Modeling extrusion conversion of starch in a single-screw extruder

Basic phenomena involved in conversion of waxy corn starch at extrusion-cooking conditions were studied and the process was simulated using a comprehensive model covering the whole length of the extruder. The basic studies were emphasized on the tribological shearing resistance of powdery starch, the starch sol-gel transition behavior, and the mechanism of starch transformation under extrusion-cooking conditions. Effects of temperature, normal pressure, moisture content, shear rate and degree of conversion of starch on the shearing resistance of powdery starch were examined and a cohesive correlation of those variables describing the tribological shearing process was obtained. The starch sol-gel transition behavior was investigated under the guideline of the percolation concept. The transition was defined using starch granular connectivity which exhibited a function of moisture content and degree of conversion of starch. A critical value of conversion was found necessary for powdery starch to change from a discrete (sol phase) into a gel phase at any given moisture content. The existence of gelatinized and melted starches at the extrusion-cooking temperature and moisture content was identified and their properties and kinetic behavior were studied.; Significant bulk density change and the associated phase change of powdery starch were observed under extrusion-cooking conditions, which led to the division of functional zones in the extruder. Four zones were identified for single-screw extruders with a tapered screw, namely a starve feeding zone, a compression zone, a tribological (transition) zone, and a rheological (metering) zone. The tribological zone is characterized by a region filled with discrete starch powder to which rheological relations do not apply. Starch conversion induced by tribological shearing in the transition zone is important in converting the powdery starch into a rheological mass.; The process was simulated on an IBM-PC with a computer program developed using information obtained from the basic studies. The process kinetic equations for starch extrusion process, the rheological relations for starch dough in the rheological zone of the extruder, and the obtained tribological relations for powdery starch in the tribological zone of the extruder were used in a one-dimensional CSTR-in-series model system (a cascade of continuous stirred tank reactors) to calculate temperature, pressure, shear stress, and the degree of conversion of starch along the down channel direction of the extruder. The predicted results were compared with the extrusion experimental results and a fairly good consistency was shown. The potential application of this study is discussed.