The freeze-drying , also known as lyophilization , lyophilization or sublimation refers to a process for the gentle drying of products. Freeze-drying is based on the physical process of sublimation : The ice crystals sublime directly into the gaseous state without the occurrence of a liquid phase in between. The end product of freeze drying is called lyophilisate .
Freeze drying is particularly useful for thermally sensitive products.
The principle of freeze drying is based on the transition of molecules from a solid phase to the gas phase, below their respective freezing points at very low pressures (a sublimation ). A freeze-drying system consists of two chambers that are connected to one another and can be closed by a valve . The product to be dried is on a heatable and coolable footprint (called hordes ) and is first at normal pressure frozen . A coil serving as a condenser is built into the second chamber through which a cold liquid (usually a brine or a refrigerant ) flows.
In the next step of primary drying , the water contained in the material is sublimed at temperatures below freezing point . This is based on the principle that water, even when frozen, has a sufficiently high vapor pressure to pass directly from the frozen to the gaseous state of aggregation . For this purpose, a vacuum is created in the chamber .
Energy is absorbed during sublimation. Because thermal energy is obtained from the ambient temperature, the temperature in the drying chamber would drop in the course of the process. In order to keep the temperature constant, only as much heat is added to the chamber as is absorbed by the water as sublimation energy. In the course of the drying process, due to the previous removal of the gases by the vacuum in the chambers, the atmosphere consists almost exclusively of water vapor , which is deposited as ice on the cold coils of the condenser. This further lowers the partial pressure of the water in the chamber and further promotes drying of the product.
During primary drying, the loosely bound adhesive and capillary water of the product to be dried is first removed. The loose bond has only a low enthalpy , so that the difference in the water partial pressures between the product and the chamber is sufficient to overcome the bond.
Secondary drying follows in the further process , during which more strongly bound water is removed from the product by further heating. These are mostly the hydration shells of the substances contained in the product. With them, the enthalpy of binding is too high to be overcome by the difference in partial pressure alone, so that additional energy input is necessary through the heating of the product. The final temperature and the application of negative or positive pressure depends on the type of chemical or physical bond between the remaining water molecules and the adsorption isotherm of the material and can also be above 0 ° C.
After drying, the water content is typically 1–4%.
For the cooling of either a cooling medium (for example, silicone oil ) used, or there is the direct application of a coolant (typically nitrogen or carbon dioxide ). The temperatures of the capacitor are typically between −60 and −80 ° C. In large industrial plants , absorption chillers with the refrigerant ammonia are often used. The condenser can then be defrosted using steam after the connection flap has been closed. The process can be carried out as a batch process (batch drying), but also continuously or semi-continuously.
Depending on the application, some auxiliary substances can be used in freeze-drying to improve the product quality:
- Cryoprotectors : Freeze-drying proteins from an aqueous solution can lead to a conformational change in the protein. When the hydrate shell of the protein is dried off, the functional groups of the protein interact with other substances instead of the water molecules. These alternative binding patterns can damage the protein. By adding max. 1% (w / w) polyethylene glycol (PEG) in the solution to be dried, this can be effectively prevented: The PEG attaches itself to the proteins with its hydroxyl groups according to the mechanism of preferential hydration and thus replaces the hydrate shell in the solution whereby the native conformation is retained. During the drying process, the PEG remains bound to the protein and protects it from harmful interactions with foreign molecules.
- Scaffolding agents : Scaffolding agents create a loose and porous structure in the dried end product (cake). They prevent the cake from collapsing or even sticking together after freeze-drying and during subsequent storage. On the one hand, this improves the stability of the product, since interactions between the molecules can be prevented. On the other hand, scaffolding agents improve the solubility of the product because the porous structure makes it easier for solvent to penetrate. Porous crystallizing sugar alcohols can be used as skeleton formers, e.g. B. Polyethylene Glycol (PEG). If PEG is already being used as a cryoprotector, no additional scaffolding is usually necessary.
A well-known example from the food industry is the freeze-drying of coffee into soluble coffee granules (instant coffee) and other instant powder beverages. Fruits for muesli flakes are also freeze-dried and thus retain their color and taste. Furthermore, the process of freeze-drying is used for herbs and spices in order to make them more durable, whereby the essential oils are retained as flavor carriers. In the animal feed industry, freeze drying is used for fish and reptile feed, for example shrimp .
Another area of application is the drying of pharmaceutical products; the pharmaceutical industry uses this process to dry drugs that would not have a long shelf life when dissolved in water. Before taking the drugs, they are dissolved in water again (for example antibiotics and newer biotechnologically manufactured drugs). Particularly in the case of parenterals, the rapid dissolving properties of the freeze-dried substances are advantageous , which result from the amorphous structure and the associated easy wettability and the high specific surface area of the freeze-dried products.
Freeze-drying is also used in archeology (for example for damp wood or leather), for freeze-dried flowers , taxidermists, libraries and restorers (documents damaged by water). The freeze-drying process can be used for all types of paper. This also works when the paper is bound in books or in the form of a document.
Freeze drying is also used in food analysis. For example, fruit or similar foods that have a high water content (melons) are removed from the water before the analysis.
A common application in ceramic production is the drying of ceramic powders, e.g. B. after grinding in water.
Freeze-drying can also be carried out as a form of burial that is not permitted in Germany ( promession ).
Another application is in space travel . In almost all previous space stations as well as space vehicles intended for longer stays , freeze-drying has been or is used in the preparation of food. The freeze-dried food is carried or delivered in shrink-wrapped portions. By adding cold (e.g. for drinks) or warm water (e.g. for meals) the food is rehydrated and can be eaten. Warm meals are usually further heated in the oven beforehand.
Disadvantages of freeze-drying, however, are the enormous expenditure of energy and the expensive, complex equipment. This is why this process is only worthwhile with very high quality products or products that cannot be preserved in any other way.
The freeze-drying of potatoes was traditionally used in the potato's countries of origin to detoxify the food, as long as no breeding alkaloid-free varieties were available. The product of freeze-drying potatoes is known as chuño in the Andes . In the highlands of Peru and Bolivia, the potatoes are exposed to frost for a long time at night at around minus ten degrees and covered with straw during the day. After a while, they are watered and re-frozen.
- the original structure of the material to be dried is retained. B. with plants, paper etc.
- gentle drying due to lower drying temperatures
- no bleeding of printing inks
- Aromas are retained better than with conventional drying processes
- very sensitive papers can be dried with this method
- better quality especially with food
- Due to the porous structure of the product, it dissolves very quickly with freeze-dried solutions
- good swelling capacity
- high energy input required
- high acquisition costs for the necessary systems
- takes a lot of time in contrast to other types of drying
- With paper, uneven tensions can occur on the individual paper fibers, which can cause the books to curl or warp - this effect is particularly pronounced with thick book covers or thick cardboard boxes
- The large surface area of the drying product increases its susceptibility to oxidation
- Stephan Alexander Maier: Development of a mini freeze dryer. Dissertation, University of Bonn, 2003
- German Coffee Association V. good explanation of the production of soluble coffee
- Pharmaceutical applications, latest technology, PAT A research group at the University of Erlangen presents freeze drying and all pharmaceutical issues
- Tour of the Space Shuttle Astronaut Ulrich Walter gives a tour of the Space Shuttle Columbia; STS-55;
- Bauer, Kurt H., 1930-, Frömming, Karl-Heinz, 1925-, Führer, Claus, 1926-2013, Lippold, Bernhard C., 1939-, Müller-Goymann, Christel, 1951-: Pharmaceutical technology: with introduction in biopharmaceutical and biotechnology . 10th, revised and updated edition. Scientific publishing company, Stuttgart 2017, ISBN 978-3-8047-3268-1 .
- DryTEC contract drying: Energy costs for own freeze drying.