Plating media of different pH were studied as to effects on enumeration of pasteurization-resistant bacteria in milk. After heat treatment, thermoduric bacteria were more tolerant of pH levels above rather than below 7, and maximum mean thermoduric counts were obtained at pH 7.5. There were some exceptions, but usually the pH levels that yielded the highest counts also produced the largest and most easily discernible colonies. Pure cultures of thermoduric bacteria grew over a much wider range of pH before than they did after laboratory pasteurization. Results indicate that, although seemingly adequate for enumeration in raw milk, the medium pH currently recommended for the standard plate count may not be satisfactory for the determination of the maximum viable bacteria population of pasteurized milk.

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Author notes

1Journal Paper No. 5289 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. Project No. 1050.

2Present address: Department of Food Science and Technology, University of California, Davis.

3Present address: Department of Dairy Science, University of Arizona, Tucson.