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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 sublime directly from the solid phase to gas. 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 partial vacuum speeds sublimation making it useful as a deliberate drying process. A cold condenser chamber and/or condenser plates provide a surface(s) for the water vapour to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures 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 causes less damage to the substance than other dehydration methods using higher temperatures. Freeze drying does not usually cause shrinkage or toughening of the material being dried. In addition, flavours and smells generally remain unchanged making the process popular for preserving food. Unfortunately, water is not the only chemical capable of sublimation and the loss of other volatile compounds such as acetic acid (vinegar) and alcohols can yield undesirable results. Freeze-dried products can be rehydrated (reconstituted) much more quickly and easily because it leaves microscopic pores. 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 can also be used to increase the shelf life of some pharmaceuticals for many years. The process has been popularized in the forms of instant coffee, and freeze dried ice cream; 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. In high altitude environments, the low temperatures and pressures can sometimes produce natural mummies by a process of freeze-drying. In chemical synthesis, products are often lyophilized to make them more stable, or easier to dissolve in water for subsequent use. Recently, some taxidermists have begun using freeze drying to preserve animals.
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