Competitive displacement of interfacial soy proteins by Tween 20 and its effect on the physical stability of emulsions

The competitive displacement of interfacial proteins, including soy protein isolate (SPI), casein, and modified SPIs (heat, sodium sulfite and succinic anhydride treatment), by Tween 20 was investigated. The structural characteristics of the soy proteins, such as the surface hydrophobicity, exposed free sulfhydryl content and ξ-potential, were analyzed to explore their effects on the protein displacement by Tween 20. The sequence of Tween 20 addition significantly affected the displacement of SPI and casein from the interfaces. The highest degrees of displacement of SPI and casein were 91.04% and 90.67%, respectively, when Tween 20 was added before homogenization. However, the corresponding values became 41.29% and 87.70% when Tween 20 was added after homogenization. The lower level of the latter value was mainly attributed to the fact that SPI formed viscoelastic interfacial films, as confirmed by interfacial rheological property measurements. Although both the ξ-potentials of the SPI emulsions and the mechanical strength of the interfacial protein films decreased due to Tween 20 addition, the physical stability of the emulsions was significantly enhanced. In regard to the modified SPIs, although a high surface hydrophobicity and a low content of exposed free sulfhydryl were associated with low degrees of protein displacement in some cases, the protein charge was the dominant factor affecting the displacement by Tween 20. The emulsion stability of both SPI and the modified SPIs was improved by Tween 20, likely because Tween 20 effectively reduced the hydrophobic interactions between the oil droplets by partially displacing proteins or binding to residual interfacial proteins.