Essential oils are highly concentrated aromatic extracts that are distilled or expressed from a variety of aromatic plant material, including flowers, flowering tops, leaves, grasses, needles and twigs, fruits/zests, seeds, resins, roots, and woods.
It is very common in the mass market today to refer to just about anything that is liquid in a brown bottle and smells as an essential oil. Even within the aromatherapy profession, many call absolutes and CO2 extracts essential oils. Technically, however, only distilled or expressed substances are essential oils. The International Organization for Standardization (ISO) in their Vocabulary of Natural Materials (ISO/ D1S9235.2) defines an essential oil as a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materials. Following the distillation, the essential oil is physically separated from the water phase.
According to Dr. Brian Lawrence “for an essential oil to be a true essential oil, it must be isolated by physical means only. The physical methods used are distillation (steam, steam/water and water) or expression (also known as cold pressing, a unique feature for citrus peel oils). There is one other method of oil isolation specific to a very limited number of essential oil plants. This is a maceration/distillation. In the process, the plant material is macerated in warm water to release the enzyme-bound essential oil. Examples of oils produced by maceration are onion, garlic, wintergreen, bitter almond, etc.”
Resins, such as frankincense, myrrh and benzoin are thick and sticky when extracted from the tree. Distilled frankincense and myrrh are more liquid and benzoin stays thick.
Two methods of extracting essential oil:
- Steam Distillation: when essential oils and hydrosols are produced
- Expression: when citrus oils are produced.
The advantage of distillation is that the volatile components can be distilled at temperatures lower than the boiling points of their individual constituents and are easily separated from the condensed water.
Preparation of plant material: some plants require preparation prior to distillation. It is sometimes necessary to crush, open, or in some other manner prepare the plant material in order for the plant to release its essence during the distillation process. Roots, woods and seeds do need preparation. Woods are usually cut into shorter lengths in order to expose the oil glands. The main purpose is to allow easier access to the volatile constituents as the steam passes through the material.
Plant material must be distilled immediately after preparation and if it is not, two things might happen:
- The total amount of oil may be reduced
- Composition and aroma will be altered because some of the constituents will evaporate
These are the three types of distillation:
- Water distillation: this method is often used with flowers (rose and orange blossoms), the plant material comes in direct contact with the water. It is the ideal type of distillation for flowers because direct steam causes these flowers to clump together.
- Water and steam: this method is used with herbs and leaves, the water remains below the plant material, which has been placed on a grate while the steam is introduced from outside the main still.
- Steam distillation: it is the most commonly used method, steam is injected in the still, usually at slightly higher pressures and temperatures than the above methods.
Hydrosols: a co-product of distillation, which contains a small amount of essential oil components. Every liter of hydrosol contains between 0.05 and 0.02 milliliter of dissolved essential oil.
Extraction is a method used for citrus essential oils: tangerine, lemon, bergamot, sweet orange and lime. Most modern expression techniques are accomplished by using machines using centrifugal force. The spinning in a centrifuge separates most of the essential oil from the fruit juice.