Damage to DNA has been identified as the probable key lethal event
in both spores and vegetative cells. In spores, however, inactivation of
germination mechanisms is also important. If this inactivation can be
bypassed in some way, then apparently dead spores may be cultured.
This has been demonstrated by the inclusion of lysozyme in recovery
media where the enzyme hydrolyses the spore cortex, replacing the
spore’s own inactivated germination system.
Deviations from log-linear kinetics in the thermal death of vegetative
cells probably reflect a greater multiplicity of target sites for thermal
inactivation such as the cytoplasmic membrane, key enzymes, RNA and
the ribosomes. This type of damage can be cumulative rather than
instantly lethal. Individual inactivation events may not kill the cell but
will inflict sub-lethal injury making it more vulnerable to other stresses. If
however injured cells are allowed time in a non-inhibitory medium, they
can repair and recover their full vigour. Examples of sub-lethal damage
can be seen when cells do not grow aerobically but can be cultured
anaerobically or in the presence of catalase, or when selective agents such
as bile salts or antibiotics, which are normally tolerated by the organism,
prove inhibitory.
Two other factors also contribute to deviations from log-linear
behaviour in vegetative organisms. Individual cells within a population
may exhibit a broader range of heat resistance than is seen with
spores and, since vegetative cells are not metabolically inert, they
may also respond and adapt to a heating regime modifying their
sensitivity.
4.1.4 Describing a Heat Process
Heating processes are neither uniform nor instantaneous. To be able
to compare the lethal effect of different processes it is necessary for us
to have some common currency to describe them. For appertization
processesthisisknownastheFvalue;aparameterwhichexpresses
the integrated lethal effect of a heat process in terms of minutes at a
given temperature indicated by a subscript. A process may have an F 121
value of say 4, which means that its particular combination of times
and temperatures is equivalent to instantaneous heating to 121 1 C,
holding at that temperature for four minutes and then cooling in-
stantly, it does not even necessarilyimply that the product ever reaches
1211 C. The F value will depend on thezvalue of the organism of
concern; if z¼ 101 C then 1 minute at 111 1 C has an F 121 ¼0.1, if
z¼ 51 CthentheF 121 valuewillbe0.01.Itisthereforenecessaryto
specify both thezvalue and the temperature when stating F. For spores
zis commonly about 10 1 CandtheF 121 determined using this value is
designated F 0.
72 The Microbiology of Food Preservation