Purpose: To compare the isothermal inactivation rate of Salmonella in a model roast beef system conditioned under three different temperature profiles immediately prior to cooking at 54.4°C.
Methods: Ground roast beef batter was inoculated with 8 log CFU/g Salmonella (5-strain mixture). One-g samples of inoculated meat were flattened into a thin film (0.5 - 1.0 mm thickness) in moisture-impermeable pouches and vacuum-packaged. Inoculated samples were then held under one of three conditions (not adapted; 4°C for 3 hours; ramp-up from 4°C to 54.4°C over 3 hours) prior to cooking at 54.4°C in a water bath. Triplicate samples were removed at 0, 10, 20, 30, 40, 60, and 90 min, immediately chilled to ≤ 4°C, and then enumerated for Salmonella survival using XLD with thin layer overlay of nonselective media to enhance recovery of injured cells. The study was replicated three times.
Results: Populations of Salmonella decreased 1.2 - 2.8 log at the end of the 3-hour ramp-up period from 4°C to 54.4°C compared to inoculation levels, whereas counts were unchanged in the other two treatments. D-values (calculated from linear regression on log reduction from the beginning of the 54.4°C cook process) were 23.4, 15.2 and 13.0 minutes for the thermal-adapted, cold-adapted, and non-adapted cells. However, when considering the initial inoculum in the roast beef batter, the final surviving populations were not significantly different among the three treatments.
Significance: Sub-lethal heat exposure associated with low-temperature cook cycles can enhance the thermal resistance of Salmonella; however, additional studies are needed to determine the practical implication for industry. Additionally, caution should be exercised when applying results from a model system to industrial processes.