Re-Interpretation of Water Activity Effects on Temperature Dependency of Salmonella Inactivation in Low-moisture Foods

Introduction: Understanding Salmonella thermal resistance in low a_w products is critical for process design and validation, as well as for developing models that are more mechanistic, robust, and reliable. Although several studies have reported secondary models for a_w and temperature effects on thermal resistance, none has linked the differences in these effects across products to underlying differences in the moisture isotherms of those products.

Purpose: The objective was to quantify how a_w affects the temperature dependency of Salmonella inactivation kinetics in low a_w products, and to evaluate commonalities and differences within and across product categories with differing equilibrium isotherm characteristics.

Methods: We recovered decimal reduction time values (D-values) reported for multiple low a_w products (including wheat flour, whey protein, and multiple peanut butter products). Those D-values were plotted and analyzed as functions of temperature and a_w. Then the temperature changes required to reduce the D-value by 90% (z_T) were calculated and analyzed in relation to a_w.

Results: The observed D-values showed a reasonable log-linear decrease with temperature, but the relationship with a_w varied among products. In peanut butter, a positive correlation (P < 0.05) was observed between z_T and a_w (i.e., z_T increasing > 60% with room-temperature a_w increasing from 0.2 to 0.8), while for low-fat peanut butter, whey protein, and wheat flour, a negative correlation was observed (P < 0.3, < 0.15, and < 0.05, respectively), with a very similar pattern across these three products.

Significance:  These results reinforce the need for a better understanding of the role of a_w in inactivation processes. The results are consistent with previously reported tendencies for low a_w products and support the possibility that a significant part of the differences observed across products might be related to physicochemical aspects of moisture equilibrium, parallel to the biological explanations that have been proposed.