EXTRUSION OF WAXY MAIZE STARCH - MELT RHEOLOGY AND MOLECULAR-WEIGHT DEGRADATION OF AMYLOPECTIN

Waxy maize starch was extruded initially using a co-rotating twin-scre w extruder. This extrudate was then equilibrated to either 18 or 23% m oisture content, and subsequently re-extruded in a single-screw extrud er at either 110 or 130 degrees C to characterize the melt viscosity. The melts exhibited shear thinning behaviour under all conditions; the power law index m increased with temperature, but did not significant ly change with moisture content. Molecular weights of selected samples were measured by multi-angle laser light scattering (MALLS) in dimeth yl sulfoxide/water (90/10 v/v). The initial extrusion pass reduced the molecular weight from 336 x 10(6) to 40 x 10(6). Molecular weight red uctions during the second pass increased with increasing specific mech anical energy, and were less drastic than the initial drop. The observ ed M-W values display a semi-logarithmic dependence on the specific me chanical energy, with r(2) = 0.925. Specific mechanical energy can the refore be used to estimate molecular weight degradation, regardless of variations in extruder type or conditions during multiple passes. Mol ecular weight distributions determined using gel permeation chromatogr aphy correlated well with the single-point molecular weight determinat ions. (C) 1997 Published by Elsevier Science Ltd.