CHAPTER III 



MILK 



The fluid known as "milk" consists of water and 

 certain solids. The latter are in solution, in suspension, 

 and in emulsion. In order to comprehend the various 

 changes which may occur in milk and to understand the 

 different methods for its examination, it is necessary to 

 have some knowledge of its physical and chemical prop- 

 erties, its microscopic appearance, the ferments or 

 enzymes it contains, and the bacteria with which it may 

 be contaminated. These points will therefore be given 

 consideration. 



CHEMICAL PROPERTIES 



Constituents. The principal chemical constituents are 

 casein, lactalbumin, lactoglobulin, fat, lactose, mineral 

 salts, and water. 



Casein is a nucleo-albumin and therefore contains 

 phosphorus. It is insoluble in water when free and un- 

 combined. But in milk it is combined with calcium in 

 the form of dicalcium caseinate. This compound, which 

 is neutral to litmus and acid to phenolphthalein, is re- 

 sponsible for the white color of milk, and in part for its 

 opacity. It is not in solution in milk, but in suspension. 

 When milk is exposed to a low temperature the calcium 

 caseinate forms flakes, which, when the temperature is 

 sufficiently low, are visible to the eye; it is also more 

 readily precipitated. This must be kept in mind when 

 the alcohol test is used (page 285). 



If an acid is added to milk the casein is precipitated. 

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