Ultrasound-Assisted Covalent Conjugation of Walnut Albumin with Bound Polyphenols: Structural Modulation and Functional Enhancement

Background: Walnut albumin (WA) possesses a balanced amino acid composition but exhibits poor solubility, limited emulsifying capacity, and low structural stability, restricting its practical applications in food systems. Methods: In this study, bound polyphenols (BPs) from jujube pomace were covalently conjugated with WA through alkaline and radical methods, with or without ultrasound assistance. Four WA–BPs conjugates were prepared, including alkaline-treated (AWA–BPs), ultrasound-assisted alkaline (UAWA–BPs), radical-treated (RWA–BPs), and ultrasound-assisted radical conjugates (URWA–BPs), to investigate the effects of different covalent assembly methods on the structural and functional properties of WA. Results: Covalent conjugation with BPs significantly altered the structural properties of WA, as evidenced by reductions in reactive groups, changes in surface hydrophobicity, fluorescence quenching, and shifts in FTIR spectra. URWA–BPs exhibited the highest grafting degree of 4.46 mg/g dry weight (DW) among the four treatment groups, indicating effective grafting and conformational rearrangement. Moreover, URWA–BPs demonstrated superior functional properties, including improved solubility, emulsifying activity, foaming properties, and antioxidant capacity. The DPPH, ABTS+, and FRAP values of URWA–BPs increased by approximately 10–17% compared with WA. In contrast, UAWA–BPs exhibited the lowest in vitro digestibility (54.90 ± 1.60%), indicating enhanced structural stability against gastrointestinal digestion. Molecular docking revealed binding free energies ranging from −5.3 to −7.6 kcal/mol, suggesting stable interactions between BPs and WA. Conclusions: The differences observed between UAWA–BPs and URWA–BPs suggest that, in addition to promoting covalent conjugation, ultrasound exerts distinct regulatory effects during alkaline and radical covalent assembly processes, resulting in different structural and functional properties. This study provides new perspectives for designing functional plant-based protein ingredients and valorizing food-processing by-products.