as key nutrients become depleted, or inhibitory metabolites accumulate,
and the culture moves into the stationary phase.
One way of representing this overall process mathematically is to
modify the basic growth Equation (3.1) so that the growth rate decreases
as the population density increases. An equation that does this and gives
us a closer approximation to the observed microbial growth curve is the
logistic equation:
dx=dt¼ðmmmmx=KÞx ð 3 : 6 ÞwhereKis the carrying capacity of the environment (the stationary phase
population) andmm, the maximum specific growth rate. Asxincreases
and approachesK, the growth rate falls to zero. Or, in its integrated
form:
x¼Kc=ðcþemmtÞð 3 : 7 Þwherec¼x 0 /(Kx 0 ).
The significance of exponential growth for food processing hardly
needs emphasizing. A single bacterium with a doubling time of 20
minutes (m¼2.1hr^1 ) growing in a food, or pockets of food trapped in
equipment, can produce a population of greater than 10^7 cells in the
course of an 8-hour working day. It is therefore, a prime concern of the
food microbiologist to understand what influences microbial growth in
foods with a view to controlling it.
The situation is complicated by the fact that the microflora is unlikely
to consist of a single pure culture. In the course of growth, harvesting/
slaughter, processing and storage, food is subject to contamination from
a range of sources (Chapter 2). Some of the micro-organisms introduced
will be unable to grow under the conditions prevailing, while others will
grow together in what is known as an association, the composition of
which will change with time.
The factors that affect microbial growth in foods, and consequently
the associations that develop, also determine the nature of spoilage and
any health risks posed. For convenience they can be divided into four
groups along the lines suggested more than 50 years ago in a seminal
review by Mossel and Ingram (Table 3.1) – physico-chemical properties
of the food itself (intrinsic factors); conditions of the storage environ-
ment (extrinsic factors); properties and interactions of the micro-
organisms present (implicit factors); and processing factors. This last
group of factors (subsumed under intrinsic properties by Mossel and
Ingram) usually exert their effect in one of two ways: either they change
an intrinsic or extrinsic property, for example slicing a product will
damage antimicrobial structures and increase nutrient availability
and redox potential, or they eliminate a proportion of the product
microflora as would occur in washing, pasteurization or irradiation.
22 Factors Affecting the Growth and Survival of Micro-organisms in Foods