Dielectric Spectroscopy of Honeydew Melons from 10 MHz to 1.8 GHz for Quality Sensing

Honeydew melons were grown and harvested with a range of maturities for measurement of tissue permittivities (dielectric constant and loss factor) to study possible correlations between the dielectric properties and soluble solids (sweetness) for nondestructive sensing of maturity. Permittivities of tissue samples from 38 melons were measured at 25°C over the frequency range from 10 MHz to 1.8 GHz along with refractometer determinations of soluble solids content (SSC), tissue density, and moisture content. A high correlation (r = 0.96) was found between SSC and the permittivity as expressed in a complex-plane plot of the two components of the relative complex permittivity, each divided by SSC. Through this mathematical relationship, SSC can be calculated from measured permittivity values independent of tissue density and moisture content. Moderate correlations were noted between dielectric constant and SSC at 10 MHz and between the loss factor and SSC at 1.8 GHz. Correlations between the dielectric properties and both moisture content and tissue density were very low. The correlation between tissue density and SSC was also very low. A high correlation was noted between SSC and moisture content, with SSC decreasing as moisture content increased. Problems in using the high correlation between permittivity and SSC for practical, nondestructive sensing of honeydew melon maturity as determined by SSC are also considered.