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 r ate and screw speed on the specific energy input during single-screw extrus ion cooking. Torque, raised by decreasing screw speed or increasing feed ra te, was found to be a power law function of the ratio of feed rate to screw speed (F-r/S-s) with r(2) > 0.94. Specific mechanical energy (SME) calcula ted from torque also was a power law function of F-r/S-s with r(2) > 0.84 a nd negative power law indices. The SME obtained was in the 225-481 kJ/kg ra nge. Thus the extruder can be considered low shear. Increasing SME raised t he die temperature and decreased both intrinsic viscosity and water absorpt ion index (WAI). The degree of gelatinization and intrinsic viscosity of ex trudates also were power law functions of F-r/S-s. The intrinsic viscosity correlated well with the degree of gelatinization, WAI, and cooking loss, a nd appeared to be a good index of the extrudate properties. Different screw profiles also affect torque measurement.