A general feature of microbial spoilage is its relatively sudden onset –
it does not appear to develop gradually, day by day a little worse, but
more often as an unexpected and unpleasant revelation. This is a
reflection of the exponential nature of microbial growth (see Section
3.1) and its consequence that microbial metabolism can also proceed at
an exponentially increasing rate. If a microbial product associated with
spoilage, for example an off odour, has a certain detection threshold, the
level will be well below this threshold for most of the product’s accept-
able shelf-life. Once reached, however, it will be rapidly passed so that in
a comparatively short time after, levels will be well in excess of the
threshold and the product will be profoundly spoiled. This is represented
in Figure 5.1 where growth of a spoilage organism is plotted on a linear
scale.
Prediction or early detection of spoilage is not always easy since the
mechanisms underlying microbiological spoilage can be quite complex. It
is generally far easier to identify the chemical responsible for a particular
off-odour than to identify the organism(s) responsible. This and the
relative speed of chemical analysis have led to the use of chemical indices
of spoilage in some areas, but more often the ultimate judgement as to
whether a food is spoiled remains subjective. Total microbial counts are
generally a poor indicator of spoilage potential. Many of the organisms
enumerated may not grow in the food and many of those that do will not
be responsible for spoilage. The value of microbial enumeration tech-
niques can be improved if they are specific to those organisms associated
Figure 5.1 Microbial growth and spoilage
120 Microbiology of Primary Food Commodities