Twin-screw extrusion of rice green gram blend: Extrusion and extrudate characteristics

A rice and green gram (Vigna radiata L.) blend (1:1, dry basis) was extruded using a co-rotating twin-screw extruder with a barrel diameter of 31 mm. The effects of barrel temperature (100-175 degrees C) and screw speed (100-400 rpm) on the extrusion system parameters (torque and specific mechanical energy) and extrudate characteristics (expansion ratio, density and maximum shear stress) were studied. The response functions (torque, specific mechanical energy, expansion ratio, extrudate density and maximum shear stress) were related (r greater than or equal to 0.870, p <0.01) to the process variables (screw speed and temperature) by second-order polynomials. The torque during extrusion was highest at the highest temperature levels. The specific mechanical energy linearly increased with screw speed. The maximum breaking (shear) stress of the product was highly sensitive to the levels of the extrusion variables and var-led up to 25-fold. Temperature and screw speed also imparted curvilinear effects on the extrusion and extrudate characteristics. High barrel temperature combined with a low screw speed is suitable for obtaining an expanded product. (C) 1997 Published by Elsevier Science Limited.