3.2. Survival of Salmonella in samples held at 21 to 80 °C
Data corresponding to the survival of Salmonella at various temperatures
in low-moisture protein powder are presented in Figs. 1
through 4. Model fit statistics for the log-linear, Baranyi, Geeraerd-tail,
Weibull and biphasic-linear models for all experimental conditions
under study are presented in Table 2, where the best statistical
parameter fits are shown in bold. The Geeraerd-tail, Weibull and
biphasic-linear models were not suitable for describing the 21 °C
data because survival numbers were maintained throughout the
experiment.
The Salmonella counts used for the data analyses,model development
and model validation, were obtained using non-selective differential
media. A comparison of non-selective with selective counts indicated
that the proportion of injured cells (Eq. (2), data not shown)was not significantly
influenced by temperature (p = 0.228), aw (p = 0.371) or
water mobility (p = 0.411). Storage time just significantly influenced
the proportion of injured cells (p = 0.044), as longer storage times led
to increasing proportions of injured cells. These results do not support a
hypothesis that the mechanism of inactivation changed from membrane
damage at lower temperatures (≤50 °C) to ribosomal degradation at
higher temperatures (N50 °C) as suggested by Aljarallah and Adams
(2007). Heating cells to temperatures just above their maximum growth
temperature causes damage to the cytoplasmic membrane, which in