Multi-scale investigation into the flaxseed oil emulsions stabilized by soybean protein isolate - (-)-epigallocatechin gallate covalent complexes: An "Interface thickening - Structure unfolding - Antioxidant reinforcement" cascade mechanism.

Plant protein-stabilized emulsions face challenges in physical and oxidative stability during storage, leading to a great limitation against the further application of these emulsions. The present study established a multiscale mechanism for enhancing flaxseed oil emulsion stability using soybean protein isolate (SPI) - (-)-epigallocatechin gallate (EGCG) covalent complexes. Complexes fabricated at varying SPI:EGCG mass ratios (200:1 to 20:1) via alkaline pH-shifting exhibited dose-dependent improvements in emulsion physical and oxidative stability. Macro-scale assessments, via droplet size, ζ-potential, and lipid oxidation measurements, revealed that higher EGCG incorporation reduced droplet size, increased ζ-potential magnitude, and suppressed lipid oxidation. Meso-scale dissipative particle dynamics simulations demonstrated EGCG-mediated interfacial thickening and uniform oil-water distribution. Molecular analysis confirmed covalent CS thiol-quinone and CN Schiff base linkages between the o-quinone form of EGCG and Lys/Cys residues in SPI, inducing SPI unfolding (β-sheet to random coil transition) and reduced surface hydrophobicity. Quantum chemical calculations revealed a narrowed HOMO-LUMO gap in covalently bound EGCG, enhancing interfacial antioxidant capacity. The stabilization cascade mechanism followed "Interface thickening - Structure unfolding - Antioxidant reinforcement" across scales. This work provides mechanistic insights for designing protein-polyphenol complexes and the wider application such as bioactive delivery and development of multiple products using emulsions stabilized by these complexes.