Milk is an interesting food of great value because it combines in itself all three of the great classes of food-stuffs,—protein, starch-sugar, and fat. Its protein is a substance called casein, which forms the bulk of curds, and which, when dried and salted, is called cheese. The fat is present in little tiny globules which give milk its whitish or milky color. When milk is allowed to stand, these globules of fat, being lighter, float up to the top and form a layer which is called cream. When this cream is skimmed off and put into a churn, and shaken or beaten violently so as to break the little film with which each of these droplets is coated, they run together and form a yellow mass which we call butter. In addition to the curd and fat, milk contains also sugar, called milk-sugar (lactose), which gives it its sweetish taste. And as a considerable part of the casein, or curd, is composed of another starch-like body, or animal starch, this makes milk quite rich in the starch-sugar group of food-stuffs.
All these substances, of course, in milk are dissolved in a large amount of water, so that when milk is evaporated, or dried, it shrinks down to barely one-sixth of its former bulk. It is, in fact, a liquid meat, starch-sugar, and fat in one; and that is why babies are able to live and thrive on it alone for the first six months of their lives. It is also a very valuable food for older children, though, naturally, it is not “strong” enough and needs to be combined with bread, puddings, meat, and fat.
