Importance of the higher retention of tocopherols and sterols for the oxidative stability of soybean and rapeseed oils.

Tocopherols and sterols were lost considerably during the refining process of vegetable oils, resulting in a dramatic decrease in the oxidation stability. However, the oxidation stability of vegetable oils was not directly proportional to tocopherol content, not mention to its synergistic interaction with sterol. Based on the peroxide values of oils with different content of tocopherols and sterols during storage at 65 °C, it was found that soybean oil (SO) had the best stability when the content of tocopherols and sterols was 0.14 and 1.09%, respectively, whereas the value in rapeseed oil (RO) was 0.06% and 1.14-2.90%. The optimal content of tocopherol in RO was lower than that in SO was due to the different tocopherol isoforms. Furthermore, it was found that the storage stability decreased significantly when adding a same content of α-tocopherol, compared with that by retaining more tocopherol isoforms existed in SO. It was suggested that retaining more tocopherol and sterol during vegetable oils refining improved the oxidation stability, however, it did not mean the more the better. The oxidation stability was dependent on both the content and the isoform. This study helps to define the effective contents of tocopherol and sterol under the moderate refining technology.