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.