Milk is the type of a complete food provided by nature. In early life it forms the exclusive food of all young mammals. It can also be employed as the chief food for adults under certain pathological conditions, which will be referred to in later chapters; but on account of the large amount that is required for daily consumption, it is not a very practicable food for healthy adults.

It is in the highest degree important for the well-being of young children to have milk which is of excellent quality at its source; and, secondly, it is equally necessary that it should not be adulterated or infected in the process of transit and distribution. Since the sources of contamination are numerous, the task of obtaining an absolutely pure supply is a difficult one, and in recent years the question of the milk-supply of our large towns has engaged more and more the attention of the profession and of civic authorities.

The milk of all animals contains the elementary substances - proteins, fats, carbohydrates, salts, and water - combined in such proportions as are best adapted for the period during which growth is most active. The proportions of these elements vary in different milks, as shown in the following table: -

Composition Of Milk Of Different Animals

Protein.

Fat.

Sugar.

Ash.

Other Nitrogenous and Unknown

Bodies.

Human . .. .

0.9

3.52

6.75

0.19

0.6

Cow ....

3.0

3.55

4.51

070

0.3

Goat ....

2.8

3.40

3.80

0.95

...

Mare ....

1.9

1.00

6.33

0.45

0.49

Ass ... . .

1.6

0.93

5.60

0.36

...

Milk is an emulsion consisting of fine oil globules suspended in a colourless liquid or plasma -

Milk an emulsion

Suspended matters

Oil globules.

Fine granular casein.

Plasma .

Proteins

in solution.

Sugar

Salt

When freshly drawn, the reaction of milk varies in different animals. Human milk is alkaline; cow's milk is neutral; and the milk of carnivora is acid in reaction. On standing, all milk becomes acid, owing to the formation of lactic acid, through the action of bacteria normally present in milk. The average proportion of the chief ingredients in cow's milk and human milk are here given for comparison: -

Cow's Milk.

Human Milk.

Proteins-

-Casein..

3.25

4.0

20

.6

Lactalbumin..

75

1.4

Fat .

3.5

3.5

Sugar.

• • •

4.0

7.0

Salts .

• • •

•7

0.2

Water

• •

87.8

87.3

It will be seen thai the chief differences in composition are the much larger amount of protein and the smaller proportion of sugar present in cow's milk. There is further an important difference in the nature of the proteins, for while in cow's milk they consist chiefly of casein, the proportion of in to lactalbumin being 4: 1, in human milk lactalbumin he chief protein, the proportion of casein to lactalbumin being approximately I: 2. The practical importance of this is readily appreciated if samples of the two milks are treated by the addition of weak hydrochloric acid. Large masses of curd are at once formed in the cow's milk, while the human milk shows a fine flocculent precipitate. Similar changes occur in the stomach. Another important difference is the sterility of human milk in contrast with the large number of bacteria normally present in cow's milk as it reaches the consumer. Human milk is further considered on p. 231. We will now consider cow's milk and its derivatives.