5.3. Extraction with organic solvents at ambient pressure

Extraction with organic solvents is principally used in the perfumery and cosmetic industries. In this method, the less polar of the components present in the herbs (the so-called oleoresins) are extracted alongside with the essential oils.

Methods involving solvent extraction are particularly difficult to control. Many plant materials require some degree of comminution before treatment with a solvent. This grinding is often accompanied by heat and may lead to volatile losses and chemical change. Green plants have a very high quantity of water and it is difficult to obtain an appropriate solvent to extract the components from the aqueous cells. Even when a very high-speed blending is used, the partition equilibrium may be slow and repeated extractions necessary to obtain a satisfactory yield.

Extraction may be improved by using dried material. But when drying is used some decomposition can occur, and aromatic materials from natural sources differ widely in their physical and chemical properties, particularly in their volatility and solubility, so the choice of the solvent may determine the success of the extraction.

The following factors should be considered when selecting a solvent for commercial uses:

solvent power (selectivity);
polarity;
boiling temperature - this should be low in order to facilitate removal of the solvent from the product;
latent heat of vaporization;
reactivity - the solvent should not react chemically with the extract, nor should it readily decompose;
viscosity - must be low;
stability to heat, oxygen and light;
safety in use - the solvent should, if possible, be non-flammable and should not present a toxicity hazard to technicians or consumers; its disposal should not endanger the environment;
the solvent should be available in substantial quantities;
cost;
suitability for re-use;
compability with local legislation about permitted solvent levels in the product - this is particularly important for food applications.

The organic solvents more frequently used are:

aliphatic hydrocarbons: propane, butane, hexane
alcohols: methanol, ethanol, 2-propanol
hydrocarbons with a carbonyl group: acetone, methyl acetate
halogen derivation: dichloro methane, dichloroethane, freons.

The main disadvantages of solvent extraction are :

poor selectivity of most solvents:
final extracts are often coloured and very viscous;

The solvent is removed from the solution by fractional distillation. The residue is termed the "concrete" and consists of volatile matter (including the essential oils) and oleoresins as well.

Industrial practice is to treat the concrete with alcohol to obtain a selective removal of the volatile constituents.