Diversity of Physicochemical Properties of Different Rice Varieties Produced in Regions of Hokkaido, Japan through Eight Years

Abstract

Although rice (Oryza sativa L.) production in Japan has remained constant in recent years, per-capita consumption has decreased (Fujibayashi, 2017). Moreover, approximately 66% of Japanese prioritize palatability over price in the rice they consume. Therefore, since Japan has become a more wealthy society, consumers have been demanding higher quality and more palatable rice (Hori et al., 2016; Ohtsubo and Nakamura, 2017). Rice quality is defined by a combination of physical and chemical properties. Physical properties comprise the grain’s external and structural characteristics (Bhattacharya, 2011a); meanwhile, chemical properties determine its sensory characteristics after cooking (Siebenmorgen et al., 2013). Therefore, because rice is handled, processed, and cooked before consumption, its physicochemical properties play a major role in influencing its quality (Bhattacharya, 2011b; Siebenmorgen et al., 2013). In Japan, physicochemical measurements and sensory testing are used to evaluate rice quality. Physicochemical measurements involve evaluating moisture, protein, and amylose content using near-infrared (NIR) spectroscopy and measuring the percentage of sound whole kernels using a visible light (VIS) grain segregator (Kondo and Kawamura, 2013). Sensory testing involves evaluating the taste. Trained panel members taste cooked rice samples and give scores for appearance, flavor, taste, hardness, stickiness, and overall sensory properties (Ohtsubo and Nakamura, 2017). Rice varieties with lower protein and amylose content, which appear as soft and sticky grains after cooking, are considered very palatable among consumers in Japan and in Northeast Asian countries. Residents in Hokkaido, Japan found milled rice of the Yumepirika variety with an amylose content of 19% and protein content of 7.5% and/or amylose content of 19% and protein content of 6.8% to be highly palatable (Kawamura et al., 2013). Similarly, brown rice with an average moisture content of 15% and sound whole kernel rate of more than 80% was considered to be of high-quality (Mizuho National Foundation, 2011). During grain filling, both kernel location in the panicle and ambient air temperature have an enormous impact on rice quality through their influence on the physicochemical properties of rice (Patindol et al., 2015). Rice grain filling depends on the flowering order of spikelets and is a long process that happens from the top of the panicle downward (Myers McClung, 2004). Therefore, it significantly affects protein and amylose content by influencing kernel thickness, weight, and maturity within the bulk of harvested rice. In general, kernels growing in the upper part of the rice panicle experience a longer growing period. They are therefore more mature, with lower levels of amylose and lower levels of protein, and are consequently of higher quality and more palatable than kernels growing in the lower part (Matsue et al., 1994a; 1994b;