Dielectric properties of whole, chopped and powdered grain at various bulk densities M.S. VENKATESH ' , E. St-DENIS', G.S.V. RAGHAVAN', P. ALVO I and C. AKYEL 2 JDepartment of Agricultural and Biosystems Engineering, Macdonald Campus of McGill University, Ste-Anne-de-Bellevue, QC, Canada H9X 3V9; and 2Poly-grames, Ecole Polytechnique de Montreal, 3333 Queen Mary, 2nd stage, Montreal, QC, Canada H3V lA2. Received 26 September 1997; accepted 1 I June 1998. Venkatesh, M.S., St-Denis, E., Raghavan, G.S.V., Alvo, P. and Akyel, C. 1998. Dielectric properties of whole, chopped and powdered grain at various bulk densities. Can. Agric. Eng. 40: 191-200. The dielectric properties of whole, chopped, and powdered corn and wheat kernels were determined at 915 MHz and 24°C by the cavity perturbation technique. Existing cubic and quadratic relations were respectively found to describe adequately the functional dependence of dielectric constant and loss on grain sample bulk density. The hypothesis that the dielectric response of particulates with non- homogeneous layers could change as a result of size reduction was investigated. The data suggest, but do not confirm, this possibility. A more detailed study including proximate analyses and moisture determinations of the fractions would be required. II est reconnu que les proprietes dielectriques des materiaux soumis aux micro-ondes varient selon Ie taux d'humidite, la temperature, la composition, et la densite des materiaux de meme que selon la frequence du champ electromagnetique. Les effets de la densite apparente et de la granulometrie sur les proprietes dielectriques des grains ont ete etudies. Les relations Iineaires existant entre Ie carre des proprietes dielectriques et la densite que l'on retrouve dans la litterature, ont ete validees avec la methode de perturbation des cavites, pour des grains concasses et en poudre. L'effect de la granulometrie sur les proprietes dielectriques des grains concasses est une relation quadratique. lin' existe cependant pas de relation "pure et exacte" entre les proprietes dielectriques et la granulometrie, comme on pouvait s'y attendre de theorique, mais les resultats peuvent tout de meme etre utilises afin de valider les modeles cubique et quadratique relies a la densite. INTRODUCTION The dielectric properties of a material explain the nature and degree of interaction between a material and an applied electromagnetic field and can be used to predict processing parameters such as the dielectric heating rate. The dielectric properties depend mainly on the moisture content, temperature, and chemical composition of the material and vary according to the frequency ofthe applied electromagnetic field. However, in predicting heating rates in a processing application, factors such as bulk density and shape must also be taken into account. Relationships between dielectric properties and air-particle mixtures were previously developed by Nelson (1983a, 1983b; 1984b, 199 I) and applied to grains. Grains are an example of a material that may be thermally processed at different bulk densities for different applications. For example, one may dry whole kernels for long-term storage, or one may need to thermally treat crushed grain or finer forms such as flour. Furthermore, for a given particle size of material, geometric factors such as bed height and width may also influence the bulk density of the bed by altering the static pressure distribution. One may also choose to pack or loosen the material prior to conveying it through the processing chamber to manipulate heating/aeration parameters. Thus, it is important to know the dielectric properties of such materials at different bulk densities. One aspect that has received little attention to date is that of a possible dependence of dielectric properties on particle size. Although it is generally considered that there should be no such dependence, we suggest that when particles with several layers of different materials are fractured, the bulk dielectric response could change due to the change in relative depth of components with different basic dielectric properties. Underlying this suggestion is the idea that the dielectric response of a single grain kernel is a function of the cross-sectional moisture and material gradients, since these gradients affect the power density attenuation. The main objective of this study was therefore to determine the relationships between the dielectric properties of com and wheat, ground to various extents and compressed to various bulk densities. A preliminary study of the possible effects of particle sizes on dielectric properties was also conducted with the same materials. ASPECTS OF MICROWAVE INTERACTIONS WITH BIOLOGICAL MATERIAL Permittivity of a material is an important parameter to consider in microwave drying because of its correlation with process variables such as material moisture content, bulk density, and temperature as well as the frequency of the alternating electromagnetic field. The permittivity E· is usually defined as (Ryynanen 1995): (I) where: E' = dielectric constant, E" =dielectric loss factor, and Eo = free space permittivity (8.854 x 10- 12 F/m). The dielectric constant E' is associated with the ability of a dielectric material to store or couple electromagnetic energy, CANADIAN AGRICULTURAL ENGINEERING Vol. 40, No. 3 July/August/September 1998 191