Distribution of Iron Between Caseins and Whey Proteins in Acidified Milk

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Food Science Technology



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The association of iron with casein and whey proteins was studied by fortifying pasteurized skim milk with 100 mg of iron (Fe3+)/L and adjusting the pH to 6.7, 6.2, 5.8, 5.3, 4.5, and 4.0. Samples were fractionated by ultracentrifugation, and the pellets and serum collected and analyzed for iron, calcium, and phosphorus contents, and for protein profile. At pH 6.7, 6.2, 5.8, and 5.3, some casein, particularly β-casein was not sedimented. More iron was sedimented at pH≤4. However, expressed on a protein basis, the pellets had lower iron contents, probably because denatured whey proteins have lower iron-binding affinities. As pH decreased, the calcium and phosphorus dissociated from casein micelles, with less solubilization of colloidal calcium phosphate occurring in the iron-fortified samples at pH 6.2, 5.8 and 5.3. In yogurt fortified with casein-chelated iron, iron remained bound to casein and was distributed within the micelles. Yogurt fortified with whey protein-chelated iron showed iron being distributed throughout the nonmicellar portion of the yogurt. Fortification with FeCl3resulted in iron being preferentially located within the casein micelles.

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