Probabilistic approach for determining Salmonella spp. and L. monocytogenes concentration in pork meat from presence/absence microbiological data

In the present study, prevalence and concentration of Salmonella spp. and Listeria monocytogenes in fresh pork cuts were determined through the analysis of twelve lots for one year. Five samples were analyzed at retail and after storage at 4 and 12 °C up to the end of shelf-life (7 days). Results obtained for Salmonella spp. indicated that a total of 15 samples (8.33%) were positive, which represents 4 (25%) sampling events positive (i.e. at least one sample was positive in at least one of the sampling scenarios). Salmonella was randomly distributed and direct correlation with storage time and temperature was not obtained. For L. monocytogenes, 26 samples (14.44%) were positive, which represents 5 (41.67%) positive sampling lots. For this pathogen, a group of samples were only positive at the end of the shelf-life but not immediately after purchasing indicating clearly that the contamination was not only heterogeneously distributed but also close to the levels of detection, and in all the cases below the limit of contamination. As neither Salmonella spp. nor L. monocytogenes was enumerated by direct plating (< 10 cfu/g) a probabilistic approach basing on Binomial and Poisson distributions was subsequently performed to estimate microbial concentration from presence/absence data. Estimated concentration values were below 40 cfu/kg for both pathogens in more than 80% of the tested lots. The data collected in this study add new knowledge on this very important and difficult to control segment of the farm-to-fork chain.