Alkyl Tail Segments Mobility as a Marker for Omega‐3 Polyunsaturated Fatty Acid‐Rich Linseed Oil Oxidative Aging

Abstract

AbstractOmega‐3 polyunsaturated fatty acid (PUFA)‐rich linseed oil (LSO) is an important component in biological systems, foods, and many other industrial products. In recent years, LSO has attracted increased attention in the field of functional foods, which has highlighted its facile susceptibility to aging by autoxidation. Common colorimetric and a long list of spectral methodologies have been used to follow after and predict LSO shelf life's quality, especially in regards to aging by autoxidation. These standard methodologies are nevertheless limited, because of the complexity of the LSO's chemical and physical changes. The goal of the present study is to develop a sensorial 1H LF‐NMR energy relaxation time application based on monitoring primary chemical and structural changes occurring with time and temperature during oxidative thermal stress for better and rapid evaluation of LSO's aging process. Using 1H low‐field NMR, the different T2 times of energy relaxations due to spin–spin coupling, and proton motion/mobility of LSO molecular segments were monitored. As previously reported, we characterized the chemical and structural changes in all phases of the autoxidation aging process. Starting from the initiation phase (abstraction of hydrogen radical, fatty acid chain rearrangement, and oxygen uptake yielding hydroperoxides products), through to the propagation phase (chain reactions resulting in tail cleavage to form alkoxy radicals, and alpha, beta‐unsaturated aldehydes formation), and a termination phase (cross linking and production of polymerization end products). The 1H LF NMR transverse relaxation approach, monitors both the covalent bond's strong forces (100–400 kJ mol−1) in LSO oxidative aging decomposition, as well as secondary relatively weak interactive forces by hydrogen bonds (~70 kJ mol−1), and electrostatic bonds (0–50 kJ mol−1) contributing to secondary crosslinking interactions leading to a LSO viscous gel of polymerized products in the termination phase. In the present paper, we show that LSO tail segments mobility in terms of T2 multi‐exponential energy relaxation time decays, generated by data reconstruction of 1H transverse relaxation components are providing a clear, sharp, and informative understanding of LSO sample's autoxidation aging processes. To support T2 time domain data analysis, we used data from high‐field band‐selective 1H NMR pulse excitation for quantification of hydroperoxides and aldehydes of the same LSO samples treated under the same thermal conditions (25, 40, 60, 80, 100, 120 °C) with pumped air for 168 hours. Peroxide value, viscosity, and self‐diffusion analyses, as well as fatty acids profile and by‐products determined by GC–MS on the same samples were carried out, and correlated with the LSO tail T2 energy relaxation time results. From these results, it is postulated that selective determination of LSO tail T2 time domain can be used as a rapid evaluation marker for following omega‐3 PUFA‐rich oils oxidative aging process within industrial and commercial products.