On the use of an internal mixer for the rheological characterization of maize starch

Abstract

The rheological behaviour of hydrated maize starch is investigated by means of a Haake internal mixer equipped with a sealed chamber. Results were obtained at temperatures between 89 and 115°C with water content between 25 and 30%. Through a proper calibration torque measurements and rotation speeds are converted to shear stress and shear rate data and this leads to the plot of a flow curve in the 10 - 1000 s-1 range. The data are compared with results of capillary rheometer and show that the mixer enables a reproducible plastification of the maize starch. The viscosity of the maize starch in the high shear rate range can be described by an apparent power law taking into account the moisture and temperature effects. On a larger range of shear rate, a Carreau law is preferred but the dependence on the temperature can only be described with shift factors that require a moisture dependent activation energy. Finally, this later discrepancy can be avoided by using a reference temperature to fulfil the iso-free volume condition by taking a constant temperature difference towards the glass transition of the samples. The glass transition is calculated by the Couschman and Karasz equation.