3.5.4. Oxidative stability index (OSI)
The results showing the effect of Senjed peel aqueous extract compared to positive control (TBHQ) and negative control (T0) samples on the oxidative stability of cold-pressed sesame oil are presented in Fig. 4. There was statistically significant differences between different treatments (p<0.05 ). As expected, samples without the addition of any antioxidants (T0) were oxidized easily, and indicated the lowest OSI values (9.05 h). In contrast, samples containing synthetic antioxidants (TTBHQ) and the highest concentration of extract (T750) showed the highest index of oxidative stability index (both 15.39 h). As can be seen, with increasing the concentration of the extract, the oxidative stability of the oil samples increases and at a concentration of 750 ppm, it is equivalent to a sample containing synthetic antioxidant TBHQ. Except T100 treatment, the OSI values of oils with added Senjed peel aqueous extract or synthetic antioxidants were found to be significantly larger (p< 0.05 ) than that of the negative control oils (T0). Incorporation of soybean, cottonseed and rice bran oils with rosemary extract resulted significantly (p< 0.05 ) higher OSI values than that of oil with added synthetic antioxidant (BHA + BHT) (Yang et al., 2016).