Shear effects on phase behaviour of the legumin-salt-water system. Modelling protein recovery

The aim of this paper is to describe the phase behaviour of aqueous protein solutions subjected to shear flow The phase behaviour of the IIS broad bean globulin (legumin) solution in 0.6 mol/dm(3) NaCl at pH 4.8 was investigated by nephelometric and rheological techniques. This system has an upper critical temperature of 21 degrees C and a critical protein concentration of 18%. A shear-induced reversible phase transition from the two- to single-phase state is observed when protein concentration exceeds 18%. Two-phase liquid systems (water-in-water emulsions) were always less viscous than single-phase systems with the same protein concentration and temperature. Presumably, this results from a 'lubricant' effect of the interfacial layer with a viscosity lower than those of the system phases. The viscosity decrease corresponding to phase separation was used to find the binodal points of the system subjected to shear forces. For the same system, the binodal obtained by viscosimetry (i.e. dynamic conditions) is asymmetrical, whilst the binodal determined by nephelometry (i.e. static conditions) is symmetrical relative to the rectilinear diameter. As a whole, the 'dynamic' binodal is located below the 'static' one. The dynamic equilibrium (between the deformation of dispersed particles and breaking-up of liquid fibrils into smaller spherical particles) in a flowing water-in-water emulsion and the Laplace pressure may be important contributory factors to phase behaviour and rheology of the system. Assuming that phase separation of the system results from incompatibility between the associated and non-associated forms of the same protein, legumin, the effect of shear forces can also be attributed to a decrease in 'melting' temperature of the protein aggregates in the concentrated phase (mesophase). Shear forces disrupting polymer-polymer interactions improve the thermodynamic quality of the solvent. The shear rate is of importance for protein precipitation and processing. The behaviour of highly concentrated liquid-dispersed droplets of protein mesophase is of applied importance for modelling protein isolation and processing, e.g. the protein recovery.