Effects of rising temperature on grain quality, palatability and physicochemical properties of rice

The effect of high temperatures on grain quality, palatability of cooked rice and physicochemical characteristics of rice was examined in a temperature gradient chamber (TGC). Experimental plots going from TGI (near the air intake side) and TG4 (near the air exhaust side) along the temperature gradient in TGC, corresponding to low and high temperature, and an open field plot (outside of TGC) were arranged. The mean and maximum air-temperatures in TG4 were 2.8 deg C and 5.9 deg C higher, respectively, than those in TG1. Brown rice yield per square m in outside (572 g) was the highest, and those in TG1 (503 g) and TG4 (180 g) were 12.1 and 68.6% lower than outside, respectively. This yield decline was due to the decrease in the number of panicles and percentage of ripened grains, and an increase in the percentage of sterile spikelets. Most of the immature grain was milky white grains (10.8-14.6%) and white berry grains (10.7-43.4%), and other white immature grain accounted for only 0-2.6%. The grains from the outside plot were separated into perfect grain and milky white grain. The overall eating quality (+3 to -3) was in the order of perfect grains (0.31) outside (0.00) milky white grains (-0.56) TGI (-1.0) TG4 (-1.44). This indicates that the palatability of perfect and milky white grains was superior to and inferior to that of normal grain, and high temperatures deteriorate the eating quality of cooked rice. In the physicochemical characteristics of rice, the protein content showed a small variation (8.6-8.9%) between normal, perfect and milky white grains, but those in TGI (9.9%) and TG4 (9.8%) were higher than that in the outside. Amylose content of each plot was in the same order as the overall eating quality. This showed that amylose content did not affect the palatability because amylose content was normally negatively correlated with overall eating quality. H/-H ratio, a texture characteristic, was smaller in perfect grains than in normal grains, and that in milky white grains was larger than normal grains. This shows that the higher the share of milky white grains, the higher the H/-H ratio. Despite the superior eating quality, H/-H in TG1 was larger than that in TG4. Thus, the effect of H/-H ratio on the palatability of cooked rice grown in TGC was not clear.