MILK PROTEIDS. 311 



With rennet the effect is very different ; the caseinogen is 

 changed into two proteids, one of which only is readily 

 coagulated, the other with difficulty. The former is at once 

 coagulated by the calcium salts (mainly phosphate) present in 

 cows' milk, and forms, with the entangled fat, the curd ; the 

 latter goes into the whey and can be coagulated by heating to 

 95 or 100. The curdling of milk by rennet is thus dependent 

 upon the presence of calcium phosphate in the milk. Ham- 

 marsten has proved that in the absence of calcium phosphate 

 or other salts of the alkaline earths rennet will not curdle milk. 



Under ordinary circumstances rennet acts best at about 35 

 and is killed or destroyed at 70. 



The albumin of milk closely resembles serum albumen of 

 blood. It is in complete solution in milk but is coagulated at 

 72, or by saturation with sodium sulphate at 30, or "ammo- 

 nium sulphate at ordinary temperatures, but not by magnesium 

 sulphate at 40. It is also precipitated by copper, mercury, or 

 lead salts, by tannin, and by alcohol. 



Its composition, according to Sebelien,* is 



% 

 Carbon - ... ... 52-19 



Hydrogen ... ... 7*18 



Oxygen ... ... 22-90 to 23-13 



Nitrogen ... ... 15'77 



Sulphur ... ... 1-73 to 1-96 



It differs in composition from casein in containing no phos- 

 phorus, more carbon, and more than twice as much sulphur. 



Many other proteids have been described as occurring in 

 cows' milk, but some doubt as to whether they are not formed 

 by the action of the reagents employed upon the casein or 

 albumen has been expressed. As an illustration of the com- 

 plexity of the method adopted for the separation and prepara- 

 tion of some of the proteids, the following account of two 

 proteids recently analysed and described by Storch may be 

 given.! Skimmed milk was mixed with three times its volume 

 of saturated solution of sodiu,m sulphate and a few drops of 

 egg albumen and heated to 100. The coagulated casein was 



* Jour. Soc. Chem. Intl. 1886, 387. 

 t Jour. Chem. Soc., abst. ii. 1897, 420; 1900, abst. i. 266. 



