heat resistance. The total picture is probably more complex than this
however, since other features of the spore such as its high content of
divalent cations, particularly calcium, are thought to make some con-
tribution to heat resistance.
Thermal sensitivity as measured by the D value can vary with factors
other than the intrinsic heat sensitivity of the organism concerned. This
is most pronounced with vegetative cells where the growth conditions
and the stage of growth of the cells can have an important influence. For
example, stationary phase cells are generally more heat resistant than
log phase cells. Heat sensitivity is also dependent on the composition of
the heating menstruum; cells tend to show greater heat sensitivity as the
pH is increased above 8 or decreased below 6. Fat enhances heat
resistance as does decreasing awthrough drying or the addition of
solutes such as sucrose. The practical implications of this can be seen in
the more severe pasteurization conditions used for high sugar or high fat
products such as ice cream mix and cream compared with that used for
milk. This effect is quite dramatic in the instance of milk chocolate
where the D 70 value ofSalmonellaSenftenberg 775 W has been meas-
ured as between 6 and 8 hours compared with only a few seconds in
milk. A more specialized example of medium effects on heat sensitivity
occurs in brewing where the ethanol content of beer has been shown to
have a profound effect on the heat sensitivity of a spoilageLactobacillus;
an observation that has implications for the pasteurization of low-
alcohol beers (Figure 4.3).
At present all thermal process calculations are based on the assump-
tion that the death of micro-organisms follows the log-linear kinetics
described by Equation 4.4. Though this is often the case, deviations from
log-linear behaviour are also often observed (Figure 4.4). Sometimes
these deviations can be rationalized on the basis of some special property
of the organism. For example, an apparent increase in viable numbers of
organisms or a lag at the start of heating may be ascribed to heat
activation of spores so that in the first moments of heating the number of
spores being activated equals or exceeds the number being destroyed.
Alternatively a lag phase may reflect the presence of clumps of cells, all of
which require to be inactivated before that colony forming unit is
destroyed. The frequently observed tailing of the curves, which has
greater practical significance, may be due to sub-populations of cells
that are more heat resistant. These deviations from the accepted model
tend to be observed more often when studying the thermal death of
vegetative organisms and in some cases may reflect inadequacy of the
logarithmic death concept in this situation.
The primary assumption which gives rise to log-linear kinetics is that
at a constant temperature each cell has an equal chance of inactivation at
any instant. This can be explained on theoretical grounds if there is a
70 The Microbiology of Food Preservation