A quantification method of retrogradation for cooked rice based on a single isolated peak in X-ray diffraction

We have established a novel quantification method for the evaluation of cooked rice retrogradation by means of X-ray diffraction (XRD). We have focused on the diffraction peak at 5.5° 2θ, which is a single isolated peak apart from the fingerprint region and corresponds to 1.6 nm periodic structure. This signal in the lower angular region is representative of the retrogradation of cooked rice, which formed wide periodic structure with high water content. Our method does not require special pre-treatment of the sample such as drying/rehydration before measurement, complicated mathematical background correction, or peak separation procedures after measurement for evaluation of the crystallinity from the acquired XRD spectra. Furthermore, this method is very sensitive for the small amount of crystal region in the sample, that is, the retrogradation in the short term after cooking, especially for amylose based structure in rice starch. Thus, our proposed method can be applied for the rapid evaluation of short term starch retrogradation, such as one day after cooking for samples having high water content.