Synergistic effect with tocopherols
The previous study (Hwang et al., 2017) found that the activity of amino acids in the absence of tocopherols was negligible while it remarkably increased in the presence of tocopherols, which indicated that the synergistic effect with tocopherols was an important mechanism for antioxidant activity of amino acids at frying temperatures. In this study, the synergistic effect of amino acid salts with mixed tocopherols was compared to that of the corresponding amino acids to examine if the improved activity of the amino acid salts was related to the synergism. The study was conducted by adding known amounts of mixed tocopherols and amino acid or amino acid salt in stripped SBO where inherent tocopherols and other polar compounds were removed. Since the previous study showed that the synergistic effect was most clearly shown when the ratio of amino acid and tocopherol was 1:1 and the heating time was 2 h (Hwang et al., 2017), these conditions were used in this study. The three strongest antioxidants, alanine K, phenylalanine K and proline K were chosen for this experiment.
Figure 3 shows results of the heating study with alanine K, phenylalanine K, and proline K with or without mixed tocopherols in stripped SBO in comparison with the corresponding amino acids. The loss of bisallylic H, of which the trend was very similar to that of olefinic H was not presented in Figure 3. Activities of amino acids and amino acid salts were very low in the absence of tocopherols while their 1:1 mixtures with tocopherols had strong antioxidant activity was consistent with the previous report while their effectiveness was not as strong as arginine and lysine (Hwang et al., 2017). From Figures 3a and 3b, it was found that the antioxidant activity of alanine K did not differ from that of alanine in the absence of tocopherols, but it was significantly stronger when combined with the same molar concentration of tocopherols indicating that synergism with tocopherols was a cause for the improved antioxidant activity of amino acid salts. The loss of olefinic H in phenylalanine and phenylalanine K treatments (Fig. 3c) followed the same trend indicating that phenylalanine K had greater synergism with tocopherols than phenylalanine. However, PTAG of phenylalanine K (Fig. 3d) did not show this trend. Since the effectiveness of 2.75 mM phenylalanine K without tocopherols in preventing the formation of PTAG was significantly greater than that of 2.75 mM phenylalanine, it could not be concluded whether the stronger activity of phenylalanine K than phenylalanine in the presence of tocopherols was due to the greater synergism or just due to an additive effect. Proline K without tocopherols also showed significantly stronger antioxidant activity than proline alone (Figs. 3e and 3f). Proline K also had stronger activity than proline in the presence of tocopherols, but the magnitude of the difference between results of proline K and proline was not greater than that in the absence of tocopherols. This indicates that proline K did not have greater synergism with tocopherols than proline. The intrinsic antioxidant activity was improved by converting the carboxylic acid group to potassium carboxylate group of proline, which was shown in the absence of tocopherols, and the improved activity was directly reflected in its antioxidant activity in the presence of tocopherols. Therefore, with the given data, it seemed that an improved synergistic effect with tocopherols or an improved intrinsic antioxidant activity of amino acid salt played a role in the improved activity of amino acid salts, in addition to their better retention of tocopherols discussed in Section 3.3. Further studies on amino acid salts are needed to better understand their activity and mechanisms.