GENERAL CONSIDERATIONS RESONANT CAVITY AS A SENSOR The resonant cavity parameters with a kernel

Principles for determining moisture content and mass of individual peanut kernels by microwave resonator measurements are discussed, and data are presented showing the application of these principles. By measuring the shift in the resonant frequency and the change in the cavity transmission characteristics when a peanut is inserted into the cavity, it is possible to obtain the moisture content of the kernel independent of its mass and apparently independent of peanut type, as far as Runner and Virginia market types are concerned. It is also possible to determine the kernel mass independent of kernel shape simultaneously with the moisture determination. Moisture contents in the range from 4 to 14%, wet basis, were determined with an uncertainty of 0.9% moisture, and the uncertainty in the mass determination was less than 0.05 g. Keywords, Moisture content. Microwaves. Moisture content is an important characteristic of peanuts, Arachis hypogaea L. , during harvesting, storage, marketing and processing. In the Southeast, peanuts are allowed to dry on the vines after being dug at harvest to an average moisture content of 18 to 22%, wet basis, before being combined. After combining, the drying process must start immediately to reduce the moisture content to less than 10.5% for marketing. They can be stored safely at this moisture level if adequate ventilation is provided. Mass of individual kernels is also an important characteristic, because it is correlated with size, and larger kernels are usually more mature and of better quality. Conventional electronic instruments for moisture content determination in grain, seeds and nuts provide a moisture content reading which is an average for the sample. This average does not provide any information about the range of individual kernel or seed moisture contents that may be present in the sample. The variation of kernel moisture contents would be valuable information at many points in the storage, handling, and processing of peanuts, such as drying and roasting. Thus, an instrument that could determine the moisture content and mass of individual kernels within samples would be useful for research and application in the peanut industry. A moisture profile meter for peanut samples was developed which consisted of a commercial crushing-roller moisture meter equipped with a millivolt recorder and a peanut singulation device to feed individual peanuts to the crushing rollers (Hutchison and Holaday, 1978). Recorded signal peaks, which resulted from the DC conductance of the kernels as they passed between the rollers, were correlated with moisture content and provided moisture Article was submitted for publication in May 1992; reviewed and approved for publication by the Food and Process Engineering Inst, of ASAE in December 1992. The authors are Andrzej W. Kraszewski, Research Electronics Engineer, and Stuart O. Nelson, Agricultural Engineer, USDAAgricultural Research Service, Athens, GA. content estimates for the individual peanuts and information on the range of kernel moisture content in a 50-kernel sample. Recently, some success has been achieved in detecting single-kernel moisture content by impedance measurements on individual peanut kernels witih capacitive sensors at radio frequencies (Kandala and Nelson, 1990; Nelson et al., 1990). Measurements of capacitance and conductance of a small parallel-plate capacitor with the peanut kernel between and in contact with the plates, performed at two frequencies (1 MHz and 4.5 MHz), allowed prediction of the kernel moisture content within ±1% of the values determined by a standard oven drying method. Resonant cavity techniques are widely used in measuring the microwave properties of materials by measuring the shift in the resonant frequency and the change in the Q factor of the cavity when the object is inserted into the cavity (Altschuler, 1963). This technique has proven to be an interesting alternative to existing methods for kernel moisture determination in soybeans and com (Kraszewski et al., 1989; Kraszewski and Nelson, 1992), because it offers nondestructive and relatively fast measurements. The purpose of this research was to determine the effects of peanut kernels of various shapes and dimensions and different moisture levels on the parameters of a microwave resonant cavity and to evaluate the feasibility for determining the moisture content and mass of individual kernels by microwave measurements. MATERIALS AND METHODS PEANUT KERNEL PREPARATION Shelled peanuts of the Runner market type (Florunner cultivar) separated into two lots according to size (jumbo and medium), were obtained for this study from the USDA, ARS, National Peanut Research Laboratory, Dawson, Georgia, after the 1990 harvest season. Forty-six peanut kernels, including those of both medium and jumbo sizes, were randomly selected for the measurements. Some kernels were lightly sprayed with distilled water to VOL. 36(1): JANUARY-FEBRUARY 1993 127 increase their moisture contents to about 16% from the initial level of 10 to 12%. The weight of wet kernels ranged from 380 to 940 mg. These kernels were permitted to dry under ambient conditions for various time intervals between the microwave measurements, and after every drying period each kernel was individually sealed in a glass vial and kept at 4° C to equilibrate, usually for 2 to 3 days, before it was measured again. In this way each kernel was measured 3 to 5 times before it was finally dried to determine its dry mass. The dry mass was determined by placing individual kernels in copper moisture dishes (Nelson and Lawrence, 1989) and drying them in a forcedair oven for six hours at 130° C (ASAE, 1991). Kernels and moisture dishes were cooled in a desiccator over anhydrous CaS04 upon removal from the oven before being weighed. Subsequently, the kernel moisture content, wet basis, was calculated as a ratio of mass of water to mass of wet kernel as determined for each microwave measurement sequence. In 1991, four additional lots, two each of the Runner (Florunner cultivar) and Virginia market types in jumbo and medium sizes, were furnished for verification tests. Ninety-one kernels were randomly selected, with weight of wet kernels ranging from 420 to 1030 mg among Florunner peanuts and 460 to 1280 mg among the Virginia peanuts. Some of the kernels were lightly splayed with distilled water to increase their moisture over the original 6% level. After that, all kernels were kept individually in glass vials at 4** C for 72 h to equilibrate, and another 24 h at 23** C for temperature stabilization before microwave measurements. The previously described procedure was then followed in every detail. ELECTRICAL MEASUREMENTS The cavity consisted of a section of standard S-band WR-284 rectangular waveguide (inside dimensions: 72 x 34 mm) 305 mm long. It was coupled with external waveguides through two identical coupling holes 20.6 mm in diameter at each end of the cavity. A Plexiglas tube of 15.8-mm outside diameter and 12.4 mm inside diameter was installed in the center of the cavity as shown in figure 1. The resonant frequency of the empty cavity operating in the Hjos (TEjos) mode was 3175.9 MHz and its Q factor (Q )̂ was 865. The cavity was located between two waveguide-tocoaxial transitions, which allowed it to be connected to an automatic network analyzer calibrated in the transmission mode. The analyzer generated 801 discrete frequencies within a range of 16 MHz spanning the resonant frequency of the cavity. This allowed measurement of the transmission through the cavity in increments of 20 kHz by reading the coordinates of a marker on the test set CRT display. The resonance of the cavity appears as a peak in transmission through the cavity. To determine the resonant frequency, the frequency of a signal coupled to the cavity is varied until the maximum of the transmission is observed. The second parameter of the resonance curve, as shown in figure 2, is its shape. The coupling devices connected to the cavity, energy losses in the cavity walls and dielectric tubing, and losses in the sample introduced into the cavity all lower the apparent Q-factor. Thus, when a peanut kernel is introduced into the cavity, its resonant frequency will decrease and the Q-factor will be lowered, causing a broader, flatter resonance curve, shifted to the left. A "marker to maximum" command automatically accomplished the determination of the resonant frequency with an accuracy better than 10 kHz and the transmission coefficient (S21) through the cavity with an accuracy of 0.02 dB. In the following experiments, the shift of resonant frequency is denoted by AF = f̂ f̂ , where subscripts o and s refer to the empty cavity and cavity loaded with a sample (kernel) at the center of the cavity, respectively. Energy dissipated in the kernel is expressed as a change in the cavity Q factor: J 1 1 [̂ o i ) A T