Soy Protein Isolate–Sodium Alginate Composite Particles for Stabilization of High Internal Phase Pickering Emulsions: Structural Characterization and Stabilization Mechanisms

High internal phase Pickering emulsions (HIPEs) stabilized with biopolymer-based particles have sparked widespread interest due to their excellent stability and potential as fat replacements in food systems. In this study, soy protein isolate (SPI) and sodium alginate (SA) were mixed to create composite colloidal particles capable of stabilizing HIPEs with an oil phase percentage of 80%. SA significantly regulated the particle size and surface hydrophobicity of the composite particles. The optimal formulation with 1.0% SA presented a uniform particle size and desirable interfacial properties. The contact angle increased from 62.3° for pure SPI to 80.8°, which effectively improved the wettability at the oil–water interface. The interfacial protein adsorption reached a maximum of 83.7%, enabling adequate coverage of oil droplets. Low-field NMR demonstrated an increase in bound water (T22) from 21.893 to 30.031 (a.u.), while CLSM images confirmed the formation of compact interfacial layers. The HIPEs possessed excellent stability against heat treatment (100 °C), freeze–thaw cycling (3 cycles), high ionic strength (up to 0.6 M NaCl), and ambient storage for 30 days. These findings demonstrate that SPI-SA complexes are excellent natural stabilizers for fabricating robust, environmentally friendly HIPEs with broad prospects for functional food applications.