safety implications in mould-ripened cheeses where mould growth
increases the pH allowing less acid tolerant organisms such asListeria
monocytogenesto grow.
Alternatively, micro-organisms may be antagonistic towards one
another producing inhibitory compounds or sequestering essential
nutrients such as iron. The best practical examples of this in food
microbiology are the lactic fermented foods which are discussed in some
detail in Chapter 9.
3.5 Predictive Food Microbiology
Understanding how different properties of a food, its environment and
its history can influence the microflora that develops on storage is an
important first step towards being able to make predictions concerning
shelf-life, spoilage and safety. The food industry is continually creating
new microbial habitats, either by design in developing new products and
reformulating traditional ones, or by chance, as a result of variations in
the composition of raw materials or in a production process. To be able
to predict microbial behaviour in each new situation and determine its
consequences for food safety and quality, we must first describe accu-
rately the food environment and then determine how this will affect
microbial growth and survival.
Characterization of a habitat in terms of its chemical and physical
properties is generally straightforward, although problems can arise if a
property is not uniformly distributed throughout the product. This can
be a particular problem with solid foods, for example the local salt
concentration may vary considerably within a ham or a block of cheese
and we have seen in Section 3.3.1 how water can migrate through a mass
of food.
A considerable amount of data is available on how factors such as pH,
aw, and temperature affect the growth and survival of microorganisms
and some of this was described in Sections 3.2 and 3.3. Much of this
information was however acquired when only one or two factors had
been changed and all the others were optimal or near-optimal. In many
foods a very different situation applies, micro-organisms experience a
whole battery of sub-optimal factors which collectively determine the
food’s characteristics as a medium for microbial growth.
Leistner described this situation as the ‘hurdle effect’, where each
inhibitory factor can be visualized as a hurdle contributing to a food’s
overall stability and safety (Figure 3.14). The analogy of a hurdle race,
while vivid, has been criticized on the grounds that it can lead to the
misapprehension that micro-organisms confront each hurdle in turn. In
reality they are usually faced with the aggregate effect of all the barriers
at once and it is this which determines whether the organisms can grow
52 Factors Affecting the Growth and Survival of Micro-organisms in Foods