Outbreaks of Salmonella linked to low water activity foods have been an increasing concern to the food industry, as Salmonella is the leading causative agent of bacterial foodborne illness in the United States. Some of the most recent foods linked to outbreaks include low water activity foods, such as peanut butter, dry cereal, and black pepper. Several studies have indicated that the thermal resistance of Salmonella at low water activity increases significantly, but very few kinetic studies have been published. The goal of this reaserch was to determine the kenetic parameters of thermal resistance of Salmonella in a low water activity food. Cereal with water activity levels between 0.1 and 0.5 (aw) was used as the model food. and the effect of sucrose addition was also assessed.
The model toasted oat cereal was inoculated with 10(8) CFU/mL of one of three Salmonella serovars, dried overnight at 40 degrees centrigrade and equilibrated to 0.11, 0.33 or 0.53 water activity. The cereal was then ground, put into sealed capillary tubes and heated at temperatures between 65 and 105 degrees centrigrade. The capillary tubes were removed at intervals, immediately cooled and then plated onto Salmonella-specific differential tryptic soy agar. The inactivation curves were the plotted and lines of best fit used were used to calculate D and Z-values for each serovar at a given temperature and water activity.
Sucrose (25%) was also added to the cereal to assess its effect on thermal resistance.
Using Salmonella serovars Typhimurium, Tennessee and Agona, at 0.53 water activity, the D-values ranged from 172 to 208 min at 65 degrees centrigrade and from 4.3 to 6.5 min 80 degrees centrigrade among the three serovars. D-values at 80 degrees centrigrade increased 4-fold when the water activity was reduced to 0.33. All serovars became more resistant to thermal treatments as the water activity decreased, and at 0.11 (aw) D-values greater than 135 min were measured at 85 degrees centrigrade.
At 0.53(aw) the D-values of each of the three serovars increased between 32 and 102% at 80 degreees centrigrade once sucrose was added to the cereal. At 0.33 (aw) the serovars'D-values increased by 95 to 152% at 85 degrees centrigrade compared to the cereal without sucrose. Once the water activity was lowered to 0.11, each of the three serovars had a decrease in D-values of between 46 and 85% at every temperature tested. The Z-values for the three serovars at 0.33 and 0.53 (aw) were approximately 10 degrees centrigrade, but this value increased from 20 to 50% if the serovars were tested at 0.11 (aw). there was not a statistically significant change in the Z-values due to the addition of sucrose.
The thermal resistance of Salmonella increased dramatically at very low water activity. This increase may render some food processes, once thought to be able to control Salmonella, ineffective at controlling these pathogenic bacteria in dry foods.
More research is required to further understand the mechanisms of the organism's activity to survive such a large increase in temperature within these low water activity environments.