COALESCENCE OF PROTEIN-STABILIZED EMULSIONS IN A HIGH-PRESSURE HOMOGENIZER

Coalescence rate constants in a high-pressure homogenizer were inferre d for pure emulsions of a neutrally buoyant mixture of carbon tetrachl oride and benzene dispersed in aqueous medium, as well as for emulsion s stabilized by sodium caseinate or whey protein, from the temporal ev olution of number of drops per unit volume in the exit stream of the h omogenizer in a recirculating system consisting of the homogenizer and a stirred tank when subjected to a negative step change in applied ho mogenizer pressure (P-h) The homogenization pressure was varied in the range 6.9 to 41.4 MPa, pH in the range 5 to 7, dispersed phase fracti on in the range 0.05 to 0.15, ionic strength in the range 0.01 to 0.05 M, and protein concentration in the range 0.01 to 0.05 wt%. Functiona l dependence of drop coalescence rate on the homogenizer pressure was derived for pure emulsions for mechanisms of drop collisions due both to turbulence and to shear. The inferred collision rate constant for p ure emulsions was found to be proportional to P-h(0.722), which was cl oser to the functional dependence for drop collisions due to turbulenc e. The coalescence rate constant was found to be higher for higher hom ogenizer pressures, larger drop sizes, and higher dispersed phase frac tions. I;or emulsions stabilized by proteins, drop coalescence rate co nstant was found to be higher at lower protein concentrations, near th e isoelectric point of the protein, and higher ionic strengths because of the smaller interdroplet colloidal repulsive forces, thus clearly demonstrating the effect of colloidal forces on drop coalescence. Sodi um caseinate was found to result in a lower coalescence rate constant than whey protein. (C) 1997 Academic Press.