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).