The suitability of handling practices used in school kitchens was evaluated using ground beef gravy that was contaminated with Clostridium perfringens. Cooked gravy was cooled to 110 F (43.5 C) and inoculated with a mixture of vegetative cells and spores of C. perfringens NCTC 8239 to provide approximately 10,000 organisms/g. After inoculation, gravy was packed in bags, refrigerated for 16 h, held for 5 h at 82 F (28 C) or 42 F (5.5 C), and then heated in a compartment steamer for 35 min or until the temperature of the gravy at the center of the pan reached 165 F (74 C). C. perfringens was enumerated at intervals during cooling, holding, and heating of the gravy. The number of viable cells after 16 h of refrigeration at 42 F (5.5 C) was influenced by the first 6 h of cooling when the temperature of the gravy was in the range that permitted growth of C. perfringens (65–122 F; 18.5–50 C). Plate counts of gravy held for 5 h at 82 F (28 C) or 42 F (5.5 C) indicated stablization of the C. perfringens population. When 165 F (74 C) was the final temperature to which the gravy was heated, no viable cells of C. perfringens were found.
The practices of handling precooked chilled gravy in school kitchens were simulated to determine if they could contribute to outbreaks of foodborne illness. Time-temperature measurements and bacteriological tests were made at intervals during chilling, holding, and heating of gravy. Sixty-six pounds of cooked ground beef gravy were packed hot (158 F, 70 C) in bags, cooled in chilled water for 1 h, and refrigerated for 16 h. The gravy was held for 5 h at 82 F (28 C) and at 42 F (5.5 C) and then it was heated in a compartment steamer for 35 min. After 1 h cooling in chilled water, the mean temperature of gravy was 82 F (28 C) and after 16 h of refrigerated storage, the temperature was 45.5 F (7.5 C). The mean temperature of chilled gravy held at 82 F (28 C) remained about 50 F (10 C) for 4 h and reached 64 F (18 C) at the end of the 5-h holding period. The mean temperature of gravy held under refrigeration for 5 h decreased 2 F, from 45.5 F (7.5 C) to 43.5 F (6.5 C). After 35 min of heating in a compartment steamer, the highest temperature was 136 F (58 C) for gravy initially at 65 F (18.5 C) and 128 F (53.5 C) for gravy initially at 47 F (8.5 C). Bacteriological tests indicated that the greatest increase in the number of total aerobic bacteria in gravy occurred during cooling rather than holding. Although some samples yielded coagulase-positive staphylococci, the numbers changed little during holding or heating. Clostridium perfringens was not found in any samples of the gravy.
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