Starch-g-poly(methyl acrylate) containing 12.3, 31.9, 51.7, and 58.3% PMA, by weight, were prepared by eerie ammonium nitrate-initiated polymerization of methyl acrylate onto granular cornstarch. The granular structures of these graft copolymers were not disrupted by steam jet cooking at 140 degrees C. At most, only 13% of the polymer was dissolved, and this soluble fraction was comprised largely of starch. The probability of crosslinking within these graft copolymer granules was considered. Physical properties of extruded ribbons depended upon whether or not granular graft copolymers were jet cooked prior to extrusion. Although tensile strengths were not greatly affected by steam jet cooking, cooked samples showed significant increases in both percent elongation and tear resistance. The effects of jet cooking upon the properties of extruded ribbons can be explained by gelatinization of starch within the grafted starch granules. Although jet-cooked granules still remain intact, gelatinization of the starch moiety causes these granules to be less rigid, more deformable, and more easily plasticized by small amounts of water. Loss of starch crystallinity after steam jet cooking was proved by both differential scanning calorimetry and X-ray diffraction.