Techniques of Herbal Drugs Extraction

This article throws light upon the top three modern techniques of herbal drugs extraction. The techniques are: 1. Supercritical Fluid Extraction 2. Ultrasound Extraction 3. Microwave Assisted Extraction.

Herbal Drugs Technique # 1. Supercritical Fluid Extraction:

During recent years, there has been a surge in the use of supercritical fluids as an extraction medi­um in the field of environmental science and nat­ural product chemistry. The existing extraction techniques by liquid solvents are of environmen­tal concern and supercritical fluid extraction tech­nology is providing reliable, safe, selective and above all, cost effective technique over that of other conventional extraction techniques.

Today, supercritical fluid extraction using carbon diox­ide as the supercritical fluid is a well-established method and widely used. SFE is an advanced sep­arating technique based on the enhanced solvating power of certain gasses above their critical point.

Above the critical temperature and pres­sure, the substance becomes a supercritical fluid and possesses the properties of gas as well as liq­uid. Thus the confined gas-like mass transfer and the liquid like solvating power of supercritical fluid is the main criteria of their use for extraction.

If a liquid is heated in a sealed system, the liquid expands and the vapor above the liquid becomes denser due to evaporation. If heating is continued or pressure applied it is possible to reach a point where the vapor phase is as dense as the liquid phase and a supercritical phase is achieved.

Supercritical fluids are produced by heating a gas above its critical temperature or compressing a liquid above its critical pressure. So a pure supercritical fluid is any compound at a temperature and pressure above the critical point. Super critical fluid exhibits a pressure tunable dis­solving power.

They possess a liquid like density, and their gas like transport properties, less viscos­ity makes them to diffuse more quickly than liq­uids into the botanical material and can there­fore extract plant products more effectively and faster than conventional organic solvents.

In su­percritical region, solvating strength is a direct function of density, which in turn is dependent on system pressure. By changing the density of the fluid through temperature and pressure, the sol­vating strength of the supercritical fluid can be altered.

At a constant temperature, extraction at lower pressure will favor non-polar analytes, and extraction at higher pressure will favor polar and higher molecular weight analytes. Thus, the ex­traction can be optimized for a particular analyte by simply changing the pressure. The use of po­larity modifiers also enhances extraction efficien­cy and selectivity to extract biologically impor­tant molecules.

Ethanol is generally used in this regard, as it is non-toxic. The amount and identity of polarity modifier and supercritical carbon dioxide density can be var­ied to optimize the extraction process. The mod­ifier can be mixed with carbon dioxide in a sepa­rate pressure chamber or it can be incorporated into the sample matrix in the extraction cell prior to extraction by supercritical fluid.

The modifier exerts its effect mainly in two basic ways:

1. By interacting with the analyte complex to promote rapid desorption into the supercritical fluid

2. By enhancing the solubility properties of supercritical carbon dioxide

Advantages:

(1) SFE is faster Mass transfer limitations letermine the rate at which the extraction can be performed.

(2) Supercritical fluids have solute diffusivities in order of magnitude higher and viscosities in order of magnitude lower than liquid sol­vents, which results in increased extraction efficiency.

(3) SFE is generally completely within 20-60 min­utes whereas liquid extraction by Soxhlet can range 10-14 hours.

(4) Elimination of multiple steps increases accu­racy and reproducibility of the extraction.

(5) The solvent strength of a supercritical fluid can easily be controlled where the solvent strength of a liquid is constant.

(6) Most of the supercritical fluids are gasses at ambient conditions. Concentration steps are greatly simplified by direct coupling of SFE to chromatographic techniques. Extraction by solid-liquid method (Soxhlet) needs to be con­centrated prior to analysis.

(7) Supercritical fluid extraction generates less waste solvents and offers less exposure of labo­ratory personnel to toxic solvents.

(8) Yields quantitative recovery of target analytes without loss or degradation during extraction.

(9) Offers selective extraction. The selective sol­vent characteristics of high-pressure carbon dioxide and the low temperature at which the extraction is carried out make it a more ver­satile extractant for the labile aroma. Selec­tive extraction can be achieved by selecting the fluid polarity and density.

(10) The technique avoids purification by adsorp­tion chromatography and keeps the other ac­tive ingredients intact in the matrix.

Disadvantages:

1. High installation cost

2. Needs specialized operator

Off Line and On Line SFE Techniques:

Off line SFE technique allows direct collection of the extracted analytes either in a solvent or passing the supercritical fluid through a column packed with chromatography adsorbent or can be collected in a cryogenic vessel for subsequent analysis. On line SFE technique, comprises of ex­traction and analysis, which has been used by many researchers as an analytical tool for the study of medicinal plants.

The extracted compo­nents are transferred directly from the SFE cell into the gas chromatography column via an on-column injector. SFE can be directly coupled with HPLC too with appropriate detector depending upon the type of analysis to be carried out.

Off line SFE is simpler to perform and after extraction the extract can be analysed by an appropriate analytical method. On line SFE re­quires an understanding of SFE and chromatograph­ic conditions and the sample extract is not avail­able for different analytical methods.

However, sampling handling between extraction and chro­matographic analysis is eliminated by an on line analysis and also loss of components can be avoid­ed.

Whereas in off line SFE, the analytes are col­lected in a solvent and loss can occur during col­lection or handling during subsequent analyses. In recent times SFE has proved out to be an effec­tive alternative tool for the extraction of various volatile compounds, essential oil, flavonoids and many steroidal compounds.

Herbal Drugs Technique # 2. Ultrasound Extraction:

The range of human hearing is from 16 KHz to 18 KHz. Ultrasound is the name given to sound waves having frequen­cies higher than those to which the human ear can respond.

Since many natural products are ther­mally unstable and many degrade during thermal extraction, so recent times have seen the growth of UAE as an efficient alternative to various con­ventional thermal extraction process.

Recent studies have shown that UAE enhances the extraction efficien­cy by increasing the yield and by shortening the time of extraction of secondary metabolites from various plant tissues. Various studies have dem­onstrated that ultrasound is capable of accelerat­ing the extraction of organic compounds contained within the plant tissues by disrupting the cell walls and enhancing mass transfer of cell contents.

The enhancement of extraction efficiency of organic compounds by ultrasound is attributed to the phe­nomenon called acoustic cavitation. As ultrasound passes through a liquid, the expansion cycles exert negative pressure on the liquid, pulling the molecules away from one an­other. If the ultrasound intensity is sufficient, the expansion cycle can create cavities or micro-bubbles in the liquid.

This occurs when the negative pressure exceeds the local tensile strength of the liquid, which varies with the type and purity of the liquid. Once formed, these bubbles will ab­sorb the energy from the sound waves and will grow during the expansion cycles and recompress during the compression cycle.

The potential en­ergy of the expanded bubble transforms into ki­netic energy in the form a liquid jet that moves through inside the bubble and penetrates the op­posite wall of the bubble.

Liquid jets driving into the surface have been observed at speeds close to 400 km/h. The impact of the jets on the solid surface is very strong. This can result in serious damage to impact zones and can produce newly exposed, highly reactive surfaces.

Scanning electron mi­crographs (SEM) have provided evidence of the mechanical effects of ultrasound, mainly shown by the destruction of cell walls and release of cell contents. In contrast to conventional extrac­tions, plant extracts diffuse across cell walls due to ultrasound, causing cell rupture over a shorter period, which ultimately increases mass transfer phenomenon.

The main advantages of ultrasound-assisted leaching over conventional Soxhlet extraction are as follows:

1. Cavitation increases the polarity of the sys­tem, including extractants, analytes and ma­trix; this increases the extraction efficiency, which can be similar to or greater than that of conventional Soxhlet extraction

2. Ultrasound-assisted leaching allows the ad­dition of a co-extractant to increase further the polarity of the liquid phase

3. It also allows the leaching of thermolabile analytes, which are generally altered under the working conditions of Soxhlet extraction.

4. The operating time is invariably shorter with faster kinetics than when compared to Soxhlet extraction.

There exist several investigation reports re­garding the reduced extraction time for UAE. The efficiency of the extraction of soyisoflavones was improved by ultrasound and complete extraction was achieved within 20 min. Similarly better efficiency than conven­tional approaches was reported for the extraction of chlorogenic acid from Eucommiaulmodies.

Higher yield of rutin and hesperidin was obtained while employing ultrasound tech­nique, which was attributed to the disruption of cell walls by cavitational effects. To achieve the same recovery of anthraquinone as that achieved by UAE, Soxhlet and Maceration took a much longer time. Many such similar reports also exist for other natural products.

The average time of ultrasonic extraction typically ranges from a few to 30 min, although it can be as long as 70 min. The recoveries obtained during this time are com­parable to those obtained after a dozen or so hours of Soxhlet extraction, carried out at the same temperature.

The extraction conditions can be op­timized with respect to time, polarity and amount of solvent, and the mass and kind of sample. The advantage of this technique is the possibility of extraction of many samples at once in an ultra­sonic bath.

The extraction is carried out at room temperature, which makes it suitable for the ex­traction of thermally labile analytes. The need for separation of the extract from the sample fol­lowing the extraction is a disadvantage of this technique.

Herbal Drugs Technique # 3. Microwave Assisted Extraction:

Microwaves are non-ionizing electromagnetic waves positioned between X-ray and infrared rays in the electromagnetic spectrum. Microwaves are made up of two oscillating perpendicular fields i.e. electric and magnetic field and the former is responsible for heating.

Unlike conventional heating which de­pends on conduction-convection phenomenon with eventually much of the heat energy being lost to the environment, in case of MAE, heating occurs in a targeted and selective manner with practically no heat being lost to the environment as the heating occurs in a closed system.

MAE offers a rapid delivery of elec­tromagnetic energy, which is efficiently convert­ed to heat energy by polar solvents. As a result, the delivery of energy takes place to the total volume of solvent and solid plant matrix with sub­sequent heating of the solvent and solid plant ma­trix.

Even though dried plant material is used for extraction in most cases, but still plant cells con­tain minute microscopic traces of moisture that serves as the target for microwave heating.

The moisture when heated up inside the plant cell due to microwave effect, evaporates and generates tremendous pressure on the cell wall due to swell­ing of the plant cell. The pressure pushes the cell wall from inside, stretching and ultimately rup­turing it, which facilitates leaching out of the active constituents from the ruptured cells to the surrounding solvent thus improving the yield of phytoconstituents.

This phenome­non can even be more intensified if the plant matrix is impregnated with solvents with higher heating efficiency under microwave. Higher tem­perature attained by microwave radiation can hydrolyze ether linkages of cellulose, which is the main constituent of plant cell wall, and can convert into soluble fractions within 1 to 2 min.

The higher temperature attained by the cell wall, during MAE, enhances the dehydration of cellu­lose and reduces its mechanical strength and this in turn helps solvent to access easily to compounds inside the cell. In order to study cell damage during the MAE experiments, tobac­co leaf samples were examined by scanning elec­tron microscopy.

Scanning electron micrographs of the untreated sample, heat-reflux extraction sample and microwave treated sample revealed that there were no structural difference between heat-reflux extraction and those of untreated sam­ples, except few slight ruptures on the surface of the sample.

However, the surface of the sample was found greatly destroyed after MAE. This observation suggests that micro­wave treatment affects the structure of the cell due to the sudden temperature rise and internal pressure increase. During the rupture process, a rapid exudation of the chemical substance with­in the cell into the surrounding solvents takes place.

This mechanism of MAE based on expos­ing the analytes to the solvent through cell rup­ture is different from that of heat-reflux extrac­tion that depends on a series of permeation and solubilization processes to bring the analytes out of the matrix.

Evidence has also been presented that during the extraction of essential oils from plant materials, MAE allows the desorption of compounds of interest out of the plant matrix.

This occurs due to the target­ed heating of the free water molecules present in the gland and vascular systems; this leads to lo­calized heating causing dramatic expansion, with subsequent rupture of their walls, allowing essen­tial oil to flow towards the organic solvent. The effect of microwave energy is strongly depend­ent on the dielectric susceptibility of both the solvent and solid plant matrix.

Most of the time, the sample is immersed in a single solvent or mixture of solvents that absorb microwave ener­gy strongly. Temperature increases penetration of the solvent into the matrix and constituents are released into the surrounding hot solvent. How­ever in some cases only selective heating of sam­ple matrix is brought about by immersing the sam­ple in a microwave transparent solvent (hexane, chloroform).

This approach is particularly useful for thermolabile components to prevent their deg­radation. The marked advantage of MAE are re­duced extraction time and solvent usage with improved yield. Microwave power, microwave exposure time, solid-solvent loading ratio and solvent polarity are some of the important fac­tors, which must be optimized to get maximum yield.

Extraction time was found to be drastically reduced from 12 h to few seconds in case of extraction of ginseng saponin using MAE. Using MAE only 30 sec was found to be sufficient for the ex­traction of cocaine when compared to several hours of conventional heating.

It took only 2 minutes for complete ex­haustive extraction of tanshinones from Salvia miltiorrhiza, whereas for the extraction of equiva­lent quantity of tanshinones it took 90, 75 and 45 minutes by Soxhlet, ultrasonic and heat reflux ex­traction respectively.

A 12-min MAE could recover 92.1 % of artemisinin from Ar­temisia annua L, while several-hour Soxhlet ex­traction could only achieve about 60% recovery.

A 4-5 min MAE (ethanol-water) of glycyrrhizic acid from licorice root achieved a higher extraction yield than extrac­tion (ethanol-water) at room temperature for 20- 24 h. MAE can also reduce solvent consumption as it requires only few ml of the solvent than when compared to large volume of solvents required in conventional extraction process.

The basic idea should be that in MAE the plant matrix should be just immersed in the extracting solvent.