Purpose: To develop a simple method to study the thermal resistance of Salmonella serotypes inoculated in oat flour at low water activity.
Methods: Outbreak and non-outbreak-associated salmonellae (n = 16) were used for the inoculation of oat flour. Dry matrices in Whirlpak bags were inoculated with lawn-grown cells at a 1:1 ratio (~18 g total) and massaged by hand until thoroughly mixed. Inoculum was packed in aluminum crucibles and stored open at 11% relative humidity and ambient temperature (23 ± 2°C). Crucibles were sealed after equilibration was achieved (3 to 6 days). A differential scanning calorimeter (DSC) was used to heat treat crucibles at 90°C for 5, 7 and 10 min and followed by cooling to 20°C for 2 min. Lids were aseptically removed from crucibles and samples diluted in buffered peptone water (BPW), then plated on tryptic soy agar with yeast extract (TSAYE) and incubated aerobically at 37°C for 24 h. Counts were expressed as CFU/g.
Results: Triplicate samples showed little variation (mean SD = 0.2 log). All serotypes exhibited enhanced thermal resistance at lower aw (~0.2) and linear inactivation behavior from which D-values were calculated that were comparable to published data. Salmonella serotypes Tennessee and Agona appeared to be among the most heat resistant with D90°C-values of 16.0 and 15.5 min, respectively.
Significance: Results indicate that a DSC can be used to generate reliable thermal kinetic data for Salmonella serotypes under low-moisture conditions. This simplified method may allow for rapid identification of the most thermally tolerant pathogens for validation of preventive controls.