Effectiveness of Salt, pH, and Diacetyl as Inhibitors for Escherichia coli O157:H7 in Dairy Foods Stored at Refrigeration Temperatures

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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 diacetyllevels of 0, 5, and 10 μg/g was determined at 4 and 12°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 dairy 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°C. The organism grew (up to a 2.2-1og increase) at pH 5.0 at 2% salt levels after 35 days of storage at 12°C but not at 4°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°C after 7 days but not after 35 days or at 4°C, when a 1- to 2-1og decline was observed.

Document Type: Research Article

Affiliations: 1: Southeast Poultry Research Laboratory, U.S. Department of Agriculture, Athens, Georgia 30605, USA 2: Center for Food Safety and Quality Enhancement, Department of Food Science and Technology, University of Georgia, Athens, Georgia 30602-7610, USA 3: University of Minia, Minia, Egypt

Publication date: September 1, 1998

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