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Survival of Escherichia coli, Strain W, during the Manufacture of Cottage Cheese¹

Department of Animal Sciences and Industry, Agricultural Experiment Station, Kansas State University, Manhattan 66506

ABSTRACT

Cottage cheese was manufactured in 10-liter experimental vats by the direct-acid-set method from milk that was inoculated with a heat resistant strain of Escherichia coli. Growth or survival of Strain W (ATCC 9637) E. coli was determined at various stages of the cheese making operation after the cheese-skim milk was inoculated to give counts of 2.5 x 10⁴ or 4.0 x 10⁵ cells/ml.

Numbers of coliform organisms remained constant at the inoculated concentration in the cheese milk up to a cooking temperature of 43 C. At 43 C, when curd was separated from the whey, the curd (not washed) had coliform counts that were two log cycles greater than the whey. These trends were in milks with both cell counts. Washing of the curd with acid and 10 ppm chlorine reduced the number of coliform cells in the curd at all cooking temperatures as compared with unwashed curd. Acid wash of the curd at pH 5 did not reduce the coliform counts below those of unwashed curd. Cooking temperatures of 54 C were necessary to destroy (< 1 cell/ml) E. coli Strain W, in either the unwashed or acid-chlorine washed curd.

Holding curd with an initial average log count of 6.26 coliform cells/ml at constant temperatures of 38, 43, 49, and 54 C confirmed that 54 C for 50 min was necessary to reduce the average count to < 1 cell/ml in isolated curd. Coliforms in whey were reduced to that concentration after 10 min at 54 C.

¹ Contribution No. 78-266-j. Department of Animal Sciences and Industry, Agricultural Experiment Station, Manhattan 66506.