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The steam distillation is one of the best methods for the isolation of the essential oil of a plant. In principle, this method takes from the difference of the boiling temperature between the essential oil and other compounds. The obtained essential oils can vary greatly in the composition of the odorous substances obtained by extraction. The steam distillation can, for example, lead to a fermantation and can therefore change the compostion of the essential oil.
The steam distillation is a gentle method for the isolation of Natural products. The isolation works at a temperature below 100 °C and is carried out at atmospheric pressure.
The essential oils were seperated due their boiling temperation. These have a boiling point from 50°C until 320°C.
It is important for this method that the insulating material is hardly or not soluble in water. This is given in case of essential oils. For non-soluble mixtures a total vapor pressure is established which is composed of the vapor pressure of the individual components (the so-called partial pressures). The vapor pressure can be increased by increasing the temperature.
Vapor pressure Ptotal = partial pressure Pwater + Pessential oil
If the vapor pressure is higher than the external pressure (usually at 1013 mbar), a liquid phase begins to boil. In insoluble mixtures thereby the vapor pressure is p, which is composed of the partial pressures Pwater and Pessential oil, increased. This leads to a lowering of the boiling point. The boiling temperature is therefore always less than 100°C, thus guaranteeing a gentle isolation. That is important for heat sensitive natural substances.
With the water steam the essential oil is carried to the condenser. That is why it is also spoken from a carrier distillation. After cooling, the aqueous phase is separated from the insoluble natural products and the essential oil can be gained.
There are three modules required for steam distillation. Firstly, a water bowl which generates the water vapor is required. The water steam is passed through the vegetable material from which the essential oil is extracted. The essential oil is carried by the water steam. In the third module, the radiator, the mixture is cooled and the distillate is gained with the two phases - the essential oil and the water.
The essential oil has a lower density than water and thus can be separated with a separating funnel from the aqueous phase.
In most cases the yields are very low. The content of essential oils are in most plants in the low single-digit percentage. This means that per kilogram of plant material often only a few mL of essential oil can be extracted.
In addition to the essential oil also the so-called hydrosol is obtained. In the distillation the essential oils are carried by the steam and it separates from the aqueous phase when they are condensing. However, the aqueous phase is capable of small amounts of polar ingredients, so for example functionalized monoterpens. This is flavoring the water. Especially for expensive starting materials, such as rose petals, the hdydrsol is sold as (rose)-hydrosol for, eg. face tonics.
Using the hydrodistillation, the plant material is added to the water which is subsequently heated. The resulting steam is then cooled and condensated. Now you can seperate the essential oil from the aqueous phase. In this method, the same physical effects occur. The steam carries the essential oil until it condensate. In contrast to the steam distillation, however, the plant material is not in contact with the water vapor but only with water, which is heated to boiling.
The researchers Boutekedjiret C., F. Bentahar, R. Belabbes and JM Bessiere published in 2003 in “Flavour and Fragrance Journal” a study in which the differences in the composition of the essential oil of rosemary is investigated depending on the type of insulation.
The main components vary depending on the type of isolation:
|1,8-Cineol||52.4 %||31.9 %|
|Campher||12.6 %||19.7 %|
|Borneol||3.4 %||12.1 %|
|α-Terpineol||2.1 %||12.8 %|
|Yield||1.2 %||0.44 %|
According to this study the two essential oils contribute the same main components in it. However, the proportions of these components are different due to the different isolation of the essential oil. The yield in the steam distillation is higher. The research group further examined the influence of the extraction time on the yield and composition of the essential oils. The steam distillation reached faster higher yields. Because of this reasons the group concludes that the method of steam distillation is superior compared to the hydrodistillation.
A steam distillation can already be done on a small scale in the laboratory within a day. It is important to note that by the low yields often only a few mL of the natural product can be obtained. In large systems, however, several liters can be won in a day. At a price of about 10 € per 10 mL, natural products worth several thousand euros can be obtained.
In the laboratory requires two bottle flasks, a riser, a Liebig condenser and a collecting vessel. In a flask, the plant material is placed in water and heated gently. With the second flaced, which is filled with water, water vapor is introduced by heating the mixture. The steam reached the condenser after the riser pipe and is the crude mixture is condensed into to collecting vessel.
In another variant (the hydrodistillation), the plant material is boiled in water and the steam is cooled afterwards. The cooled water, which served as a carrier material, is returned to the starting solution so that the small amounts of natural substances which are dissolved in water are recycled.
In a large-scale plant, hundreds of kilos of plant material can be processed per batch. The generation of water vapor is done by a steam boiler. The resulting steam is passed through a vessel with the plant material. The plant material (often dried) must be packed tightly enough in the vessel so that the steam can not escape through a gap.
Again, the water is then cooled and the natural products are gained.
 C. Boutekedjiret, F. Bentahar, R. Belabbes, J. M. Bessiere; Extraction of rosemary essential oil by steam distillation and hydrodistillation. Flavour and Fragrance Journal, 2003, 18, 481-484. http://onlinelibrary.wiley.com/doi/10.1002/ffj.1226/abstract