Predicting Growth–No Growth of Listeria monocytogenes on Vacuum-Packaged Ready-to-Eat Meats

Abstract

Compliance with U.S. Department of Agriculture (USDA) composition-based labeling standards often has been regarded as evidence of the shelf stability of ready-to-eat (RTE) meats. However, the USDA now requires further proof of shelf stability. Our previous work included development of equations for predicting the probability of Staphylococcus aureus growth based on the pH and a(w) of an RTE product. In the present study, we evaluated the growth-no-growth during 21 degrees C storage of Listeria monocytogenes on 39 vacuum-packaged commercial RTE meat products with a wide range of pH (4.6 to 6.5), a(w) (0.47 to 0.98), and percent water-phase salt (%WPS; 2.9 to 34.0). Pieces of each product were inoculated with a five-strain cocktail of L. monocytogenes and vacuum packaged, and L. monocytogenes levels were determined immediately after inoculation and after storage at 21 degrees C for up to 5 weeks. L. monocytogenes grew on 13 of 14 products labeled "keep refrigerated" but not on any of the 25 products sold as shelf stable. Using bias reduction logistic regression data analysis, the probability of L. monocytogenes growth (Pr) could be predicted as a function of pH and a(w): Pr = exp[-59.58 + (4.67 x pH) + (35.05 x a(w))]/{1 + exp[-59.58 + (4.67 x pH) + (35.05 x a(w))]}. Pr also could be predicted as a function of pH and %WPS: Pr = exp[-20.52 + (4.10 x pH) - (0.51 x %WPS)]/{1 + exp[-20.52 + (4.10 x pH) - (0.51 x %WPS)]}. The equations accurately predicted L. monocytogenes growth (Pr values of 0.68 to 0.99) or no growth (Pr values of <0.01 to 0.26) and with our equations for predicting S. aureus growth will be useful for evaluating RTE meat shelf stability.