Effect of different electric fields on temperature rise, energy efficiency ratio, and solids content during electro-osmotic dewatering of tofu residue (okura)

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.