Interfacial shear rheological behaviour of proteins in three-phase partitioning systems

Three-phase partitioning process (TPP) is a relatively recent, but fast developing technique, which is used for the accumulation of proteins from multicomponent extracts. In this process a coherent, protein-rich middle layer is formed between two immiscible liquid phases. The two liquid phases are developed by phase separation of systems consisting of tert-butanol, ammonium sulphate and water, and the system is characterised by low interfacial tension. Interfacial rheological properties of four different proteins (bovine serum albumin, ovalbumin, β-lactoglobulin and lysozyme) were investigated in model systems of TPP by means of a two-dimensional Couette-type interfacial rheometer used in the oscillatory mode. Ovalbumin and β-lactoglobulin showed the highest interfacial shear elasticity and viscosity, whilst the rheological parameters of lysozyme were hardly measurable. Shear viscoelastic behaviour of protein film adsorbed at the interface and of thin emulsion layer obtained by slight mixing of the liquid phases were compared as a function of time. The interfacial rheological results and also the microscopic observation of the middle layer support the crucial role of emulsification and emulsion stability related to viscoelastic character of the interfacial film in the protein separation during TPP.