Rheology of Sodium Caseinate Stabilized Oil-in-Water Emulsions

We report on shear rheological measurements at 30°C of fine oil-in-water emulsions (volume-surface average diameter < 0.5 &mgr;m) prepared at pH 6.8 with sodium caseinate as the sole emulsifier (1-6 wt%) and n -tetradecane as the dispersed phase (10, 35, or 45 vol%). Strong sensitivity of rheological behavior to total protein concentration was indicated by both steady-state viscometry and small-deformation oscillatory experiments. The behavior can be classified into three types, depending on the protein/oil ratio. (1) Emulsions containing insufficient protein for (near-) saturation protein surface coverage develop a time-dependent increase in low-stress apparent viscosity and associated shear-thinning behavior; this can be attributed to bridging flocculation. (2) Emulsions having full protein surface coverage but relatively little excess unadsorbed protein in the continuous phase are stable Newtonian liquids. (3) Emulsions containing a substantial excess of unadsorbed sodium caseinate exhibit considerable pseudoplasticity which can be attributed to depletion flocculation. Taken as a whole, the time-dependent rheological properties for this set of emulsions as a function of protein content and oil volume fraction are largely consistent with our previous results on the creaming stability and the particle gel microstructure for these same emulsion systems. In particular, the reversible flocculation of emulsion samples of high protein content is readily explicable in terms of depletion flocculation of droplets by unadsorbed protein existing in the form of approximately spherical caseinate submicelles.