Limited hydrolysis as a strategy to improve the non-covalent interaction of epigallocatechin-3-gallate (EGCG) with whey protein isolate near the isoelectric point

This study was aimed to investigate the interaction mechanisms and structural changes of whey protein isolate (WPI) and whey protein isolate hydrolysates (WPIHs) with epigallocatechin-3-gallate (EGCG) near the isoelectric point through multiple spectroscopic techniques and field emission scanning electron microscopy. Fluorescence spectra results indicated that limited hydrolysis endowed WPIHs with higher affinity for the EGCG but the increased degree of hydrolysis led to an opposite result. Thermodynamic analysis revealed that EGCG bound WPI primarily through hydrogen bonds and van der waals forces, while the hydrophobic force was the main driving force in the interaction of EGCG with WPIHs. Synchronous fluorescence and three-dimensional spectra confirmed that EGCG induced conformational alterations of WPI and WPIHs, which was further supported by Ultraviolet-Visible spectra. Raman spectra indicated that binding to EGCG resulted in changes in the microenvironment of tryptophan residues, CH bending vibration and the secondary structure arrangements of WPI and WPIHs. Furthermore, compared with a sheet-like structure of WPI-EGCG complexes, the morphology of WPIHs with limited hydrolysis presented an uneven blocky structure after complexing with EGCG. Our findings might be helpful to better understand the interactions of milk protein hydrolysates-EGCG and suggest the potential application of the formed complexes as bioactive ingredients in food industry.