ANS, a hydrophobic fluorescence probe, is widely used to study conformational changes in proteins and characterize surface exposure of hydrophobic sites (Alizadeh-Pasdar & Li-Chan, 2000). A decrease in surface hydrophobicity of protein–EGCG complexes was shown in Table 2, which suggested that the surface of milk proteins became hydrophilic. Since changes in surface hydrophobicity can be explained as a combined effect of the number and distribution of non-polar groups, the differences of hydrophilic/hydrophobic character may be explained by following deductions: (ⅰ) introduction of the hydrophilic hydroxyl groups of EGCG; (ⅱ) blocking of hydrophobic residues; (ⅲ) exposure of some previously buried hydrophilic regions. When non-covalent interactions occur between EGCG and proteins, proteins have an increased polarity around the tyrosine residues and a rearranged tryptophan microenvironment, and a lot of hydroxyl groups are introduced to the protein surface, thus decreasing the surface hydrophobicity (Yang et al., 2014). Besides, covalent modification of proteins can block relatively few hydrophilic groups such as amino groups and sulfhydryl groups and introduce a large amount of hydroxyl groups, thus making the surface more hydrophilic. As shown in Table 2, surface hydrophobicity of covalent protein–EGCG complexes was lower than that of non-covalent ones, which suggested that covalent interactions might introduce more hydrophilic groups to the protein surface than non-covalent interactions.