INTRODUCTION:
Plants have, at one time, supplied virtually all cultures with food, clothing, shelter and medicine. It is estimated that approximately 10-15 % of the roughly 300,000 species of higher plants have a history of use in traditional medicinal. By contrast, only 1 % of plant species have a history of food use.1 Most of the crude drugs are obtained from plants only a small number comes from animal and mineral origins. Drugs obtained from the plants consist of entire plants or their parts. In recent years, investigation of natural products has produced large number of potential drugs and many of them are used for several other purposes in various industries. Drugs from natural origin are being used suitably in confectionaries, food industries and beverages; as spices and condiments and for other purposes as technical products. The medicinal importance of plants in the treatment of human ailments is immense and has been so since the dawn of civilization.2 The use of traditional medicine and medicinal plants in most developing countries, as a normative basis for the maintenance of good health, has been widely observed. Furthermore, an increasing reliance on the use of medicinal plants in the industrialized societies has been traced to the extraction and development of several drugs chemotherapeutics from these plants as well as from traditionally used herbal remedies.3
World Health Organization (W.H.O.) currently encourages, recommends and promotes traditional/herbal remedies in National Health Care Programmes because such drugs are easily available at low cost, are comparatively safe and the people have faith in such remedies. The W.H.O. assembly in a number of resolutions emphasized the need to ensure quality control of medicinal plant products by using modern techniques and applying suitable standards. For pharmaceuticals purposes, the quality of medicinal plant material must be as high as that of other medicinal preparations.4
Plants (in addition to compounds that are necessary for the growth and reproduction such as carbohydrates, proteins and lipids) synthesize a tremendous number of so called secondary metabolites, which do not appear to be strictly necessary for the survival. Often these secondary metabolites are produced as a response to external stimuli, e.g., infection or nutritional or climatic changes, and they may be accumulated in only certain parts of the plants.5 Indigenous cultures have learnt to exploit the properties of secondary metabolites in many ways, e.g., specific plants or parts of them have been used as poisons, analgesics, stimulants, preservatives, colorants, tanning agents for tanning leather, etc.6 As our understanding of chemistry and other natural sciences has increased, the active chemical compounds of these traditionally used plants have been successfully isolated and identified. Nowadays, instead of using, e.g., pastes or crude extract prepared from plant material, the tendency is to use pure compounds, irrespective of the intended use be analgesia or for coloring fabrics.7 the first step in obtaining the secondary metabolites from biogenic materials is to release them from the matrix by means of extraction. Due to the often very complex composition of the material and minute amounts of some of the constituents present, the choice of the extraction method is of great importance. Obviously, an incorrect choice will cause the entire isolation process to fail if some or all of the desired components of the material cannot be released satisfactorily from the matrix.8
1 EXTRACTION:
Extraction involves the separation of medicinally active portion of plants or animal tissues from the inactive or inert components by using selective solvents in standard extraction procedure. The solvent used for the extraction purpose is known as menstruum and residue left after extracting the desired constituent is known as marc.9 The products obtained after the extractions are relatively impure liquids, semisolids or powders and intended only for the oral or external use. These include decoctions, infusions, fluid extracts, tinctures, pilular (semisolid) extract and powdered extract. These types of preparations are known as Galenicals. Extraction pursues to be of interest in order to improve the yield of drugs derived from plants and animal sources.10
1.1 METHODS OF EXTRACTION:
1.1.1 OLDER METHODS:
1.1.1.1 Mechanical pressing:
This method is applied to the extraction of oils from oil seeds. This method needs no extraction medium. This process may be combined with some form of pre treatment like cleaning before extraction but in general the only equipment needed is hydraulic press.11, 12
1.1.1.2 Hydro distillation:
In this method the botanicals are fully submerged in water, producing a soup, the steam of which contains the aromatic plant molecule.13
1.1.1.3 Enfleurage:
This is an extraction with cold fat. It is used mainly for the extraction of fragrance from flowers.12, 14
1.1.2 CONVENTIONAL/TRADITIONAL METHODS:
The Conventional/Traditional methods of extraction are given below:
1.1.2.1 Infusion:
The drug to be extracted is placed at the bottom of the infusion pot, water added and the content stirred occasionally. Alternatively the drug wrapped in a muslin cloth may be suspended just below the level of the water for about fifteen minutes. Examples are infusion of Senna and infusion of Quassia.
Advantage: It is a simple and speedy process.
Limitations:
a) It is used for the soft drug.
b) The active constituents are water soluble.
c) Infusions must be freshly prepared and consumed within 24 hours of its preparation. However concentrated infusion can also be prepared using Alcohol, thus can be stored for an appreciable period of time.
1.1.2.2 Decoction:
This process extracts water soluble and heat stable constituents from crude drugs by boiling in water for 15 minutes, cooling, straining and passing sufficient cold water through the drug to produce the required volume. At present no decoction is official in I.P. or B.P.
Advantages:
a) Hard and woody drugs can be extracted.
b) Less time consuming.
Limitations:
a) Only water soluble and heat stable constituents are extracted.
b) Only freshly prepared Decoction is dispensed and consumed.
1.1.2.3 Digestion:
In this process the crude drug is gently heated. This method is a modified maceration process.
Advantage:
There is increased solvent efficiency of the menstruum.
Limitation:
This method cannot be used for the thermolabile constituents.9
1.1.2.4 Steam Distillation:
This method is primarily used to obtain essential oils from plant materials. In this method, a packed bed of plant materials is continuously flushed with steam and the volatile organic compounds present in the material are taken up by the vapour phase. Compounds carried by the vapour steam are then separated after decreasing the temperature of the vapour by condensation.14
1.1.2.5 Maceration:
In this process the drug is placed in contact with the menstruum in a stoppered container for 2-7 days with frequent stirring. Then filtration is done, after draining most of the liquid, the filter residue is washed with sufficient quantity of solvent and the filtrates are combined. Examples include tincture of lemon, tincture of squill, etc.
Advantages:
a) For more effective extraction multiple maceration can be done.
b) Both organized and unorganized drug can be extracted.
c) Thermolabile constituents of the drug can be extracted with ease.
Limitation:
This extraction process is time consuming.
1.1.2.6 Percolation:
In this process a suitable comminuted drug is made evenly and uniformly damped with the prescribed solvent, and then it is packed in percolator. The drug is allowed to macerate for 24 hours and then after adding more of menstruum percolation is started. Examples include tincture of belladonna, compound tincture of cardamom and strong tincture of ginger.
Advantages:
a) It is comparatively less time consuming.
b) The process can be modified to increase the efficiency.
Limitations:
a) The process requires a particular apparatus, percolator.
b) The process is not suitable for soft drugs which may block the percolator.
1.1.2.6.1 Continuous hot percolation or Soxhlet extraction.
This process is used for those drugs where the imbibition of the menstruum into the cellular tissues is very slow and the solute is not readily soluble into the solvent and the menstruum quantity is less. This process uses Soxhlet extractor, where small volume of hot menstruum is passed over the drug repeatedly in order to exhaust the drug.9
Advantages:
a) Less solvent is needed.
b) The drug is extracted with pure solvent ensuring maximum extraction.
Limitations:
a) In general the drug must be powdered.
b) The method is unsatisfactory with drugs having thermolabile constituents.
c) The method is restricted to pure boiling solvent or to azeotropes.15
1.1.3 IMPROVED/ NON CONVENTIONAL METHODS OF EXTRACTION:
The conventional extraction processes are time consuming e.g., maceration done for 2-7 days; involve bulk amount of solvents and ultimately there might be thermal decomposition of the target molecule like in the case of Soxhlet extraction.16 The demand for new extraction techniques has encouraged the development of alternative extraction techniques such as Ultrasonic Assisted Extraction (UAE), Microwave Assisted Extraction (MAE), Supercritical Fluid Extraction (SCF) and Accelerated Solvent Extraction (ASE). These techniques have enabled automation, shortened extraction time and reduced organic solvent consumption.17
Thus all efforts in developing improved extraction methods need to be focused in order to increase the extraction efficiency, which leads to increased yield and/or shorter extraction time. A brief review of some of the improved methods of extraction of plants constituents is given below.
A study done in Bulgaria reported that Ultrasound extraction of biological active components of Propolis provided high extraction yield, requires shorter time frame and less labour when compared to the maceration extraction or even the microwave assisted extraction.18 Dai et al. (2001) recognized the importance of non conventional extraction method of Azadirachtin-related lemonoids (AZRL) from various parts of the Neem tree. The result showed that Microwave assisted extraction process (MAP) enhances the extraction of AZRL.19 Lu et al. (2006) compared the various extraction methods of Glycyrrhizic acid in Licorice and found that the non conventional methods like Ultrasound method, Microwave assisted method and Supercritical fluid extraction method are more effective than the conventional method due to the short extraction time, low consumption of solution and energy.20
1.1.3.1 Ultrasound assisted extraction (UAE) method:
Ultrasound is like the ordinary sound that we hear except that its pitch is beyond our range. Ultrasound extraction uses the energy of Ultrasound for accelerating the rate of extraction. Ultrasound, i.e., frequencies above 20,000 Hz., may be produced with :
i) Magnetostrictive ultrasonic transmitters
ii) Piezoelectric ultrasonic transmitters.21
Ultrasound causes rapid extraction due to:
i) increase in the permeability of the cell wall,
ii) spontaneous formation of bubbles in the liquid below its boiling point, i.e., cavitation effect, due to dynamic stressing and
iii) increase in the mechanical stressing, i.e., internal friction of the cells.16, 21
There are various studies in which the UAE method has been compared with the conventional methods and it has been found to increase the extraction efficiency.22-24 UAE method was compared with the traditional shake-flask extraction method and it was found that UAE significantly reduces the extraction time.25 The comparison between hydrodistillation and ultrasonic solvent extraction was done for the isolation of volatile compounds from two unifloral honeys of Robinia pseudoacacia L. and Castanea sativa L. It was found that UAE gave the most representative profile of all volatiles.26
1.1.3.2 Microwave assisted extraction (MAE) method:
Microwaves are high frequency electromagnetic waves in the same frequency band as radar waves. The most frequently used microwave frequency for industrial and scientific use is 2.45 GHz. This method uses electron tubes or transistors for the production of low frequency microwave and a magnetron for high frequency.
Magnetron is a metallic tube consists of cathode and anode, and is surrounded by a permanent magnet frame. The pathway for the production of microwave is outlined as below:
Voltage cathode emit electrons
anode resonators polarity reverse microwaves.27
MAE has been used as an alternative in the extraction of organic compounds from plant materials and foods. It is based upon the selective and rapid localized heating of moisture in the sample by microwaves. Due to the localized heating, pressure builds up within the cells of the sample, leading to a fast transfer of the compounds from the cells into the extracting solvent.28 In the case of extraction; due to dipole rotation there is disruption of the weak hydrogen bonds in the solvent. Furthermore, the migration of dissolved ions increases solvent penetration into the matrix and thus facilitates the solvation of analytes.29
In comparison with conventional extraction methods employed to extract alizarin and purpurin in rubiaceae plants, the main advantages of MAE procedure are the low consumption of organic solvent and, particularly, the rapid extraction which was performed in only 20 min.30 MAE of volatile organic acids in tobacco showed significantly better recoveries than those obtained by the conventional methods.31
1.1.3.3 Supercritical fluid extraction (SFE) method:
A supercritical fluid is any substance at a temperature and pressure above its thermodynamic critical point. It has a unique ability to diffuse through solids like a gas and dissolve materials like a liquid. Moreover, it can readily change in density upon minute change in temperature or pressure. These properties make it suitable as a substitute for organic solvents in SFE. Carbon dioxide and water are the most commonly used supercritical fluids.32 The supercritical fluid extract of Curcuma aerogenosa, Citrus hystrix and Azadirachta indica can be applied as a constituent of cosmetic product and medicines because of their antimicrobial activity.33 The yield of Naringin from the peel of Citrus paradise was higher than that attained by the conventional technique of maceration.34
1.1.3.4 Solid phase extraction (SPE) method:
This method involves the usage of a small bed of a specific adsorbent material contained in the barrel of a disposable hypodermic syringe.35 Yang et al. (2007) successfully used this method for the extraction of roots of Salvia miltiorrhiza.36
1.1.3.5 Solid phase micro extraction (SPME) method:
This method uses a thin silica capillary fibre coated on the outer surface with a layer of stationary phase. The silica capillary is immersed into a small volume of stirred water sample for 2-15 minutes during which organic material partitions into the stationary phase.35 Richter and Schellenberg (2007) found SPME method to be most time saving method for the rapid determination of the aroma compound composition in marjoram, caraway, sage and thyme.37
1.1.3.6 Circulatory extraction:
The efficiency of the extraction in a maceration process can be increased by arranging for the solvents to be continuously circulated through the drug.
i) Multiple stage extraction: The extractor is filled with drug; solvent is circulated and is run off to the first receiver. Extractor is refilled with the solvent, procedure is repeated and solvent is collected in the second receiver. Similarly in the third receiver and so on. Now the drug is removed from the extractor and is recharged. The solution one is made to extract the fresh drug and is removed to evaporate. Then solution two extracts the drug and becomes solution one. Similarly solution three becomes solution two. Fresh solvent is added and is run off to receiver three. Drug is removed, recharged and the cycle is repeated.
ii) Extraction Battery: This equipment is based on the countercurrent movement of the solvent and the drug. In this method, the solution contacts the fresh drug before discharge, giving maximum concentration and the drug contacts fresh solvent before dumping, ensuring exhaustion. The apparatus consists of number of vessels with interconnecting pipe works, such that solvent can be added to and product taken from any vessel. The vessels can be arranged into series with any of the vessels as the first of the series.
1.1.3.7 Rotary Film Evaporator:
It consists of a narrow cylindrical vessel. A bladed rotor is placed inside. In this, a film is formed and agitated mechanically due to which there is a good heat transfer. Evaporation occurs as the liquid passes down the wall, vapour is taken off to a condenser and the concentrated liquid is withdrawn at the bottom of the vessel.15
1.1.3.8 Spouted bed extraction:
It consists of a cylinder tapered at both ends and containing the drug, e.g., seeds of Bixa orellana at lower end through which a jet of hot air is forced. The seeds are propelled into the space above from where the seeds fall back to be recirculated and the annatto powder is collected.38
1.1.3.9 Bio-Chelation:
Bio-Chelation includes the use of cold extraction, where the plant parts are extracted without being exposed to excessive amount of heat. In addition the process incorporates the use of an exclusive technique that removes much of the alcohol used during the maceration phase, replacing it with vegetable glycerin instead.39
1.1.3.10 Phytonic extraction:
This method uses a new type of benign non-chlorofluorocarbon (CFC) gaseous solvent, R134a also called as florasols.13
1.1.3.11 Forced flow solid-liquid extraction (FFSLE):
Such type of technique is employed by many methods like medium pressure solid-liquid extraction and rotation planar extraction. Here the extraction solvent is forced through the sample bed either by means of pressure or by centrifugal force thus increasing the efficiency of the extraction process.40
1.1.3.12 Pressurized Liquid extraction or Accelerated solvent extraction:
This method works according to the principle of static extraction with superheated liquids.41 The method uses an organic solvent at high pressure and temperature above the boiling point.42 There is increased efficiency because of higher solubilities of analytes in solvents at higher temperature, higher diffusion rate and disruption of the strong solute-matrix interaction. Warburton et al. (2007) found this method to be efficient in extracting kavain from the powdered roots of Piper methysticum.43
1.1.3.13 Electrical Discharge Extraction Technique:
In this method, cavitation phenomenon is achieved by the electrical discharge. The electrical source charges the capacitor which discharge through the spark gap and electrodes. This high frequency discharge causes cavitaton. This method has been used for the extraction of Rauwolfia and Belladona.21
CONCLUSION:
Requirements for herbal medicines have been established within the last few years, and the trend is to define the dosage form with uniform amount of extract. There is increasing scientific interest in the extraction and isolation of secondary metabolites from plants. The correct extraction process is of great importance because of the fact that only small amount of active constituents are present. Hence, efforts to develop an efficient extraction method have begun worldwide. The use of non- conventional methods has increased rapidly because of its apparent advantages over traditional methods. Such kind of efforts will not only improve the extraction efficiency, be it in terms of yield or amount of solvent used or time taken, but will also give rise to the discovery of new effective compounds from phyto- pharmaceutical sources. More research is needed to understand the underlying principles and to remove the technical problems in order to further improve the extraction efficiency.
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Received on 05.01.2010 Modified on 20.02.2010
Accepted on 12.03.2010 © RJPT All right reserved
Research J. Pharm. and Tech.3 (3): July-Sept. 2010; Page 644-649