Fate of Listeria monocytogenes during the Manufacture, Ripening and Storage of Feta Cheese

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Abstract
The ability of Listeria monocytogenes to grow during the Feta cheese-making process, and to survive during ripening, and storage of the cheese was examined. Pasteurized whole cow's milk was inoculated to contain ca. 5.0 × 103 L. monocytogenes [strain Scott A or California (CA)] cfu/ml and made into Feta cheese according to standard procedure. Lactobacillus bulgaricus, and Streptococcus thermophilus (1:1, v/v) were used as starter culture (1%, v/v). Fresh cheese was placed into sterile 12% salt brine and was held at 22°C for 24 h. Then it was placed into sterile 6% salt brine and held 4 d at 22°C after which it was stored in the same brine at 4°C. Milk, curd, whey, cheese, and brine were tested for numbers of L. monocytogenes and pH. Duplicate samples were used to enumerate L. monocytogenes by surface-plating on McBride Listeria Agar. Selected Listeria colonies were confirmed biochemically, L. monocytogenes was entrapped in curd during cheese-making with the population in curd being 0.92 Log10 cfu/g greater than in the inoculated milk; the whey contained an average of 3.2% of the initial inoculum. L. monocytogenes in cheese increased in numbers by ca. 1.5 Log10 cfu/g during the first 2 d of ripening, the population was 2.33 (S.D. ± 0.12) Log10 cfu/g greater in cheese than in the inoculated milk, with a maximum number of 1.5 × 106 cfu/g. The pH value of 2-d-old cheese decreased to 4.6 and then growth of L. monocytogenes ceased. Both strains of L. monocytogenes survived in Feta cheese for more than 90 d even at the low pH of 4.30 (S.D. ± 0.05) that Feta cheese had after ripening. Strain CA was significantly (P<0.006) less tolerant than strain Scott A, of conditions in the cheese during storage at 4°C.

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