136 CHEMISTRY OF PLANT LIFE 



alcohols. They form acetyl esters, or acetates, which can be 

 separated from each other and identified by their crystal forms 

 and melting points. Because of this fact and of the further fact 

 that they are present in detectable quantities in practically all 

 fats and oils, they afford a qualitative means of distinguishing 

 between fats of animal and of plant origin. This possibility is 

 the most interesting fact known concerning these complex alco- 

 hols; although their presence as esters in all plant and animal 

 fats indicates that they must have some biological function. 



Phytosterol is not a single alcohol, but a mixture of at least two, 

 which have been separated and studied as sitosterol, C27H4sOH, 

 and stigmasterol, CsoH^gOH. As has been said, these are found 

 in small proportions in all vegetable fats, being present in largest 

 amounts in oily seeds, especially those of the legumes. 



The saponification of esters of cholesterol and phytosterol is a 

 difficult and unsatisfactory process; but since this affords the only 

 known means to distinguish between fats of plants and of animal 

 origin, its technique has been fairly well worked out, and the 

 process used in the study of the changes which take place in plant 

 fats when they are used by animals as food. 



HYDROLYSIS AND SYNTHESIS OF FATS 



The reaction for the hydrolysis of fats has been discussed in 

 connection with the process for the manufacture of glycerine. 

 This reaction takes place very slowly with cold water alone, can 

 be easily brought about by the action of superheated steam, 

 and much more easily and rapidly in the presence of some catalyst 

 (sulfuric acid is an especially effective catalyst for this purpose). 



Fats can be artificially synthetized by heating mixtures of 

 glycerol and fatty acids, under considerable pressure, for some time 

 at temperatures of 200 to 240 C.; or by heating a mixture of the 

 disulfuric ester of glycerol with a fatty acid dissolved in sulfuric 

 acid. Recently, fatty acids have been prepared from carbohy- 

 drates, by first breaking the hexoses down into three-carbon 

 compounds, then carefully oxidizing these to pyruvic acid, 

 CHs-CO-COOH, which can then be condensed into acids having 

 longer chains. The violent reagents and long-continued processes 

 which must be employed for the artificial hydrolysis or synthesis 

 of the fats are in sharp contrast with the easy and rapid transition 



