The mechanistic pattern of LSO oxidation can be seen in the viscosity measurements of all the samples described in Fig. 3, at different oxidative temperatures and time. The two lowest temperature samples (25 and 40oC) marked as Group A show a minimal or no heating effect and remain at a low constant viscosity over all the time tested from time 0 up to 168 hours. The LSO samples treated with higher temperatures from 60oC up to 120oC marked as Group B, clearly show an increase of viscosity over time, however it is possible to differentiate between each temperature. There is a period of very minimal ”heating effect” of 96, 72, 48 and 24 hours for 60, 80, 100 and 120oC, respectively. Following this early phase there is a dramatic increase of level of viscosity of the samples closely dependent of the temperature. It is interesting to note that non of the samples reach a peak point and may further increase if the experiment might be extended. The viscosity of the LSO sample treated with air pumping and heated to 120oC, reached to a maximal level of viscosity (0.8 Pa.s) and lost its fluidity already after 72 hours. The sample treated with 100oC reached also a similar level of viscosity after 120 Hours and the other two samples of Group B reach a level of viscosity of 0.4 Pa.s at the end of the experiment. The fact that all the samples treated with air and increase temperature (> 60oC) reached a point of viscous gel-like products suggest of a significant temperature depended polymerization phase, as the termination step of the autoxidation process. These results are in agreement with the literature reported on termination phase of LSO oxidation (Douny et al., 2016; Vieira et al., 2017; Resende et al., 2019).
The fact that in all cases there was a ”leg time” or a ”induction period” that it length is depended of the temperature level may suggest that a certain level of energy introduction to the sample is required to release the weak interactions kept by van der Waals and hydrogen bonds (H abstraction) in the original liquid LSO, so that oxygen from the pumped air may be able to interact with the rearranged conjugated diene segments of the PUFAs of the LSO. In other words, the heating ”induction period” open and stimulates the autoxidation process of LSO by making some delicate structural changes. The ability to monitor these changes may open the way to evaluate the progress of the oxidation of LSO from relatively early stages until the termination phase. Based on previous publications (Berman et al., 2015, Meiri et al., 2015) discussing the relationship between 1H NMR T2 and weak forces effects on fatty acids structure and assembly in different temperatures, we suggest that LSO tail T2 changes during the period of minimal ”heating effect” described above (Fig. 1c) are providing good information required for evaluation of the chemical and structural changes during LSO autoxidation. Furthermore, correlation of tail T2 vs. viscosity show that only after several testing time points (depending of heating temperature) both parameters well correlate (see supplemental information 2).