Protein–saponin interfacial interactions direct the stability and oxidative protection of Platycodon grandiflorum saponin-loaded soy protein nanoemulsions

This study aimed to develop a green nanoemulsion system stabilized by soy protein isolate (SPI) to encapsulate Platycodon grandiflorum saponins (PGS), significantly enhancing their stability and bioavailability. The PGS–SPI composite nanoemulsions exhibited favorable physicochemical properties, robust interfacial interactions, and enhanced environmental stability. At a PGS loading of 60 mg/mL, droplets had a uniform size of 295.3 nm and a high absolute zeta potential of 58.17 mV, with an encapsulation efficiency (EE) of 83.64%, indicating strong electrostatic stabilization and efficient encapsulation. The formulations remained stable across neutral to moderate alkaline pH, at NaCl concentrations up to 0.2 M, and up to 70 °C. Long-term storage at 37 °C for 28 days demonstrated sustained encapsulation retention (64.89%) and delayed lipid oxidation, suggesting excellent shelf-life performance. Spectroscopic and thermodynamic analyses revealed that hydrogen bonding and hydrophobic interactions between PGS and SPI establish a robust interfacial layer that mitigates droplet coalescence and oxidative degradation. These results demonstrate that an SPI-stabilized nanoemulsion is a promising delivery platform for PGS, offering a novel strategy for designing effective, stable, and bioavailable formulations for functional-food and nutraceutical applications.