Soy protein supplementation increases the nutritional value of starch-based expanded snacks. A systematic study was conducted to serve as baseline for optimizing the addition of soy protein concentrate (SPC). Physical and microstructural properties of native corn starch-soy protein concentrate (CS-SPC) extrudates were investigated in relation to the macromolecular changes in starch during extrusion. The effects of extruder screw speed (230 and 330 rpm) and SPC concentration (0%, 5%, 10%, 15%, 20%) on the abovementioned parameters were determined. Increasing screw speed resulted in higher specific mechanical energy (SME) and expansion, and lower mechanical strength. On the other hand, addition of 5-20% SPC led to lower SME and expansion, and higher mechanical strength. X-ray micrographs showed smaller yet more cells, and cell wall thickening with SPC addition. Water absorption index increased and water solubility index decreased with increase in screw speed and SPC level. Increasing screw speed resulted in a slight shift towards smaller molecular weight fractions of starch, as determined by gel permeation chromatography. (c) 2008 Elsevier Ltd. All rights reserved.