Impact of Temperature Dependence of Water Activity on Salmonella Inactivation in a Multicomponent Food System

Introduction: Salmonella, which is often inactivated by heat-based processes, has greater thermal resistance at low water activity (a_w). Prior research revealed that a_w did not change appreciably with increasing temperature in a multicomponent food system formulated with 55.9% fat; however, a_w increased in high-protein and high-carbohydrate formulations and decreased in high-fat formulations. Though a_w is temperature dependent and impacted by food composition, the influence of these factors on Salmonella inactivation in low a_w multicomponent foods is not well understood and often ignored in process validation. 

Purpose: To determine the influence of food composition and temperature dependence of a_w on thermal resistance of Salmonella in a multicomponent food system.

Methods: Food matrices, comprised of rice flour, soybean oil and soy protein powder, were formulated to contain high-carbohydrate (67.3%), high-protein (85.8%) and different levels (55.9% and 87.0%) of fat content. Each matrix was inoculated with Salmonella Agona then stored under controlled humidity to equilibrate a_w. Samples, ~0.7g, were loaded into aluminum test cells, which were heated at 85^oC in a water bath.  Samples were cooled, diluted in buffered peptone water, and enumerated on tryptic soy agar supplemented with yeast extract.  D-values were calculated and results analyzed by t-test.

Results: Thermal resistance of Salmonella was lower in formulations where a_w increased and higher in the formulation where a_w decreased at elevated temperature. D-values for Salmonella in the high-carbohydrate and high-protein formulations (3.09±0.11 min and 3.13±0.10 min) were significantly lower (p<0.05) than in the 55.9% fat matrix (7.22±0.31 min). The D-value of Salmonella in the 87% fat formulation (11.08±0.36 min) was significantly higher (p<0.05) than in other formulations. 

Significance: Since significantly higher D-values for Salmonella in low a_w foods may result at elevated temperature, food composition and temperature dependence of a_w should be considered when validating thermal inactivation processes.