Residence Time Distribution for Evaluating Flow Patterns and Mixing Actions of Rice Extruded with Thermostable α-Amylase

Abstract

The effect of enzyme concentration, feed rate, screw speed, moisture content, and barrel temperature on residence time distributions (RTDs) of rice flour in a co-rotating twin-screw extruder had been broadly investigated. Feed rate and screw speed as the most important operating variables affecting the mean residence time (MRT) were further emphasized by using a second-order central composite design for achieving high-efficiency enzymatic extrusion with other variables constant. Increasing enzyme concentration to 1‰ (db of starch), the MRT increased probably due to the largely reduced viscosity of extrudate caused by rapid gelatinization and degradation of starch. Limited range of moisture content and barrel temperature had only slight impact on MRT and axial mixing considering the required high enzymatic activity relying on their levels sensitively. Flow pattern of extruded rice with thermostable α-amylase was close to perfect mixing flow. Seven flow models were compared to fit the RTD data, and Yeh-Jaw simplified model showed the best fitting results for enzymatic extrusion. Overall, the mechanism of introduced enzyme affecting MRT and mixing actions of extrusion process is of industrial implication for main ingredients reactions and nutrients to retain during enzymatic extrusion.