REVIEW PAPERAn Overview of Microwave Processing and Dielectric Properties of Agri-food Materials
Section snippets
Microwave processing: fundamentals and application
In North America, only four microwave and three radio frequencies are permitted by the Federal Communications Commission (FCC) for dielectric heating applications (Decareau, 1985). The allotted microwave frequencies are 915, 2450, 5800 MHz and the radio frequencies are 13·56, 27·12, and 40·68 MHz. Although both North and South America permit 915 MHz, most of the commercial microwave processing equipment is designed for operation at 2450 MHz, which reflects commercial emphasis on home microwave
Microwave–material interaction aspects
When microwaves are directed towards a material, part of the energy is reflected, part is transmitted through the surface, and of this latter quantity, part of it is absorbed. The proportions of energy, which fall into these three categories, have been defined in terms of the dielectric properties. The fundamental electrical property through which the interactions are described is the complex relative permittivity of the material ε*. It is mathematically expressed as:where ε′ is the
Quantities expressed in terms of dielectric properties
The proportion of reflected energy Pr is a function of the dielectric constant ε′ and the angle of incidence. For an angle of incidence of 90°, it is simply:
For example, the value for ε′ of water is 78 at room temperature. The reflectivity is therefore greater than 0·64. However, the reflected energy will also reflect from the walls of the chamber and impinge on the water over and over again, resulting in a specific reflectivity of about 0·20. Thus, the target material absorbs
Temperature and moisture content dependencies
The dielectric properties of most materials vary with several different factors. In hygroscopic materials such as agri-foods, the amount of water in the material is generally a dominant factor. The dielectric properties depend on the frequency of the applied alternating electric field, the temperature of the material, and on the density, composition, and structure of the material. In granular or particulate materials, the bulk density of the air–particle mixture is another factor that
Applications of dielectric properties of measurements
As mentioned in the introduction, the dielectric properties of materials depend on many factors, including some that are related to chemical composition. Once fundamental data on the relationships between the dielectric properties and other factors have been established, the rapidity with which the dielectric properties can be measured, and the non-destructiveness of the methods, can lead to better methods of quality analysis or monitoring of relevant properties or states, before, during or
Dielectric behaviour of soils
Important factors in the behaviour of soils exposed to microwaves are the dielectric properties of the components of the soil. Only limited information is available on these properties in the literature, particularly at microwave frequencies. Hoekstra and Delaney (1974) studied the dielectric properties of a fine sand, a silt, a silty clay and a clay soil (f=100 MHz–26 GHz). Their data revealed the following general relationships: the values for ε′ increase with an increase in temperature, but
Agriculture
Recently, there have been relatively few attempts to apply microwave power to agriculture; more investigations were carried out in the late 1960s and the early 1970s. The areas of application include drying of grains (Bhartia et al., 1973; Copson, 1962; Fanslow & Saul, 1971; Rzepecka et al., 1972; Wesley et al., 1974), insect control (Nelson, 1973; Nelson et al., 1974), and seed germination (Jolly & Tate, 1971). Such exotic uses of microwave energy as protection of plants form cold were also
Conclusions
In this study, an attempt was made to capture the potential of electromagnetic energy-based processing through an extensive and comprehensive literature overview in the subject area as it mainly summarises several microwave processing aspects and its usefulness in understanding the microwave energy–material interaction and the role of dielectric properties. This comprehensive coverage of the knowledge will be useful for academic, scientific and industrial community with respect to treating and
Acknowledgements
The authors thank Mr. P. Alvo for editorial assistance and the Natural Science and Engineering Council of Canada (NSERC) for the financial support.
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