268. Are The Compounds Similar?

We have seen that the classes of compounds and the ash constituents are alike in mother's and cow's milk, but the question naturally arises whether the compounds themselves are similar. Are the protein bodies and the fats alike in the two milks? A negative answer must be given to this question. There are several differences. As we know, the protein of milk is a mixture of several nitrogen compounds, casein, albumin, globulin, and others. These bodies do not exist in the same proportions in the two milks under discussion, the proportion of casein in the protein of cow's milk being 80 per cent, which is nearly twice as large as in human milk, the soluble bodies like albumin and globulin being proportionately larger in the latter milk.1 The significance of this fact lies in the unlike behavior of casein and albumin toward acids and coagulating ferments. Casein is coagulated at ordinary temperatures by the combined action of acid and pepsin, while albumin is not, and partly for this reason the milks under comparison must behave quite differently in the human stomach when they come in contact with the gastric juice. It appears, too, that the casein of cow's milk is not quite the same compound as in mother's milk. This is shown by a difference in the proportions of the several elements in the casein from the two sources, especially of the phosphorus and sulfur.2

As with protein, the fat of milk is not a single body, but is a mixture of several individual fats, and these exist in the fat of the two milks in quite different proportions. As previously explained, the fats consist of fatty acids united with glycerine. When freed from the glycerine, some are solid, and some are liquids, at ordinary temperatures; some are volatile and gradually pass into the air, especially when heated; and some are non-volatile, or fixed. Cow's milk contains more than ten times as large a proportion of the volatile acids as does human milk, while the percentage of oleic acid, an oil at room temperature, is much greater in the latter. The combined effect of these differences is that mother's milk fat melts at the lower temperature, a fact that is doubtless of some importance as related to the ease of digestion.1

1 "Handbuch der Milchkunde," Sommerfield, pp. 782-784. 2 Loc. cit., p. 787.

269. Comparison Of Physical Condition

Certain other differences should be noted that show a marked difference in the physical character of the two milks. It is well understood that the casein of milk is not in solution therein, but is held in suspension as colloidal particles. When cow's milk is examined with the ultra microscope, it is possible to see these particles, which either by their constitution or their abundance completely hide the fat globules. When a similar examination is made of mother's milk, no casein particles are visible, and the fat globules show in a dark and apparently otherwise empty space. This difference can scarcely be due to the less amount of casein in the mother's milk, but shows rather that the constitution of the particles is unlike in the two cases. The fat globules of the mother's milk are much smaller than in the other.