Orthokinetic flocculation of caseinate-stabilized emulsions: Influence of calcium concentration, shear rate, and protein content

Calcium-induced flocculation of caseinate-stabilized soybean oil-in-water e mulsions in conditions of Couette flow was studied. A concentrated emulsion (20% oil, 0.5-2.0% sodium caseinate in 20 mM imidazole, pH 7) was diluted 20 times in buffer containing concentrations of CaCl2 between 9 and 17 mM: and sheared at rates between 335 and 1340 s(-1). The average particle size (d(43)), measured by integrated light scattering, increased in a sigmoidal manner with shearing time. An increased shear rate resulted in an increased flocculation rate, because of the increased number of collisions between p articles, but a decreased value of the maximum d(43), because higher shear rates increasingly disrupted the flocs. The flocculation rate was increased by increasing the calcium concentration, indicating an increased collision efficiency. The orthokinetic stability of the emulsions was increased with increased protein content, and it is postulated that the increased surface coverage and hydrodynamic thickness of the adsorbed protein layer increase d steric repulsion between droplets, so that higher calcium concentrations were necessary to induce sufficient conformational change of the proteins t o allow flocculation. At high caseinate concentrations, calcium may also in duce precipitation of unadsorbed caseins from the serum to the oil-water in terface, thereby increasing steric repulsion and hence increasing orthokine tic stability.