Temperature and moisture dependent dielectric properties of legume flours associated with dielectric heating

Authors: GUO, W - Northwest Agricultural University; WANG, S - Washington State University; TIWARI, G - Washington State University; Johnson, Judy; TANG, J - Washington State University

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 6/3/2009
Publication Date: 6/21/2009
Citation: Guo, W., Wang, S., Tiwari, G., Johnson, J.A., Tang, J. 2009. Temperature and moisture dependent dielectric properties of legume flours associated with dielectric heating. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). Paper No. 095910.

Interpretive Summary: A major problem in the storage and marketing of legumes is infestation by insect pests, causing importing countries to require phytosanitary disinfestation treatments such as fumigation. Consumer and environmental concerns over the use of chemical fumigants has generated interest in non-chemical alternatives such as heat treatments using radio frequency and microwave energy. To develop such heat treatments for legumes, the dielectric properties that govern the interaction between the electromagnetic energy and the legumes must be determined. Dielectric properties of chickpea, green pea, lentil and soybean flour samples at different frequencies, temperatures and moisture contents were measured by an open-ended coaxial-line probe and impendence analyzer. The dielectric constant and loss factor of the four legume flours decreased with increasing frequency but increased with increasing temperature and moisture content. The effective electrical conductivity of legume flours at the highest moisture content and temperature was 5-6 orders of magnitude smaller than that in high moisture vegetables, fruits and other foods at room temperatures. The difference in loss factors among the four legumes was clear at high moisture contents at 27 MHz and became negligible at 915 MHz for all moisture contents and temperatures. The penetration depth decreased with increasing frequency, temperature and moisture content, which was large enough at 27 MHz to develop largescale industrial RF treatments.

Technical 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.