under a curve describing a plot of lethal rate against time gives the
overall process lethality, F 0 (Figure 4.6).
F 0 ¼Z
LRdt ð 4 : 11 ÞThis procedure has safeguards built into it. If the slowest heating point
receives an appropriate treatment then the lethality of the process
elsewhere in the product will be in excess of this. A further safety margin
is introduced by only considering the heating phase of the process; the
cooling phase, although short, will also have some lethal effect.
Process confirmation can also be achieved by microbiological testing
in which inoculated packs are put through the heat process and the
spoilage/survival rate determined. Heat penetration studies though give
much more precise and useable information since inoculated packs are
subject to culture variations which can affect resistance and also recovery
patterns.
A change in any aspect of the product or its preparation will require
the heat process to be re-validated and failure to do this could have
serious consequences. An early example of this was the scandal in the
mid-nineteenth century when huge quantities of canned meat supplied to
the Royal Navy putrefied leading to the accusation that the meat had
been bad before canning. It transpired that the problem arose because
cans with a capacity of 9–14 lb were being used instead of the original
2–6 lb cans. In these larger cans the centre of the pack took longer to heat
and did not reach a temperature sufficient to kill all the bacteria. More
recently, replacement of sugar with an artificial sweetener in hazelnut
puree meant that spores ofC. botulinumsurviving the mild heat process
Figure 4.6 A lethal rate plot.KProduct temperature;lethal rate
76 The Microbiology of Food Preservation