Introduction
Linseed oil (LSO) contains high levels of omega-3 polyunsaturated fatty
acid (PUFA), and lesser amounts of other fatty acids (e.g., of 48-60%
of linolenic acid (18:3), 14-19% of linoleic acid (18:2), and mono and
nonsaturated 14-24% of oleic acid (18:1), 3-6% of stearic acid (18:0)
and 6-7% of palmitic acid (16:0) (Lazzari and Chiantore, 1999). LSO is
used in various industries (e.g., paints, wood finish, linoleum
production, nutritional supplements and foods) wherein the LSO’s
autoxidation aging processes is an important product factor that are
influenced by the air/oxygen supply, and elevated temperatures resulting
in some cases in a viscous gel like semi-solid end product (Zhang et
al., 2012; Kaleem et al, 2015). It is well established that oxidation
takes place by a free radical mechanism on the polyunsaturated fatty
acid’s double bonds and tail segments of the alkyl chain resulting in
some low molecular weight molecules and cross-linking polymerization of
components forming 3D networks (Zhang et al., 2012; Douny et al., 2016).
A considerable amount of research has been performed on elucidation of
the autoxidation mechanism, since lipid oxidation is important for
numerous products and is also known to result in wood and linoleum
fires, food spoilage, and biologically tissue injuries and degenerative
diseases (Gorkum and Bouwman, 2005; Budularto and Kamal-Eldin, 2015).
PUFAs are highly susceptible to thermal autoxidation due to the presence
of readily removed bisallylic hydrogen atoms (Zhang et al., 2012; Vieira
et al., 2017), of relatively low bond dissociation energies of about 71
KJmole-1 (Juita et al., 2013), resulting in radical
chain initiation of decomposition and crosslinking polymerization
reactions (Gorkum and Bouwman, 2005). The LSO omega-3 PUFA-rich
oxidative thermal aging process is demonstrated in Scheme 1. Initial
formation of lipid peroxide on a LSO linolenic acid bisallylic carbon 11
is followed by a molecular rearrangement yielding conjugated diene.
Oxygen uptake initially forms a hydroperoxide that initiates the
propagation phase, with similar chain reactions of the surrounding
PUFAs. The propagation phase terminates with cleavage of the PUFA alkyl
tails and release of alpha, beta unsaturated aldehydes (e.g., acrolin
(2-propenal), cortonaldehyde (2-butenal; 4-hydroxy-trans-2-nonenal
(HNE); 4-hydroxy-trans-2-hexanal (HHE); malonaldehyde (MDA)). Previous
research showed that in heated vegetable oils with PUFA components,
significant concentrations of these aldehydes could be generated (Vieira
et al., 2017). Small amounts of the low molecular weight aldehydes are
volatized and large amounts of polymerized by crosslinking, nonvolatile
viscous products remain in the oxidized oil sample (Gorkum and Bouwman,
2005).