pH andaw. Micro-organisms can be classified into several physiological
groups based on their cardinal temperatures. This is a useful, if rather
arbitrary, convention, since the distribution of micro-organisms through
the growth temperature range is continuous. To take account of this and
the effect of other factors, it is more appropriate to define cardinal
temperatures as ranges rather than single values (Table 3.12).
In food microbiology mesophilic and psychrotrophic organisms are
generally of greatest importance. Mesophiles, with temperature optima
around 37 1 C, are frequently of human or animal origin and include
many of the more common foodborne pathogens such asSalmonella,
Staphylococcus aureusandClostridium perfringens.
As a rule mesophiles grow more quickly at their optima than psych-
rotrophs and so spoilage of perishable products stored in the mesophilic
growth range is more rapid than spoilage under chill conditions. Because
of the different groups of organisms involved, it can also be different in
character.
Among the organisms capable of growth at low temperatures, two
groups can be distinguished: the true or strict psychrophiles (‘cold
loving’) have optima of 12–15 1 C and will not grow above about 20 1 C.
As a result of this sensitivity to quite moderate temperatures, psych-
rophiles are largely confined to polar regions and the marine environ-
ment. Psychrotrophs or facultative psychrophiles will grow down to the
same low temperatures as strict psychrophiles but have higher optimum
and maximum growth temperatures. This tolerance of a wider range of
temperatures means that psychrotrophs are found in a more diverse
range of habitats and consequently are of greater importance in the
spoilage of chilled foods.
Thermophiles are generally of far less importance in food microbio-
logy, although thermophilic spore formers such as certainBacillusand
Clostridiumspecies do pose problems in a restricted number of situations
(see Chapter 4).
Another feature evident from Figure 3.13 is that the curve is asym-
metric – growth declines more rapidly above the optimum temperature
than below it. As the temperature is decreased from the optimum the
Table 3.12 Cardinal temperatures for microbial growth
Temperature ( 1 C)Group Minimum Optimum Maximum
Thermophiles 40–45 55–75 60–90
Mesophiles 5–15 30–40 40–47
Psychrophiles (obligate psychrophiles) 5toþ 5 12–15 15–20
Psychrotrophs (facultative psychrophiles) 5toþ 5 25–30 30–35
(Adapted from ICMSF 1980)
Chapter 3 47