Formation and Characterization of Noncovalent Walnut Protein Hydrolysate-Polyphenol Complexes: Role of Polyphenol Diversity in Functional Modulation

The valorization of walnut processing byproducts is crucial for developing sustainable functional ingredients. This study examined noncovalent complexes between walnut protein hydrolysate (WPH) and walnut polyphenols (EA, GA, FA, CA, EC). Binding induced concentration-dependent fluorescence quenching and structural changes, including a reduced β-sheet and increased β-turn content. Hydrophobic interactions dominated complex formation, except for WPH-EC, which was governed by electrostatic forces. These interactions enhanced functional properties; all complexes showed elevated solubility, lower interfacial tension, and improved emulsifying activity, with WPH-EA exhibiting the highest antioxidant capacity and WPH-GA showing the strongest inhibition of α-amylase and α-glucosidase. Rheological tests confirmed increased viscosity and viscoelasticity, indicating stronger network formation. In vitro digestion revealed that complexation reduced protein digestibility but significantly improved polyphenol bioaccessibility. These findings support the potential of WPH-polyphenol complexes as multifunctional, sustainable ingredients.