OPA method has proved to be a sensitive method for analyzing the free amino groups of proteins, which can give some information about lysine side chains (Church et al., 1983). As shown in Table 1, the number of free amino groups in the covalent complexes significantly (p < 0.05) declined compared with that of control milk proteins. Since the determination was conducted in the presence of 1% SDS, which is able to destroy non-covalent protein interactions ( Kroll, Rawel, & Rohn, 2003), it can be speculated that the decreases in free amino groups indicate covalent reaction of ɛ-amino groups of lysine with EGCG. Table 1 also gives the free sulfhydryl group content of the protein samples. For LG, the free SH content of covalent LG–EGCG complex (3.89 ± 0.07 μM/g) was significantly (p < 0.05) lower than that of control LG (43.34 ± 0.18 μM/g). Similarly, Ellman's assay showed almost complete disappearance of the free sulfhydryl groups in other covalent complexes. The results indicated that free sulfhydryl groups of the four proteins showed strong reactivity with EGCG. Since 8 M urea can prevent the conversion of sulfhydryl groups to disulphide bridges, it may be deduced that the sulfhydryl groups indeed react with EGCG.