SHEAR EFFECTS ON PHASE-BEHAVIOR OF THE LEGUMIN-SALT-WATER SYSTEM - MODELING PROTEIN RECOVERY

The aim of this paper is to describe the phase behaviour of aqueous pr otein 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 s ystem has an upper critical temperature of 21 degrees C and a critical protein concentration of 18%. A shear-induced reversible phase transi tion from the two- to single-phase state is observed when protein conc entration exceeds 18%. Two-phase liquid systems (water-in-water emulsi ons) 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 th an those of the system phases. The viscosity decrease corresponding to phase separation was used to find the binodal points of the system su bjected to shear forces. For the same system, the binodal obtained by viscosimetry (i.e. dynamic conditions) is asymmetrical, whilst the bin odal determined by nephelometry (i.e. static conditions) is symmetrica l relative to the rectilinear diameter. As a whole, the 'dynamic' bino dal is located below the 'static' one. The dynamic equilibrium (betwee n the deformation of dispersed particles and breaking-up of liquid fib rils into smaller spherical particles) in a flowing water-in-water emu lsion and the Laplace pressure may be important contributory factors t o phase behaviour and rheology of the system. Assuming that phase sepa ration of the system results from incompatibility between the associat ed and non-associated forms of the same protein, legumin, the effect o f shear forces can also be attributed to a decrease in 'melting' tempe rature of the protein aggregates in the concentrated phase (mesophase) . Shear forces disrupting polymer-polymer interactions improve the the rmodynamic quality of the solvent. The shear rate is of importance for protein precipitation and processing. The behaviour of highly concent rated liquid-dispersed droplets of protein mesophase is of applied imp ortance for modelling protein isolation and processing, e.g. the prote in recovery.