Factors influencing the dielectric properties of agricultural and food products.

Dielectric properties of materials are defined, and the major factors that influence these properties of agricultural and food materials, namely, frequency of the applied radiofrequency 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 is 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.

Effects of "natural" water and "added" water on prediction of moisture content and bulk density of shelled corn from microwave dielectric properties.

Dielectric properties of samples of shelled corn of "natural" water content and those prepared by adding water were measured in free space at microwave frequencies and 23 degrees C. Results of measurements of attenuation, phase shift and dielectric constant and loss factor at 9 GHz show no difference between the samples with "natural" water and those in which water was added artificially. Bulk densities and moisture contents predicted from calibration equations expressed in terms of dielectric properties of both natural and added water samples agreed closely, and standard errors were less than 1% for moisture content and relative error for bulk density was less than 5%.

Fundamentals of dielectric properties measurements and agricultural applications.

Dielectrics and dielectric properties are defined generally and dielectric measurement methods and equipment are described for various frequency ranges from audio frequencies through microwave frequencies. These include impedance and admittance bridges, resonant frequency, transmission-line, and free-space methods in the frequency domain and time-domain and broadband techniques. Many references are cited describing methods in detail and giving sources of dielectric properties data. Finally a few applications for such data are presented and sources of tabulated and dielectric properties data bases are identified.

Influence of water content on RF and microwave dielectric behavior of foods.

The importance of dielectric properties of food materials is discussed with respect to their influence on the heating of materials by radio-frequency and microwave energy and their use for rapid, nondestructive sensing of quality characteristics of such materials. Data are presented graphically showing the frequency and temperature dependence of the dielectric constant and loss factor of wheat, fresh chicken breast meat, several fresh fruits, and apple juice, representing food materials with a wide range of moisture content. The influence of moisture or water content on the dielectric behavior of these materials is discussed, and that behavior is explained in terms of dipolar relaxation and ionic conduction.

Dielectric properties of honeydew melons and correlation with quality.

Dielectric properties of three honeydew melon cultivars, grown and harvested to provide a range of maturities, were measured with an open-ended coaxial-line probe and impedance analyzer over the frequency range from 10 MHz to 1.8 GHz. Probe measurements were made on the external surface of the melons and also on tissue samples from the edible internal tissue. Moisture content and soluble solids content (SSC) were measured for internal tissue samples, and SSC (sweetness) was used as the quality factor for correlation with the dielectric properties. Individual dielectric constant and loss factor correlations with SSC were low, but a high correlation was obtained between the SSC and permittivity from a complex-plane plot of dielectric constant and loss factor, each divided by SSC, for both the external surface and internal tissue measurements. However, SSC prediction from the dielectric properties by these relationships was not as high as expected. Permittivity data (dielectric constant and loss factor) for the melons are presented graphically to show their relationships with frequency for external surface and internal tissue measurements. A dielectric relaxation for the external surface measurements, which is attributable to bound water and Maxwell-Wagner relaxations, is also illustrated. Coefficients of determination for complex-plane plots, moisture content and SSC relationship, and penetration depth are also shown graphically. Further studies are needed for determining the practicality of sensing melon quality from the dielectric properties.

Dielectric spectroscopy of fresh fruit and vegetable tissues from 10 to 1800 MHz.

Dielectric spectroscopy data from measurements on tissue samples of nine fresh fruits and vegetables were used to study their dielectric behavior over the frequency range from 10 MHz to 1.8 GHz at 5 to 65 degrees C. Dielectric constant and loss-factor data are presented graphically for apple, avocado, banana, cantaloupe, carrot, cucumber, grape, orange, and potato, showing dielectric constants ranging from values of several hundred at 10 MHz to less than 100 at 1.8 GHz and loss factors on the order of one thousand at 10 MHz to less than 20 at 1.8 GHz. The dielectric loss factor increased consistently with increasing temperature at frequencies below 1 GHz. The dielectric constant increased with temperature at lower frequencies, but it decreased with temperature at the higher frequencies. This reversal of the sign of the temperature coefficient occurred at some point in the frequency range between 20 and 120 MHz where the temperature dependence of the dielectric constant was zero. At frequencies below this point, ionic conduction dominates the dielectric behavior, but above that point dipolar relaxation appears to control the behavior. Multiple linear regression provided equations for calculation of the loss factor in the frequency range from 10 to 300 MHz at temperatures from 5 to 65 degrees C. The data provide new information useful in understanding dielectric heating behavior and evaluating dielectric properties of such agricultural products for quality sensing applications.

Microwave permittivity determination in agricultural products.

Applications of microwave measurement techniques, including frequency-domain and time-domain methods, for determining the permittivity of agricultural products (seeds, plants, grains, fruits, meat, etc.) are reviewed. Graphical data illustrating the dependence of the dielectric constants and loss factors of such products on moisture content, frequency of the applied fields, temperature, and bulk density of granular materials are provided for selected materials. Finally, recommendations are offered for further study and applications of permittivity measurements.

Agricultural applications of dielectric spectroscopy.

A brief account of interest in dielectric properties of agricultural materials is presented, and some examples of dielectric spectroscopy applied to agricultural problems are discussed. Included are wide frequency range (250 Hz to 12 GHz) permittivity, or dielectric properties, measurements on adult rice weevils and hard red winter wheat, for the purpose of assessing selective dielectric heating of the insects, and broadband (200 MHz to 20 GHz) permittivity measurements on tissues of fresh fruits and vegetables. Similar measurements are shown for tree-ripened peaches, which were obtained to assess possibilities for a permittivity-based maturity index. Broadband (10 MHz to 1.8 GHz) permittivity measurements are shown for several fruits and vegetables as a function of temperature from 5 to 95 degrees C. Measurements over the same frequency range and similar temperature ranges are presented for two other food products, whey protein gel and apple juice. A few comments are offered on likely future dielectric spectroscopy applications in agriculture.

Principles for microwave moisture and density measurement in grain and seed.

The importance of cereal grain moisture content in determining time of harvest and in preserving grain quality is described. Techniques for moisture measurement, including electronic moisture meters, are also discussed briefly, and the need for better moisture sensing techniques for modern agricultural on-line moisture monitoring is described. Principles of microwave free-space measurements involving attenuation and phase-shift determination are explained, and density-independent functions of the grain permittivity are presented that permit reliable moisture sensing applicable to moving grain in which bulk density variation occurs. Three different approaches are discussed for determining moisture content and bulk density of grain from microwave measurement of the dielectric properties. A new moisture calibration function, based on complex-plane plots of dielectric constant and loss factor, each divided by bulk density, is described, which promises a universal calibration for different types of grain and seed. This important advantage should encourage commercial development for practical use.