Thermal Death Kinetics of Three Representative Salmonella enterica Strains in Toasted Oats Cereal

Several reports have indicated that the thermal tolerance of Salmonella at low-water activity increases significantly, but information on the impact of diverse food matrices is still scarce. The goal of this research was to determine the kinetic parameters (decimal reduction time, D; time required for the first decimal reduction, δ) of thermal resistance of Salmonella in a previously cooked low water activity food. Commercial toasted oats cereal (TOC) was used as the food model, with or without sucrose (25%) addition. TOC samples were inoculated with 10⁸ CFU/mL of a single strain of one of three Salmonella serovars (Agona, Tennessee, Typhimurium). TOC samples were ground and equilibrated to a_w values of 0.11, 0.33 and 0.53, respectively. Ground TOC was heated at temperatures between 65 °C and 105 °C and viable counts were determined over time (depending on the temperature for up to 6 h). Death kinetic parameters were determined using linear and Weibull regression models. More than 70% of Weibull's adjusted regression coefficients (Radj2) and only 38% of the linear model's Radj2 had values greater than 0.8. For all serovars, both D and δ values increased consistently at a 0.11 a_w compared to 0.33 and 0.53. At 0.33 a_w, the δ values for Typhimurium, Tennessee and Agona were 0.55, 1.01 and 2.87, respectively, at 85 °C, but these values increased to 65, 105 and 64 min, respectively, at 0.11 a_w. At 100 °C, δ values were 0.9, 5.5 and 2.3 min, respectively, at 0.11 a_w. The addition of sucrose resulted in a consistent reduction of eight out of nine δ values determined at 0.11 a_w at 85, 95 and 100 °C, but this trend was not consistent at 0.33 and 0.53 a_w. The Z values (increase of temperature required to decrease δ-value one log) were determined with modified δ values for a fixed β (a fitting parameter that describes the shape of the curve), and ranged between 8.9 °C and 13.4 °C; they were not influenced by a_w, strain or sugar content. These findings indicated that in TOC, high thermal tolerance was consistent among serovars and thermal tolerance was inversely dependent on a_w.