Status of Zinc in Cow's Milk

Abstract

A colorimetric method employing dithizone was developed and validated for determination of Zn in milk. The Zn content of cow''s milk was found to range from 3 to 6 [mu]g/ml. Very little of the Zn in milk is associated with the fat phase, inasmuch as the Zn content of skimmilk is virtually identical to that of the whole milk from which it is derived. An average of abuut 12% of the total Zn is present in dissolved, ultraf ilterable form. The remainder is associated with the caseinate particles. None appears to be associated with the noncasein or whey proteins, inasmuch as the bound Zn is all sedimented with the caseinate under conditions of ultracentrifugation which do not sediment the whey protein. The casein-bound Zn is all released by acidification to th 2.0, but only about 1/2 of it is removed by dialysis of skimmilk at pH 6.6 or by extraction with dithizone in acetone. Thus, a portion of the Zn appears to be tightly and another loosely bound to constituents of the caseinate complex. The solubilization of bound Zn by adding graded amounts of EDTA to skimmilk further supports the conclusion that Zn is bound in 2 different manners.