Temperature resistance of Salmonella in low-water activity whey protein powder as influenced by salt content

Salmonella can survive in low-water activity (a(w)) foods for long periods of time. Water activity and the presence of solutes may affect its survival during heating. Low-a(w) products that contain sodium levels above 0.1 % (wt/wt) and that have been involved in major Salmonella outbreaks include peanut products and salty snacks. Reduced a(w) protects against thermal inactivation. There is conflicting information regarding the role of salt. The aim of this study was to determine whether NaCl influences the survival of Salmonella in low-a(w) whey protein powder independent of a(w) at 70 and 80 °C. Whey protein powders of differing NaCl concentrations (0, 8, and 17 % [wt/wt]) were equilibrated to target a(w) levels 0.23, 0.33, and 0.58. Powders were inoculated with Salmonella, vacuum sealed, and stored at 70 and 80 °C for 48 h. Cells were recovered on nonselective differential media. Survival data were fit with the Weibull model, and first decimal reduction times (δ) (measured in minutes) and shape factor values (β) were estimated. The influence of temperature, a(w), and salinity on Weibull model parameters (δ and β) was analyzed using multiple linear regression. Results showed that a(w) significantly influenced the survival of Salmonella at both temperatures, increasing resistance at decreasing a(w). Sodium chloride did not provide additional protection or inactivation of Salmonella at any temperature beyond that attributed to a(w). The Weibull model described the survival kinetics of Salmonella well, with R2 adj and root mean square error values ranging from 0.59 to 0.97 and 0.27 to 1.07, respectively. Temperature and a(w) influenced δ values (P < 0.05), whereas no significant differences were found between 70 and 80 °C among the different salt concentrations (P > 0.05). β values were not significantly influenced by temperature, a(w), or % NaCl (P > 0.05). This study indicates that information on salt content in food may not help improve predictions on the inactivation kinetics of Salmonella in low-a(w) protein systems within the a(w) levels and temperatures studied.