Fate of Listeria monocytogenes During the Manufacture and Ripening of Blue Cheese

Abstract

The ability of Listeria monocytogenes to grow during the manufacture of blue cheese and to survive during its ripening was examined. Pasteurized skim milk was standardized to a milk fat content of 3.7% by addition of pasteurized homogenized cream (35% milk fat), was inoculated to contain ca. 1.0–2.0 × 10³ L. monocytogenes [strain Scott A or California (CA)] cfu/ml, and was made into blue cheese according to the modified Iowa method. Blue cheese was ripened at 9–12°C and a relative humidity of 90–98% for 84 d, and then cheese was stored at 4°C. Duplicate samples of milk, curd, whey, and cheese were tested for pH and for numbers of Listeria by surface plating of appropriate dilutions [made in Tryptose Broth (TB) with 2% sodium citrate] on McBride Listeria Agar (MLA). Initial TB dilutions were stored at 4°C and surface-plated on MLA after 2, 4, 6, and 8 weeks, if the pathogen was not quantitated in the original sample. Selected Listeria colonies were confirmed biochemically. L. monocytogenes was entrapped in curd during cheese-making with the population in curd before hooping being ca. 1.0 log₁₀ cfu/g greater than in the inoculated milk; whey contained an average of 3.6% of the cells in the initial inoculum. L. monocytogenes in cheese increased in numbers by 0.58 to 1.22 log₁₀ cfu/g during the first 24 h of the cheese-making process. Only modest growth (0.12 to 0.30 log₁₀ cfu/g) was noted in two lots with rapid acid production. Growth of L. monocytogenes ceased when the pH of cheese dropped below 5.0. Populations of both strains of the pathogen decreased significantly (P≤ 0.005) during the first 50 d of ripening, by an average of 2.68 log₁₀ cfu/g compared to populations of 1-d-old cheese. From days 50 to 120 the environment of blue cheese became more favorable (pH of cheese increased because of growth by Penicillium roqueforti), and this resulted in improved survival but no growth of the pathogen. Strain Scott A survived without any more substantial decrease in numbers during days 50 to 120 of storage. Strain CA survived during days 50 to 80, and then populations of the pathogen decreased gradually so that direct plating at 110 d (one trial) and 120 d gave negative results, but the same samples gave positive results after cold enrichment.