are tested. Such very low rates of defectives are realistic for pathogens
which occur infrequently and at low concentration.
From Table 2 it can be seen that even a sampling plan with 60
sample units has quite a low probability of detecting contamination
rates of 1 or 2% as the probabilities of detection are only 45 and 70%,
respectively. For example in a batch of 100,000 chocolate bars of
which 1% (i.e. 1000 bars!) are contaminated with Salmonella, the
probability that this rate of contamination would be detected with
60 sample units is only 45%, meaning that such a batch will go
undetected in 55% of the cases. Obviously, Salmonella contamination
rates of 1 or 2% in chocolate would be unacceptably high. Also,
the statistics presented in Table 2 assume that the contamination is
homogenously distributed throughout the batch, and that Pdefective
is equal for every sample unit taken.
5. Sampling and control in a typical food production process
Typically, in production processes, (1) the raw material undergoes
inactivation to eliminate or reduce the level of microorganisms
which are present, (2) recontamination from the
processing environment may occur during the industrial processing
and (3) growth may occur during transport and storage (either
in a professional setting or at the consumer level) before the food is
consumed (Fig. 2). The order of the inactivation, recontamination
and growth can be different as in this scheme. Microbial testing can
be performed by sampling the food as raw material, during processing,
and after processing or at the end of shelf-life in case of
perishable foods. Also, the production environment can be sampled
and tested to identify the potential for recontamination.
Fig. 2 represents a general flow chart of these typical elements of
a food production process from raw materials to consumption. It is
a strong simplification because inactivation, recontamination and
growth can occur at several steps of the process. The flow chart
stresses that if inactivation eliminates microorganisms and recontamination
is prevented, production is under control. If microorganisms
are still present at low numbers, prevention of growth (e.g.
by short storage times at low temperature) will keep the level low
until consumption.
Changes in concentrations of microorganisms can be expressed
mathematically. If Nrm is the concentration in the raw material, Nf1
is the concentration in the food product after inactivation, Nf2 is the
concentration in the food product after recontamination and Nend is
the concentration in the finished product, they relate as:
log Nf1 ¼ log Nrm log Red (2)
Nf2 ¼ Nf1 þ Rec (3)
log Nend ¼ log Nf2 þ log Growth (4)
where log Red is the log reduction by inactivation, Rec is the concentration
increase due to recontamination expressed as cfu/g, cfu/