Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2013
Publication Date: 3/16/2013
Citation: Hwang, H.-S., Doll, K.M., Winkler-Moser, J.K., Vermillion, K., Liu, S.X. 2013. No evidence found for Diels-Alder reaction products in soybean oil oxidized at the frying temperature by NMR study. Journal of the American Oil Chemists' Society. 90(6):825-834.
Interpretive Summary: Correct oxidation mechanisms and accurate identification of products in the oxidized oil are important to finding new natural antioxidants and evaluating potential toxicity. The Diels-Alder reaction is traditionally accepted as one of the major reaction mechanisms producing cyclic degradation products during the heating process of vegetable oil. Since its introduction during the 1930’s-1960’s this reaction has been cited in many research articles and books and taught to young students. However, our research using a modern analytical method, NMR, indicated no evidence of the widely accepted Diels-Alder reaction in the oxidized vegetable oils. As a result, other scientists are urged to look at alternative reaction mechanisms and to use modern analytical methods. This finding is significant because it will contribute to the use of correct oxidation mechanisms in developing new antioxidants for frying oil and to providing accurate identification of cyclic degradation products and their potential toxicity in the oxidized oil.
Technical Abstract: 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 hrs with 1.45 surface area-to-volume ratio showed 36.1% polymer peak area in gel permeation chromatogram. However, the NMR DEPT (Distortionless Enhancement by Polarization Transfer) 135 spectrum didn’t show any signals of possible Diels-Alder products. A fraction separated from the oxidized soybean oil by column chromatography contained 97.8% 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 27.2% and 63.0% 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%.