Deterioration of natural antioxidant such as phenolic compounds
and tocopherols is observed when virgin olive oil and sunflower oil
are heated repeatedly [34]. Evuen et al. [35] conducted a study to
investigate the toxicological effects of heating of vegetable oils on their
natural antioxidant levels. The oils were repeatedly heated for three
consecutive days. Refined, deodorized palm olein, groundnut oil,
congealed and locally made vegetable oil samples showed a reduction
in alpha-tocopherol and beta-carotene levels as the frying oils were
repeatedly heated [35]. The effect of antioxidants on the stability of
rapeseed oil during heating at 80°C and during deep-fat frying were
evaluated by determination of the production of polymers, its peroxide
index and tocopherol content [36]. Repeated heating reduced the
stability of the rapeseed oil, with a lowering of the tocopherol content
and an elevation in the levels of lipid peroxidation products. A study
carried out by Koh et al. [37] demonstrated that with increased frying
cycles, antioxidant activities reduced significantly in palm oil and
rice bran oil. Tocotrienol and tocopherol concentrations decreased
in both vegetable oils. However, it was reported that tocotrienol is
more susceptible to degradation when compared to tocopherol.
Both vitamin E homologues are potent antioxidants. Nevertheless,
tocotrienol was shown to possess greater antioxidant capacity [31,32].