Interfacial Reinforcement Rather than Adsorption Quantity Determines the Stability of Soy Protein Isolate (SPI)/High-Methoxyl Pectin (HMP) Bilayer Emulsions

Bilayer emulsions have gained increasing attention in modern food industry due to their tunable interfacial structure and resistance to environmental stresses. However, the mechanisms linking interfacial properties and macroscopic emulsion stability remain elusive. In this study, Soy Protein Isolate (SPI) and High-Methoxyl Pectin (HMP) bilayer emulsions with varied mass ratios were fabricated, and the relationship between interfacial properties and emulsion stability was systematically investigated. Laser diffraction particle size analysis, Confocal laser scanning microscopy, Turbiscan Stability and rheological characterization were explored to evaluate the emulsion stability, while quartz crystal microbalance with dissipation monitoring (QCM-D) was adopted to investigate interfacial adsorption behavior and the viscoelastic properties of the interfacial layers. The results demonstrated that the r = 1:2 formulation achieved the highest interfacial adsorption mass and greatest film thickness, whereas the r = 1:3 formulation exhibited the highest stability. The r = 1:3 ratio yielded the strongest binding affinity (Δf3 = 15.02 Hz), energy dissipation (ΔD3 = 7.1941 × 10−6), and interfacial elasticity (43.15 kPa), indicating the formation of a highly hydrated and mechanically reinforced interfacial layer. The findings highlight that interfacial reinforcement efficiency rather than interfacial adsorption quantity is the key determinant governing the stability of SPI/HMP bilayer emulsions. This study would offer new insights into the structure–function relationship between interfacial characteristics and macroscopic stability in protein–polysaccharide bilayer emulsions, and provide theoretical guidance for the development of plant-based emulsion systems.