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Freeze Drying is basically a 3 stage process.
In the first stage, the sample is frozen so that the “free” water present is converted to ice, and so the phase change from liquid to solid is achieved. This stage is known as PRE-FREEZING.
In the second stage of the freeze drying process, the ice formed in the pre-freezing step is removed from the sample by the direct conversion of the ice (solid phase) to a vapour (vapour phase), without passing through a liquid phase, by a process called sublimation. This stage is known as the PRIMARY DRYING.
Typically most aqueous samples/solutions show an increase in concentration as the product temperature is reduced and the water is converted to ice, during the pre-freeze step.
These are known as EUTECTIC FORMING solutions.
Some solutions however do not exhibit this quick transformation from a liquid phase to a solid phase, when the temperature is reduced, they just become more viscous. These are known as GLASS FORMING solutions. Most sugars solutions or samples containing sugars exhibit this phenomena.
Finally in the third stage, any water that is “molecular bound” to the solids of the sample, is converted to vapour and removed from the sample. This stage is known as SECONDARY DRYING and usually involves increasing the temperature and pressure environment i.e. the shelf temperature / vacuum inside the chamber, to provide the energy to” break the molecular bonding”. This process is called DESORPTION.
Both in Freeze Drying & Vacuum Concentration, a low pressure environment is required to allow these processes to take place, therefore a high quality vacuum pump is necessary.
In Freeze Drying, in order to start the removal of the water, the pressure inside the sample chamber has to be below the “triple point” value, whilst maintaining the temperature of the sample below its freezing point.
Whilst in Vacuum Concentration, where we want to keep the sample in a “liquid phase” during the process, vacuum is required to reduce its” boiling point” to a level whereby the liquid sample can be evaporated at low temperatures without denaturing the sample. The centrifugal force exerted on the sample inhibits any bubbling/boiling action whilst concentrating any solutes at the bottom of the tube.