Residence time distributions of starch with high moisture content in a single-screw extruder

Starch was extruded with sodium trimetaphosphate and sodium hydroxide in a single-screw extruder with a mixing element at 28.5%, 35.5%, and 41.2% moisture content (wet basis) and 90, 140 and 190 rpm screw speeds. The effects of moisture content of starch on mean residence time, spread of residence time distribution curve, and flow pattern were determined at three screw speeds. Increasing the screw speed reduced mean residence time at the moisture contents between 28.5% and 41.2%. Increasing the moisture content from 28.5% to 41.2% did not significantly affect the mean residence time. The spread of residence time distribution curve was reduced as screw speed was increased, but was not affected as moisture content was increased from 28.5% to 41.2%. The flow pattern was between plug flow and mixed flow. Two flow models were used to represent the flow pattern in the extruder. The parameters of each model were determined by a nonlinear regression, and the fitness of experimental data to the flow models was examined. (C) 2004 Elsevier Ltd. All rights reserved.