Determination of the effect of tocopherol isomers and thymoquinone on the oxidative stability of purified black cumin (Nigella sativa) oil

In this study, the effect of minor constituents (α-, γ-tocopherol and thymoquinone) and the binary and ternary mixtures of these constituents on the oxidative stability of purified black cumin oil was evaluated. A purified black cumin oil was spiked with several concentrations of these compounds and, then, subjected to an accelerated oxidation tests in a Rancimat apparatus at 90 °C and in an oven at 60 °C. The induction periods of black cumin triacylglycerols containing minor constituents were higher than that of purified black cumin oil. K232, K270, conjugated diene content, peroxide value, p-anisidine value and polymer triglyceride content of oil samples were measured during storage at 60 °C. Triacylglycerols containing individual and binary mixture of γ-tocopherol were more stable against oxidation than control. Especially, ternary mixture of minor constituents were more effective to retard oxidation than the binary mixtures. During storage at 60 °C, α-tocopherol disappeared fastly followed by γ-tocopherol. The more resistant minor compound towards oxidation was thymoquinone and a little decrease was observed at the end of storage. Addition of thymoquinone to purified triacylglycerols with tocopherol isomers was retarded the decrease of tocopherol isomers. To determine the effect of tocopherol different concentrations on the synergistic effect between the tocopherol isomers and thymoquinone, tocopherol isomers and thymoquinone (1000 mg/kg) were added to the pure triglycerides at 50 and 100 mg/kg, and the samples were subjected to the oxidation test in an oven at 60 °C. K232, K270 and conjugated diene values of oil samples taken during oxidation were observed to increase. According to the results, increasing concentration of tocopherol isomers resulted in increased oxidative stability when used alone or in binary mixtures.