<p dir="ltr">Extrusion is a complex process that is difficult to model due to the complex geometry. In addition, modeling the flow of a shear and thermal sensitive material such as waxy corn starch further complicates the problem. Starch undergoes three main transformations during processing: 1. Gelatinization, 2. Melting, and 3. Fragmentation. The first two can be combined into starch conversion and have been studied in detail, along with their effect on viscosity.</p><p dir="ltr">This work extruded waxy corn starch using the “NASA” autogenous single screw extruder with and without steam locks at moisture contents of 30% w.b. and 35% w.b. as well as screw speeds of 300 rpm and 600 rpm. A Brabender single screw was used at 100 rpm, 35% w.b., and 140°C to obtain starch at another molecular weight. Molecular weight was measured using HPSEC-MALLS-RI, and viscosity was measured using a capillary rheometer. Starch conversion was checked by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC).</p><p dir="ltr">The work extended upon previous rheological models by separating the lumped SME (specific mechanical energy) parameter into degree of starch conversion and molecular weight reduction. The new viscosity model can be combined with kinetics to predict rheology in computational fluid dynamics models that model the extrusion process. The approach can aid in designing the extrusion process and other unit operations by predicting extrusion characteristics without having to build the new design using a trial-and-error approach.</p><p dir="ltr">Additionally, a method was investigated to decouple SME into an average shear rate and mean residence time by setting SME equal to SEC (specific energy consumption). Shear history was found from the decoupled average shear rate and mean residence time. Shear history was a worse predictor for molecular weight reduction than SME alone because it was derived from SME with approximated average values from the extrusion trials.</p><p dir="ltr">Finally, the effect of steam locks in the “NASA” extruder was investigated and found to marginally reduce molecular weight, reduce the mass flow rate and increase the mean residence time for every condition except at a screw speed of 600 rpm and moisture content of 30% w.b. The work as a whole demonstrates the importance of understanding how materials change during processing.</p>