Experimental study of screw configuration effects in the twin-screw extrusion-cooking of maize grits

The effects of screw configuration on the product properties, die pressure and screw torque in the twin-screw extrusion-cooking of maize grits is described. Extrudate bulk density, water solubility and water absorpiton were measured for a set range of extrusion moisture contents and barrel temperatures for each of four screw configurations of different conveying efficiencies. It is shown that screw configuration is a potent variable in determining product properties. The variation in water solubility with specific mechanical energy input conforms to Meuser's model. The screw configuration controls the specific mechanical energy input range. Screw configurations of low conveying efficiency give rise to low-viscosity melts which expand into low-bulk-density products. Dead-stopped runs showed that the conveying efficiency controls the degree of fill in the extruder barrel. Typically, the mean residence time was increased from 56 to 94 s by decreasing the conveying efficiency of the screws.