Effect of Rapid Desiccation on Thermal Resistance of Salmonella in Wheat Flour

Introduction:  Salmonella is able to survive in low moisture environments, and has been shown to become more resistant to heat as the water activity (a_w) of the product decreases. However, it is unknown how rapidly the resistance changes if the product water activity is rapidly altered, as can occur in certain processes.  

Purpose:  The purpose of this study was to evaluate the effect of rapid desiccation on the thermal resistance of Salmonella.

Methods:  Wheat flour was inoculated with Salmonella Enteritidis PT30 (~8.0 log CFU/g), then divided into three treatment groups. Groups A and B were equilibrated over ~4 d in controlled-humidity chambers to 0.6 and 0.3 a_w, respectively. Group C was equilibrated to 0.6 a_w, then rapidly dried to 0.3 a_w (< 4 min), using desiccated room temperature air in a small fluidized bed drying system. Samples (~1 g) then immediately (within ~1 min) were isothermally treated (80°C) in aluminum test cells for varying durations, immediately cooled in ice water, serially diluted, and plated on modified trypticase soy agar with yeast extract for enumeration of survivors. D-values were calculated and compared via ANOVA.

Results:  The mean D-values for groups A, B, and C were 1.33, 7.32, and 5.73 min, respectively. The rapidly desiccated group (C) and the group initially equilibrated to 0.3 water activity (B) were not significantly different (P > 0.05), but both were significantly greater than for the group initially equilibrated to 0.6 water activity (P< 0.05).

Significance:  Salmonella in the rapidly desiccated flour (0.3 a_w) was as thermally resistant as that which previously had been equilibrated to 0.3 a_w. These results suggest that the observed enhanced thermal resistance of Salmonella at lower a_w is a state function that requires negligible adaption time.