Modeling the Effect of Temperature and Water Activity on the Thermal Resistance of Salmonella Enteritidis PT 30 in Wheat Flour.

Salmonella continues to be a problem associated with low-moisture foods, particularly given enhanced thermal resistance at lower water activity (aw). However, there is a scarcity of thermal inactivation models accounting for the effect of aw. The objective of this study was to test multiple secondary models for the effect of product (wheat flour) aw on Salmonella enterica Enteritidis phage type 30 thermal resistance. A full-factorial experimental design included three temperatures (75, 80, and 85°C) and four aw values (~0.30, 0.45, 0.60, and 0.70). Prior to isothermal treatment, sample aw was achieved by equilibrating samples in a humidity-controlled conditioning chamber. Two primary models (log linear and Weibull type) and three secondary models (second-order response surface, modified Bigelow type, and combined effects) were evaluated using the corrected Akaike information criterion and root mean squared errors. Statistical analyses of the primary models favored the log-linear model. Incorporating the three secondary models into the log-linear primary model yielded root mean squared errors of 2.1, 0.78, and 0.96 log CFU/g and corrected Akaike information criterion values of 460, -145, and -19 for the response surface, modified Bigelow, and combined-effects models, respectively. The modified Bigelow-type model, which exponentially scaled both temperature and aw effects on thermal inactivation rates, predicted Salmonella lethality significantly better (P < 0.05) than did the other secondary models examined. Overall, aw is a critical factor affecting thermal inactivation of Salmonella in low-moisture products and should be appropriately included in thermal inactivation models for these types of systems.

Effect of rapid product desiccation or hydration on thermal resistance of Salmonella enterica serovar enteritidis PT 30 in wheat flour.

Salmonella is able to survive in low-moisture environments and is known to be more heat resistant as product water activity (aw) decreases. However, it is unknown how rapidly the resistance changes if product aw is altered rapidly, as can occur in certain processes. Therefore, the objective was to determine the effect of rapid product desiccation or hydration on Salmonella thermal resistance. Two dynamic moisture treatments were compared with two static moisture treatments to determine the effect of time-at-moisture on the thermal resistance of Salmonella enterica serovar Enteritidis phage type 30 (PT 30) in wheat flour. After inoculation, two static moisture groups were equilibrated to 0.3 and 0.6 aw over 4 to 7 days, and two dynamic moisture groups then were rapidly (<4 min) desiccated from 0.6 to 0.3 aw or hydrated from 0.3 to 0.6 aw. Samples then were subjected to isothermal (80°C) heat treatments, and Salmonella thermal resistance was compared via decimal reduction times (i.e., D80°C-values). The D80°C-value in flour that was rapidly desiccated from 0.6 to 0.3 aw was statistically equivalent (P > 0.05) to the D80°C-value in flour previously equilibrated to 0.3 aw, but both were greater (P < 0.05) than the D80°C-value in flour previously equilibrated to 0.6 aw. Similarly, the D80°C-value in flour rapidly hydrated from 0.3 to 0.6 aw was statistically equivalent (P > 0.05) to the D80°C-value in flour previously equilibrated to 0.6 aw, and both were less than the D80°C-value in flour previously equilibrated to 0.3 aw. Therefore, Salmonella in the rapidly desiccated flour (0.3 aw) was as thermally resistant as that which previously had been equilibrated to 0.3 aw, and Salmonella in the rapidly hydrated flour (0.6 aw) responded similarly to that in the flour previously equilibrated to 0.6 aw. These results suggest that the response period to new aw is negligible, which is critically important in applying thermal resistance data or parameters to industrial pasteurization validations.