Essential Oils: From Definition to Applications

Essential Oils: From Definition to Applications

Essential oils are complex mixtures of volatile organic compounds, typically with a fragrant aroma. They are commonly found in plants, with a few exceptions found in animals. Essential oils typically share the following general characteristics:

• Volatility at room temperature: Due to their primarily volatile compound composition.

• Fragrant aroma: The components of essential oils create the unique scent associated with each type.

• Lower density than water: This causes essential oils to usually float on water.

• Insoluble in water: But soluble in organic solvents like alcohol and ether.

• Steam distillable: A common method for extracting essential oils from raw materials.

Components of Essential Oils:

Essential oils consist of dozens to hundreds of volatile compounds, called constituents. These constituents typically have low molecular weights (<300 Da) and are mostly in liquid form. Some constituents have high concentrations (a few percent to a few dozen percent), while others may only be present in trace amounts.

Classification of Constituents:

• Monoterpenes: Includes non-oxygenated monoterpenes (possessing a certain degree of fragrance) and oxygenated monoterpenes (important aromatic compounds).

• Terpenoids: Formed from isoprene units (hemiterpen = C5H8) linked together following a head-to-tail rule.

• Nitrogen and Sulfur derivatives: Examples include Allicin, isothiocyanates, and Methyl anthranilate.

• Non-essential oil volatile components: Examples include Coumarin.

Physical Properties:

• State: Essential oils are typically viscous liquids, with viscosity increasing upon oxidation.

• Odor: Essential oils often possess a characteristic “pleasant” aroma, while some have a pungent, unpleasant smell.

• Taste: Usually pungent, hot, with some having a spicy or sweet taste.

• Volatility: Essential oils are volatile at room temperature, contributing to their unique aroma properties.

• Freezing point: The freezing point depends on the composition and proportion of the components.

• Boiling point: No fixed boiling point, but rather a range of boiling points.

• Solubility: Essential oils are easily soluble in non-polar solvents, soluble in polar solvents but sparingly soluble in water.

• Density: Most essential oils have a density < 1,000, but some have a density > 1,000.

• Optical rotation: Many essential oil constituents are optically active.

• Refractive index: n D 20 : 1,450 – 1,560.

• UV spectrum: Constituents with conjugated double bonds, aromatic compounds: have UV absorption.

Chemical Reactions:

• Oxidation-polymerization: Essential oils with conjugated double bonds are susceptible to oxidation by atmospheric oxygen, forming polymerization products.

• Color reactions: Tests can be conducted using sulfuric acid, vanillinhydrochloric or sulfuric acid, and anisaldehyde-sulfuric acid reagents.

Analytical Methods:

• Thin-layer chromatography (TLC): Commonly used for qualitative analysis of essential oils. Note: samples are prone to evaporation.

• Gas chromatography (GC): Often used in modern laboratories.

• Chemical indices:

• Acid value: The number of mg KOH needed to neutralize the free acid content per 1g of sample.

• Ester value: The number of mg KOH needed to saponify the free acid content per 1g of sample.

• Acetyl value: The number of mg KOH needed to neutralize the acetic acid released after saponification per 1g of acetylated sample.

Methods for Essential Oil Extraction:

• Steam distillation: The most common method, suitable for most essential oils.

• Enfleurage: Utilizing fats, paraffin, or adsorbents to extract essential oils.

• Expression: Using force to extract essential oils, suitable for plants with oil glands on the surface.

• Solvent extraction: Employing solvents to exhaustively extract essential oils.

Preservation of Essential Oils:

• Store in airtight containers (glass, metal, plastic) to protect from direct sunlight and high temperatures.

Applications of Essential Oils:

• Pharmaceuticals: Essential oils are used to treat various ailments such as colds, flu, headaches, digestive issues, infections, etc.

• Fragrance: Essential oils are key ingredients in the production of perfumes, cosmetics, soaps, etc.

• Food: Essential oils are used as flavorings and aroma compounds for food and beverages.

Common Essential Oils:

• Orange: Citrus sinensis.

• Lime: Citrus aurantifolia.

• Lemon: Citrus limon.

• Grapefruit: Citrus maxima.

• Lemongrass: Cymbopogon.

• Litsea cubeba: Litsea cubeba.

• Cardamom: Amomum aromaticum.

• Amomum: Amomum longiligulare.

• Peppermint: Mentha.

• Pine: Pinus.

• Camphor: Cinnamoum camphora.

• Cassia: Cinnamum cassia.

• Tea Tree: Melaleuca.

• Eucalyptus: Eucalyptus.

• Ginger: Zingiber officinale.

• Basil: Ocimum.

• Clove: Syzygium aromaticum.

• Star Anise: llicium verum.

Note:

• Essential oils can cause skin irritation, so use them in appropriate dosages and follow professional guidance.

• Choose essential oils from reputable sources to ensure quality.