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Salmonella Thermal Resistance during Desiccation and Rehydration in Low Water Activity Foods


Low water activity in foods can reduce bacterial growth and metabolism. Reduced water activity in turn can induce a number of "stress response" mechanisms in the bacteria that consequently enhance the heat resistance and prolong their survival in the product. Salmonella have been implicated in contamination of a wide variety of low water activity food products such as peanut butters. However, the physiology of Salmonella under desiccation stress and specific molecular mechanisms of its survival and thermal resistance in low water activity foods have not been fully explored.<P> The overall goal of this project is to elucidate the mechanisms of survival and stress response of Salmonella in low water activity foods with a focus on post-harvest processing and practical methods to reduce pathogen load. <P>Specifically, we aim to evaluate the survival and death rates before and after the exposure of Salmonella to desiccation and rehydration conditions in peanut butter, and identify effective combinations of rehydration time, water activity, and heating temperatures that would be adequate to inactivate desiccated, heat-resistant Salmonella strains in peanut butter. We also aim to explore the mechanisms of Salmonella thermal resistance and identify genetic factors that mediate Salmonella survival under the desiccation stress. <P>Collectively, results from this proposed research will lead to developing a framework of more effective processing and intervention strategies to reduce Salmonella contamination in low water activity foods such as peanut butter.

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Non-Technical Summary: Salmonella is frequently found in the intestinal tract of a wide variety of animals and survives extremely well under dry conditions and is therefore a concern as a potential environmental contaminant of a wide range of dried foods. Once in a dried or reduced water activity state the bacterium becomes much more heat resistant, making effective decontamination measures technically challenging. Here in the U.S., suspected contamination of dried products and ingredients frequently leads to a large number of recalls, which in recent years has included such products as nuts, dietary supplements, fermented sausage, pet food, tea, cereals, snack foods and herbs and spices. The recent outbreak of salmonellosis attributed to contaminated peanut butter has served to illustrate the complexity of the food supply chain. The proposed work will use microbiological challenge studies and new molecular tools to study the stress response of Salmonella during desiccation and rehydration prior to and during industrial thermal processing. This work will provide important data on the heat resistance of Salmonella after different treatments of desiccation and rehydration. More importantly, this work will provide a mechanistic understanding of the response to Salmonella under conditions of desiccation stress prior to heat stress which will allow the design of effective validation studies. <P> Approach: We will use the Salmonella Typhimurium strains that FDA collected from the 2008 peanut butter outbreak for evaluation of Salmonella thermal resistance under select pasteurization and peanut roasting temperatures, and its survival and re-growth before and after the exposure to desiccation and rehydration in peanut butter products. Proper combinations of rehydration time, water activity, and heating temperatures that would be adequate to inactivate heat-resistant Salmonella cells will be identified. In addition, we will apply molecular and genomic analyses including Salmonella whole-genome DNA microarrays that we recently developed in our laboratories to further explore the underlying mechanisms of Salmonella desiccation stress response and thermal resistance and to identify genetic factors such as regulatory genes that may mediate Salmonella survival under desiccation stress and thermal treatments. Experimental data that will be generated in this study, such as processing parameters, effective combination of rehydration time, water activities and heating temperature, and D/z-values, will provide the underpinning bases for food manufacturers to validate and redesign the processing and intervention schemes. The molecular mechanisms underlying Salmonella thermal resistance will improve our basic understanding of how this pathogen manages to survive in low water activity foods.

Zhang, Wei
Illinois Institute of Technology
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