Evaluation of a post-lethality treatment against Listeria monocytogenes on Ricotta salata cheese

Abstract

As a control measure for Listeria monocytogenes contamination a post-lethality heat treatment was applied in vacuum packaged Ricotta salata, a traditional whey cheese obtained from sheep’s milk. In Ricotta salata L. monocytogenes usually occurs as post-process contamination and its concentration can increase during storage at refrigeration temperature. Ricotta salata wheels were challenged spraying the rind with a L. monocytogenes suspension, obtaining a contamination level of ca. 106 cfu g−1. In order to take into account strains differences in growth, the inoculum contained a mixture of the reference strain NCTC 10887 (serovar 1/2b) and two wild type (serovar 1/2a) strains. 87 artificially contaminated Ricotta wheels were randomly selected and subjected to water bath heat treatment at 85 °C for 90 min, while 21 served as negative controls. Samples were stored at refrigeration temperatures until analysis. L. monocytogenes, background microflora and physico-chemical properties were assessed 6 h, 2, 4, 6 and 12 months after the artificial contamination. Pulsed filed gel electrophoresis was conducted to distinguish between the inoculated and recovered strains. L. monocytogenes cells were below the limit for the enumeration method in samples submitted to the lethal treatment. Heat treatment was effective to achieve a 6 log10 reduction of L. monocytogenes cells.