Submitted to: International Journal of Food Properties
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2010
Publication Date: 1/31/2013
Citation: Guo, W., Zhu, X., Nelson, S.O. 2013. Permittivities of watermelon pulp and juice and correlation with quality.. International Journal of Food Properties. 16:475-484. Interpretive Summary: Techniques for nondestructively determining 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, 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 pulp and juice samples were measured over the frequency range from 10 to 4500 MHz along with refractometer determinations of soluble solids content (SSC), moisture content, and pH. Dielectric properties of edible pulp and juice were similar within a few percentage points, but neither the dielectric constant nor the loss factor showed high correlations useful for predicting melon quality. Also, high loss factors prevent sufficient penetration of the melon tissue by electromagnetic energy for possible nondestructive sensing of watermelon quality. Thus, the development of a melon quality meter based on these principles seems unlikely.
Technical Abstract: Permittivities from 10 to 4500 MHz at 24 'C were measured on pulp and juice of watermelons of different maturities with an open-ended coaxial-line probe and network analyzer. The dielectric constant of both materials decreased monotonically with increasing frequency and loss factor had minimum at about 1000 MHz. Moreover, the pulp and juice had very similar loss factor values at each frequency. Ionic conduction is the major loss mechanism at lower frequencies in this range, while dipolar losses are dominant at the higher frequencies. The best linear coefficients of determination, 0.8 and 0.69, were found at 4500 MHz for correlations between the loss tangent of juice and soluble solids content and between loss tangent of juice and soluble solids/pH, respectively. The penetration depth of electromagnetic energy in pulp and juice at 4500 MHz was 4mm. The study offers useful information in assessing watermelon quality from dielectric properties.