Thermally induced isomerization of linoleic acid in soybean oil

The molecular mechanisms of the thermally induced cis/trans isomerization of the non-conjugated linoleic acids (C18:2) were investigated in a combined experimental and computational study of C18:2 isomers. C18:2 isomers in soybean oil heated at two temperatures (180 and 220 °C) were analyzed by GC method. C18:2-9c,12t and C18:2-9t,12c were both the main trans isomers in heated soybean oil. Two alternative isomerization schemes via the proton transfer paths were developed. The geometries in ground states, transition states, and intermediates were optimised using the density functional theory (DFT) at B3LYP/6-31G* level. The formation of C18:2-9t,12t had two barriers; the first of which was the formation of C18:2-9c,12t or C18:2-9t,12c. Zero-point energy corrections of each isomer was calculated at B3LYP/6-311++G** level. The intrinsic reaction coordinates (IRCs) were obtained to examine the transition states and intermediates. The activation energy differences between the cis and trans isomers of C18:2 possibly contributed to the distribution of the final ratio of the isomers. The DFT calculations indicated that the obtained experiment data was well explained by the isomerization mechanism developed.

► Heat treatment induced the formation of trans linoleic acid in soybean oil.
► C18:2-9c,12t and C18:2-9t,12c are the major trans isomers in heated soybean oil.
► Cis to trans isomerization is energetically a more favourable process.
► The isomerization process proceeds via a proton transfer paths.