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Status of Zinc in Cow's Milk¹

Department of Biochemistry, University of Minnesota, St. Paul

ABSTRACT

A colorimetric method employing dithizone was developed and validated for determination of zinc in milk. The zinc content of cow's milk was found to range from 3 to 6 µg/milliliter. Very little of the zinc in milk is associated with the fat phase, inasmuch as the zinc content of skimmilk is virtually identical to that of the whole milk from which it is derived. An average of about 12% of the total zinc is present in dissolved, ultrafilterable form. The remainder is associated with the caseinate particles. None appears to be associated with the noncasein or whey proteins, inasmuch as the bound zinc is all sedimented with the caseinate under conditions of ultracentrifugation which do not sediment the whey proteins. The casein-bound zinc is all released by acidification to pH 2.0, but only about half of it is removed by dialysis of skimmilk at pH 6.6 or by extraction with dithizone in acetone. Thus, a portion of the zinc appears to be tightly and another loosely bound to constituents of the caseinate complex. The solubilization of bound zinc by adding graded amounts of EDTA to skimmilk further supports the conclusion that zinc is bound in two different manners.

¹ Paper no. 6037 Scientific Journal Series, Minnesota Agricultural Experiment Station.