Extrusion has been widely used for the processing of cereal grains because this thermoplastic technology is continuous and saves unit operations and energy. In extrusion, the materials are subjected to heating, mixing, and shearing, resulting in physical and chemical changes during its passage through the extruder. The major advantages of extrusion include: improvement of starch digestibility and reduction of its molecular weight, production of free sugars and dextrins, changes in the native structure of both starch granules and proteins and reduced viscosity of fermentation broths. Therefore, extrusion could be an effective process to improve the bioconversion rate of sorghum starch (Zhan et al., 2006).

An innovative processing technology patented by researchers of Cornell University combines extrusion process and supercritical-fluid technology. The main difference between supercritical-fluid (SCFX) and conventional extrusion is the injection of supercritical carbon dioxide, which replaces water as blowing agent for expansion. The injection of supercritical- fluid carbon dioxide breaks the intimate bonds between starch granules and protein matrix and results in the improvement of starch availability (Zhan et al., 2006). These researchers suggested that SCFX produces molecular degradation of starch during extrusion of sorghum. This process also increased about 8% the protein digestibility, the measurable starch content, the free sugar concentration and gelatinized starch and other parameters that increased ethanol yield (+5%) and boosted fermentation efficiency compared to the non­extruded counterpart. The SCFX cooking also affected the crude fiber, chemical fraction that after microscope examination showed disruption and fissures. These authors describe the sorghum extrudates with "porous structure". Thus, this thermoplastic procedure was indeed effective as pretreatment to improve bioconversion of sorghum into ethanol.