Effect of the Drying Process on the Physical Properties and Cooking Quality of Japonica Aromatic Rice

Rice is a major economic crop in Japan, where it is usually dried using a batch-type dryer until the final moisture content (FMC) reaches below 9%. The influence of low FMC (about 9%) produced by four drying temperatures (30, 40, 50, and 60℃) on the physical properties, milling, and cooking qualities of short grain aromatic rice were investigated. The obtained data were also compared with standard FMC (about 12%) treatments which were dried in a batch type dryer. The experimental result showed that for low FMC samples, head rice yield (HRY) was increased by 4% at drying temperatures of 30℃ and 40℃ but was reduced by about 20% at 60℃ in comparison with control samples (25℃, 60% RH). The water uptake ratio (WUR) and volume expansion ratio (VER) were decreased significantly as the drying temperatures were increased. We can conclude that low FMC (9%) could produce higher HRY, if the drying temperature is 40℃ or less. In addition, high drying temperatures (50℃ and 60℃) caused increases in the number of fissured kernels, energy consumption and decreases in HRY, WUR, and VER. Maintaining the HRY and keeping a cooking quality at maximum level are the main task for the optimization of drying process. It was occurred at drying temperature less than 40℃ with a low FMC for aromatic rice. Therefore, a drying temperature of more than 40℃ would result in deterioration of quality of japonica type aromatic rice, followed by low FMC.
イランではバッチ式の乾燥機により主要な穀物である籾水分を9%まで乾燥する.香り米籾の最終水分が, 約12% (標準水分籾) と約9% (低水分籾) になるように籾温度30, 40, 50, 60℃の4つの処理区で試料籾を乾燥処理し, 得られた籾, 玄米, 精米の物理的性質および炊飯特性を調べた.低水分籾の歩留まりの結果として, 対照処理区 (乾燥処理条件: 温度25℃, 相対湿度60%) の籾と比べて, 60℃の乾燥処理区では歩留まりが20%低く, 30℃と40℃の乾燥処理区では4%高かった.炊飯特性試験における加熱吸水率と膨張容積比は, 乾燥処理の温度が高くなると低下した.低水分籾では, 温度が40℃もしくはそれ以下の乾燥処理の場合, 高い歩留まりを示すことが明らかになった.加えて, 温度 (50-60℃) の乾燥処理は, 胴割れ粒, 乾燥エネルギを増加させ, 歩留まり, 加熱吸水率および膨張容積比を低下させる原因となる.高い歩留まりの維持と炊飯特性試験の指標が, 乾燥プロセス最適化の目安となり, 香り米を低水分まで40℃以下の温度で乾燥処理する条件によってもたらされる.物理的性質と炊飯特性試験の結果から, 40℃以上の高い温度での乾燥処理は, 日本型香り米の品質劣化をもたらす.