Conclusions
Experiments with four model molecules (stearic acid,
trans-7-tetradecene, methyl oleate and oleic acid) under high frying
thermal oxidation (210°C, 3h, in air) demonstrated that oxidative
polymerization predominates only when there are olefin and carboxylic
acid functions on the same molecule. Oleic acid is thus an ideal
model molecule for investigating the initial polymerization events in
vegetable oil.
NMR spectroscopy experiments identified the first major (non-volatile)
thermal oxidation products generated. Under our oxidation conditions,
with model molecules, we evidenced: stearic acid forms a bit of internal
ester, trans-7-tetradecene forms (30%) epoxide, methyl oleate largely
forms (90%) epoxide and oxidized monomers while oleic acid forms (40%)
much larger species and fragments. Changes in NMR signal intensities
reveal oxidation occurs at the double bond. Long-range13C-1H correlation experiments confirm there are no
species with oxygen near a double bond.
Monomeric epoxide species (M2), formed first during oxidization of oleic
acid at 210°C, undergo rapid di-acylation through inter-molecular ester
cross-links (T1 in Figure 11 ). Fragmentation of oleic acid
during oxidation was revealed by formation of methylene groups bonded to
oxygen (CH2-O). Observation of these unprecedented
fragments (CH2-O rather than CH2-C(O)O)
along with the di-ester trimer demonstrate experimentally the first
steps in vegetable oil polymerization.
Ester cross-links are seldom contemplated in vegetable oil polymers.
Drying and poly-unsaturated oils are thought to increase molecular size
by ether cross-links. Here we show, for a mono-unsaturated oil, the
reactive olefinic groups form epoxides which then cross-link through
ester groups. Surprisingly, this knowledge, from experimental NMR
observations and model chemical reactions, is being realized only now .
The first trimer formed for oleic acid is still reactive since it
possesses both oleic and carboxylic acid functions. This allows
propagation of deleterious polyesters at longer reaction times.
Facile formation of polyesters provides a new perspective on oxidation
of lipids and vegetable oils. This report dispels traditional, widely
accepted, reaction schemes involving ether cross-links and
hydroxy-olefin species.