482 



FOOD 



[CH. XXX. 



coagulates- like white of egg when boiled. Microscopically, cells 

 from the acini of the mammary gland are seen, which contain fat 

 globules in their interior ; they are called colostrum corpuscles. 



Reaction and Specific Gravity. The reaction of fresh cow's 

 milk and of human milk is amphoteric ; that is, it turns blue litmus 

 red, and red litmus blue. This is due to the presence of both acid and 

 alkaline salts. All milk readily turns acid or sour as the result of 

 fermentative change, part of its lactose being transformed into lactic 

 acid. The specific gravity of milk is usually ascertained with the 

 hydrometer. That of normal cow's milk varies from 1028 to 1034. 

 When the milk is skimmed the specific gravity rises, owing to the 

 removal of the light constituent, the fat, to 1033 to 1037. In all 

 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 and a little cream to make it approximately equal to 

 natural human milk. 



The Proteins of Milk. The principal protein 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 

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

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

 temperature, etc.) from serum-albumin ; it is called lact-albumin. 



The Coagulation of Milk. Kennet is the agent usually employed 

 for this purpose : it is a enzyme 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 protein called whey -protein, 

 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. 



