pH-Regulated Microstructural and Rheological Properties of Curdlan-Soy Protein Thermo-Irreversible Gels

Understanding the interactions between polysaccharides and plant proteins is essential for controlling texture in the formulation of alternative food products. This study investigates the influence of pH on rheology and microstructure of composite gels formed from curdlan and soy protein isolate (SPI). Composite gels, prepared at a 1:1 weight ratio of curdlan to SPI to eliminate the influence of mixing ratio, were compared with their respective pure components across pH 5, 7, and 9. In pure curdlan gel, a higher pH (pH 9) resulted in increased linear viscoelasticity, which can be linked to enhanced curdlan solubility, as revealed by scanning electron microscopy. In contrast, the curdlan-SPI composite gels exhibited the strongest gelation at pH 5 (G'~7535 Pa). Microstructural analysis showed that aggregated SPI particles were embedded within the curdlan network at this pH, thus reinforcing its gel strength. At pH 9, the increased solubility of SPI hindered associations of curdlan molecules, leading to weakened gel structure. Under large amplitude oscillatory shear, the composite gel at pH 9 exhibited an earlier onset of nonlinear response compared to the other pH conditions, consistent with diminished structural integrity. X-ray diffraction analysis confirmed the presence of both single- and triple-helix conformations of curdlan, which contribute to gel network formation. However, incorporation of SPI reduced the degree of triple-helix ordering, indicating disruption of curdlan's structural organization. These findings underscore the critical role of pH in modulating the phase behavior and rheological properties of nonionic polysaccharide-protein gels. Such insights are valuable for optimizing the formulation of plant-based gel systems with tailored textural properties for the development of alternative food products.