Amylose content and chemical modification effects on the extrusion of thermoplastic starch from maize

The effects of starch structural properties and starch modification on extruder operation were monitored via die pressure, motor torque, mean residence time and specific mechanical energy (SME). The structural properties studied involved variations in the ratios of amylose and amylopectin as well as the effect of a hydroxypropylated starch on the fore mentioned extruder properties. A full factorial design of experiments (DOE) was used to then determine the influence of starch type (unmodified starches with 0%, 28%, 50% and 80% amylose; 80% amylose hydroxylpropylated starch) and screw speed (250, 300 and 350 rpm) on these processing parameters. The effects of starch type and screw speed on extrusion operation that were systematically investigated using the DOE and have provided valuable insight into the relationships between starch structure and processing. The design of experiments showed that starch type for both unmodified and modified maize had a statistically significant effect on parameters such as torque, die pressure and specific mechanical energy and that screw speed also significantly effected specific mechanical energy. Residence time distributions differed according to starch type (amylose content, hydroxypropylation) and screw speed. The additional study of residence time distribution also gave an indication of the degree of mixing in the extruder. Starch type variations were apparent at low screw speed however at higher screw speed the influence of starch type decreased significantly. (C) 2008 Elsevier Ltd. All rights reserved.