Shelf life and growth potential of Listeria monocytogenes in steak tartare

The evaluation of the microbiological shelf-life under refrigeration conditions of steak tartare showed Total Viable Counts (excluding LAB) around 4 Log CFU/g at the beginning of the study, with a gradual increase reaching 7 Log CFU/g after 12 days. LAB represented the main microflora. A significant acidification was detected at the end of the shelf life. Total Volatile Basic Nitrogen concentration changed slightly during the shelf life. After an initial increase (until T5), all the colour indices were stable. The growth potential of Listeria monocytogenes was also evaluated in accordance with EURL Lm guidelines by conducting challenge tests. The growth potential calculated during the challenge test performed on three different batches indicated the absence of a significant growth (δ < 0.5 Log CFU/g). When considering intermediate sampling times, a maximum increase of 0.51 Log CFU/g was detected at T5 in one batch. Finally, daily thresholds (maximum tolerable L. monocytogenes counts) were calculated to highlight the maximum acceptable load of remaining shelf life to avoid the overcoming of the legal limit of 100 CFU/g. Considering the worst-case scenario, a total increase of 0.89 Log CFU/g in 12 days was estimated, obtaining a “safety initial concentration” of 1.11 Log CFU/g of the pathogen.