94 HOW CBOPS GROW. 



Casein. Animal Casein is the peculiar albuminoid of 

 milk, in which it exists dissolved to the amount usually 

 of 3 to 6 per cent. By saturating milk with magnesium 

 sulphate the casein separates as an opaque white precipi- 

 tate. Thus obtained it is freely soluble in water. Casein 

 is also precipitated from milk by adding a little acetic or 

 other acid, but is then nearly insoluble in water, has 

 a decided acid reaction, and reddens blue litmus. The 

 spontaneous curdling of milk, after standing at or- 

 dinary temperatures for some time, appears to be directly 

 due to the lactic acid which develops from milk-sugar as 

 the milk sours. When milk is swallowed by a mamma- 

 lian animal it curdles directly, and in the making of cheese 

 the casein of milk is coagulated by the use of rennet, which 

 is an infusion of tbe membrane lining the calf's stomach. 

 Coagulated casein, though insoluble in water, dissolves 

 in very dilute acids, and also in very dilute alkalies. 



The coherent cheese curd which is separated from milk 

 by rennet is doubtless a decomposition-product of casein, 

 and carries with it a considerable portion of the phosphates 

 and other salts of the milk. These salts are not found in 

 the casein precipitated by acids, being kept in solution 

 by the latter, but casein appears to contain a small amount 

 of phosphorus (equivalent to 0.9 per cent phosphoric 

 oxide) in organic combination. Skim-milk cheese, when 

 new, consists mainly of coagulated casein with a little 

 fat. Cheese made from entire milk contains most of the 

 fat of the milk. 



Exp. 50. Observe the coagulation of casein when milk is treated 

 with a few drops of dilute hydrochloric acid. Test the curd with 

 nitrate of mercury. 



EXP. 51. Boil milk with a little magnesium sulphate (Epsom salts) 

 until it curdles. 



Vegetable Casein. Several distinct substances have 

 been described as vegetable caseins. Our knowledge with 

 regard to them is in many important respects very defi- 

 cient. Even their elementary composition is a matter of 

 uncertainty. 



