Submitted to: Measurement Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/26/2007
Publication Date: 6/5/2007
Citation: Nelson, S.O., Guo, W., Trabelsi, S., Kays, S.J. 2007. Dielectric Spectroscopy of Watermelons for Sensing Quality. Measurement Science and Technology. 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. Electrical characteristics of fruit tissue known as dielectric properties can be sensed with electric fields for possible detection of fruit quality. Therefore, four miniature watermelons were grown and harvested with a range of maturities for dielectric spectroscopy measurements of the melon tissue to learn whether there might be differences in the dielectric properties that could be correlated with quality. The best criterion for quality is the content 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 the watermelon tissues 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 was calculated from measured dielectric properties, but predictions were not as high as desired, and further research is needed to assess the dielectric technique for sensing watermelon quality. The challenge in using this new principle for practical nondestructive sensing of honeydew melon quality is 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 four small-sized watermelon varieties, 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. However, SSC prediction from the dielectric properties by this relationship was not as high as expected (coefficient of determination about 0.4). Permittivity data (dielectric constant and loss factor) for the melons are presented graphically to show their relationships with frequency for the four melon cultivars and 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.