HIGH-MOISTURE TWIN-SCREW EXTRUSION OF SAGO STARCH .1. INFLUENCE ON GRANULE MORPHOLOGY AND STRUCTURE

Effects of barrel temperature (81-149 degrees C) and screw speed (315- 486rpm) on extrusion processing of sage starch in a co-rotating twin-s crew extruder under a high moisture system (34-47%) were investigated using response surface methodology. Structural changes were characteri sed by measuring water solubility index (WSI), water absorption index (WAI), degree of gelatinisation (DG), dextrose equivalent (DE) and hig h performance size-exclusion chromatography (HPSEC) profiles of the ex trudates. Thermomechanical processing of sage starch in the twin-screw extruder at the high moisture (34-47%) system led to shear-induced li mited degradation and starch phase transitions (a composite melting ge latinisation process). Strong positive correlations between WAI, WSI a nd DG showed that gelatinisation was the fundamental mechanism in this high moisture system rather than dextrinisation. Processing-induced s olubility increased at the expense of water absorption. Low WSI (4.5-1 8.1%) is ascribed to the presence of structures of either granular cry stallite remnants or rearrangement of bonds during extrusion. Copyrigh t (C) 1996 Published by Elsevier Science Ltd