Page:The New International Encyclopædia 1st ed. v. 13.djvu/553

* MILK. 499 MILK. seventeentli century fat, casein, and whey were the only constituents recognized. In the early part of the eighteenth century Leeuwenhoeck discovered the microscopical characteristics of milk, and about the same time Boerhaave made a qualitative examination. The lirst quantitative analysis of milk recorded was made in 1737 by Geolt'rey, who determined with considerable accu- racy the casein, milk sugar, and mineral matter. Woman's milk contains 87. 4 per cent, of water and 12. ti per cent, of solids, the latter including 3.8 per cent, of fat, 1.0 per cent, of casein, 1.3 per cent, of albumin, G.2 per cent, of milk sugar, and 0.3 per cent, of ash (mineral matter left after burning). The milk from different animals varies considerably in composition, as shown in the following table: minus tile water are designated total solids. The projjortion of total solids is a general indication of the richness of the milk, .mong the solids the chief importance attaches to the fat. First, it is the measure of the value of milk for butter-mak- ing, and to a very great extent for cheese-making also; second, it is the constituent which more than any other gives milk its appearance of rich- ness; third, it fluctuates more widely than any other constituent. .Milk fat is a familiar sub- stance in the form of butter, which containa about 85 per cent, of fat, some water, salt, and casein. The fat in milk is in the form of minute globules held in suspension, and on standing it rises to the surface as cream (q.v.), which eon- tains some of the other constituents in smaller proportion. The globules may be readily seen by Composition of Milk of Different Animals KIND OP ANIMAL Cow Buffalo.., Goat Ewe Llama.... Mare Aes Camel Sow Elephant. Porijoise. Dog Cat Water Per cent. 87.2 81.4 85.7 80.8 80.5 91.5 89.6 86.6 84.0 67.9 41.1 75.4 82.1 Total solids Per cent. 12.8 18.6 14.3 19.2 13.5 8.5 10.4 13.4 16.0 32.1 58.9 24.6 17.9 In total solids Fat Per cent. 3.7 7.5 4 8 6.9 3.2 1.2 1.6 3.1 4.0 19.6 45.8 9.6 3.3 Casein Per cent. 3.0 6.8 3.2 5.0 3.0 1.2 ,7 4,6 7.2 3.1 11.2 6.1 3.1 Albumin Milk sugar Per cent. .5 .3 1.1 1.5 ,9 .1 1,6 4.0 7.2 3.1 11.2 5.1 6.0 Per cent. 4.9 4.1 4.4 4.9 5.6 5.7 6.0 5.6 3.1 8.8 1.3 3.1 4.9 Ash Per cent. ,7 ,9 .3 .5 .7 1.1 .6 .6 ,7 .6 The above are general averages of a greater or less number of analyses, depending upon the kind of milk animal, each kind of which gives milk that varies more or less with the individual as well as with the species. There is, further- more, some difficulty in arriving at a general average for any kind of mammal, since normal milk is obtained with difficulty from animals unaccustomed to being milked. Properties of Cows' Milk. Since, in general properties and composition, cows' milk is typical of all milks, is of chief commercial interest, and has been studied much more in detail, it will be the main subject of this article. The milk se- creted immediately after parturition is termed culnxiniiti (q.v.), or berintinfix. and difl'ers consid- erahly from normal milk in linth physical and chemical characters. Milk from which the fat has been removed by skimming or by the separator is called skim milk (q.v.), and the residue left after churning cream is known as buttermilk (q.v.). Whey (q.v.) is the liquid remaining after the curd of the milk has been separated. Cows' milk is an opaque, whitish liquid, some- times faintly yellow or bluish, with a slight alka- line reaction and a sweetish taste wlien fresh. It is heavier than water, its specific gravity ranging usually from 1.029 to 1.035, the average for mixed milk being about 1.032, By removing the fat (skimming the cream) the specific grav- ity is raised, and by adding water it is lowered. This is the basis of a simple, but (when taken alone) unreliable, test of the quality of milk and of the practice of skimming. Chemically, milk consists of an aqueous solution of milk sugar, casein, albumin, and ash, with the fat in suspen- sion, forming an ennilsion. The water and the constituents dissolved in it constitute the milk serum, and the constituents (i.e. the dry matter) means of a microscope. For a long time fat glob- ules were believed to be surrounded by a njem- brane or proteid coating, which was destroyed by churning the cream, and thus allowed the fat to unite into a solid mass. This view is still held by some, but the prevailing belief at present is that the globules are free and owe their spherical form to the surface tension. The glob- ules vary greatly in size, being from -j-j'jt; to 155(51; of 'ii 'i''h '1 diameter., pint of average milk has been estimated to contain not far from a million globules. The size of the globules varies with the period of lactation, diminishing toward the close, and to some extent with the breed and the individual. The globules in the Jersey and Guernsey milk are relatively large; in Holstein milk very small. The large globules rise more rapidly, and milk containing them creams more readily and completely. Milk fat is a pale yellow substance consisting of a mixture of the glycerides of 8 or more fatty acids. Of the.se olein constitutes about 35 per cent., palmitin 25.7, myristin 20, laurin 7.5,' butyrin 3.85. caprin 3.0 per cent., and the re- mainder is principally caprylin and stearin. By the action of caustic alkali these glycerides are broken up into their respective fatty acids and glycerin, and a certain relatively small ]u-opor- tion of these, including the butyric, caproic, and caprvlic acids, are volatile. The characteristic flavor and aroma of milk and butter are due largely to butyrin, and this decomposes readily, forming butyric acid, which is evident in rancid or 'frowy' butter. The chief nitrogenous or albuminoid constituent of milk is casein, which is of prime importance in cheese-making. It is coagulated by rennet and by acids, and this is what gives sour milk its thick curdled appear- ance. The acid developed in souring precipitates