MILK, AN INVALUABLE FOOD.—It has been said that there is no one food except milk which cannot be eliminated from the diet. Milk is the only food for which there are no easily found substitutes. The housekeeper or one who plans the food for the family should purchase daily, if possible, a pint of milk for each adult and a quart for each child under ten years. She should see to it that this amount of milk is entirely used either as a beverage or in cooked foods. If one must economize in foods, less should be spent for meat, and more for milk.

Although more than 4/5 of milk is water, it contains only a little more water than do potatoes and lean meat. The value of milk is due to the fact that it contains: (a) Proteins of "excellent quality." An authority on diet says [Footnote 48: See "The Newer Knowledge of Nutrition," by McCollum, p. 74.]: "There can be no doubt that the proteins of milk are far superior to those of any foods derived from vegetable sources." The most important protein existing in milk is called casein.

Casein is a complete protein and is very important for growth. It has a peculiar property; it precipitates when acid is added to milk. When milk sours, the sugar contained in the milk changes to an acid, and this acid causes the casein to precipitate. Casein is also clotted by an enzyme occurring in the digestive juice of the stomach.

(b) Valuable ash. Lime which is so essential to bodybuilding is one of the minerals in milk. The following diagram from United States Food Leaflet No. 11 shows that milk is especially rich in lime. (Lime is calcium oxide.)

[Illustration]

(c) Vitamines. These are substances contained only in certain foods. They are essential for maintaining life and health. Milk is rich in these indispensable materials (see Division Seven).

Milk also contains fat and carbohydrate. The presence of the foodstuffs in milk is shown by the following:

EXPERIMENT 44: SEPARATION OF MILK INTO FOODSTUFFS.

(a) By means of a cream dipper, remove the cream from a bottle of milk. Place a drop of the cream on a piece of paper. Let the paper dry. What foodstuff is indicated by the stain on the paper?

(b) Take 1/4 cupful of the skimmed milk. Heat it to blood temperature (test by dropping the milk on the wrist, see Junket Custard). Crush 1/8 junket tablet and add it to the warm milk. Stir until the powder is dissolved. Let the milk stand in a warm place until it is clotted. Heat the clotted milk and boil 1 minute. Pour it into a filter paper. Catch the filtrate in a beaker. What is the foodstuff that remains in the filter paper (see Proteins of "excellent quality")?

(c) Put 15 cubic centimeters of Fehling's Solution [Footnote 49: NOTE TO THE TEACHER.—Fehling's Solution is made as follows: Prepare a solution of Rochelle salts,—175 grams of Rochelle salts, 50 grams of sodium hydroxide, and 250 cubic centimeters of water. Prepare a solution of copper sulphate,—57.73 grams of copper sulphate, 250 cubic centimeters of water, and 0.4 cubic centimeter of sulphuric acid. Then combine 1 part of the alkaline Rochelle salt solution, 1 part of copper sulphate, and 4 parts of water. Boil the mixture.

This solution deteriorates readily. The best results are obtained by using a "fresh" mixture for testing sugar and by boiling just before using.] in a flask. Boil for 2 minutes. Add 2 cubic centimeters of the filtrate from (b) and boil one minute. To what color does the blue mixture change? A red precipitate indicates sugar. What foodstuff does this test indicate that milk contains?

(d) Put the remainder of the filtrate from (b) in a custard cup and evaporate over hot water to dryness. Note the residue. What foodstuff other than sugar is contained in the residue?

(e) What foodstuff has passed off in the form of vapor during evaporation?

(f) As mentioned above, milk also contains vitamines.