Moisture distribution of soaked rice grains observed by magnetic resonance imaging and physicochemical properties of cooked rice grains

Moisture distribution in individual polished rice grains was observed during soaking by magnetic resonance (MR) imaging, and a nuclear magnetic resonance (NMR) signal intensity profile (SI-profile) was generated from the MR image. The water penetration pattern during soaking roughly showed dissimilar trends between different varieties of japonica and japonica-indica hybrid rice. NMR signal intensity at the completion of water absorption varied within each grain; high at the periphery and the central region and low in the area between the periphery and the central region. High moisture content within the central region is due to loosely packed starch granules. The SI-profile was congruent for grains of the same variety harvested in different regions and years and characterized a grain moisture distribution for each variety. Moisture distribution was compared using SI-profiles for varieties with different amylose contents and new varieties bred for specific end-uses in Japan. The NMR signal intensity, which is related to the moisture content, at the surface of soaked grain was negatively correlated to the grain amylose content. The NMR signal intensities at the surface of soaked grain negatively correlated with the overall hardness of the cooked rice grain as measured by the single-grain low-high compression test.     

Water penetration into rice grains during soaking observed by gradient echo magnetic resonance imaging

The water penetration into a rice grain during soaking was monitored by three-dimensional gradient echo magnetic resonance imaging, which has the advantage of a short measurement time and a sufficient sensitivity for low-moisture samples. The water penetration route was compared between the milled and brown rice grains of two japonica cultivars. In normal milled rice grains of Koshihikari, water first penetrated the ventral side surface and the embryo attachment site of the endosperm, then migrated along the central line and transverse cracks, and finally diffused to all parts of the endosperm. In milled rice grains of Yamadanishiki with a white core, water quickly infiltrated into the cracks or chalky parts on the dorsiventral line and then diffused to the lateral side. In brown rice grains of both cultivars water penetrated extremely slowly and did not infiltrate directly into transverse cracks or the white core due to inhibition by the pericarp and seed coat. The central part of every grain allowed more water to penetrate. The route, pattern and speed of water penetration were determined by the morphological structure, crack formation and hardness distribution associated with the packing of the starch granules in the grain.     

Magnetic resonance imaging and analyses of tempering processes in rice kernels

This research involved a magnetic resonance imaging (MRI) technique that was applied to examine water distribution and migration in single rice kernels during the tempering process. The imaging experiments were performed in a Bruker 9.4T MRI system. Three-dimensional spin-echo (SE) imaging sequences were optimized by adjusting the scanning parameters of echo time (TE) and repetition time (TR) to obtain images with maximum contrast. The MR images showed that the moisture distribution in the rice kernel is non-uniform and compartmental. The embryo region exhibited much higher MR signal intensity than the starchy endosperm portion. The tempering process was analyzed with spatial-temporal signal intensities of the endosperm following the drying process of the rice kernel. The transient change of the signal intensities in the endosperm was well fitted with a double exponential function suggesting that both convection and diffusion contributed to the reduction of the moisture gradient within the rice kernel during tempering. This hypothesis was further supported by the experimental data of the insulated rice kernel whose convective mass transfer was excluded. The experimental results revealed that MR imaging of rice kernels could be used as an efficient tool to examine the mechanisms of moisture migration within cereal grains.     

Starch gelatinization distribution and peripheral cell disruption in cooking rice grains monitored by microscopy

Starch gelatinization kinetics governs rice cooking behaviour (cooking time and texture). Starch gelatinization however occurs unevenly in the cooking grain. The aim of this study was to investigate the dynamics of starch gelatinization topography in rice kernels cooked in excess water at two temperatures: 75 °C and 95 °C, for times ranging from 5 to 30 min. Gelatinization front position was assessed over time on 40 μm cross sections using four different tracking methods: directly or after iodine staining using a microscope or a stereomicroscope under normal or polarized light. The four methods gave similar results and the obtained kinetics can be used to model starch gelatinization during grain cooking.     

Soaking time of rice in semidry flour milling was shortened by increasing the grains cracks

To shorten the soaking time of rice grains in the process of semidry-milling, the changes in moisture absorption, damaged starch, and magnetic resonance images during soaking, as well as effect of increasing grains cracks on moisture absorption were investigated. As the results, the soaking time to reach the maximum moisture of Glutinous and Indica rice grains was 40 min and 20 min, respectively, but was 3 h and 30 min to reach the minimum damaged starch contents in rice flour. The moisture penetrated quickly from the embryo attachment site, ventral side, and cracks into the inside, and then diffused through the cracks. The result showed that homogeneous distribution of moisture in the rice grains was necessary to reduce damaged starch. Moreover, owing to the increase of grains cracks, the soaking time of Glutinous and Indica rice grains was shortened from 3 h to 1 h, and from 30 min to 10 min, respectively, by increasing drying temperature to 60 °C.