No Evidence Found for Diels–Alder Reaction Products in Soybean Oil Oxidized at the Frying Temperature by NMR Study

It has been generally accepted that the Diels–Alder reaction mechanism is one of the major reaction mechanisms to produce dimers and polymers during heating process of vegetable oil. Soybean oil oxidized at 180 °C for 24 h with 1.45 surface area‐to‐volume ratio showed 36 % polymer peak area in gel permeation chromatogram. However, the NMR DEPT (Distortionless Enhancement by Polarization Transfer) 135 spectrum did not show any signals of possible Diels–Alder products. A fraction separated from the oxidized soybean oil by column chromatography contained 98 % polymers, but again, showed no signals of proposed Diels–Alder products in the DEPT 135 spectrum. Methyl oleate and triolein without a diene required for the Diels–Alder reaction produced 27 and 63 % of total polymers, respectively, under the same condition. This indicates that the polymers must be produced by reactions other than the Diels–Alder reaction for these oils. This study shows that the Diels–Alder reaction is not the major reaction to produce polymers during oxidation of soybean oil, within the DEPT 135 spectroscopy sensitivity level, about 5 mol %.