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 to allow the frozen water in the material to sublimate directly from the solid phase to gas in a manner similar to that which causes unused ice cubes to shrink in a frost-free freezer. The greatly reduced water content that results inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance.
The application of high vacuum in freeze drying causes ice to sublimates much more quickly, making it useful as a deliberate drying process. A cold condenser chamber and/or condenser plates provide a surface(s) for the vapour to re-solidify on. These surfaces must be colder than the temperature of the surface of the material being dried, or the vapour will not migrate to the collector. Temperatures for this ice collection are typically below -50 °C.
If a freeze-dried substance is sealed to prevent the reabsorption of moisture, the substance may be stored at room temperature without refrigeration, and be protected against spoilage for many years. Freeze drying tends to damage the tissue being dehydrated less than other dehydration methods, which involve higher temperatures. Freeze drying doesn't usually cause shrinkage or toughening of the material being dried, and flavours/smells also remain virtually unchanged.
Liquid solutions that are freeze-dried can be rehydrated (reconstituted) much more quickly and easily because it leaves microscopic pores in the resulting powder. The pores are created by the ice crystals that sublimate, leaving gaps or pores in its place. This is especially important when it comes to pharmaceutical uses. Lyophilization also increases the shelf life of drugs for many years.
The process has been popularized in the form of freeze dried ice cream and as an example of astronaut food. It is also popular and convenient for hikers because the reduced weight allows them to carry more food and reconstitute it with available water. Freeze drying is used in the manufacture of instant coffee as well as some pharmaceuticals.
In high altitude environments, the low temperatures and pressures can sometimes produce natural mummies by a process of freeze-drying.