Handbook of Hygiene Control in the Food Industry

theirhighcontamination level. Thesesamplesare largelyaboveeveryFSOthat
should be set. Therefore one could arguethat it is then not so important whereto
set the limit,but howone controls the compliance, and especially howone can
detect and,moreimportantly, preventthese low-frequency,extremelevels.

2.3.2 Quantitative methods
To estimatethe valuesin the FSOequation one can use microbiologicalmethods
or use quantitativemicrobiology. Characteristicnumbers (Zwietering,2002)
showing the changein log numbers can supplythe necessarynumbersfor the
equation in a direct way for every stagein the chain, withthe first characteristic
number, the step characteristic(SC):

SCà

kt

lnÖ 10 Ü

for growth (G) or inactivation (R) Ö 2 : 2 Ü

in whichkis the specific growthrate or inactivationrate (dependingon the
temperatureand otherfactors)andtis the time.
It should be notedthatSCis onlycondition' dependent, i.e. the effectof a heattreatmentremains the same whether the initiallevelof microorganismsis 103 organisms/g or 1 organism/g,e.g. a 6D reduction.Therefore, growthand inactivationareadditive'on a logarithmicscale.If growthand inactivation
processes are considered to followfirst orderkinetics, it is possible to expressa
process without recontaminationas:

NàN 0 expÖk 1 tÜexpÖk 2 tÜexpÖk 3 tÜexpÖk 4 tÜ... Ö 2 : 3 Ü

with k the specific growth or inactivation rate, depending on the actual
conditionsin the stage.
On a log scalethesekinetics becomeadditive:

Fig. 2.6 FSO:link limitswithend result.

40 Handbookof hygiene controlin the foodindustry