Permittivities of Watermelon Pulp and Juice and Correlation with Quality Indicators

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 a 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 content/pH, respectively. The penetration depth of electromagnetic energy in pulp and juice at 4500 MHz was 4 mm. The study offers useful information in assessing watermelon quality from dielectric properties.