Nondestructive monitoring of freeze drying and spray freeze drying endpoint by temperature-pressure method

The determination of the drying endpoint is crucial for the success of the freeze-drying process. However, nondestructive determination methods of the freeze-drying endpoint are often not applicable in spray freeze-drying equipment and process. To address this issue, an integrated spray freeze-drying equipment was developed that atomizes and freezes materials in the drying room through an atomizer, and freezes the frozen particles in the same drying room for vacuum freeze-drying to achieve one-step drying to prepare freeze-dried powder. The temperature and pressure method is used for real-time nondestructive monitoring of the entire drying process while ensuring no pollution in the materials. Experimental studies using pure water and milk as typical materials showed that a heating temperature of 45 °C and a pressure drop to 4.4 Pa can be considered as the drying endpoint, applicable to the monitoring of spray freeze-drying process. The temperature pressure method can be used as a method for judging the sublimation endpoint of spray freeze-drying. The relationship between temperature and pressure of the drying chamber can reflect the dry basis water content of freeze-dried materials.