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.