Amino acid salts
Figure 1 shows the reaction of amino acids with NaOH or KOH to produce amino acid salts and molecular structures of amino acids used in the present study. Although the apparent change by this reaction was to convert the carboxylic acid group to sodium or potassium carboxylate group, since amino acids exist as Zwitterions in a neutral aqueous solution and in solid state (Sinha, et al., 2013), the other way to describe this reaction is that the ammonium group (-NH3+) of the Zwitterion was converted to an amino group (-NH2) while the cation of the base (Na+ or K+) bound the carboxylate (-COO-). We hypothesized that these amino acid salts would have stronger antioxidant activity than the corresponding amino acids because the resultant amino group can inhibit the oxidation of oil by its radical scavenging activity and by antioxidants produced from reactions with oxidation products (Farag, et al., 1978; Choe & Min, 2009) and the conversion of the carboxylic acid group to the carboxylate group can improve the antioxidant activity by preventing the negative effect of the carboxylic acid group.
For glutamic acid containing two carboxylic acid groups, glutamic acid di-sodium salt was also prepared by adding two molar equivalent amounts of NaOH. Nine amino acid sodium salts and four potassium salts were prepared as powder. Yields ranged from 54.1 to 84.1% for sodium salts and from 50.0 to 65.8% for potassium salts. It should be noted that the yields can be further optimized since this study did not attempt to optimize the yields but more focused on producing fine powder.