Fabrication and characterization of walnut protein-polyphenol conjugates from hickory by-products: Structure and interaction mechanism analysis

Abstract

The protein-polyphenol interaction mechanisms are affected by component chemical structures, environmental conditions, and processing methods. In this study, walnut protein (WP)-polyphenol complexes or conjugates were prepared using five approaches: physical mixing, heat treatment, alkali treatment, free radical grafting (FRG), and ultrasound-assisted free-radical grafting (UFRG). All samples had amorphous characteristics and FRG/UFRG groups showed a distinct ester bond absorption peak (FTIR: 1739 cm−1). Atomic force microscopy showed differences in the aggregation states of complexes and conjugates, with the UFRG group having the smallest aggregates. Raman spectroscopy indicated that under the UFRG treatment, the WP-HSC sample had the highest random coil content (74.30%), highest tryptophan residue exposure (I₇₆₀/I₁₀₀₃ = 1.10), and a near equilibrium conformation of disulfide bonds (g-g-t/t-g-t). Correlation analysis showed the random coil content of WP was significantly positively correlated (P < 0.05) with grafting efficiency, turbidity, contact angle and surface hydrophobicity. In summary, this study provides valuable insights into factors regulating plant protein-polyphenol conjugate formation.