Temperature and Moisture Dependent Dielectric Properties of Legume Flours Associated with Dielectric Heating

Abstract

Dielectric properties of flour samples from four legumes (chickpea, green pea, lentil, and soybean) at four different moisture contents were measured using an open-ended coaxial probe and impedance analyzer at frequencies of 10 to 1800 MHz and temperatures of 20 to 90°C. The dielectric constant and loss factor of the legume samples decreased with increasing frequency but increased with increasing temperature and moisture content. At low frequencies and high temperatures and moisture contents, negative linear correlations were observed between the loss factor and the frequency on a log-log plot, which was mainly caused by the ionic conductance. At 1800 MHz, the dielectric property data could be used to estimate the legume sample density judging from high linear correlations. Loss factors for the four legume samples were similar at 27 MHz, 20°C and low moisture contents (e.g. <15 %). At the highest moisture content (e.g., 20%) soybean had the highest loss factor at 27 MHz and 20°C, followed by green pea, lentil and chickpea. The difference in loss factor among the four legumes did not show clear patterns at 915 MHz. Deep penetration depths at 27 MHz could help in developing large-scale industrial RF treatments for postharvest insect control or other applications that require bulk heating in legumes with acceptable heating uniformity and throughputs.