¹H LF‐NMR Energy Relaxation Time Characterization of the Chemical and Morphological Structure of PUFA‐Rich Linseed Oil During Oxidation With and Without Antioxidants

Abstract

A major stability issue of food and other products containing lipids is their susceptibility to oxidation and the efficacy of endogenous or exogenous antioxidants that are sensitive to internal morphological and chemical structures which are difficult to characterize with current analytical methods. This work develops signal reconstruction of 1H LF‐NMR spin‐lattice (T1) and spin‐spin (T2) energy relaxation times into 2D T2 versus T1 peak graphs of the internal chemical and morphological arrangements within lipid materials, with the potential for efficient structural studies of lipid oxidation and antioxidant efficacy in complex foods. This novel NMR inverse Laplace transformation (ILT) signal processing reconstruction is now applied in a PUFA‐rich linseed oil's thermal autoxidation and antioxidant efficacy study. To characterize antioxidant efficacy, it is important to understand the mechanism of oxidation and the chemical and morphological structural aspects. The initial steps of oxidation show a clear peak shift to shorter T2 versus T1 peak values, for the different poly‐unsaturated fatty acids (PUFA), which for mono‐unsaturated and saturated FAs remain unchanged. These peak shifts are within an oxidation time period needed for hydroperoxides formation. For longer oxidation times a new set of T2 versus T1 peaks are generated with shorter T2 and stable and/or increased T1 values, suggesting formation of aldehydes and polymerized oxidation end products. The chemical changes of the fatty acids of linseed oil are confirmed by their gas chromatography analysis during the studied time interval. Similarly peroxides and aldehydes formation trends are confirmed by PV and p‐ansidine tests, respectively for the same time points. Based on the oxidation study the control of T1 and T2 relaxation times by α tocopherol antioxidant is characterized. In effect α tocopherol antioxidant maintains peak stability during linseed oil's thermal autoxidation, in a 2D T2 versus T1 relaxation time graph of peaks assigned to chemical morphological structures, demonstrating α tocopherol's ability to minimize linseed oil's oxidation. Practical Application: Monitoring of food products oxidation and quality control. 1H LF‐NMR T1 and T2 energy relaxation times reconstructed with PDCO into 2D spectra of material analysis has proven to be an advantageous tool to monitor complex chemical and morphological structural changes during the autoxidation of PUFA‐rich linseed oil. It is suggested that an initial oxidation phase generates a shift, of the original linseed peaks along the T1 = T2 diagonal due to hydroperoxide formation. At longer oxidation times there is an accumulation of peaks below the diagonal line assigned to oligomeric/polymeric termination products, generating a significant “bending effect.” α tocopherol addition at effective concentrations is able to change this pattern of PUFA‐rich linseed oil oxidation by blocking early peaks shift and late bending effect.

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

“…The present study was carried out as a follow‐up of our previous publications presenting the 1 H LF‐NMR 2D T 1 –T 2 graphical maps for butter, rapeseed oil, soybean oil, and linseed oil (Resende et al, 2019a, b). These articles showed that the composition of the saturated/unsaturated fatty acids affects the triacylglyceride (TAG) oil's chemical and morphological energy relaxation time domains, which could be correlated to its oxidative propensity.…”

“…Autoxidation experimental design was based on previous studies (Berman et al, 2016; Meiri et al, 2015; Resende et al, 2019a, b; Wiesman et al, 2018). Briefly, six samples of 250 mL of linseed oil in 500 mL beakers were heated on a hot plate at different temperatures (25, 40, 60, 80, 100, and 120 °C).…”

“…The field of 1 H LF‐NMR energy time relaxometry is a powerful tool for identifying molecular species and to study their dynamics even in complex materials (Berman et al, 2013a, 2015; Resende et al, 2019a, b; Rudszuck et al, 2019; Wiesman et al, 2018). This relates to the measurement of energy time relaxation values as a consequence of interactions among nuclear spins and between spins and their surroundings (matrix).…”

“…Current technologies are not effective in characterizing the morphological and chemical structural domains of saturated, monounsaturated fatty acids (MUFA) and PUFA materials, or how the morphological structures of fatty acids, at the meso, nano, and molecular levels, affect their oxidation mechanisms. 1 H LF‐NMR energy relaxation time technology consisting of L 1 /L 2 norm regularization (Campisi‐Pinto et al, 2018, 2020; Resende et al, 2019a, b), is proposed as a tool to analyze PUFA oils undergoing thermal oxidation. This technology can generate two‐dimensional (2D) chemical and morphological spectra using a recently modified and developed primal‐dual interior method for the convex objectives (PDCO) optimization solver for computational processing of the energy relaxation time signals T 1 (spin–lattice) and T 2 (spin–spin): With carefully chosen reconstruction parameters, the data signals can be reconstructed into 2D graphics of the different energy relaxation times assigned to the mobility of different chemical structures, and their adjacent environments (Wiesman et al, 2018).…”

“…The technology of the 1 H LF‐NMR energy relaxation time proved to be an effective tool to characterize and monitor PUFA oxidation (Resende et al, 2019a). The use of 2D graphic reconstruction of T 1 vs. T 2 as compared to only one dimensional (1D) has the ability to increase peak separation on the diagonal (T 1 = T 2 ) and when new polymerized oxidation products, new peaks appear below the diagonal (same constant T 1 but decreased T 2 ) during later stages of the oxidation process (Resende et al, 2019a, b).…”