Physical characterization of starch extrudates as a function of melting transitions and extrusion conditions

The objective of this study was to investigate the relationships between starch melt transition characteristics, extrusion conditions, and final product properties. Cornstarch was extruded using a corotating twin-screw extruder at varying moisture content, medium/high screw configuration, and 300/400-rpm screw speeds. Extrudates were evaluated for bulk density, expansion ratio, cell structure, bending strength, thermal and pasting properties. Temperature change ((delta)T) was defined as the temperature difference between the native starch melting temperature and its melt temperature just behind the die during extrusion at a given moisture content. (delta)T was significantly affected by starch moisture content and mechanical shearing, which controlled the melting behavior of the starch in the barrel. Amylose-lipid complex formation during extrusion increased as (delta)T increased and leveled off at (delta)T of about 20(degree)C. Pasting peak viscosity of the starch extrudates decreased as (delta)T increased and leveled off at (delta)T of about 15(degree)C. Within the same range of mechanical shearing intensity, extrudate bulk density and bending strength decreased linearly as (delta)T increased. Extrudate expansion was negatively correlated to bulk density.