Hydrogenation of a menhaden oil: I. Fatty acid and C20 monoethylenic isomer compositions as a function of the degree of hydrogenation

US menhaden oil is rich in long-chain polyethylenic fatty acids, chiefly C₂₀ (eicosapentaenoic) and C₂₂ (docosahexaenoic) fatty acids, unlike Canadian herring oil, which is rich in long-chain (C₂₀ and C₂₂) monoethylenic fatty acids. An examination of the product fatty acids from hydrogenation of menhaden oil therefore comple-ments studies previously published for herring oil. During a commer-cial hydrogenation of menhaden oil, iodine value (IV) 159.0, on nickel catalyst, samples were collected at IV 150.0, 140.0, 131.5, 120.5, 96.5, 90.0 and 84.5. The fatty acid compositions were deter-mined using a combination of mercuric adduct fractionation and gas liquid chromatographic (GLC) analyses, and the totaltrans content by infrared spectroscopy. The partial hydrogenation resulted in the disappearance of the pentaenoic and hexaenoic fatty acids, a de-crease in tetraenes, and a definite increase in trienes, 8.3% at IV 84.5 compared to 4.2% at IV 159.0. The dienoic fatty acids in-creased to 13.2% at IV 84.5 compared to 4.1% at IV 159.0, and the monoenoic fatty acids increased to 34.2% from 24.0%. No impor-tant changes in the saturated acids were observed, 43.8% at IV 84.5 compared to 41.6% at IV 159.0. The totaltrans content varied from 3.4% at IV 150.0 to 45.1% at an IV of 84.5. The isomer composi-tions of thecis andtrans C₂₀ monoethylenic fatty acids were deter-mined using a combination of preparative GLC, AgNO₃ thin layer chromatography and ozonolysis. Thecis 20:1 acids at IV 84.5 still retained 27.5% of the major isomer (All) originally present at 72%. The parent A5, A8, All, A14 and A17 bonds of the 20:5 originally present could be detected in thecis 20:1 isomers at IV 96.5 but not at IV 84.5. At IV 84.5, 58% of the 20:1was trans, the major isomer being All (9.4% of total 20.1), accompanied by important quanti-ties of Δ10 and Δ12, respectively, 6.9% and 6.6% of the total 20:1. Presented in part at the 73rd annual AOCS meeting, Toronto, 1982.