PROPERTIES or Cow's' ,11n.s. 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 colostrum (q.v.), or beastings, and differs consid erably from normal milk iu both physical and ehemical 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 when fresh. It is heavier than water, its speeifie 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 simple. hut (when taken alone) mu•eliable, test of the quality of milk and of the practice of skimming. Chemically, milk consists of an aqueous solution of milk sugar. easein, albumin, and ash, with the fat in suspen sion, forming an emulsion. 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 mem 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 -f556 to of an inch in diameter. A 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 Bolstein 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 eonsisting of a mixture of the glycerides of 8 or more fatty acids. Of these olein constitutes about 35 per cent., palmitin 25.7. myristin 20, laurin butyrin 3.55. ea pr in 3.6 per cent., and the re mainder is principally caprylin aml stearin. By the action of caustic alkali these glycerides are broken up into their respective fatty acids and glycerin, and a certain relatively small propor tion of these, including the butyric. caproie. and caprylie 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' hotter. The chief nitrogenous or alhuminoid 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 the casein, which gradually separates from the soluble constituents, imc•losing much of the fat. Casein is held in solution in milk by the presence of lime salts, and lime water causes it when curdled to separate in a nitwit more finely di vided eondithin. The albumin of milk is curdled by acids or rennet, but is acted upon by heat. 'Fibrin, similar to that of blood, globu lin, nuclein, and several other nitrogenous bodies. have been found in milk in small quantities, but are of little importance. The sugar in milk, chemically known as lactose, is not so sweet as cane sugar. It is in solution. The ash of Milk consists of a mixture of a num ber of salts, but is composed principally of the phosphates of lime and potash, the chlorides of potash and soda, and small amounts of phosphate of iron and magnesia. Some of the phosphate of lime appears to lie associated with the casein, which also contains some sulphur. Alost of the salts are in solution. Besides the constituents named, milk contains normally a coloring mat ter, a trace of citric acid, urea, and several other bodies. The fat and the albumin of milk are the most variable eonstituents, the ash and the sugar the least so. The casein bears a quite constant ratio to the fat. rising and falling with it.