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Influence of NaCl Content and Cooling Rate on Outgrowth of Clostridium perfringens Spores in Cooked Ham and Beef

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The effect of NaCl concentration and cooling rate on the ability of Clostridium perfringens to grow from spore inocula was studied with the use of a process that simulates the industrial cooking and cooling of smoked boneless ham and beef roasts. NaCl was added to ground cooked hams A and B (which were commercially obtained) to obtain levels of 2.4, 3.1, 3.6, and 4.1% (wt/wt) and 2.8, 3.3, 3.8, and 4.3% (wt/wt), respectively, and to raw ground beef to obtain levels of 0, 1, 2, 3, and 4% (wt/wt). Ham C, a specially formulated, commercially prepared product, was supplemented with NaCl to obtain levels of 2.0, 2.5, 3.0, and 3.5%. The samples were inoculated with a three-strain mixture of C. perfringens spores to obtain concentrations of ca. 3 log10 CFU/g. Portions of meat (5 g each) were spread into thin layers (1 to 2 mm) in plastic bags, vacuum packaged, and stored at -40°C. Thawed samples were heated at 75°C for 20 min and subsequently cooled in a programmed water bath from 54.4 to ≤8.5°C in 15, 18, or 21 h. For the enumeration of C. perfringens, samples were plated on tryptose-sulfite-cycloserine agar and incubated in an anaerobic chamber at 37°C for 48 h. Population densities for cooked ham and beef increased as cooling time increased, and NaCl exerted a strong inhibitory effect on the germination and outgrowth of C. perfringens. For beef, while 3% NaCl completely arrested growth, pathogen numbers increased by ≥3, 5, and 5 log10 CFU/g in 15, 18, and 21 h, respectively, when the NaCl level was <2%. C. perfringens did not grow during cooling for 15, 18, or 21 h in ham samples containing ≥3.1% NaCl. Results obtained in this study suggest that a 15-h cooling time for cooked ham, which is normally formulated to contain >2% NaCl, would yield an acceptable product (with an increase of <1 log10 CFU/g in the C. perfringens count); however, for beef containing <2% NaCl, C. perfringens populations may reach levels high enough to cause illness.


Document Type: Research Article

Affiliations: Microbial Food Safety Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, 600 East Mermaid Lane, Wyndmoor, Pennsylvania 19038, USA

Publication date: September 1, 2003

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