Freeze-drying is sometimes called lyophilization and is a promising preservation process. In freeze-drying, a product is first frozen and then the pressure is lowered and the ice is removed by sublimation. Sublimation is the transformation of a substance from a solid to a gaseous state without first liquefying. Unlike other preservation processes that evaporate water through heat, 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. This is why freeze-drying is mainly used in biomedicine and food processing, because nutrients and active ingredients are particularly well preserved.

The First Experiments

Freeze-drying developed separately in early communities in the Americas, East Asia and Northern Europe. Tribes in Peru were already freezing tubers and potatoes in the cold peaks of the Andes. They then placed the frozen food in the sun, allowing the ice to evaporate. From this they obtained a traditional staple food that formed the basis for many different meals. The indigenous peoples could then provide 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

In 1890, tissue was freeze-dried to perfection for the first time in a laboratory and then rehydrated. In the following decades, freeze-drying was further researched and scaled up, so that during the Second World War, blood plasma could be safely preserved and transported with the help of freeze-drying. Somewhat later, the technique was also used to transport vaccines. Almost all food for NASA's Apollo missions were also preserved by freeze-drying.

The Freezing

There are various methods that can be used to freeze the product. Freezing can be done in a cold bath with liquid nitrogen, in a shockfroster, a regular freezer, or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that the product is not thawed but sublimated. In this way, the shape of the product is optimally preserved.

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 cell walls of the product, resulting in a lower quality end product. To prevent this, the freeze process is carried out as quickly as possible.

Freeze-Drying Process Through Drying Cycles

The second phase of freeze-drying is the so-called primary drying (sublimation), in which the pressure is lowered (a vacuum is created) and heat is added to the product so that the water can sublimate. The vacuum promotes sublimation. The condenser provides a surface for the water vapor. In the case of WAVE freeze dryers, 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 vapor. Too much heat at once can affect the structure and shape of the material. We recommend adding heat in small steps in order not to overheat the material as the ice will melt and the structure will be lost.

This is followed by further phases of freeze-drying, secondary drying (adsorption), in which water molecules that are ionically bound are removed. By setting the temperature higher in small steps than in the primary drying phase, the bonds between the material and the water molecules are broken. These drying stages can take place at 5°C to 10°C intervals, for example, and can be flexibly adjusted by the Siemens control of the WAVE freeze dryers. 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.

● Treated products retain their original shape and structure

● Aromas are better preserved than with other drying processes

● Better taste and consistency for food

● A gentle process that retains vitamins, nutrients and active ingredients well

● Less weight despite original structure

● Can be rehydrated

● Avoid risk of mold during drying

● Very fast drying process

● Freeze drying will kill almost all germs and bacteria in the process

● The material can be stored for up to 20 years

Freeze-drying is most commonly used in pharmaceutical science to extend the shelf life of important 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 became particularly popular after freeze-dried ice cream was made as a snack for NASA astronauts. Recently, a variety of products have been created as the freeze-drying process has become cheaper, unlocking unlimited creative potential. Hard-to-sell foods such as crooked asparagus are being turned into new products such as asparagus powder for the food industry. Farmers can process the vegetables and fruit and sell freeze-dried strawberries or apricots, for example. Confectioners are refining their creations with freeze-dried ingredients. There are many different creative approaches to how freeze-drying can enrich our lives.