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Relation between Lactic Acid and Direct Microscopic Counts for Bacteria in Nonfat Dry Milk¹

Biological Sciences Branch, USDA, Beltsville, Maryland

ABSTRACT

About 50 dry milk samples with known direct microscopic counts for bacteria were analyzed for lactic acid by Ion exchange, Golden State, Davidson, and Ling methods. Precision was poor on all but the Ion exchange method. All methods gave correlation coefficients of about .8 between apparent lactic acid and bacteria counts. The Ion exchange method was found to be the most accurate and also the slowest. The Ling method was definitely the fastest. The value of lactic acid content as an estimate of bacteria count was calculated. The expected range of bacteria counts for a given amount of lactic acid was too large for lactic acid to be a satisfactory indicator of bacterial contamination. Roller-dried samples gave unexpectedly high analyses for lactic acid.

¹ Presented in part before the American Dry Milk Institute Research Conference in Madison, Wisconsin, on October 13, 1958.