High pressure processing effect on different Listeria spp. in a commercial starter-free fresh cheese

In this study, both microbial inactivation and growth of Listeria spp. inoculated in commercial free-starter fresh cheese was evaluated after high-pressure processing (HPP). HPP conditions (300, 400, 500 and 600 MPa at 6 °C for 5 min) and inoculum level (3-4 or 6-7 log CFU/g of cheese), as well as differences among strains inoculated (Listeria innocua, L. monocytogenes CECT 4031 and L. monocytogenes Scott A) were investigated. Inactivation and generation of sublethal injury were determined after HPP using ALOA (Agar Listeria according to Ottaviani and Agosti) and TAL (Thin Agar Layer) plating methods, respectively. Listeria inactivation increased with the pressure applied, presenting some statistical differences between the employed strains, inoculum level and sublethal injury. The highest lethality values were obtained at 600 MPa for the three strains tested, although the 500 MPa treatment presented high lethality for L. innocua and L. monocytogenes CECT 4031. After treatment, L. innocua and L. monocytogenes CECT 4031 counts in fresh cheese increased gradually during cold storage. By contrast, counts in cheeses inoculated with L. monocytogenes Scott A did not change significantly (p ≥ 0.05), being this strain the most pressure resistant and with the slowest growth rate. The manuscript present information supporting that, strains with high-level resistance should be employed during inactivation studies, instead of surrogate microorganisms. Application of HPP treatments of 500 MPa and especially 600 MPa on fresh cheeses would be effective to eliminate the most resistant microorganism to a level that should not present a public health risk under normal conditions of distribution and storage.