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.