Physico-ecological studies on quality formation of rice kernel. I. Effects of nitrogen top-dressed at full heading time and air temperature during ripening period on quality of rice kernel.

Abstract

Effects of nitrogen top-dressed at full heading time and air temperature during ripening period on rice quality were investigated from the viewpoints of the texture of cooked rice and protein, amylose and fat-by-hydrolysis contents. The protein content of milled rice increased correspondingly with an increase in nitrogen fertilizer. In terms of textural characteristics of cooked rice, chewiness value increased with that of protein content. Stickiness and textural palatability index (TPI) values were highest at 8.5% protein content. Amylose and fat-by-hydrolysis contents had no relationship with protein content. These results suggested that protein, especially the water-insoluble kind, had a great effect on the texture of cooked rice when the changes in the contents both of amylose and fat-by-hydrolysis were small. A lower temperature treatment during ripening period remarkably increased chewiness value, but stickiness and TPI values markedly decreased in nonwaxy rice. However, these features were not seen in waxy rice. A lower temperature treatment increased amylose and fat-by-hydrolysis contents in nonwaxy rice, but this was not detected in waxy rice. The protein content decreased at a lower temperature in both nonwaxy and waxy rice. It was concluded that air temperature during ripening period affected the texture of nonwaxy cooked rice by changing the amylose and fat-by-hydrolysis contents, but not the protein content. However, the texture of waxy cooked rice was unaffected by air temperature.