Thermal Resistance of Salmonella enterica in a High-protein Matrix at Varying Water Activity

Introduction: Legacy baking processes may need to be validated to satisfy Food Safety Modernization Act (FSMA) requirements. During baking, products typically start out at a high water activity and end at a low water activity. At present, information is lacking on the influence of product composition and dynamic water activity on the thermal resistance of Salmonella enterica.

Purpose: The objectives of this work were to assess the influence of water activity and temperature on thermal resistance of S. enterica serovar Agona 447967 in a model high-protein food matrix.

Methods: Plate-harvested cells were used to inoculate a high-protein matrix with a product composition ratio of 43:38:11 (carbohydrate:protein:fat). Varying volumes of buffered peptone water (BPW) diluent were added to adjust water activity (0.50 to 0.98), while holding inoculation level constant.  The matrix was hand massaged until visually uniform and then held at ambient conditions (23±2ºC) for one hour to equilibrate. Triplicate samples were packed into aluminum test cells under controlled water activity conditions. Thermal resistance was evaluated under isothermal conditions in an oil bath (54 to 90ºC). Treated samples were serially diluted using BPW, plated on trypticase soy agar with yeast extract, and incubated at 37±2ºC for 48 h. Positive samples were confirmed on xylose lysine deoxycholate agar.

Results: D_75°C-values were 0.002 to 35.30 min and increased with a reduction in a_w as expected. The z_T-values for a_w 0.501, 0.621, 0.721, 0.803, 0.901, and 0.978 were 9.64, 9.93, 9.89, 10.08, 7.33, 5.22ºC, respectively. Za_w-values exhibited non-linearity and temperature dependence.

Significance: Results indicate that a correlation exists between temperature and water activity and must be accounted for when predicting inactivation of Salmonella under dynamic processing conditions.