The effect of surfactants on the solubility, zeta potential, and viscosity of soy protein isolates

The interactions of soy protein isolate (SPI, 5 wt%) with an anionic (sodium dodecyl sulfate, SDS) and a nonionic (polyoxyethylene sorbitan monolaurate, Tween 20) surfactant were studied as a function of pH. The solubility of the Tween 20 and surfactant free samples reached a minimum close to the isoelectric point of the protein (pH 4-5) whereas the SDS-containing sample was highly soluble over the whole range studied. The viscosity of the Tween 20-containing and surfactant-free samples were low (∼ 3 mPa s) and largely pH independent while the SDS-containing samples were much more viscous, particularly at high pH (> pI) and high [SDS] (>critical micellar concentration (CMC)). The effect of the surfactants and pH on the δ-potential of more dilute protein suspensions was measured by phase analysis light scattering. charge on the SDS samples was high and negative at all pH values while the Tween 20 and surfactant-free samples showed a typical but distinct sigmoidal changes from positive to negative with increasing pH and a neutral point corresponding to the isoelectric point and the point of minimum solubility. The δ-potential changed linearly with added SDS from a pH-determined value at 0 mM to approximately - 50 mV at 7.5 mM. These observations are discussed in terms of the changing colloidal interactions of soy protein particles.