Phase diagram, rheology and interfacial tension of aqueous mixtures of Na-caseinate and Na-alginate

In this work, the system Na-alginate/Na-caseinate/water has been characterised in terms of phase diagram, rheological properties of the equilibrium phases, and interfacial tension as a function of tie-line position. In water, the two biopolymers are poorly miscible and the system separates in two phases, each highly enriched of only one of the two biopolymers. The rheological properties of the equilibrium phases are essentially determined by the biopolymer present to a larger extent. Thus, as for the aqueous solutions of each biopolymer, the Na-caseinate rich phases are Newtonian, while the Na-alginate rich phases show shear-thinning behaviour at shear rates larger than 1 s−1. In the region close to the critical point, the Na-alginate rich phases are more viscous than the Na-caseinate rich ones. The opposite trend is found as the total amount of biopolymers increases. The interfacial tension between the equilibrium phases was measured by analysing drop retraction upon cessation of shear flow. The interfacial tension decreases as the tie-line approaches the critical point. A linear relation was found to hold between interfacial tension and tie-line length.