Comparison of post-lethality thermal treatment conditions on the reduction of Listeria monocytogenes and sensory properties of vacuum packed ricotta salata cheese

Ricotta salata is a whey protein cheese produced in Sardinia that in the last decades has been linked to several recalls and in 2012 to a severe human listeriosis outbreak. Contamination of ricotta salata with Listeria monocytogenes mainly occurs during post-process handling and generally origins from the processing environment. The application of water bath heat treatment in vacuum packed ricotta salata is a possible strategy to control L. monocytogenes superficial contamination. The objective of the present study was to select a heat treatment able to inactivate L. monocytogenes count of at least 5 log. Nine temperature time combinations, 75 °C, 85 °C and 90 °C applied for 15 min, 25 min and 40 min each were tested in ricotta wheels artificially contaminated with a mixture of 5 L. monocytogenes strains. Inactivation was assessed respectively one day and 30 days after heat treatment. The efficacy of treatments was evaluated based on the reduction in L. monocytogenes counts, on the impact on sensory properties and on the cost of the treatment. Two out of nine treatment combinations, i.e. 85 °C for 40 min and 90 °C for 40 min, were effective in reducing L. monocytogenes contamination level of 5 log. No significant difference was observed in sensory properties after the heat treatments. Therefore both combinations are eligible to conduct a successive study aimed to extend the shelf-life of ricotta salata up to several months.