Freeze Drying
From http://en.wikipedia.org/wiki/Freeze_Drying
- Freeze drying (also known as lyophilization) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas. In the freezing step, it is important to cool the material below its eutectic point, the lowest temperature at which the solid and liquid phase of the material can coexist. The freezing phase is the most critical in the whole freeze drying process, because the product can be spoiled if badly done. During the primary drying phase the pressure is lowered (to the range of a few millibar) and enough heat is supplied to the material for the water to sublimate. This phase may be slow (can be several days in the industry), because if too much heat is added the material’s structure could be altered. The secondary drying phase aims to sublimate the water molecules that are adsorbed during the freezing process, since the mobile water molecules were sublimated in the primary drying phase. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0°C, to break any physico-chemical interactions that have formed between the water molecules and the frozen material. After the freeze drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.
