THE PBODUCTS OF THE BAIBY. 183 



stearin, olein, and butyrin. These may be classified as 

 fixed, that is, those which remain on heating, and as 

 volatile, those which may be driven off by heat : fixed 

 fats are also of two kinds, solid and liquid. 



The volatile fats affect the flavor of dairy products 

 more than the fixed, and it is the proportion of liquid 

 fat (olein) which affects the solidity of butter. The 

 liquid fats increase with succulent foods, and the solid 

 with dry foods. 



Albumen and Casein. — These two substances con- 

 stitute the chief albuminoids of milk; and while they 

 resemble each other in composition, they possess different 

 properties. The albumen, which is contained in small 

 amounts not usually exceeding one-half per cent, is co- 

 agulated by heat and not by acids, while casein is coagu- 

 lated by acids and not by heat. This property of casein 

 is very important in the manufacture of cheese. 



Milk Sugar, called by chemists "lactose,'^ possesses 

 practically the same food value as other sugars. It dif- 

 fers from cane sugar in appearance and in its properties. 

 When it is crystallized, it is very hard, and it does not 

 possess as high a sweetening power. 



Ash, or Mineral Salts, consists of phosphates of lime, 

 magnesia, and iron, and chlorides and sulphates of soda 

 and potash. 



Average Composition of Milk. — Milk is not a pro- 

 duct of fixed composition. Both the total amount and 

 the proportion of the constituents are influenced by a 

 variety of conditions, the chief of which are : breed of 

 the animal ; her age, health, and individuality ; the method 

 of feeding and kind of food ; period of lactation, and time 



