The Influence of Fat Content on the Survival of Salmonella in a Low-water Activity Model Food System at 70°C

Introduction: Low-water activity (a_w) foods and ingredients are naturally dry or have been dried through processing.  Low a_w is a barrier to growth for many vegetative pathogens, including Salmonella spp. However, Salmonella has demonstrated the ability to survive in low-water activity foods for long periods of time. The role of fat in the survival of Salmonella in low-water activity foods remains unclear.    

Purpose: The purpose of this study is to evaluate the role of fat in the survival of Salmonella in a whey protein isolate model food system at 70°C and at constant water activities of 0.18 and 0.57.  

Methods: Peanut oil was homogenized with whey protein isolate to achieve 5, 10 and 20% (w/w) fat concentration and was equilibrated to water activity levels of 0.57 and 0.18. A dried cocktail including S. Typhimurium, S. Tennessee, S. Agona, and S. Montevideo was inoculated and treated at 70°C for 24 hours (0.18 a_w) and 16 hours (0.57 a_w). Survivors were recovered at various times using supplemented Tryptic Soy Agar. Log CFU/g of surviving Salmonella in 0, 5, 10 and 20% peanut oil at each time point were compared using one-way and two-way ANOVA.

Results: Overall survival was not affected by fat content after 16 hours at 0.57 a_w and 24 hours at 0.18 a_w (P = 0.841, df =3, α=0.05). Survival of Salmonella differed with water activity (P = 0.001). In addition there were no differences in survival at individual time points (P > 0.05). The average log reduction of Salmonella was 4.93 at 0.57 a_w after 16 hours and 2.67 at 0.18 a_w after 24 hours.  

Significance: An understanding of the role of fat in the survival of Salmonella in low water activity foods will improve the ability to predict and control the behavior of Salmonella in these foods.