Factors Affecting Antioxidant Activity of Amino Acids in Soybean Oil at Frying Temperatures

Recently it is reported that some amino acids have strong antioxidant activity in frying oil showing potential as new natural antioxidants. This study attempts to examine some proposed mechanisms and to understand a few factors affecting antioxidant activities of amino acids. First, to understand the effect of the two major functional groups in an amino acid, carboxylic acid, and amino groups; β‐alanine and their derivatives are examined. Methyl 3‐aminopropanoate having an ester group (‐COOCH3) in place of the carboxylic acid group (‐COOH) of β‐alanine has stronger activity than β‐alanine, presumably, due to increased lipophilicity. 3‐(Dimethylamino)propionic acid containing a dimethylamino group (‐N(CH3)2) in place of the amino group (‐NH2) of β‐alanine also has slightly increased activity than β‐alanine. In general, antioxidant activity increases with increasing alkyl chain length of an amino acid. However, DL‐2‐aminobutyric acid (C4) and DL‐norleucine (C6) has higher activity than other analogs with a longer alkyl chain. Chelation of metals by an amino acid does not play an important role under the conditions used in this study. Dipeptides do not show stronger activity than the two constituent amino acids alone in soybean oil (SBO). Practical Application: Amino acids can practically be used as natural antioxidants for frying. Since this finding is very recent, very little is known about the antioxidant activity of amino acids, which must be investigated for the practical application. In this study, the understanding of several factors affecting their activity, which can be used in the practical application is attempted. It is found that, in general, antioxidant activity increases with increasing lipophilicity of an amino acid. Dipeptides do not show stronger activity than the two constituent amino acids alone. Therefore, a single effective amino acid instead of peptides or protein hydrolysates are recommended. Antioxidant activity of DL‐alanine derivatives with a longer alkyl chain. In general, higher lipophilicity provides higher antioxidant activity. However, DL‐2‐aminobutyric acid (C4) and DL‐norleucine (C6) has higher activity than other analogs with a longer alkyl chain, presumably, due to the combination of their lipophilicity and mobility. Antioxidant activity of DL‐alanine derivatives with a longer alkyl chain. In general, higher lipophilicity provides higher antioxidant activity. However, DL‐2‐aminobutyric acid (C4) and DL‐norleucine (C6) has h