Effect of Shear Stress Extrusion Intensity on Plasticized Corn Flour Structure: Proteins Role and Distribution

Abstract : Plasticized corn flour-based materials were prepared by extrusion and injection molding. Extrusion of corn flour blends (75% wet basis (wb)-glycerol (5 or 10% wb)-water) was performed in a twin-screw extruder with either one or three shearing zones. Native corn flour is mainly composed of corn starch granules surrounded by proteins layers. Therefore, the destructuration of corn flour by thermomechanical treatments was analyzed (i) by techniques essentially allowing to monitor corn starch amorphization (differential scanning calorimetry, X-ray diffractometry, determination of water sorption isotherms, susceptibility to hydrolysis by amylolytic enzymes) (ii) and via proteins layers role and distribution observed by confocal scanning laser microscopy and comparing the susceptibility of corn starch to hydrolysis by amylolytic enzymes in the presence or not of a protease. Both corn starch granules amorphization and proteins dispersion and aggregation were more pronounced for materials extruded in a screw profile with three shearing zones. For materials extruded in a screw profile with one shearing zone, the amorphization of starch was higher in materials made with 5% wb glycerol, whereas the proteins dispersion and aggregation was more pronounced in materials made with 10% wb glycerol. A barrier role of proteins to hydrolysis of corn starch by amylolytic enzymes was demonstrated and discussed.