Identification of minor C18 triene and conjugated diene isomers in hydrogenated soybean oil and margarine by GC-MI-FT-IR spectroscopy

Vegetable oils are partially hydrogenated in order to produce palatable products of suitable plasticity. The constituents of these new dietary products are complex mixtures of fatty acid isomers with different nutritional properties. A rapid method is described for separating and identifying fatty acid methyl ester (FAME) isomers of linolenic (octadecatrienoic, 18:3) acid and of conjugated octadecadienoic (18:2) acid, minor species found in hydrogenated soybean oil and margarine, by capillary gas chromatography-matrix isolation-Fourier transform-infrared (GC-MI-FT-IR) spectroscopy. FAMEs of 18:3 acid isomers in margarine, soybean oil hydrogenated in our laboratory, and isomerized linolenic acid were identified by this method, and MI-FT-IR spectra of FAME geometric isomers of octadecatrienoic and conjugated octadecadienoic acids are reported for the first time. Five major C18 triene GC peaks are found in chromatograms of isomerized methyl linolenate, representing species with tri-cis and tri-trans configurations and three species with cis-trans mixed configurations. FAME isomers with these configurations are also found for a hydrogenated soybean oil having an iodine value of 111. Methyl linolenate (tri-cis) is no longer found when soybean oil is further hydrogenated to an iodine value of 96. IR spectra characteristic of a tri-trans isomer are obtained for two test samples with iodine values of 111 and 96. Besides methyl linolenate, only isomers with a mono-trans di-cis configuration are found for the margarine analyzed. Conjugated cis-trans and trans-trans 18:2 FAME isomers are also found in all the hydrogenated soybean oil and margarine analyzed.