Modeling shearing resistance of powdery starch for simulation studies of extrusion cooking processes
1998
Qu, D. | Wang, S.S.
Modeling extrusion cooking of starch requires the use of appropriate intrinsic kinetic equations. Knowledge of shear stress profile in a particulate system such as that in the transition zone of an extruder is crucial for calculation of degrees of conversion (gelatinization and melting) of starch. Shearing resistance of moistened powder starch as affected by temperature, normal pressure, effective shear rate, moisture content, and degree of starch conversion was studied in a model system. A modified vane shear testing device was used for experimental measurement of each individual effect. Within the experimental ranges studied (temperature 30 to 80 degrees C, moisture content 16 to 35 %, effective shear rate 0 to 550 s(-1), starch conversion 0 to 0.36 (36 %), and normal pressure 0 to 83 kPa) parameter values in each functionality were evaluated. A single mathematical model useful for calculating shear stress was obtained by combining these effects in a single relationship. A good correlation was shown when the calculated shear stress was plotted against the experimentally measured. The methodologies developed here are useful for the studies of powdery food materials. The potential application of this correlation to the simulation of extrusion processes was discussed.
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