Behaviour of gelatinisation and viscosity in soy sweet potato mixtures by single screw extrusion: A response surface analysis

Mixtures of soybean and sweet potato (Ipomoea batatas L. Lam) flour, contai ning 18% moisture on dry matter were extruded in a single screw extruder: T he feed supply consisted of different sweet potato/soybean ratios. The vari able process conditions used were screw speed (rpm) and die diameter Cold p aste, hot end viscosity and degree of gelatinisation (starch damage) were d etermined. Quadratic response surfaces were fitted to these responses. At s mall die diameters an increase in both screw speed and percentage of sweet potato raised cold paste viscosity, whereas at large die diameters a reduct ion was found for hot end viscosity. A quadratic effect of sweet potato was found, with a minimum at +/-76%. At small die diameters an increase in the percentage of sweet potato raised the degree of gelatinisation, whereas an opposite effect was found at large die diameters. (C) 1999 Elsevier Scienc e Limited. All rights reserved.