Influence of pH and heating on physicochemical properties of whey protein-stabilized emulsions containing a nonionic surfactant

The influence of polyoxyethylene sorbitan monolauarate (Tween 20) on droplet flocculation in 20 wt % soybean oil-in-water emulsions stabilized by whey protein isolate (WPI) was investigated by light scattering, rheology, and creaming measurements. In the absence of Tween 20, emulsions were prone to droplet flocculation near the isoelectric point (pI) of the proteins (4 < pH < 6) but were stable at higher and lower pH values. Flocculation led to an increase in emulsion particle size, viscosity, and creaming index. Addition of Tween 20 at low concentrations (R < 0.1, where R is the surfactant/protein molar ratio) had little effect on emulsion properties, but at higher concentrations (R > 5) it decreased the amount of flocculation and shifted it to lower pH values. The higher concentrations of Tween 20 also reduced the extent of droplet flocculation in emulsions heated at 78 degrees C for 30 min in the presence of 100 mM NaCl. The results are explained in terms of competitive adsorption, protein-surfactant interactions, and colloidal interactions. This study shows that the physicochemical properties of protein-stabilized emulsions can be controlled by adding nonionic surfactants.