Freeze drying (also known as lyophilization or cryodesiccation) 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.
Amorphous (glassy) materials do not have an eutectic point, but do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.
Large objects take a few months to freeze dry.
In this phase, pressure is controlled through the application of partial vacuum. The vacuum speeds sublimation making it useful as a deliberate drying process. Furthermore, 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 (−60 °F).
It's important to note that in this range of pressure, the heat is mainly brought by conduction or radiation, the convection effect can be considered as insignificant.
After the freeze drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.
At the end of the operation, the final residual water content in the product is around 1% to 4%, which is extremely low.
Freeze drying also 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 the process 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.
Freeze drying is used to preserve food and make it very lightweight. The process has been popularized in the forms of 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. Instant coffee is sometimes freeze dried, despite high costs of freeze dryers. The coffee is often dried by vaporization in a hot air flow, or by projection on hot metallic plates. Freeze dried fruit is used in some breakfast cereal. However, the freeze-drying process is used more commonly in the pharmaceutical industry.
In bioseparations, freeze drying can also be used as a late-stage purification procedure, because it can effectively remove solvents. Furthermore, it is capable of concentrating substances with low molecular weights that are too small to be removed by a filtration membrane.
Freeze-drying is a relatively expensive process. The equipment is about three times as expensive as the equipment used for other separation processes, and the high energy demands lead to high energy costs. Furthermore, freeze drying also has a long process time, because the addition of too much heat to the material can cause melting or structural deformations. Therefore, freeze drying is often reserved for materials that are heat-sensitive, such as proteins, enzymes, microorganisms, and blood plasma. The low operating temperature of the process leads to minimal damage of these heat-sensitive products.
Organizations such as the Document Conservation Laboratory at the United States National Archives and Records Administration (NARA) have done studies on freeze drying as a recovery method of water-damaged books and documents. While recovery is possible, restoration quality depends on the material of the documents. If a document is made of a variety of materials, which have different absorption properties, expansion will occur at a non-uniform rate which could lead to deformations. Water can also cause mold to grow or make inks bleed. In these cases, freeze drying may not be an effective restoration method.
Advanced ceramics processes sometimes use freeze drying to create a formable powder from a sprayed slurry mist. Freeze drying creates softer particles with a more homogenous chemical composition than traditional hot spray drying, but it is also more expensive.
In high altitude environments, the low temperatures and pressures can sometimes produce natural mummies by a process of freeze-drying.
Rotary freeze dryers are usually used with liquid products, such as pharmaceutical solutions and tissue extracts.
Manifold freeze dryers are usually used when drying a large amount of small containers and the product will be used in a short period of time. A manifold dryer will dry the product to less than 5% moisture content. Without heat only primary drying (removal of the unbound water) can be achieved. A heater needs to be added for secondary drying, which will remove the bound water and will produce a lower moisture content.
Tray freeze dryers are more sophisticated and are used to dry a variety of materials. A tray freeze dryer is used to produce the driest product for long-term storage. A tray freeze dryer allows the product to be frozen in place and performs both primary (unbound water removal) and secondary (bound water removal) freeze drying, thus producing the driest possible end-product. Tray freeze dryers can dry product in bulk or in vials. When drying in vials, the freeze dryer is supplied with a stoppering mechanism that allows a stopper to be pressed into place sealing the vial before it is exposed to the atmosphere. This is used for long term storage, such as vaccines.