The composition of milk varies within wide limits, so that it is impossible to gauge within 20 per cent, the amount of nutritive materials contained in a given quantity. The main variations are in the amount of fat and protein present. The composition, quality, and quantity of are influenced by a variety of circumstances: -

(1) The influence of breed. Alderneys give a milk rich in fat; Longhorns a milk proportionately rich in casein.

(2) The milk with a first calf contains less water, and is richer, but the quantity yielded is less than in subsequent pregnancies.

(3) The oftener the mammae are emptied the richer the milk becomes in casein, and the milk withdrawn at tie end of milking is always richer in butter than at the beginning. The evening milk contains a larger proportion of butter than the morning.

(4) The nature and quality of the food exercise considerable influence over the amount and quality of the milk. Fresh green pastures produce the sweetest flavoured and best milk. beetroot and carrots increase the amount of sugar. Turnips, brewers' grains, and oilcake impart a peculiar flavour and deepen the colour. The colour, the odour, and even poisonous properties may pass from the vegetables or plants consumed into the milk secreted.

(5) The degree to which the milk has been adulterated by the addition of water, sugar, or of preservatives such as formalin, salicylic acid, and the like.

The specific gravity of milk is from 1026 to 1035. This is, however, of no value as a test of its quality, because skimming milk raises the specific gravity, since fat is lighter than water. Hence, milk can be skimmed and water added to keep the specific gravity the same. An estimation of the fat is ential for determining the nutritive value of milk. Good milk should contain 4 per cent, of fat.

The proteins of milk consist of casein and a small amount of lactalbumin.

Casein is the principal protein, and is kept in a state of solution by the phosphate of lime present in milk. This protein is distinctive in containing no nuclein, and seems to yield no uric acid when split up. Casein further differs from other proteins in not being coagulated by heat; it is readily coagulated by acids and also by rennet, a ferment secreted by the stomach. From casein a substance resembling flour has been prepared, and is in the market under the name protene. Another substance - Nutrose - is casein in combination with a soda salt. Plasmon is a similar preparation. These preparations of casein are used to increase the nutritive value of gruels, soups, teas, and puddings. They are further referred to on p. 158.

Lactalbumin is present only in very small quantity in milk; it coagulates slowly when boiled, and forms the "skin" on boiled milk.

The carbohydrate of milk is in the form of lactose or milk sugar. This is less soluble than ordinary sugar, from which it also differs in having no sweet taste. Unlike glucose, it is not readily fermented by yeasts, but it is readily split up by certain micro-organisms, with the production of lactic acid, a process which occurs in the souring of milk. Many cases of diarrhcea in infancy are due to the irritation of lactic acid produced in this manner. The absence of carbohydrate in milk makes it a specially useful food in cases of severe diabetes.

The fat of milk is in the form of an exceedingly fine emulsion, being suspended in the plasma of milk in the form of globules, which impart to milk its white, opaque appearance. The proportion of fat should not be less than 3 per cent. On standing, the fat globules run together and float on the surface in the form of cream. This finely divided form of fat is one of the most easily digestible fats. When the cream is removed skimmed milk is left; this, however, may still contain 1/2 to 1 per cent, of fat. If the cream be removed by a separator, the fat is much more completely abstracted, and the milk so prepared is known as separated milk.

The mineral matters are of great importance, the most important salts being the phosphate of lime and phosphate of potassium. The one important mineral constituent that milk lacks is iron, and this has to be borne in mind when giving a prolonged course of strict milk diet to adults. The deficiency of iron in milk is of less importance in the feeding of infants because of the large reserve amount of iron normally stored in the foetal liver.

Citric acid is present in milk, chiefly combined with lime as calcium citrate. This is only sparingly soluble. It is stated that a good cow yields as much citric acid in a day as would be found in two or three lemons. It has been suggested that the citric acid is the antiscorbutic element of milk.

The remaining ingredient is water. This keeps the solid constituents in solution, but makes milk a dilute and bulky food.

As was shown by table on p. 29, the relative proportions of the different constituents of milk vary considerably in different animals.

Goat's milk is rich in fat and protein. It is poor in sugar, and is not much liked on account of its peculiar smell, due to the characteristic acid. Still records the case of an infant who was fed on goat's milk for several months; the goat cost fourteen pence a week to keep, and yielded at least two to three pints of milk twice a day. Goat's milk requires dilution in the same way as cow's milk.

Mare's milk resembles human milk very closely, but is rather poorer in fat and proteins. Like human milk, it is richer in sugar than the other varieties. The richness in sugar is taken advantage of to prepare koumiss by alcoholic fermentation.

Ass's milk is the poorest in solids, but it is a sweet milk. The deficiency of fat and casein makes it easily digestible for infants and invalids who are unable to digest cow's milk. The small amount of fat present makes it unsuitable for continued use.

Cow's milk is remarkable for its large proportion of protein and fat, and the comparatively small amount of sugar.