SENSING METHODS & INSTRUMENTATION FOR RAPID DETERMINATION OF MOISTURE CONTENT... AND OTHER QUALITY ATTRIBUTES OF GRAINS, SEEDS, & NUTS
Location: Quality and Safety Assessment Research Unit
Title: Factors Influencing the Dielectric Properties of Agricultural and Food Materials
Submitted to: Journal of Microwave Power and Electromagnetic Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 30, 2012
Publication Date: June 30, 2012
Citation: Nelson, S.O., Trabelsi, S. Factors Influencing the Dielectric Properties of Agricultural and Food Materials. Journal of Microwave Power and Electromagnetic Energy. Volume 46, No.2, pp.93-107, 2012.
Interpretive Summary: Dielectric properties of materials are those electrical characteristics of poorly conducting materials that determine their interaction with electric fields. The two properties of major interest are the dielectric constant and the dielectric loss factor of materials. These properties are important in any processes involving radio-frequency or microwave dielectric heating. They determine how well energy can be absorbed from the high-frequency alternating electric fields and thus how rapidly the materials will be heated. Since dielectric properties of materials are highly correlated with the amount of water in materials such as agricultural products and food materials, sensing the dielectric properties can be used for rapid measurement of their moisture content.
Dielectric properties of agricultural and food products are influenced by other factors in addition to moisture content. Temperature of the material is an important factor, and its influence must be understood in either moisture sensing or dielectric heating applications. Bulk density of particulate materials such as grain and seed is subject to considerable variation, and its influence, too, must be understood for satisfactory applications. Composition of materials, in addition to water content, can have an influence on dielectric properties. The dielectric properties are also highly variable with the frequency of the alternating fields applied to them.
This paper presents information on the influence of important factors affecting the dielectric properties of agricultural and food products. The dependence of the dielectric properties of these products on the frequency of applied radio-frequency and microwave fields, temperature of the products, their moisture contents, and bulk densities are described, based on fundamental principles. The nature of those dependencies of the dielectric properties of fruits and fruit juice, grains, peanuts. eggs, whey protein gell, and chicken meat are illustrated graphically from measurement data on these commodities and some of their products. The dielectric constant always remains constant or decreases with increasing frequency, whereas the loss factor may increase or decrease with increasing frequency, depending on the operating frequency and dielectric relaxations and ionic-conduction phenomena present in the materials. With increasing temperature of the products, the dielectric constant and loss factor may increase or decrease depending on the frequency range and the amount of water present in the materials. For granular and particulate materials, both the dielectric constant and loss factor tend to increase linearly with increasing bulk density of the materials. Generally, the dielectric constant and loss factor both increase with increasing moisture content, especially in lower moisture materials such as cereal grains.
The information presented will be helpful to those considering applications of dielectric properties for sensing quality attributes of agricultural products and for those considering microwave or radio-frequency dielectric heating applications as well.
Dielectric properties of materials are defined, and the major factors that influence these properties of agricultural and food materials, namely, frequency of the applied radio-frequency or microwave electric fields, and water content, temperature, and density of the materials, are discussed on the basis of fundamental concepts. The dependence of measured dielectric properties on these factors are illustrated graphically and discussed for a number of agricultural and food products, including examples of grain, peanuts, fruit, eggs, fresh chicken meat, whey protein gel, and a macaroni and cheese preparation. General observations are provided on the nature of the variation of the dielectric properties with the major variables.