イネ登熟期が記録的な猛暑となった平成22年産米のデンプン特性及び蒸米酵素消化性

摘要

The mean temperature in August 2010 was the highest ever recorded in Japan. Previous studies showed that higher temperatures during rice grain filling cause a higher retrogradation of gelatinized starch due to longer side chains of amylopectin, as well as lower enzymatic digestibility during sake making. Thus, the effect of extremely high temperatures during the 2010 season on starch characteristics of rice grains for sake making and the enzyme digestibility of steamed rice grains was examined. The gelati-nization temperatures of rice samples harvested during the 2010 season were higher than those harvested in the 2007 (a hot summer but cooler than 2010) and 2009 (a cooler summer) seasons. The decreasing degree of enzymatic digestibility of steamed rice grains after partial retrogradation of starch was higher in 2010 than in 2007 and 2009. On the other hand, even in the rice samples harvested in the same prefectures, gelatinization temperatures and the enzyme digestibility of rice grains varied among samples. Especially, the rice samples whose heading date was late showed lower gelatinization temperatures and higher enzyme digestibility of steamed rice grains than early heading samples. Moreover, the polished rice grains harvested during the 2010 season showed lower alkali solubility than those harvested in 2009. The present study suggests that the enzyme digestibility of steamed rice grains can be predicted by mean air temperature after heading even in historic high air temperature such as the 2010 summer, and the alkali solubility method can be useful for examining the starch characteristics of rice grains conveniently.%イネ登熟期が全国的に記録的な猛暑となった平成22年産米のデンプンの性質およぴ蒸米の酵素消化性について検討した。これまでの報告から予測されるとおり，猛暑となった平成22年産米はデンプン糊化温度が高く，蒸米酵素消化性は気中放置により低下しやすかった。一方，同一県内で採取された試料でも,糊化温度と酵素消化性にバラツキがみられた。特に出穂時期が遅い試料は酵素消化性の低下がゆるやかであつた。米粒のアルカリ崩壊性について観察したところ平成21年産米が大きく崩壊したのに対し平成22年産米はアルカリに抵抗性を示した。以上，平成22年のような記録的な猛暑であっても蒸米の酵素消化性が悪くこれまでの知見が適用できることを確認するとともに，アルカリ崩壊性により簡便にデンプンの性質を明らかにできる可能性が示唆された。