A comparative heat inactivation study of indigenous microflora in beef with that of Listeria monocytogenes, Salmonella serotypes and Escherichia coli O157:H7.
Thermal inactivation of a mixture of five strains of Listeria monocytogenes, four strains of Escherichia coli O157:H7 and eight serotypes of Salmonella were compared with that of indigenous microflora in 75% lean ground beef. Inoculated meat was packaged in bags that were completely immersed in a circulating water bath and held at 55, 57.5 and 60 degrees C for predetermined lengths of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soya agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. D-values, determined by linear regression, in beef were 77.49, 21.9, and 10.66 min at 55, 57.5, and 60 degrees C, respectively, for indigenous microflora (z = 5.81 degrees C). When either of the three pathogens were heated in beef, their D-values calculated were significantly lower (P < 0.05) than those of indigenous microflora at all temperatures. The slope of the thermal death time curve for L. monocytogenes, E. coli O157:H7 and indigenous microflora were similar. Using a survival model for nonlinear survival curves, the D1-values at all temperatures for L. monocytogenes were significantly higher (P < 0.05) compared with those for Salmonella serotypes, E. coli O157:H7 or indigenous microflora. However, higher recovery of a subpopulation of the indigenous microflora in beef exposed to heating at 55, 57.5 or 60 degrees C resulted in significantly higher (P < 0.05) D2-values at all three temperatures, compared with those of the three pathogens at the same test temperatures. If the thermal process is designed to ensure destruction of indigenous microbial flora, it should also provide an adequate degree of protection against L. monocytogenes, Salmonella serotypes or E. coli O157:H7. The results of this study will assist the retail food industry in designing acceptance limits on critical control points that ensure safety, without introducing pathogens in a retail food environment, against L. monocytogenes, E. coli O157:H7 and Salmonella in cooked ground beef.