Protein solubility is a prime factor for understanding the performance of protein functionality in complex food systems, since protein solubility is a prerequisite for other functional properties such as emulsifying properties. Fig. 5 shows solubility profiles of protein–EGCG complexes and control milk proteins. Control LA, LG and LF maintained good solubility (≥80%) even around their isoelectric points mainly because they had low surface hydrophobicity, while SC precipitated in large amount around its isoelectric point. In comparison with corresponding control proteins, the non-covalent complexes exhibited similar solubility profiles, while the covalent complexes exhibited the lowest solubility at lower pH values, around which the covalent complexes almost totally precipitated. Meanwhile, the solubility of LA, LG and SC was more affected by the covalent modification than that of SC. The observed solubility changes of the covalent complexes may be due to loss of positively charged groups. As lysine is a hydrophilic amino acid, the free available amino groups tend to locate at the surface of milk proteins. The covalent reaction of different milk proteins with EGCG was accompanied by a corresponding blocking of the positively charged amino groups, thus leading to a change in the net charge of the protein molecule surface, which could furthermore affect the solubility of the covalent complexes at different pH values. Since a protein usually has the least solubility at the isoelectric point (Franco & Filho, 2011), it can be assumed that isoelectric points of the covalent complexes change to lower pH values, which can also be confirmed by zeta potential analysis of protein samples (data not shown).