Freeze Drying & Vacuum Concentration

Vacuum concentration and freeze drying are related methods used for the gentle drying or preservation of thermally sensitive materials. This often involves the removal of aqueous media. As can be seen from the state diagram of water, which can also serve as an example for other solvents, above a certain process pressure (in this case 6.1 hPa for H2O) drying occurs from the liquid phase. At pressures below this level, the water temperature drops below 0 °C.

Drying occurs directly from the frozen state, bypassing the liquid phase. The triple point (in this case p = 6.1 hPa and T = 0 °C for H2O) can be regarded as a sort of boundary between vacuum concentration (or drying) and freeze drying. Ice, water and vapour can be observed simultaneously in the chamber. The key features of the two methods are listed in the following table.


Vacuum Concentration

Freeze Drying

Evaporation Temperature Range

- 5C to- + 20C

- 60C to 0C


Usually dissolved; residue is a powder or a crystalline substance

Solids (including ceramics, meat, archaeological objects, bones, plants and books)


Various solvent types (some exotic)

Aqueous solutions, only small quantities or special solvents

Water / Solvent Content

Approx. 5% final context

Low residual moisture possible (<1%)

Process Time

Minutes to hours

0.5 - 3 days; Several weeks in some case

Christ systems also allow the two methods to be combined in a Rotational Vacuum Concentrator (RVC). Starting at a relatively high pressure (e.g. 20 hPa) and then reducing it to a lower pressure (e.g. 1 hPa) concentrates an aqueous DNA solution to a small residual volume, which then freezes in the device at the lower pressure. Final drying occurs by sublimation.

Product concentration by sedimentation at the bottom of the test tube is a desirable side effect.
This means that Rotational Vacuum Concentration (RVC) not only protects the specimen against superheating but also prevents diffuse product distribution in the test tube, which can lead to low recovery rates.

Application Overview: Lyophilization

Lyophilization, also called freeze drying, is by far the gentlest product drying method. It is based on the physical phenomenon of sublimation, which means a direct transition from the solid state to the vapour state without passing through the liquid state. The frozen products are dried under vacuum without thawing. This method offers a wide spectrum of potential applications:

  • for preserving the characteristics of the original substances (e.g. pharmaceutical products or coffee)
  • for preserving the original form (e.g. animal preparations, archaeological objects, flowers or books)
  • for conditioning the material (e.g. freeze-dried fruit in yoghurt)
  • for the sample preparation for chemical or biochemical analysis (e.g. investigation of trace elements in foods, sludge or soil)

Freeze dryers are generally divided into two categories: those for batch operation and those for continuous operation. Systems for batch operation are commercially available with capacities from 2 kg to approximately 1 tonne per drying cycle. Christ focuses on this product segment and is the only manufacturer in the world that covers the full spectrum from standard laboratory units over pilot freeze dryers to large production machines.

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