Freeze drying, sometimes called lyophilization, is a promising preservation method. In freeze drying, a product is first frozen, then the pressure is reduced and the ice is removed by sublimation. Sublimation is the transformation of a substance from a solid to a gaseous state without first liquefying it. Unlike other preservation methods that use heat to evaporate water, freeze drying removes water in the form of ice.
The low temperature allows for a higher quality end product. Not only does the product retain its original shape, but the process is also very gentle on vitamins. Therefore, freeze-drying is mainly used in biomedicine and food processing, because nutrients and active ingredients are preserved particularly well.
The First Attempts
Freeze drying developed separately in early communities in America, East Asia, and Northern Europe. Tribes in Peru were already freezing tubers and potatoes in the frigid peaks of the Andes. Then they put the frozen food in the sun, which allowed the ice to evaporate. From this they gained a traditional staple food that formed the basis for many different meals. The indigenous peoples were thus able to supply themselves with preserved, tasty food all year round. Like the Peruvians with potatoes, the monks of Japan with tofu and the Vikings with cod, for example, benefited from their knowledge of freeze-drying.
milestones of science
In 1890, tissue was first perfectly freeze-dried in a laboratory and then rehydrated again. In the decades that followed, freeze-drying was further explored and scaled up so that during World War II freeze-drying was used to safely preserve and transport blood plasma. A little later, the technique was also used to transport vaccines. Almost all food for NASA’s Apollo missions was also preserved by freeze drying.
There are various methods to freeze the product. Freezing can be done in a cold bath, in a refrigerator, or directly on a shelf in the freeze dryer. Cooling the material below its triple point ensures that the product is sublimated rather than thawed. The shape of the product is thus retained in the best possible way.
Large ice crystals created by slow freezing are the easiest for the freeze dryer to dry. However, if the crystals are too large, they can destroy the product’s cell walls, resulting in a lower quality end product. To prevent this, the freezing process is carried out as quickly as possible.
Freeze drying process through drying cycles
The second stage of freeze drying is called primary drying (sublimation), where the pressure is reduced (a vacuum is created) and heat is applied to the product to allow the water to sublimate. The vacuum promotes sublimation. The condenser provides a surface for the water vapor, in the case of the WAVE freeze dryer the vacuum chamber wall is the condenser to which the water vapor adheres and solidifies in the form of ice. The condenser also protects the vacuum pump from the water vapour. Too much heat at once can affect the structure and shape of the material.
This is followed by the further phases of freeze drying, the secondary drying (adsorption), in which water molecules that are ionically bound are removed. By increasing the temperature in small steps higher than in the primary drying phase, the bonds between the material and the water molecules are broken. These drying stages can, for example, take place at intervals of 10°C and can be flexibly adjusted using the Siemens control of the WAVE freeze dryer. Freeze-dried materials retain a porous structure. After the freeze drying process is complete, the vacuum can be broken before the material is sealed airtight. Most materials can be dried to 1-5% residual moisture thanks to this process.
● The treated products retain their original shape and structure.
● Arostaffe are preserved better than with other drying methods.
● Better taste and texture for food.
● A gentle process that preserves vitamins, nutrients and active ingredients.
● Less weight despite faithful structure.
● Can be rehydrated.
The most common use of freeze drying is in pharmaceutics to extend the shelf life of essential products such as vaccines and other injectables. By removing the water from the material and sealing it in a vial, the material can be easily stored, shipped and later rehydrated to its original form for injection.
Freeze drying is used to preserve food and make it very light. The process grew in popularity, particularly after freeze-dried ice cream was made as a snack for NASA astronauts. A large number of products have recently been created because the freeze-drying process has become cheaper and creative potential has been released. Food that is difficult to sell, such as crooked asparagus, is processed into new products: asparagus powder for the food industry. Farmers can process the vegetables and fruit and sell freeze-dried strawberries or apricots, for example. Confectioners refine their creations with freeze-dried ingredients. Many different
The duration of the freeze-drying process
The freeze drying time is of crucial importance for the utilization of a freeze dryer. However, freeze drying times vary greatly between different materials as water content is the main reason for the differences. But other factors also play a role. Oranges and tangerines, for example, have extremely waterproof skin and store water very well. In this case, slicing the fruit is key – smaller pieces are easier to freeze-dry. Berries with a thick skin are generally also good at storing water, e.g. E.g. blueberries. Strawberries, on the other hand, are very easy to freeze-dry.
The table below shows the drying times for medium-sized pieces of approx. 10 mm at a final temperature of 25°C, 40°C and 60°C.
The higher the temperature, the faster the drying process, but slower drying retains more vitamins and better structure. The temperatures and drying cycles must therefore be individually adjusted depending on the desired quality of the end product, which can be guaranteed by the flexible user interface of the Wave freeze dryer.