Protein aggregation induced by phase separation in a pea proteins–sodium alginate–water ternary system

The physicochemical properties of a native, globular plant protein–linear anionic polysaccharide aqueous system at 20 °C were investigated in conditions where biopolymers carry a net negative charge (pH 7.2, 0.1 M NaCl). The pea proteins–sodium alginate mixtures showed a phase separation mainly by thermodynamic incompatibility, characterized at both the macroscopic and microscopic scale. Phase diagram was established and confocal laser scanning microscopy (CLSM) provided accurate data on the microstructure morphology of the system, regarding its phase behavior. In admixture, sodium alginate induced a protein aggregation, certainly by a local depletion of the polysaccharide. Protein aggregates were present in both single-phase and biphasic mixtures, while increasing the sodium alginate concentration provided larger and denser protein microdomains, leading to a non-equilibrium state. By phase separation, the pea protein aggregates entrapped a part of the sodium alginate phase, thus modifying the protein volume fraction. Along a tie-line, a phase inversion phenomenon was detected, from a sodium alginate to a pea proteins-continuous phase. Rheological properties of the mixed systems depended on the biopolymer composition and were modified with respect to individual components. The protein-enriched phase volume fraction modified the flow behavior to non-Newtonian for starting concentrated mixtures; a shear-thinning at high shear rates was evidenced, while mixtures with a particularly high sodium alginate concentration (>0.8 wt%) had an apparent yield stress for low shear rates. Mechanical spectra showed that both protein enrichment within microdomains in the presence of alginate and stronger protein–protein interaction with time impacted the viscoelastic properties (G′ and G″ moduli) of the whole mixture.