462 FOOD [CH. XXVIIT. 



cases the specific gravity of water, with which other substances are 

 compared, is taken as 1000. 



Composition Bunge gives the following table, contrasting the 

 milk of woman, and the cow : 



Hence, in feeding infants on cow's milk, it is necessary to dilute it, 

 and add sugar to make it approximately equal to natural human 

 milk. 



The Proteids of Milk. The principal proteid in milk is called 

 caseinogen ; it is precipitable by acids such as acetic acid, and also by 

 saturation with magnesium sulphate, or half saturation with ammonium 

 sulphate, so resembling globulins ; it is coagulated by rennet to form 

 casein. Cheese consists of casein with the entangled fat. The other 

 proteid in milk is an albumin. It is present in small quantities only ; 

 it differs in some of its properties (specific rotation, coagulation 

 temperature, and solubilities) from serum-albumin ; it is called lad- 

 albumin. 



The Coagulation of Milk. Rennet is the agent usually employed 

 for this purpose : it is a ferment secreted by the stomach, especially 

 in sucking animals, and is generally obtained from the calf. 



The curd consists of the casein and entangled fat: the liquid 

 residue called whey contains the sugar, salts, and albumin of the milk. 

 There is also a small quantity of a new proteid called whey -proteid 

 which differs from caseinogen by not being convertible into casein ; 

 this is produced by the decomposition of the caseinogen molecule 

 during the process of curdling. 



The curd formed in human milk is more finely divided than that 

 in cow's milk; and it is more digestible. In feeding children and 

 invalids on cow's milk, the lumpy condition of the curd may be ob- 

 viated by the addition of lime water or barley water to the milk. 

 There appears to be no chemical difference between the caseinogen of 

 human and that of cow's milk ; variations in the amount of calcium 

 salts, and of citric acid account for the differences described. 



The addition of rennet produces coagulation in milk, provided 

 that a sufficient amount of calcium salts is present. If the calcium 

 salts are precipitated by the addition of potassium oxalate, rennet 

 causes no formation of casein. The process of curdling in milk is a 



