaminationlevel that is associated witha 9-dayshelf-life,
an additionalcontamination by 1 cfu/Lcan be seenas a twofoldincreasein GNP
level,whichcorrespondsto a reductionof shelf-lifeby only5 hours.It is thus
clearthat the risk associated with theB. cereuscontaminationin the pasteuriser
is the highest, as it reduces shelf-life most.
20.4 Future trends
The development of risk assessmentin foodhygieneoverthe past decadehas
beenvery rapid.The two maindriversfor this fast development werethe SPS
Table20.2 Contaminationcontributionof varioussourcesof (re)contamination
Source B. cereus Gram-negativepsychrotrophs
contribution(cfu/L) contribution(cfu/L)
Rawmilk 100 0
Pasteuriser 100 0
Constantpressurevalve 0.25
Deadleg aroundtemperaturesensor 0.05
Air in buffertank 0.1
Air aroundfiller 0.2
Wateraroundfiller 0.2
Packagingmaterial 0.2
Total 200 1
Riskassessmentin hygiene management 321