Extraction can be defined as the removal of soluble materials from an insoluble residue, either liquid or solid, by treatment with a liquid solvent. There are different methods process of extraction they are :-

1. Infusion
2. Decoction
3. Maceration
4. Percolation

Infusion:- The weighed quantity of drug is kept in contact with known quantity of menstrum for a specified period of time and at the end of the period, the supernatant liquid are collected and poured into receiver. Hence menstrum may be either cold or hot depending on drug material.
E.g Fresh quassia infusion.

Decoction :- It is not a official preparation in pharmacopoeia. Here known quantity of drug is boiled with the menstrum for a specified period of time (15 mins) at the end of which the supernatant liquid is collected.
E.g, Coffee or tea decoction.

Maceration:- It is defined as that phase the solid materials with whole of menstrum in a closed vessel and allowed to stand for 7 days shaking occasionally strain, presss the marc, and mix the liquid obtained with the mother liquor and clarify the filtration.
e.g. For preparing tinctures form organized drugs.

Percolation:- There are 3 types of percolation.
1. Percolation process used for tinctures
2. Percolation process used for concentrated preparations
3. continuous hot percolation or soxhelation.

In percolation process there are 3 main stages.
a. Imbibition : The powdered drug is moistened with some of the menstrum and allowed to stand for 4 hours. During this time the drug swells and the menstrum penetrates the cell walls.
b. Maceration :- The moistened drug is left in contact with the menstrum for 24 hours. During this time the menstrum is saturated with the soluble constituents of durg.
c. Percolation:- This consists of the downward displacement of the saturated solution formed in above maceration and extraction of the remaining soluble matter by slow passage of menstrum through the column of drug.

Liquid/Liquid Extraction:- This method will be familiar from the purification procedures used in alkaloidal assays , and large scale processes are based on similar principles.
A solution of substance, it is desired to extract is brought into contact with another solvent for the substance that is immiscible with the first solvent. A concentration gradient is set up between the phases and mass transfer will occur until an equilibrium is established and the distribution of the solute depending on the distribution co efficient.
Solid/Liquid extraction :- The extraction of a soluble constituent from a sold by means of a solvent is commonly referred as leaching. The general term extraction is most frequently used. Certain operations involve the extraction of a soluble substance from an insoluble mineral material. But this is subjected to control by the usual factors affecting solution and will not be considered further.

EXTRACTION OF PLANT MATERIALS.
All plant materials used should be properly authenticated. The choice of extraction procedure depends on the nature of the plant material and the components to be isolated. Dried materials are usually powdered before extraction, where as fresh plants (leaves etc) can be homogenized or macerated with a solvent such as alcohol. The latter is also particularly useful for stabilizing fresh leaves by dropping them in the boiling solvent. Alcohol is a general solvent for many plant constituents (most fixed oils expected) and as such may give problems in the sub-sequent elimination of pigments, resins etc. Water, immiscible solvents are widely used. Light petroleum (essential and fixed oils, steroids) Ether and chloroform (alkaloids, quinones). The extraction of organic bases e.g alkaloids usually necessitates basification of the plant material if a water immiscible solvent is to be used, for aromatic acids and phenols acidification may be required.
Ultra sound may enhance the extraction process for some plant materials an the B.P. uses this in the preparation of a 50% ethanolic solution of opium for the assay of alkaloids.
The general methods of plant extractns are :-

Super critical fluid extraction
The plant products which have medicinal flavoring and cosmetic interest. In 1822, cagniard de la tour reported that above a certain temperature and pressure, single substances do not condense or evaporate but exists as a fluid. Under these conditions the gas and liquid phases both posses the same density and no division exists between the two phases. This is the critical state. For water the critical conditions for temperature (tc) and pressure (pc) are 374 degrees and 220 atm respectively and for carbon di oxide tc=31 degrees and pc=74 atm. In practice conditions somewhat above the critical temperatures and pressure for a particular substance are usually used and these super critical fluids exhibits properties intermediate between those of the liquids and gaseous phase.

It also allows a low temperature process and has proved of value for the extraction of labile expensive fragrances and medicinal phytochemicals. To render it more polar a small amount of modifier. E.g. methanol may be added to CO2. The high pressure and for some substance the high temperature involved in super critical fluid extraction are the principal disadvantages of the technique.

Examples of more recent studies involving the extractions of phytochemicals with supercritical CO2.
1. Alkaloids:- De caffeination of green coffee. (Industrial application). Isolation of vidoline from Catharanthus roseus.
2. Ditopene:- extraction of taxol from Taxus brevtocia. (Extraction more selective than conventional ethanol extraction) also from T. cuspidate
3. Fixed oils :- Extaction of oil from evening primrose (shift in triglyceride composition oxidation of γ-linolenic acid during extraction reduced.)
4. Pigments :- Extraction of annatto seeds.
5. Sesquiterpene lactones:- In conjunction with gas chromatography for the isolation of parthenolide from fever few ( extracts contained significant amounts of camphor and chrysanthenol acetate.) addition of methanol or methyl cyanide as CO2 modifiers gave higher yields but produced co-extractives.
6. Volatile oils and resins :- hops (commercial application) Transkinase and myrrh piper nigram muntok (superior aroma of oil : yield 2.8% volatile oil compared with 0.6% steam distillation.) Rose petals richer in relevant fragrance compounds compared with steam distillation). Rosemary also studied : angellca root. Coriander, Illecium,. Verum, juniper, Berries etc.

Sepration and Isolation of constituents :-
The techniques which are constantly being developed n the separation and isolation of constituents.

Separation and isolation of constituents.
1. Sublimation :- Sublimation may sometimes be possible on the whole drug, as in the isolation of caffeine from Tea or for the purification of materials present in a crude extract modern equipment employs low pressures with a strict control of Temperature. Sometimes substances when heated pass directly from the solid to the vapor state with out melting when cooled it results in the solid substance is known as sublimation.

2. Distillation:- Fractional distillation has been traditionally used for the separation of the components of volatile mixtures. It has been widely used for the isolation of the components of volatile oils. Steam distillation is much used to isolate volatile oils and hydrocyanic acid from plant material.

3. Fractional liberation:- Some groups of compounds tend themselves to fractional liberation from mixture. As an example a mixture of alkaloid salts in aqueous solution when treated with aliquots of alkali, will give first the weakest base in the Free State followed by base liberation in ascending order of basicity. If the mixture is shaken with an organic solvent after each addition, then a fractionated series of tests will be obtained. A similar scheme can be used or organic acids soluble in water immiscible solvents, in this case, starting with a mixture of the acid salts. It is possible to fractionally liberate the acids by addition of mineral acids.

About Author / Additional Info:
I Am M.Pharma in cognosy and M.Sc in chemistry