Physicochemical analysis of casein solubility in water-ethanol solutions

Abstract

Due to the change in the apparent acid dissociation constant, when ethanol was added to water solutions of an industrially produced spray-dried sodium caseinate, the solution's pH increased proportionally to the reciprocal relative dielectric constant (1/ε). The pH increased by 1 unit when the ethanol volume fraction reached about 0.60. The casein solubility profile at 20°C as a function of pH (3-8) and the ethanol volume fraction (0-0.75) was quite well represented by the equation describing the ionisation process. The pH inflection point and the slope coefficient were approximately linearly related to 1/ε. The evolution of the electrostatic surface potential (ψ0) and the zeta potential (ζ) explained the changes in casein solubility as a function of pH and the dielectric constant of the solvent. The increase in ψ0 by 37 ± 5.2 mV or in ζ by 8.6 ± 1.4 mV raised casein solubility from 10 to 90%. © INRA. EDP Sciences, 2006.