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Primary Drying

During the primary drying stage, the ice in the sample is con­verted directly to a vapour, at low temperature and pressure, by the process of sublimation and is converted back to ice in the condenser, which is at a lower temperature and pressure than that of the sample chamber.
 
The “energy” required for this process to occur is provided by a gentle increase in sample temperature (heat). In vacuum con­centration the sample is not normally in a frozen state, therefore this stage of the cycle is evaporation rather than sublimation.
 
The amount of “energy” required to initiate this step in the freeze drying / vacuum concentration cycle, is equivalent to that needed to melt the ice and/or evaporate the liquid. The resulting vapour is collected by the condenser/cold trap, which has a lower temperature and pressure than the product and hence is converted back to ice on the condenser surfaces.
 
If too much energy (heat) is applied to the sample during this Pri­mary Drying step, such that sublimation takes place too rapidly, the condenser may not be able to convert the vapours to ice fast enough , the temperature inside the condenser will increase along with its vapour pressure and this results in the sample melting.
 
Optimising the heat applied to the sample in the Primary Drying step can only be achieved by experimentation and is essential so that the capacity of the condenser is not exceeded or over­loaded, and that the product temperature does not rise above its “melting” point thereby destroying it. 

Secondary Drying

After the removal of the “free” water/ice in the Primary Dry­ing stage by sublimation or evaporation in the case of vacuum concentration, there is usually residual water present which is strongly bound to the molecules of the sample.
 
This water bound to the molecular structure invariably has a va­pour pressure lower than that in water in its “free” form.
 
Removal of this bound water is performed in the Secondary Dry­ing stage at higher product temperature because now any biologi­cal activity of the sample/product will not be impaired or effected.