Formation of traces of conjugated tetraenoic and trienoic constituents in autoxidized linolenic and linoleic acids and vegetable oils during alkali‐isomerization

Summary It has been shown that the low‐intensity absorption bands characteristics of conjugated tetraenoic and trienoic fatty acids frequently encountered in the ultra‐violet spectra of alkali‐isomerized vegetable oils prepared by ordinary commercial or laboratory extraction techniques have their probable origin in oxidation products of linolenic and linoleic acid, respectively. Similar bands are found in the spectra of mildly autoxidized preparations of pure linolenic and linoleic acids after either alkali‐isomerization or heating at 180°C. in neutral ethylene glycol. Tetraenoic and trienoic conjugation formed from oxidation products of linolenic and linoleic acids during alkali‐isomerization can be differentiated from the tetraenoic and trienoic conjugation produced by alkali‐isomerization of arachidonic and linolenic acids, respectively, by spectrophotometric examination of the sample after heating in neutral ethylene glycol. Essentially equal amounts of conjugation are formed from the fatty acid oxidation products on heating and on alkali‐isomerization while no conjugation is obtained from arachidonic and linolenic acids on heating in the absence of alkali.