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results indicate that, while undetectable at the end of sell hold, there were some low levels of spores, and there was some outgrowth of C. perfringens spores during the designated chill procedure and during the designated cold hold procedure. Clostridium perfringens does not multiply at temperatures below 50°F (10°C) (3); therefore, this apparent growth may be a result of spore outgrowth and increase in vegetative cells during slow cooling.

Clostridium perfringens: Post retherm (reheat) and hold
The mean C. perfringens count was 3.67 log CFU/g following the controlled vapor oven retherm process, indicating that the retherm process allowed some multiplication of C. perfringens.

DISCUSSION
This 6-hour cooking process of beef roasts in a controlled vapor oven meets regulatory requirements for a safe roast product. The oven relative humidity was in the range of 20 to 40% for approximately the first two hours of cooking and then increased to 82 to 91% for the remaining cooking and post-cook hold. The experiment shows that this is sufficient to assure the destruction of Salmonella on the surface of the meat. The meat surface had a greater than 6.5-log Salmonella reduction at the end of 6 hours. The center temperature of the meat was always lower than the temperature of the oven by a few degrees, and destruction of Salmonella in the center of the roast was not as rapid. If roasts are cooked to a center temperature of 130°F (54.4°C) there is a greater than 5 log reduction in the center of the roasts at the end of the 6 hour cook. Note that the log reduction after

121 minutes of hold was slightly lower (4.7 log), but within sampling variation. Salmonella is not a food safety concern in meat that is cooked in a controlled vapor oven. The oven used in this study effectively reduced Salmonella 5 logs and more. When the meat was cooked, held for 28 hours, and cooled, C. perfringens was undetectable. However, during hot hold after cooling and holding, a small number of vegetative cells of C. perfringens were detected (probably due to spore germination and outgrowth during cooling). This means that the safest use of a slow-cook oven is not to remove food from the oven to cool it and then later retherm (reheat) it. The safest procedure is to maintain roasts in the oven at a temperature of 130°F (54.4°C) or slightly above until the entire roast is served. The controlled vapor oven is capable of stable operation at 130°F (54.4°C), and at this temperature, Salmonella and C. perfringens in cooked roasts will not multiply.

ACKNOWLEDGMENTS
This research was supported and funded by Winston Industries, LLC, Louisville, KY. Research for this study was conducted by ABC Research Corp., Gainesville, FL.

REFERENCES
1. Goodfellow, S. J., and W. I. Brown. 1978. Fate of Salmonella inoculated into beef for cooking. J. Food Prot. 41:598–605 2. Hsieh, P. Y., and R. Labbe. 2007. Influence of peptone source on sporulation of Clostridium perfringens Type A. J. Food Prot. 70:1730– 1734. 3. International Commission of Microbiological Specifications for Foods 10.

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(ICMSF). 1996. Microorganisms in Foods 5, Microbiological specifications of food pathogens. Blackie Academic & Professional, New York, NY. Kang, D.- H., and D. Y. C. Fung. 1999. Thin agar layer method of recovery of heat-injured Listeria monocytogenes. J. Food Prot. 62:1346–1349. Labbe, R. 1989. Clostridium perfringens, p. 191–235. In M. P. Doyle (ed.), Foodborne bacterial pathogens. Marcel Dekker, Inc., New York, NY. Labbe, R.G. 2001. Clostridium perfringens, Chapter 34, p. 325–330. In F. P. Downes, and K. Ito (ed.), Compendium of methods for the microbiological examination of foods. 4th ed. American Public Health Association, Washington, D.C. Shigehisa, T., T. Nakagami, and S.Taji. 1985. Influence of heating and cooling rates on spore germination of Clostridium perfringens in media and in roast beef. Jpn. J. Vet. Sci. 47:259–267. US Food and Drug Administration. 2005. Food Code. Available at: http://www.cfsan.fda.gov/~dms/ fc05-toc.html. Accessed 19 March 2009. Willardsen, R. R., F. F. Busta, and C.E. Allen. 1977. Dialysis technique for containment of microbial populations inoculated into food systems. Appl. Environ. Microbiol. 34:240–241. Willardsen, R. R., F. F. Busta, C. E. Allen, and L. B. Smith. 1977. Growth and survival of Clostridium perfringens during constantly rising temperatures. J. Food Sci. 43:470–475. Zimmerman, O.T., and I. Lavine. 1964. Industrial research service's psychrometric tables and charts. Industrial Research Service, Inc. 2nd ed. Dover, NH.

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