The analysis of an average specimen of cow's milk, according to Letheby, is:—
| Nitrogenous matter | 4.1 |
| Fat | 3.9 |
| Sugar of milk | 5.2 |
| Mineral matter | 0.8 |
| Water | 86.0 |
If a drop of milk be examined with a microscope, it will be seen as a clear liquid, holding in suspension a large number of minute globules, which give the milk its opacity or white color. These microscopic globules are composed of fatty matter, each surrounded by an envelope of casein, the principal nitrogenous element found in milk. They are lighter than the surrounding liquid, and when the milk remains at rest, they gradually rise to the top and form cream. Casein, unlike albumen, is not coagulated by heat; hence when milk is cooked, it undergoes no noticeable change, save the coagulation of the very small amount of albumen it contains, which, as it solidifies, rises to the top, carrying with it a small portion of the sugar and saline matter and some of the fat globules, forming a skin-like scum upon the surface. Casein, although not coagulable by heat, is coagulated by the introduction into the milk of acids or extract of rennet. The curd of cheese is coagulated casein. When milk is allowed to stand for some time exposed to warmth and air, a spontaneous coagulation occurs, caused by fermentative changes in the sugar of milk, by which it is converted into lactic acid through the action of germs.
Milk is sometimes adulterated by water, the removal of more or less of the cream, or the addition of some foreign substance to increase its density.
The quality of milk is more or less influenced by the food upon which the animal is fed. Watery milk may be produced by feeding a cow upon sloppy food.
The milk of diseased animals should never be used for food. There is no way by which such milk can invariably be detected, but Prof. Vaughan, of Michigan University, notes the following kinds of milk to be avoided:
1. Milk which becomes sour and curdles within a few hours after it has been drawn, and before any cream forms on its surface. This is known in some sections as 'curdly' milk, and it comes from cows with certain inflammatory affections of the udder, or digestive diseases, or those which have been overdriven or worried.
2. "Bitter-sweet milk" has cream of a bitter taste, is covered with 'blisters,' and frequently with a fine mold. Butter and cheese made from such milk cannot be eaten on account of the disagreeable taste.
3. 'Slimy milk' can be drawn out into fine, ropy fibers. It has an unpleasant taste, which is most marked in the cream. The causes which lead to the secretion of this milk are not known.
4. 'Blue milk' is characterized by the appearance on its surface, eighteen or twenty-four hours after it is drawn, of small, indigo-blue spots, which rapidly enlarge until the whole surface is covered with a blue film. If the milk be allowed to stand a few days, the blue is converted into a greenish or reddish color. This coloration of the milk is due to the growth of microscopic organisms. The butter made from 'blue milk' is dirty-white, gelatinous, and bitter.