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United States Department of Agriculture

Agricultural Research Service

Research Project: MOLECULAR APPROACHES FOR THE IDENTIFICATION AND CHARACTERIZATION OF ANTIMICROBIAL RESISTANCE IN FOODBORNE PATHOGENS

Location: Bacterial Epidemiology and Antimicrobial Resistance

Title: Relative survival of four serotypes of Salmonella enterica in low-water activity whey protein powder held at 36 and 70°C at various water activity levels

Authors
item Santillana Farakos, S -
item Hicks, J -
item Frye, Jonathan
item Frank, J -

Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 7, 2014
Publication Date: July 1, 2014
Citation: Santillana Farakos, S.M., Hicks, J.W., Frye, J.G., Frank, J.F. 2014. Relative survival of four serotypes of Salmonella enterica in low-water activity whey protein powder held at 36 and 70°C at various water activity levels. Journal of Food Protection. 77(7):1198-1200.

Interpretive Summary: Salmonella enterica is a leading cause of foodborne illness in the United States. Although Salmonella is not able to grow in low-moisture conditions, where water activity is below 0.94, it can survive in low-moisture foods for long periods of time. Temperature, water activity, available nutrients, and serotype also affect the survival of Salmonella. The aim of this study was to evaluate the influence of temperature and water activity on the relative persistence among four serotypes of Salmonella enterica in low-moisture whey protein powder. Whey protein powder samples at a water activity of 0.18±0.02 and 0.54±0.03 were inoculated with a mixture of Salmonella (S. Agona, S. Tennessee, S. Montevideo and S. Typhimurium), vacuum-sealed and stored at 36 °C for six months and 70 °C for 48 hours. Sample were spread on growth media in petri dishes and Salmonella colonies (30-32 per plate) were randomly picked from each plate at the end of each survival study. A polymerase chain reaction (PCR) assay, Salmonella Multiplex Assay for Rapid Typing, was used determine the serotype of each Salmonella picked. A statistical test (Chi-square) was used to test for significant differences in serotype survival under the different conditions. Results showed significant differences among Salmonella serotypes in low-moisture whey protein powder at a distinct water activity (p<0.0001) and different storage temperatures (p<0.0001). S. Agona was more persistent at the lower temperature (36 °C), while at 70 °C, S. Tennessee was the most predominant serotype. Similarly, at lower water activity levels (<0.2), S. Agona was found to be the most persistent serotype at both temperatures while at higher water activities (>0.5) and higher temperature, S. Tennessee was the predominant serotype. S. Montevideo and S. Typhimurium were the serotypes with the lowest persistence. When developing predictive models for survival of Salmonella in low-moisture foods, the difference in survival between serotypes should be taken into consideration.

Technical Abstract: Salmonella enterica is the leading cause of health burdens in the United States. Although the pathogen is not able to grow at aw levels below 0.94, it can survive in low-moisture foods for long periods of time. Temperature, aw, substrate and serotype affect its persistence. The aim of this study was to evaluate the influence of temperature and aw on the relative persistence among four serotypes of Salmonella enterica in low-moisture whey protein powder. Whey protein powder was equilibrated to aw 0.18±0.02 and 0.54±0.03, inoculated with a cocktail of Salmonella (S. Agona, S. Tennessee, S. Montevideo and S. Typhimurium), vacuum-sealed and stored at 36 °C for six months and 70 °C for 48 hours. Presumptive Salmonella colonies (30-32) were randomly picked from each plate at the end of each survival study. Multiplex PCR was used to amplify the DNA and the resulting fluorescently labeled amplicons were separated by capillary electrophoresis. Genemapper software was used to analyze the size of the amplicons and using the Salmonella Multiplex Assay for Rapid Typing code, the serotypes of the isolates were determined. A Chi-square test for independence was used to test for significant differences in serotype frequency distribution. Results showed significant differences in prevalence exist among Salmonella serotypes in low-moisture whey protein powder of distinct aw (p<0.0001) and stored at different temperatures (p<0.0001). S. Agona was more persistent at the lower temperature (36 °C), while at 70 °C, S. Tennessee was the most predominant serovar. Similarly, at lower aw levels (<0.2), S. Agona was found to be the most persistent serovar at both temperatures while at higher water activities (>0.5) and higher temperature, S. Tennessee was the predominant serotype. S. Montevideo and S. Typhimurium were the serovars with the lowest persistence. These results should be considered when developing predictive models for survival of Salmonella in low-moisture foods.

Last Modified: 9/23/2014
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