Protein Concentration and Protein-Exposed Hydrophobicity as Dominant Parameters Determining the Flocculation of Protein-Stabilized Oil-in-Water Emulsions

DLVO theory is often considered to be applicable to the description of flocculation of protein-stabilized oil-in-water emulsions. To test this, emulsions made with different globular proteins (β-lactoglobulin, ovalbumin, patatin, and two variants of ovalbumin) were compared under different conditions (pH and electrolyte concentration). As expected, flocculation was observed under conditions in which the zeta potential is decreased (around the isoelectric point and at high ionic strength). However, the extent of flocculation at higher ionic strength (>50 mM NaCl) decreased with increasing protein-exposed hydrophobicity. A higher exposed hydrophobicity resulted in a higher zeta potential of the emulsion droplets and consequently increased stability against flocculation. Furthermore, the addition of excess protein strongly increased the stability against salt-induced flocculation, which is not described by DLVO theory. In the protein-poor regime, emulsions showed flocculation at high ionic strength (>100 mM NaCl), whereas emulsions were stable against flocculation if excess protein was present. This research shows that the exposed hydrophobicity of the proteins and the presence of excess protein affect the flocculation behavior.