Rheological properties of calcium treated hydroxypropyl rice starches

Unisolated rice starch was used for hydroxypropylation and calcium complexation. Viscosity was measured at different starch concentrations, temperatures, and shear rates. The viscosity decreased with increase of calcium concentrations and shear rates. Temperature, starch concentrations and levels of Molar Substitution (MS) enhanced the viscosity. Hysteresis behaviour was markedly influenced by levels of calcium and MS. Higher viscosity was observed in hydroxypropyl starch. It is assumed that starch. molecules exist in an expanded and/or extended forms which may result in higher viscosity. Calcium treated starch shows lower viscosity. Calcium ions possibly squeezed the starch molecules into 8 tighter configuration and/or calcium may compete with starch for water causing the limitation of water thus resulting in reduction of viscosity. Higher viscosity was observed at higher temperatures. Higher temperature cause the starch granules swelling thus produce higher frictional forces responsible for higher viscosity.