Effect of Freeze-Drying and Spray-Drying on Emulsibility and Stability of Emulsions Stabilized by Covalent and Non-Covalent Whey Protein–Dextran Complexes

This study focuses on the stability of whey protein isolate–dextran (WPI–D) complexes, exploring how their modification modes (covalent grafting/non-covalent mixing) and drying processes (spray-drying [SD]/freeze-drying [FD]) regulate structural properties and stability. Covalent WPI–D grafts (WPI-D-G) exhibit a higher degree of glycosylation, with significant fluorescence quenching and red shift. Fourier transform infrared (FTIR) spectral analysis reveals reduced absorbance of amide bands, collectively forming rigid cross-linked structures. Compared to non-covalent WPI–D mixtures (WPI–D-M), these structures exhibit superior emulsifying activity index (EAI), emulsion stability index (ESI), as well as stronger tolerance to environmental stresses (thermal stability: FD WPI–D-G (WPI–D-GF) maintain a particle size of 0.447 µm at 80°C, whereas SD WPI–D-G (WPI–D-GS) tend to aggregate; salt tolerance: stable at NaCl ≤ 20 mg/mL; pH stability: robust across pH 2–7). WPI–D-GF retain intact cross-linked structures, exhibiting enhanced emulsifying performance and colloidal stability (higher absolute zeta potential), whereas WPI–D-GS, despite forming smaller uniform spherical particles, undergo partial structural degradation. Regarding comparative stability under different conditions: WPI–D-GF outperforms WPI–D-GS in thermal stability, whereas covalent grafts from both drying methods demonstrate comparable salt and pH tolerance. These findings elucidate the structure–stability relationship of WPI–D complexes, thereby providing a theoretical foundation for regulating their stability through process optimization.