Regarding the stability of tocopherol, for pure soybean oil at high temperature, the literature suggests the following stability sequence: d > g > b > a (Kamal-Eldin and Appelqvist 1996). However, we found that, soybean oil free from antioxidants, stability decreased in the order d > b > a = g. Steel et al. (2005) also found a greater degradation of a and g-tocopherol in soybean oil that had been heated for 10 h. A rapid degradation of a-tocopherol was reported by Verleyen (2001). Furthermore, Warner and Gehring (2009) observed a higher degradation of a- and d-tocopherol in soybean oil subjected to 65 h of frying. Rosemary extract increased the stability of the tocopherol fractions to a considerable extent. The difference in stability between the tocopherols was evident in the oil samples that contained rosemary extract, and the sequence of stability was changed to d > a > b > g. When reducing losses of a-tocopherol, the stabilizing effect of the rosemary extract is particularly marked, and it is clear that the rosemary extract was slightly more active than the TBHQ at the concentrations used. Gordon and Kourimská (1995) reported that rosemary extract has the ability to slow the loss of tocopherols that are naturally present in vegetable oils. No synergistic effect was observed between rosemary extract and
TBHQ, because the treatment with only the natural antioxidant resulted in higher amounts of tocopherols.