Radio Frequency (RF) dielectric properties of honeydew melon and watermelon juice and correlations with sugar content

Authors: GUO, WEN-CHUAN - UGA; Nelson, Stuart; Trabelsi, Samir; KAYS, STANLEY - UGA

Submitted to: Transactions of the Chinese Society of Agricultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2008
Publication Date: 5/30/2008
Citation: Guo, W., Nelson, S.O., Trabelsi, S., Kays, S.J. 2008. Radio Frequency (RF) dielectric properties of honeydew melon and watermelon juice and correlations with sugar content. Transactions of the Chinese Society of Agricultural Engineering.

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, honeydew melons and 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 melon tissues were measured over the frequency range from 10 MHz to 1.8 GHz along with refractometer determinations of soluble solids content (SSC) and moisture content. 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. Dielectric properties of juice extracted from the melons was also measured and correlations with quality factors are reported in this paper. The challenge in using dielectric properties for practical nondestructive sensing of 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 juice of three honeydew melon cultivars and four watermelon cultivars of different maturities were measured with an open-ended coaxial-line probe and impedance analyzer over the frequency range from 10 MHz to 1.8 GHz. Soluble solids content (SSC) of fruit juice and moisture content (MC) of fruit pulp were also measured, and correlations with dielectric properties were analyzed. The dielectric constant and loss factor of fruit juice decreased monotonically as frequency increased. The best linear relationships were found between loss tangent and SSC, and between loss tangent and MC for honeydew melons at 1.8 GHz, for which coefficients of determination were 0.784 and 0.772, respectively. The coefficients of determination for correlations between dielectric properties and quality of watermelons were less than 0.4. Obvious linear relationships between SSC and MC were found in honeydew melons and watermelons, with coefficients of determination of 0.973 and 0.906, respectively. Further studies are needed for determining relationships between internal qualities and dielectric properties of intact fruits, fruit pulp and juice. Key words: Honeydew melon, watermelon, juice, dielectric properties, soluble solids content, moisture content