Effect of different electric fields on temperature rise, energy efficiency ratio, and solids content during electro-osmotic dewatering of tofu residue (okara)
A large amount of tofu residue (okara) is created by the traditional soybean food (e.g. tofu and soymilk) manufacturing industry every year in Japan. For wide utilization of okara, a high-efficiency dewatering technique is essential. The present study investigated the feasibility of electro-osmotic dewatering (EOD) of okara under pressure (70 kPa) with a horizontal electric field (both electrodes were titanium sieves, 14 mesh) that was characterized by a DC, an AC (with square waveform), and a duty ratio (DR; with rectangular waveform) voltage (20 V RMS). The results confirmed that the temperature rise could not be ignored under any kind of electric field. It was found that the water within the sample moved towards both electrodes simultaneously, especially in the initial EOD period. The mass of drainage increased with an increase in DC voltage. However, the frequency and the DR had no significant effect on the total drainage. The energy efficiency ratio, defined as the ratio of the energy consumed for evaporating the excess removed water to the energy input to the electric field for EOD, exceeded one within the initial period; however, it decreased quickly and was lower than one with dewatering. Although the Joule heat losses resulted in a decreased energy efficiency ratio during EOD, the temperature rise had the positive effects of pasteurization and deactivation on some antinutritional factors, especially trysin inhibitor. The solids content of the drainage removed by the EOD course was lower than that removed by mere pressure; it decreased with an increase in DC voltage, but increased with an increase in frequency. However, the DR of the voltage had no significant effect on the solids content under the experiment conditions. The results of this study should be helpful not only to okara dewatering, but also to the reuse of other high moisture content byproducts in the food industry.日本で毎年多く発生する豆腐・豆乳製造時の副産物であるオカラについては, その有効利用のための効率的脱水技術の開発が求められている. 筆者らは, 今まで, 電気浸透処理を用いたオカラの脱水処理について検討を加えてきた. ここでは, 電気浸透処理時の電界条件がオカラの脱水時の温度上昇やエネルギー効率, 固形率に及ぼす影響について報告する. 実験は, 水平に設置した電気浸透脱水処理装置で実施した. 電極は, チタニウム製のメッシュタイプを用いて, 加圧条件は70kPaとした. 電界条件としては, 直流, 交流 (矩形波) , さらに通電率を制御したもの (矩形波:以下DR処理) を用いて有効電圧は20Vとした. この電界条件では, オカラの脱水時の温度上昇は無視できるレベルであった. またオカラ内部の水分変化は, 両面の電極に向けて脱水が進行し, 特に処理初期においてこの傾向は顕著であった. 直流では, 電圧の上昇に伴って脱水率が進行したが, 交流, DR処理では, あまり電圧の影響は認められなかった. エネルギー効率 (電気浸透処理のために投入したエネルギーに対して電気浸透効果による脱水量の蒸発に必要なエネルギーの比により算出) は, 初期において大きく1を上回ったが, 処理時間の経過によって急激に低下して, 最終的には1以下となった. 脱液部の固形分については, 直流処理では電圧増加によって減少し, 交流処理では, 周波数の増加に応じて増加したが, DR処理では条件による変化はほとんどなかった. これらのオカラの脱水処理に関する電気浸透法での基礎諸言は, 同様に高水分系の食品副産物の再資源化のための効率的な脱水操作として有益な情報である