Submitted to: American Institute of Chemical Engineers Annual Meeting
Publication Type: Proceedings
Publication Acceptance Date: November 14, 1996
Publication Date: N/A
Interpretive Summary: Extrusion is a process commonly used to convert starch into finished products, in both the food and plastics industries. In the plastics industry, it is common for materials to be processed more than one time before conversion to final product. These processes can cause breakdown of starch, so it is important to understand the influence of multiple processing steps on starch structure. We have found that the degradation of starch is strongly correlated with certain processing parameters, so that degradation during multiple processing steps can be estimated. This allows the relationship between extrusion conditions and final product properties to be predicted more accurately. These results are useful to researchers who use extrusion to process starch and other process-sensitive materials, and scientists involved in the study of processing of polymers in general.
Technical Abstract: The degradation of starch during extrusion and the role of specific mechanical energy (SME) in this process have been widely studied for single pass extrusion. Multiple extrusion histories are not uncommon in the plastics industry, but little if any has been reported on their effects on starch. Native waxy maize starch (app. 98% amylopectin) was initially converted to a thermoplastic by twin screw extrusion. This extrudate was equilibrated to either 18% or 23% moisture content, and subsequently re-extruded in a single screw extruder (3:1 compression screw) at 110 deg C or 130 deg C. Melt viscosity data were calculated using the output-pressure data from the second pass. The melts exhibited shear thinning behavior; the power law index increased with temperature, and slightly with moisture content. Molecular weights of selected second-pass extrudates, as well as the native starch and the first-pass extrudate, were measured by light scattering in dimethyl sulfoxide/water. The initial extrusion pass reduced the molecular weight from 300 million to 50 million. Molecular weight reductions in the second pass increased with increasing SME. A first order expression was shown to fit the MW-SME data with a correlation coefficient of 0.91. Implications of the degradation on extrusion processing of starch and the use of single screw extruders for rheological characterization will be discussed.