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We'll start in this first section with a discussion of the natural origins of essential oils and how we obtain them from the plants that produce them.

1.1 Definition & Role in Plants

Essential oils are highly concentrated, volatile, aromatic essences found in many plants. The role they play in each plant is varied and not entirely understood but includes attracting beneficial insects and repelling unwanted ones and protecting the plant from harmful bacteria. Essential oils are stored in specialized cells or reservoirs in the plant. Depending on the plant these oils can be found in the root, leaf, flowers, fruit or seed

To obtain the oil it must be released from storage pockets in the plant. Take a fresh peppermint leaf and handling it carefully to avoid bruising, smell it. Little or no fragrance will be detected. Now rub the leaf between your fingers to break the cell walls and smell the leaf again – the strong, crisp scent of peppermint oil is now released.

1.2 Distillation of Essential Oils from Plants

The primary method of producing essential oils is through steam distillation. Distillation is an age-old process thought to have originated in Persia somewhere around the 10th century AD. While the crude still of the past is often replaced with modern, stainless steel stills, the process (and often the simple country still) basically remains unchanged. Water is heated to boiling and its steam passes through fresh plant material stacked on a rack above the boiling water. This causes the cell walls of the plant material to break down and release the essential oil. The water and essential oil vapor then passes through a cooler that condenses the steam and the oil into a liquid. The liquid is collected and the oil is separated from the water. Most oils are lighter than water and thus collect on the surface of the water where they are siphoned off. Oils heavier than water sink to the bottom of the collector where they are removed. Some stills use direct distillation where the plant material is mixed with the boiling water with the same effect. The leftover water from a steam or water distillation is sometimes saved and sold as an aromatic hydrosol. Aromatic hydrosols contain water-soluble constituents not found in the essential oil and are most often used in skin care. Most hydrosols are mildly astringent and make good skin toners. They are also soothing and refreshing when used in a facial spritzer and make good bases for masks and lotions. Because they are mostly water, however, they often contain preservatives (if not, they should be stored in the refrigerator after opening).

1.3 Other Methods of Obtaining Essential Oils from Plants

Expression is a mechanical method of pressing citrus peels such as lime, lemon, orange and grapefruit to remove the essential oils. Expressed oils contain the essential oils along with a tiny amount of part of the fruit such as colorants and waxes from the fruit. Although an essential oil without any of these additional components can be created through distillations, citrus oils are especially fragile and the resulting oil does not have the same “fresh fruit” aroma and quality of the expressed oil.

Absolutes are usually made today by first using a solvent to extract the essential oil from the plant part (concrete), then washing with alcohol to separate the essential oil from the solvent. This method is used when the heat of distillation would damage delicate oils or when the quantity of oil in the plant is so low that distillation would not be an effective method of extraction. Examples of absolutes are rose, carnation, hyacinth, violet and jasmine. There is also an older method of creating absolutes called enfleurage.

Carbon dioxide extraction is a relatively new, but efficient, method that uses CO2 under high pressure to remove the essential oils. Because of the low temperatures used, the aromas of the oils produced under this method are exceptionally true to those of the plant. However, the high cost of the equipment and complexity of the process makes the cost of these oils prohibitive in most cases.

1.4 Chemistry of Essential Oils

Each essential oil is a unique combination of many different aromatic chemicals. Over 30,000 of these aroma chemicals have been identified, and most essential oils contain around 100. It's this unique combination of components that is responsible for each essential oil's characteristic aroma. Variation in the amount of each chemical constituent can occur in an essential oil because of variations in climate, soil, seasonal growing conditions, harvesting methods and even handling techniques. As a rule, these variations fall within a specified range. For some plants, however, a difference in the chemical composition occurs due to differences in genetics. The result is a chemotype of that plant. A chemotype is usually identified by a constituent that is present naturally in a relatively high amount and, because of its higher concentration, affects how the essential oil is used. For example, thyme (T. vulgaris) has many chemotypes. The thymol chemotype is strongly antibacterial, but is irritating to the skin and mucous membranes because of the higher amount of thymol present. The milder geraniol chemotype, on the other hand, also has antibacterial properties, but does not cause the irritation of the thymol thyme.

Essential oils often share the same chemical constituents. Knowing which constituents and at what levels they're present in the oil can give clues to the oil's properties as well as its hazards. For example, essential oils that contain eucalyptol (such as eucalyptus and tea tree) are useful for relieving respiratory tract congestion. And oils with even low levels of thujone—a neurotoxin—should be used with caution (sage, yarrow), and those with higher amounts (mugwort, wormwood) avoided entirely. However, the constituents present at the greatest level don't always have the most impact on the aroma, or even the action, of the oil. Both lemon oil and orange oil contain 90% or more limonene, but lemon oil's aroma comes from the 3% citral and sinensal in its makeup, and other trace components are responsible for orange oil’s scent. There are many intriguing aspects to the way essential oil constituents work together synergistically to create the characteristic fragrance and properties of each essential oil.