Freeze drying (also known as lyophilization or cryodessication) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Dehydration ( hypohydration) is the removal of Water ( hydro in ancient Greek) from an object Food preservation is the process of treating and handling Food in a way that preserves its edibility and nutrition value 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. For freezing as a method of food preservation see Frozen food. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage
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Freeze drying protectants
Similar to cryoprotectants, some molecules protect freeze dried material. A cryoprotectant is a substance that is used to protect Biological tissue from Freezing damage (damage due to Ice formation Known as lyoprotectants, these molecules are typically polyhydroxy compounds such as sugars (mono-, di-, and polysaccharides), polyalcohols, and their derivatives. Sugar is a class of edible Crystalline substances mainly Sucrose, Lactose, and Fructose. Monosaccharides (from Greek monos: single sacchar: sugar are the most basic unit of Carbohydrates They consist of one sugar and A disaccharide is a Sugar (a Carbohydrate) composed of two Monosaccharides 'Disaccharide' is one of the four chemical groupings of carbohydrates ( Polysaccharides are relatively complex Carbohydrates They are Polymers made up of many Monosaccharides joined together by Glycosidic bonds A sugar alcohol (also known as a Polyol, polyhydric alcohol, or polyalcohol) is a hydrogenated form of Carbohydrate, whose Trehalose and sucrose are natural lyoprotectants. Trehalose, also known as mycose, is a natural alpha-linked Disaccharide formed by an α α-1 1-glucoside bond between two α-glucose units Solubility of Pure SucroseTemperature(Cg Trehalose is produced by a variety of plant, fungi, and invertebrate animals that remain in a state of suspended animation during periods of drought (also known as anhydrobiosis). Suspended animation is the slowing of life processes by external means without termination Cryptobiosis is an ametabolic state of life entered by an animal's Organisms in response to adverse environmental conditions such as Desiccation, Freezing
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, called a shell freezer, which is cooled by mechanical refrigeration, dry ice and methanol, or liquid nitrogen. Dry ice is solid Carbon dioxide. It is commonly used as a versatile cooling agent Methanol, also known as methyl alcohol, carbinol, wood alcohol, wood naphtha or wood spirits, is a Chemical compound Liquid nitrogen (liquid density at the Triple point is 0707 g/mL is the liquid produced industrially in large quantities by Fractional distillation of On a larger-scale, freezing is usually done using a freeze-drying machine. In this step, it is important to cool the material below its eutectic point, the lowest temperature at which the solid and liquid phase of the material can coexist. This 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. Annealing, in Metallurgy and Materials science, is a Heat treatment wherein a material is altered causing changes in its properties such as strength However, in the case of food, or objects with formerly living cells, large ice crystals will break the cell walls. As discovered by Clarence Birdseye, when food is frozen at −40 °C to −45 °C or below, then it tastes better. Clarence Frank Birdseye II ( December 9, 1886 - October 7, 1956) was an American Inventor who is considered the founder Usually, the freezing temperatures are between −50 °C and −80 °C. The freezing phase is the most critical in the whole freeze drying process, because the product can be spoiled if badly done.
Amorphous (glassy) materials do not have a 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.
Primary drying
During the primary drying phase the pressure is lowered (to the range of a few millibars) and enough heat is supplied to the material for the water to sublimate. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. Sublimation of an element or compound is a transition from the Solid to Gas phase with no intermediate liquid stage The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation. In Thermochemistry, latent heat is the amount of Energy in the form of Heat released or absorbed by a substance during a change of phase In this initial drying phase about 95% of the water in the material is sublimated. This phase may be slow (can be several days in the industry), 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. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. 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.
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. Adsorption is a process that occurs when a gas or liquid Solute accumulates on the surface of a solid or a liquid (adsorbent forming a film of molecules or atoms (the This part of the freeze-drying process is governed by the material’s adsorption isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0 °C, 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 (typically in the range of microbars, or fractions of a pascal). 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.
At the end of the operation, the final residual humidity in the product is around 1% to 4%, which is extremely low.
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. Room temperature (also referred to as ambient temperature) is a common term to denote a certain Temperature within enclosed space at which humans are accustomed Preservation is possible because the greatly reduced water content inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance. A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins Decomposition (or spoilage) refers to the break down of tissue of a formerly living Organism into simpler forms of matter
Freeze drying also causes less damage to the substance than other dehydration methods using higher temperatures. Dehydration ( hypohydration) is the removal of Water ( hydro in ancient Greek) from an object 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 (responsible for the smell of 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.
Applications of freeze-drying
Pharmaceutical and biotechnology
Pharmaceutical companies often use freeze drying to increase the shelf life of products, such as vaccines and other injectables. By removing the water from the material and sealing the material in a vial, the material can be easily stored, shipped and later reconstituted to its original form for injection.
Food industry
Freeze drying is used to preserve food and make it very lightweight. Food is any substance usually composed primarily of Carbohydrates Fats water and/or Proteins that can be eaten or drunk by an The process has been popularized in the forms of freeze-dried ice cream, an example of astronaut food. Freeze-dried ice cream, also known as astronaut ice cream or space ice cream, is a brick of dehydrated Ice cream that is always ready to eat with no need An astronaut or cosmonaut (космона́вт) is a person trained 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 word 'hiking' is understood in all English-speaking countries but there are differences in usage Instant coffee is sometimes freeze dried, despite high costs of freeze dryers. Instant coffee is a beverage derived from brewed Coffee beans Through various manufacturing processes the coffee is dehydrated into the form of powder or granules Freeze drying (also known as lyophilization or cryodesiccation) is a Dehydration process typically used to preserve a perishable material 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. Military forces may use freeze dried food, known as Meal, Ready-to-Eat (MRE). The meals are typically prepared simply by adding hot water, which rehydrates and reconstitutes the food and may also cook it slightly.
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 In Chemistry, a solution is a Homogeneous Mixture composed of two or more substances Water is a common Chemical substance that is essential for the survival of 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 substances with low molecular weights that are too small to be removed by a filtration membrane. Filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases by interposing a medium to fluid flow through which the fluid
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. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually Blood plasma is the Liquid component of Blood, in which the Blood cells are suspended The low operating temperature of the process leads to minimal damage of these heat-sensitive products.
Other uses
Recently, some taxidermists have begun using freeze drying to preserve animals, such as pets. Taxidermy ( Greek for "skin arrangement" is the art of mounting or reproducing Animals for display (e A pet is an Animal kept for companionship and enjoyment or a househeld animal as opposed to Livestock, Laboratory animals Working animals
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. The word ceramic is derived from the Greek word κεραμικός ( keramikos) Freeze drying creates softer particles with a more homogenous chemical composition than traditional hot spray drying, but it is also more expensive. Spray drying is a commonly used method of drying a liquid feed through a hot gas
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 Flesh have been preserved by either intentional or Incidental exposure to Chemicals extreme
Freeze-drying equipment
There are essentially three categories of freeze dryers: rotary evaporators, manifold freeze dryers, and tray freeze dryers.
Rotary freeze dryers are usually used with liquid products, such as pharmaceutical solutions and tissue extracts. Tissue is a cellular organizational level intermediate between cells and a complete organism
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.
See also
References
- Harris, E. Food preservation is the process of treating and handling Food in a way that preserves its edibility and nutrition value Supercritical drying is a process to remove liquid in a precisely controlled way similar to Freeze drying. 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.
- http://www.archives.gov/preservation/conservation/drying-methods-01.html
External links
- http://www.freezedryinginfo.com/
- 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.aspx / Passage FreezeDrying Software for Lyophilizating Process
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