Electrical properties of wheat bran structural layer powder

Wheat bran is a good source for processing whole wheat food. Due to the nutrition enrichment of aleurone in wheat bran tissues, separating wheat aleurone from wheat bran could increase its potential value. Electrostatic separation (ES) is a new dry separation technology with high efficiency, low energy consumption and few waste water, based on electrical properties of materials, including corona charging, tribo-charging and dielectrophoresis. However, few studies were conducted on the dielectricity, resistivity and corona charging property of the powders from different wheat bran tissues, which are important for ES. Moreover, the influence of water content and particle size on corona charging properties of bran particles is not clear yet. To clarify the electrical properties of wheat bran tissues and the potential of ES on separating cereal bio-material , the dielectric, conductivity and corona charging properties of wheat bran powders were investigated by using bran particles with different composition, particle sizes and water content as the starting material. The results showed that, both the dielectric constant of aleurone-rich and outer-rich pericarp particles decreased with the frequency increased. When the frequency was up to 20 MHz, the dielectric constant of aleurone and outer pericarp-rich fractions finally tended to be similar. When the frequency was 300 kHz, the dielectric constant of outer pericarp-rich fractions was similar to air, while the largest difference between the relative permittivity of aleurone-rich and outer pericarp-rich particles appeared at 1.3 MHz with a ratio of 5.3. The results of resistance analysis suggested that, wheat bran powder was a good insulation material. The resistivity of aleurone-rich powder was about 2 300 MΩ/cm, which was five times more than that of outer pericarp-rich fraction at the same condition. The result of corona charging property indicated that, as the voltage increased, the charge amount obtained by wheat bran powders increased gradually, among which, the charge amount obtained by aleurone-rich particleswas about 1.7 times more than that of outer pericarp-rich fraction. Moreover, the charge amount obtained by wheat bran particles decreased with the increasing of water content in wheat bran, while the charge amount increased with the decreasing of the particle size of wheat bran. In brief, the electrical properties of wheat bran tissues, including aleurone and outer pericarp, were significantly different. The dielectric and conductivity of aleurone-rich particles are greater than that of outer pericarp, and the charges obtained by wheat bran particles after corona charging were greater than that of outer pericarp particles. Thus, wheat bran tissues, i.e., aleurone and outer pericarp can obtain different charge amount by tribo-charging and corona charging, then can be separated and enriched in electric field. The bran tissues can be separated by using dielectrophoresis and corona charging in theory, but some pretreatments on the particle size, water content and delivery process need to be done, to avoid adhesion and gathering between the bran particles, and finally the bran tissues can be separation and enriched efficiently by ES.