In the event of mother's milk being unavailable for the infant, it becomes essential to provide a substitute for it, and we look first naturally to the milk of other mammals. Animals, other than the cow, from which milk may be obtained for the nutrition of the infant, are the ass, the goat, and the mare; but for all practical purposes cow's milk is of pre-eminent importance.

The chief characteristics of mother's milk are purity, ready digestibility, and suitability for adequate nutrition and development; and though cow's milk is lacking in many particulars, yet it is the nearest natural substitute for mother's milk. All patent and artificial foods are wanting in the essential quality of freshness. None contain the elements of the foodstuffs necessary for the infant's healthy nutrition, in a form so closely resembling mother's milk, as does cow's milk; many differ from it very markedly in quality, many are deficient in certain constituents, and many contain totally unsuitable constituents.

The composition of cow's milk varies according to many of the conditions of life of the animal, and thus the milk from a herd is more constant in composition than that from a single cow. The milk of Jersey, Guernsey, and Alderney cows is, as a rule, unsuitable for infant feeding, because it contains a high percentage amount of fat, and the nature of this fat is more unlike the fat of human milk than is the fat in the milk of other breeds of cows.

In comparing the average composition of human milk and cow's milk the following figures are sufficiently accurate for all practical purposes, as the variations in the composition of commercial milk are wide.

Comparison Of The Percentage Composition Of Human And Cow's Milk

The varieties of protein present in cow's milk are, like those in human milk, chiefly cascinogen and lactalhumin, but they are present in double the amount of those in human milk. The difference between the protein of the two milks is not, however, merely the amount. The bulk of the protein present in maternal milk consists of lactalhumin, whereas in cow's milk it consists of cascinogen.

Although the amount of fat in cow's milk is approximately the same as that in human milk, yet it differs from the latter in chemical composition and is not so readily digested Sugar occurs in solution, as in human milk, but in considerably less quantity, and thus, associated with an excess of protein, there is a deficiency of carbohydrate in cow's milk.

The salts in cow's milk are about three times as plentiful as in human milk; but as they occur largely in states of combination different from those found in human milk, their absorption takes place less readily.

Cow's milk, as supplied to the infant, is impure and stale. Fresh milk possesses the property of inhibiting bacterial development, and this power is considerable where the infant receives its mother's milk. Fresh cow's milk has slight power in this respect; but milk which is old, or milk which has been heated, lacks the power, while milk which has been merely frozen after milking retains it to some degree. Owing to the fact that lactic acid bacilli swarm in cow's milk, this milk when stale becomes sour; but even before a degree of sourness has been reached which is readily appreciable by the examiner, these bacilli may have caused such changes in the milk as to have rendered it very indigestible for the infant. Practically all milk as drawn from the udder of the cow contains many bacteria, and milk is a fluid in which bacteria grow readily. The presence of lactic acid bacilli tends to hinder the growth of other organisms, and in the case of an infant large numbers of bacteria are always dangerous, quite irrespective of their nature.

Tuberculosis is a very common disease among cows, and in the milk of infected animals tubercle bacilli are very frequently found. These bacilli from a bovine source are undoubtedly capable of causing tuberculosis in the human being; yet, while milk containing them is a dangerous food for the infant, a still more dangerous means of infection is through contamination of cow's milk with tubercle bacilli from a human source - e.g. sputum.

With the object of remedying or minimising the disadvantages attendant on artificial feeding, various methods of modification of the milk are commonly employed, and these may be suitably grouped under the following three headings: -

1. Modifications to counteract impurity.

2. Modifications to alter the percentage composition of the different constituents.

3. Modifications to increase or favour its digestibility.

I. Modiftcations To Counteract Impurity

The danger from impurity of milk is obviated by the application of heat, and to this end various degrees of heat are employed.

Thus we may pasteurise, i.e., heat to a temperature of 150°-160° F. for twenty or thirty minutes; or scald, i.e., heat to boiling-point; or sterilise, i.e., expose the milk to a temperature of 212° F. for forty minutes - in each case rapid cooling succeeding the application of heat.

For practical purposes, scalding is the most satisfactory procedure. The effect of pasteurisation is uncertain, and prolonged sterilisation causes marked changes in the composition of the milk, without satisfactory compensatory advantages in the matter of sterility.

The effects of the three processes may be summarised shortly as follows: -

The Results Of The Application Of Heat To Milk

Consequent on the above changes induced in the milk by the different degrees of heat, alterations occur in its digestion. With large precipitation of calcium salts, rennin curd forms slowly in the stomach, and, consequently, is a softer and more flocculent curd the higher the degree of heat to which the milk has been subjected. The flocculent curd of highly heated milk is not so fitted for gastric digestion as that of milk heated at a lower temperature, and it passes more readily onwards to the intestine.