(199g) Effect of the Dispersed Phase Volume Fraction On Emulsion Drop Size Distributions Obtained From High Pressure Homogenization
Oil-in-water emulsions are ubiquitous dispersed phase systems with diverse applications including natural and processed foods. Emulsified products exhibit a wide range of physicochemical and sensory characteristics based on the system formulation, which influence dispersed and continuous phase properties such as density, viscosity, pH, electrical conductivity and interfacial tension. Process operations such as homogenization also have a substantial effect on product properties, including appearance, taste, mouthfeel, odor and safety. Emulsion system formulation and processing operations both impact the drop size distribution, a key property that influences dispersion rheology, stability, texture, appearance and food safety. In high pressure homogenization, a coarse emulsion is passed through a small orifice under very high pressure. The fluid stream passes radially through the narrow gap formed between the piston and the valve seat at high velocity, creating a local environment of high turbulence and shear stress that causes drop deformation and breakage. Under typical industrial conditions where the oil/surfactant ratio is large to reduce manufacturing costs, drop coalescence is prevalent due to insufficient surfactant to stabilize newly formed drops.
The population balance equation (PBE) modeling framework is particularly well suited for emulsification processes because functions describing single droplet events such as breakage and coalescence can be incorporated within a fundamental number balance equation to predict the evolution of the drop size distribution. While PBE models have been developed for a wide variety of emulsification processes including high pressure homogenizers, these models are typically formulated for prediction at constant and relatively low oil fractions. In our previous work (Maindarkar et al., 2012), we incorporated coalescence into breakage-only PBE model of high pressure homogenization to allow prediction of drop size distributions at high oil-to-surfactant ratios. While the model was capable of accurate prediction at a constant oil concentration, extensibility to other oil concentrations proved to be unsatisfactory.
In this presentation, we describe the development of a new PBE model that allows prediction of homogenized drop size distributions over a wide range of oil concentrations. Our model oil-in-water emulsion system consisted of 1% Pluronic F68 non-ionic surfactant and 10-50% vegetable oil. Our previous PBE model was modified in two important ways: (1) a new mechanistic function for drop breakage due to turbulent shear was developed and combined with a previous breakage function for drop collisions with turbulent eddies; and (2) the constant continuous phase viscosity used in the breakage and coalescence functions was replaced with an emulsion viscosity calculated using a model that describes the shear thinning behavior of the emulsion as a function of the oil concentration. Nonlinear optimization was used to estimate viscosity model parameters and adjustable parameters in the breakage and coalescence functions from viscosity and drop distribution data collected at 10% oil. The resulting model was shown to produce satisfactory emulsion viscosity and drop size distribution predictions at 30% and 50% oil without re-estimation of the model parameters.
Maindarkar S.N., Raikar N.B., Henson M.A., “Incorporating emulsion drop coalescence into population balance equation models of high pressure homogenzation”, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 396 (2012), 63-73.