Effect of Thymus vulgaris L. Essential oil on Oxidative Stability of Virgin Olive Oil and its Synergy with Citric Acid

Since usage of synthetic antioxidants has been under question due to their toxicity and possible carcinogenicity, there is still interest in developing plant-derived natural antioxidants, especially from edible plants. This study investigated the effect of Thymus vulgaris L. essential oil on virgin olive oil oxidation in comparison with tocopheryl acetate and butylated hydroxytoluene (BHT). Furthermore, the synergistic activities of citric acid with natural and synthetic antioxidants were determined. T. vulgaris essential oil was added to the virgin olive oil at a concentration of 1000 ppm. The BHT and tocopheryl acetate were added to the virgin olive oil at 100 ppm concentration. Virgin olive oil samples were stored at 60°C for 16 days. The peroxide value, p-anisidine value, K232, and K268 values were measured every 4 days. Changes in chlorophyll and carotenoid contents of virgin olive oil samples were determined at the beginning and end of storage period. T. vulgaris essential oil significantly reduced the peroxide value, p-anisidine value, K232, and K268 values of virgin olive oil. The effect of T. vulgaris essential oil on retarding virgin olive oil oxidation was similar to the effect of BHT. By comparison, tocopheryl acetate was less effective in reducing the oxidation of virgin olive oil. Furthermore, T. vulgaris essential oil could preserve chlorophyll and carotenoid of virgin olive oil during storage period. T. vulgaris essential oil can be applied to increase the oxidative stability of virgin olive oil, and can protect the chlorophyll and carotenoid of virgin olive oil.