High temperature water activity as a key factor influencing survival of Salmonella Enteritidis PT30 in thermal processing

Salmonella in low-moisture foods has enhanced thermal tolerance and is difficult to control. The objective of this research was to study relationship between thermal tolerance of Salmonella Enteritidis PT30 and water activity (a(w)) of food matrices measured at elevated temperatures during thermal processing. Three different foods were selected for this study. They were wheat flour (WF), almond flour (AF) and whey protein (WP), representing carbohydrate-, fat-, and protein-rich food systems, respectively. Pre-equilibrated powders were inoculated independently with S. Enteritidis PT30 and conditioned to a(w), of 0.25, 0.45, 0.60, and 0.80 at room temperature (similar to 20 degrees C). Aluminum thermal death time test cells (TDT cells) and newly designed thermal a cells (TAC, with controlled a(w)) were heated at 80 degrees C to determine D-values (the time needed to active 90% of target bacteria) of S. Enteritidis PT30 in the three powders. Water activities of powders in the TDT cells at 80 degrees C were calculated to be between 0.41 and 0.89, while in the TAC were controlled to 0.32, 0.50, 0.63, and 0.81, respectively. Results showed that D-80 degrees C-values of S. Enteritidis PT30 decreased exponentially with increasing a(w) of foods at the treatment temperature 80 degrees C regardless of the food matrices and the testing methods. Thus, it is critical to understand how a(w) of a food matrix changes with temperature when selecting appropriate treatment conditions for thermal control of Salmonella in low-moisture foods.