Antioxidant activity in other vegetable oils
Antioxidant activity of phenylalanine K was also compared with that of phenylalanine in other vegetable oils. High oleic oils including olive oil, canola oil, HO SBO, and avocado oil, which draw great interest as frying oils due to their oxidative stability, were chosen in this study. Corn oil, which is widely used as frying oil, was also chosen. Table 2 shows fatty acid composition and tocopherols in these oils. Trace amounts of minor fatty acids including C11:0, C14:0, C15:0, C17:0, C18:2 (c9, t12), C18:2 (t9, c12), C18:3 (9,12,15), and C20:1 (c11) were also detected in some oils, but they are not listed in Table 2.
Table 3 shows the heating study results with 5.5 mM phenylalanine or phenylalanine K in these vegetable oils at 180 °C. Due to the very small bisallylic H signal in olive oil, canola oil, HO SBO, and avocado oil, loss of allylic H was determined instead of bisallylic H for these oils. Phenylalanine K had significantly stronger antioxidant activity than phenylalanine in olive oil and HO SBO. It showed slightly stronger activity than phenylalanine in canola oil, avocado oil, and corn oil. It is noteworthy that phenylalanine K had very strong antioxidant activity in HO SBO showing very low losses of olefinic H and allylic H (2.43% and 0.95%, respectively) and PTAG (1.26%) compared the control oil (13.27% loss of olefinic, 9.62% loss of allylic H, and 9.45% PTAG).
Understanding what made phenylalanine K more effective in some oils than in other oils would be very useful for the development of new natural antioxidants. For that, correlation tests were conducted between loss of olefinic H and PTAG using %improvements from control and three major fatty acids (C16:0, C18:1 (c9), and C18:2 (c9,12)), ratio of saturated fatty acids to unsaturated fatty acids, three major tocopherols (α-, γ-, and δ-), and total tocopherols (Table 4). No correlation of the antioxidant activity with any fatty acid or the ratio of saturated fatty acids to unsaturated fatty acids was observed. In contrast, δ-tocopherol had relatively strong positive correlations with loss of olefinic H (r = 0.8823, p = 0.0200) and PTAG (r = 0.8899, p = 0.0175) of phenylalanine and loss of olefinic H (r = 0.8432, p = 0.0349) and PTAG (r = 0.8225, p = 0.0445) of phenylalanine K. γ-Tocopherol had weak positive correlations with the antioxidant activity (r = 0.5869, p = 0.2207 and r = 0.6046, p = 0.2036 for loss of olefinic H and PTAG of phenylalanine, respectively, and r = 0.5305, p = 0.2789 and r = 0.3009, p = 0.5623 for loss of olefinic H and PTAG of phenylalanine K, respectively). The correlations with total tocopherols were weak but positive, indicating that the activity of phenylalanine and phenylalanine K is likely related to the concentration of tocopherols in oil. It is interesting that α-tocopherol had negative correlations with loss of olefinic H (r = -0.9200, p = 0.0094) and PTAG (r = -0.7397, p = 0.0928) of phenylalanine and loss of olefinic H (r = -0.8756, p = 0.0222) and PTAG (r = -0.7194, p = 0.1071) of phenylalanine K. This was likely caused by the fact that the two oils, olive and avocado oils, with a high content of α-tocopherol happened to have the lowest total tocopherols. Correlation tests in this study had a limitation that tests were conducted with only six oils, but the interesting trends found in this study could be further investigated, which can be important for the application of amino acid salts in frying oil.