Influence of Water Mobility on the Survival of Salmonella spp. in Low-moisture Whey Protein Powder at 80°C

Introduction: Salmonella can survive in dry food for long periods of time. Reduced microbial inactivation in low moisture food during heating is believed to be due to the interaction of cells and water, and is often thought to be related to water activity (a_w). Water mobility is a measure of the ability of water molecules to translocate in the food. Little is known about the role of water mobility in influencing the survival of Salmonellain low moisture foods.

Purpose: The aim of this study is to determine the influence of water mobility on the survival of Salmonellain low moisture whey protein powder held at 80°C.

Methods: Whey protein powder (95% protein) of differing water mobilities was produced by pH adjustment (2, 5, and 7), heat denaturation and equilibration to various a_w levels between 0.18 and 0.52. Powders were inoculated with a four-strain cocktail of Salmonella, vacuum-sealed and immersed in a circulating water bath at 80 °C for 48 hr. Cells were recovered at various times on nonselective differential media. Microbial survival data were analyzed using ANOVA.

Results: Water activity significantly influences the heat inactivation of Salmonella (P < 0.0001). Treatment after 4 hours showed average log reduction values of 2.7, 4.6, 5.0, 5.2 and 5.1 at a_w levels of 0.18, 0.29, 0.36, 0.42 and 0.52, respectively. Water mobility had a significant effect on microbial death (P = 0.030) at higher a_w levels (0.36 to 0.52), with average log reductions of 4.8 and 5.4 at lower and higher mobilities, respectively. Water mobility did not significantly influence inactivation in the a_w range of 0.18 to 0.29 (P = 0.598).

Significance: Water mobility influences the survival of Salmonella spp. in low moisture protein powder at a_w levels between 0.36 and 0.52 during treatment at 80 °C.