Investigation of polymers and alcohols produced in oxidized soybean oil at frying temperatures

Authors: Hwang, Hong-Sik; BALL, JAMES - Ford Motor Company; Doll, Kenneth - Ken; ANDERSON, JAMES - Ford Motor Company; Vermillion, Karl

Submitted to: Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2020
Publication Date: 2/10/2020
Citation: Hwang, H.-S., Ball, J.C., Doll, K.M., Anderson, J.E., Vermillion, K. 2020. Investigation of polymers and alcohols produced in oxidized soybean oil at frying temperatures. Food Chemistry. 317:126379. https://doi.org/10.1016/j.foodchem.2020.126379.
DOI: https://doi.org/10.1016/j.foodchem.2020.126379

Interpretive Summary: Deep frying is a popular cooking method in the food industry and restaurants because of the desirable flavors and textures of fried foods. However, oxidation of the frying oil is accelerated at these high temperatures. Ingestion of thermally oxidized oil during frying is associated with adverse effects on bone health, kidney disease, levels of cholesterol and phospholipid in plasma, and cancer. Especially, polymers accumulate in oil during frying and, therefore, are of concerns and are the subject of this study. Since oil oxidation involves many complicated chemical reactions, it is not well understood what kinds of polymers are produced and how they are formed. It is important to understand these to understand their toxicity and to prevent polymerization. Recently, ether and ester bonds were proposed as linkages in polymers of oil and fatty acid esters heated at lower temperatures than frying temperatures. In this study, we prepared oxidized soybean oil samples by frying French fries for 4 day and used modern analytical methods such as NMR to examine if ester and ether bonds were formed. We found ester bonds in polymers of soybean oil, but not ether bonds. We also confirmed some oxidation products previously proposed, verified uncertain compounds, and found one new kind of oxidation product using NMR and other techniques. This information is very valuable for understanding chemical structures of oxidation products during frying, which is critical to understand safety of oxidized foods and for strategy to prevent oxidation.

Technical Abstract: Although significant amounts of polymers that are associated with adverse health effects in oils are produced during frying, the chemical bonds that form during the polymerization have not been well understood. This study revealed that ester bonds were responsible for the polymerization of soybean oil during frying and heating at 175 ºC. Ester value of soybean oil increased during frying up to day 3 and slightly decreased at day 4 indicating that esterification and hydrolysis concomitantly occurred. The ^13^C NMR spectra showed further evidence of the formation of ester bonds. This study also examined unidentified chemical bonds forming polymers other than ester bonds with NMR spectroscopy. No NMR signals indicating ether bonds were observed. The NMR study after the reaction of oxidized soybean oil with acetyl chloride clarified uncertain assignments of proton signals, confirmed some previous assignments, and assigned a new proton signal as an alcohol.