Purpose: The objective of this study was to evaluate how water activity (aw) and fat level affect the thermal destruction of Salmonella Tennessee in model food systems made from peanut flour and oil.
Methods: Sixteen peanut paste combinations of four fat concentrations (47, 50, 53 and 56%) and four aw (0.3, 0.4, 0.5 and 0.6), were inoculated with dry inoculum on talc (~1 x 106CFU/g) and tested for thermal resistance at 70 and 75°C. A modified thermal death test method using magnetic copper plates to compress the samples at ≤ 1 mm thickness was used. Cell counts were obtained by plating samples onto tryptic soy agar. Counts of survivor cells were fitted to the Weibull model using the Geeraerd and Van Impe Inactivation Model Fitting Tool.
Results: Greater inactivation of Salmonella Tennessee was achieved at 75 than 70°C. After 50 minutes of treatment at 70°C, the greatest heat resistance (≤ 2.1 log-reduction) of Salmonella Tennessee was observed in the formulations at 47% fat and 0.3 and 0.6 aw, and in the formulation containing 53% fat and 0.6 aw. At 75°C, significantly greater heat resistance occurred in the formulations at lower fat levels (47 and 50%) than at the higher levels (53 and 56%). The inactivation curves were upwardly concave, indicating rapid death in the first 3-6 min, followed by lower death rates and an asymptotic tail.
Significance: Understanding how the properties of food, such as fat content and aw can affect the heat resistance of Salmonella in food will allow the food processors to formulate foods and food processes that will destroy the organism. The inactivation data from this study will be used in predictive modeling.