Rice drying has to be properly carried out not only to arrest spoilage but also to prevent or minimize kernel fissuring during and after drying. It is necessary to strike a balance between drying for premium paddy quality and for lower drying cost. This research was conducted to investigate advanced rice drying techniques and fissure occurrence in heated air drying to find scientific bases for increasing drying efficiency and minimizing kernel fissure occurrence in rice drying. The component studies and their respective results and conclusions are as follows:

　1. Experiments on thin-layer dehumidification drying of high-moisture, raw rough rice were carried out in a closed, circulating air system and an open air system. Results showed that dehumidifying the drying air could provide high drying rates up to 1.9% per hour in the closed system and 2.2%/h in the open system, without necessarily heating the air beyond 40°C. The effect of temperature rise on the hourly drying rate was more significant than the decrease in relative humidity. In the closed system, the drying rate exponentially increased as temperature rose, while in the open system the drying rate increase with temperature was almost linear. For effective dehumidification drying, air temperatures should not fall below 20°C since at 20°C and above, any reduction in humidity significantly increased drying rate. In the closed system where medium-grain Mutsuhomare was used, fissured grain percentage after drying increased linearly with increasing drying rate, whereas in the open system where the medium-grain Tsugaruroman was used, fissure percentage rose exponentially. This variation might be due to varietal differences in kernel fissuring resistance. Based on the 3.0% limit for heavy fissure occurrence in paddy drying, the maximum permissible dehumidification drying rate for Mutsuhomare should be about 1%/h and for Tsugaruroman, 1.5%/h.

　2. A one ton capacity, radial-flow, circular bin dryer was fabricated and equipped with a used window-type air-conditioner (7 kilowatt cooling capacity), converted into a dehumidifier. The dryer configuration was based on a 1-ton capacity, in-store paddy dryer, which employed an electric resistance heating system. Test results showed that, by using dehumidified air, long-grain paddy with 26% moisture could be dried to 14% in 24 hours (0.5%/h). The open-air drying system's average specific moisture extraction rate (SMER) was 2.2 kg H2O/kilowatt-hour and the dehumidifier's coefficient of performance was 1.34, which were within the usual range cited for dehumidification dryers. This SMER was also 38% higher than that of the electric resistance dryer. The quality of the dehumidifier-dried paddy, in terms of minimal grain fissure occurrence, was similar to naturally dried paddy. Under humid tropical conditions, the dehumidification system was found to be a technically feasible, more energy-efficient and safer alternative to resistance heating in conditioning ambient air for drying.

　3. Video microscopy of a drying brown rice kernel was carried out along with single-grain layer, heated air drying of rough rice to investigate the formation of fissures during the drying process and to determine the relationship between fissure occurrence and drying rate. Results confirmed that the drying grain shrinks unequally; however, this did not necessarily cause fissure formation. Fissuring is initiated around the central part of the grain during drying. The degrees of fissures during drying were more indistinct or weak compared with those caused by moisture adsorption. However, fissures occurred at high percentage after drying when the final storage moisture was reached. Fast drying rates at air temperatures over 40°C resulted in more fissured grains. It was therefore inferred that fissure occurrence depends upon the difference in water vapor pressure between the central part and the surface layer of the grain at the end of drying since this gradient became steeper as the drying rate increased.

　　　4. Single-grain layer, heated air drying experiments were also performed to investigate the possibility of increasing the drying rate for selected rice cultivars beyond the standard limit of 0.8%/h without exceeding the 3% limit for heavy fissure occurrence. Results showed that varietal differences in fissure resistance were unexpectedly great. Of the five varieties used in the study, Yaeminori was the weakest, with many light fissures occurring even in natural shade drying. Koshihikari was the strongest among the medium grain varieties used since in drying it at over four times the standard rate, the heavy fissure limit was not exceeded. In applying this result to practical deep-bed drying, a rate twice the standard could possibly be realized. For Hitomebore and Tsugaruroman the limits to which the rate could be raised were 1.2 and 1.5%/h, respectively. The long grain IR-64 remarkably showed the highest fissure resistance and for this variety the standard rate may easily be doubled. It is recommended that the drying ability of commercial dryers should not be set at a uniform rate but be made adjustable according to the variety to be dried.

　5. A rotary drum dryer prototype equipped with a built-in rice husk furnace was designed, fabricated and tested to combine convection drying with conduction heating of paddy to increase grain moisture reduction rates. Results showed that moisture removal from conduction-heated paddy could be abetted by aeration of the heated grains. Fresh ambient air forced inside the drum in a counter-flow direction at 1-2 m/s dryerated the heated. Only a single pass through the rotary drum with an average grain retention time of 82 s was needed to reduce paddy moisture from 26% to 18-19%. The new prototype's average pre-drying capacity was 230 kg/h, five times greater than that of the benchmark prototype. Its thermal efficiency (33%) was also 50% higher. It was found that high-rate, high-temperature, first stage drying using the dryer did not affect paddy quality in terms of total milling recovery and head rice yield when the initial grain moisture was high (26%). However, when paddy with 21% moisture was used, the milling and head rice yields were lower than those of paddy dried in a heated (43°C) air dryer.

　6. Single-grain layer, heated-vacuum drying of brown rice and rough rice was conducted to clarify the relations between drying rate and fissure occurrence and to determine the differences in fissuring characteristics and magnitude between rough rice and brown rice under reduced pressure condition. In brown rice, the vacuum pressure and temperature strongly influenced drying rate. The initial relative humidity at the start of vacuum operation did not show much effect. When vacuum pressure was released the thin film cover on the surface of brown rice was ruptured. Surface cracks were typically caused by water vapor compressing the surface due to the rapid change to atmospheric pressure. Internal fissure occurrence in brown rice immediately after drying was low, but increased as time elapsed. If the drying rate could be gradually decreased during the drying period until the final moisture is reached to recline the vapor pressure gradient between the grain and ambient air, then fissure occurrence would be prevented or minimized. In rough rice, surface cracks were far less in number than in brown rice even if the pressure was rapidly released. This could be attributed to the protection from high ambient temperature or low humidity afforded by the hull to rough rice. As in conventional heated air drying, fissure occurrence three days after drying started to rise as the drying rate increased beyond 1%/h.

（和訳)

　米の乾燥は変質を抑制するばかりでなく、乾燥中と後の穀粒胴割れを防止または最少にするため、適切に行われなければならない。優良な米質と低コストの乾燥のため、乾燥の間にバランスをとる必要がある。本研究は、米乾燥における乾燥効率の向上と胴割れ発生率低減のための科学的な基礎を見い出そうと、加熱通風乾燥における先駆的乾燥技術と胴割れ発生を検討したものである。研究の構成と各結果および結論は次のとおりである。

　1. 高水分生籾の薄層における除湿乾燥の実験は、閉鎖循環気系と開放通気系で実施された。結果は、乾燥用空気を除湿することにより、空気を40℃以上高めることはなく、閉鎖循環気系では1.9 %/h、開放通気系では2.2 %/hの高い乾燥速度を得ることができた。乾燥速度に関して加熱温度を高めた効果は、相対湿度を低下させたよりも顕著であった。乾燥速度は、閉鎖循環気系で加熱温度を高めることによって指数関数的に、また開放通気系ではほとんど直線的に上昇した。除湿乾燥を効果的に行うには、通気温度が20℃以上で除湿は乾燥速度を有為に高めたので、20℃以下に下げるべきではない。中粒種の"むつほまれ"を用いた閉鎖循環気系で、乾燥後の胴割れ率は乾燥速度が早くなるにつれて増え、中粒種の"つがるロマン"を用いた開放通気系では、胴割れ率が指数関数的に増大した。この差は穀粒の胴割れ抵抗性に関する品種間差異に基づくものと思われる。米乾燥における胴割れ発生の上限を3%におくと、除湿乾燥における許容最大乾燥速度は"むつほまれ"でおよそ1%/h、"つがるロマン"で1.5%/hとすべきである。

　2. 1トン容量を有する軸流通風円形ビンを製作し、中古のウインド型空気調整器 (冷却容量7kW) を除湿器にかえて取り付けた。乾燥機の外形は、電気抵抗加熱システムを使った屋内用1トン容量の米乾燥機に基づいている。実験結果は、水分26%の長粒種に除湿空気を用いることにより、24時間で水分14%へ0.5%/hの速度で乾燥した。この屋外通気乾燥システムの平均水分除去率 (SMER) は1kWh当り水分2.2kgにあり、また除湿器の動作係数は1.34と除湿乾燥機の既往の文献に見られる通常の範囲内にあった。このSMERの値は電気抵抗式乾燥機のものよりも38%高かった。除湿乾燥した米の品質は、胴割れ率で自然乾燥米に匹敵するものであった。熱帯の湿潤条件下の除湿システムは、乾燥のために周囲空気を調整するなかで、電気抵抗式加熱に替わり技術的に実行可能で、しかもエネルギー効率が高く、より安全性のある選択肢であると見られた。

　3. 乾燥中の玄米粒1個の顕微鏡ビデオカメラによる検査により、乾燥経過中の胴割れ形成が検討され、また乾燥速度と胴割れ発生率との関係を知るために、単粒層においては籾の熱風乾燥で行われた。結果は、乾燥中の穀粒は等しく収縮せず、必ずしも胴割れを形成するものではなかった。乾燥中の胴割れ程度は、吸湿で起こるものと比較して不明瞭あるいは弱いものであった。しかし、胴割れは終期貯蔵水分に到達した後に高い割合で発生した。40℃を超える通気温度で乾燥速度は早く、胴割れ粒をさらに多く発生した。したがって、胴割れ発生は乾燥終了時における穀粒の中央部と表層間の水蒸気圧差によるものであり、この勾配が乾燥速度の増大につれて強くなったことから推察される。

　4. 単粒層の加熱通風実験が、重胴割れ率3%の許容上限を超えずに0.8%/hの標準乾燥速度を上回る可能性を探るため、選抜された米の品種について行われた。結果は、胴割れ抵抗性の品種間差異が意外に大きいことを示した。本実験に供試した5品種のうち"ヤエミノリ"は最も弱く、自然の陰干しでさえ多くの軽胴割れを発生した。"コシヒカリ"は供試した中粒種中で最強を示し、標準乾燥速度の4倍速でも重胴割れ率の許容上限を越えなかった。この結果を実際の厚層乾燥へ適用するに当り、標準乾燥速度の倍速は実現可能と思われる。"ひとめぼれ"と"つがるロマン"は速度を1.2と1.5%/hにそれぞれ引き上げることが可能と思われる。長粒種の"IR-64"は最も強い胴割れ抵抗性を示し、この品種にとって倍速は容易と思われる。市販乾燥機の乾燥能力は一律の速度をセットすべきでなく、乾燥しようとする品種によって調節可能にすべきことを提案する。

　5. 籾殻燃焼炉を取り付けた回転ドラム式乾燥機の試作機が設計・製作され、穀粒水分の乾減率を高めるため籾の伝導熱に対流熱を組み合わせて試験を実施した。結果は、伝導加熱された籾からの水分除去が加熱穀粒への通気によって促された。周囲の新鮮空気は、加熱穀粒に対し向流で風速1～2 m/sによりドラムの中へ強制的に送風された。回転ドラムの中を穀粒平均滞留時間82秒間で通過すると、籾水分が26%から18～19%へ乾減した。この新しい試作機の平均予乾燥能力は基準試作機の5倍大きい230kg/hである。その熱効率33%は50%高かった。この乾燥機を用いて初期穀粒水分が26%と高い時に、高速、高温の第1段階の乾燥では、搗精率で示した米質と1等米比率に影響しなかった。水分21%の籾が使われた時に、その搗精率と1等米比率は43℃の加熱通風乾燥機を用いて乾燥したものよりも低かった。

　6. 玄米と籾の単粒層における加熱-真空乾燥が、乾燥速度と胴割れ発生率の関係を明らかにするため、また減圧条件下で籾と玄米の胴割れ特性と発生率の差を明らかにするために行われた。玄米では、真空下の圧力と温度が乾燥速度に強く影響した。真空操作開始時の初期の相対湿度はあまり大きく影響しなかった。真空が解除されたとき、玄米表面を被う薄膜の破れることがあった。表面割れは大気圧へ急速に変化させることにより、表面を圧縮していた水蒸気によって特徴的に引き起こされた。乾燥直後の玄米の内部胴割れは低く、時間の経過につれて増大した。もし、終期水分が穀粒と周囲空気の間の水蒸気圧勾配を緩和するに至るまで、乾燥期間中に乾燥速度を徐々に下げられるならば、胴割れ発生は抑制されるか最低になると思われる。籾では、真空が急速に解除されても、表面割れの数は少なかった。このことは籾に対し高い周囲温度から、あるいは籾殻による低湿度から守られているためである。慣行の加熱通風乾燥におけるように、乾燥後3日目の胴割れ率は乾燥速度が1%/hを超えて高くなった時に、上昇しはじめた。