The combined presence of sodium caseinate and SDS has
been found to enhance the stability of hydrocarbon oil-in-water
emulsions. This is especially apparent in cases in which the presence
of excess surfactant alone would result in destabilization
of the emulsion through depletion flocculation caused by unadsorbed
SDS micelles. The caseinate-induced stabilization is
manifest in a substantial enhancement in the creaming stability.
The formation of mixed caseinate–SDS structures in the bulk
aqueous phase seems to be the most probable explanation for
the stabilizing action of the caseinate. Addition of salt leads to
flocculation, as reflected in much greater shear-thinning character
of SDS-stabilized emulsions, and significantly enhanced
creaming. Increase in caseinate levels in these high ionic strength
systems stabilizes emulsions by reducing the extent of flocculation
and shear-thinning rheology (rendering them practically
Newtonian).
In summary, we have shown that the surface-active agents
SDS and sodium caseinate can act together to enhance emulsion
stability. This contrasts with the negative synergism displayed
by Tween 20 and caseinate: combination of these two latter substances
was found to reduce the creaming stability of emulsions,
while increasing the extent of shear-thinning rheology. This contrast
in behavior is attributed to a substantial difference in the
protein–surfactant interactions for the two cases.