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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