CONCLUSIONS
Converting the carboxylic acid group of an amino acid into its sodium
salt (-COO-Na+) or potassium salt
(-COO-K+) increased its
effectiveness in preventing the oxidation of SBO at 180 °C. The
antioxidant activity order of glutamic acid salts was di-sodium salt
> mono sodium salt > glutamic acid further
confirming that removing the negative effect of the carboxylic acid
group increased the antioxidant activity. Potassium salts had stronger
antioxidant activity than sodium salts, presumably, due to their higher
lipophilicity than sodium salts. Alanine K, phenylalanine K and proline
K (5.5 mM) were more effective in preventing the oxidation of SBO than
TBHQ (0.02%, 1.1 mM) at 180 °C. It was found that the stronger
antioxidant activity of amino acid salts than the corresponding amino
acids was attributed to their more effective retention of tocopherols in
SBO. A study with mixtures of mixed tocopherols and alanine K,
phenylalanine K, or proline K in stripped SBO indicated that the
stronger antioxidant of amino acid salts than the corresponding amino
acids was attributed to the synergistic effect with tocopherols or their
stronger intrinsic activity. The strong activity of amino acid potassium
salts was also confirmed in a frying study. Phenylalanine K was much
more effective than phenylalanine in preventing the oxidation of other
vegetable oils such as olive oil and high oleic soybean oil. It also had
slightly stronger activity than phenylalanine in canola oil, avocado
oil, and corn oil. This study showed that converting the carboxylic acid
group of an amino acid to a sodium or potassium carboxylate group
significantly improved the antioxidant activity of amino acid at frying
temperatures. Further studies will focus on the preparation of more
amino acid salts, their activities, and their effects on sensory
properties of fried foods and on the formation of toxic volatiles.