Retention of tocopherols
In the previous study (Hwang et al., 2017), it was found that amino
acids having stronger antioxidant activity more effectively retained
tocopherols in SBO, and that retention of tocopherols was one important
factor for effectiveness in preventing the oxidation of oil. To examine
whether the stronger antioxidant activity of amino acid salts over the
corresponding amino acids was attributed to better retention of
tocopherols during heating studies, tocopherols in SBO containing 5.5 mM
alanine, alanine K, phenylalanine, or phenylalanine K were analyzed
after 2, 4, and 8 h at 180 °C.
Figure 2 shows the concentrations of three tocopherols (α-, γ-, and
δ-tocopherols) remained in SBO during heating. Average values at 2 h and
4 h were compared by Tukey-Kramer Honestly Significant Difference test
(P < 0.05). At 4 h, no
α-tocopherol remained in the
control SBO while 7.2% γ-tocopherol and 35.3% δ-tocopherol were left.
Alanine did not improve the retention of α-tocopherol, but slightly
improved the retention of γ-tocopherol and δ-tocopherol at 2 h (Figs.
2a-2c). At 4 h, SBO with alanine did not have higher concentrations of
tocopherols than the control and the concentration of δ-tocopherol was
even lower than that in the control at 8 h. This result is somewhat
different from the previous study (Hwang et al., 2017), which reported
that alanine was effective in retaining tocopherols throughout the
course of the heating study. This difference might have been caused by
different sources of alanine and SBO and the heating environments such
as humidity. Nevertheless, it is noteworthy that alanine K retained the
three tocopherols significantly better than alanine (Figs. 2a-2c)
indicating that the stronger antioxidant activity of alanine K than
alanine is attributed to better retention of tocopherols in SBO.
Phenylalanine effectively retained tocopherols except for δ-tocopherol
at 8 h (Figs. 2d-2f). Phenylalanine K retained tocopherols significantly
better than phenylalanine confirming that the stronger antioxidant
activity of potassium salt of amino acids is attributed to better
retention of tocopherols than the corresponding amino acids.