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).