Fig. 1 Omega-3 linolenic acid-rich LSO decomposition pattern and main released aldehydes as determined by GC-MS analysis (a) selective exponents of LSO segments with emphasis of segment of tail T2 time domain relaxation determination (b) and graphic presentation of LSO tail T2 values changes during heating at 25, 40, 60, 80, 100, 120oC together with air pumping for 168 hours (25 and 40oC designated as Group A and 60, 80, 100, 120oC designated as Group B) (c).
Fig. 2 presents the band selective HF 1H NMR pulse excitation that provide quantification of hydroperoxides (Fig 2a) and aldehydes (Fig 2b) of the same LSO samples used for determining tail T2 transverse relaxation shown in Fig 1c, treated under the same thermal oxidation conditions (25, 40, 60, 80, 100, 120oC) with air pumping up to 168 h. The same samples, the peroxide level of LSO at 60oC is the highest with a concentration peak after 96 h (380 mmol/kg). Peroxide levels of LSO at 80oC are significantly lower than at 60oC and peaks after 48 h (190 mmol/kg). Peroxide levels of LSO at 100 and 120oC are significantly lower than at 80oC and peak after 24 and 9 hrs, respectively. Peroxide levels at 40 and 25oC is minimal throughout the experiment, and reach low values of 35 and 20 mmol/kg, respectively at 168 h (Fig. 2a). It should be noted that the peroxide level at the beginning of the experiment using stored LSO sample is about 7 mmol/kg. This data is in good agreement with our PV tests of the LSO samples of the present study (not shown) and also supported by previous LSO reports (Resende et al., 2019b; Douny et al., 2016). This indicates that even at room temperatures of moderate storage condition, there are some oxidation reactions of the LSO ’s PUFAs, but at a relatively low level.
Selective band HF 1H NMR aldehyde analysis of LSO under thermal oxidation shows a clear temperature pattern (Figure 2b). In the LSO samples that were oxidized at 40 and 25oC, however there was not any significant accumulation of aldehydes. From 60oC up to 120oC, aldehyde levels continuously increased. In LSO samples heated at the highest temperature tested of 120oC, aldehyde levels peak after 48 h (95 mmol/kg). In LSO samples oxidized at 100, 80 and 60oC, the aldehyde levels continuously increased and did not peak after 168 h (85, 60, 25 mmol/kg, respectively). These last data points are in agreement with the reports saying that the chain reactions of oxidation may last for relatively long time periods and the rate of these reactions depends on the environmental conditions dominated by temperature (Lazzari & Chiantore (1999).
It is well known that due to the high speed of peroxide generation and turn over, even when using the powerful methodology of band selective1H NMR pulse excitation it is difficult to provide a detailed view of the data of peroxide levels in the very early stages of oil oxidation, in particularly for the higher thermal oxidation conditions (> 80oC), as shown in Fig. 2A.
It also should be noted that in the tail T2 TD relaxation test (Fig. 1c) a clear change of the T2 could be observed at 60, 40 and 25 oC after 24, 48 h and 72 h, respectively. However even in the case of the selective tail T2 methodology results, it is difficult to view a clear and detailed pattern of changes in the very early phase of the first 10 hours of the oxidation process.
To further and better study and verify the changes of chemical composition, GC-MS analysis of all the LSO samples used in the present study was carried out. Table 1 presents the changes of the non-oxidized saturated (SAT), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids during LSO thermal autoxidation (25, 40, 60, 80, 100, 120oC) for 0, 3, 6, 9, 24, 48, 72, 96, 120, 168 h. At low temperatures of 25 and 40oC the profile of the three groups of fatty acids seem to be stable, while the profile of 60oC demonstrate significant decrease/loss of PUFA at the later time periods (120 and 168 h) and relative increase in MUFA and SAT. At 80oC there is a moderate decrease of PUFA and an increase of MUFA and SAT after 48 up to 168 h. At the higher temperatures of 100 and 120oC a similar pattern is observed, but with a faster rate of PUFA loss starting from 24 h up to 120 h and 96 h, respectively, in which the original liquid sample is transformed to a viscous gel.