Guraya R, Frank J F, Hassan A N
Southeast Poultry Research Laboratory, U. S. Department of Agriculture, Athens, Georgia 30605, USA.
J Food Prot. 1998 Sep;61(9):1098-102. doi: 10.4315/0362-028x-61.9.1098.
The behavior of Escherichia coli O157:H7 inoculated in 10% rehydrated nonfat dry milk adjusted to pH levels between 3.8 and 5.4 with lactic acid, salt levels of 0 to 6%, and diacetyl levels of 0, 5, and 10 micrograms/g was determined at 4 and 12 degrees C. Cell populations were determined by surface plating on tryptic soy agar after 7 and 35 days of incubation. Survival was also determined using retail cultured diary products. E. coli O157:H7 did not survive in skim milk at pH 3.8 and was reduced by 3 log cycles at pH 4.1, regardless of salt, diacetyl, and temperature levels. At pH levels above 4.4, survival was observed at lower salt concentrations for up to 35 days at both 12 and 4 degrees C. The organism grew (up to a 2.2-log increase) at pH 5.0 at 2% salt levels after 35 days of storage at 12 degrees C. Diacetyl at a concentration of 10 ppm had no effect on survival and growth. In all but one case, E. coli O157:H7 was inactivated in yogurt, sour cream, and buttermilk at a rate similar to or greater than what was consistent with the acidified skim milk data. Also consistent with the skim milk data, growth occurred in two of the three cottage cheese samples at 12 degrees C after 7 days but not after 35 days or at 4 degrees C, when a 1- to 2-log decline was observed.
测定了接种于用乳酸将pH值调节至3.8至5.4、盐含量为0至6%以及双乙酰含量分别为0、5和10微克/克的10%复水脱脂奶粉中的大肠杆菌O157:H7在4℃和12℃下的行为。培养7天和35天后，通过在胰蛋白胨大豆琼脂上进行表面平板计数来确定细胞数量。还使用市售的发酵乳制品来测定存活率。无论盐、双乙酰和温度水平如何，大肠杆菌O157:H7在pH 3.8的脱脂牛奶中无法存活，在pH 4.1时数量减少3个对数周期。在pH值高于4.4时，在12℃和4℃下，较低盐浓度下可观察到长达35天的存活情况。在12℃下储存35天后，该菌在2%盐含量、pH 5.0时生长（数量增加高达2.2个对数）。浓度为10 ppm的双乙酰对存活和生长没有影响。除一个案例外，在酸奶、酸奶油和酪乳中，大肠杆菌O157:H7的失活速率与酸化脱脂牛奶数据一致或更快。同样与脱脂牛奶数据一致的是，三种农家干酪样品中有两种在12℃下培养7天后生长，但在35天后或4℃下未生长，此时观察到数量下降1至2个对数。
