Rheological properties and phase transition of cornstarches with different amylose/amylopectin ratios under shear stress

The rheological properties and phase transition of cornstarch with low moisture content under shear stress was investigated using a mixer with twin-roller rotors by the variation of torque as functions of temperature and time. Cornstarches with different AM/AP ratios (wx: 4:96, maize: 29:71, Gelose 50: 62:38, and Gelose 80: 77:23) were used as model materials in the experimental work. The results showed that, as the AM content increased from 0 to 77%, the maximum torque increased from about 54 to 66 N/m and the final torque increased from about 27 to 47 N/m, respectively, and a longer mixing time was required to achieve a stable state (constant torque value). Microscopic and differential scanning calorimetry (DSC) studies showed that the phase transition reflected by changes of birefringence and endotherms could well be associated with the torque curve measured by the mixer. The higher gelatinization temperatures of the starches with higher AM content, and especially the existence of multi-phase transitions at high temperatures, could explain the rheological behaviors detected by the mixer. The technique can be used to study the starch melting and destructuration under shear stress with low moisture content (<45%), which represents the most of extrusion conditions for starch-based materials.