Oxidative and thermal stabilities
Oxidative stability index (OSI) measured according to AOCS standard
method Cd 12b-92 is one of the most widely used methods to determine
susceptibility to oxidation of edible oils (Márquez-Ruiz et al., 2008).
TGA determining the thermal stabilities of oils has been used to predict
the onset of oil oxidation during
heating (Borugadda & Goud, 2014; Gao & Birch, 2016; Kalam et al.,
2017; Mokbli et al., 2018). Thermal stability of oil can be determined
by the temperature where 5 or 90% mass loss occurs (Mokbli et al.,
2018) or by the onset temperature of weight loss, which is defined as
the temperature where oil starts to decompose (Kalam et al., 2017).
These two methods were used to evaluate oxidative stabilities of
silflower oil in this study. Table 3 shows that the OSI at 110 °C of
silflower oil (2.42 h) and the onset temperature of degradation of
silflower oil (282.5 °C) were significantly lower than those of
sunflower oil (4.35 h and 298. 2 °C). Apparently, these results
indicated that oil extracted from unhulled silflower seed with hexane
had a lower oxidative stability than that from unhulled sunflower seed.
The two major factors determining the oxidative stability of oil are the
fatty acid composition and antioxidants present in the oil (Gao &
Birch, 2016). The amount and activity of antioxidants in
oil may be more important factors
than the fatty acid composition for the different oxidative stabilities
of these two oils because their fatty acid compositions were very
similar as shown in Table 1. Silflower oil contained higher total
phenolics than sunflower oil, but lower α-tocopherol (Table 1).
Therefore, the tocopherol level might have a significant effect on the
oxidative stability of these oils. In addition, another factor to
consider for the oxidative stability of these oils is the chlorophyll
level because chlorophyll is known to have prooxidant activity in edible
oils (Usuki et al., 1984), and silflower oil contained much higher
chlorophyll than sunflower oil. The level of chlorophyll can be
controlled by some ways such as harvest time, ripening stage, and
storage conditions (Yilmaz & Gökmen, 2016) and also by oil refining
processes. In this study, a few refining processes were evaluated to
decrease the level of chlorophyll and then, oxidative stabilities of
refined oils were evaluated.