Extraction is the process in which the plant tissues are treated with specific solvents whereby the medicinally active constituents are dissolved out, cell tissues and most of inactive or inert components remain undissolved. The plant material used for extraction should be properly identified. The choice of the plant material for extraction depends on its nature and the components required to be isolated. The solvents used for extraction purposes is known as “Menstruum” and residue left after extracting the desired constituents is known as “Marc”.
The medicinal value of natural herb of the plant is due to the active constituents. It is generally advantages to extract the active constituents to formulate a control doses form of that active constituents rather than using the bulk quantity.
From the stand point of pharmacy, the purpose of a solvent is to remove from a solid, either in part or in its entirety such substances that may be rendered to a liquid.
When the material has extracted, the “Menstruum” is known as “Vehicle” or “Carrier” of the extracted materials. Solvents differ widely from each other, not only in differing boiling points, but how they act or react with substances in which they come in contact.
An Ideal Solvent for the Extraction of the Herbs should meet the Following Criteria:-
1. It should be non-toxic and selective, i.e. it should dissolve only the required constituent with minimum amount of the inert materials.
2. It should not cause the extract to complex or dissociate.
3. It should be preservative in action.
4. It should promote rapid physiologic absorption of the extract.
5. It should be easily evaporated at low heat.
Alcohol (Ethanol) will meet all above criteria.
There are large number of solvents (Menstruum) used for extraction of herbs, but the selection of the suitable solvents capable of extracting the active constituents depends upon the chemical properties of active constituents as well as the qualities of the solvent. The solvents commonly used for the extraction of the herbs include water, alcohol and there different dilutions.
a. Water: - It is a good solvent for the extraction of many types of active constituents such as alkaloidal salts, colouring agents, glycosides, gums, sugars, anthraquinone derivatives and tannins. It can also act as menstruum for many organic acids and small proportions of volatile oils.
Water is not a suitable menstruum (Solvent) for constituents like waxes, fats, fixed oil and alkaloidal bases due to there insolubility in water. Water is not selective as it can dissolve a wide range of substances and leads to hydrolysis of many substances.
Water soluble herbs are aloe, glycyrrhiza, linseed, senna leaves, senna pods, ginger etc.
b. Alcohol: - Alcohol or ethanol can dissolve a large number of chemical constituents such as alkaloids, alkaloidal salts, glycosides, tannins, anthraquinone derivatives, volatile oils and resins, but constituents like albumin, gums, waxes, fats, fixed oils and sucrose are insoluble in alcohol.
Generally dilute alcohols (hydroalcoholic solutions) are used for many extractions, but in some cases stronger alcohol may be used to prevent the extraction of unwanted substances such as gums.
It is non-toxic in the quantities present in medicinal substances. It is reasonably selective. In a herb containing a number of chemical substances such as alkaloidal salts, glycosides, albumin and gum, water will dissolve all the substances. Whereas dilute alcohol will dissolve only the alkaloidal salts and glycosides.
Alcohol soluble herbs are benzoin, asafoetida, ginger, valerian, myrrh etc.
Other solvents Used for Extraction of Herbs Include Ether (Anaesthetic Ether), chloroform, glycerin, light petroleum, benzene, propylene glycol, acids such as acetic acid and tannic acid.
c. Ether: - Soluble Constituents are oils, fats, waxes, resins and alkaloidal bases. Highly inflammable produces physiologically effects. Ether soluble herbs are capsicum, male fern, linseed, nutmeg etc.
d. Chloroform: - Soluble constituents are oils, fats, waxes, resins and alkaloidal bases. Non inflammable.
e. Glycerin: - Soluble constituents are tannins. Non inflammable and viscous liquid.
f. Light Petroluem: - Soluble constituents are oils, fats, waxes, resins and alkaloidal bases.
Highly inflammable and very volatile.
g. Fixed Oils: - Soluble constituents (Arachis Oil) can act as menstruum for camphor. Non inflammable and viscous.
h. Propylene Glycol: - Soluble constituents are progesterone, phenobarbitone sodium. Clear, colorless, odourless, viscous liquid, miscible with water, alcohol and chloroform.
Extraction of organic bases like alkaloids usually necessitates basification of plant material, if a water immiscible solvent is to be used, whereas for aromatic acids and phenols, acidification may be required. The glycosides are soluble in water and alcohol but are insoluble in non-polar solvents. Tannins are phenolic matter soluble in water, alcohol and ethyl acetate.
Cell inclusions (Ergastic substances of plants) - The non living substances of plant metabolism are known as ergastic substances. They may be reserve foods, secretory and excretory or end products of metabolism.
(a) Reserve Foods: - The materials which occur as reserve foods in cell are the carbohydrates, proteins and lipids. They are present in insoluble forms. Their conversion to soluble form is covered by enzymatic processes.
(b) Secretory Products: - Enzymes, coloring substances, nectar are the examples of secretory products.
Enzymes are nitrogenous water soluble compounds carrying out hydrolysis of carbohydrates and proteins.
Coloring substances or pigments like chlorophyll are essential for photosynthesis and several others, flavonoid glycosides giving attractive color to various flowers and leaves.
Nectar is the sugary solution secreated by many flowers in special cells or glands to attract the insects for effective pollination.
(c) Excretory Products (compounds): - Animals are able to get rid of their excretory products in liquid or solid form. Plants being unable to do so, their wastes are excreted in the form of insoluble products and are stored inside the cells only. Excretory products (compounds) of the plants are named as tannins, resins, latex, volatile oils, chemicals like alkaloids, glycosides and mineral crystals, such a as calcium oxalate, calcium carbonate, silica, etc.
In extraction, a solvent is employed which is capable of penetrating the tissues of the herb and dissolve the active principles contained in it cell. Any method will be a good technique, if it will accelerate:
1. Wetting of the surface of the herb particles.
2. Permeability of cell walls.
3. Rate of dissolution of cell contents in the solvent.
4. Outward diffusion of the solution.
Article Composed by Vishal Gupta
References:R.M. Mehta Text Book Of Pharmaceutics