Incompatibility and Phase Separation in a Bovine Serum Albumin/β-Casein/Water Ternary Film at the Air–Water Interface

Abstract

Thermodynamic incompatibility and two-dimensional phase separation in a bovine serum albumin (BSA)/β-casein/water ternary film at the air–water interface has been studied using an epifluorescence microscopy technique. The incompatibility between BSA and β-casein at the air–water interface was deduced from deviation of the experimental equilibrium composition of the proteins in the mixed saturated monolayer film from that predicted by the Langmuir-type competitive adsorption model at various bulk concentration ratios. Fluorescence microscopy of the mixed monolayer film showed distinct phase-separated BSA-rich and β-casein-rich regions coexisting with inhomogeneous mixed regions. BSA always tended to be the dispersed phase and β-casein the continuous phase. It is suggested that because the free energy at the “interfaces” between the phase-separated regions is generally higher than at other regions of the film, they may act as zones of instability in protein-stabilized foams and possibly emulsions.