Casein content of human milk.

Three methods for estimating the casein content of human milk were tested; isoelectric precipitation with washing and correction for co-precipitating proteins, sedimentation by ultracentrifugation, and indirect analysis (ie analyzing for the content of the major whey proteins and subtracting these from the total protein content). Gel electrophoresis and amino acid analysis were used to confirm some of the results. The casein content (mg/ml) of mature human milk (n = 9) was 2.33 +/- 1.69 by isoelectric precipitation, 1.80 +/- 0.48 by sedimentation and 2.96 +/- 1.08 by the indirect approach. A probable partition of nitrogen in breast milk would be casein N: whey protein N: non-protein N of 20:50:30; ie the correct ratio of casein nitrogen: whey nitrogen is approximately 20:80. Analysis of trace elements and minerals demonstrates that of total Ca 10%, Mg 5%, Zn 28%, Cu 17%, and Fe 27% is bound to casein when prepared by ultracentrifugation while isoelectric precipitation causes a redistribution of some of these elements. Since the protein ratio of human milk is considered a guideline when manufacturing infant formulas, these findings should be considered with regard to infant nutrition.