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

•Ester bonds are responsible for the formation of polymers of soybean oil during frying.•Ether bonds were not found in polymers of oxidized soybean oil.•Reaction of oxidized oil with acetyl chloride could verify alcohols in oxidized oil.•One new NMR signal was found by the reaction with acetyl chloride. Although significant amounts of polymers associated with adverse health effects in oils are produced during frying, the chemical bonds forming these polymers are not well understood. This study revealed that ester bonds are responsible for the polymerization of soybean oil during frying and heating at 175 °C. The ester value of soybean oil increased during frying up to day 3 of the experiment and slightly decreased on day 4 of the experiment indicating that esterification and hydrolysis concomitantly occurred. The 13C NMR spectra showed further evidence of the formation of ester bonds. This study also examined unidentified chemical bonds in the polymer products, 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 assignments of proton signals, confirming some previous assignments, and assigning a new proton signal as an alcohol.