Time-course changes of the fine structure of rice starch grains during cooking process by scanning electron microscopy

Fine structural changes accompanied with the gelatinization of starch granules in a milled rice grain during cooking were observed with a scanning electron microscope using three rice cultivars, Koshihikari, Kirara 397 and Thailand rice (Indica type). In cv. Koshihikari, the change of starch grains starred from the surface of the amyloplast envelope after 10 minutes of cooking (temperature at the central part of rice cooker was 45 degreesC). After 15 minutes of cooking (51.3 degreesC), starch granules in the first cell layer of a milled rice grain became swollen and a fibrous structure was observed on the surface of each starch granule. By the development of the mesh-like structure on the surface of starch granules, each amyloplast changed to one unit body after 20 minutes of cooking (98.5 degreesC). Then: the bodies in each endosperm cell were fused and developed into the unit of irregular gel-like structure. The swelling of starch granules and the Formation of irregular gel-like and mesh-like structures during cooking processes were faster in the outer layer than at the central part of a grain. These changes occurred faster in cv, Koshihikari than in Kirara 397 and Thailand rice. The pore size in the mesh-like structure was larger in the outer layer of a grain. We suggest thar, in the gelatinization processes of starch, water molecules are incorporated into the insoluble high density amylopectin molecules as the main components of a starch grain resulting in swelling of starch and change to the soluble structure with a low molecular density.