Abstract
The hydration and swelling of starch in cooked rice was observed with scanning electron microscopy (SEM) with the following four sample preparation methods : rapid freezing after soaking in 50% ethanol (cryo-SEM); chemical fixation with tannin-osmium (chemical fixation); directly freeze-drying (freeze-drying); observation at a vacuum pressure of 30 Pa without freezing or fixation (variable-pressure SEM).
No information on the hydration and swelling of the starch in cooked rice could be obtained for the samples prepared by chemical fixation and variable-pressure SEM. Only a dense structure, which is considered as having been the original structure, was apparent. The samples prepared by freeze-drying showed a network structure in the whole part of the endosperm cell which is thought to have been a secondary structure caused by ice crystal formation because the size of the network depended on the freezing rate. On the other hand, the fine structure of cooked rice could be clearly observed by rapid freezing of the sample after soaking in 50% ethanol. This method was most appropriate for examining the hydration and swelling of the starch in cooked rice, even though it had a freezing process in the preparation procedure.
