CHAPTER III 

 THE CHEMISTRY OF NEUTRAL FATS 



1. Introduction 



Although the chemistry and the metabohsm of fats are usually con- 

 sidered to be identical with those of their component fatty acids, there are 

 many reasons why they should be discussed separately. Actually, fatty 

 acids are of minor importance per se in the naturally occurring fats and oils. 

 In fats prepared from animal or vegetable sources, free fatty acids comprise 

 only a fractional part of the total present as the triglyceride. The free 

 acids may be found in appreciable amounts only in rancid fats or in freshly 

 prepared oils from sources like the olive, where a high concentration of 

 lipase is present in the pulp from which the oil is processed. On the other 

 hand, the fat which is found in the storage organs of animals is largely 

 neutral fat or is in the form of the corresponding phospholipids. The ques- 

 tion has been raised by Fraser^ whether or not the hydrolysis of the fat to 

 fatty acid (or soap) and glycerol is a necessary prerequisite to absorption of 

 fat from the gastrointestinal tract, as has been generally accepted for many 

 years. 



It has long been recognized that certain properties of neutral fats are 

 quite distinct from those of the component fatty acids. In the case of the 

 simple triglycerides, the melting points and solubilities are quite comparable 

 to those of the component fatty acids. On the other hand, where mixed 

 triglycerides occur, i.e., those in which several varieties of fatty acids are 

 combined with a single glycerol molecule, the melting point of the resulting 

 compound may vary greatly from that of a mixture of the component fatty 

 'acids. In addition, the melting point may show marked variation accord- 

 ing to the relative positions of the component fatty acid residues. 



Isomerism is considerably more complex in the neutral fat than in the 

 single fatty acid. In the case of the triglyceride one is concerned not 

 only with such positional isomerism of the fatty acids as the location of the 

 unsaturated linkage and such types of stereoisomerism as cis-trans ar- 

 rangement, but also with optical isomerism resulting from unsymmetrical 

 arrangements of the fatty acids, as well as with other variations which 

 occur when dissimilar fatty acid residues occupy the different positions in 

 glycerol. 



The most marked difference in properties between the triglycerides and 



1 A. C. Fraser, Physiol. Revs., 26, 103-119 (1946). 



165 



