Fate and survival of Listeria monocytogenes and Escherichia coli O157:H7 during ripening of cheddar cheeses manufactured from unpasteurized raw milk

Abstract

Cheese products may serve as vehicles for transmission of foodborne pathogens, causing disease outbreaks, particularly when unpasteurized raw milk is used for cheese production. Therefore, the adequacy of a 60-day aging period for raw milk Cheddar cheese to eliminate Escherichia coliO157:H7 and Listeria monocytogenes was evaluated. The changes in cheese components and characteristics of pH, Aw, coliform, and LAB were examined during the ripening process. Cheddar cheese was manufactured from raw milk artificially spiked with 1, 3, or 5 log CFU/mL L. monocytogenes or E. coliO157:H7. When compared with the pasteurized samples, the unpasteurized samples exhibited slightly lower pH values and similar aw values. No coliform were observed in the pasteurized cheese, whereas over 5-log CFU/g of coliform were found in the cheese made from unpasteurized milk during the early phases of ripening. However, coliform were completely eliminated from the cheese made from unpasteurized milk within five weeks. In contrast, while the level of LAB was initially 2-log lower in the cheese made from pasteurized milk than that derived with unpasteurized milk, the number of LAB between the two cheese preparations equalized within five weeks. Although L. monocytogenes numbers decreased significantly to below the detection limit (1-log CFU/g) during ripening, viable cells were isolated from all the inocula. E. coliO157:H7 cells also significantly decreased to below the detection limit during ripening at all the inoculation levels. However, viable cells were still detected. Thus, a 60-day ripening process alone may not be sufficient for elimination of contaminating pathogens.