Methods For Extracting Essential Oils

The essential oils are volatile aromatic compounds and hydrophobic secondary metabolites which are produced by plant species. They are important as flavourings and perfumes as well as having a plethora of therapeutic, cosmetic, pharmaceutical and nutritional benefits. A single compound might have a distinct aroma but when describing essential oils from a plant for example, it is a composition of hundreds of different aroma chemicals. Their combination is usually characteristic of a particular plant. When it comes to extracting essential oils we can employ a range of technologies to obtain the best quality product.

Many essential oil preparations remain unresearched in therapeutic studies. Their efficacy is still being explored in the treatment of a range of medical conditions.

Types Of Essential Oils And Aromas

Typical examples of single chemicals include methyl butyrate which evokes a fruity smell in apples and pineapple. Benzaldehyde on the other hand is associated with almonds whilst cinnamaldehyde conveys the smell of cinnamon. Anethole is associated with the highly distinctive aroma of aniseed or liquorice. Methone is associated with a range of mint smells. The stereochemistry of these molecules is highly important in the type of smell conveyed. 

Without exception, essential oils are liquid at room temperature although a few form a wax on occasion. The most appropriate extraction method is chosen depending on the volatility or ease of evaporation of the aromas or essential oils in question. Coupled to this property is the degree of polarity of each molecule which is a measure of its hydrophilicity (desire to be extracted in water or a highly polar solvent) or its hydrophobicity of the compounds to be extracted.

Techniques For Extracting Essential Oils

The classic techniques for extraction are the following:

• hydrodistillation and steam distillation
• solvent extraction
• ultrasound
• microwave
• ohmic heating
• supercritical fluid extraction
• pulsed electric field extraction

The conventional methods of extraction:

Hydrodistillation

Hydrodistillation is a very common and easily commercialised method for essential oil extraction. The method involves immersion of plant material in a water bath. The mixture is heated to boiling point at atmospheric pressure. With heating, the aromas are released from plant cells to form an azeotropic mixture. Many of the components have boiling points above 100 ºC and many are mechanically driven out of solution with the water vapour. 

The water vapour laden with aromas is condensed by cooling which produces a separation of the mixture water. The essential oils are decanted away from this mixture.

The ‘Clevenger’ system adopted by the European Pharmacopoeia involves recycling of the aqueous phase of the distillate in the boiler through a cohobage system. It was developed by Clevenger in the 1920s. The water and volatile molecules are separated on the basis of density difference. The process of steam distillation takes between 3 and 6 hours.

Steam Distillation

Steam distillation is still the most popular method and now supercedes hydrodistillation in many instances. The method takes advantage of a compounds’ volatility to evaporate (or volatilize) when heated in dry steam and the hydrophobicity of this compound to separate into an oil phase during condensation.

The plant material is placed in a still and pressurized steam is forced through the plant material. Hot steam helps release aroma chemicals from the plant cells and these are mechanically removed in the water vapour.

The steam containing the essential oils passes through a condenser where it forms a distillate of water and aroma. The essential oil usually forms an oily layer on the surface of the water and is separated mechanically. The water which is left containing any aromas is known as floral water.

Solvent Extraction

Solvent extraction is usually needed to extract aroma molecules which are sensitive to the harsher techniques of hydrodistillation or steam distillation.

Solvent extraction involves exploiting the distribution of non-polar molecules into a non-polar or slightly polar solvent. Originally, petroleum spirits was used but now hexane and toluene are preferred. Other types of solvent include alcohols such as methanol and ethanol. A novel but well developed approach uses carbon dioxide in the form of supercritical carbon dioxide extraction.

The addition of a solvent causes the essential oils to partition between the  two phases and they usually preferentially separate into the petroleum or ethanolic solvent.

The aromas in the extracting solvent are called ‘absolutes’ and are readily concentrated further. They are not actually essential oils in this context until they are processed further.

The Non-Volatile Fractions in Essential Oils

In essential oils, non-volatile chemicals refer to those components that do not readily evaporate at room temperature or below. Unlike the volatile compounds, which are responsible for the characteristic aroma of the essential oil and easily dissipate into the air, non-volatile chemicals are typically heavier and remain in the oil even after the volatile components have evaporated. These non-volatile constituents play essential roles in the overall composition and properties of the essential oils, contributing to their stability, color, texture, and potential therapeutic benefits. In this article, we will explore some of the common non-volatile chemicals found in essential oils and their significance in aromatherapy, perfumery, and other applications.

1. Fatty Acids: Fatty acids are a group of non-volatile compounds commonly found in essential oils. They are long-chain carboxylic acids and contribute to the oil’s viscosity and texture. Fatty acids are often present in carrier oils, which are vegetable oils used to dilute essential oils before topical application. Examples of fatty acids found in essential oils include oleic acid, linoleic acid, and palmitic acid. These fatty acids have various skin-nourishing and emollient properties, making them beneficial in skincare and massage applications.
2. Waxes: Waxes are another class of non-volatile constituents found in essential oils. They impart a thick, waxy consistency to the oils and can contribute to their color. Beeswax, for example, is a common wax found in essential oils like beeswax absolute. Waxes provide stability to essential oil blends and are often used as fixatives in perfumery to slow down the evaporation of the more volatile aromatic compounds, thereby prolonging the fragrance’s longevity.
3. Pigments: Essential oils can contain various pigments that contribute to their color. Chlorophyll, carotenoids, and anthocyanins are examples of pigments found in certain essential oils. For instance, chlorophyll gives the green color to oils like parsley or coriander, while carotenoids can lend a yellow or orange hue to oils like carrot seed or sweet orange. Pigments have antioxidant properties and may provide additional benefits when the oils are applied topically.
4. Resins and Gums: Certain essential oils, such as frankincense and myrrh, contain non-volatile compounds like resins and gums. These constituents are responsible for the oils’ thick, sticky texture. Resins and gums have been used historically in incense and ceremonial applications, and they contribute to the grounding and calming effects often associated with these oils in aromatherapy.
5. Tocopherols (Vitamin E): Tocopherols, commonly known as vitamin E, are non-volatile antioxidants present in some essential oils, especially those derived from seeds. Vitamin E has protective properties for the skin and can help extend the shelf life of the essential oil by preventing oxidation and rancidity.
6. Sterols: Sterols are non-volatile compounds found in essential oils that possess various biological activities. They have been studied for their potential anti-inflammatory, antimicrobial, and immune-modulating properties. Some essential oils, such as wheat germ oil, contain significant amounts of sterols.
7. Fixed Aliphatic Alcohols: Fixed aliphatic alcohols, also known as long-chain alcohols, are non-volatile alcohols found in some essential oils. Examples include cetyl alcohol and stearyl alcohol. These alcohols contribute to the texture and consistency of the oils and can have emollient and moisturizing effects on the skin.
8. Flavonoids: Flavonoids are a group of polyphenolic compounds found in certain essential oils, particularly those derived from citrus fruits. They have antioxidant properties and contribute to the oils’ potential health benefits.
9. Sugars: Some essential oils contain small amounts of sugars, which contribute to their overall sweetness and viscosity. Sugars may also have humectant properties, helping the skin retain moisture.
10. Lignans: Lignans are non-volatile compounds with antioxidant and potential estrogenic properties. They are found in some essential oils like flaxseed oil and can contribute to the oil’s therapeutic effects.

Essential oils contain a diverse array of non-volatile chemicals that play important roles in their overall composition and properties. These constituents contribute to the oils’ texture, color, stability, and potential therapeutic benefits. While the volatile compounds are primarily responsible for the characteristic aroma of the oils, the non-volatile chemicals provide valuable contributions to the oils’ overall profile and make essential oils versatile and valuable in various applications, including aromatherapy, perfumery, skincare, and more. Understanding the presence and significance of non-volatile constituents in essential oils allows for a deeper appreciation of their complexity and potential health benefits.

Precipitates Formed in Essential Oil Manufacture

In essential oils, precipitates are solid or semi-solid substances that separate from the oil and settle at the bottom of the container or appear as floating particles. These precipitates can form due to various factors and can vary depending on the type of essential oil and its specific composition. Here are some common precipitates that can be found in essential oils:

1. Wax: Many essential oils, especially those obtained from flowers and certain citrus fruits, contain natural waxes. Over time or under certain temperature conditions, these waxes may solidify and form a layer or small particles in the essential oil. This is particularly common in cold-pressed citrus essential oils like orange, lemon, and grapefruit.
2. Resins and Oleoresins: Some essential oils are obtained from resinous materials, such as frankincense and myrrh. These oils can contain resinous particles that separate and settle, giving the oil a cloudy appearance.
3. Undissolved Solids: During the extraction process of certain essential oils, particularly absolutes or CO2 extracts, small undissolved particles of the plant material or other substances may remain in the oil, forming precipitates.
4. Chemical Reactions: Essential oils can be sensitive to heat, light, and air, leading to chemical reactions that cause the formation of solid particles or sediment. This is more likely to occur in aged or improperly stored essential oils.
5. Essential Oil Crystals: Some essential oils, like menthol (from peppermint or spearmint) and thymol (from thyme), can naturally crystallize at low temperatures or with time, leading to the formation of visible crystals in the oil.
6. Water Content: Essential oils that have not been properly dried after distillation or extraction might contain traces of water. These water droplets can form small emulsions or precipitates in the essential oil.
7. Particulate Contamination: Sometimes, foreign particles or contaminants may find their way into the essential oil during the production, packaging, or storage process, leading to the formation of precipitates.

It is essential to note that the presence of precipitates does not necessarily indicate a problem with the essential oil’s quality or purity. Precipitates are a natural occurrence in many essential oils and do not typically affect their therapeutic properties or fragrance.

To deal with precipitates in essential oils, users can gently warm the oil by placing the closed bottle in a bowl of warm water for a few minutes, or they can roll the bottle between their hands to disperse the particles. However, if the essential oil appears contaminated or the precipitate looks unusual, it is best to contact the manufacturer or a knowledgeable expert to ensure the oil’s integrity and safety. Storing essential oils in a cool, dark place and tightly closing the containers after each use can help prevent the formation of precipitates and maintain the oil’s quality.

When it comes to extracting essential oils we have a range of technologies available to us. The supply chain is much more complex than just manufacturing but it’s worth considering what technology is available and how it is developing because it is such a valuable industry.