therefore a risk assessment of an imaginary production line of pasteurised milk.
It is basedon the experience of the presentauthorsand on the workof Eneroth
(1999). The mathematics usedin the assessment is of an approximate nature and
is meantto giveroughestimates only.Throughout the examplecontamination
levels are expressedas colony-formingunits(cfu) eitherper mL or per L;
0.1 cfu/Lshouldbe readas 1 cfu/10L.
The production line to be assessedis schematicallyshownin Fig. 20.1.This
plant produces 10^8 litresof pasteurisedmilkper year,whichcorrespondsto
200 000 L per batch. It consists of silo tanks for raw milk,a pasteuriser, buffer
tanks and a filling machine. Numerous valves and sensorsare present on the
production line.Thisexample-assessmentconsidersa constantpressurevalve
(CPM valve)present downstreamof the pasteuriser and a temperature sensor
placed on a T-piecedownstreamof this valve. The fillingmachine fills gable-top
cartons(`tetras') of 1 L and is in opencontact withthe surrounding air. Water is
usedfor chilling the fillers, for lubricatingthe conveyer chains, and for rinsing
of milkresidues.
The risk assessmentis basedon the four stepsaccording to the CAC(1999).
In thehazardidentificationstep, the microbiological hazardsassociatedwith
pasteurisedmilkare identified. Fromliteraturereviews and past experienceit is
clear that the mainorganismsof interestcan be dividedin two groups: (i) Gram-
negative psychrotrophs (such as Pseudomonas, Enterobactereaceae and
Aeromonas) and (ii) Gram-positivesporeformers(mainlyBacilluscereus).
The Gram-negative psychrotrophs(GNPs) are all killedby a suitablyperforming
pasteurisation step,but theymaybe present in the finalconsumerpackages due
to recontamination before sealing the gable-top package. B. cereus spores
survive pasteurisation,and are typically presentin pasteurisedmilk.
Thehazardcharacterisationstep discusses the effectthat above-mentioned
microbiological species haveon quality and safetyof pasteurised milk.Thisstep
is totallybasedon literaturereviewsand is not dependent on the details of the
production line.The groupof GNPsincludes well-known pathogens,suchasE.
coliO157:H7 andAeromonasspecies. Moreover, manyspeciesof this groupcan
causeproduct off-flavours, i.e. spoilage.It is thereforegenerally understoodthat
the level of these bacteria in the final productmustbe below 106 to 10^7 cfu/mL
Fig. 20.1 Productionline for pasteurisedmilk.318 Handbookof hygiene controlin the foodindustry