Ultrastructural Changes Occurring During Thermoplastic Extrusion of Soybean Grits

Changes occurring in the ultrastructure of soybean grits during thermoplastic or cooking extrusion were studied with the scanning electron microscope. The extruded product was obtained with a Wenger X‐5 laboratory extruder under one set of conditions (feel moisture content: 25%; screw speed: 800 rpm; max barrel temp: 145° C; temp at die: 120°C). At steady state, the extruder barrel was disassembled and samples taken from seven preselected areas along the screw. Comminution and mixing predominate first until cooking temperatures are reached in the last turns of the screw. In this zone, completely disrupted cells are arranged into strands that orient into thin fibrils at the die. Pressure release upon exit of the extrudate from the unit causes radial flow with the formation of new, more extended surfaces. Only the combined effect of shear, heat and pressure release is shown to be responsibte for texturization.