Freeze drying is the drying technique of removal of the frozen solvent by sublimation process and bound water removal through the desorption process. Freeze drying is known as lyophilization, the method used for heat-sensitive proteins, microbes, pharmaceuticals, tissues, and plasma. Sublimation occurs during freeze-drying which directly converts ice (solid) into vapor without any liquid phase. The product to be dried is firstly frozen, then placed in freeze dryer under deep vacuum, below the triple point of water. Heat energy is applied which leads to sublimation of ice. The steps in freeze drying are pretreatment/ formulation, loading in containers, freezing, primary drying (sublimation), secondary drying (desorption) and taking off dried product from freeze dryer.
The freeze-drying process has to be designed in such a way which should be robust and repeatable with defined parameters like temperature, pressure and time for each step. These parameters are vital for getting the desired product in terms of visual and functional characteristics. Main equipment of freeze dryer comprises of the refrigeration system, vacuum system, control system, product chamber, and condenser.
Recipe of freeze drying has to be designed depending upon the unique characteristics of the product, volume, and container. There is not any fixed recipe which will work for every product. Individual temperature, pressure and time setting have to be done.
Freeze dried products are very hygroscopic in nature, they should be well sealed and stored in airtight containers for prevention of rehydration from the atmospheric surrounding.