Study On The Minor Components In Different Vegetable Oils And Their Relation With Antioxidant Capacity

Minor components such as tocochromanols, polyphenols, phytosterols consist an very important part in vegetable oils. Though minor in composition, these components which can exert major influence on the oxidative stability of vegetable oils are closely related with the quality of oils. Oxidation can cause the degradation of vegetable oils, so the antioxidant capacity in vitro can be used as an index to evaluate the quality of vegetable oils indirectly. In this study the fatty acid composition and minor components of several vegetable oils were analyzed, the antioxidant capacity of vegetable oils were investigated, and then the potential associations of antioxidant capacity with minor components of vegetable oils were discussed to provide a data base for the comprehensive evaluation of edible oils.Firstly, the compositions of 13 different vegetable oils samples include fatty acid and minor components were analyzed qualitatively and quantitatively by GC, HPLC and GC-MS. The minor components in vegetable oils can be categorized into three main parts include tocochromanols, polyphenols and phytosterols. The total tocopherol content of 13 vegetable oils ranged from 173.21 to 2573.69 mg/kg, the α- and γ-tocopherols were the most abundant in most oil samples. Wheat germ oil contained the highest content of tocopherol 2573.69 mg/kg. Soybean oil contained γ-tocopherol and δ-tocopherol. Rice bran oil and palm oil also contained tocotrienols. The total polyphenol content of 13 vegetable oils ranged from 4.78 to 387.40 mg gallic acid /kg oil. Sesame oil had the highest content of polyphenol.β-sitosterol, stigmasterol and campesterol are the main phytosterol in 13 different vegetable oils. β-sitosterol is the most abundant sterol in all oil samples. Wheat germ oil contained the highest content of phytosterols. Some vegetable oils also contained other minor components such as squalene and γ-oryzanol. The different composition creates the different qualities of vegetable oils.Secondly, the quality of different oil samples was evaluated by Rancimat. It showed that several oil samples with high content of minor components have longer induction time than others. Then the antiradical capacity of polar fraction of oils were evaluated by ORAC, ABTS, FRAP, and DPPH method in methanol. The results show that vegetable oils which contain large amount of minor components such as olive oil, sesame oil and wheat germ oil also have better antiradical capacity. On the contrary, camellia oil and coconut oil show no antiradical activity. The antiradical capacity of non-polar fraction of in vegetable oils and whole oils were evaluated by DPPH method in ethyl acetate. It shows that wheat germ oil, rice bran oil and soybean oil which contained large amounts of tocopherol have good antiradical capacity. The correlation between the total polyphenol and antiradical capacities measured by four methods were high and the highest with ORAC. It suggested that the polyphenol was the main antioxidants in the polar fraction of vegetable oils. The correlation between the total tocopherol and antiradical capacities measured by DPPH was statistically significant(P<0.01). It suggested that the tocopherols were the main antioxidants in the non-polar fraction of vegetable oils.Finally, the variation of minor components and antioxidant capacity were monitored during 4 months storage under 25 ℃, light conditions. The tocopherols in vegetable oils decreased during storage, α-tocopherol decreased faster than γ-tocopherol. The total polyphenol decreased during storage. The phytosterol showed no variation during storage. The antioxidant capacity of polar fraction and non-polar fraction decreased during storage. The polyphenols had a high correlation with the antiradical capacity of polar fraction during storage. The tocopherols in vegetable oils had a high correlation with the antiradical capacity measured by DPPH in ethyl acetate during storage.