Salmonella Growth in Cooked Beef at Selected Cooling Rates

Abstract

Results of this study support the present USDA Food Safety Inspection Service (FSIS) cooling requirement for cooked meat products and remind the consumer to refrigerate such products. USDA FSIS requires food processors to cool certain cooked meat products between 4 and 49°C within 2 h. Our study evaluated the adequacy of that requirement by determining how cooling rates affected growth of salmonellae in cooked meats. Two strains of Salmonella sp. showing resistance to multiple antibiotics were compared with a susceptible strain, and were shown to be similar in growth capabilities. These antibiotic resistant strains were inoculated in ground beef or beef cubes. In experiments simulating precooking contamination, heavily inoculated (109 CFU/g) ground beef meatballs were cooked to 63°C (145°F) and cooled to either 23 or 4°C (40°F) within 2 to 6 h. Increases in the numbers of the surviving pathogen were small (ca. 0.1 log10/g) when the product was cooled to 4°C within 2 h. Surviving salmonellae increased greater than tenfold when the meats were cooled over intervals of 6 h. A 4-h cooling interval permitted an intermediate growth rate. Salmonella held in ground beef at 23°C for 6 h showed less than 1-log10 increase per gram. Experiments with Salmonella inoculated onto the surface of beef cubes after cooking also indicated that the 2-h cooling interval prevented substantive proliferation.