The influence of three ingredients (bran, sucrose, magnesium carbonate) commonly used in extrusion-cooking of wheat flour-type formulations was studied. Extrusion-cooking was carried out in a twin-screw corotating extruder. The effects of the ingredients were analyzed by determining extrusion parameters, physical properties (apparent density, modulus of deformability and breaking stress) and structural properties (computerized image analysis of the transverse cut of extrudate). An increase in apparent density occurred when concentration of any ingredient increased. Cell number per pixel area increased greatly while average cell size decreased as bran concentration increased from 0 to 16%. Average cell size increased as magnesium carbonate increased from 0 to 0.4%, but cell size decreased above 0.4%.