Water activity change at elevated temperatures and thermal resistance of Salmonella in all purpose wheat flour and peanut butter

• We generated water sorption isotherms of two low-moisture foods at 20 and 80 °C.
• Water activity changed considerably at 80 °C in those foods compared to 20 °C.
• We determined thermal resistance of Salmonella in those foods at 80 °C.
• Water activity change may be partly attributed to the change in thermal resistance.

Water activity (aw) is a major factor affecting pathogen heat resistance in low-moisture foods. However, there is a lack of data for aw at elevated temperatures that occur during actual thermal processing conditions, and its influence on thermal tolerance of pathogens. The objective of this study was to gain an in-depth understanding of the relationship between temperature-induced changes in aw and thermal resistance of Salmonella in all purpose flour and peanut butter at elevated temperatures (80 oC). Equilibrium water sorption isotherms (water content vs. water activity) for all purpose wheat flour and peanut butter over the range of 20 to 80 °C were generated using a vapor sorption analyzer and a newly developed thermal cell. The thermal resistance (D80-values) of Salmonella in all purpose wheat flour and peanut butter with initial aw of 0.45 (measured at room temperature, ~ 20 °C) was determined via isothermal treatment of small (< 1 g) samples. When increasing sample temperature from 20 to 80 °C in sealed cells, the aw of all purpose flour increased from 0.45 to 0.80, but the aw of peanut butter decreased from 0.45 to 0.04. The corresponding estimated D80-values of Salmonella in all purpose flour and peanut butter with 20 oC aw of 0.45 were 6.9 ± 0.7 min and 17.0 ± 0.9 min, respectively. The significantly (P < 0.05) higher D80-value of Salmonella in peanut butter than in all purpose flour may be partially attributed to the reduced aw in peanut butter in comparison to the increased aw in all purpose flour at 80 °C. The improved understanding of temperature-induced changes in aw of low-moisture products of different composition provides a new insight into seemly unpredictable results, when using heat treatments to control Salmonella in such food systems.