Purpose: To determine the temperature-death times of Listeria monocytogenes, Salmonella, and Shiga toxin-producing E. coli (STEC) in ready-to-eat roast beef at 54.4, 60, 65.6, and 71.1 °C.
Methods: Ground roast beef (containing 1.0% salt, 0.35% sodium phosphates, 0.75% sugar, 20% water) was inoculated with 8 log CFU/g L. monocytogenes or Salmonella (5-strain mix) or STEC (7-strain mix). One-g portions (0.5-1.0 mm in moisture-impermeable vacuum pouches) were heated at one of four temperatures (54.4, 60, 65.6, or 71.1 °C) in a water bath. Triplicate samples were immediately removed and chilled to 4 °C when meat reached target temperature and at seven additional times. Surviving L. monocytogenes, Salmonella, or STEC were enumerated using Modified Oxford, XLD, or Sorbitol MacConkey agar base, respectively, with thin layer overlay of nonselective media to enhance recovery of injured cells. Each study was replicated twice.
Results: Inactivation rates for STEC were similar to Salmonella at all four temperatures, and were comparable to or less than the times reported in Appendix A. In contrast, L. monocytogenes showed greater thermotolerance than Salmonella and STEC under all conditions. For example, > 5-log reduction of Salmonella and STEC was achieved at 71.1 °C instantaneous, whereas L. monocytogenes was inactivated within 15 seconds. At 65.6 °C, STEC and Salmonella populations decreased 5 log within 60 seconds, whereas ca. 5 minutes was required to inactivate 5 log L. monocytogenes.
Significance: Results support Appendix A as an acceptable tool for Salmonella and STEC lethality and provide new thermal processing guidance to appropriately address pathogenic bacteria in RTE roast beef. Additional studies will be completed in ham and turkey to confirm inactivation.