Endogenous antioxidants contained in vegetable oil provide a
natural resistance to oxidative deterioration. The antioxidant activity
of the phenolic extract of virgin olive oil was found to be very low
after the sixth frying process [28]. Cooking oil is more susceptible
to oxidation following repeated heating due to the increased
concentrations of polar compounds, and oxidized triacylglycerol
monomers, dimers and polymers [28]. Similarly, total loss of
antioxidant activity due to deep-fat frying after the 12th frying
processes has been reported [28]. Vitamin E consists of tocopherols
and tocotrienols isomers, which are the major antioxidants of
vegetable oils [29]. Adam et al. [30] reported that reheating palm and
soybean oils significantly reduced the content of the various vitamin E
fractions. The stability of the vitamin E isomers varies during heating
because it depends on the type of oil and the content of vitamin E
in those edible oils. Palm oil is rich in tocopherols and tocotrienols.
Tocotrienol has been exhibited to have more potent antioxidant
activity [31,32] than tocopherol, which is found in soybean oils. In
addition, soybean oil is high in PUFA content compared to palm oil,
which has approximately 1:1 ratio of saturated and unsaturated fatty
acids with lower PUFA levels. Hence, soybean oil is more prone to
oxidation than palm oil following repeated heating [33].