Maturity effects on dielectric properties of apples from 10 to 4500 MHz

Dielectric properties of external surface, internal tissue and juice of Fuji apples during the last two months of tree-ripening were measured with an open-ended coaxial-line probe and a network analyzer at 24 °C from 10 to 4500 MHz. The firmness, soluble solids content (SSC), pH, moisture content and electrical conductivity were also measured to determine whether permittivity is related to apple quality. During the tree-ripening period, the permittivity and electrical conductivity did not reveal obvious trends. Moisture content and SSC remained essentially constant, while the firmness decreased and pH increased with maturity. No obvious correlations were found between permittivity and firmness, moisture content or pH. The linear relationship between surface permittivity and SSC was poor (R2 < 0.2). The best R2 for linear regression between loss tangent of tissue and SSC and between dielectric constant of juice and SSC was 0.61 and 0.67, respectively, at 4500 MHz. Further studies are needed to assess the potential usefulness of dielectric properties for sensing apple maturity or internal quality.