Antioxidant Activity of Methanolic Extract of Olive Leaf on Oxidative Stability of Sunflower Oil

Approximately 90% of the fatty acids in sunflower oil are unsaturated. This study evaluated the antioxidant activity of leaf methanol extract of four Iranian olive cultivars on the oxidative stability of sunflower oil. Leaf methanolic extracts of four Iranian olive cultivars (Zard, Roghani, Shiraz, and Dezfool) were prepared by microwave extraction method. Total phenolic content, diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity, and reducing power of the extracts were determined. Peroxide and anisidine values of sunflower oil treated with the extracts were measured during 30 days of storage at 60°C. The concentration of methanolic extract of the Roghani cultivar (46.27±2.76 μg/mL) required to scavenge 50% of the initial DPPH radicals (IC50) showed a significant difference (p<0.05) with the IC50 of butylated hydroxytoluene (BHT) (112.90±14.81 μg/mL), meaning that the percent inhibition of the DPPH radical of the methanol extract of the Roghani cultivar was higher than that of BHT (p<0.05). During the 30 days storage period, sunflower oil samples without antioxidants showed significantly higher anisidine and peroxide values than samples treated with olive leaf extracts (p<0.05). In all treated samples, as the concentration of the extracts increased, peroxide and anisidine values significantly decreased (p<0.05). This study showed that the methanolic extracts of olive leaves had the ability to limit the oxidation of lipids and can be considered as a potential antioxidant source of natural origin. The methanolic extract of Roghani cultivar had the highest effect on the oxidative stability of sunflower oil.