Figure 1. Trans-esterification of triacylglycerol (triolein) to its
fatty acid methyl ester (biodiesel) and glycerol.
Oxidation of culinary oils has been studied for decades, detailing the
numerous minor species that form (e.g. hydroperoxides, epoxides,
aldehydes, acids, ketones) . Multitudes of reports confirm that
oxidation and oligomerization occur more rapidly for oils with increased
olefinic character. But the oligomers formed during thermal oxidation
lack elucidation. Crosslinks in heated oils are usually attributed to
C-C bonds without oxygen atmosphere and to ether C-O bonds open to air .
Clearly, oxidative polymerization of oil is inconsistent with a
Diels-Alder type reaction to give C-C crosslinks . Supposedly,
poly-unsaturated “drying oils” (e.g., linseed), used for centuries,
polymerize spontaneously in air by ether cross-links .
Clearly, the situation is complex for unsaturated triacylglycerols.
Along with ester hydrolysis, glycerol and fatty acids form reactive
alkoxyl and peroxyl radicals which undergo polymerization and scission
reactions simultaneously . Molecular size increase during lipid
oxidation is easily measured by chromatographic methods. But this gives
no cross-link identity
Recently evidence was presented for dispelling ether cross-links in
thermally oxidized vegetable oils. Model reactions between carboxylic
acids and alcohols (90°C with aeration) showed facile ester formation .
NMR spectroscopy found ester bond formation in thermally oxidized oleic
acid and soybean oil . Although poorly sensitive, NMR spectroscopy is
the most widely utilized spectroscopic technique. It can identify the
chemical groups providing the first cross-links and measure molecular
size (diffusion rate) of the major products. Information indispensable
for unravelling the polymerization reaction.
Due to NMR spectral degeneracy, ester cross-links are difficult to
detect starting with tri-acyl glycerides. Hwang et al. had to
chromatographically separated the polar fraction of oxidized soybean oil
(potatoes frying, 175°C, 24h) to observe 13C NMR
signals of the new ester species. Also, they observed primary alcohols
by 1H NMR, from fragmentation reactions, in common
with our oleic acid oxidation .
We continue simplified stepwise approach to study oil polymerization.
This work investigates mono-unsaturated acyl chains (e.g. oleyl) with
our standard treatment, 210°C for 3h exposed to air. Oleic acid
represents vegetable oil since hydrolysis is an early reaction step. In
addition, without NMR signals from acylglycerides and glycerol, spectra
are more intense, more resolved, and easier to interpret. Since focus is
on characterizing the first larger-sized species formed in
polymerization (oligomers), high frying (210°C) above normal frying
(175°C), was used for a short time (3h) in open-air containers.
This standard oxidation process is applied to three other “model”
molecules (stearic acid, trans-7-tetradecene and methyl oleate) closely
related to oleic acid, to evidence the roles of olefinic and carboxylate
species in oligomerization. NMR studies provided information on both
structures (1H, 13C) and sizes
(DOSY) of the non-volatile molecules produced by auto-oxidation and
polymerization. One important structural feature provided by NMR
spectroscopy is 3-bond C-H connections which identify the nuclei near
the reactive centers (olefin, acid, alcohol) of triacylglycerides.