274 ESSENTIAL FATTY ACIDS 



of preparation of shortenings from liquid vegetable oils, the resultant prod- 

 ucts contain essential fatty acids in quantities in the same order as that 

 found in other edible fats.^" 



The essential fatty acids, which contain only cis double bonds, can be 

 catalytically converted by selenium isomerization to trans forms. The iso- 

 merization has been accomplished on both linoleic-' and linolenic^^ acids. 

 The isomers thus foiTned have no biological value, although they are 

 metabolized by the animal. ^^ 



Treatment of the essential acids with alkali at high temperatures causes 

 the double bonds to shift, yielding conjugated isomers. Because such con- 

 jugated polyenes possess strong ultraviolet absorption, this reaction has 

 been made the basis of a spectrophotometric method for the analysis of 

 the essential acids. The conjugated acids themselves have no biological 

 potency. 



The saponification and esterification reactions are the only important 

 carboxyl reactions of the essential fatty acids. However, the differences in 

 saponification or esterification rates of the different long-chain acids are 

 so small as to be of no practical importance. For discussion of these re- 

 actions, the reader is referred to Markley.^^ 



E. OXIDATION 



The polyene acids are highly susceptible to oxidative rancidity. After a 

 variable induction period, the oxygen absorption increases autocatalyti- 

 cally. The maximum rates of oxidation of the various pol5ainsaturated acids 

 increase with each additional double bond in the molecule, so that with 

 arachidonic acid autoxidation is very rapid. In Fig. 2 the rates of oxidation 

 of the esters of essential fatty acids are compared with ethyl oleate. The 

 rates of oxidation are roughly 1 :40: 100:200 for oleate, linoleate, linolenate, 

 and arachidonate, respectively.^* 



The autoxidation of the essential fatty acids is accompanied by radical 

 changes in the ultraviolet absorption spectrum. ^^ The major changes are 

 development of a strong absorption in the conjugated diene region (2340 A.) 

 and a smaller increase around 2700 A. The former has been shown to be 

 due to the conjugated diene hydroperoxide which is the primary product 

 of oxidation, and the latter is probably due to unsaturated ketonic sec- 



20 H. J. Deuel, Jr., S. M. Greenberg, L. Anisfeld, and D. Melnick, /. Nutrition 45, 

 535 (1951). 



21 J. P. Kass and G. O. Burr, J. Am. Chem. Soc. 61, 1062 (1939). 



22 J. P. Kass, J. Nichols, and G. O. Burr, J. Am. Chem. Soc. 63, 1060(1941). 



23 R. T. Holman, Proc. Soc. Exptl. Biol. Med. 76, 100 (1951). 



24 K. S. Markley, Fatty Acids. Interscience Publishers, New York, 1947. 

 26 R. T. Holman and O. Elmer, J. Am. Oil Chemists' Soc. 24, 127 (1947). 

 26 R. T. Holman and G. O. Burr, /. Am. Chem. Soc. 68, 562 (1946). 



