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Encyclopedia > Freeze drying
In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Freeze drying (blue arrow) brings the system around the triple point, avoiding the direct liquid-gas transition seen in ordinary drying (green arrow).
In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Freeze drying (blue arrow) brings the system around the triple point, avoiding the direct liquid-gas transition seen in ordinary drying (green arrow).

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. Image File history File links Drying. ... Image File history File links Drying. ... In physical chemistry and materials science, a phase diagram is a type of graph used to show the equilibrium conditions between the thermodynamically-distinct phases. ... In physics, the triple point of a substance is the temperature and pressure at which three phases (gas, liquid, and solid) of that substance may coexist in thermodynamic equilibrium. ... Dehydration (hypohydration) is the removal of water (hydro in ancient Greek) from an object. ... For other meanings of the word jam, see Jam (disambiguation) Jam from berries Jam is a type of fruit preserve. ... In physics and chemistry, freezing is the process of cooling a liquid to the temperature (called freezing point) where it turns solid. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin, Canberra. ... Sublimation of an element or substance is a conversion between the solid and the gas phases with no intermediate liquid stage. ...

Contents

The Freeze-drying process

There are three stages in the complete freeze-drying process: Freezing, Primary Drying, and Secondary Drying.


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. Larger crystals are easier to freeze dry. To produce larger crystals the product should be frozen slowly or can be cycled up and down in temperature. This cycling process is called annealing. Dry ice is a genericized trademark for solid (frozen) carbon dioxide. ... Methanol, also known as methyl alcohol, wood alcohol or wood spirits, is a chemical compound with chemical formula CH3OH. It is the simplest alcohol, and is a light, volatile, colourless, flammable, poisonous liquid with a distinctive odor that is somewhat milder and sweeter than ethanol (ethyl alcohol). ... General Name, Symbol, Number Nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15 (VA), 2 , p Density 1. ... A eutectic or eutectic mixture is a mixture of two or more phases at a composition that has the lowest melting point, and where the phases simultaneously crystallise from molten solution at this temperature. ...


Amporphous (glassy) materials do not have a eutectic point, but do have a critical temperature, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.


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. This article or section does not cite its references or sources. ...


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. Look up Vacuum in Wiktionary, the free dictionary. ...


Secondary Drying

The secondary drying phase aims to sublimate the water molecules that are adsorbed 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. Adsorption is a process that occurs when a gas or liquid or solute (called adsorbate) accumulates on the surface of a solid or more rarely a liquid (adsorbent), forming a molecular or atomic film (adsorbate). ... An isotherm is a line of equal or constant temperature on a graph, plot, or map; an isopleth of temperature. ...


After the freeze drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.


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. To meet Wikipedias quality standards, this article may require cleanup. ... A cluster of Escherichia coli bacteria magnified 10,000 times. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ... Spoilage redirects here. ...


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. Dehydration (hypohydration) is the removal of water (hydro in ancient Greek) from an object. ... Sublimation has three separate meanings: Sublimation (physics), the change from solid to gas without passing the liquid state Sublimation (psychology), the transformation of emotions Dye sublimation, the transference of printed images to a synthetic substrate by the application of heat Category: ...


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. Pharmacology (in Greek: pharmacon is drug, and logos is science) is the study of how chemical substances interfere with living systems. ...


Uses of Freeze-drying

Freeze-drying is used in many different industries, sometimes for different reasons.


Food Industry

A package of Freeze-dried ice cream, sold as a novelty item.
A package of Freeze-dried ice cream, sold as a novelty item.

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. Currently, the freeze drying process is used more commonly in the pharmaceutical industry. Image File history File links SpaceIceCream. ... Image File history File links SpaceIceCream. ... Freeze-dried ice cream Freeze-dried ice cream was developed by NASA for the Apollo missions and is still used today. ... Freeze-dried ice cream Freeze-dried ice cream was developed by NASA for the Apollo missions and is still used today. ... For other uses, see Astronaut (disambiguation). ... Two hikers in the Mount Hood National Forest Eagle Creek hiking Hiking is a form of walking, undertaken with the specific purpose of exploring and enjoying the scenery. ... Instant coffee Instant coffee is a beverage derived from brewed coffee beans. ...


Technological Industry

In chemical synthesis, products are often lyophilized to make them more stable, or easier to dissolve in water for subsequent use. In chemistry, chemical synthesis is purposeful execution of chemical reactions in order to get a product, or several products. ... Dissolving table salt in water This article is about a chemical solution; for other uses of the term solution, see solution (disambiguation). ... Water is a chemical substance that is essential to all known forms of life. ...


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. In chemistry, alchemy and water treatment, filtration is the process of using a filter to mechanically separate a mixture. ...


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. A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Neuraminidase ribbon diagram An enzyme (in Greek en = in and zyme = blend) is a protein, or protein complex, that catalyzes a chemical reaction and also controls the 3D orientation of the catalyzed substrates. ... A microorganism or microbe is an organism that is so small that it is microscopic (invisible to the naked eye). ... Blood plasma is the liquid component of blood, in which the blood cells are suspended. ...


Other Uses

Recently, some taxidermists have begun using freeze drying to preserve animals. A taxidermied snow leopard. ...


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 physical deformations. Water can also cause mold to grow and prompt image media to become soluble causing bleeding. In these cases, freeze drying may not be an effective restoration method.


In high altitude environments, the low temperatures and pressures can sometimes produce natural mummies by a process of freeze-drying. A mummy is a corpse whose skin and dried flesh have been preserved by either intentional or accidental exposure to chemicals, extreme cold, very low humidity, or airlessness. ...


Freeze-drying Equipment

Benchtop mainfold freeze dryer.
Benchtop mainfold freeze dryer.

There are essentially three categories of freeze dryers: rotary evaporators, manifold freeze dryers, and tray freeze dryers. Image File history File linksMetadata Download high-resolution version (2036x2304, 444 KB) I took the photo and edited myself. ... Image File history File linksMetadata Download high-resolution version (2036x2304, 444 KB) I took the photo and edited myself. ...


Rotary freeze dryers are usually used with liquid products, such as pharmaceutical solutions and tissue extracts. Biological tissue is a collection of interconnected cells that perform a similar function within an organism. ...

Unloading trays of freeze-dried material from a small cabinet-type freeze dryer
Unloading trays of freeze-dried material from a small cabinet-type freeze dryer

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. Image File history File links Download high-resolution version (2670x1755, 758 KB) Unloading a small lyophilizer (freeze dryer), from the USDA/ARS image gallery. ... Image File history File links Download high-resolution version (2670x1755, 758 KB) Unloading a small lyophilizer (freeze dryer), from the USDA/ARS image gallery. ...

Production freeze dryer
Production freeze dryer

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. Image File history File linksMetadata Download high-resolution version (1618x1536, 427 KB) I took and edited the photo to release it to the public. ... Image File history File linksMetadata Download high-resolution version (1618x1536, 427 KB) I took and edited the photo to release it to the public. ...


External links

References

See also


 
 

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