Impact of pH, ionic strength and chitosan charge density on chitosan/casein complexation and phase behavior

The interaction between polysaccharides and proteins has attracted great interests and the phase behavior and colloidal properties of complexes between chitosan, a positively charged polysaccharide, and proteins still need careful investigation. Here, we use chitosan/casein as model system and monitor their complex size, zeta-potential, light scattering intensity and phase behavior as functions of chitosan deacetylation degree, pH and ionic strength. Chitosan flocculates casein at low polysaccharide concentrations, while stabilizes casein at moderate to high concentrations. Such flocculation and stabilization ability are dependent on the linear charge density of chitosan, where high charge density chitosan saturates casein surface charge and stabilizes the complexes at lower polysaccharide concentration. The complexes are stable at acidic pH where chitosan is highly protonated and the addition of NaCl leads to the reduction in both size and zeta-potential and facilitates complex aggregation. The phase maps are constructed to provide guidelines for further studies on chitosan/casein and other chitosan/protein complexes.