In the case of trans isomers, small changes were observed in
their formation at lower frying temperatures (≤200 C, 8 h)
(Table 3). However, when comparing the relative percentage of
TFAs during the frying sessions at a higher temperature of 250 C, a
clear increase in TFAs with 18 carbon atoms was observed (Fig. 2).
The total contents of trans C18:2 isomers in frying oils were
significantly increased with an increase in the length of deep frying
time (P < 0.05). The final concentrations of total trans C18:2 in fried
palm oil was 3.64 mg/g at 250 C, which was increased by 53.02%. A
linear relationship between the amount of total trans-linoleic acids
and the frying time was also observed ( R2 = 0.984). Whereas the
trans C18:1 isomer appeared only after 4 h frying at 250 C in palm
oil, the trans C18:3 isomers were found after 2 h frying at 250 C.
Once produced, the amount of ctt/cct C18:3 was found to be very
stable afterward throughout the repeated frying process. In the
case of 9t 12c 15c C18:3, its accumulation increased slightly with the
increasing frying cycles. The formation of trans isomers at elevated
temperatures indicated that a specific amount of energy was
required to transfer double bonds from cis to trans configuration,
and when the numbers of cis double bonds increased, the activation
energy for isomerization decreased. This result was consistent
with a previous study (Tsuzuki et al., 2008), which found a substantial
increase in trans 18:2 FAs and in trans 18:3 FAs in safflower
oil, when the oils were heated at 250e350 C.