Effect of extrusion processing history on the rheology of corn meal

Slit and capillary dies attached in-line to an extruder are a popular configuration for obtaining the flow curves of food doughs during extrusion-cooking. However, it is demonstrated in this paper that the use of screw speed, especially for a single-screw extruder, to vary the shear rate through the die can result in erroneous flow curves for single-screw extruders. Furthermore, the use of a sidestream valve to vary the flow rate for a given screw speed is shown to result in flow curves that are different and better behaved. Such a procedure will be helpful in single-screw extruder systems, not only in obtaining the true flow curves of the food dough, but also in rheological measurements with in-line rheometers. Data for hole pressure (a measure of the first normal stress difference) are also presented to demonstrate the validity of using the sidestream valve and the error in using the screw speed to obtain rheological measurements during extrusion of starchy foods.