Modeling the Influence of Temperature, Water Activity and Water Mobility on the Persistence of Salmonella in Low-moisture Food

Introduction: Salmonella is able to survive in low-moisture foods for many weeks or months. Heat resistance is affected by many factors including water activity (a_w). Low a_w environments protect Salmonella from thermal inactivation. Water mobility is different from a_w, and is a measure of the ability of water to translocate in a food. Little is known about the role of water mobility in influencing the survival of Salmonella in low-moisture foods.

Purpose: This study developed mathematical models that predict the behavior of Salmonella in low-moisture foods as influenced by a_w, temperature and water mobility.

Methods: Whey protein powders of differing water mobilities were equilibrated to various a_w levels between 0.19 and 0.54. Powders were inoculated with a four-strain cocktail of Salmonella (all previously involved in outbreaks in dry foods). Powders were vacuum-sealed and stored at temperatures ranging from 21°C to 80°C. Survival data was fitted to the log-linear, Geeraerd-tail, Weibull, biphasic-linear and Baranyi models. Secondary linear models relating the time required for first decimal reduction (δ) and shape factor values (β) to a_w, temperature and water mobility were fit using multiple linear regression. The models were validated in dry non-fat dairy and grain products, as well as low-fat peanut and cocoa products.

Results: Water activity significantly influenced the survival of Salmonella in low-moisture food (P < 0.05) at all temperatures while water mobility had no effect independent of a_w (P > 0.05). The Weibull model provided the best description of survival kinetics. Secondary models were useful in predicting the survival of Salmonella in tested low-moisture foods (R = 0.94), providing a more accurate prediction in non-fat food (R = 0.95) as compared to food containing low-fat levels (R = 0.91).

Significance: The models developed in this study provide baseline information to be used for research on risk mitigation strategies for low-moisture foods.