Effects of feed rate and screw speed on operating characteristics and extrudate properties during single-screw extrusion cooking of rice flour

Rice flour (37% moisture content) was used to examine the effects of feed rate and screw speed on the specific energy input during single-screw extrusion cooking. Torque, raised by decreasing screw speed or increasing feed rate, was found to be a power law function of the ratio of feed rate to screw speed (F(r)/S(s)) with r2 > 0.94. Specific mechanical energy (SME) calculated from torque also was a power law function of F(r)/S(s) with r2 >0.84 and negative power law indices. The SME obtained was in the 225-481 kJ/kg range. Thus the extruder can be considered low shear. Increasing SME raised the die temperature and decreased both intrinsic viscosity and water absorption index (WAI). The degree of gelatinization and intrinsic viscosity of extrudates also were power law functions of F(r)/S(s). The intrinsic viscosity correlated well with the degree of gelatinization, WAI, and cooking loss, and appeared to be a good index of the extrudate properties. Different screw profiles also affect torque measurement.