Hydrogenation of a menhaden oil: II. Formation and evolution of the C20 dienoic and trienoic fatty acids as a function of the degree of hydrogenation

Abstract

AbstractTo complement studies on monoethylenic fatty acids produced from the major polyunsaturated fatty acid (20:5A5,8,11,14,17) during hydrogenation of a menhaden oil of iodine value (IV) 159, the C10 dienoic and trienoic fatty acid isomers of partially hydro‐genated menhaden oils (PHMO) of IV 131.5, 96.5 and 85.5 were isolated by a combination of preparative gas liquid chromatography (GLC), mercuric adduct fractionation, and silver nitrate thin layer chromatography (AgNO3‐TLC). The 20:2 fatty acid methyl esters of the three PHMO samples were transformed to the corresponding alcohols and ozonized in BF3‐MeOH, followed by GLC analysis of the ozonolysis fragments. During the hydrogenation process, re‐sidual ethylenic bonds in the 20:2 isomers tend to migrate both towards the carboxyl group and towards the methyl end of the molecule. The hydrazine reaction results revealed that thetrans ethylenic bonds in the 20:2 and 20:3 isomers were distributed all along the the carbon chain, but thecis ethylenic bonds were more localized in the Δ11,Δ14 and Δ17 positions of the preexisting major menhaden oil component 20:5Δ5,8,11,14,17. Iatroscan analyses on AgNO3‐chromarods revealed that, as a result of the hydrogenation process, almost half of the 20:2 isomers were non‐methylene‐interruptedcis, trans/trans, cis structures.