Freeze drying
From Wikipedia, the free encyclopedia
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 and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas.
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[edit] The Freeze-drying process
There are three stages in the complete freeze-drying process: Freezing, Primary Drying, and Secondary Drying.
[edit] Freezing
The freezing process consists of freezing the material. In a lab, this is often done by placing the material in a freeze-drying flask and rotating the flask in a bath of dry ice and methanol, or liquid nitrogen. On a larger-scale, freezing is usually done using a freeze-drying machine. In this step, it is important to freeze the material at a temperature below the eutectic point of the material. Since the eutectic point occurs at the lowest temperature where the solid and liquid phase of the material can coexist, freezing the material at a temperature below this point ensures that sublimation rather than melting will occur in the following steps.
[edit] Primary Drying
During the primary drying phase the pressure is lowered and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation. In this initial drying phase about 98% of the water in the material is sublimated. This phase may be slow, because if too much heat is added the material’s structure could be altered.
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.
[edit] Secondary Drying
The secondary drying phase aims to sublimate the water molecules that are adsorped during the freezing process, since the mobile water molecules were sublimated in the primary drying phase. This part of the freeze-drying process is governed by the material’s adsorption isotherms. In this phase, the temperature is raised even higher than in the primary drying phase to break any physico-chemical interactions that have formed between the water molecules and the frozen material. Usually the pressure is also lowered in this stage to encourage sublimation. However, there are products that benefit from increased pressure as well.
After the freeze drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.
[edit] Properties of Freeze-dried Products
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. Preservation is possible because the greatly reduced water content that results inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance.
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 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.
[edit] Uses of Freeze-drying
Freeze-drying is used in many different industries, sometimes for different reasons.
[edit] Food Industry
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. The instant coffee is never freeze dryed, due to the excessive cost of the freeze dryers. The coffee is dryed by vaporisation in a hot air flow, or by projection on hot metallic plates. Nowaday, the freeze drying process is much more used in the pharmaceutical industry.
[edit] Technological Industry
In chemical synthesis, products are often lyophilized to make them more stable, or easier to dissolve in water for subsequent use.
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 molecules with low molecular weights that are too small to be filtered out 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.
[edit] Other Uses
Recently, some taxidermists have begun using freeze drying to preserve animals. The library used freeze dryers to save book damaged by flood.
In high altitude environments, the low temperatures and pressures can sometimes produce natural mummies by a process of freeze-drying.
[edit] Freeze-drying Equipment
There are essentially three categories of freeze dryers: rotary freeze dryers, manifold freeze dryers, and cabinet freeze dryers. 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. Cabinet freeze dryers are the most common, because they can be used to dry a variety of materials.
[edit] External links
- http://home.howstuffworks.com/freeze-drying.htm/printable
- http://inventors.about.com/library/inventors/blfrdrfood.htm
- http://www.fda.gov/ora/inspect_ref/igs/lyophi.html
- http://www.rpi.edu/dept/chem-eng/Biotech-Environ/LYO/index.html
- http://www.technalysis.us/freeze_dry_software.htm / CAE freeze drying processes
[edit] References
- Harris, E. L. V. and S. Angal (1989). Protein Purification Methods. Oxford University Press. ISBN 0-19-963003-8
- Kennedy, John F. and Joaquim M. S. Cabral (1993). Recovery Processes for Biological Materials. John Wiley & Sons Ltd.
