Freeze Drying

Freeze drying (sublimation drying): consist of reducing the temperature and pressure to value below the triple point under these conditions any heat transferred is used as latent heat and the ice sublimes directly to the vapor state.

 Uses Of Freeze Drying:

The method is applied only to biological products:

a.  Antibiotics (other than penicillin)

b.  Blood products

c.   Vaccine such as

smallpox.

d.  Enzyme preparation such as hyaluronidase.

e.  And microbiological culture.

The Following Aspects Must Be Taken Into Account:

1.  The temperature must be kept well below the triple point and it is usual to work in the range -10 to -30^oC.

2.  The pressure must be below the triple point  and pressures between 10 and 30N/m² are used.

Stages Of The Drying Process:

Usually the material is frozen before application of vacuum to avoid foaming under these conditions the primary drying stage by sublimation can reduce the moisture content to 0.5 to 1%. If it is necessary to remove the final traces the temperature is raised or a desiccant used, so the secondary drying is an ordinary vacuum drying process.

Advantages:

1.  Drying takes place at very low temperatures, so the enzyme action is inhibited, and decomposition minimized.

2.  The solution is frozen, so that the final dry product occupying the same volume as the original solution.

3.  Under high vacuum, there is no contact with the air, and oxidation is minimized.

Disadvantage:

1.  The process is very slow and used complicated plant, which is very expensive.

Methods are used to affect freezing while ensuring that a large surface is produced.

1.  Shell freezing: the bottle is used only part filled with product and is rotated slowly, almost horizontal, in a refrigerated bath this give a thin layer, a large area of sublimation ( in fact, the area increase → drying increase.

Vertical spin freezing: the bottles are first chilled and then spun individually in a vertical position in a stream of very cold air