DEPLETION FLOCCULATION OF EMULSIONS CONTAINING UNADSORBED SODIUM CASEINATE

The influence of unadsorbed protein on the stability of fine oil-in-wa ter 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 pr otein concentration (2% w/w) around that required for nearly saturatio n 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 considerabl y more protein than required for saturation coverage, creaming stabili ty 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 th e formation of a weak particle network gel. Complementary rheological studies have shown that increasing protein concentration results in ps eudoplastic 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 rema ining unadsorbed in the aqueous phase. It is suggested that aggregatio n of caseinate to form sub-micelles causes depletion by small particle s in a manner similar to that observed previously for micelle-containi ng small-molecule surfactant-stabilized systems.