Submitted to: Journal of Microwave Power and Electromagnetic Energy
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/19/2007
Publication Date: 7/18/2007
Citation: Wen-Chuan, G., Nelson, S.O., Trabelsi, S., Kays, S.J. 2007. Dielectric Properties of Honeydew Melons and Correlation with Quality. Journal of Microwave Power and Electromagnetic Energy. Interpretive Summary: Techniques for nondestructive determination of quality of agricultural products are helpful to producers, handlers and processors, those marketing the produce, and consumers. Visible and physical characteristics of many fresh fruits and vegetables are available for correlation with quality, and some of these, such as color, size, weight, density, elasticity, and firmness are used in automatic sorting of some produce into different categories for the market. For the honeydew melon, however, no useful characteristics of this type have been found for reliable correlation with quality. Electrical characteristics of fruit tissue known as dielectric properties can be sensed with electric fields for possible detection of fruit quality. Therefore honeydew melons were grown and harvested with a range of maturities for dielectric spectroscopy measurements of the melon tissue to learn whether differences in the dielectric properties might exist that could be correlated with maturity. Dielectric properties were measured both on the external surface of the melons and on internal tissue samples. The best criterion for quality is the measurement of soluble solids, which are mostly sugars and therefore a measure of sweetness. This requires the extraction of tissue samples from the melons and measurement of expressed juice with a refractometer instrument, which has been calibrated to indicate percentage of soluble solids. The dielectric properties of mature honeydew melon tissue were measured over the frequency range from 10 MHz to 1.8 GHz along with refractometer determinations of soluble solids content (SSC). A high correlation was found between SSC and the dielectric properties as expressed in a complex-plane plot of the dielectric constant and loss factor, each divided by SSC. Through this mathematical relationship, SSC can be calculated from measured dielectric properties independent of tissue density, moisture content, and temperature. Problems in using this new principle for practical nondestructive sensing of honeydew melon quality are considerable, but development of such a melon quality meter would be helpful to melon growers and handlers in the marketing of high quality produce for American consumers.
Technical Abstract: 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, 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 their dielectric properties.