Pulse NMR study of refrigerated cooked rice and comparison with physicochemical properties

The behavior of water molecules in refrigerated cooked rice was examined by measuring spin-spin relaxation time T-2 using a pulse NMR spectrometer. T-2 decay curve was analyzed using two-component and non-linear regression analysis and the presence of three kinds of water components with different T-2 were revealed. In the case of fresh cooked rice, the observed T-2 and the relative population were 0.9 ms (short T-2) and 13%, 26 ms (intermediate T-2) and 76%, and 200 ms (long T-2) and 11%, respectively. The short T-2 component was ascribed to the water molecules hydrated directly on the starch molecules. Intermediate T-2 component was dominant and ascribed to the water molecules accommodated in the starch matrix inside the rice grain. The mobility of water molecules was suppressed remarkably during the storage at 0degreesC while it was almost independent of the refrigerated storage below -18degreesC. The long T-2 component was ascribed to those grains accommodated in the surface luster film that was formed in the initial stage of the cooking process and composed of low molecular weight polysaccharides. The relaxation property observed by NMR was well consistent with the physicochemical properties.