thumb|Freeze-dried strawberries
Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature dehydration process that involves freezing the product and lowering pressure, thereby removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.
Because of the low temperature used in processing, If the product to be dried is a liquid, as often seen in pharmaceutical applications, the properties of the final product are optimized by the combination of excipients (i.e., inactive ingredients). Primary applications of freeze drying include biological (e.g., bacteria and yeasts), biomedical (e.g., surgical transplants), food processing (e.g., coffee), and preservation. The Inca people also used the unique climate of the Altiplano to freeze dry meat.
The Japanese koya-dofu, freeze-dried tofu, dates to the mid-1500s in Nagano and the 1600s on Mount Koya.
Modern freeze drying began as early as 1890 by Richard Altmann who devised a method to freeze dry tissues (either plant or animal), but went virtually unnoticed until the 1930s. In 1909, L. F. Shackell independently created the vacuum chamber by using an electrical pump. No further freeze drying information was documented until Tival in 1927 and Elser in 1934 had patented freeze drying systems with improvements to freezing and condenser steps.
Early uses in food
thumb|[[Freeze-dried ice cream]]
Freeze-dried foods became a major component of astronaut and military rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate, were transformed into hot meals in space by improving the process of rehydrating freeze-dried meals with water. The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids. How rations are chosen and developed is based on acceptance, nutrition, wholesomeness, producibility, cost, and sanitation. Additional requirements for rations include having a minimum shelf life of three years, being deliverable by air, being consumable in worldwide environments, and providing a complete nutritional profile.
Freezing and annealing
During the freezing stage, the material is cooled below its triple point, the temperature at which the solid, liquid, and gas phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps. To facilitate faster and more efficient freeze drying, larger ice crystals are preferable. The large ice crystals form a network within the product which promotes faster removal of water vapor during sublimation.
Amorphous materials do not have a eutectic point, but they do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying.
Structurally sensitive goods
In the case of goods where preservation of structure is required, like food or objects with formerly-living cells, large ice crystals break the cell walls, resulting in increasingly poor texture and loss of nutrients. In this case, rapidly freezing the material to below its eutectic point avoids the formation of large ice crystals. biologics, and other injectables. By removing the water from the material and sealing the material in a glass vial, the material can be easily stored, shipped, and later reconstituted to its original form for injection. Another example from the pharmaceutical industry is the use of freeze drying to produce tablets or wafers, the advantage of which is less excipient as well as a rapidly absorbed and easily administered dosage form.
Freeze-dried pharmaceutical products are produced as lyophilized powders for reconstitution in vials, and more recently in prefilled syringes for self-administration by a person.
Examples of lyophilized pharmaceutical drugs include:
- Vancomycin, an intravenous antibiotic for treatment of complicated skin infections, bloodstream infections, endocarditis, bone and joint infections, and meningitis.
- Activase, an intravenous "blood clot buster" used for treatment of ischemic stroke.
- Carmustine, a chemotherapy drug used for treatment of glioblastoma, brainstem glioma, and other brain tumors.
Biological products
Examples of lyophilized biological products include:
- Many vaccines such as live measles virus vaccine, typhoid vaccine, and meningococcal polysaccharide vaccine groups A and C combined.
- This also include antibodies, some of which are blockbuster drugs: etanercept (Enbrel by Amgen), infliximab (Remicade by Janssen Biotech), rituximab, and trastuzumab (Herceptin by Genentech).
- Cell extracts that support cell-free biotechnology applications such as point-of-care diagnostics and biomanufacturing are also freeze-dried to improve stability under room temperature storage.
Lyophilized biologics can be pressed into pellets and tablets for anhydrous and high-density, solid-state storage of biological products.
In bioseparations, freeze-drying can be used also 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.
Live material
Some live cell cultures can be freeze-dried, stored for an extended period, and then reconstituted into a live, functional state. An excipient, more specifically a cryoprotectant, may be required.
- Prokaryotes and yeast are relatively easy to freeze-dry and then resuscitate.
- Live vaccines (described above) are also examples of this class.
- The simpler blood cells (red blood cells and platelets) have been freeze-dried. With the right protection, recovery rates are as high as 90%.
Even if the cell is damaged beyond resuscitation, it is preserved. This can be helpful for later research too: although the type strain culture for Vampirovibrio chlorellavorus could not been resuscitated, it contained enough DNA for its genome to be sequenced.
Technological industry
In chemical synthesis, products are often freeze-dried to make them more stable, or easier to dissolve in water for subsequent use.
In nanotechnology, freeze-drying is used for nanotube purification to avoid aggregation due to capillary forces during regular thermal vaporization drying.
Food
thumb|Freeze dried [[bacon bars]]
thumb|Freeze-dried Bulgarian [[apricot, melon, meatball soup, tarator]]
thumb|Freeze dried ice cream and chocolate, and spaghetti with bacon
The primary purpose of freeze drying within the food industry is to extend the shelf-life of the food while maintaining the quality. It can also preserve raw ingredients such as egg whites for baking.
NASA and military rations
Because of its light weight per volume of reconstituted food, freeze-dried products are popular and convenient for hikers, as military rations, or astronaut meals. An instant coffee can be produced by freeze-drying a water extract of roasted beans.
Insects
Freeze-drying is used extensively to preserve insects for the purposes of consumption. Whole freeze-dried insects are sold as exotic pet food, bird feed, fish bait, and increasingly for human consumption. Powdered freeze-dried insects are used as a protein base in animal feeds, and in some markets, as a nutritional supplement for human use. Freeze-drying is also used as a means to memorialize pets after death. Rather than opting for a traditional skin mount when choosing to preserve their pet via taxidermy, many owners opt for freeze-drying because it is less invasive upon the pet's body.
Other uses
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 homogeneous chemical composition than traditional hot spray drying, but it is also more expensive.
Advantages
Freeze-drying is viewed as the optimal method of choice for dehydration of food because of the preservation of quality, meaning characteristics of the food product such as aroma, rehydration, and bioactivity, are noticeably higher compared to foods dried using other techniques. An example of this is a viral hepatitis A outbreak that occurred in the United States in 2016, associated with frozen strawberries. If the product is not properly packaged and/or stored, the product can absorb moisture, allowing the once inhibited pathogens to begin reproducing as well.
Equipment and types of freeze dryers
thumb|Unloading trays of freeze-dried material from a small cabinet-type freeze-dryer
thumb|A residential freeze-dryer, along with the vacuum pump, and a cooling fan for the pump
There are many types of freeze-dryers available, however, they usually contain a few essential components. These are a vacuum chamber, The freeze-dryer can run for a few hours or days depending on the product.
Contact freeze dryers
Contact freeze dryers use contact (conduction) of the food with the heating element to supply the sublimation energy. This type of freeze dryer is a basic model that is simple to set up for sample analysis. One of the major ways contact freeze dryers heat is with shelf-like platforms contacting the samples. The shelves play a major role as they behave like heat exchangers at different times of the freeze-drying process. They are connected to a silicone oil system that will remove heat energy during freezing and provide energy during drying times.
Additionally, the shelf-fluid system works to provide specific temperatures to the shelves during drying by pumping a fluid (usually silicone oil) at low pressure. The downside to this type of freeze dryer is that the heat is only transferred from the heating element to the side of the sample immediately touching the heater. This problem can be minimized by maximizing the surface area of the sample touching the heating element by using a ribbed tray, slightly compressing the sample between two solid heated plates above and below, or compressing with a heated mesh from above and below.