Fate ofMycobacterium aviumsubsp.paratuberculosisin Swiss Hard and Semihard Cheese Manufactured from Raw Milk

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Abstract
Raw milk was artificially contaminated with declumped cells ofMycobacterium aviumsubsp.paratuberculosisat a concentration of 104to 105CFU/ml and was used to manufacture model hard (Swiss Emmentaler) and semihard (Swiss Tisliter) cheese. Two different strains ofM. aviumsubsp.paratuberculosiswere tested, and for each strain, two model hard and semihard cheeses were produced. The survival ofM. aviumsubsp.paratuberculosiscells was monitored over a ripening period of 120 days by plating out homogenized cheese samples onto 7H10-PANTA agar. In both the hard and the semihard cheeses, counts decreased steadily but slowly during cheese ripening. Nevertheless, viable cells could still be detected in 120-day cheese.Dvalues were calculated at 27.8 days for hard and 45.5 days for semihard cheese. The most important factors responsible for the death ofM. aviumsubsp.paratuberculosisin cheese were the temperatures applied during cheese manufacture and the low pH at the early stages of cheese ripening. Since the ripening period for these raw milk cheeses lasts at least 90 to 120 days, theDvalues found indicate that 103to 104cells ofM. aviumsubsp.paratuberculosisper g will be inactivated.