Food Protection Trends - April 2010 - 224

INTRODUCTION
The purpose of this study was to determine the effectiveness of controlled vapor oven processing procedures for the reduction of pathogenic bacteria such as Salmonella and Clostridium perfringens in large cuts of meat, using FDA 2005 Food Code (8) §3-401.11 (B) (1) whole meat roast cooking guidelines as a reference. The safety of controlled vapor oven cooking procedures was evaluated on the basis of the ability of the procedures to produce a reduction of a Salmonella inoculum on roast surfaces and in minced beef contained within dialysis tubes placed in the center of beef roasts, as well as the effect of cooking, holding, cooling, storage, and retherm (reheating) procedures on the outgrowth of spores and destruction of vegetative cells of C. perfringens in minced beef contained in dialysis tubes placed in the center of the roasts.

MATERIALS AND METHODS
Equipment and product
A CVap oven (Model CAC507) (Winston Industries, LLC, Louisville, KY) was used for this experiment. Operator accessible controls of this equipment are the Wet Bulb Temperature (Twb) and Dry Bulb Temperature (Tdb) (11). Twb is controlled by a temperature-controlled water evaporator at the bottom of the food chamber. Tdb is controlled by temperature-controlled air heaters in the upper food chamber. Seven boneless beef rib roasts (USDA Choice) weighing 13.0 to 14.0 lb (5.89 to 6.35 kg) were procured from a local distributor. The roasts were stored at 32 to 34°F (0 to 1.1°C) and used within 3 to 7 days of procurement.

7.2 (BPB). Each strain was then resuspended, concentrated, and combined in BPB to obtain a final cocktail suspension containing equal concentrations of each strain (i.e., ca. 1010 CFU/ml), which was used to inoculate the product. The three strains of C. perfringens used in this study were NCTC 8238 (ATCC 12916), NCTC 8239 (ATCC 12917), and NCTC 10240 (ATCC 14810). Each strain was individually cultured in fresh Fluid Thioglycollate Broth (FTB) and then transferred to, and individually cultured in, a medium designed to promote the maximum formation of spores (2). Spores from each culture were harvested by centrifugation at 10,000 × g for 10 min. and the pellets resuspended in sterile distilled water and stored at 35 to 38°F (1.7 to 3.3°C). Aliquots of the spore suspensions were combined to prepare a final working (cocktail) suspension containing approximately equal numbers of spores of each strain (i.e., ca. 10,000 CFU/ml), which was used to inoculate the product. The final working suspension was heat treated (i.e., 20 min. at 167°F (75°C) and immediately cooled to 39°F (4°C)) just prior to inoculation to ensure that only spores were present. The final working suspension was enumerated on Tryptose Sulfite Cycloserine (TSC) agar without egg yolk.

Sample inoculation and preparation
The boneless beef rib roasts contained insertions of the Salmonella cocktail and C. perfringens spore cocktail, using the dialysis tubing technique for containment of microbial populations (1, 9). This was accomplished by mincing, inoculating, and mixing approximately 300 g of the raw product (beef roast) with ca. 30 ml of C. perfringens spore cocktail. Another ca. 300-g portion of raw product was minced, inoculated, and mixed with 3.0 ml of the Salmonella cocktail inoculum. For each pathogen, three 10-to15-g aliquots of the inoculated minced product were analyzed microbiologically to establish initial counts. The respective inoculated product portions had ca. 5 × 103 CFU/g of C. perfringens and ca. 2 × 108 CFU/g of Salmonella. For C. perfringens, the remaining inoculated, minced product was portioned

into 18 sample aliquots (ca. 10 g each) that were placed into moistened 20.4 mm diameter dialysis tube segments. For Salmonella, the remaining inoculated minced product was portioned into 9 sample aliquots (ca. 10 g each) that were placed into moistened 20.4 mm diameter dialysis tube segments. The dialysis segments were closed at each end with thread, resulting in a small tube of inoculated product about 3 cm long. The dialysis sample units were color coded to differentiate the Salmonella from the C. perfringens inoculum and were kept refrigerated until insertion into the beef roasts, which was performed within 24 hours. Incisions were made in the geometric center of three roasts. Three dialysis sample units of both Salmonella and C. perfringens were carefully inserted through each incision and were placed at the approximate geometric center of each roast. The incision was then closed. A 10 cm2 surface site on each of the 3 roasts containing the dialysis sample units of Salmonella and C. perfringens was inoculated with 0.1 ml of the Salmonella suspension in 0.01 ml droplets. The inoculated surface sites were marked to identify the sites for subsequent microbial analysis. This inoculum technique delivered about 109 CFU/cm2 to each designated surface site. After surface inoculation, the roasts were held refrigerated for at least 30 minutes to allow bacterial attachment and consistent temperature equilibration prior to control sampling (time-zero) or cooking.

Experimental cooling cycle development
Procedures were tested for cooling and holding cooked roasts in a refrigerator with an air temperature of 38 ± 2°F (3.3 ± 1°C), from 130 to 41°F (54.4 to 5°C) in a time that would allow germination and multiplication of C. perfringens, over approximately 24 h. This cooling time was chosen to be representative of abusive cooling of roasts that occurs in retail food establishment refrigeration units and is sufficient for the growth of C. perfringens. For the roast cooling experiment, 2 roasts were fitted with ther-

Test microorganisms
Three serotypes of Salmonella were used: Enteritidis (ATCC 13076), Typhimurium (ATCC 14028), and Montevideo (ATCC 8387). Each Salmonella serotype was grown separately at 95°F (35°C) for 24 hours in Tryptic Soy Broth (TSB) and underwent at least two serial transfers. Bacterial cells for each culture were harvested by centrifugation at 10,000 × g for 10 min. and washed with Butterfield’s Phosphate Buffer, pH

224 FOOD PROTECTION TRENDS | APRIL 2010

Food Protection Trends - April 2010

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