Rheological modelling of enzymatic extrusion of rice starch

Abstract

A rice starch mixture pre-mixed with thermostable α-amylase was extruded in a pilot co-rotating twin-screw extruder, with the die replaced by a slit rheometer. The apparent viscosity at the slit wall was determined for a variety of operating conditions. The factors studied were barrel temperature (70–110 °C), water content (550–650 g/kg rice mixture), flow rate (8 × 10−7 to 2.5 × 10−6 m3/s) and enzyme concentration (0·01–0·1 g/kg). The same screw configuration and rotational speed (225 rpm) were used. A rheological model incorporating the integrated effect of extrusion and enzymatic activity was developed with a stepwise regression of the experimental data, analysing after each step the variability of the model parameters with the different factors. It was found that for the range of experimental conditions in question the influence of each factor could be considered separately with good results, leading to a power-law type model with a constant behaviour index. The parameters obtained describing the influence of water content, temperature and shear rate were within the range of available literature data for non-enzymatic extrusion and the influence of the enzyme concentration was well described by a power-law dependency.