Milk contains all the elements which are necessary to maintain life; and constitutes a complete diet for infants.  It will sustain life in an adult for several months.  Although milk furnishes a useful food, it is not essential to a diet required for active bodily exercise.  It is seldom given to athletes while in active training.  Adults who are able to eat any kind of food are kept in better health by abstaining from milk, except as used for cooking purposes.  An occasional glass of hot milk taken as a stimulant for tired brain and nerves is sometimes beneficial.  Milk is composed of water, salts, fat, milk sugar or lactose, albumen and casein.  Average milk has from 8 to 10 per cent. of cream.  Good milk should form a layer of cream about 2-1/2 in. thick as it stands in a quart bottle.  Lactose (milk sugar) is an important ingredient in milk.  It is less liable to ferment in the stomach than cane sugar.  In the presence of fermenting nitrogenous material it is converted into lactic acid, making the milk sour.  Casein is present in milk chiefly in its alkaline form, and in conjunction with calcium phosphate.  Milk absorbs germs from the air and from unclean vessels very readily.  Good, clean, uncontaminated milk ought to keep fresh, exposed in a clean room at a temperature of 68 deg.  F., for 48 hours without souring.  If the milk is tainted in any way it will sour in a few hours.  Boiled milk will keep fresh half as long again as fresh milk.  Milk absorbs odors very quickly, therefore should never be left in a refrigerator with stale cheese, ham, vegetables, etc., unless in an air-tight jar.  It should never be left exposed in a sick room or near waste pipes.  Absolute cleanliness is necessary for the preservation of milk; vessels in which it is to be kept must be thoroughly scalded with boiling water, not merely washed out with warm water.

Methods of Preserving Milk.

STERILIZED MILK.

Milk to be thoroughly sterilized and germ free must be heated to the boiling point (212 deg.  F.).  This may be done by putting the milk into perfectly clean bottles and placing in a rack, in a kettle of boiling water, remaining until it reaches the necessary degree of heat.  The bottle should be closely covered immediately after with absorbent cotton or cotton batting in order to prevent other germs getting into the milk.

PASTEURIZED MILK.

The difference between pasteurizing and sterilizing is only in the degree of heat to which the milk is subjected.  In pasteurizing, the milk is kept at a temperature of 170 deg.  F. from 10 to 20 minutes.  This is considered a better method for treating milk which is to be given to young children, as it is more easily digested than sterilized milk.  All milk should be sterilized or pasteurized in warm weather, especially for children.

CHEESE.

Cheese is one of the most nutritious of foods, and when meat is scarce makes an excellent substitute, as it contains more protein than meat.  Cheese is the separated casein of milk, which includes some of the fats and salts.