Electrohydrodynamic (EHD) Drying Characteristic of Okara Cake

Abstract

Drying of okara cake was investigated in an oven at 105°C with or without the electrohydrodynamic (EHD) technique. The okara cakes (77 g in weight and 81.57% ± 0.14%wb initial moisture) were put in Petri dishes of 93 mm in internal diameter and 18 mm in depth. One sample was exposed to a high electric field (HEF) established with needle and plate electrodes and the other was treated as control. During drying, the voltage was applied at −20 ± 0.1 kV, number of electrodes in point electrodes were 1 and 3, and the electrode gap was 35, 50, and 65 mm. Varying the number of needles in the point electrodes and the electrode gap in HEF drying, the initial drying rate was found to be 1.7–3.2 times higher than the control sample at the same time. The results indicated that the range of the drying rate ratio changed with the moisture content was one to two during drying process. The results also suggested that the drying kinetics of okara cake treated both with and without an electric field were better fitted with Page's model than the Fickian diffusion model. Moreover, the falling rate period could be divided into two parts. When the ratio of moisture content was equal to e−e (= 0.066), the falling rate period changed from the first stage into the second stage. The drying time under HEF condition reduced by 15% to 40% compared to control measured at the final moisture content of 10% wb. The HEF energy efficiency ratio changed with the moisture content was always more than 1 except for a short period before the end of drying. It indicated that the heat energy should be transported to the okara cake exposed to the HEF more than to the control from the oven.