EFFECTIVENESS OF SALT, PH, AND DIACETYL AS INHIBITORS FOR ESCHERICHIA-COLI O157-H7 IN DAIRY FOODS STORED AT REFRIGERATION TEMPERATURES

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 mu g /g was determined at 4 and 12 degrees C. Cell populations were determi ned by surface plating on tryptic soy agar after 7 and 35 days of incu bation. Survival was also determined using retail cultured dairy produ cts. E. coli O157:H7 did not survive in skim milli at pH 3.8 and was r educed by 3 log cycles at pH 4.1, regardless of salt, diacetyl, and te mperature levels. At pH levels above 4.4, survival was observed at low er salt concentrations for up to 35 days at both 12 and 4 degrees C. T he organism grew (up to a 2.2-log increase) at pH 5.0 at 2% salt level s after 35 days of storage at 12 degrees C but not at 4 degrees C. Dia cetyl at a concentration of 10 ppm had no effect on survival and growt h. In all but one case, E. coli O157:H7 was inactivated in yogurt, sou r 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 sa mples at 12 degrees C after 7 days but not after 35 days or at 4 degre es C, when a 1- to 2-log decline was observed.