Enhancing Food Safety Through Control of Food-Borne Disease Agents

NON-TECHNICAL SUMMARY: Although cooking is thought to be a simple process, undercooked meat has been identified as one of the most frequent causes of E. coli 0157:H7 infection in the United States for over 20 years. The USDA and FDA provide guidelines for the safe cooking of meat and poultry products, yet there is still a need to verify that products have been safely cooked. This project will develop and validate non-linear models to predict lethality of microbial pathogens in meat products during cooking. <P>
APPROACH: Ground beef and turkey will be obtained from a local supplier. Proximate composition and pH will be determined. Meat will be vacuum packaged, frozen and irradiated at 10 kGy to kill existing vegetative cells and stored at -20C. Irradiated meat will be checked for sterility. The Salmonella cocktail will contain S. Thompson (FSIS 120), S. Enteritidis phage type 13A (H 3527), S. Enteritidis phage type 4 (H 3502), S. Typhimurium DT104 (H 3380), S. Hadar (MF 60404), S. Copenhagen (8454), S. Montevideo (FSIS 051) and S. Heidelberg (F5038BG1). The Listeria cocktail will contain eight L. monocytogenes strains of different ribotype patterns isolated from retail ground meat.The E. coli O157:H7 cocktail will consist of 8 strains previously isolated from ground beef. Strains will be maintained as frozen stock cultures. Cultures will be transferred into trypticase soy broth and 0.6% yeast extract (TSB-YE) and incubated for 24h at 35C. Strains will be transferred to a second tube and incubated to obtain cells in the early stationary growth phase. The optical density of each culture will be determined and adjusted to ca. 1.0 (10-9 CFU/ml) using TSB-YE with the exact number of cells determined by plating on TSA-YE. Equal volumes of these cultures will be combined to obtain a cocktail containing equal numbers of the test strains. Ground meat will be inoculated with a cocktail to contain about 10-9 CFU/g. Thermal inactivation experiments under isothermal conditions will be performed in a water bath between 50 to 75C. Nonisothermal inactivation experiments will be performed in a programmable thermocycler. Heating protocols will be based on USDA safe harbor guidelines. Heating and cooling rates will be controlled. Each experiment will be performed in triplicate and analyzed for statistical significance. To enumerate pathogen survivors, cooked meat will be aseptically transferred and homogenized with sterile 0.1% peptone water. Bacterial counts will be determined by decimal dilution and plating on TSA-YE. Plates will be counted after 48 h at 30C. The equation parameters of Salmonella, L. monocytogenes and E. coli O157:H7 in beef and turkey determined in the isothermal experiments will be used to devise a systematic procedure for predicting microbial inactivation in nonisothermal processes using non-linear models. We will critically evaluate and improve the predictive capabilities of the model, by comparing the predicted results to actual nonisothermal data. The dynamic isothermal method will be investigated as another approach to model kinetic parameters from the nonisothermal data obtained in the experiments described above. The ability of the non-linear and dynamic models to predict microbial lethality under nonisothermal conditions will be compared.

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PROGRESS: 2003/10 TO 2006/09<BR>

The microbiological safety of food products is important to consumers around the globe. Although first-order models have been used to describe microbial inactivation during cooking, both linear and non-linear survival curves have been reported. In this project, we a) examined the inactivation behavior of Salmonella spp., Listeria monocytogenes and Escherichia coli O157:H7 in ground turkey breast, turkey thigh, pork shoulder and beef when exposed to isothermal and non-isothermal cooking temperatures, b) evaluated the ability of the Weibull distribution to predict pathogen inactivation under non-isothermal conditions, c) evaluated pathogen inactivation in ground meat patties exposed to industry style cooking conditions, and d) investigated the ability of a fluorescent protein complex, R-phycoerythrin (PE), to serve as a marker to indicate pathogen destruction during processing. Ground pork, turkey breast, turkey thigh and beef were inoculated with pathogens and cooked isothermally (50-72C) and non-isothermally (54, 60 and 66C at a rate of 5 and 10C/min) to determine reduction in counts. Surviving pathogens were enumerated at intervals throughout each cooking process. Isothermal survival curves were described by the both the Weibull distribution and its simplified linear form (n=1), log S=-bt, to determine pathogen characteristics. Predicted and experimental values were compared to determine the relationship between isothermal and non-isothermal data and verify fitness of the Weibull model. Non-linearity was observed when experimental and predicted isothermal survival curves of Salmonella spp. in turkey breast, turkey thigh and pork were graphed. The Weibull distribution described Salmonella inactivation better than its simplified linear form at temperatures of 58C and above. The n values were greater than one (1.2-2.0) for most curves. A linear model better predicted isothermal inactivation of Salmonella spp. and E. coli in ground beef at 58C and above. A strong positive correlation was found between cooking temperature, lethality, and pathogen destruction in the ground meat patties. A weaker correlation was observed between R-PE fluorescence and pathogen inactivation. Experiments in ground turkey, pork and beef patties indicated that recommended safe harbor temperatures and hold times are insufficient to meet lethality performance standards for Salmonella inactivation published in Title 9 Code of Federal Regulations.
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IMPACT: 2003/10 TO 2006/09<BR>
Successful completion of this study will yield a computer model that can be used to compare the effect of different nonisothermal temperature histories on microbial destruction. Processors could use these models to determine the efficacy of any new/modified thermal process. A better understanding of microbial lethality and better process control will ultimately lead to a safer food supply, fewer illnesses and deaths, and reduce economic losses to the food industry.