PAT METABOLISM 687 



OH (hydroxyl) groups, the resulting ester called triglyceride is neu- 

 tral fat. Tripalmitin has the formula : 



CH 2 -OOC-C 15 H 31 

 CH -OOC-C 1B H 31 

 CH 2 -OOC-C 15 H 31 . 



By boiling neutral fats with alkali the fatty acid radicles are split off 

 as soaps, leaving the glycerol. This process is called saponification, and 

 it may be effected in many other ways, as for example by heating with 

 steam or by the action of special enzymes called Upases, which are widely 

 distributed in plants and animals. 



The natural fats are usually a mixture of triglycerides, and their dif- 

 ferences in properties are dependent upon the relative amounts of fatty 

 acids present. The three most important in animal fats are tripalmitin, 

 tristearin and triolein. It is essential in the study of fat metabolism that 

 we should know the most important methods ~by which the proportion of 

 fatty acids present in a mixed fat is determined. These methods are as 

 follows : 



1. The melting point. Olein is liquid at C. ; palmitic acid melts at 

 62.6 C.; and stearic at 69.3 0. The solidity of animal fats depends on 

 the proportion of olein, palmitin and stearin present. Mutton fat, for ex- 

 ample, is much stiffer than pig fat because it contains less olein and more 

 stearin. The melting points of fats from different parts of the body may 

 also vary. 



2. The acid number indicates the amount of free fatty acid mixed with 

 the fat, and is determined by titrating a solution of a weighed quantity of 

 the fat in alcohol with a N/10 alcoholic solution of KOH, phenolphtha- 

 lein being used as indicator. 



3. The saponification value indicates the total amount of fatty acid 

 present, both that which is free and that combined with glycerol. It is 

 determined by heating a weighed amount of fat with an exactly known 

 amount of alcoholic KOH (determined by titration with standard acid). 

 After saponification is complete, titration of the mixture shows how much 

 alkali has been used to combine with the fatty acid. This is the saponi- 

 fication value. 



4. The ester value indicates the amount of fatty acid combined with 

 glycerol, and is obtained by subtracting the acid value from the saponi- 

 fication value. 



Besides these there are two values, known as the iodine and the Reichert- 

 Meissl values, that are of importance because they depend on certain char- 

 acteristics of the fatty-acid radicles. 



