The ratio of soy protein isolate to cornstarch was studied in the extrusion of four different formulations of soy protein plastic in order to qualitatively examine the surface quality upon extrusion.Levels of glycerol and moisture were constant across all samples, and no other additives were used in the mixtures.Mixtures were made using a planetary mixer, and then extruded using a single-screw extruder equipped with a 10.16 centimeter sheeting die with a 0.152-centimeter opening.The surface of the extruded plastic became smoother with increasing starch content, but the processability became too difficult with very high starch levels.