288 ESSENTIAL FATTY ACIDS 



express the results in a relative manner, standard values based on certain 

 assumptions^ have been employed, but it should be understood that the 

 results, as expressed, are tentative relative approximations. No attempt has 

 been made to extend the calculations to include linoleic and linolenic acids. 



Objection has been raised that the spectral methods are not specific for 

 the essential fatty acids but will detect unnatural isomers as well. This 

 apparently is the case, judging from the results of a study of metabolism 

 of unnatural isomers of the essential fatty acids in which spectrophoto- 

 metrically detectable polyunsaturates were found in animals suffering from 

 fat-deficiency symptoms.^ Even so, this is not a serious criticism of the 

 method, for it is used largely on natural substances where unnatural isomers 

 are not often encountered. 



The spectrophotometric method is the best current method for deter- 

 mination of linoleic and linolenic acids in natvu'al vegetable oils. The method 

 as employed by Brice and Swain, ^ requiring 100-mg. samples, is excellent 

 for use on vegetable oils where linolenic acid is the most unsaturated acid 

 present. Some micro modifications of the method have been developed for 

 smaller samples,^' ^ but these are of more limited application. When pure 

 hexaene and pentaene acids have been isolated and their responses measured 

 under various conditions, a modified method will be available for the quanti- 

 tative measurement of more highly unsaturated members of the group. ^'^ 

 However, it seems doubtful that a spectrophotometric method of precision 

 for the simultaneous determination of all members of the polyunsaturated 

 group will be forthcoming. 



B. CHEMICAL METHODS 



1. Thiocyanogen Value 



The estimation of the content of oleic, linoleic, and linolenic acids in a 

 fat can be made if the thiocyanogen value, iodine value, and saturated fatty 

 acid content are known. Theoretically, thiocyanogen adds to the double 

 bond of oleic acid, one of the two double bonds of linoleic acid, and two 

 of the three in linolenic acid. The halogen in the iodine value reagent adds 

 to all double bonds. Correction for the total saturated acid content of the 

 fat then allows the setting up of a set of simultaneous equations for the 

 calculation of the contents of the three unsaturated fatty acids. Although 



^ C. Widmer, Master's Thesis, Agricultural and Mechanical College of Texas, 1949. 

 6R. T. Holman, Proc. Soc. Exptl. Biol. Med. 76, 100 (1951). 



7 L. C. Berk, N. Kretchmer, R. T. Holman, and G. O. Burr. Anal. Chem. 22, 718 

 (1950). 



8 P. W. O'Connell and B. F. Daubert, Arch. Biochem. 25, 444 (1950). 



'» Recently S. F. Herb and R. W. Riemenschneider have made a detailed study of 

 the effect of alkali on the conjugation of polyunsaturated fatty acids, and have es- 

 tablished some constants for pentaene and tetraene acids. /. A77i. Oil Chemists Soc. 

 29, 456 (1952). 



