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.

中文翻译：

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使用开放式同轴微波谐振器监测冷冻干燥过程中的初级干燥

摘要 设计并应用了一种开放式同轴微波谐振器，用于通过微波谐振光谱监测冷冻干燥过程中的初级干燥阶段。具有内腔和圆柱形磁芯的不锈钢圆柱形谐振器被设计用于感测谐振器上方空间中的介电常数和磁导率（即介电特性）。在 2500-3000 MHz 频率范围内获得的频谱模式和峰值强度对探头上不同体积的冰做出敏感反应。通过跟踪峰位移和/或所选峰频率的强度，成功地实现了对糊精溶液和切片苹果的冷冻干燥的监测。从微波谐振器估计的干燥终点完全对应于从干燥过程中的产品温度估计的点。这种传感装置有可能应用于冷冻干燥过程的在线监测，而无需与产品接触，尤其是在冷冻干燥过程中。