Salmonella enterica is a frequent food contaminant and the leading cause of foodborne bacterial illnesses in the United States. Our study... Show moreSalmonella enterica is a frequent food contaminant and the leading cause of foodborne bacterial illnesses in the United States. Our study demonstrated that a 5-strain S. enterica cocktail displayed increased heat resistance in peanut butter of low water activity (aw). Significant differences (P < 0.05) were found between the survival rates of Salmonella enterica and Escherichia coli O157:H7 in peanut butter with different formulations and water activity. High carbohydrate content in peanut butter and low incubation temperature resulted in higher levels of bacterial survival during storage but lower levels of bacterial resistance to heat treatment. Furthermore, we also compared the relative heat resistance of three individual strains of S. enterica representing serotypes Typhimurium, Enteritidis and Tennessee and the 3-strain cocktail treated at both 90oC and 126oC in two different peanut butter formulations with varied fat and carbohydrate contents and adjusted water activities (aw from 0.2 to 0.8). When treated at 90oC, increased water activity in peanut butter significantly (P < 0.05) reduced the heat resistance of desiccation-stressed S. enterica cells. Differences in heat resistance were also detected among the three S. enterica serotypes and between the two peanut butter formulations. When treated at 126oC, the differences in bacterial heat resistance among serotypes and adjusted water activities were less notable (P > 0.05). Based on the Weibull model, an average of 52 to 132 min was required to achieve a 5-log reduction of the 3-strain cocktail at 90oC in peanut butter with an aw of 0.2. When aw was increased to 0.6, to achieve the same 5-log reduction required only 23-27 min. At aw of 0.8, S. enterica could be completely killed in less than 10 min in peanut butter with a fat content of 48.49%. Using scanning electron microscopy, we observed minor morphological changes xiii of S. enterica cells during desiccation and rehydration processes in peanut oil, which was used as a surrogate for peanut putter. Results from this study collectively suggest that water activity plays a critical role in determining S. enterica heat resistance in peanut butter. The variability that exists among the heat resistance of different S. enterica serotypes in different peanut butter formulations should also be taken into consideration for developing and validating effective intervention and mitigation strategies in peanut butter production. PH.D in Biology, May 2014 Show less
