Protective cultures and high-pressure treatment to preventL. monocytogenesgrowth in Ricotta Salata cheese

The general aim of this thesis was to evaluate the efficacy of the protective cultures (PCs) and High Hydrostatic Pressures (HPP) as new strategies to controlListeriamonocytogenes (Lm) on Ricotta salata cheese (RS). Maximum Lm growth rate (µmax) in RS stored at +8°C was determined to evaluate the worst-case scenario. Lm µmax value was about 0.02 log/hours that fit the estimates obtained from an open source predictive software (ComBase) An in vitro test to select commercial protective cultures suitable for the challenge test was also carried out. From the 5 PCs tested,Lactobacillus plantarumandCarnobacteriumspp. were chosen. A challenge test was performed on 180 under vacuum wedges of RS divided into 4 groups of 45 RS: negative controls without Lm or PCs; positive controls inoculated with a pool of 5 strains of Lm at concentration of about 100 cfu g-1; two different experimental groups inoculated with Lm strains andL. plantarumorCarnobacteriumspp (106 cfu g-1). The results showed that Lm growth in RS wedges was not affected by PCs. To test the HPP treatment at 500 MPa at 10°C for 300 seconds, 108 RS wedges were inoculated with two different concentrations (103 and 105 cfu g-1) of a pool of 5 strains of Lm. While the low concentration simulated a natural contamination, the high one permitted to evaluate the reduction of Lm after the treatment. The reduction was about 3 log, but Lm was able to recovery and replicate during storage at refrigerated temperature (+4°C).