pH shift combined with high-pressure homogenisation induced remodelling of the air-water interface: Effect of structural and viscosity changes of soy protein isolate on interfacial properties

The high viscosity of soy protein isolate (SPI) limits its performance at the air-water interface in multiphase colloidal systems. Since structure and aggregation state of proteins strongly influence their interfacial behaviour, this study explores the impact of the physicochemical properties of SPI on its interfacial adsorption mechanism, with a particular focus to broaden its applications in soft food materials, such as foams or colloids. Low-viscosity SPI with high interfacial activity was obtained through the combined use of pH shift (pH 9) and high-pressure homogenisation (60, 80, and 100 MPa). This treatment (100 MPa) promoted protein unfolding and reduced average particle size, thereby lowering steric hindrance and internal friction. As a result, viscosity (16.9 mPa·s) decreased in treated SPI compared to untreated SPI. These structural changes reduced surface tension of SPI, which in turn accelerated the rate of penetration and rearrangement at the air-water interface, and improved interfacial activity in SPI. Correlation analyses further showed that smaller, more flexible, and disordered proteins generally exhibited better interfacial properties compared to larger, more rigid, and ordered proteins. This study provides new insights into the interplay between structure, viscosity, and interfacial behaviour of SPI, providing a theoretical basis for improving its use as an interfacial stabiliser in foam-based-foods.