Effects of thymoquinone on the oxidative stability of refined sunflower oil and corn oil

In this study, the effect of thymoquinone on the oxidative stability of sunflower and corn oil was investigated. 2,2′-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, reducing power, antioxidant activities with 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) test, β-carotene bleaching test and conjugated diene method in linoleic acid model system were determined at different concentrations (10, 25, 50, 100, 250, 500, 1000 ppm) of thymoquinone solutions and compared with those of BHA, BHT and α-tocopherol. Antioxidant activity of thymoquinone increased as the concentration increased in all antioxidant activity methods. In general, thymoquinone had lower antioxidant activity than BHA, BHT and α-tocopherol. Refined sunflower oil and corn oil were purified by adsorption on activated carbon followed by column chromatography method. The initial specific extinction (K232, K268) values, peroxide values, p-anisidine values, total tocopherol contents and induction periods of purified sunflower and corn triglycerides were lower than refined sunflower and corn oils. Induction periods of sunflower oil triglycerides and corn oil triglycerides supplemented with 1000 ppm thymoquinone were 8.85 and 5.30 hours, respectively, while triglycerides supplemented with 100 ppm BHT were 15.61 and 15.01 hours, respectively in Rancimat. In accelerated oxidation test performed at 60 °C, refined oils were stored for 21 days and purified oils were stored for 7 days. Oxidation was investigated with K232 and K268 values, peroxide value, p-anisidine value and total content of tocopherol. In the oxidation of refined oils and triglycerides, the primary and secondary oxidation products increased over time, while the total amount of tocopherol decreased during the storage period. As in the antioxidant activity tests, thymoquinone showed a concentration-depended activity in oxidation tests. In oven test, the effect of BHT was low in refined oils, whereas in triglycerides it was very high. Similarly, the effect of thymoquinone on oxidative stability was more pronounced in purified triglycerides than refined oils. In general, all additives were more effective in refined corn oil than in refined sunflower oil.