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United States Department of Agriculture

Agricultural Research Service

Title: Effect of Screw Geometry and Processing Conditions on Extruded Starch

Authors
item Jasberg, Brian
item Willett, Julious

Submitted to: Society of Plastics Engineers Proceedings
Publication Type: Proceedings
Publication Acceptance Date: May 9, 1996
Publication Date: N/A

Interpretive Summary: Three corn starches (normal maize, waxy maize, and high amylose) were extruded with three different screws at different conditions to see if starch was degraded differently. Normal maize and waxy maize run at low speeds and low temperatures had "ghosts" (remnants of starch granules) present. Higher temperature or screw speed reduced or eliminated the ghosts. In general, results indicated that as more work was put into the starch in the form of torque, the starch product was degraded more. Higher screw speed, higher temperature, and smaller screw clearances increased the degradation. These results will help researchers and producers of starch-based biodegradable items understand how processing can affect starch structure and properties, an important factor in product strength and flexibility.

Technical Abstract: Normal, waxy, and high amylose corn starches at 22.5% moisture were processed in a Brabender 19 mm single screw extruder. Three screws (30/1 L/D) were evaluated: a 1/1 compression ratio screw, a 3/1 compression ratio screw, and a screw with a spiral fluted dispersive mixing section. Each screw was tested at three speeds (25, 50, 100 rpm) and two temperatures (120 deg C and 160 deg C). While the mean residence times were similar for all three screws, the mixing screw had a narrower residence time distribution. Extrudate morphology was examined using scanning electron microscopy (SEM) and X-ray diffraction. At 120 deg C and low screw speed, normal and waxy starches possessed residual granule structure, and the waxy starch retained significant crystallinity. Increasing the screw speed eliminated the granule structure in the normal starch, but some residual crystallinity remained in the waxy starch. At 160 deg C, the residual structure of all three starches was disrupted regardless of the screw. Intrinsic viscosity measurements on the normal starch after extrusion showed that the degree of degradation increased with increasing specific mechanical energy and increasing temperature. These results show that a balance between thermal and shear energy is required when extruding starch.

Last Modified: 11/23/2014
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