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S. E. SMITH¹, J. L. Maurer, A. Orta-Ramirez, E. T. Ryser, and D. M. Smith. (1) Department of Food Science and Human Nutrition, Michigan State University, East Lansing, MI 48824

JUSTIFICATION: USDA-FSIS processing regulations include a lethality performance standard for destruction of Salmonella in beef. A 6.5-log decrease in Salmonella is required for roast beef processing, while a 5-log reduction is proposed for meat patties. Use of an 8-strain cocktail of Salmonella is recommended for verification of the standard.

OBJECTIVES: The objectives were to 1) compare the heat resistance of several Salmonella serotypes and Escherichia coli O157:H7 in ground beef of two fat contents and 2) assess the effects of microbial growth phase and freezing on bacterial heat resistance.

METHODS: Thermal inactivation parameters for E. coli O157:H7, S. senftenberg, three strains of S. Typhimurium DT104, and a cocktail comprised of 8 serotypes of Salmonella were determined in ground beef (5% and 19% fat). Irradiated beef was inoculated, placed in plastic pouches (1-2 mm thickness) and heated at four temperatures (55-66C). Bacterial counts were determined using Petrifilm Aerobic Count Plates. D and z values were calculated by linear regression analysis. Cocktail survivors were identified using API 20 E test strips. Experiments were performed in triplicate.

RESULTS: S. senftenberg was the most heat resistant organism. E. coli was less heat resistant at ³58C than the salmonellae, but at 55C the D value (22.5 min) was similar to that for the DT104 strains (22.0 min) and higher than the cocktail (16.3 min). Inactivation of E. coli (z=3.6C) was more temperature dependant than the cocktail (z=4.0C) and DT104 (z=4.3C). E. coli and DT104 strains were more heat resistant in beef of high fat content. The cocktail was more thermally stable in the stationary phase (D_58C=3.39 min) than in the log phase (D_58C=2.72 min). Freezing of inoculated raw meat decreased heat resistance of the cocktail (D_58C=1.43 min). At least five of the eight cocktail serotypes were identified after heating.

SIGNIFICANCE: The 8-strain cocktail in stationary phase can be used to verify the adequacy of a commercial process at ³58C, but should not be used at lower temperatures.