Papageorgiou Demetrios K, Marth Elmer H
Department of Food Science and the Food Research Institute, University of Wisconsin-Madison, Madison, Wisconsin 53706.
J Food Prot. 1989 Jul;52(7):459-465. doi: 10.4315/0362-028X-52.7.459.
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 × 10 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 log 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 log cfu/g during the first 24 h of the cheese-making process. Only modest growth (0.12 to 0.30 log 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 log 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.
研究了单增李斯特菌在蓝纹奶酪制作过程中生长以及在成熟过程中存活的能力。通过添加巴氏杀菌均质奶油（乳脂含量35%）将巴氏杀菌脱脂乳的乳脂含量标准化至3.7%，接种使其含有约1.0 - 2.0×10个单增李斯特菌[斯科特A菌株或加利福尼亚（CA）菌株]cfu/ml，并按照改良的爱荷华方法制作成蓝纹奶酪。蓝纹奶酪在9 - 12°C、相对湿度90 - 98%的条件下成熟84天，然后在4°C下储存。对牛奶、凝乳、乳清和奶酪的重复样本进行pH值检测，并通过将适当稀释液[用含2%柠檬酸钠的胰蛋白胨肉汤（TB）配制]表面接种在麦克布莱德李斯特菌琼脂（MLA）上检测李斯特菌数量。如果原始样本中未对病原体进行定量，则将初始TB稀释液保存在4°C，并在2、4、6和8周后表面接种在MLA上。对选定的李斯特菌菌落进行生化鉴定。在奶酪制作过程中，单增李斯特菌被困在凝乳中，装模前凝乳中的菌量比接种牛奶中的菌量约高1.0 log cfu/g；乳清中平均含有初始接种菌量的3.6%。在奶酪制作的前24小时内，奶酪中的单增李斯特菌数量增加了0.58至1.22 log cfu/g。在两个产酸迅速的批次中仅观察到适度生长（0.12至0.30 log cfu/g）。当奶酪的pH值降至5.0以下时单增李斯特菌的生长停止。在成熟的前50天内，两种病原体菌株的菌量均显著下降（P≤0.005），与1日龄奶酪的菌量相比平均下降了2.68 log cfu/g。从第50天到120天，蓝纹奶酪的环境变得更有利于病原体存活（由于罗克福青霉生长导致奶酪pH值升高），这使得病原体的存活率提高但没有生长。在储存的第50至120天，斯科特A菌株存活且菌量没有进一步大幅下降。CA菌株在第50至80天存活，然后病原体菌量逐渐下降，以至于在110天（一次试验）和120天直接接种呈阴性结果，但相同样本经冷增菌后呈阳性结果。
