The casein complexes (casein micelles) of human skim milk were evaluated by electron microscopy. Fourteen samples of human milk were obtained from seven donors whose stage of lactation varied from 1 to 23 months. Composition and biochemical parameters were measured to evaluate the nature of these samples. For mature human milks, 2-13 months of lactation, average percentage of solids-not-fat was found to be 8.32 .+-. 0.16. Protein compositions of the skim milks were studied by the Coomassie blue dye binding method and by SDS-gel electrophoresis; total protein and casein were found to be 0.62 .+-. 0.18 and 0.29 .+-. 0.02 percent, respectively. The lysozyme and lactoferrin contents were also determined. The total casein of the skim milks appeared to increase slightly as lactation progressed; an exception occurred in milks obtained from one donor at 22 and 23 months postpartum which exhibited decreased casein and elevated lysozyme and lactoferrin contents. The overall morphology of the casein micelles was evaluated by electron microscopy using platinum shadowing, negative staining, and thin sectioning methods. Platinum shadowing was largely unsuccessful, but negative staining showed a discrete substructure for human casein micelles. Average corrected diameters of 43.0 nm from the area and 47.5 nm from circumference measurements were obtained from analysis of fixed and pelleted human milk micelles.
