Milk

Milk is an opaque white fluid secreted by the mammary glands of the females of the class Mammalia, after they have brought forth their young, and during the period in which their offspring are too immature to live upon ordinary food. It is devoid of odour, except for a short time after its extraction; is of a slightly sweet taste, most commonly of a slightly alkaline reaction (except in the Carnivora, in which it is acid); and its average specific gravity (in the case of human milk) is 1032.

When examined under the microscope milk is found to consist of numberless transparent globules, of very minute size, floating in a clear colourless fluid, the milk plasma. These globules are composed of fat, and they are each enclosed by a thin envelope of an albuminous material termed casein. When milk has stood for some time, the larger globules rise to the surface and form a layer of cream, which is therefore rich in fat and poor in other nutritive substances (presently to be described) that are found in the milk plasma. When the cream is removed skimmed milk remains. If this, or better still the unskimmed milk, be agitated in a churn, the envelopes which surround the fat globules are broken, the fat runs together, and we have milk fat or butter. The albuminous casein, which according to some encloses the fat globules, but according to other observers exists in solution in the plasma, is an albuminous substance combined with calcium phosphate. This calcium phosphate is necessary for its solution, and if its union with the albumen be interfered with, as by the addition of an acid, the casein separates out in microscopic filaments which interlace, enclosing the milk globules, and forming a more or less solid clot. If milk be allowed to remain in an open vessel and in warm weather, a few hours will produce this result; the casein clots in little masses, and we say 'the milk has turned.' It is acid or sour to the taste, and contains micro-organisms (bacterium lacticum), by whose agency these changes are brought about. These little microbes have the power of converting the milk sugar into lactic acid, which in its turn coagulates the casein. These microbes do not exist in milk freshly passed from the mammary glands; they must find their way into the milk, where they rapidly multiply; and, as their germs are very freely distributed, this occurs sooner or later. The dairy-keeper, by efficient ventilation and scrupulous cleanliness, endeavours to keep his dairy and his milk as free from these organisms as possible, and the careful nurse scalds out the infant's bottle in order that they may not multiply, as they will readily do, in any stale milk, rapidly infecting the fresh milk each time the bottle is used. The casein is not only clotted by acids, but a secretion of the stomach called rennet has a similar action. A teaspoonful of a commercial infusion of rennet will cause half a pint of milk, at a summer temperature, to form a beautiful white clot, which subsequently contracts, expressing the whey. This occurs in the stomach when we drink milk, and this is one reason why milk may disagree: in order to render it more digestible it may be 'sipped' or it may be taken with lime-water, for in this way the formation of large clots within the stomach may be avoided. The curds which form after the addition of rennet can be made into cheese. In cheese, therefore, we have a rich supply of nitrogenous matter (casein) together with fatty matter derived from the milk globules held fast in the curd.

Milk contains a sugar—milk sugar—in solution, and in addition a rather large proportion of inorganic salts. It contains all that a child requires for the growth and nourishment of its body, and is manufactured at great expenditure of the mother's strength. The first milk that flows from the breast at the beginning of a lactation period is termed the colostrum, and is rich in fat but poor in casein. After a few days, during which time the child feeds chiefly on its own tissues and loses weight, the secretion becomes thoroughly established. In a healthy, well-fed woman this continues for some months, after which time the drain upon the energy of the mother's body renders the milk poorer and less nutritious. The milk contains the salts, chiefly of lime, from which the infant builds its skeleton. Where the children are ill nourished and rickety, doctors often recommend the dilution of the milk with lime-water, ignorant of the fact that milk contains a considerably larger quantity of lime than the lime-water itself. The lime-water diminishes acidity, and renders the milk digestible, but hardly adds lime; rickets is generally due not to lack of lime-salts in the food, but to want in the child's system of the power to assimilate them. It is well known that many medicines taken by the mother are excreted in the milk, and this point must be borne in mind by mothers suckling their infants; much nonsense is, however, believed in regarding the fatal and sudden injury done to children as a result of severe mental excitement on the part of the wet-nurse. Owing to inability of the mother either to nourish her offspring herself or to provide it with a wet-nurse, it may be necessary to bring it up on the milk of an animal; see INFANT (FEEDING OF), BREASTS.

The following table, which is based on researches of Vernois and Becquerel, show the density and composition of 1000 parts of milk:

It is preferable to use the milk of one animal and not the mixed milk of a dairy, as in that way we minimise the chances of infection. It should be diluted with about one-third of water, and perhaps a pinch of sugar added. It then forms a good substitute for the mother's milk. But disease is very frequently transmitted by milk, not only from using contaminated water for washing the milk cans and for adulterating it, but also from the cow itself (see TYPHOID FEVER, SCARLATINA). It is not improbable that many obscure tubercular conditions are thus acquired by children.

Condensed milk is generally prepared from that of the cow, sweetened by the addition of ordinary cane sugar, and evaporated to about $\backslash frac\{1\}\{4\}$th of its bulk. While hot it is poured into tins and sealed up. When used for food the milk may be diluted with six or seven times its volume of water, but in the case of infants the dilution must be more liberal. During the first month it may be diluted with twelve or fourteen volumes, and later on with ten volumes of water. It is often found to agree with children better than cow's milk, but it is a fatal mistake to rear an infant on condensed milk entirely, as the diet will suffer from too great uniformity. It is a safe rule when a child has to be brought up on animal milk, and when the household milk does not agree, to change the dairy. If this does not succeed, the ordinary cow's milk may be tried, say for the morning or afternoon feeding, with condensed milk at night. In all cases it is better to err in freely diluting milk, for nothing is so apt to disagree with a child as a surfeit of rich milk; dilution will do no particular harm.

Milk is frequently adulterated, chiefly with water (see ADULTERATION). In this case a given volume of milk will contain an abnormally small number of milk globules. As these milk globules are the cause of the opacity of milk, a thin layer of the adulterated milk will be less opaque than a similar layer of unadulterated milk. Many forms of lactoscopes have been invented for testing the opacity and consequent dilution of milk (see LACTOMETER). Unskimmed milk should yield in standing 12 to 24 parts per cent. of cream, and its specific gravity should be 1.028 to 1.034. Skimmed milk is heavier—1.032 to 1.040 (see also DAIRY).—Milkweed is a local name for the genus Asclepias; for Milk-tree, see COW-TREE.