Evaluation of starch retrogradation by X-ray diffraction using a water-addition method

The X-ray diffraction patterns of dry samples of starch often contain broad, unresolved peaks. When starch samples contain water, the diffraction peak of recrystallized starch becomes clear, although the peak intensity depends on the water content. Therefore, in this study, the water content of starch was controlled by adding a certain amount of water to the freeze-dried sample to reduce the measurement error. By adding water to the freeze-dried samples of cooked rice, the diffraction peak became clear and the increase in starch retrogradation with time could be measured more accurately. Accordingly, this water-addition method was confirmed to be effective in improving the measurement accuracy of starch retrogradation. Furthermore, with water addition, the tiny difference in retrogradation caused by the addition of α-glucosidase and a branching enzyme could be evaluated. The increased retrogradation of A-type, B-type, and C-type starches over time was also evaluated.