Freeze drying is a process of water vapours from a frozen silution by sublimation
Processing of freeze drying
(i) Pre-treatment: This step is done to reduce the volume of the solution to be introduced into the container. The solution is pre-concentrated under normal vacuum tray drying. Sometimes solid or liquid desiccants are also used for this purpose. This reduces the actual drying by 8-10 times.
(ii) Pre-freezing: This is done to solidify water. The ampoules. vials and bottles in which aqueous solution is packed, are frozen in cold shelves at a temperature below -50°C.
(iii) Primary drying: Moisture removed by applying vacuum drying. The temperature and pressure is kept below the triple point of water (0.0098°C and 4.58mm of mercury for the sublimation of water. Heat is supplied which transfers as latent heat and ice sublimes directly into vapour state which are ultimately removed. Primary drying help to remove about 98-99% moisture.
(iv) Secondary drying: The vacuum drying is done at a temperature of 50°-60°C. The rate of drying is very slow and its takes about 10-20 hours.
(v) Packing: The biological products dried by freeze drying are usually required to be in a sterile condition for injection. Freeze dried products to ensure protection from moisture. The containers should be closed under the controlled, atmospheric condition and also observing aseptic conditions. The containers are labeled and packed in card- board boxes.
Also read: Fluid bed dryer
Advantage of Freeze Drying Process:
1. The product obtained is light and porous having excellent solubility.
2. Drying takes place under vacuum; hence, oxidation is minimized as there is no contact with air.
3. The heat sensitive materials can be dried.
4. The loss of volatile material is minimum.
Disadvantages of Freeze Drying Process:
1. The process is very expensive because a complicated plant is used.
2. The product obtained by freeze drying process is very hygroscopic So packaging requires special precautions.
3. The period of drying is quite long. It is usually not less than 10 hours.