Soxhlet extractor : An Important apparatus for isolation of bioactive compounds from medicinal plants

Sanjeet

Several approaches can be employed to extract the plant material. Although water is used as an extractant in many traditional protocols, organic solvents of varying polarities are generally selected in modern methods of extraction to exploit the various solubilities of plant constituents.

A Soxhlet extractor is a piece of laboratory apparatus invented in 1879 by Franz Von Soxhlet. It was originally designed for the extraction of a lipid from a solid material. However, a Soxhlet extractor is not limited to the extraction of lipids. Typically, a Soxhlet extraction is only required where the desired compound has a limited solubility in a solvent, and the impurity is insoluble in that solvent. If the desired compound has a significant solubility in a solvent then a simple filtration can be used to separate the compound from the insoluble substance. Normally a solid material containing some of the desired compound is placed inside a thimble made from thick filter paper, which is loaded into the main chamber of the Soxhlet extractor. The Soxhlet extractor is placed onto a flask containing the extraction solvent. The Soxhlet is then equipped with a condenser. The solvent is heated to reflux.

The solvent vapour travels up a distillation arm and floods into the chamber housing the thimble of solid. The condenser ensures that any solvent vapour cools, and drips back down into the chamber housing the solid material. The chamber containing the solid material slowly fills with warm solvent. Some of the desired compound will then dissolve in the warm solvent. When the Soxhlet chamber is almost full, the chamber is automatically emptied by a siphon side arm, with the solvent running back down to the distillation flask. The thimble ensures that the rapid motion of the solvent does not transport any solid material to the still pot. This cycle may be allowed to repeat many times, over hours or days. During each cycle, a portion of the non-volatile compound dissolves in the solvent. After many cycles the desired compound is concentrated in the distillation flask. The advantage of this system is that instead of many portions of warm solvent being passed through the sample, just one batch of solvent is recycled. After extraction the solvent is removed, typically by means of a rotary evaporator, yielding the extracted compound. The non-soluble portion of the extracted solid remains in the thimble, and is usually discarded.

In this method the sample is dried, ground into small particles and placed in a porous cellulose thimble. The thimble is placed in an extraction chamber (2), which is suspended above a flask containing the solvent (1) and below a condenser (4). The flask is heated and the solvent evaporates and moves up into the condenser where it is converted into a liquid that trickles into the extraction chamber containing the sample. The extraction chamber is designed so that when the solvent surrounding the sample exceeds a certain level it overflows and trickles back down into the boiling flask. At the end of the extraction process, which lasts a few hours, the flask containing the solvent and lipid is removed. In some device a funnel (3) allows to recover the solvent at the end of the extraction after closing a stopcock between the funnel and the extraction chamber. The solvent in the flask (1)  is then evaporated and the mass of the remaining lipid is measured. The percentage of lipid in the initial sample can then be calculated. 

Other methods are:

Maceration

This simple widely used procedure involves leaving the pulverized plant to soak in a suitable solvent in a closed container .simple maceration is performed at room temperature by mixing the ground drug with the solvent (drug solvent ratio : 1:5 or 1:10) and leaving the mixture for several days with occasional shaking or stirring. The extract is then repeated from the plant particles by straining . The process is repeated for once or twice with fresh solvent .Finally the last residue of extract is pressed out of the plant particles using a mechanical press or a centrifuge.kinetic maceration differe from simple one by continous stirring.

1. The method is suitable for both initial and bulk extraction.
2. The main disadvantage of maceration is that the process can be quite time-consuming, taking from a few hours up to several weeks

Ultrasound-assisted solvent extraction

This is a modified maceration method where the extraction is facilitated by the use of ultrasound . The plant powder is placed in a vial. The vial is placed in an ultrasonic bath, and ultrasound is used to induce a mechanical stress on the cells through the production of cavitations in the sample. The cellular breakdown increases the solubilization of metabolites in the solvent and improves extraction

yields.

1. it is mostly used for the initial extraction of a small amount of material.

Percolation

the powdered plant material is soaked initially in a solvent in a percolator . Additional solvent is then poured on top of the plant material and allowed to percolate slowly (dropwise) out of the bottom of the percolator. Additional filtration of the extract is not required because there is a filter at the outlet of the percolator.  Percolation is adequate for both initial and large-scale extraction.

The main disadvantages are :

1. Fine powders and materials such as resins and plants that swell excessively (e.g., those containing mucilages) can clog the percolator. 
2. If the material is not distributed homogenously in the container, the solvent may not reach all areas and the extraction will be incomplete.

Pressurized solvent extraction

The powdered plant material is loaded into an extraction cell, which is placed in an oven. The solvent is then pumped from a reservoir to fill the cell, which is heated and pressurized at programmed levels for a set period of time. The cell is flushed with nitrogen gas, and the extract, which is automatically filtered, is collected in a flask. Fresh solvent is used to rinse the cell and to solubilize the remaining components. A final purge with nitrogen gas is performed to dry the material. offers a more economical and environment-friendly alternative to conventional approaches.

Extraction under reflux and steam distillation :

plant material is immersed in a solvent in a round-bottomed flask, which is connected to a condenser. The solvent is heated until it reaches its boiling point. As the vapor is condensed, the solvent

is recycled to the flask.

• It is commonly applied to the extraction of plant essential oils.
• The main disadvantage is that thermolabile components risk being degraded.

Extration with supercritical fluids

Supercritical fluids (SCFs) are increasingly replacing organic solvents, e.g., n-hexane, dichloromethane, chloroform, and so on, that are conventionally used in industrial extraction operations because of regulatory and environmental pressures on hydrocarbon and ozone-depleting emissions.

Most of the currently available Solvent FreeExtraction systems utilize CO2, which is generally considered as safe for solvent-free extraction processes. The fundamental steps involved in SFE are as follows:

(1) Liquid CO2 is forced into supercritical state by regulating its temperature and pressure.

(2) Supercritical CO2 has solvent power and extracts predominantly lipophilic and volatile compounds.

(3) Gaseous CO2 returns to CO2 tank. After a full round, the new extraction starts with circulating CO2.

Countercurrent extraction

This is a continuous process in which the plant material moves against the solvent. It is suitable procedure for production of large amounts of extracts on an industrial scale. Several types of extractors are available. In the screw extractor the plant material is transported by a screw through a tube and meets the solvent which is pumped in the opposite direction.

Source: Wikipedia