Improved open-ended coaxial probe for temperature-dependent permittivity measurements of foodstuff at radio frequencies

Accurate determination of permittivity is a crucial factor for successfully developing new food processing technologies at radio frequencies. An improved measurement test fixture based on an open-ended coaxial probe was developed to determine the permittivity of lossy food products at radio frequencies as a function of temperature. In order to increase the sensitivity of the permittivity measurements at these frequencies, especially when using a vector network analyzer as a measuring device, the dimensions of the coaxial cell were carefully modeled to consider the range of dielectric losses estimated for food materials. The electromagnetic model was used to retrieve permittivity values from reflection measurements where the entire geometry of the test fixture was considered assuming there was no open radiation or infinite flanges and without the need of reference materials to calibrate the probe. The new cell was validated through permittivity measurements of six saline solutions and then, employed to determine the dielectric properties of cheese sauces and commercial milk samples at 40.68 MHz up to 120 °C. The ionic content and temperature had a limited influence on the dielectric constant but highly determined the loss factor.