Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: June 25, 2012
Publication Date: July 29, 2012
Citation: Trabelsi, S., Lewis, M.A., Nelson, S.O. 2012. Microwave meter for rapid, nondestructive determination of in-shell peanut kernel moisture content from dielectric measurements on cleaned and uncleaned pod samples. ASABE Annual International Meeting. ASABE Paper No. 121338109. Interpretive Summary: Vidalia onions are a type of sweet onion grown in a region of southeast Georgia that are highly valued in the marketplace for their desirable characteristics. The 2010 Vidalia onion crop was valued at more than $126 million. Prior to storage, onions must be cured. Curing is an essential step that seals the moisture in the interior of the onion from the outside environment. This forms a barrier to the intrusion of fungal infections during post-harvest storage and shipping. Currently, the curing process is controlled simply by time and visual inspection. However, varietal and year-to-year differences require adjustments in the curing procedure. If a method for rapidly sensing the curing of the onions could be developed, it would enable a more efficient curing process. Since dielectric properties of many agricultural products can be used for rapid sensing of moisture content, a study of the dielectric properties of onions was initiated for potential use in sensing moisture content as related to the curing process. Dielectric properties of fresh Vidalia onions were measured at a frequency of 13 GHz over moisture contents ranging from 6% to 92%. The onions were minced to provide uniform materials for the measurements. Variation of the dielectric constant and dielectric loss factor of the onions and a combination of the two, identified as a density-independent function, were studied, and two moisture ranges, those below about 40% and those above that level were identified that had differing dielectric behavior. This study demonstrated that dielectric properties can be used to predict a wide range of moisture contents. The correlation of the dielectric properties was improved markedly by expressing the relationship between the density-independent function of the dielectric properties and moisture content. This discovery permitted the prediction of moisture content from the dielectric properties with a high degree of accuracy. Thus, microwave sensing of moisture content might be developed for use in following the moisture changes in onions during the curing and providing information useful in improving the curing process, which would be beneficial for growers and handlers in providing high quality produce for consumers.
Technical Abstract: Microwave Sensing provides a means for nondestructively determining the amount of moisture in materials by sensing the dielectric properties of the material. In this study, dielectric properties of Vidalia onions were analyzed for moisture dependence at 13.36 GHz and 23°C for moisture content between 6% and 92%. Dielectric properties were obtained using an open-ended coaxial-line probe connected to a network analyzer. Linear regression models were developed using the dielectric constant, dielectric loss factor, and a density-independent function. Models were evaluated for calibration effectiveness in predicting moisture content using coefficient of determination and standard error of calibration In order to validate the models, a standard error of performance was calculated from a set of observations that were not used in the calibration. Preliminary results showed use of a density-independent function was more effective in prediction of moisture content than using dielectric properties alone. The measurements and models presented in this paper constitute the foundation for the development of a sensor for determining the degree of curing an onion has undergone.