Freeze-drying (or lyophilization) is a dehydration process during which the product that needs to be dried is frozen first, then, with the surrounding pressure reduced, very slowly heat is added in order to allow the frozen water in the material to “disappear”. More precisely, the ice is allowed to sublimate directly from the solid phase to the gas phase, avoiding the liquid phase.
The detailed description of the freeze-drying procedure is the following: If the pressure is lowered to a few millibars, water sublimates from its solid state (i.e. ice), meaning that when heat is supplied, ice doesn’t turn into water, but transitions to its gas state without passing through an intermediate liquid phase. This occurs at very low pressure, usually under 6 millibars. Therefore, first the product is frozen, placed into a vacuum chamber where the surrounding pressure is reduced to less than 6 millibars. After this, no heating is needed: It is sufficient to bring the product to room temperature very slowly. For instance, in order to make strawberries from frozen strawberries, not less than 20-24 hours is required!
There are two main advantages of freeze-drying compared to other traditional drying methods. First, no considerable amount of heat is used; therefore, heat doesn’t damage the valuable substances of the material. Vitamins don’t become dissolved, the product retains its color and taste, and the fructose content doesn’t become destroyed either. Second, during the freeze-drying process there is no form of liquid water, in other words, no evaporation occurs. Therefore, valuable substances, such as the nutritional content, don’t leave the fruit with the evaporating water but remain entirely in the fruit. Afterwards nothing needs to be “restored” or supplemented, thus freeze-dried products do not contain any added aromas, sugars, coloring agents, vitamins, or mineral substances—everything is “original” in them.