In this study, the effects of gallic acid (GA) on trypsin digestion of commercial alpha-casein (alpha-CN), which contains alpha(s1)-CN and alpha(s2)-CN, and the peptides released during digestion were investigated. Gallic acid showed no effect on the initial rate of digestion. However, the apparent degree of hydrolysis achieved its maximum value after 1 h, then decreased in the presence of GA, suggesting the cross-linking between peptides once released from alpha-CN during digestion. In the presence of GA, three peaks derived from alpha(s1)-CN disappeared and three new peaks appeared in high-performance liquid chromatography (HPLC) analysis. In these peptides, two Met residues corresponding to the Met(135) and Met(196) in alpha(s1)-CN were oxidized to Met sulfoxide residues. The oxidation of Met(196) was quicker than that of Met(135). The inhibitory activity of TTMPLW (alpha(s1)-CN 193-199) against angiotensin I-converting enzyme was reduced slightly by the oxidation of its Met residue. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.