Viscosity modeling of aquafeed dough in a single screw extruder

Abstract

During extrusion, the shearing effects of screws with thermal energy generated by viscous dissipation modifies the rheological properties of raw materials due to the physical and chemical changes of biopolymers. Therefore, it is essential to investigate the rheological properties of dough during extrusion. In the present study the viscosity of dough within the extruder was analyzed and the effect of extrusion processing parameters on the viscosity was evaluated. A viscosity model was developed to study the influence of soy white flakes during extrusion of fish feed and the effect of extrusion processing parameters on mass flow rate, torque, specific mechanical energy and viscosity of dough inside a single screw extruder were evaluated. Full factorial design was used for three variables each at three levels which generated total 27 experimental runs. Process conditions concern different screw speed (80, 120 and 160 rpm), barrel temperature (100, 120 and 140 oC) and soy white flakes (SWF) contents (30, 40 and 50% db). Round capillary die rheometer was used to determine the rheological data of the dough within the extruder. It was observed that increase in the soy white flakes content from 30% to 50 % resulted in a 39%, 40% and 23% increase in mass flow rate, torque and specific mechanical energy and 39% decrease in the apparent viscosity respectively. The specific mechanical energy, mass flow rate is increased by 15%, 41% respectively and viscosity is decreased by 31.2% when screw speed is increased from 80 rpm to 160 rpm. Higher barrel and die temperature led to decrease in the apparent viscosity of the dough, torque and specific mechanical energy. The viscosity model for soy white flakes-based dough developed in this study can be used to simulate extrusion of aquafeed.