mixer. In a high-pressure homogenizer, the oil and water Coalescence rate constants in a high-pressure homogenizer were mixture is subjected to intense turbulent and shear flow inferred for pure emulsions of a neutrally buoyant mixture of fields. Turbulence is the predominant mechanism for emulsicarbon tetrachloride and benzene dispersed in aqueous medium, fication (1) even though laminar shear and cavitation may as well as for emulsions stabilized by sodium caseinate or whey also play a role. Turbulence leads to breakup of the dispersed protein, from the temporal evolution of number of drops per unit phase into small droplets. The relative motion between the volume in the exit stream of the homogenizer in a recirculating drops results in their collision, leading to their coalescence.
system consisting of the homogenizer and a stirred tank when
There is usually a dynamic equilibrium between breakage subjected to a negative step change in applied homogenizer presand coalescence. Consequently, the drop size distribution of sure (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 fraction the emulsion will be influenced by the rates of breakage and in the range 0.05 to 0.15, ionic strength in the range 0.01 to 0.05 coalescence. The shelf life as well as the texture of the M, and protein concentration in the range 0.01 to 0.05 wt%. Funcemulsion would greatly depend on the drop size distribution tional dependence of drop coalescence rate on the homogenizer which can be controlled by controlling the rates of drop pressure was derived for pure emulsions for mechanisms of drop breakage and coalescence during emulsion formation. This collisions due both to turbulence and to shear. The inferred colliwould require knowledge of the effect of operating paramesion rate constant for pure emulsions was found to be proportional ters and formulation of the emulsion on drop breakage and to P 0.722 h , which was closer to the functional dependence for drop coalescence. Proteins and surfactants are added to help procollisions due to turbulence. The coalescence rate constant was duce small drops. Surfactants decrease the interfacial tension found to be higher for higher homogenizer pressures, larger drop between the oil and water phases through adsorption at the sizes, and higher dispersed phase fractions. For emulsions stabiinterface. This results in less energy required for drop breaklized by proteins, drop coalescence rate constant was found to be higher at lower protein concentrations, near the isoelectric point age, thus leading to smaller droplets. In addition, surfactant of the protein, and higher ionic strengths because of the smaller and proteins inhibit coalescence of colliding drops through interdroplet colloidal repulsive forces, thus clearly demonstrating repulsive electrostatic and steric interaction forces. These the effect of colloidal forces on drop coalescence. Sodium caseinate repulsive interactions minimize coalescence by slowing was found to result in a lower coalescence rate constant than whey down the drainage of the intervening continuous film during protein. α§ 1997 Academic Press a drop pair encounter. The magnitude of the interaction Key Words: protein-stabilized emulsions; drop coalescence; highforces depends on the amount and the nature of protein and/ pressure homogenizer; emulsification; effect of colloidal forces.
or surfactant adsorbed at the oil-water interface, pH, ionic strength, and temperature. Also, the rupture of the intervening continuous phase film, necessary for drop coalescence,