The stability of lipid oxidation of fat-rich food products is one the most important factors to preserve the quality of food products. Therefore this study aimed to enhance the oxidative stability of Nigella sativa oil (NS) oil for 15 days at 50^°C, by using different concentrations of extracts from olive leaves (OL), tomato by-product (TP), orange peel (OP) and water melon peel (WP) as naturally bioactive compounds, in addition to assess the quality of mayonnaise containing NS oil as a functional ingredient.  The natural phenolic compounds of these agro- industrial wastes were extracted and identified using HPLC. The results showed that OL extract had the highest total phenolic content (121.09 mg GAE/g dw) which represented about 1.76, 3.91 and 13.34 times higher than the phenolics in the TP, OP and WP extracts, respectively. The results of the oxidative stability measurements indicated that the addition of all extracts to NS oil retarded the oxidative rancidity compared to the control as measured by the peroxide value, p-anisidine value, and thiobarbituric acid. Infrared analysis demonstrated the antioxidant power of the OL extract at 400 ppm and the TP at 600 ppm in preventing rancidity conditions, which were similarly effective as the synthetic antioxidant Tertiary-butyl hydroquinone (TBHQ) (200 ppm). This demonstrated the potential for using them as natural antioxidants to prevent rancidity in stored oil. The results of quality and sensory evaluation of NS oil mayonnaise showed an increase in viscosity and a change in colour parameters that were imperceptible to the human eye as the ΔE values were less than 3, and all mayonnaise products were “accepted" by the panelists.
Keywords: Lipid oxidation, Rancidity, Functional ingredients and   Synthetic antioxidant.
The stability of lipid oxidation of fat-rich food products is one the most important factors to preserve the quality of food products. Therefore this study aimed to enhance the oxidative stability of Nigella sativa oil (NS) oil for 15 days at 50^°C, by using different concentrations of extracts from olive leaves (OL), tomato by-product (TP), orange peel (OP) and water melon peel (WP) as naturally bioactive compounds, in addition to assess the quality of mayonnaise containing NS oil as a functional ingredient.  The natural phenolic compounds of these agro- industrial wastes were extracted and identified using HPLC. The results showed that OL extract had the highest total phenolic content (121.09 mg GAE/g dw) which represented about 1.76, 3.91 and 13.34 times higher than the phenolics in the TP, OP and WP extracts, respectively. The results of the oxidative stability measurements indicated that the addition of all extracts to NS oil retarded the oxidative rancidity compared to the control as measured by the peroxide value, p-anisidine value, and thiobarbituric acid. Infrared analysis demonstrated the antioxidant power of the OL extract at 400 ppm and the TP at 600 ppm in preventing rancidity conditions, which were similarly effective as the synthetic antioxidant Tertiary-butyl hydroquinone (TBHQ) (200 ppm). This demonstrated the potential for using them as natural antioxidants to prevent rancidity in stored oil. The results of quality and sensory evaluation of NS oil mayonnaise showed an increase in viscosity and a change in colour parameters that were imperceptible to the human eye as the ΔE values were less than 3, and all mayonnaise products were “accepted" by the panelists.