Investigation of dielectric sensing for fruit quality determination

Permittivities of honeydew melons and watermelons, grown to provide a range of maturities, were measured with an open-ended coaxial-line probe and impedance analyzer at frequencies from 10 MHz to 1.8 GHz. Measurements on fresh apples were also made over a ten-week storage period. Data were taken on the external surfaces of the fruit and on tissue samples from the edible internal tissue. Soluble solids content (SSC), indicative of sweetness, was used for melons as the quality factor for correlation with permittivity. 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, quality prediction from this relationship was not very high. Dielectric properties of apples were relatively constant during the ten-week storage period even though firmness of apple tissues decreased significantly over that period.