11. EXTRACTION

EXTRACTION

The process of separating medicinally active constituents of plant and animal tissues by using selective solvents and standard procedures is termed extraction. The extracted products of plant tissues obtained in liquid or semisolid state (after removing the solvent) or in dry powdered form are complex mixtures of metabolites; these products are meant for oral or external use. The extracted preparations include decoctions, infusions, fluid extracts, tinctures, pilular (semisolid) extracts, or powdered extracts; these preparations are named as galenicals after Galen (a Greek physician of 2nd century). The standardised extraction procedures involve treatment with a selective solvent (menstruum) to yield the therapeutically active constituents of crude drugs, removing the inactive ones. The undissolved residue left behind is termed marc.

STEPS OF DRUG EXTRACTION PROCESS

1. The solvent penetrates the drug.
2. The drug constituents dissolve in the solvent.
3. The solution within the cells diffuses out.
4. The dissolved portion separates from the exhausted drug.

FACTORS AFFECTING EXTRACTION EFFICIENCY

1. Nature and properties of drug and its extractable constituents.
2. Particle size of the powdered drug.
3. Solvent nature.
4. The state of contact between the solvent and drug particles.

PHYSICAL FACTORS IN EXTRACTION

Drug extraction process is also influenced by various physical factors, like gravitation, diffusion, osmosis, adhesion, capillarity, convection, solubility, and surface tension.

ADVANTAGES OF EXTRACTION AT ELEVATED TEMPERATURES

1. Decreased viscosity reduces the boundary layers.
2. Convection currents act similar to agitation during extraction.
3. Diffusion coefficients are directly proportional to absolute temperature and inversely proportional to viscosity, thus diffusion rate is influenced by increased temperature.
4. Solubility of the constituents in the menstruum increases with increased temperature.

PROPERTIES OF AN IDEAL MENSTRUUM

1. Chemically and physically inert.
2. Non-toxic.
3. Inexpensive.
4. Selective, i.e., it should dissolve the desired active constituents with a minimum of the inert material.

Water, ethanol, and their mixtures are most commonly used as they fulfil the above mentioned considerations.

CLASSIFICATION OF EXTRACTION METHODS

• Infusion (Fresh infusion, Concentrated infusion)
• Decoction
• Maceration (Simple maceration, Modified maceration, Multiple maceration)
• Percolation (Simple percolation, Percolation for concentrated preparations, Modified percolation)
• Continuous hot extraction (Soxhlet extraction)
• Digestion
• Microwave-assisted extraction
• Ultrasound-assisted extraction

FACTORS AFFECTING CHOICE OF EXTRACTION PROCESS

1. Nature of the Drug: For hard and tough drugs (e.g., nux vomica), percolation method is used; while for soft and parenchymatous drugs (e.g., gelatin), drugs which cannot be easily powdered (e.g., squill), and unorganised drugs (e.g., benzoin), maceration method is preferred.
2. Therapeutic Value of the Drug: For less therapeutically important drugs (e.g., flavours like lemon, bitters like gentian), maceration method is used; while for drugs with significant therapeutic value (e.g., belladonna), percolation method is preferred.
3. Stability of the Drug: For drugs whose constituents are thermolabile, continuous extraction method should be avoided.
4. Cost of the Drug: For costly drugs (e.g., ginger), percolation method is used (as it is economical); while for cheaper drugs, maceration method is preferred.
5. Solvent: For drugs whose desired constituents require a solvent other than a simple boiling solvent or an azeotrope, reserved percolation method (and not continuous extraction) is used.
6. Concentration of the Product: Dilute products (like tinctures) can be prepared by maceration or percolation method. In case the solvent is suitable and the desired constituents are heat-stable, continuous extraction method is used.

MACERATION AND ITS MODIFICATIONS

Simple Maceration

This method is used for preparing tinctures from organised drugs, e.g., roots, stems, leaves, etc. Solid ingredients and solvent are taken in a stoppered container, and left undisturbed for at least 3-7 days with frequent agitation. When the soluble matter dissolves in the solvent, the resultant mixture is passed through sieves or nets. The marc retained in the sieves is pressed, the liquids are combined, and filtered or decanted after standing. The ratio of drugs to menstruum should be 1:10. Examples: Tincture of myrrh, compound tincture of benzoin.

Modified Maceration

This method is used for preparing tinctures from unorganised drugs, e.g., oleo-resins and gum resins. During the extraction of unorganised drugs, the marc forms a compact mass and retains no macerate; thus pressing the marc is not required. The final product is adjusted to a definite volume. Since the unorganised drugs lack cellular structure, the soluble components are in direct contact with the menstruum, thus quickening the extraction process.

Multiple Maceration

This method is used for preparing concentrated extracts. This method includes double maceration and triple maceration.

DIGESTION

Digestion is a modified maceration process. It involves extraction at such a high temperature which does not put adverse effects on the active ingredients. Higher temperature enhances the solvent action of menstruum and constant mechanical agitation of the system speed up the attainment of equilibrium. If at the used temperature the menstruum gets volatilised easily, a reflux condenser should be attached to the vessel in which the digestion process is being performed; this facilitates the condensation of menstruum, so that it can be recovered and returned back to the container.

PERCOLATION (SOXHLET EXTRACTION)

Procedure

1. Size Reduction of the Drug to a Suitable Degree: The degree of size reduction of the drug to be extracted should be established by conducting experiments.
2. Moistening or Imbibition: The drug should be moistened with the menstruum for around 4 hours.
3. Packing: After completing the moistening of drug, it should be packed layer-wise in the percolator. The drug occupies two-thirds of the column.
4. Maceration: The menstruum is added in the percolator leaving at least 2 cm layer at the top. The percolator is closed for 24 hours.
5. Percolation: With the completion of maceration step, percolation is initiated by opening the lower stopcock. The percolation rate should be kept moderate.
6. Pressing the Marc: The marc left behind in the percolator is removed and pressed using a tincture press or a hydraulic press.
7. Adjustment of Volume: The liquid obtained after marc expression is mixed with the percolate, and the mixture is added with more menstruum to make up the desired volume.
8. Clarification: It is carried out by subsidence or filtration to obtain a clear finished preparation.

CONTINUOUS HOT EXTRACTION (SOXHLET EXTRACTION)

Merits

• It is a simple and clear design.
• It is a continuous process of production.
• It can be easily monitored by viewing.
• It allows reusability after stripping and distillation.

Limitations

1. Physical Character of the Drug: The method is not used if the drug to be extracted is of such physical nature that it would block the Soxhlet apparatus, e.g., opium, gum, resin, orange peel.
2. Solvent: The method can be used only with pure solvents or constant boiling mixtures (like alcohol-water).
3. Chemical Constituents of the Drug: The method is not used for thermolabile active constituents, e.g., enzymes, alkaloids, anthraquinone derivatives, esters.

INFUSION

Fresh Infusion

This type of infusion is an aqueous solution of active constituents of a vegetable drug prepared by the process of infusion. For example, fresh infusion of Quassia. It is stated in pharmacopoeia that fresh infusion should be used within 12 hours after its preparation as it gets spoiled due to growth of fungi or bacteria.

Concentrated Infusion

This type of infusion is 8 times stronger than fresh infusion and is prepared by double or triple maceration process. Unlike fresh infusion, alcohol is used as menstruum in the concentration of 20-25%. They can be stored for a longer time period as alcohol acts as a preservative. For example, concentrated infusion of Quassia and concentrated compound infusion of chirata.

DECOCTION

Decoction is a solution containing water soluble components of a plant drug. This process is mainly used for the extraction of vegetable drugs (of hard and woody nature) containing water-soluble and non-volatile components. Ideally, it should be freshly prepared. Procedure: The drug is boiled with water for a specific time period (usually 10 minutes). Then, the liquid is strained by passing water through the contents of the strainer to make up the required volume.