Distribution of Natural and Added Copper and Iron in Milk

Abstract

Summary A radioactive tracer was used to determine the distribution of copper in milk. For comparison, the distribution of radioactive iron also was determined. The method can readily distinguish the sources (natural or added) of copper or iron. Marked differences were observed in the distribution of natural and added copper. Naturally occurring copper was concentrated ( ca. 10–35% of the copper) at the surface of the fat globules; whereas, most of the added copper was uniformly associated with the skimmilk proteins. Only 2–3% of the total added copper was associated with the fat globules. Natural and added copper in association with the fat globules were recovered with the fat globule membrane proteins. The distribution of natural iron, like natural copper, favored the fat globules, but none of the added iron became associated with the fat globules. Natural iron not associated with the fat globules was more concentrated with the whey proteins than with casein. Natural copper and iron were not dialyzable. Added copper and iron in the skimmilk were slightly dialyzable, and the amount of unbound metal increased as pH was decreased to 3.0. Added copper associated with fat globules was not dialyzable.