Antioxidant activity of OOE in stripped SBO during storage
Antioxidant activity of OOE was evaluated in stripped SBO at two different temperatures: 25 ºC as a typical storage temperature of oil and 40 ºC as a possible high temperature during transportation and manufacturing processes. Figure 1 shows the oxidation of stripped SBO containing 0.1% OOE compared to control (stripped SBO without antioxidant), 0.02% BHT, 0.1% RE, and 0.1% Toco. As mentioned in the introduction section, to avoid relying on a single analytical method, PV, CDV and p -AV were used to determine oil oxidation. PV is an indicator for the early stages of oxidation, and p -AV is an effective indicator for secondary stages of lipid oxidation, while CDV is useful for early and intermediate stages.
Figure 1a shows PV of stripped SBO with four different treatments stored at 25 ℃. Since the PV of 0.1% Toco and the control reached a peak value at day 14, reasonable comparisons between treatments could not be made beyond day 14. Therefore, we focused on the comparison among treatments during the lag phase, which were days 5 and 14. On day 5, SBO samples with 0.1% OOE and 0.02% BHT had lower PV (0.68 ± 0.09 and 0.73 ± 0.10 meq/Kg, respectively) than those with 0.1% RE (1.92 ± 0.20 meq/Kg) and 0.1% Toco (2.09 ± 0.03 meq/Kg) while the control had the highest PV (4.20 ± 0.19 meq/Kg). On day 14, the PV of the sample with 0.1% Toco sharply increased (12.09 ± 0.22 meq/Kg) and became close to that of the control (12.72 ± 0.34 meq/Kg). Oil samples with 0.1% OOE and 0.02% BHT had the lowest PV (2.62 ± 0.25 and 2.78 ± 0.07 meq/Kg, respectively) followed by 0.1% RE (6.02 ± 0.30 meq/Kg) on day 14. Figure 1b shows the CDV of SBO samples stored at 25 ℃ up to 50 days. CDV of the control sharply increased during storage up to day 21 and gradually increased between day 21 and day 50. All antioxidant treatments significantly lowered the CDV. Since differences between treatments became obvious on day 21, the Tukey-Kramer HSD test (at P < 0.05 for CDV) was used to compare mean CDV only for data from days 21 and 50. The 0.1% Toco was found to have significant antioxidant activity in comparison with the CDV of the control, but it had significantly higher CDV (5.38 ± 0.27 mmol/L) than 0.1% OOE, 0.02% BHT and 0.1% RE (3.23 ± 0.17, 2.72 ± 0.06 and 3.38 ± 0.18 mmol/L, respectively) on day 21. There was no significant difference between 0.1% OOE and 0.02% BHT throughout the study. The 0.1% RE treatment had a significantly higher CDV (4.64 ± 0.17 mmol/L) on day 50 compared to 0.02% BHT (2.93 ± 0.26 mmol/L), but didn’t significantly differ from the 0.1% OOE treatment (3.42 ± 0.33 mmol/L). Figure 1c shows p -AV of SBO samples with statistical analysis of results for day 21 and day 50. While p -AV of the control increased to 134.05 ± 4.35 on day 50, those containing 0.1% OOE, 0.02% BHT, 0.1% RE, and 0.1% Toco did not significantly increase throughout the study. This indicates that all the antioxidants were very effective in preventing the formation of aldehydes, the secondary oxidation products, even though significant amounts of primary oxidation products, hydroperoxides and conjugated dienes, formed during storage up to 50 days.
Figure 2 shows the oxidation of SBO samples at 40 ℃. The PV of the control and SBO with 0.1% Toco reached peak values on day 7, and so the treatment comparisons were focused on days 1, 3, and 7 (Fig. 2a). There was no significant difference between the 0.1% OOE and the 0.02% BHT treatments through Day 7. PV of SBO with RE was significantly higher than those with OOE and BHT on days 3 and 7. The PV of Toco increased more rapidly than the other three antioxidant treatments. CDV analysis also showed the antioxidant activity order of BHT ≈ OOE > RE > Toco at 40 ℃ (Fig. 2b), which was observed with PV. On day 7, SBO samples containing 0.1% OOE, 0.02% BHT and 0.1% RE exhibited similar CDV (3.40 ± 0.05, 3.38 ± 0.12 and 3.90 ± 0.28 mmol/L, respectively) while CDV of SBO with 0.1% Toco (6.00 ± 0.12 mmol/L) was higher than those with other treatments indicating the weakest activity of Toco. On day 20, SBO with OOE and BHT had the lowest CDV (3.60 ± 0.10 and 3.27 ± 0.07 mmol/L, respectively) followed by RE (6.49 ± 0.25 mmol/L), and then Toco (13.81 ± 0.85 mmol/L). As observed at 25 ℃, samples with all the treatments showed much lower p -AV than the control during the course of 20-day storage at 40 ℃. Only at day 20, SBO containing 0.1% Toco showed a slightly, but not significantly, higherp -AV (6.55 ± 0.55) than SBO with 0.1% OOE, 0.02% BHT, and 0.1% RE (1.38 ± 0.55, 1.19 ± 0.25 and 3.48 ± 0.10, respectively).
The trend observed with SBO samples at 25 ℃ was very similar to that at 40 ℃. The overall antioxidant activity order was BHT ≈ OOE > RE > Toco during storage of SBO at 25 ℃ and 40 ℃. It is noteworthy that 0.1% OOE showed stronger antioxidant activity than the commercial natural antioxidants, RE at the manufacturer’s recommended highest concentration (0.1%) and Toco at 0.1%.