Dielectric properties of in-shell and shelled sunflower seeds (Helianthus annuus L.) and pine nuts (Pinus pinea L.) at different temperatures for radio and microwave frequencies

Abstract

The bulk permittivity (εbulk) and particle permittivity (εpart) of in-shell and shelled sunflower seeds (Helianthus annuus L) and pine nuts (Pinus pinea L.) were determined in a temperature range of 20–60°C and a frequency range of 27–5000 MHz using the transmission line method. Additionally, the samples were analyzed for moisture content, water activity (aw,), fat content, color, and densities (bulk, tapped, and particle). The dielectric constant (ε’) decreased with increasing temperature for sunflower seeds (from 3.04 to 2.63 for in-shell samples) and increased with temperature for pine nuts (from 4.41 to 5.48, also for in-shell seeds), due to the differences in the aw values. For all samples, the ε’ and loss factor (ε’”) decreased with increasing frequency. ε” and ε’” values increased with higher bulk density; for instance, at frequency of 915 MHz, in-shell sunflower seeds (ρbulk = 0.341 g/cm3) had ε” = 1.49 and ε’” = 0.02 at 20°C, while in-shell pine nuts (ρbulk = 0.601 g/cm3) had ε” = 2.53 and ε’’ = 0.03 at 20°C. Higher penetration depth (dp) values were found at lower frequencies, for example, shelled sunflower seeds at 60°C exhibited dp = 7.52 at 27 MHz, but dp = 1.20 m at 5000 MHz. Results are valuable for designing further radiofrequency and microwave treatments for these seeds and nuts.