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When the electrical properties of materials are well correlated with other properties or characteristics of those materials that are important, it is often possible to use properly designed instruments for rapidly measuring those nonelectrical properties of interest. This may be accomplished by the use of electric fields in sensing the electrical properties of the material and then obtaining estimates of the nonelectrical properties through appropriate correlations. In the agricultural and food industries, these kinds of measurements have been used for many years for rapid determination of moisture content in grain and seed. Research has been conducted seeking useful correlations between quality attributes of other food materials and their electrical properties, but they have generally been less successful than the determination of moisture content. In this article, an overview of the historical development of such measurement methods and techniques is presented with respect to grain and seed and other agricultural products and food materials. The article covers use of electrical resistance or conductance of grain samples, radio-frequency capacitance measurements, and microwave measurements of dielectric properties for sensing the moisture content of grain and seed. It deals with the principles involved rather than descriptions of specific instruments that have been developed for rapid measurement of grain and seed moisture content. [ABSTRACT FROM PUBLISHER]

A planar transmission-line configuration for rapid, nondestructive, wideband permittivity measurements of liquid and semisolid materials at microwave frequencies is described. The transmission-line propagation constant of the proposed configuration is determined with the multiline technique from scattering parameter measurements of two lines with a known length difference. Guidelines for the appropriate selection of the line dimensions and a technique for resolving the phase ambiguity associated with the multiline technique are presented. The material permittivity is related to the propagation constant by closed-form expressions that allow the permittivity to be determined by a relatively simple numerical inversion procedure. The proposed sensor has the advantage of not requiring the sample to have uniform edges, nor that they are perpendicular to the planar line. The technique used for determining the line propagation constant has the advantage of not requiring any transmission-line discontinuities to be explicitly modeled. To illustrate the technique, a coplanar waveguide transmission-line sensor was fabricated and used to measure the permittivity of six liquids and four gelatin samples from 1 to 5 GHz. These measurements agree very well with coaxial-line probe measurements, demonstrating the accuracy of the proposed configuration. The sensitivity of the results to dimensional tolerances is demonstrated. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

The importance of moisture measurement in grain and seed is discussed and a brief history of the development of electrical moisture sensing instruments, based on sensing the electrical properties of these materials, is presented. Data are presented graphically on the permittivities or dielectric properties of grain and seed, showing their variation with frequency, moisture content, temperature, and bulk density, and references are cited for further information. More recent developments in higher frequency and microwave measurements for moisture content and bulk density sensing are briefly described, and numerous studies are cited providing sources of information on these newer techniques. [ABSTRACT FROM AUTHOR]

The importance of moisture measurement in grain and seed is discussed, and a brief history of the development of moisture sensing instruments, based on sensing of dielectric properties of these materials, is presented. Data are presented graphically on the permittivities or dielectric properties of grain and seed showing their variation with frequency, moisture content, temperature, and bulk density, and references are cited for further information. More recent developments on microwave measurements for moisture content and bulk density sensing are briefly described, and numerous studies are cited providing sources of information on these newer techniques. [ABSTRACT FROM AUTHOR]

Several cultivars of cantaloupe, honeydew melons, and watermelons were planted and harvested with a range of maturities for dielectric properties measurements and moisture and soluble solids content (SSC) determination. Permittivities (dielectric constants and dielectric loss factors) were determined over the frequency range from 200 MHz to 20 GHz with an open-ended coaxial-line probe and network analyzer for both interior tissue and surface measurements. Permittivity data are presented graphically for all three types of melons. High correlations were noted between SSC and moisture content in the tissues of all three kinds of melons, with SSC increasing linearly with decreasing moisture content of the edible tissues. Dielectric properties determined by measurements on the external surface of the melons had lower values than those of the internal tissues. Dielectric properties were similar for all three melon types, and they reflect the influence of the dielectric behavior of free water. No obvious correlations were noted between the dielectric properties and the SSC (sweetness) for sensing the quality of the melons. [ABSTRACT FROM AUTHOR]

A new method for quantitative "reading" of dielectric data of two-phase dielectric mixtures is suggested. This method is based on ideas related to the application of the generalized mean value (GMV) function to random data series (statistics of fractional moments). The GMV function allows transformation of arbitrary random data series to smooth curves that in turn can be fitted by an analytical function with a limited number of parameters. These fitting parameters are sensitive to the influence of an external factor, so the dependence of these parameters on the external factor can be used as calibration curves. In this instance we analyzed dielectric data measured for ground hard red winter wheat with 12.5%, 17.9% and 21.2% moisture contents in the temperature range from 2 °C to 76 °C. This system is a complex system from the viewpoint of the complexity of the dielectric data interpretation. The common treatment of these dielectric spectra does not provide a monotonic calibration curve. We treated these spectra as random data series by the use of the GMV function. As a result of this treatment, we obtained the monotonic temperature dependence of several fitting parameters for the given moisture contents, and these relationships can be fitted by an analytical function for calibration use. We hope that this new method will find application for analysis of other complex systems. [ABSTRACT FROM AUTHOR]

Dielectric spectroscopy data measured for several fresh fruits, fresh chicken breast, and hard red winter wheat are presented, and the dielectric behavior is discussed with respect to important variables. A brief review of reported measurements on these materials and description of the methods of measurement are included. Frequency ranges for the data are 10 MHz to 1.8 GHz and 200 MHz to 20 GHz. Variation of the dielectric constant and loss factor with frequency and temperature are shown graphically, and the influences of dipolar relaxation and ionic conduction are discussed. [ABSTRACT FROM AUTHOR]

The dielectric properties, consisting of the dielectric constant (ε′) and loss factor (ε″), were measured with an open-ended coaxial-line probe and impedance analyzer for uncooked broiler breast muscle pectoralis major and pectoralis minor, deboned at 2- and 24-h post- mortem, over the frequency range from 10 to 1,800 MHz at temperatures ranging from 5 to 85°C. The dielectric property profiles of chicken breast muscle are dependent upon the radio-wave and microwave frequencies and temperature. Increasing frequency from 10 to 1,800 MHz results in decreasing values of the dielectric constant and loss factor regardless of temperature in this range, chicken breast muscle type, or deboning time. However, the response to temperature varies with the frequency, muscle type, and deboning time. There are no differences in the dielectric constant and loss factor values at frequencies of 26 or 1,800 MHz between samples deboned at 2- and at 24-h postmortem. However, the muscle type significantly affects the average values of the dielectric constant and loss factor, with pectoralis minor having significantly higher average values. Both the deboning time and muscle type significantly affect the average values of the loss tangent (tan δ = dielectric loss factor/dielectric constant) at 26 and 1,800 MHz, with pectoralis minor having higher values than pectoralis major and 2-h samples having higher values than 24-h samples. Our quality measurements also show there are significant differences in chicken meat quality characteristics, including color, pH, drip loss, water holding capacity, and texture (Warner-Bratzler shear force value) between the different muscle types and between different deboning times in the same test. These results suggest that there is a probable potential for using dielectric property measurements to assess the quality of chicken meat. [ABSTRACT FROM AUTHOR]

Honeydew melons were grown and harvested with a range of maturities for measurement of tissue permittivities (dielectric constant and loss factor) to study possible correlations between the dielectric properties and soluble solids (sweetness) for nondestructive sensing of maturity. Permittivities of tissue samples from 38 melons were measured at 25°C over the frequency range from 10 MHz to 1.8 GHz along with refractometer determinations of soluble solids content (SSC), tissue density, and moisture content. A high correlation (r = 0.96) was found between SSC and the permittivity as expressed in a complex-plane plot of the two components of the relative complex permittivity, each divided by SSC. Through this mathematical relationship, SSC can be calculated from measured permittivity values independent of tissue density and moisture content. Moderate correlations were noted between dielectric constant and SSC at 10 MHz and between the loss factor and SSC at 1.8 GHz. Correlations between the dielectric properties and both moisture content and tissue density were very low. The correlation between tissue density and SSC was also very low. A high correlation was noted between SSC and moisture content, with SSC decreasing as moisture content increased. Problems in using the high correlation between permittivity and SSC for practical, nondestructive sensing of honeydew melon maturity as determined by SSC are also considered. [ABSTRACT FROM AUTHOR]

A brief historical perspective on dielectric properties of agricultural products is presented, including their use for rapid measurement of moisture content in grain and in considering potential dielectric heating applications. General principles are discussed as they relate to dielectric properties of materials and energy absorption from radio-frequency and microwave electric fields. Measurement principles and techniques for determining the dielectric properties of agricultural materials are described, and appropriate references are cited for detailed information. Dielectric properties data for some grain, insects, fruits, and vegetables are presented that relate to applications for selective heating of insects in grain and potential maturity and quality sensing in fresh fruits and vegetables. Principles are discussed for the use of microwave dielectric properties for sensing moisture content in grain, and relationships between dielectric properties of granular and powdered materials and their bulk densities are described. [ABSTRACT FROM AUTHOR]

Relationships between the permittivities of powdered or granular solid materials and their bulk densities (density of the air-particle mixture) are discussed. Linear relationships between functions of the permittivity and bulk density are identified that are useful in determining permittivity of solids from measurements of the permittivity of pulverized samples. The usefulness of several dielectric mixture equations for calculating solid material permittivity from measured permittivity of pulverized samples is also discussed. Results of testing linear extrapolation techniques and dielectric mixture equations on pulverized coal, limestone, plastics, and granular wheat and flour are presented. Recommendations are provided for reliable estimation of solid material permittivities or changes in permittivities of powdered and granular materials as a result of changes in their bulk densities. [ABSTRACT FROM AUTHOR]

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 °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 °C. The data provide new information useful in understanding dielectric heating behavior and evaluating dielectric properties of such agricultural products for quality sensing applications. [ABSTRACT FROM AUTHOR]

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 °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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A simple and accurate technique is described for measuring the uniaxial permittivity tensor of biological materials with a coplanar waveguide transmission-line configuration. Permittivity tensor results are presented for several chicken and beef fresh meat samples at 2.45 GHz. [ABSTRACT FROM PUBLISHER]

Production of pelleted biomass for fuel is an emerging industry in the United States. Moisture content is a primary quality attribute of pelleted biomass materials because it is critical in binding, storage, combustion, and the pricing of pelleted biomass. To produce pellets of high quality, moisture content must be tightly controlled. A microwave system was designed for moisture sensing in flowing bulk material and used to determine feasibility of sensing moisture content in pelleted biomass from measurement of the dielectric properties at microwave frequencies. Samples of pelleted biomass derived from peanut hulls and pine sawdust were used for moisture content determination. Moisture contents of pine sawdust pellets ranged from 5.4%–9.9% (wet basis), and the range for peanut hull pellets was 8.9%–14.5%. At each moisture content, three different material flow rates were tested, and moisture content predictions were compared to those obtained with static measurement. Moisture content of flowing material was predicted by using a permittivity-based density-independent moisture calibration function. Root-mean-square deviations were computed for comparisons between reference moisture content, and predicted moisture contents for both static and flowing materials. Results showed that predicted moisture contents under static and flowing conditions were comparable. [ABSTRACT FROM AUTHOR]

A rapid dielectric-based method was used to non-destructively determine the moisture content and bulk density of peanut-hull pellets from free-space measurement of their dielectric properties at microwave frequencies. For moisture content determination, a permittivity-based function was used that permits moisture prediction independent of bulk density from measurement of the dielectric properties at microwave frequencies. For bulk density determination, an analytical expression was obtained from a representation of the dielectric properties in the complex plane. Results of moisture and density prediction from measurement of the dielectric properties are shown at 5, 10, and 15 GHz at 22 °C. [ABSTRACT FROM AUTHOR]

Abstract: Dielectric properties of external surface, internal tissue and juice of Fuji apples during the last two months of tree-ripening were measured with an open-ended coaxial-line probe and a network analyzer at 24 °C from 10 to 4500 MHz. The firmness, soluble solids content (SSC), pH, moisture content and electrical conductivity were also measured to determine whether permittivity is related to apple quality. During the tree-ripening period, the permittivity and electrical conductivity did not reveal obvious trends. Moisture content and SSC remained essentially constant, while the firmness decreased and pH increased with maturity. No obvious correlations were found between permittivity and firmness, moisture content or pH. The linear relationship between surface permittivity and SSC was poor (R 2 < 0.2). The best R 2 for linear regression between loss tangent of tissue and SSC and between dielectric constant of juice and SSC was 0.61 and 0.67, respectively, at 4500 MHz. Further studies are needed to assess the potential usefulness of dielectric properties for sensing apple maturity or internal quality. [ABSTRACT FROM AUTHOR]

Abstract: Microwave sensing offers an opportunity to determine nondestructively the amount of moisture in materials by sensing the dielectric properties of the material. Dielectric properties of Vidalia onions grown in southeastern Georgia were measured with an open-ended coaxial-line probe and network analyzer in the range from 200MHz to 20GHz. Frequency dependence and moisture dependence of dielectric properties were analyzed for moisture contents between 8% and 91%. Moisture content was linearly correlated with the dielectric constant at higher frequencies for the entire moisture range. A density-independent function that incorporates both the dielectric constant and loss factor was tested across multiple frequencies and moisture ranges. Use of this function enabled prediction of moisture content with high accuracy (R 2 =0.99) up to 40% moisture content. [Copyright &y& Elsevier]