LIPIDES (fats and RELATED SUBSTANCES) 



75 



a high fat content, while those of the temperate zone are usually high 

 in starch. This is only a general rule since there are many exceptions; 

 rice, which is a tropical product, contains little fat, and soybean, which 

 contains much fat, grows in temperate regions. The fats of commerce 

 are removed from the material in which they occur either by mechanical 

 means or by extraction with suitable solvents. The mechanical methods 

 arc more generally used. They may vary widely — churning in the making 

 of butter, heating and filtering in the rendering of lard and tallow, 

 pressing, with or without the aid of heat, in obtaining olive, cottonseed, 

 and other oils. 



Elementary composition 



The fats are comparatively high in carbon and low in oxygen. This 

 is in sharp contrast to the carbohydrates, which contain a high percentage 

 of oxygen. A comparison of the elementary composition of fat, starch, 

 and protein is as follows: 



From the above table it is evident that fats have a considerably higher 

 heat value than either proteins or carbohydrates. On oxidation in the 

 body, fats give two and one-fourth times as much heat as the other 

 foodstuffs. This is because of the higher content of carbon and hydrogen 

 in fats. 



Lipides such as lecithin and cephalin, closely related to the true fats, 

 contain, in addition, the elements phosphorus and nitrogen. As a class, 

 however, lipides (and carbohydrates) do not contain nitrogen and thus 

 are sharply differentiated from proteins. 



Products on hydrolysis 



Some idea regarding the nature of a fat may be obtained by breaking 

 down the fat into its constituent parts — that is, by hydrolyzing it. When- 

 ever a natural fat is hydrolyzed by acids or enzymes, glycerol and a 

 number of fatty acids are obtained. The fatty acids are divided into 

 two series, the saturated and the unsaturated. A study of these acids 

 is indispensable to a proper understanding of the fats themselves. 



