Observation by Photomicroscopic and X-ray Diffraction Method of Heat-Moisture Treatment on Starch Granules

Potato, corn, waxy corn, rice and waxy rice starches were heated under 100% humidity at temperatures ranging from 120 to 135°C. The morphology and the crystal structure of these treated starches were investigated by microscopic observations and X-ray diffraction measurements. Although all starches changed more or less their morphological features and granular structures, the mode and extent of change depended on the kind of starch and the condition used for the treatment. Socalled annealing, which was detected as changes in shape and peak intensity of X-ray diffraction pattern, was observed for the starch, treated at temperature near 130°C ; whereas for the starch treated at higher temperature melting behavior was observed. The shape of potato starch granulesremained almost intact, even by 130°C treatment, but that of other starch granules did not remain, especially, waxy rice and waxy corn starch granules which showed a paste-like feature. It was found that the difference in damage, judging from the morphological feature of starches, was not consistent with the extent of depolymerization of starch molecules. Potato and corn starch molecules were easily depolymerized compared to rice and waxy corn starch molecules. It was also found that the def atted starch was more labile to the heat-moisture treatment than undef atted starch.