IRON-SUPPLEMENTED COW MILK - IDENTIFICATION AND SPECTRAL PROPERTIES OF IRON BOUND TO CASEIN MICELLES

Because transition metals may cause “oxidized” flavors and odors in dairy products, the physical chemistry of iron bound to casein phosphoproteins may greatly influence the nutritional and organoleptic properties of iron-fortified milk. Centrifugal, spectrophotometric, and chromatographic evidence is presented to determine the distribution of iron in milk supplemented with ionic, chelated, or polynuclear iron complexes. With most iron donors, iron added at low concentration sedimented with the casein micelle and could be recovered with isoelectric casein. With the nitrilotriacetate (NTA) or lactobionate chelates of iron(III), the casein fraction of skim milk became saturated after addition of 10-20 mmol of Fe/L of milk. α-Casein was the principal iron-binding protein in milk. Iron donated by ferrous salt or ferric NTA was bound as the iron(III)-oxyphosphate complex on the phosphorylserine residues of casein. Ferrous salts may cause organoleptic deterioration of supplemented milk because the iron not bound to casein is capable of interacting with oxidizable milkfat. This oxidative instability may be reduced by use of chelated iron(III) supplements such as ferric nitrilotriacetate and ferric lactobionate that donate iron rapidly and specifically to the casein phosphoproteins, which effectively remove iron from the lipid phase. © 1979, American Chemical Society. All rights reserved.