Introduction to Volatile Oils

• Lecture 1 covers an introduction to volatile oils, their chemistry, and isolation methods.
• Ahmed Elissawy, Ph.D., Associate Professor of Pharmacognosy at the Faculty of Pharmacy, Ain-Shams University, presents the lecture.
• Contact information: [email protected]

Introduction to Volatile Oils

• Volatile oils are complex liquid mixtures of odoriferous compounds with varying chemical compositions.
• They evaporate easily when exposed to air at ordinary temperatures.
• Used for specific therapeutic activity or their aroma they are also known as essential oils or essence.
• Volatile oils are hydrocarbons and oxygenated compounds.
• They are non-greasy and resinify upon exposure to O₂.
• They are soluble in organic solvents like chloroform, ether, and alcohol, but do not saponify with alkali.
• They leave a transient stain on filter paper.
• Essential oils are isolated from 17,500 aromatic species.
• They are found in families like Myrtaceae, Lauraceae, Rutaceae, Lamiaceae, Astraceae, Apiaceae, Poaceae, and Zingiberaceae.
• Essential oils accumulate in different vegetable organs such as flowers (jasmine), leaves (eucalyptus), barks (cinnamon), woods (sandalwood), rhizomes (ginger), and fruits (anise).
• Essential oils can be located in oil cells, glandular trichomes, secretory cavities, and secretory canals within plants.
• They can act as allelopathic agents, protect against predators, and attract pollinating species.
• Volatile oils are typically colorless, pleasant-smelling liquids that are volatile at room temperature.
• They are steam distillable, have a high refractive index, and are mostly optically active.
• Their density is usually less than water, and they are immiscible with water but can impart fragrance to it, creating aromatic water.
• They are soluble in alcohol and common organic solvents.
• Volatile oils darken in color upon exposure to air and light, undergoing resinification.
• On cooling, volatile oils separate into a liquid fraction called oleoptene (hydrocarbons) and a solid fraction called stearoptene (oxygenated compounds).
• Oils of cinnamon and clove are heavier than water.
• Oils of anise and rose solidify just below room temperature at 15-18°C.
• Oils containing azulenes are colored, for example, chamomile oil is blue.

Chemistry of Volatile Oils

• Essential oils are complex and highly variable mixtures mainly belonging to the "Terpenes" group.
• Terpenes defined as natural products with structures divided into isoprene units.
• The Isoprene unit is a branched-chain, 5-carbon unit containing 2 unsaturated bonds.
• During the formation of terpenes, the isoprene units are linked in a head-to-tail fashion.

Classification of Terpenes

• Hemiterpenes: 5 carbons, 1 isoprene unit, C5H8.
• Monoterpenes: 10 carbons, 2 isoprene units, C10H16.
• Sesquiterpenes: 15 carbons, 3 isoprene units, C15H24
• Diterpenes: 20 carbons, 4 isoprene units, C20H32.
• Triterpenes: 30 carbons, 6 isoprene units, C30H48.
• Terpenes can be classified according to cyclization, the number of carbons, and the presence of oxygen.

Isolation of Volatile Oils

• Methods of isolation include distillation, expression, extraction, and enzymatic hydrolysis.
• Selection of a suitable isolation method depends on the plant material's condition, oil localization, oil amount, and the nature of the oil constituents.
• Distillation methods: hydrodistillation, water & steam distillation, direct steam distillation, and steam distillation by microwaves.

Distillation Methods Applications

• It is the application for thermostable oils and oils present in large amounts like turpentine, peppermint, thyme, cardamon, anise, and eucalyptus.
• In hydrodistillation, the plant material is boiled in water using a "Clevenger apparatus" to release volatile oils from plant tissue.
• The vapor mixture of water and oil is condensed, and the distillate is separated.
• Steam Distillation: steam travels through biomass, transferring essential oils and water-soluble compounds into the vapor stream which then condenses, with the essential oil layer separating in the receiver.
• Direct Steam Distillation: fresh plant material is directly cut and put in distilling chambers.
• Steam Distillation by Microwaves: the plant is selectively heated by microwave radiation under vacuum, distilling the essential oil with water vapor.

Advantages of Steam Distillation by Microwaves

• Fast.
• Consumes little energy.
• Yields higher quality than traditional methods.
• Distillation should be done just after comminution to prevent loss or deterioration.
• Coarse comminution increases hydrodiffusion for better yield.
• Sensitive constituents may be affected by high temperatures and water, creating artifacts.
• Water-soluble constituents may remain in the distillation chamber.
• Aromatic water (hydrosols) is a distilled aqueous layer saturated with oil.
• Cohobation is the return of aromatic water to the distillation chamber to recover dissolved oil.

Expression Methods

• Include the sponge method and scarification method (Ecuelle a'piquer).
• Mechanical procedures at room temperature involve puncturing and squeezing plant material to collect oil.
• Used for heat-sensitive oils and oils in outer peels of fruits like citrus.
• The sponge method squeezes the rind of fruits like oranges; the fruit pulp is removed, the peel is soaked, inverted, and a sponge is used to absorb expressed oils.
• The sponge is then squeezed to release the volatile oil.
• In the Scarification method, the instrument, is called "Ecuelle a'piquer", and it consists of a funnel made of copper containing metal pins to penetrate the epidermis of the peels. The stem serves as a receiver for the oils.

Extraction Methods

• Involves the use of volatile solvents (hexane, ether), non-volatile solvents (lard, tallow), or supercritical gases (carbon dioxide) to extract volatile oil from plant material.
• It is used for delicate flower oils like jasmine but it should be present in very small amounts and made out of thermolabile constituents.
• Floral Concrete is concentrated and purified volatile oil products containing odoriferous principles, waxes, albuminous materials, and colored pigments.
• Absolutes are obtained by dissolving floral concretes in absolute alcohols and concentrating the alcohol solution.
• Enfleurage method in non-volatile extraction involves placing petals between layers of purified animal fat, saturating it with flower oil.
• The pneumatic method involves passing hot air through flowers, with the air carrying volatile oils then passed through melted fat to absorb the oil.
• Supercritical fluid (SCF) extraction uses a fluid with liquid density and gas viscosity above the critical point (32°C and 7.4 MPa).
• Carbon dioxide is the gas of choice for its chemical inertness, non-flammability, non-toxicity, and ability to extract pure essential oils without residues.
• Supercritical gas extraction provides high yields of volatile oils without decomposition and adjustable solvation power with no hydrolysis or rearrangement.

Enzymatic Hydrolysis of Glycosides

• Useful for specific oils present in glycosidic combination.
• Specific enzymes hydrolyze glycosides to release volatile components.
• Distillation follows hydrolysis to obtain volatile components.
• Examples include Vanillin, Benzaldehyde, and Methyl Salicylate (Winter green oil).