ANTIOXIDANT EFFECTS OF D-TOCOPHEROLS AT DIFFERENT CONCENTRATIONS IN OILS DURING MICROWAVE-HEATING

The effects of d-tocopherols at different concentrations (50 to 1000 ppm) on the oxidative stability of ethyl linoleate and tocopherol-stripped oils were investigated under microwave heating conditions. Purified substrate oils were prepared by aluminum oxide column chromatography. After the addition of tocopherols (alpha-, beta-, gamma- or delta-) to the oils, peroxide, carbonyl and p-anisidine values were measured in the samples after heating in a microwave oven. Further, the residual amount of tocopherol homologues in the oils after heating was determined by high-performance liquid chromatography for evaluation of their effects at different concentrations on oxidative deterioration. Microwave heating resulted in some acceleration in the oxidation of the purified substrate oils. Optimum concentrations of tocopherols required to increase oxidative stability were 100 ppm for alpha-, 150-200 ppm for beta- or gamma- and 500 ppm for delta- tocopherol, respectively. The antioxidant effect of tocopherols decreased in the order alpha> beta is-approximately-equal-to gamma > delta at each level, in all substrates. Therefore, alpha-tocopherol was consumed first, followed by beta- or gamma-tocopherol, and delta-tocopherol was consumed more slowly. The tocopherols had no further significant antioxidant activity (P > 0.05) at concentrations higher than 500 ppm.