Additional bleaching processes
Since silflower oil had very high chlorophyll even after the general refining process, silflower oil was further refined to reduce the level of chlorophyll and evaluated for the oxidative stability. Treatments with acid-activated bentonite (Sarioğlan et al., 2010; Srasra et al., 1989), acid-activated sepiolite (Saneei et al., 2015), and Tonsil® (Sabah & Çelik, 2005) have been reported to be effective to remove chlorophyll, and therefore, these methods were employed in this study. Table 6 summarizes the properties of silflower oils that were refined by these three methods. Bentonite was the most effective in removing chlorophyll (4.2 mg/kg) followed by Tonsil® (38.5 mg/kg), and then sepiolite (474.5 mg/kg). Since oil samples had been exposed to oxidation during the multiple refining processes, peroxide value (PV) and conjugated diene value (CDV) were determined from this point to examine oil oxidation. PV and CDV decreased after the treatment, rather than increased. PV was significantly lowered by bentonite and Tonsil® while it was only slightly decreased by sepiolite. CDV significantly decreased by all the three methods. Total phenolics value and TPC also decreased after further refining. Radical scavenging ability determined by ABTS assay increased for silflower oil that were treated with bentonite and Tonsil®. The ABTS assay slightly (not statistically significantly) decreased after the treatment with sepiolite. The level of α-tocopherol decreased after the treatments with sepiolite while the increased by bentonite and Tonsil®. Again, some studies found that the contents of tocopherols could increase after refining processes (Pestana et al., 2008; Rossi et al., 2001; Van Hoed et al., 2006) although the mechanism for an increase in tocopherols was not well known. The FFA level increased after treating with bentonite while it decreased by sepiolite and remined similar after the treatment with Tonsil®.
Oxidative stability indicated by OSI decreased by refining with bentonite and sepiolite while it slightly increased by Tonsil®. Thermal stability of silflower oil significantly increased after the treatments with all three bleaching agents. The thermal stability of silflower oil treated with Tonsil® was slightly higher than refined sunflower oil. However, OSI values of further refined silflower oils were lower than refined sunflower oil. Further studies are needed to understand the factors affecting the oxidative stability of silflower oil and to improve the properties of silflower oil.
The color of further bleached silflower oils shown in Figure 3 well reflected the amount of chlorophyll. Chlorophyll content had statistically significant (p <0.05) correlation coefficients with L* (r = -0.995) and a* (r= 0.995) values of the further refined oils. Silflower oil refined with bentonite had the lightest color followed by that with Tonsil®, and then that with sepiolite, which was also shown in lightness (L* ) in Table 7. The negative a* values for oils treated with bentonite and Tonsil® indicates that other compounds responsible of red color were also removed along with chlorophyll.