general rule, storage temperature should be as low, and as tightly
controlled, as possible.
The ability of organisms to grow at low temperatures appears to be
particularly associated with the composition and architecture of the
plasma membrane (see Section 3.3.2). As the temperature is lowered, the
plasma membrane undergoes a phase transition from a liquid crystalline
state to a rigid gel in which solute transport is severely limited. The
temperature of this transition is lower in psychrotrophs and psych-
rophiles largely as a result of higher levels of unsaturated and short
chain fatty acids in their membrane lipids. If some organisms are allowed
to adapt to growth at lower temperatures they increase the proportion of
these components in their membranes.
There seems to be no taxonomic restriction on psychrotrophic organ-
isms which can be found in the yeasts, moulds, Gram-negative and
Gram-positive bacteria. One feature they share is that in addition to their
ability to grow at low temperatures, they are inactivated at moderate
temperatures. A number of reasons for this marked heat sensitivity have
been put forward including the possibility of excessive membrane fluidity
at higher temperatures. Low thermal stability of key enzymes and other
functional proteins appears to be an important factor, although thermo-
stable extracellular lipases and proteases produced by psychrotrophic
pseudomonads can be a problem in the dairy industry.
Though mesophiles cannot grow at chill temperatures, they are not
necessarily killed. Chilling will produce a phenomenon known as cold
shock which causes death and injury in a proportion of the population
but its effects are not predictable in the same way as heat processing. The
extent of cold shock depends on a number of factors such as the organ-
ism (Gram-negatives appear more susceptible than Gram-posi-tives), its
phase of growth (exponential-phase cells are more susceptible than
stationary phase cells), the temperature differential and the rate of
cooling (in both cases the larger it is, the greater the damage), and the
growth medium (cells grown in complex media are more resistant).
The principal mechanism of cold shock appears to be damage to
membranes caused by phase changes in the membrane lipids which
create hydrophilic pores through which cytoplasmic contents can leak
out. An increase in single-strand breaks in DNA has also been noted as
well as the synthesis of specific cold-shock proteins to protect the cell.
Since chilling is not a bacteriocidal process, the use of good micro-
biological quality raw materials and hygienic handling are key require-
ments for the production of safe chill foods. Mesophiles that survive
cooling, albeit in an injured state, can persist in the food for extended
periods and may recover and resume growth should conditions later
become favourable. Thus chilling will prevent an increase in the risk from
mesophilic pathogens, but will not assure its elimination. There are
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