Rheological properties of whey protein/gum arabic coacervates

Abstract

Complex coacervation in whey protein (WP)/gum arabic (GA) mixtures occurred in a specific pH window between 2.5 and 4.8. After phase separation, a concentrated polymer (also called coacervate) phase was obtained, whose viscoelastic properties were studied at various pH values. The viscosity of the WP/GA coacervate exhibited a surprisingly low shear-rate dependence, especially at pH 4.0, from 0.3 to 30 s−1 and a shear thinning above 30 s−1, indicating a structural change. Hysteresis in the flow curve was measured at the pH values at which the electrostatic interactions were the strongest. Hysteresis was due to a slow structural rearrangement of the coacervate phase and, with time, the initial viscosity was completely recovered, showing that structural changes were reversible. In frequency sweep experiments, the values of the viscous modulus G″ were up to ten times higher than the values of the elastic modulus G′, indicating the highly viscous character of the coacervates. pH 4.0 appeared to be the pH at which the coacervate phase was the most concentrated in biopolymer (Cp=32%), and at which the highest viscosity was measured. By decoupling the effect of biopolymer concentration and electrostatic interactions, it appeared that the high viscosity of the WP/GA coacervates was mainly due to the strong electrostatic interactions between WP and GA at low pH. The weaker the electrostatic interaction was, the lower the viscosity, especially at pH 4.5 and 3.0. The viscous behavior of the coacervates showed parallels with that of concentrated latex dispersions. WP/GA particles would consist of a GA polymer chain (as in latex) but now crosslinked by the electrostatic interactions with WP.