Sublimation
Sublimation is the transition of a substance from the solid state to the vapour state, without passing through the liquid state.
Steps:
The basic steps of freeze-drying consists of:
1. Freezing the food so that the water in the food become ice;
2. Under a vacuum, sublimating the ice directly into water vapour;
3. Drawing off the water vapour using a condenser;
4. After this, there is about 4-5% of moisture left in the material. To reduce this final moisture content, low temperature heat is applied to the frozen material to accelerate sublimation and pull off the remaining moisture.
Benefits
If a freeze-dried food is sealed to prevent the reabsorption of moisture, it may be stored at room temperature without refrigeration, and be protected against spoilage for many years. Preservation is possible because the greatly reduced water content inhibits the action of microorganisms and enzymes that would normally spoil or degrade the food.
Freeze-drying also causes less damage to the substance than other drying methods using higher temperatures. Freeze-drying does not usually cause shrinkage or toughening of the material being dried. In addition, flavours, smells and nutritional content generally remain unchanged.
In the freeze-drying process, microscopic pores are created by the ice crystals that sublimate, leaving gaps or pores in their place. Freeze-dried products can be rehydrated (reconstituted) much more quickly and easily.
Other References:
Wikipedia
Freeze drying water damaged papers at the National Library of New Zealand
Very High-tech Lyophilization Plant
Freeze Drying 101
