Effect of protein aggregates on the complex coacervation between β-lactoglobulin and acacia gum at pH 4.2

Abstract

The influence of protein aggregates on complex coacervation between β-lactoglobulin (β-lg) and acacia gum (AG) has been studied at pH 4.2 using phase diagrams, electrophoretic mobility measurements, laser light scattering and phase contrast optical microscopy. Removal of protein aggregates by centrifugation (pH 4.75, 1 h, 10,000g) led to an aggregate-free β-lg dispersion (AF-BLG) exhibiting a narrow polydispersity in molecular masses (M(w) ~ 18,500 and 64,500) and hydrodynamic radii (R(h) ~ 2.51 nm) as determined by GPC and QELS measurements, respectively. Phase diagram determination for the AF-BLG system revealed a two-phase region located in the water-reach corner. In addition, the highest total biopolymer concentration leading to phase separation (4.5 wt%) and the tie-lines shape typically accounted for complex coacervation. The position and extend of the two-phase region differed from that obtained using BLG dispersions (containing protein aggregates). This was partly due to the lowest M(w) and R(h) polydispersity of the AF-BLG, but also to the decrease of the electrophoretic mobility (μ(E)) of the AF-BLG induced by the protein aggregates elimination (loss of cations in the dispersion). The variation of the μ(E) of the AF-BLG/AG complexes with pH was similar to that of the BLG/AG ones, but of lower intensity due to the decrease of cations in the system. The size distribution of the BLG/AG complexes obtained at pH 4.2 was mainly governed by the presence of large-sized aggregates (10-50 μm). By contrast, AF-BLG/AG coacervates exhibited a volume average diameter (d43) that reached 220 μm for the 2:1 Pr:Ps ratio at pH 4.2, revealing less stability of particles in the system. These results were in agreement with phase contrast optical microscopy that revealed marked differences in the structure of BLG/AG complexes and microparticles as compared to that of AF-BLG/AG coacervates. (C) 2000 Elsevier Science Ltd.