7. The solids contained in milk are proteins, fat, carbohydrate in the form of sugar, and mineral salts, besides which, of course, water occurs in large quantities. The sugar and fat of milk serve as fuel; the mineral salts are chiefly valuable for the growth of bones and teeth and for their effect on the liquids of the body; and the proteins, like the fat and sugar, serve as fuel, but they also make and repair the muscular tissues of the body.
In considering the food substances of milk, it will be well to note also that they vary according to the breed, feeding, and individual characteristics of the cow. Jerseys and Guernseys give milk rich in fat and total solids, and while Holstein cows give a greater quantity of milk, such milk has a smaller proportion of fat and total solids. As a rule, though, the composition of milk may be considered as approximately 3.3 per cent. protein, 4 per cent. fat, 5 per cent. carbohydrate, and .7 per cent. mineral matter, making a total of 13 per cent. This indicates the quantity of actual food material in milk, the remainder, or 87 per cent., being water.
8. PROTEIN IN MILK.--Because of the double usefulness of protein--to serve as fuel and to make and repair muscular tissue--this element is regarded as an important ingredient of milk. The protein in milk is called _casein_. The opaque whiteness of milk is largely due to the presence of this substance. As long as milk remains sweet, the lime salts it contains hold this casein in solution; but when it sours, the salts are made soluble and the casein thickens, or coagulates. In addition to casein, milk contains a small amount of protein in the form of _albumin_. This substance, upon being heated, coagulates and causes the formation of the skin that is always found on the top of milk that has been heated. The skin thus formed contains everything that is found in milk, because, as it forms, casein is dried with it and sugar and fat, too, are caught and held there. It is the protein of milk and its characteristic coagulation that are made use of in the making of cheese. In cooking, the protein of milk is probably more affected than any of the other substances, but the degree to which the digestion of milk is thus affected is not definitely known, this being a much disputed question.