Effect of the antioxidant activity of methanolic and acetonic extracts of olive leaves on the shelf life of soybeen oil

Different intrinsic and extrinsic factors may initiate the oxidation of lipids. The initial products of oxidation are tasteless and odorless and after degradation and production of secondary products, the off-flavors and off-odors will appeare in edible oils. This is a great concern in food industry, because it decreases the shelflife of food products. Free radicals are produced during chain reactions in lipid oxidation process. To avoid this, synthetic antioxidants are usually used which are sensible to heat and are hazardous to human health and may cause cancer. Polyphenols have antioxidant activity and absorb free radicals. Thus, the vegetable oils rich in polyphenols can affect human health. In this research, we aimed to investigate the application of natural extract of olive leaves as a natural antioxidant to avoid soy oil oxidation and to compare it with synthetic antioxidants. Matherials and In this project, methanol and acetone solvents in an ultrasonic apparatus were used to extract the phenolic compounds of the leaves of Mary and Yellow varieties of olive. The amount of phenolics, as well as their antioxidant effect on the oxidation of crude soybean oil, was investigated. The phenolic contents were measured by Folin–Ciocalteu method and the antioxidant activities of the extracts were determined by two controlling methods of DPPH radical-scavenging activity and Ferric reducing power, and then the results were compared with antioxidant activities of synthetic antioxidants.The antioxidant effects of different concentrations of the extracts on delaying the oxidation of crude oil were studied by determining their peroxide and thiobarbitoric acid values. The leaves of Mary variety showed the highest amount of phenolics (101.382mg/g), as determined by measureing the phenolic contents. The results suggested that the methanolic extract of Mary variety had a better antioxidant activity than the other natural antioxidants, compared to synthetic antioxidants BHT and TBHQ. In addition, it was shown that the radical scavenging activity was directly correlated with the increase of the extract concentration. Then the methanolic extract of Mary variety at three levels 100, 200 and 500 ppm and BHT and TBHQ antioxidants at 100 and 200 ppm were added to antioxidant-free soy oil. The control oil, which lacked antioxidants, showed the highest peroxide and thiobarbitoric acid values. The methanolic extract of the Mary variety showed the highest extraction efficiency as well as the most antioxidant activety. In addition, it was shown that the radical scavenging activity was increased by the increase of extract concentration. The methanolic extract of Mary variety at the concentration of 500 ppm was comparable with the synthetic antioxidants at the levels of 100 and 200ppm.