Introduction
Oils are important natural products found in many plants and animals. In pharmacognosy, oils mainly fall into two groups: fixed oils and volatile (essential) oils. Fixed oils are non-volatile, do not evaporate on heating and leave a permanent stain on paper. Volatile oils evaporate when exposed to air, contain strong aroma and are responsible for the fragrance of many medicinal plants. This chapter gives a simplified and clear understanding of oils, especially volatile oils, as commonly required in the Pharm.D curriculum.
Types of Oils
1. Fixed Oils
Fixed oils are triglyceride esters of fatty acids. They are non-volatile and leave oily stains on paper. Examples include castor oil, groundnut oil and almond oil.
2. Volatile Oils (Essential Oils)
Volatile oils are fragrant, volatile substances obtained from different parts of aromatic plants. They contain terpenes, alcohols, aldehydes, ketones, esters and phenols. They are responsible for the characteristic aroma of plants like clove, cinnamon, peppermint and eucalyptus.
Characteristics of Volatile Oils
- Evaporate when exposed to air.
- Have strong aromatic smell.
- Do not leave greasy stains on paper.
- Soluble in alcohol and organic solvents.
- Slightly soluble in water (form aromatic waters).
- Oxidize on exposure to light and air, becoming resinous.
Chemical Constituents of Volatile Oils
- Terpenes: e.g., limonene, pinene.
- Alcohols: menthol, borneol.
- Aldehydes: citronellal, cinnamaldehyde.
- Ketones: carvone, camphor.
- Phenols: thymol, eugenol.
- Esters: linalyl acetate, geranyl acetate.
Uses of Volatile Oils
- Used as flavouring and aromatic agents.
- Have antiseptic, carminative and expectorant actions.
- Used in perfumes and cosmetics.
- Used as preservatives due to antimicrobial activity.
- Act as starting materials for chemical synthesis of drugs.
Extraction Methods for Volatile Oils
Volatile oils are extracted using several methods depending on the plant nature and oil content.
1. Steam Distillation
Most common method. Plant material is exposed to steam, which releases essential oil. The vapour is condensed, and oil separates from water.
Examples: eucalyptus oil, clove oil, peppermint oil.
2. Hydrodistillation
Plant material is boiled in water. Steam carries the volatile oil, and condensation separates it.
3. Expression (Cold Pressing)
Used mainly for citrus fruits. The rind is mechanically pressed to release oil.
Example: orange oil, lemon oil.
4. Solvent Extraction
Used for delicate flowers that cannot undergo distillation. Organic solvents extract the oil, forming a waxy mass called “concrete,” which is later purified to produce “absolute.”
5. Enfleurage (Old Method)
Used for flowers like jasmine. Odor is absorbed by fatty layers and later extracted with alcohol.
Important Volatile Oil–Containing Drugs
1. Clove
Obtained from dried flower buds of Syzygium aromaticum. Contains eugenol.
Uses: analgesic in toothache, antiseptic, carminative.
2. Cinnamon
Bark of Cinnamomum zeylanicum. Contains cinnamaldehyde.
Uses: carminative, flavouring agent, antimicrobial.
3. Peppermint
Leaves of Mentha piperita. Contains menthol and menthone.
Uses: cooling agent, carminative, flavouring.
4. Eucalyptus
Leaves of Eucalyptus globulus. Contains cineole.
Uses: expectorant, antiseptic, inhalation therapy.
5. Fennel
Fruits of Foeniculum vulgare. Contains anethole.
Uses: carminative, digestive aid.
Evaluation of Volatile Oils
Quality testing of volatile oils is essential to detect adulteration and ensure purity.
1. Physical Tests
- Odour: characteristic smell.
- Colour: varies with plant source.
- Solubility: soluble in alcohol.
- Optical rotation: important for oils like lemon and peppermint.
2. Chemical Tests
- Phenolic content: e.g., ferric chloride for eugenol.
- Aldehyde tests: Schiff’s reagent.
- Ester value: indicates ester content.
- Iodine value: measures unsaturation.
Adulteration of Volatile Oils
- Mixing with cheap oils (e.g., adding clove stem oil to clove bud oil).
- Adding synthetic aromatic chemicals.
- Using turpentine or kerosene as extenders.
Storage of Volatile Oils
- Store in airtight, amber-coloured glass containers.
- Keep away from heat, light and air.
- Fill container completely to prevent oxidation.
- Use cool storage for oils rich in aldehydes and phenols.
Detailed Notes:
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