54 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



gravity of water, with which other substances are compared, is taken 

 as 1,000. 



Composition. Bunge gives the following table, contrasting the 

 milk of woman and cow : 



Hence, in feeding infants on cow's milk, it will be necessary to 

 dilute it, and add sugar to make it approximately equal to natural 

 human milk. 



The Proteins of Milk. The principal protein in milk is called 

 caseinogen : this is the one which is coagulated by rennet to form 

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

 protein 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 lact- 

 albumin. 



The Coagulation of Milk. Eennet is the agent usually employed 

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

 by 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 protein called whey- 

 protein, which differs from caseinogen by not being convertible into 

 casein. It 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 : hence it is more digestible. In feeding children and 

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

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



Considerable discussion has taken place as to whether the 

 caseinogen of human milk may not be a different protein from that 

 of cow's milk, especially in relation to the amount and manner of 

 combination of its phosphorus. The differences, however, appear to 

 be explicable on the hypothesis that they are due to variations in the 

 amounts of calcium salts and of citric acid which are present. 



Caseinogen itself may be precipitated by acids such as acetic acid, 



