Dehydration is the removal of water or the elements of water, in the correct proportion, from a substance or system or chemical compound. The elements of water may be removed from a single molecule or from more than one molecule, as in the dehydration of alcohol, which may yield ethylene by loss of the elements of water from one molecule or ethyl ether by loss of the elements of water from two molecules:
CH3CH2OH ? CH2=CH2 + H2O
2CH3CH2OH ? CH3CH2OCH2CH3 + H2O
The latter reaction is commonly used in the production of ethers by the dehydration of alcohols.
Vapor-phase dehydration over catalysts such as alumina is also practiced. Hydration of olefins to produce alcohols, usually over an acidic catalyst, produces substantial quantities of ethers as by-products. The reverse reaction, ethers to alcohols, can be accomplished by recycling the ethers over a catalyst.
In food processing, dehydration is the removal of more than 95% of the water by use of thermal energy. However, there is no clearly defined line of demarcation between drying and dehydrating, the latter sometimes being considered as a supplement of drying.
The term dehydration is not generally applied to situations where there is a loss of water as the result of evaporation. The distinction between the terms drying and dehydrating may be somewhat clarified by the fact that most substances can be dried beyond their capability of restoration.
Rehydration or reconstitution is the restoration of a dehydrated food product to its original edible condition by the simple addition of water, usually just prior to consumption or further processing.