Purpose: The purpose of this study was to gain in-depth understanding of the relationship between temperature-induced changes in aw and thermal resistance of Salmonella in wheat flour and peanut butter at elevated temperatures.
Methods: Equilibrium isotherms (moisture content vs. water activity) for wheat flour and peanut butter at 20 to 80°C were generated using a vapor sorption analyzer and a newly developed thermal cell with relative humidity sensor, and compared using ANOVA. The thermal resistance (D80C-values) of Salmonella in wheat flour and peanut butter with initial aw of 0.4 (measured at room temperature) were determined via isothermal treatment of small (< 1 g) samples in aluminum test cells in a temperature-controlled water bath.
Results: When increasing sample temperature from 20 to 80°C (at constant moisture content), the aw of wheat flour increased significantly from 0.4 to 0.7 (P < 0.05), but the aw of peanut butter decreased from 0.4 to 0.01 (P < 0.05). The corresponding estimated D80C-values of Salmonella in wheat flour and peanut butter with room temperature aw of 0.4 were 5.9 min and 14.9 min, respectively. The significantly higher D80C-value of Salmonella in peanut butter than in wheat flour may be partially attributed to the reduced aw in peanut butter in comparison to the increased aw in wheat flour at 80°C, given equivalent aw values measured at room temperature.
Significance: Improved understanding of the temperature-induced changes in aw of low-moisture products is critically important in designing and validating thermal processing methods to control Salmonella in low-moisture foods.