Extraction involves the separation of medicinally active portions of plant or animal tissues from the inactive or inert components by using selective solvents by standard extraction procedures.

Extraction methods include solvent extraction, distillation method, pressing and sublimation according to the extraction principle. Solvent extraction is the most widely used method.

The extraction of natural products progresses through the following stages: \

• (1) the solvent penetrates into the solid matrix;
• (2) the solute dissolves in the solvents;
• (3) the solute is diffused out of the solid matrix;
• (4) the extracted solutes are collected.

Efficiency of extraction depends upon following factors

• The properties of the extraction solvent,
• The particle size of the raw materials,
• The solvent-to-solid ration,
• The extraction temperature &
• The extraction duration

Note: Expression is the physical act of applying pressure to squeeze out oils or juices from plants. This was normally achieved with a tincture press.

Menstruum: 

The solvent used for extraction is known as a menstruum

Marc

Inert insoluble material that remains after extraction

Galenical: 

All preparations that are prepared by using one of the various extraction processes is known as Galenicals.

The products obtained from plants are relatively impure liquids, semisolids, or powders intended only for oral or external use. These include classes of preparations known as decoctions, infusions, fluidextracts, tinctures, pilular (semisolid) extracts, and powdered extracts. Such preparations have been called galenicals.

Steps Involved in the Extraction of Medicinal Plants

In order to extract medicinal ingredients from plant material, the following sequential steps are involved-

1. Size reduction
2. Extraction
3. Filtration
4. Concentration
5. Drying

Solvent/menstruum  used in extraction process

Other Solvents Used for Extraction

3. Chloroform

• Nonpolar solvent
• Useful in the extraction of compounds such as terpenoids, flavonoids, fats, and oils.

 Advantages-

• Colorless, sweet smell, and is soluble in alcohols.
• Also absorbed and metabolized in the body

Disadvantages– sedative and carcinogenic property.

4. Ether

• Nonpolar solvent
• Useful in the extraction of compounds such as alkaloids, terpenoids, coumarins, and fatty acids.

Advantages

• Miscible with water.
• Has low boiling point.
• tasteless in nature.

Disadvantages- highly volatile and flammable in nature.

5. Ionic liquid (green solvent)

• It is highly polar and extremely heat stable (remain in a liquid state even at 3,000.
• It is Miscibility with water and other solvent.

 Advantages-

• Excellent solvent that attracts and transmit microwave
• It is nonflammable

Various type of Methods of Extraction

1. Infusion method

Drug is placed at  bottom of the pot +  add water/hot water and stirred three to four times,  stands a usually 15 minutes. After 15 minutes, filtering off the liquid without pressing the marc.

or

Drug is Enclosed in muslin bag& suspended in the water/hot water in a pot. stirring is not required due to slow circulation.

2. Decoction

• In this process, the crude drug is boiled in a specified volume of water for a defined time. It is then cooled and strained or filtered.
• This procedure is suitable for extracting water-soluble, heat-stable constituents from hard & woody materials.
• The starting ratio of crude drug to water is fixed, e.g. 1:4 or 1:16; the volume is then brought down to one-fourth its original volume by boiling during the extraction procedure. Then, the concentrated extract is filtered and used as such or processed further.
• This process is typically used in preparation of Ayurvedic extracts called “quath” or “kawath”.

3. Maceration

• In this process, the whole or coarsely powdered crude drug is placed in a stoppered container. Then the mesntruum is poured on top until completely covered the drug material. The container is then closed & allowed to stand at room temperature for a period of at least 3 days with frequent agitation until the soluble matter has dissolved. The mixture then is strained, the marc (the damp solid material) is pressed, and the combined liquids are clarified by filtration or decantation after standing.
• This method is convenient and very suitable for thermolabile plant material.

Note: Plant material in fine powder form is never used, as it makes subsequent clarification/filtration of the extract difficult.

Process of extraction works by molecular diffusion, which is very slow. Occasional shaking assists diffusion and also ensures dispersal of the concentrated solution accumulating around the surface of the particles, thereby bringing fresh menstruum to the particle surface for further extraction.

A closed vessel is used to prevent evaporation of the menstruum during the extraction period and thus avoids batch to batch variation.

Maceration Process for Organized and Unorganized Crude Drugs

• Organized drugs have a defined cellular structure whereas unorganized drugs are non-cellular.
• Bark and roots are examples of organized crude drugs, while gum and resin are unorganized crude drugs.

4. Percolation

Percolation is used mostly to prepare tincture & fluid extract. It is also known as Exhaustive Extraction.

A percolator (a narrow, cone shaped vessel open at both ends) is used to extract active ingredients from the crude drugs.

Various types of Percolation

a) Simple percolation

b) Percolation process for concentrate preparation

c) Continuous hot percolation/Soxhlet

Steps involved in percolation process:

I. Size Reduction

Size reduction increases the surface area of the crude drug and more surface area of the crude drug will be available to react with the menstruum to insure complete exhaustion of the crude drug. Size reduction is also helpful for uniform packing of the crude drug in a percolator and reducing the moment of menstruum in the percolator.

II. Imbibition

In the Imbibition step, the powdered crude drug is moistened with menstruum in well closed container for 4 hrs to swell and to escape entrapped air.  During imbibition step, the menstruum penetrates the cell wall. This initial moistening of the crude drug powder is very important because it reduces the chances of choking of percolator though the dried drug swells when comes into contact with menstruum. Imbibitions also prevent the washing of fine particles of the crude drug during percolation.

.III. Packing

Moistened drug is packed in percolator & additional menstruum is added to form shallow layer.

The lower end of the percolator should be plucked with cotton and then place the drug powder layer by layer. Two third of the percolator should be covered with the drug on which place the piece of filter paper and wash sand should be placed on the top of the filter paper. This prevents any type of disturbance of the crude drug by the flow of menstruum.

IV. Maceration

V. Percolation

VI. Pressing the marc

5. Hot Continuous Extraction (Soxhlet)

The Soxhlet extraction is an automatic continuous extraction method with high extraction efficiency that requires less time and solvent consumption than maceration or percolation.

Principle: Soxhlet utilizes the principle of reflux and siphoning to continuously extract the herb with fresh solvent.

Soxhlet extraction method is suitable when

• Menstruum has less penetration into the cellular tissue.
• Active constituents are not readily extracted with a solvent. 

Note: The high temperature and long extraction time in the Soxhlet extraction will increase the possibilities of thermal degradation. 

Fixed oil from seed, alkaloid from drug are  extract by soxhlet extraction process using Benzene, chloroform and petroleum, Ether etc.

Apparatus-

The apparatus consist of three important parts:

• A . Flask (round bottom) containing the boiling solvent.
• B . Soxhlet Extractor in which the drug to be extracted is packed.It has a side tube which carries the vapours of the solvent from the flask to the condenser and a syphon tube which syphons over the extract from soxhlet extractor to the flask.
• C . A condenser in which the vapours of the solvent are condensed again into solvent

Procedure:-

• Placed  finely ground crude drug which is packed into a porous bag/thimbe (made of filter paper) in a soxlet extractor..
• The solvent is placed in the flask. & extracting solvent is boiled. It gets converted into vapours. & vapour is condensed in reflux condensor.
• When solvent is boiled on heating the flask,it gets converted into vapours.These vapours enter into the condenser through the side tube and get condensed into hot liquid which falls on the column of the drug.When the extractor gets filled with the solvent ,the level of syphon tube also raise up to its top.The solvent containing active constituents of the drug in the syphon tube syphon over and run into the flask ,thus emptying the body of extractor .This alternation of filling and emptying the body of extractor goes on continuously. The soluble active constituents of the drug remain in the flask while the solvent is repeatedly volatilised. The process of filling and emptying of the extractor is repeated until the drug is exhausted. Normally the process is repeated about 15 times for complete exhaustion of the drug.

Advantage:

• Large amount of drug can be extracted with much smaller quantity of solvent(fewer solvents).
• Tremendous economy in terms of time, energy & ultimately financial inputs.
• Small scale used a batch-process.
• Becomes more economical when converted into continuous extraction.

Demerits:

Thermolabile drugs and constituents are not suitably extracted by this method.

6. Digestion

• Drug is extracted by heating at a particular pressure. This will increase the penetration power of the menstruum so that there is complete extraction of the drug.
• Digestor: Metal vessel

7. Counter-current Extraction

In counter-current extraction (CCE), wet raw material is pulverized using toothed disc disintegrators to produce a fi ne slurry. In this process, the material to be extracted is moved in one direction (generally in the form of a fi ne slurry) within a cylindrical extractor where it comes in contact with extraction solvent. The further the starting material moves, the more concentrated the extract becomes. Complete extraction is thus possible when the quantities of solvent and material and their flow rates are optimized. The process is highly efficient, requiring little time and posing no risk from high temperature. Finally, sufficiently concentrated extract comes out at one end of the extractor while the marc (practically free of visible solvent) falls out from the other end.

7. Ultrasound Extraction (Sonication)

The procedure involves the use of ultrasound with frequencies ranging from 20 kHz to 2000 kHz; this increases the permeability of cell walls and produces cavitation. Although the process is useful in some cases, like extraction of rauwolfi a root.

Its large-scale application is limited due to the higher costs.

One disadvantage of the procedure is the occasional but known deleterious effect of ultrasound energy (more than 20 kHz) on the active constituents of medicinal plants through formation of free radicals and consequently undesirable changes in the drug molecules

8. Supercritical Fluid Extraction

The different component in the supercritical fluid extraction is the first one is a fluid reservoir which contains the fluid-like gas cylinder in the case of CO₂. Another one is the pump; the pump may be a syringe pump or reciprocating pump. Another important is the extractor which may be made of stainless steel and can with a stand to high pressure like 300 to 600 atmosphere. Another important component of the supercritical fluid extractor is the restrictor. It is important for the controlled and systematic release of the pressure inside the extractor vesicle. The restrictor may be a fixed type or variable type. The isolated constituent is collected in a collector where it can be detected with the help of a detector. Sometimes modifiers are also added into the supercritical fluid to increase its versatility like one to ten percent of methanol is added in CO₂.

9. Hydrodistillation

It is method popular methods to separate the essential oil from crude drugs.

In order to isolate essential oils by hydrodistillation, the aromatic plant material is packed in a still and a sufficient quantity of water is added and brought to a boil; alternatively, live steam is injected into the plant charge. Due to the influence of hot water and steam, the essential oil is freed from the oil glands in the plant tissue. The vapor mixture of water and oil is condensed by indirect cooling with water. From the condenser, distillate flows into a separator, where oil separates automatically from the distillate water.

Three are three types of hydrodistillation for isolating essential oils from plant materials:

1. Water distillation
2. Water and steam distillation
3. Direct steam distillation

Read more about Hydrodistillation click here…

A brief summary of various extraction methods

Read More…

Factors Affecting the Choice of Extraction Process

Principles and Mechanisms Involved in Crude Drug Extraction by Maceration, Percolation and Infusion

MCQ Methods of Extraction

1) When a drug is extracted by heating at a particular pressure, the process is called as __________.

(Pharmacist MPPEB 2017, MP)