Steam distillation is a method of extracting compounds that decompose at high temperatures by introducing steam into the raw material. It is used for compounds that decompose below their boiling points and can be used to isolate essential oils from plant material.
Steam distillation is a method of isolating compounds that decompose at high temperatures by distilling them in such a way that steam is introduced into the raw material. Normally, distillation is employed to separate one substance from another substance with a higher boiling point. The mixture is heated to the boiling point of the substance to be extracted and the vapor is cooled and condensed. This technique, however, is not suitable for compounds that decompose below their boiling points. Steam distillation can be used for these compounds, provided the compound is not miscible with water.
The boiling point of a liquid depends on the ratio of its vapor pressure to atmospheric pressure. Vapor pressure increases with temperature and as it heats up, the liquid will boil at the point where the vapor pressure equals the pressure of the atmosphere. Thus boiling can be achieved by heating the liquid or by reducing the atmospheric pressure. Some liquids, however, decompose below their boiling point at normal atmospheric pressure. One solution to this problem is to use vacuum distillation or distill under reduced pressure, so that the liquid boils at a lower temperature; another solution is steam distillation.
Where the compound of interest, normally an oil, is immiscible with water and has a higher boiling point, the vapor pressure of the two combined is the sum of their separate vapor pressures and therefore will be greater than that of the compound alone. The increase in vapor pressure achieved by mixing the compound with water or steam reduces the boiling point, allowing it to be distilled without decomposition. Normally, much more water is distilled than oil, but as they cool and condense in a receiver, the two form distinct layers from which the oil can easily be separated. To maximize the amount of oil that is extracted, it is normal to have an excess of water over oil. In some cases, the oil-containing material is simply mixed with water and heated, while in other cases, the water may be boiled in a separate container and the resulting steam admitted to the material for distillation.
A common application for steam distillation is the isolation of essential oils from plant material, such as herbs and spices. These oils can be used in flavors, perfumes and aromatherapy. Some have medical uses. Examples of essential oils often extracted by steam distillation are eucalyptus, clove, orange, mint, and lemongrass.
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