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Distillation is the main method
by which essential oils are
extracted from plants. Indeed,
according to some authorities, it is the only method that produces
essential oils as correctly defined - those obtained by other methods
being known as essences or absolutes.
Distillation involves heating the
plant material, either by placing it in water, which is then brought to
the boil, or placing the plant material on a rack or grid and heating
the water beneath it, so that steam passes up through it. Leaves,
twigs, berries, petals and other parts of the plant may be used. If the
plant material is placed in the water, the process is known as direct
distillation, and if it is put on a grid and the steam passed through
it, the system is known as steam distillation.
In either method, the heat and
steam cause the walls of the specialised plant cells in which the plant
essence is stored, to break down and release the essence in the form of
a vapour. This vapour, together with the steam involved in the
distilling process, is gathered into a pipe which passes through
cooling tanks, and this causes the mixed vapours to return to liquid
form so that they can be collected in vats at the end of the process.
The steam condenses into a watery distillate, while the essence from
the plant becomes an essential oil. This, being lighter than water,
collects in the upper part of the vats and can easily be separated from
the watery part. In some cases, the watery distillate is also a
valuable product, and is sold as a flower-water or herbal water. In
France these distillates are usually described as a hydrolat.
With one or two plants, the
amount of essential oil that can be obtained by distilling is
insignificant, and is regarded as a byproduct
of the production of rosewater or
orange-flower water, for example. Other methods, such as enfleurage
or solvent
extraction are used to obtain the essences from these and other
delicate flower petals.
The process of distillation has
been known and used for obtaining essential oils since at least the
10th Century A.D. and is thought to have originated in Persia, where
the oils were highly prized as perfumes (Shakespeare's 'perfumes of
Arabia'). However, recent archaeological digs in Italy have uncovered
simple stills which suggest the Romans already knew this technique.
Some of the stills in use today,
especially in less developed countries, and at small-scale rural
distilleries in Europe, differ very little from the earliest stills
known, but in areas where essential oil production is an important
industry, they may be very large and complex, though the basic
principles of production involved are identical. Stainless steel is
often used in the construction of modern stills to avoid any
contamination of the distillate, and this may produce better quality
oils, though it is non-proven.
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