High moisture twin-screw extrusion of sago starch: 1. Influence on granule morphology and structure

Abstract

Effects of barrel temperature (81-149°C) and screw speed (315-486rpm) on extrusion processing of sago starch in a co-rotating twin-screw extruder under a high moisture system (34-47%) were investigated using response surface methodology. Structural changes were characterised by measuring water solubility index (WSI), water absorption index (WAI), degree of gelatinisation (DG), dextrose equivalent (DE) and high performance size-exclusion chromatography (HPSEC) profiles of the extrudates. Thermomechanical processing of sago starch in the twin-screw extruder at the high moisture (34-47%) system led to shear-induced limited degradation and starch phase transitions (a composite melting gelatinisation process). Strong positive correlations between WAI, WSI and DG showed that gelatinisation was the fundamental mechanism in this high moisture system rather than dextrinisation. Processing-induced solubility increased at the expense of water absorption. Low WSI (4.5-18.1%) is ascribed to the presence of structures of either granular crystallite remnants or rearrangement of bonds during extrusion. Copyright © 1996 Published by Elsevier Science Ltd.