pH-responsive W1/O/W2 emulsions synergistically stabilized by shellac nanoparticles and chitin nanocrystals: stabilization mechanism, response characteristics and application in co-delivery of astaxanthin and ergothioneine.

In this study, a pH-responsive W₁/O/W₂ emulsion was constructed for the co-delivery of astaxanthin (AST) and ergothioneine (EGT) using polyglycerol polyricinoleate (PGPR) as the primary emulsifier and a synergistic mixture of shellac nanoparticles (SNPs) and chitin nanocrystals (ChNCs) as the secondary particulate stabilizer. By adjusting the outer aqueous phase (W₂) content, the emulsion structure and stability were effectively tuned. A W₁/O:W₂ ratio of 2:1 yielded the most stable, viscoelastic double emulsion with a well-defined triple-compartment architecture. The system maintained interfacial integrity under acidic conditions, began to undergo interfacial disassembly at neutral pH, and exhibited severe droplet coalescence under alkaline conditions, demonstrating pH-responsive behavior. Encapsulation efficiency was optimized at 1.0 mg/mL AST and 1.5 mg/mL EGT, with minimal impact on droplet characteristics. Compared to single emulsions, the double emulsion significantly enhanced the stability of both AST and EGT during storage and under UV irradiation. In vitro digestion studies demonstrated that the emulsion effectively prevented premature gastric release and enabled pH-triggered intestinal release. Notably, the double emulsion achieved AST and EGT bioaccessibility of 46.32% and 52.31%, respectively, representing 1.33-fold and 1.35-fold enhancements over the O/W₂ and W₁/O emulsions. This work presents a novel strategy for developing pH-responsive W₁/O/W₂ double emulsions as effective co-delivery carriers for hydrophilic and hydrophobic bioactive compounds.