Complex coacervation of ovalbumin-carboxymethylcellulose assessed by isothermal titration calorimeter and rheology: Effect of ionic strength and charge density of polysaccharide

Abstract

The objective of this study was to investigate the complex coacervation between ovalbumin (OVA) and carboxymethylcellulose (CMC) with different degrees of substitution (CMC 0.7 and CMC 1.2). Turbidity titration showed that complexes or coacervates could form between OVA and CMC by electrostatic interactions depending on pH changes. Specially, the critical pH values (pHc, pHφ1) increased with the salt ion concentration raising from 0 to 20 mM, while further increasing the ionic strength reduced the formation of complex coacervates. The ITC results also showed that when CNaCl = 20 mM, the complex coacervation between OVA and CMC exhibited the highest stoichiometric ratio and binding constant. When CNaCl≥200 mM, the complex coacervation changed from spontaneous exothermic (ΔG<0, ΔH<0) to endothermic (ΔG<0, ΔH>0) due to the shielding effect by high concentration of salt ions. In addition, dynamic rheological properties showed that the OVA/CMC coacervates displayed the strongest elastic modulus at CNaCl = 20 mM, and the elastic modulus of all samples were greater than the viscous modulus. These findings all suggested a salt-enhanced effect at lower salt concentrations or a salt-reduced effect at higher salt concentrations. On the other hand, the stoichiometric and viscoelastic properties of OVA/CMC 1.2 coacervates were greater than OVA/CMC 0.7 coacervates, indicating that CMC 1.2 had stronger protein binding capacity owing to higher charge density.