Pickering emulsion stabilized by whey protein isolate and psyllium husk powder: Interface properties and...

In this study, whey protein isolate (WPI) and psyllium husk powder (PHP) were used as raw materials to prepare WPI-PHP composites, which were subsequently employed as emulsifiers to construct emulsion systems. The effects of PHP content (0-0.18%) and environmental pH (3-11) on the structural properties of the composites, as well as on the performance and stability of the resulting emulsions, were systematically investigated. The composites and emulsions were characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), interfacial tension measurement, three-phase contact angle analysis, zeta potential, particle size analysis, rheological testing, and multi-stability assessment. The results indicate that polysaccharides and proteins form an ordered composite film at the oil-water interface through the synergistic effects of electrostatic, hydrogen bonding, and hydrophobic interactions. Under optimal conditions (0.09% polysaccharide content, pH 7), the emulsion exhibited the smallest particle size, the highest absolute zeta potential (approximately -30 mV), and an interfacial contact angle of 95.05°, indicating a moderately hydrophobic state favorable for long-term droplet stability. FTIR and XRD analyses confirmed that, under these conditions, the polysaccharides and proteins formed structurally ordered intermolecular complexes. Rheological analysis revealed that this emulsion system behaves as a viscous fluid with optimal energy dissipation capacity. Stability tests further confirmed that the optimized emulsion showed the smallest change in particle size after 28 days of storage at room temperature. It displayed no significant instability following thermal treatment (90 °C), freeze-thaw cycles, or pH adjustments, demonstrating exceptional tolerance to multiple environmental stresses. Notably, at the protein isoelectric point (pH 5), the emulsion particles were largest, the zeta potential was lowest, and the stability was poorest. In contrast, an excessive polysaccharide content (≥0.12%) increased the system's viscosity but impaired long-term storage stability by inducing interfacial reorganization and relaxation. In summary, this study identifies the optimal formulation conditions for WPI-PHP composite emulsions (0.09%PHP, pH 7) and elucidates the underlying mechanisms by which polysaccharide concentration and pH dynamically modulate interfacial structure and macroscopic properties through the regulation of intermolecular interactions. These findings provide both theoretical guidance and a practical basis for the design of highly efficient and stable natural emulsifier systems.