FOODS 



53 



MILK 



Milk is often spoken of as a ' perfect food/ and it is so for infants. 

 For those who are older it is so voluminous that unpleasantly large 

 quantities of it would have to be taken in the course of the day to 

 insure the proper supply of nitrogen and carbon. Moreover, for adults 

 it is relatively too rich in protein and fat. It also contains too little 

 iron (Bunge) ; hence children weaned late become anaemic. 



The microscope reveals that it consists of two parts : a clear fluid 

 and a number of minute particles that float in it. These consist of 

 minute oil globules, varying in diameter from 0*0015 to O'OOo milli- 

 metre. 



The milk secreted during the first few days of lactation is called 

 colostrum. It contains very little caseinogen, but large quantities 



fat. (Heidenhain.) 



FIG. 10. Microscopic appearance of milk in the 

 early stage of lactation, showing colostrum 

 corpuscles (a) in addition to fat globules. (Yeo.) 



of globulin instead. 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. This is due to the presence of 

 both acid and alkaline salts ; the latter are usually in excess. Milk 

 readily turns acid or sour as the result of fermentative change, part 

 of its lactose being transformed into lactic acid (see p. 19). The 

 specific gravity of milk is usually ascertained with the hydrometer. 

 That of normal cow's milk varies from 1,028 to 1,034. When the milk 

 is skimmed the specific gravity rises, owing to the removal of the 

 light constituent, the fat, to 1,033 to 1,037. In all cases the specific 



