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.