Insights into Polymerization of Vegetable Oil: Oligomerization of Oleic Acid

Abstract

Viscous gel-like species form in vegetable oils during frying. After fatty acid formation, thermal oxidation of unsaturated chains causes polymerization. Heat treatment of oleic acid was employed to produce and identify the initial polymerization products. Spectroscopic studies revealed that heating oleic acid (210 C, open to air) causes the formation of oligomers cross-linked by ester groups. The absence of esters, before thermal treatment, facilitated observation of cross-links (ester groups) that propagate to produce heavier insoluble products.

Thermal oxidation of model molecules to reveal vegetable oil polymerization studied by NMR spectroscopy and self‐diffusion

“…Thermal oxidation of oleic acid at 210 C was reported previously (Bonetti & Parker Jr, 2019). "Clean" observation of a methine carbon bonded to the oxygen ( CHO ) of an ester drove communication of the discovery, Figure 7 and Figure S14.…”

“…Hwang and co-workers had to chromatographically separate the polar fraction of oxidized soybean oil (potatoes frying, 175 C, 24 h) to observe 13 C NMR signals of the newly formed ester species (Hwang et al, 2020). In addition, they observed primary alcohols by 1 H NMR, from fragmentation reactions, in common with our oleic acid oxidation (Bonetti & Parker Jr, 2019).…”

“…Four model molecules: methyl oleate, trans‐7‐tetradecene, oleic acid, and stearic acid were purchased from Aldrich. Each substance was studied pure (dissolved in CDCl 3 ) and after oxidation at 210°C for 3 h in containers open to air with stirring as previously (Bonetti & Parker Jr, 2019).…”

“…Model reactions between carboxylic acids and alcohols (90 C with aeration) showed facile ester formation (Ball et al, 2019). NMR spectroscopy found ester bond formation in thermally oxidized oleic acid (Bonetti & Parker Jr, 2019) and soybean oil (Hwang et al, 2020). Although poorly sensitive, NMR spectroscopy is a powerful spectroscopic technique.…”

A versatile method to fingerprint and compare the oxidative behaviour of lipids beyond their oxidative stability

“…As in polyunsaturated systems, in monounsaturated ones, peroxides and other oxidation intermediates are formed and they break down to a wide range of secondary oxidation products which include volatiles compounds 61 . Differently from oleic acid based oils and triglycerides, the experimental mass change of methyl oleate shows only a slow mass loss (Fig.…”