Monitoring of primary drying in the freeze-drying process using an open-ended coaxial microwave resonator

Abstract

An open-ended coaxial microwave resonator was designed and applied for monitoring the primary drying stage in the freeze-drying process by microwave resonance spectroscopy. A stainless steel cylindrical resonator with an inner cavity and cylindrical core was designed to sense the electrical permittivity and permeability (i.e., dielectric property) in the space over the resonator. The obtained spectrum patterns and peak intensities in the frequency range of 2500–3000 MHz responded sensitively to different volumes of ice over the probe. Monitoring of freeze-drying of a dextrin solution and sliced apple was successfully achieved by tracking the peak shift and/or the intensity of selected peak frequencies. The end-point of drying estimated from the microwave resonator perfectly corresponded to the point estimated from the product temperature during drying. This sensing device can potentially be applied for in-line monitoring of the freeze-drying process without contact to product, especially in a freeze-drying process.