Alpha-Tocopherol, a Powerful Molecule, Leads to the Formation of Oxylipins in Polyunsaturated Oils Differently to the Temperature Increase: A Detailed Study by Proton Nuclear Magnetic Resonance of Walnut Oil Oxidation

Abstract

Lipid oxidation causes food degradation and the formation of toxic compounds. Therefore, the addition to foods of compounds able to avoid, delay or minimize this degradative process is a commonly used strategy. Nevertheless, neither the identity of most of the formed compounds in this complex process nor the way in which their formation is affected by the strategy used are well known. In this context, the effect the temperature increase and the enrichment level in alpha-tocopherol on the evolution of the walnut oil oxidation, as a model of an oil rich in polyunsaturated omega-6 acyl groups, submitted to storage conditions, are tackled by 1H NMR. The study has allowed knowing the degradation kinetic of both the oil acyl groups and alpha-tocopherol, the identification of a very high number of oxylipins and the kinetic of their formation. The temperature increase accelerates the formation of all oxylipins, favouring the formation of hydroperoxy conjugated E,E-dienes and related derivatives versus that of the Z,E-isomers. The enrichment in alpha-tocopherol accelerates the formation of hydroperoxy conjugated Z,E-dienes and related derivatives, and delays in relation to the formation of the former that of the E,E-isomers and related derivatives, hindering, to a certain extent, the formation of the latter in line with the enrichment level.

Individual and Joint Effect of Alpha-Tocopherol and Hydroxytyrosol Acetate on the Oxidation of Sunflower Oil Submitted to Oxidative Conditions: A Study by Proton Nuclear Magnetic Resonance

“…The identification of the compounds present in the different samples was carried out on the basis of the assignment of the 1 H NMR signals to the hydrogen atoms of the different structures, like in previous studies [ 26 , 27 , 28 , 29 , 30 , 31 , 52 ], with the support of the standard compounds, indicated in Table S1 (Supplementary Material) . These signals, their chemical shifts and their assignments to the various hydrogen atoms are given in Tables S2–S15 (Supplementary Material) .…”

“…The estimation of the concentration of linoleic acyl group, of αT, and HTy-Ac, as well as of the several oxidation compounds present in the several samples over the storage time, in relation to that of triglycerides (TG) was carried out as in previous studies [ 29 , 30 , 31 ].…”

“…The statistical analysis and graphical representations were performed using Microsoft Office Excel 2016, as in previous studies [ 30 , 31 ]. The statistical analysis was used for two purposes.…”

“…Recently, however, the development of a new methodology based on Proton Nuclear Magnetic Resonance, 1 H NMR, allows, in a very simple way, the monitoring and study of the oxidation processes of oils, and of lipids in general, in an exhaustive and complete way. This makes it very suitable for the in-depth study of the antioxidant or prooxidant capacity [ 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. The information provided by this methodology allows us to know the progression of the oxidation process in a global way from the point of view of the compounds degraded, and from the point of view of the oxylipins formed and their evolution.…”

“…The study was carried out using 1 H NMR spectroscopy. Alpha -tocopherol has shown, in previous studies, to have antioxidant or prooxidant ability, depending on the lipid system and the experiment conditions [ 14 , 17 , 21 , 22 , 26 , 27 , 30 , 37 , 38 , 39 ]. Hydroxytyrosol acetate is a minor component of virgin olive oil with a well-recognized antioxidant ability [ 31 , 40 , 41 , 42 , 43 , 44 ].…”