MILK (O. Eng. meoluc; from a common Indo-European root, cf. Lat. mulgere, Gr. AjueXYetc), the fluid secreted by the mammary glands of the division of vertebrate animals called Mammalia (see MAMMARY GLAND), and primarily devised for the nourishment of their own young.
The milk of various domesticated animals is more or less used by man for food. The milk of the cow, which may be taken as typical of all others, and is indeed by far the most important and valuable of all (see DAIRY AND DAIRY FARMING), is, when newly drawn, an opaque white fluid, with a yellowish tinge, soft, bland and sweetish to the taste, and possessed of a faintly animal odour. This odour, according to Schreiner, is due to the presence of sulphuretted hydrogen, and disappears after a short exposure. The specific gravity of milk ordinarily ranges from 1-029 to 1-033, verv seldom reaching 1-035 or falling so low as 1-027. ! In chemical constitution it consists of an emulsion of fatty globules (cream) in a watery alkaline solution of casein, and a variety of sugar, peculiar to milk, called lactose. The fat (which when separated we know as butter) and the lactose constitute the carbonaceous portion of the milk regarded as food. The casein, which forms the principal constituent of cheese, and a certain proportion of albumen which is present, form the nitrogenous, while the complex saline substances and water are the mineral constituents. These various substances are present in the proportions which render milk a perfect and typical food suitable to the wants of the young of the various animals for whom it is provided by nature. The milk of animals, so far as is known, contains them, although they are present in somewhat different proportions. It is probable that the milk of ruminants possesses certain physical and physiological distinctions from that of non-ruminant animals, which will account for the virtues attributed to the milk of the ass and mare.
In addition to these constituents milk contains small proportions of the gases carbonic acid, sulphuretted hydrogen, nitrogen and oxygen, and minute quantities of other principles, the constant presence and essential conditions of which have not been determined. These consist of galactin and lactochrome, substances peculiar to milk, discovered by Winter Blyth, with certain animal principles such as leucin, pepton, kreatin, tyrosin, etc. The salts in milk consist, according to the average of numerous analyses by Fleischmann, of the following constituents:
Potash I7-34 Magnesia .... 4-07 Ferric oxide . . . 0-62 Phosphoric acid Chlorine Lime Soda 28-31 16-34 27-00 10-00 Milk thus is not to be regarded as a definite chemical compound nor even as a mixture of bodies in fixed and invariable proportions. Not only does the milk of different races and breeds of cows vary within comparatively wide limits; the milk of the same animal is subject to extensive fluctuation. The principal causes of variation in the individual are age, period of lactation, nature and amount of food, state of health, and treatment, such as frequency of milking, etc. The following table indicates the range of normal variations:
Water 90-00 to 83-65 Fat . . .-' 2-80 4-50 Casein and albumin 3-30 5-55 Sugar 3-00 ,, 5-50 Ash 0-70 0-80 The average quantity of milk yielded by cows is also highly variable, both in individuals and breeds.
Milk and Disease. Although the milk of a perfectly healthy cow may be absolutely sterile, it is difficult to obtain it in that condition. In the report of the joint committee appointed for the purpose by the county boroughs of Bradford, Hull, Leeds, Rotherham and Sheffield in 1908, the following conclusions were drawn: (l) Cows' milk freshly drawn from the udder by ordinary methods contains bacteria. They are more numerous in the first flow of the milk. (2) There is a great increase in contamination in the milk at each stage before it reaches the customer. This is due to (a) the dirty condition of the' cows' udders, (V) the imperfect cleansing of the cans and of the hands of the milkers. The committee recommend:
1 Ewe's milk is exceedingly variable, especially in its percentage of fat. The above analysis is one of nine by Dr Arthur Voelcker, in which the fat was found to range from about 2 to I2|%.
" (i) The washing of the udder and flanks with soap and water, and similar attention to the hands of the milker. (2) Efficient sterilization of all vessels by steam if possible, or by abundance of boiling water. (3) Rejection of the first draw of the milk from each teat. (4) Avoidance of any work raising dust immediately before or during milking. (5) Removal of the milk of each cow immediately from the shed. (6) Ventilation and cleanliness of the cowsheds. This provides for the reduction as far as possible of contamination during the milking process itself. As any bacteria present in the milk tend to multiply rapidly on the way to the consumer, it is mainly a question of the time which elapses before consumption. It is, therefore, further recommended (a) that the milk be rapidly cooled or chilled, as the lower the temperature the less dp the bacteria multiply, (i) that contamination during railway transit be avoided by dustproof locked milk cans.
By treating milk at a temperature of 60 C. for one hour, 70 C. for ten minutes, and 95 C. for one minute, tubercle bacilli, if present, will certainly be killed. Cholera and typhoid organisms are less resistant, and are killed more quickly than tubercle bacilli at the above temperatures. Only a single pathogenic species can withstand the short boiling to which milk is ordinarily treated in domestic management, and this is the anthrax bacillus containing spores. The danger from this source is remote, as the microbe does not form spores within the animal body. Even in the worst cases, therefore, only vegetable forms, easily destroyed by boiling, can find their way into the milk from the body of the cow.
The lactic acid bacillus, always present in unboiled milk (to which the souring of milk is due), is easily destroyed by heat; but the bacillus mesentericus, often found in it, forms spores, which are not destroyed by ordinary boiling, and germinate when the milk is kept at a moderately warm temperature, producing a brisk fermentation whereby a large volume of gas is liberated. The fundamental idea of Soxhlet's method for sterilizing milk is to boil it for forty minutes in small bottles holding just enough for one meal, and closing the same with an impervious stopper, which is only removed just before use. Milk so treated will keep at the ordinary room temperature, as the spores of the B. mesentericus do not develop below 15 C.; but if it be introduced into the alimentary canal of a child the spores will rapidly multiply, and in such cases large quantities of gas, giving rise to flatulency, will be formed, and possibly also poisonous decomposition products of albuminoid matter. To render milk sterile in the strict sense of the word it is necessary to raise it to a temperature of about 120 C. for twenty minutes. Under these conditions the lactose decomposes into dark-brown fission products, the fat loses its emulsified condition and separates out as cream which cannot be made to diffuse again even by shaking, and the albuminoids are converted into a form very difficult of digestion.
In short, there is the greatest difficulty in freeing milk on a large scale from germs without at the same time seriously prejudicing its. flavour and nutritive value. Since, then, the destruction of the hardy germs is so difficult, the greater care should be taken, by washing the udder, hands and milk vessels, to secure extreme cleanliness in the preparation of milk intended for infant consumption. Sterilization then becomes an easier task, the milk drawn under these conditions being very poor in spore-forming bacteria. It is imperative that cream destined for butter-making should be free from pathogenic organisms. The organisms of cholera, typhoid fever and tuberculosis present in butter retain their vitality for a long time. As butter is consumed in the raw state, a trustworthy preliminary treatment of the cream is in the highest degree desirable. Schuppan has shown that it is possible to produce good butter from Pasteurized or even sterilized cream, and Weigmann introduced the plan of artificially souring cream by means of pure cultures of B. acidi lactici.
Since Metchnikoff's introduction (see LONGEVITY) of the use of soured milk for dietetic purposes the lactic acid bacillus destroying pathogenic bacteria in the intestine a great impetus has been given to the multiplication of laboratory preparations containing cultures of the bacillus; and in recent years much benefit to health has, in certain cases, been derived from the discovery.
See also the articles ADULTERATION ; DAIRY AND DAIRY FARMING ; INFANCY; DIETETICS; FOOD and FOOD PRESERVATION; in the last of which the preparation of condensed milk is described.