Fig. 2 Demonstration of LSO (a) and FO (b) 2D T1-T2 TD fingerprinting map after 96 h heating at 55 oC.
The LSO and FO 1H LF NMR energy relaxation time spectra after undergoing oxidation (Fig. 2) has more peaks than the four characteristic peaks of the original LSO and FO (Fig. 1). These new peaks correspond to the oxidation products. The spectrum of a non-oxidized oil (Fig. 1) is characterized by 4 major peaks with the T1 and T2 of similar values with the peaks being aligned in the diagonal of the map. After oxidation the map of Fig. 2 not only present new peaks related to the oxidation products but there is also a bending effect in the curve from the diagonal because of decreasing T2 values, indicating oxidation (Resende et al., 2019a,b). In order to confirm the oxidation of the LSO and FO after 96 hours of thermal oxidation observed in the1H LF-NMR energy relaxation time map of Fig. 2, these material were also analyzed by peroxide value (PV) tests. The PV test results (Table 2) support the oxidation of the LSO and FO wherein the fish oil shows a higher PV value, indicating a higher degree of oxidation than the LSO. This is in agreement with the GC-MS results of changes of fatty acids profile during thermal heating (not shown) and previously published literature (Resende et al., 2019), because of the higher number of double bonds in the fish oil increase its propensity to oxidation.
Table 2: Peroxide values (PV) of LSO and FO in thermal conditions (55oC)