11. Extraction in Pharmacy: Principles, Methods, and Factors Affecting Efficiency

EXTRACTION

Extraction is the process of separating medicinally active constituents from plant and animal tissues by using selective solvents and standard procedures. The extracted products obtained from plant tissues may be in liquid, semisolid, or dry powdered form after removal of the solvent. These preparations are complex mixtures of metabolites intended for oral or external use. Common examples include decoctions, infusions, fluid extracts, tinctures, semisolid extracts, and powdered extracts. Such preparations are known as galenicals, and the insoluble residue left behind after extraction is called marc.

In pharmaceutical practice, extraction is used to obtain the therapeutically active constituents of crude drugs while removing unwanted or inactive components. The process depends on the choice of solvent, the nature of the drug, the degree of contact between the solvent and drug particles, and the method employed. Because many herbal and natural products contain a mixture of useful and inactive materials, extraction is an important step in preparing standardised medicinal products.

STEPS OF DRUG EXTRACTION PROCESS

• The solvent penetrates the drug.
• The drug constituents dissolve in the solvent.
• The solution within the cells diffuses out.
• The dissolved portion separates from the exhausted drug.

These steps occur in sequence, although they may overlap during the actual extraction process. The efficiency of each step depends on particle size, solvent choice, temperature, and the nature of the material being extracted.

FACTORS AFFECTING EXTRACTION EFFICIENCY

• Nature and properties of the drug and its extractable constituents.
• Particle size of the powdered drug.
• Nature of the solvent.
• The state of contact between the solvent and drug particles.

A finer particle size generally increases surface area and improves extraction, while the wrong solvent may fail to dissolve the desired constituents. Good mixing and proper contact between the drug and menstruum also improve the overall yield.

PHYSICAL FACTORS IN EXTRACTION

Drug extraction is also influenced by physical factors such as gravitation, diffusion, osmosis, adhesion, capillarity, convection, solubility, and surface tension. These factors control how the solvent enters the material, how the dissolved substances move out, and how efficiently the active principles are transferred into the menstruum. Temperature, viscosity, and density differences also contribute to extraction behaviour.

ADVANTAGES OF EXTRACTION AT ELEVATED TEMPERATURES

• Decreased viscosity reduces the boundary layers.
• Convection currents act similar to agitation during extraction.
• Diffusion coefficients increase with temperature and decrease with viscosity, so diffusion becomes faster.
• Solubility of constituents in the menstruum increases with temperature.

For heat-stable drugs, elevated temperature can improve both rate and yield of extraction. However, thermolabile constituents may be damaged by excessive heat, so temperature must be selected carefully.

PROPERTIES OF AN IDEAL MENSTRUUM

• Chemically and physically inert.
• Non-toxic.
• Inexpensive.
• Selective, so it dissolves the desired active constituents with minimum inert material.

Water, ethanol, and their mixtures are commonly used because they satisfy many of these requirements. The choice of menstruum depends on the solubility of the active principles and the intended final product.

CLASSIFICATION OF EXTRACTION METHODS

• Infusion (fresh infusion, concentrated infusion).
• Decoction.
• Maceration (simple, modified, multiple).
• Percolation (simple, concentrated preparations, modified).
• Continuous hot extraction (Soxhlet extraction).
• Digestion.
• Microwave-assisted extraction.
• Ultrasound-assisted extraction.

FACTORS AFFECTING CHOICE OF EXTRACTION PROCESS

• Nature of the Drug: Hard and tough drugs such as nux vomica are usually extracted by percolation, while soft, parenchymatous, unorganised, or difficult-to-powder drugs are better suited for maceration.
• Therapeutic Value of the Drug: For less therapeutically important drugs, maceration may be used, while important drugs are often extracted by percolation for better efficiency.
• Stability of the Drug: Thermolabile constituents should not be exposed to continuous hot extraction.
• Cost of the Drug: Costly drugs are generally extracted by percolation because it is more economical.
• Solvent: If a special solvent system is needed, reserved percolation may be preferred over continuous extraction.
• Concentration of the Product: Dilute products such as tinctures may be prepared by maceration or percolation, while heat-stable materials may be processed by continuous extraction.

MACERATION AND ITS MODIFICATIONS

Simple Maceration

Simple maceration is used for preparing tinctures from organised drugs such as roots, stems, and leaves. The solid drug and solvent are placed in a stoppered container and left undisturbed for 3 to 7 days with frequent agitation. After extraction, the mixture is strained, the marc is pressed, and the liquids are combined, then filtered or decanted. The usual drug-to-menstruum ratio is 1:10. Examples include tincture of myrrh and compound tincture of benzoin.

Modified Maceration

Modified maceration is used for unorganised drugs such as oleo-resins and gum resins. In this method, the marc may form a compact mass and retain little or no macerate, so pressing is not necessary. The final product is adjusted to a definite volume. Because such drugs do not have a cellular structure, the soluble components come into direct contact with the menstruum, which speeds up extraction.

Multiple Maceration

Multiple maceration is used for preparing concentrated extracts and includes double maceration and triple maceration. The process improves efficiency by using fresh menstruum in successive stages to obtain more complete extraction of active constituents.

DIGESTION

Digestion is a modified maceration process that uses a higher temperature which does not adversely affect the active ingredients. The increased temperature enhances the solvent action of the menstruum, and constant mechanical agitation speeds up attainment of equilibrium. If the menstruum volatilises easily at the working temperature, a reflux condenser is attached so that vapours are condensed and returned to the vessel.

PERCOLATION (SOXHLET EXTRACTION)

Procedure

• Size Reduction of the Drug: The drug is reduced to a suitable size based on experimental requirements.
• Moistening or Imbibition: The drug is moistened with menstruum for about 4 hours.
• Packing: The moistened drug is packed layer-wise into the percolator and occupies about two-thirds of the column.
• Maceration: Menstruum is added, leaving a layer of about 2 cm at the top, and the percolator is kept closed for 24 hours.
• Percolation: After maceration, the lower stopcock is opened and the percolation rate is kept moderate.
• Pressing the Marc: The residue is removed and pressed using a tincture press or hydraulic press.
• Adjustment of Volume: The expressed liquid is combined with the percolate and made up to the desired volume.
• Clarification: Clarification is done by subsidence or filtration.

Percolation is a widely used extraction method because it generally gives better control over contact between the drug and solvent. It is suitable for many organised drugs and is preferred when a more efficient and economical process is needed.

CONTINUOUS HOT EXTRACTION (SOXHLET EXTRACTION)

Merits

• It has a simple and clear design.
• It is a continuous process.
• It can be easily monitored visually.
• It allows reuse after stripping and distillation.

Limitations

• Physical Character of the Drug: The method is not suitable for drugs that block the Soxhlet apparatus, such as opium, gum, resin, and orange peel.
• Solvent: It can be used only with pure solvents or constant-boiling mixtures such as alcohol-water.
• Chemical Constituents of the Drug: It is not suitable for thermolabile constituents such as enzymes, alkaloids, anthraquinone derivatives, and esters.

INFUSION

Fresh Infusion

Fresh infusion is an aqueous solution of active constituents of a vegetable drug prepared by infusion. It is intended for immediate use and should be consumed within 12 hours of preparation because it may spoil due to microbial growth. Fresh infusion of Quassia is a common example.

Concentrated Infusion

Concentrated infusion is about eight times stronger than fresh infusion and is prepared by double or triple maceration. Alcohol in the range of 20 to 25 percent is used as the menstruum, which also acts as a preservative and allows longer storage. Examples include concentrated infusion of Quassia and concentrated compound infusion of chirata.

DECOCTION

Decoction is a liquid preparation containing water-soluble components of plant drugs, usually of hard and woody nature. It is mainly used for drugs that contain non-volatile, water-soluble constituents. The drug is boiled with water for a specific time, usually about 10 minutes, and then strained. The resulting liquid is passed through the contents of the strainer and adjusted to the required volume.

Decoctions are generally prepared freshly because prolonged storage may lead to deterioration. This method is simple and effective for extracting constituents from tough plant materials that are not easily exhausted by cold infusion.