82 GENERAL BIOCHEMISTRY 



some vegetable oils are solid at this temperature while some animal 

 fats are semisolid or even liquid at 2()°C. Moreover such a classifica- 

 tion overlooks entirely the marine and bacterial lipides which, depend- 

 ing upon their origin, may be solid or liquid at 20°C. 



Fats are lighter than water and usually have a density between 

 0.8 and 0.9 g./cm.^ The fact that they are poor conductors of heat 

 makes them particularly usefid as body insulators. When pure, fats 

 are colorless, virtually odorless, and possess an extremely bland taste. 

 They are capable of easily absorbing a variety of odors and hence 

 flavors during storage. For example, the young housewife soon dis- 

 covers that the flavor of an onion will quickly permeate butter that 

 is stored with the tuber in a refrigerator. In some instances this char- 

 acteristic absorption of odor and flavor is used to advantage. For 

 example, the delicate perfumes of some flowers can be isolated by 

 placing the petals in contact with fat for a period of time, then extract- 

 ing the fat with cold alcohol and concentrating the essence. 



Structure and Chemical Properties 



Fats are complex mixtures of glycerides. The degree of complexity 

 of the mixture depends on the number of diff^erent fatty acids and the 

 amoimt of each acid available for the formation of glycerides. In 

 addition, the difference in the nature of the fats produced by different 

 organs of a plant or animal suggests that such organs enjoy a prefer- 

 ence for certain fatty acids and hence for certain glyceride mixtures. 



The principal difference between animal and vegetable fats lies 

 in the amotuit and type of unsaturated fatty acids in the glycerides. 

 Both animal and vegetable fats contain a considerable amount of 

 oleic acid, but only vegetable fats can be considered rich in the poly- 

 unsaturated acids, linolic and linolenic. The presence of appreciable 

 quantities of these unsaturated acids in the glycerides of vegetable 

 fats accounts for their liquidity at 20°C. On the other hand, the 

 predominance of saturated glycerides in animal fats explains their 

 solid state at room temperature. 



Although a number of saturated fatty acids may be isolated from 

 vegetable fats, palmitic acid predominates. In animal fats a larger 

 variety of saturated acids may be found although stearic acid proves 

 to be the most abundant. In any case, it is easy to understand that 

 variations in the number of fatty acids, in the type of fatty acids, and 

 in the amounts of each acid will result in the formation of structurally 

 different fats, each possessing its own characteristic physical and 

 chemical properties. 



Since fats are organic esters, they are easily hydrolyzed. Hydrolysis 



