Investigation of rheological properties and conformation of cassava starch in zinc chloride solution

Zinc chloride aqueous solution is a non-derivatizing solvent for starch, and can be used as a media for homogeneous starch modification. Here, the rheological properties of starch in ZnCl₂ aqueous solution under different conditions are considered. Specifically, a large number (120) of viscosity–shear-rate curves for starch–ZnCl₂ solutions with different starch contents, ZnCl₂ concentrations, and temperatures were obtained and modeled using a master curve method. In particular, the power-law index (n) from the modeling could accurately classify the starch molecules in ZnCl₂ aqueous solutions into an insoluble state, a dilute state, a semi-dilute state, and a sticky state, reflecting the different degrees of chain interactions in the solution. In particular, the sticky state would be mostly likely be controlled by the zinc–starch complex, which made the rheological behavior entirely different from that at low ZnCl₂ concentrations. The knowledge obtained from this study can be used to guide the homogeneous modification of starch and the fabrication of starch-based materials using green solvents such as aqueous ZnCl₂ with desired properties.