Depletion flocculation of emulsions containing unadsorbed sodium caseinate

The influence of unadsorbed protein on the stability of fine oil-in-water emulsions containing sodium caseinate has been investigated. Time-dependent creaming profiles were determined at 30 degrees C using the ultrasound velocity scanning technique. The results show that, at a protein concentration (2% w/w) around that required for nearly saturation coverage of droplets, emulsion stability with respect to creaming is good (30 days), with no indication of flocculation or coalescence. At a higher protein concentration (4% w/w), corresponding to considerably more protein than required for saturation coverage, creaming stability is reduced, with rapid development of a serum layer within 20 h of emulsion preparation. Further increase in protein concentration (eg to 6% w/w) results in partial restabilization of the emulsion through the formation of a weak particle network gel. Complementary rheological studies have shown that increasing protein concentration results in pseudoplastic emulsions with significantly higher low-shear viscosities. This behaviour, directly confirmed by light microscopy, is attributed to reversible depletion flocculation of the emulsions by protein remaining unadsorbed in the aqueous phase. It is suggested that aggregation of caseinate to form sub-micelles causes depletion by small particles in a manner similar to that observed previously for micelle-containing small-molecule surfactant-stabilized systems.