Modeling the Effect of Water Activity on Thermal Resistance of Salmonella in Wheat Flour

MODELING THE EFFECT OF WATER ACTIVITY ON THERMAL RESISTANCE OF SALMONELLA IN WHEAT FLOUR By Danielle F. Smith Salmonella is able to survive in low-moisture environments, is known to be more heat resistant as product water activity decreases, and recently has been implicated in several outbreaks and/or recalls associated with low-moisture foods. Therefore, the specific objectives were to: (1) Evaluate the effect of rapid product desiccation and hydration on the thermal resistance of Salmonella Enteritidis PT 30 in wheat flour, and (2) Test multiple secondary models for the effect of product (wheat flour) water activity on Salmonella thermal resistance. A custom-built fluidized-bed drying and hydrating system was used to rapidly (1-4 min) change the sample’s water activity (0.6 to 0.3, or 0.3 to 0.6) prior to thermal treatment, and the results were compared with those from samples with longer equilibration times (4-7 days). Desiccation or hydration rate did not affect Salmonella thermal resistance; instead, product water activity at the time of thermal treatment controlled Salmonella thermal resistance, regardless of the water activity “history”. An additional isothermal inactivation study, with three water activities and three temperatures, generated data used to evaluate the effectiveness of three secondary models (response surface, modified Bigelow-type, and a combined-effects model) in accounting for product water activity. The combined-effects model best accounted for water activity when modeling microbial inactivation under low-moisture conditions, when considering goodness-offit, phenomenological basis, and model utility.