The Chemistry and Biochemistry of
Essential Oils – A Glossary

Developed for the ChemAroma Website by Dr. William E. Kurtin, Ph. D.,

Department of Chemistry, Trinity University

I. Definitions

Amphipathic - having both hydrophilic and hydrophobic properties

Aroma Detection (Olfactory stimulation) - a bimolecular process involving the interaction of an airborne odorant molecule with the complementary site of a receptor molecule that occurs at a boundary between nerve cells. As a result of this molecular interaction an electrical signal is sent to the brain. The decoding of this electrical impulse provides the quality and strength of any aroma.

Distillation - a process that consists of driving a vapor from a liquid by heating and then condensing the vapor to liquid by cooling; used especially for purification.

Essential Oil - a product made by distillation with either water or steam or dry distillation of natural materials (plants), Following distillation, the essential oil is separated from the water phase (hydrosol).

Gas Chromatography - Mass Spectrometry (GC-MS) - a kind of "fingerprint" analysis, at the molecular level used for analysis of the components of essential oils.

Gas Chromatography – an analytical chemical method that separates components of oils in time based on differences in volatility. The more volatile compounds go through the instrument the fastest, and thus come out in the shortest time.

Hydrophilic - "water-loving"

Hydrophobic - "water-hating"; sometimes used interchangeably with the term lipophilic ("oil-loving")

Hydrosol - the distillation water, which will contain plant compounds that are volatile but also water-soluble (most essential oils have some water solubility). Because it contains water-soluble compounds, the hydrosol is usually milder than the pure essential oils.

LD₅₀ - a toxicological parameter used to indicate the potential harmful effects of chemical substances on biological organisms. For example, if the LD₅₀ of a compound is 887 mg/kg in rats, this means that the compound was administered orally to laboratory rats, and 50% of the rats died on a dosage of 887mg per kilogram of body weight.

Lipids – the class of biological molecules that are not very soluble in water

Mass Spectrometry – an analytical chemical method that identifies each component of an essential oil as it exits a gas chromatograph. Each compound is bombarded by a beam of high-energy electrons, which breaks it up into smaller pieces. The pattern of these pieces is a unique "fingerprint" for that compound.

Odorant – a volatile component of an essential oil.

Oil - any of numerous unctuous combustible substances that are liquid, or at least liquefiable on warming, are soluble in ether but not in water, and leave a greasy stain on cloth or paper.

Olfactory Region - an area of about 2.5 square centimeters in each nasal passage. The surface is coated with a mucous layer that is rich in amphipathic molecules. Embedded in the mucous are extensions of the olfactory nerves (neurons) that are called "cilia". The cilia are the sites where molecular interaction with odorant molecules occurs.

Organic Chemistry - chemistry of compounds containing primarily the chemical elements carbon, hydrogen, nitrogen, and oxygen.

Phenylpropanes – a chemical class of essential oils, such as cinnamaldehyde

Receptor – A large protein molecule that sits on the surface of an olfactory cell, and is designed to interact specifically with only one, or a very small set, of odorant molecules.

Steam Distillation – A variation on simple distillation, the difference being that heat is not applied directly to plant material. Instead a column of steam is generated in a separate vessel and passed through the sample. The advantage is that distillation occurs at a lower, and thus gentler, temperature.

Terpenes – the largest chemical class of essential oils, such as spearmint

Transdermal Absorption - Rubbing or massaging an essential oil directly into the skin to provide a high concentration of the oil's ingredients at a desired location. This occurs by diffusion and absorption of the components into the capillaries and specialized receptors that span the various layers of the skin.

Volatility – the tendency of chemical substances to go from the liquid to the vapor state

II. – Practical Scientific Information

English to Metric Conversion Units

Dry Measurement                                             Liquid Measurement

1 pound(lb) = 16 ounces(oz)                             1 gallon(gal) = 4 quarts(qt)

1 lb = 454 grams(g)                                          1 qt = 2 pints(pt)

1 kilogram(kg) = 1000g                                    1 liter(L) = 1.06 qt

1 milligram(mg) = 0.001g                                  1 milliliter(mL) = 0.001L

1 g = 0.035oz                                                   1 fluid oz » 30 mL

1 oz » 28.4 g                                                    1 cup = 8 fl.oz » 237 mL

2 Tablespoon(Tsp) = 1/8 cup = 1 oz                 1 teaspoon(tsp)  » 5 mL

                                                                        1 ml » 20-30 drops (varies with oil)

Oil Concentration and Dosage

Example: Apply 2 mL of 5% spearmint oil (carvone) to skin

Concentration

The 5% means 5 parts essential oil to 95 parts carrier oil. Eg., 5 mL of essential oil with 95 mL of carrier oil, to yield 100 mL of mixed solution.

Dosage

            Dosage refers to the actual mass of essential oil ingested orally or applied to the skin, divided by the mass of the recipient.

            The mass of oil is obtained from the volume of oil used by multiplying by the density of the oil (which must be obtained from chemical tables). For this example, 5 mL of pure spearmint oil (density = 0.96g/mL) were mixed with 95 mL of carrier. Thus the mass of oil in the mixture is

            Volume (mL) x density (g/mL) = 5 mL x 0.96 g/mL = 4.8 g

            Thus, the concentration of oil in the mixture is 4.8g per 100mL of solution, or

            4.8g/100mL = 0.048g oil per mL of solution.

            The mass of oil applied is the volume of mixture used multiplied by the concentration of the oil in the mixture, or, in this example

            2 mL mixture x 0.048 g/mL = 0.096g oil applied. If the oil was applied to an adult weighing 60 kg (132 lb), the dosage would then be 0.096g divided by 60 kg, or

            0.096g/60kg = 0.0016g/kg

The actual dosage would likely be less than this since some of the oil would likely evaporate from the skin surface before being absorbed.

The dosage may be compared with the LD₅₀ for the oil to evaluate potential toxicity. For example, the LD₅₀ for spearmint oil is about 1.6g/kg.

©William E. Kurtin and Lyn Belisle, 2006
Use only with permission

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