Emulsion stability and dilatational rheological properties of soy/whey protein isolate complexes at the oil-water interface: Influence of pH

The effects of pH (2.0-11.0) on the interaction of soy protein isolate (SPI) and whey protein isolate (WPI) at the oil-water interface and the stability of SPI-WPI emulsions were studied by analyzing their interfacial tension, dilatational and steady-state rheological behaviors, particle size, and creaming stability. Within a pH range of 2.0-8.0, the SPI-WPI emulsions were very viscous, exhibiting extensive droplet flocculation (3405.00 +/- 202.40 nm at pH 4.0) and poor creaming stability (73.58 +/- 0.94% at pH 4.0), thereby indicating an unstable emulsion. The enhanced viscoelasticity and decreased interface pressure between SPI and WPI at the interface indicated a fine interface behavior. This behavior may be due to the strong electrostatic attraction interaction between the SPI and WPI at the interface. This co-adsorption at the interface increases the amount of adsorbed proteins, leading to increased viscoelasticity of the emulsion. However, at pH 8.0 to 11.0, the weaker droplet flocculation (288.50 +/- 0.26 nm at pH 11.0) and fine creaming stability (no stratification) indicated that a stable emulsion was formed. This may be attributed to electrostatic repulsion between the SPI and WPI at a pH above the isoelectric point (pI) of the proteins, which inhibits droplet flocculation. This study offers a basis for the selection of pH treatment of SPI-WPI composite emulsions with different stabilities and rheological behaviors, as well as a potential application for bioactive compound delivery systems.