Freeze-drying—technically known as lyophilisation, lyophilization, or cryodesiccation—is a dehydration process typically used to preserve a perishable  material or make the material more convenient for transport. Freeze-drying works by freezing the materialfrozen surrounding pressure to allow the  and then reducing the  water in the material to sublimate directly from the solid  phase to the gas phase.

 
 

Properties of freeze-dried products

     If a freeze-dried substance is sealed to prevent the reabsorption of moisture, the substance 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 substance. 

     Freeze-drying also causes less damage to the substance than other dehydration 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, making the process popular for preserving food. However, water is not the only chemical capable of sublimation, and the loss of other volatile compounds such as acetic acid (vinegar) and alcohols can yield undesirable results.

     Freeze-dried products can be rehydrated (reconstituted) much more quickly and easily because the process leaves microscopic pores. The pores are created by the ice crystals that sublimate, leaving gaps or pores in their place. This is especially important when it comes to pharmaceutical uses. Freeze-drying can also be used to increase the shelf life of some pharmaceuticals for  many years.    

 

Reference : https://en.wikipedia.org/wiki/Freeze-drying