Fate of Listeria innocua during production and ripening of smeared hard cheese made from raw milk

The fate of 2 different Listeria innocua strains was analyzed during the production and ripening of smeared raw milk Greyerzer cheese (Gruyere). These strains were used as surrogates for the pathogenic Listeria monocytogenes, as they are physiologically very similar. Bacterial cells were added to the cheese milk at levels of 10(5) cfu/mL. During the first 24 h of cheese making, the number of the test strains decreased to a level of below 102 cfu/g. Obviously, the cooking temperature of 56 degrees C and the subsequent slight temperature decrease to 50 degrees C within 70 min contributed to a distinct reduction of Listeria counts. The counts in the cheese cores did not exceed 10(3) cfu/g within 12 wk of cheese ripening and Listeria was not detectable after 24 wk. In contrast to the cores of the cheeses of the 4 batches in this study, their rinds always contained a high listerial load of approximately 10(6) to 10(8) cfu/g throughout the entire ripening period. The smeared surface showed an increase of pH to alkaline values, corresponding to smear microbiota development. Coryneforms and Staphylococcus counts were stable at >10(7) cfu/cm(2) over 175 d, whereas yeast counts decreased to about 10(5) cfu/cm(2) at the end of ripening. The study shows that the smear culture had no noticeable anti-listerial potential. When removing the rind or portioning such smeared cheese loaves with a cutting device, a postprocess contamination of the core might occur, thus presenting a major hygienic risk.