Modeling macromolecular degradation of corn starch in a twin screw extruder

Macromolecular degradation of starch in a twin screw extruder was modeled. A shear cell having well-defined flow conditions described earlier was used to measure peak viscosity of corn starch melts at various moisture contents and temperatures. Shear rate and elongation rate distributions in the extruder were estimated from numerical calculations from literature and elongational viscosity was estimated using the Trouton ratio. In this way, stresses in the extruder were calculated and, using relations on maximum stress vs. intrinsic viscosity obtained in earlier work, the expected relative intrinsic viscosity was calculated. The model gave a good prediction of relative intrinsic viscosity after extrusion at various temperatures (97-130 °C), moisture contents (23-45%) and screw speeds (90 or 140 rpm). This suggests that the use of pilot scale equipment having a well-defined flow pattern can be useful for understanding complex processes such as extrusion.