Heat pretreatment was investigated as a way to improve the alpha-amylase-catalyzed hydrolysis of granular corn starch at high concentration (45%, w/w). Native and preheated starches were hydrolyzed in the granular state for 1 h at 40 degrees C using a commercially available a-amylase. The initial dextrose equivalent (DE) value of the native granular starch (8.0%) increased significantly (to 12.6%) after pretreatment at 60 degrees C for 15 min. Microscopic analysis of the starch granules indicated that heat pretreatment increased the number and size of the pores and pinholes on the granule surface, which facilitated the adsorption and subsequent penetration of enzyme molecules. Furthermore, the amylose present in amorphous regions leached rapidly when corn starch was incubated at 60 degrees C, suggesting weaker interactions within the granules. The DE value of the hydrolysate showed a linear dependence on the amount of amylose that leached during the course of heat pretreatment (R-2 = 0.9771). Mobilization of amylose chains within the granule, caused by the heat pretreatment, may have allowed the enzyme to make greater use of amylose, which is its primary substrate during starch granule hydrolysis. In addition, a 35% reduction in the K-m value showed that heat pretreatment increased the affinity of a amylase for the starch granules. This can be attributed to the mobilization of amylose chains within the granule and expansion of the surface pinholes. These effects demonstrate that proper heat pretreatment of granular starch before amylolysis has great potential in industry.