Physicochemical analysis of casein solubility in water-ethanol solutions

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/epsilon). The pH increased by 1 unit when the ethanol volume fraction reached about 0.60. The casein solubility profile at 20 degrees 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/epsilon. The evolution of the electrostatic surface potential (psi(0)) and the zeta potential (zeta) explained the changes in casein solubility as a function of pH and the dielectric constant of the solvent. The increase in psi(0) by 37 +/- 5.2 mV or in zeta by 8.6 +/- 1.4 mV raised casein solubility from 10 to 90%.