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Detection of Nonfat Dried Milk in Whole Milk

U.S. Department of Health, Education, and Welfare, Food and Drug Administration, Cincinnati, Ohio 45226

ABSTRACT

Proteins precipitated from milk by acetic acid at pH 4.7 were reacted with phosphomolybdic acid, and the blue color imparted to them was measured by reflectance speotrophotometry. The computed curves for unknown samples were: Per cent nonfat dried milk (fresh) = 789.2 x OD - 129.8; per cent nonfat dried milk (old) - 575.5 x OD - 112.5, where OD is optical density (reflectance). The minimum amount of detectable nonfat dried milk was 10%, and the precision of the method was 0.014 OD.

Sequential analyses of the fresh-raw, pasteurized, condensed, and dried skimmilk samples revealed significant change in the proteins during drying. Dialysis of milk or washing of the precipitated proteins prior to the reaction with phosphomolybdic acid considerably decreased the blue color imparted to the proteins. Similarly, increased fat in milk progressively decreased the color reaction. Storage of fresh nonfat dried milk at 4 and 22 C had no effect on the test up to 8 and 4 weeks.