614 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



IV. THE RELATIVE VALUE OF DIFFERENT KINDS OF MILK. 



V. SOLIDS IN A PINT OF MILK. 



Nitrogenous constituents, 23.9 grammes. 



Fatty 22.7 



Saccharine " . . . . .80.3 " 

 Salts, 4.0 



2. CASEIN AND MILK COAGULATION. Casein is a proteid body of an 

 acid reaction which is scarcely soluble in water but soluble in dilute acids 

 and alkalies. In milk its solution is rendered possible by its combina- 

 tion with soluble alkali albuminates and through the presence of calcium 

 phosphate. In amount it varies from 3 to 5 per cent., the relative pro- 

 portions of casein and albumen being from 1.87 per cent, to 4.68 per 

 cent, of the former and from 0.60 per cent, to 1.77 per cent, of the latter. 



Casein, although closely similar in composition to alkali albumen, is 

 not identical with it, but is probabty to be regarded as a combination of 

 alkali albuminate and nuclein. Casein may be obtained from milk by 

 dilution with four times its volume of water, the addition of dilute acetic 

 acid (0.1 per cent.) until a precipitate begins to appear, th.en passing a 

 current of carbonic acid gas, filtering, and washing the precipitate with 

 water, alcohol, and ether. 



Casein may also be obtained from milk by the addition of magnesium 

 sulphate to saturation. 



When freed by ether from fats after its preparation by the latter 

 process and dissolved in water, casein is a snow-white powder and in 

 solution rotates yellow light eighty degrees to the left; in dilute alkaline 

 solution, seventy-six degrees to the left; in strong alkaline solution, 

 ninety-one degrees to the left ; and in dilute hydrochloric acid, eighty- 

 seven degrees to the left. It leaves no ash on incineration. 



When milk is boiled the serum-albumen of milk becomes coagulated, 

 while the skim formed is due to the deposition of a thin pellicle of casein 

 due to evaporation, and which is renewed as fast as it is removed. 



The coagulation of milk depends upon the precipitation of casein. 

 Everything, therefore, which causes casein to become insoluble causes 

 coagulation of milk. Such agents are acids, rennet, tannin, alcohol, and 

 mineral salts. 



