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.