Survival of foodborne pathogens during frozen storage of cheese made from artificially inoculated milk

As cheese is often frozen prior to microbiological analysis, the aim of this study was to determine the survival of foodborne pathogens to freezing in cheese. A semi-soft cheese was produced, in independent triplicate for each pathogen mix, using milk inoculated with two pathogen mixes: Listeria monocytogenes + Staphylococcus aureus and Escherichia coli O157:H7 + Salmonella typhimurium. Three different strains were used for each pathogen, except for E. coli O157:H7 for which two strains were used. Cheeses were manufactured, wrapped in plastic bags and frozen at -20 A degrees C. For the E. coli O157:H7 + S. typhimurium pathogen mix, the effect of different freezing conditions on survival was studied. In all cases, the numbers of starter bacteria and pathogens were monitored in fresh cheese and after 2, 7, 30 and 90 days of storage. Two methods of thawing were compared after 30 days (14 h at 4 A degrees C and 4 h at 20 A degrees C). The numbers of L. monocytogenes, S. aureus and starter bacteria did not change significantly during frozen storage at -20 A degrees C, but E. coli and S. typhimurium decreased significantly after 2 days. There was no significant (p > 0.05) influence of the thawing method. Freezing of cheese at -80 A degrees C or flash freezing in liquid nitrogen only facilitated survival of E. coli O157:H7 and S. typhimurium for 1 day. The study shows that cheese samples should not be frozen prior to analysis for detection or enumeration of E. coli or S. typhimurium.