Dielectric Measurement of Agricultural Grain Moisture—Theory and Applications

Abstract

Moisture content is a critical variable for the harvesting, processing, storing and marketing of cereal grains, oilseeds and legumes. Efficient and accurate determination of grain moisture content even with advanced nondestructive techniques, remains a challenge due to complex water-retaining biological structures and hierarchical composition and geometry of grains that affect measurement interpretation and require specific grain-dependent calibration. We review (1) the primary factors affecting permittivity measurements used in practice for inferring moisture content in grains; (2) develop novel methods for estimating critical parameters for permittivity modeling including packing density, porosity, water binding surface area and water phase permittivity and (3) represent the permittivity of packs of grains using dielectric mixture theory as a function of moisture content applied to high moisture corn (as a model grain). Grain permittivity measurements are affected by their free and bound water contents, chemical composition, temperature, constituent shape, phase configuration and measurement frequency. A large fraction of grain water is bound exhibiting reduced permittivity compared to that of free water. The reduced mixture permittivity and attributed to hydrophilic surfaces in starches, proteins and other high surface area grain constituents. The hierarchal grain structure (i.e., kernel, starch grain, lamella, molecule) and the different constituents influence permittivity measurements due to their layering, geometry (i.e., kernel or starch grain), configuration and water-binding surface area. Dielectric mixture theory offers a physically-based approach for modeling permittivity of agricultural grains and similar granular media.

Complex Permittivity of Ex-Vivo Human, Bovine and Porcine Brain Tissues in the Microwave Frequency Range

“…The increase in free water causes a decrease in white matter integrity [ 41 ]. Additionally, free water has a larger permittivity than bound water as bound water exhibits a permittivity value closer to that of dry matter, as reported in [ 42 ]. Thus, a reasonable explanation for the difference in permittivity of brains H1 and H2 could be in the changes that were ongoing in the elderly brain, causing it to have an increase in extracellular free water, especially because the biological tissue permittivity in the gigahertz frequency range is mostly influenced by the polarization of free water molecules [ 43 ].…”

Laboratory dielectric measurements to evaluate the conductivity change in the presence of chelating agent with different brines

“…These sensors are designed to automate irrigation and optimize the utilization of water, energy and labor. Jones et al 19 discussed the utilization of dielectric measurements used for inferring moisture content in grains. As measuring the moisture content is crucial for the harvesting, processing, storing and marketing of cereal grains, oilseeds and legumes.…”

A metamaterial based sensor for moisture and density of cereal flours

“…The interaction of the moist sample with the em field is entirely different from the dry sample. 41 Moisture content determines the storage conditions of cereal grains and hence it plays a very vital role in the shelf life of food samples. The maximum allowed moisture content of cereal grains as per international standards is 10% to 15% on wet basis for safe storage.…”