Effect of thickness and appearance quality of brown rice on palatability and physicochemical properties of rice grown under shading and high-temperature treatments

We examined the influences of grain-thickness and mixing ratio of perfect grains to white-immature grains on the palatability and physicochemical properties of milled rice grown in shading, high-temperature and control plots using Nipponbare and Hinohikari. The thinner the grain, especially less than 1.9 mm, the lower the palatability. The decreasing tendency in palatability was marked in the high-temperature plot than in the control plot, and in Nipponbare than in Hinohikari. In both cultivars and in each plot, the palatability of the grains thicker than 2.0 mm showed a plateau. The thinner the grain, the higher was the protein content. The protein content was higher in the order of high-temperature plot > shading plot > control. On the contrary, the thinner the grain, the lower was the amylose content. The amylose content was lower in the high-temperature plot than in either the control plot or shading plot. The maximum viscosity and breakdown values of amylographic characteristics decreased the thinner the grain. The higher the mixing ratio of white-immature grains, the lower the palatability. The mixing ratio of white-immature grains showed negative correlation with the overall eating quality, amylose content, maximum viscosity and breakdown, but a positive correlation with protein content. However in the control plot, the protein content of perfect grains was not different from that of perfect grains and white-immature grains, suggesting that the lower palatability of white-immature grains was not caused by the protein content. The thinner the grain and the higher the mixing ratio of white-immature grains under the shading and high-temperature treatments, the lower was the palatability due to the increase in protein content despite the decrease in amylose content.