Extraction Methods and Principles

Overview and Definitions of Extraction

• Definition of Extraction: Extraction is a method used to remove active constituents from insoluble residues (which may be solid or liquid) through treatment with a liquid solvent. It involves the separation of medicinally active portions of plant or animal tissues from inactive or inert components using selective solvents.
• Mechanism: In this process, desired components are dissolved by selective solvents, while the unwanted matter is subsequently removed.
• Key Terminology:   - Extract: Preparations derived from crude drugs containing all constituents soluble in the specific solvent used.   - Menstruum: The selective solvent used during the extraction process to dissolve the wanted components.   - Marc: The undissolved, inert solid residue remaining after the extraction process is complete.   - Micelle: The extract solution (solvent plus dissolved active constituents) before the solvent is evaporated.
• Preparation Mediums: Extracts are typically prepared using ethanol or other suitable solvents.
• Classification of Extract Types:   - Dry Extracts: Used for the production of tablets and capsules.   - Soft Extracts: Utilized in semi-solid preparations like ointments and suppositories.   - Liquid Extracts: Prepared as tinctures.

Ideal Properties of Extraction Solvents

To ensure an efficient and safe extraction, the solvent (menstruum) should ideally possess the following six characteristics:

1. High Selectivity: It must be highly selective for the specific compound intended for extraction.
2. Chemical Inertness: It should not react with the extracted compounds or any other compounds present in the plant material.
3. Economic Feasibility: It should have a low price.
4. Safety: It must be harmless to humans and the environment.
5. Volatility: It should be completely volatile to allow for easy removal via evaporation.
6. Low Viscosity: It should have minimum viscosity to facilitate penetration and flow.

Factors and Types of Extraction Processes

• Factors Affecting Extraction:   - Nature of the drug.   - Solvent selection.   - Temperature.   - $pKa$.   - Particle size of the material.
• Extraction Classifications by Phase:   - Liquid-Liquid Extraction (Solvent Extraction):     - Components of a liquid mixture are separated by contact with a suitable insoluble solvent that preferentially dissolves one or more components.     - Separation relies on the unequal distribution of components between two immiscible liquids.     - Phase 1: Known as the feed solution.     - Phase 2: The solvent used for extraction.     - The process involves contacting the feed and solvent (which form a homogeneous mixture) to separate one liquid preferentially. This is widely used for plant actives and aromatic compounds.   - Solid-Liquid Extraction (Leaching):     - This involves removing constituents from a mixture of solids by bringing the solid material into contact with a liquid solvent that dissolves the constituents.     - Purposes: To produce a concentrated solution of an active principle or to free insoluble constituents from soluble contaminants.   - Solid Phase Extraction.

Methodologies of Extraction

The following methods are utilized in the field of extraction:

• Maceration
• Percolation
• Infusion
• Digestion
• Decoction
• Soxhlet Extraction
• Sonication Extraction
• Microwave Assisted Extraction
• Accelerated Solvent Extraction
• Supercritical Fluid Extraction (SFE)

Maceration: Process and Principles

• Definition: Maceration involves soaking coarsely powdered plant material in a container with a solvent.
• Experimental Case Study (Standard Procedure):   - Initial Step: Add $100\,g$ of dry powder to $500\,mL$ of ethanol.   - First Extraction: Keep for $4\,hours$ with continuous shaking on a magnetic stirrer. Filter the mixture to obtain Filtrate-1 and a residue.   - Second Extraction: Add another $500\,mL$ of ethanol to the residue. Keep for $4\,hours$ with continuous shaking. Filter to obtain Filtrate-2 and a new residue.   - Third Extraction: Add another $500\,mL$ of ethanol to the residue. Keep overnight with continuous shaking. Filter to obtain Filtrate-3.   - Consolidation: Pool all filtrates (Filtrate-1 + Filtrate-2 + Filtrate-3).   - Finalization: Evaporate the combined filtrates to obtain the extract. In this case, the yield is specified as $16.47\,g$.
• Standard Procedure for Plant Material (Leaves, Stem Bark, Root Bark):   1. Place material in a container and cover completely with menstruum.   2. Close the container and sit for at least $3\,days$.   3. Stir or shake periodically.   4. Separate the micelle from the marc via filtration.   5. Evaporate the menstruum using a water bath or oven.
• Material Preparation Details:   - Plants must be washed to remove soil, rocks, and weeds.   - Material should be cut into small pieces to increase solvent contact.   - Finely powdered material risks: Loss of volatile active ingredients (essential oils) and difficulty in filtration post-maceration.   - Course material risks: If pieces are too large, the solvent cannot penetrate inner cells effectively.
• Merits and Demerits:   - Merits: Simple, convenient, saves energy, and is suitable for thermolabile (heat-sensitive) compounds.   - Demerits: Very slow process, requires a large volume of solvent, and may not comprehensively extract the drug.

Soxhlet Extraction

• Apparatus Components:   1. Round bottom flask (distillation flask).   2. Soxhlet extractor (main chamber).   3. Condenser (with water inlet and outlet).   4. Thimble (made of cellulose, holding the source material).   5. Side tube and Siphon tube.
• Assembly:   - Place source material inside the thimble and load it into the Soxhlet extractor.   - Solvent is placed in the distillation flask on a heating mantle.   - The extractor is placed on the flask, and the reflux condenser is placed on the extractor.
• Steps of Operation:   1. Start the hot plate.   2. Solvent starts boiling.   3. Solvent vapors rise.   4. Vapors travel through the side tube toward the condenser.   5. Condenser cools the solvent vapors.   6. Condensed solvent droplets fall into the thimble.   7. The thimble containing the sample becomes saturated with solvent.   8. The solvent-extract mixture travels through the Siphon tube once it reaches the top of the bend.   9. The mixture falls back into the round bottom flask (RBF), and the cycle repeats.
• Merits:   - Extracts large amounts of drug with much smaller quantities of solvent.   - Significant economy in time, energy, and financial input.   - Continuous extraction makes it economical for large-scale procedures.
• Demerits:   - Limited by the physical nature of the drug.   - Only non-polar solvents can be used.   - Dependent on the chemical constituents of the drug.

Supercritical Fluid Extraction (SFE)

• Definition of Supercritical Fluid (SCF):   - A material used in a state above its critical temperature and critical pressure where gas and liquid phases coexist and are indistinguishable.   - At the critical point, the density of gas and liquid phases is identical.   - Properties: Densities are liquid-like; transport properties are gas-like. This provides excellent penetrative and extractive abilities.
• Standard SCF Parameters:   - Carbon dioxide ($CO_2$): Critical Temperature = $304.1\,K$, Critical Pressure = $73.8\,bar$. (Most commonly used SCF).   - Ethane: Critical Temp = $305.4\,K$, Critical Pressure = $48.8\,bar$.   - Ethylene: Critical Temp = $282.4\,K$, Critical Pressure = $50.4\,bar$.   - Propane: Critical Temp = $369.8\,K$, Critical Pressure = $42.5\,bar$.   - Propylene: Critical Temp = $364.9\,K$, Critical Pressure = $46.0\,bar$.   - Trifluoromethane (Fluoroform): Critical Temp = $299.3\,K$, Critical Pressure = $48.6\,bar$.   - Chlorotrifluoromethane: Critical Temp = $302.0\,K$, Critical Pressure = $38.7\,bar$.
• 7-Step Operation (Example using Ground Hemp):   1. The extractor chamber is filled with ground hemp.   2. A pump forces pressurized $CO_2$ (at optimum temperature in a liquid supercritical state, $SCCO_2$) into the extractor chamber.   3. The $SCCO_2$ acts as a solvent, dissolving desired compounds in the plant material.   4. The $SCCO_2$ carries botanical oil particles past a pressure release valve into a separator.   5. In the separator, lower pressure causes the $CO_2$ to separate from the botanical oil.   6. The $CO_2$ is recycled back into the $CO_2$ tank for a closed-loop system.   7. Captured botanical oils, waxes, and resins descend to a collection vessel.

Ultrasound-Assisted Extraction (UAE)

• Definition: A fast, effective method using ultrasonic waves (frequency $> 20,000\,Hz$) to extract components.
• Mechanism:   - Ultrasound generates cavitation bubbles in the solvent.   - Collapse of these bubbles disrupts plant cell walls.   - This increases solvent penetration and accelerates the release of compounds.
• Apparatus: Simple and open assembly involving a conical flask or beaker and an ultrasound probe (e.g., set at $20.3\,KHz$ or $20.4\,KHz$).
• Advantages:   - Does not generate significant heat; ideal for heat-labile (thermolabile) substances.   - Inexpensive, simple, and efficient.   - Increases extraction yield and features faster kinetics.
• Disadvantages:   - May cause undesirable changes in drug molecules through the formation of free radicals.