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 recrys-tallized 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 alpha-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.