assume the incidence of salmonella in the raw milk to be 1 colony-forming
unit (cfu) 1^1 , then after pasteurization it will be reduced to 10^10 cfu 1^1
or 1 cfu (10^10 1)^1. This means that if the milk is packed in 1 litre
containers, one pack in 10 billion (10^10 ) would contain salmonella. If
however the level of salmonella contamination was higher, say 10^4 cfu 1^1 ,
then the same heat treatment would result in contamination of one in a
million packs. Such simple calculations underestimate the true lethal effect
since they assume instantaneous heating and cooling.
As the temperature is increased so the D value decreases. This is an
exponential process over the range of temperatures used in the heat
processing of food so that plotting log D against temperature gives a
straight line. From this we can derive another important parameter in
heat processing,z:the temperature change which results in a tenfold (1
log) change in D(Figure 4.2).
z¼ðT 2 T 1 Þ=ðlogD 1 logD 2 Þð 4 : 6 ÞKnowledge of an organism’szvalue is important if we are to take into
account the lethal effect of the different temperatures experienced during
a heat process.
4.1.3 Heat Sensitivity of Micro-organisms
The heat sensitivity of various micro-organisms is illustrated by Table 4.3
which shows their D values. Generally psychrotrophs are less heat
resistant than mesophiles, which are less heat resistant than thermophiles;
and Gram-positives are more heat resistant than Gram-negatives. Most
vegetative cells are killed almost instantaneously at 100 1 C and their D
values are measured and expressed at temperatures appropriate to pas-
teurization.
Figure 4.2 The z-value
68 The Microbiology of Food Preservation