Performance comparrison of three density-independent calibration functions for microwave moisture sensing in unshelled peanuts during drying

Authors: Lewis, Micah; Trabelsi, Samir

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/10/2020
Publication Date: 10/15/2020
Citation: Lewis, M.A., Trabelsi, S. 2020. Performance comparrison of three density-independent calibration functions for microwave moisture sensing in unshelled peanuts during drying. Applied Engineering in Agriculture. 36,5,667-672. https://doi.org/10.13031/aea.13703.
DOI: https://doi.org/10.13031/aea.13703

Interpretive Summary: Harvesting of peanut crops requires digging the peanuts, leaving them to dry in windrows, and then combining them when the farmers believe they are dry enough to deliver to the buying points. Often additional drying is done at the buying points if kernel moisture content exceeds 10.49%. Samples of peanuts must currently be shelled and cleaned before kernel moisture content can be tested as part of the peanut grading process, which must be completed before they are sold. Through ARS, USDA research, microwave peanut moisture meters have been developed that can determine kernel moisture content rapidly and nondestructively by microwave measurements on unshelled peanuts. In new research on peanut drying, this technique has been adapted for monitoring moisture content of pod peanuts, as well as kernel moisture content if desired, during drying. Because bulk density variation of the pod peanuts influences moisture content determination, new research has been conducted on density-independent moisture calibration equations for use in sensing peanut moisture contents. Three density-independent calibration functions were used to determine peanut pod (unshelled peanuts) moisture content in real-time during drying. Two of the functions are based on dielectric properties of the peanuts, which are highly correlated with moisture content, and the other is based on the measured microwave attenuation and phase shift. The stability and accuracy of the three functions in their use for pod moisture content determination were compared. As peanuts dry, temperature, moisture and density change continuously. Temperature and moisture can be measured effectively; however, density can not be measured accurately at all locations. Therefore, it is necessary to use such equations for the dielectric properties sensing that are not affected by changing density. Measurements of dielectric properties were taken with a microwave moisture sensor developed within USDA. Calibration measurements were taken from 6.5 to 19% peanut pod moisture content (wet basis) and from 10 to 40 °C. After a successful calibration, 150 lbs. (68 kg) of pods were conditioned to 15% moisture content, and they were dried using a quarter-scale peanut drying system also developed within USDA. Pod moisture content was measured with the microwave sensor every 12 seconds as the peanuts dried for approximately six hours. During drying, samples were extracted to obtain oven moisture contents by the standard oven-drying method. At the completion of the experiment, the oven moisture contents for the pods were compared with those measured by the microwave sensor for each density-independent moisture calibration function. The results showed that the standard error of performance for all functions was less than 0.55% moisture content. This value is within the error normally reported for the microwave moisture sensor when predicting moisture content in nuts. This study demonstrated that real-time moisture content determination in peanut pods during drying is achievable with low error through free-space transmission measurements of the dielectric properties and the utilization of density-independent calibration functions. These findings will be useful in continued research on peanut drying, which may be helpful in controlling drying processes to save energy costs and improve quality retention for the benefit of producers and consumers as well.

Technical Abstract: A microwave moisture sensor, developed within USDA ARS, was used to determine moisture content in unshelled peanuts during drying. Relative complex permittivities of the peanuts obtained from free-space transmission measurements at 5.8 GHz were used for the moisture determination. Drying causes changes in moisture content, temperature, and density. Therefore, moisture content was estimated, independent of bulk density, with three density-independent calibration functions. One of the functions is based on the measured attenuation and phase shift, and the other two are permittivity based. The accuracy and stability of these three functions for moisture content determination were evaluated over a temperature range of 10 – 40 °C and a moisture content range of 6.5% – 19% wet basis (w.b.); and estimated moisture content was compared to the reference oven drying method. Statistical analysis showed high coefficients of determination (r2), = 0.97 for the calibration with each function. The standard error of performance (SEP) for the three functions was = 0.55% moisture content. Therefore, microwave sensing is a viable solution for nondestructive, real-time determination of moisture content in peanuts in dynamic situations such as drying.