3.6.2. Theory of freeze drying

3.6.2. Theory of freeze drying

The boiling point of water depends on the overhead atmospheric pressure. At normal atmospheric pressure of 1 bar (i.e., 760 mg of Hg) it boils at 100ºC. However, if it is held in a sealed container and overhead atmospheric pressure is lowered than one by using vacuum pumps, then it boils at less than 100ºC. It is observed that the lower the pressure (or the better the vacuum) the lower is the boiling point of water but the energy needed for evaporation of the same quantity of water is more. The pressure, the corresponding boiling point of water and latent heat of evaporation are presented in the following table.

Pressure    (bar)

 Boiling Point  of Water OC     

Latent heat of vaporization (kj/kg) of water

1  

100   

2257

0.5  

 81 

2305

 0.2 

60   

 2358

0.1 

46    

2392

0.01

7    

 2485 

   At OOC water exists an ice (solid phase) and if the overhead pressure is sufficiently low, solid ice will directly sublime as water vapour without going into liquid  phase. To understand the phenomenon more clearly it is essential to refer to the phase diagram of water.

 Pressure units conversion table :

Torr 

 mm Hg 

Pascal

1   

1 

133.3

10-1 

 0.1

 13.33

10-2 

0.01  

1.333

         From the phase diagram it is clear that if the temperature of water and pressure is below the triple point it exists as ice and it will directly pass into vapour phase.

If the overhead pressure of a food is held below 4.58 Tor (610.5 Pa) and the food is frozen, when the frozen food is heated solid ice sublimes directly to vapour phase without melting. Water vapour is continuously removed from the food by keeping the pressure in the freeze drier cabinet below the vapour pressure at the surface of ice by removing the water vapour with a vacuum pump and condensing it on refrigeration coils or plate or drum. As the drying proceeds, the sublimation process continues as long as latent heat of sublimation is provided/supplied. The latent heat of sublimation is either conducted through the food to the sublimation front or produced in the food by microwaves. The pressure, sublimation temperature in the frozen state and corresponding latent heat of sublimation are presented below.  

In freeze drying, foods are dried in two stages, first by sublimation to approximately 15% moisture content (on wet weight basis) and then by evaporative drying of unfrozen water to 2%  of moisture content (W.W.B). Desorption is achieved by raising the temperature in the drier to near ambient temperature while retaining the low pressure. 

The rate of drying depends mostly on the resistance of the food to heat transfer and on the resistance to vapour flow.

Last modified: Saturday, 24 December 2011, 11:27 AM