Predicting growth-no growth of Staphylococcus aureus on vacuum-packaged ready-to-eat meats

U.S. Department of Agriculture (USDA) composition-based labeling standards for various ready-to-eat (RTE) meat products typically specify maximum product pH and/or moisture:protein ratio and less often maximum water activity (a(w)). Compliance with these standards often has been regarded as proof of shelf stability. However, the USDA now requires additional proof, e.g., challenge study results, of shelf stability. The pathogen most likely to grow on vacuum-packaged, reduced-moisture products is Staphylococcus aureus. Therefore, vacuum-packaged RTE products that do not support S. aureus growth at room temperature could be considered shelf stable. We developed mathematical equations for predicting whether S. aureus would grow under such conditions. Twenty-four commercial RTE meat products and 10 intentionally misprocessed products (insufficient drying, fermentation, and/or salt) were inoculated with a five-strain cocktail of S. aureus, vacuum packaged, and stored at 21 degrees C. Initial, 7-day, and 28-day S. aureus counts were recorded. Product pH, a(w), moisture:protein ratio, and percentage of water-phase salt (%WPS) also were determined. S. aureus grew only in the intentionally misprocessed products and in some commercial products labeled "keep refrigerated." Using bias reduction logistic regression data analysis, the probability of S. aureus growth (Pr) could be predicted by either of two equations. The first was based on pH and a(w) values: Pr = exp[-59.36 + (5.75 x pH) + (28.73 x a(w))]/{1 + [exp(-59.36 + (5.75 x pH) + (28.73 x a(w))]}. The second was based on pH and %WPS: Pr = exp[-26.93 + (5.38 x pH) + (-0.61 x %WPS)]/{1 + exp[-26.93 + (5.38 x pH) + (-0.61 x %WPS)]}. These equations accounted for observed S. aureus growth-no growth results and will be a useful tool for evaluating the shelf stability of RTE meats.