Influence of Cooling Rate on Outgrowth of Clostridium perfringens Spores in Cooked Ground Beef

The ability of Clostridium perfringens spores to germinate and grow was studied to determine a safe cooling rate for cooked beef. Beef samples were inoculated with a cocktail of three strains of heat-shocked C. perfringens spores (NCTC 8238, NCTC 8239 and ATCC 10288), vacuum-packaged, and cooked in a stirred water bath to an internal temperature of 60°C in 1 h. Then, samples were cooled through the temperature range of 54.4°C to 7.2°C at rates varying from 6 to 18 h. Samples were removed at various times during cooling to determine if the spores had germinated and multiplied. The samples were plated on tryptose-sulfite-cycloserine agar and incubated anaerobically at 37°C for 48 h. Minimal growth was observed with cooling periods of up to 15 h. However, with the time to achieve 7.2°C extended to 18 h, C. perfringens spores germinated and grew from an inoculum of approximately 1.5 log₁₀ to about 6.0 log₁₀ CFU/g. This study indicated that pasteurized cooked beef must be cooled to 72°C in 15 h or less to prevent C. perfringens foodborne disease outbreaks.