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per cent., with an average of about 5 per cent., of milk-sugar. 

 This average may be reduced to about 4 per cent, in sour 

 milk. Sour milk is a result of the action of bacteria upon the 

 milk-sugar. The bacteria decompose the milk-sugar and one 

 of the products of this decomposition is lactic acid, which 

 curdles the milk or precipitates the casein. If milk is kept 

 free of bacteria, or if all the bacteria in milk are destroyed by 

 sterilization and the milk is thereafter kept free from germs,, 

 it will remain sweet indefinitely. 



Casein and albumen are the chief protein compounds of 

 milk. When the milk is first drawn from the udder the 

 casein is in the form of caseinogen, but it is soon changed 

 into casein. Casein contains phosphorus and sulphur, which 

 chemical elements are not found in any of the other protein 

 compounds of milk. Dilute acids precipitate the casein and 

 thus curdle the milk. If the acid is neutralized by some 

 alkali (lime water or soda) the casein will be redissolved. 

 Rennet will also precipitate casein, and the curd thus formed 

 is used in making cheese; but this curd cannot be redis- 

 solved by adding lime water or soda. The quantity of casein 

 in cow's milk will vary from 2 to B.5 per cent. 



The alounien of milk is somewhat similar to that in blood 

 and in the white of an egg. It is not precipitated by dilute 

 acids or by rennet, but it can be coagulated by heating the 

 milk to 170 degrees F.; it then collects in a film on the surface 

 of the milk. The qiintity of albumen in cows' milk will range 

 from .5 to .8 per cent. 



Tnere are other unstable, and somewhat indefinite, protein 

 compounds in milk, but they are small in quantity and the 

 chemists do not agree as to their properties. 



The average amaunt of total protein constituents in milk is 

 3.3 per cent, of the entire milk. ''Milk with a low fat content 

 will contain more casein and albumen than fat, while the 

 reverse is generally true in case of milk containing more than 

 3.5 per cent, of fat." 



The ash or mineral matter is made up of "chlorides and 

 phosphates of [sodium, potassium, magnesium and calcium ; 

 iron oxide, and sulphuric and citric acid are also present in 



