"Essential oils are popular aromatics for scenting a space or setting a mood, but they can be so much more beneficial to our health.” ― Amy Leigh Mercree
Introduction | Extraction Methods| Azeotropic Extraction | Cold Pressing | Solvent Extraction | Contamination | Shelf Life | Final Thoughts | Takeaway Points | References | Disclaimer
Introduction
The selection of plant essences on the current market is very vast. There are essential oils, absolutes, and CO2 essences, just to mention a few. Their extraction methods, concentrations, and uses vary for each of them. The quality of the oil they produce, its purity, and its aromatic profile are all influenced by the extraction methods, making them particularly intriguing. Understanding the pros and cons of azeotropic extractions, cold pressing, and solvent extractions can help us, the consumers, make a more informed decision when building our essential oil case.
The types of extraction methods
The extraction of essential oils from herbs, plants, and plant components is a very delicate process. There is no one universal isolation technique that would work equally well for all the botanical materials. Not only are the essential oils obtained from different plant parts, but these plant components can also store their aromatic molecules in various places within their cellular structure. The placement of these aromatic substances and the properties of the plant material all determine the best method of their extraction. Essential oil isolation methods include:
Azeotropic extraction or distillation methods, which include steam distillation, hydrodiffusion, and hydrodistillation;
Expression methods such as cold pressing.
Solvent extractions include CO2 extraction, enfleurage, maceration, and food-grade solvents.
Steam distillation is the method that is most frequently used to extract essential oils from their respective plant sources. The vast majority of essential oils, or 93%, are obtained through a distillation process that makes use of steam. The remaining 7% of essential oils are obtained through a variety of other processes[4].
Azeotropic extraction
Azeotropic distillation methods are industrial isolation techniques that allow for the separation of the components of a mixture and, in the end, the extraction of, in this case, plant extracts. There are three main azeotropic extraction techniques. Steam distillation is by far the most commonly used isolation method for the production of essential oils. Hydrodiffusion and hydrodistillation are less frequently used; however, they are not uncommon.
Steam Distillation
Steam distillation is a straightforward but sensitive process. It uses hot steam to evaporate oils from the plant material. The steam is introduced into the processing still from the bottom of the plant material. The temperature of the steam has to be well controlled and maintained at optimal levels for the process to succeed, so that the botanical elements do not burn but the aromatic particles evaporate into the steam. The vapor obtained in this process is later cooled and condensed into liquid form. At the next stage, the water and the oil are separated. The final products of this action are the purest form of essential oil and hydrosol [floral water].
Steam distillation can be used on most of the plant parts used for oil extraction. It should not, however, be used on plants whose oils are sensitive to heat.
Hydrodiffusion
Hydrodiffusion is a very similar process to steam distillation. It uses the same equipment and requires the same steam temperature control. The main difference is in the way the steam is introduced into the still. Hydrodiffusion relies on the osmotic action of the steam, which comes from the top of the botanical elements, not from the bottom. This makes it more efficient, less time-consuming, and yields higher-quality oils.
Hydrodistillation
Hydrodistillation, or water distillation, is a process where plant material is immersed in water and then brought to a boil. The water that surrounds the plant parts protects the separated oil from overheating and therefore losing its properties. At the next stage, the water-oil mixture is cooled and eventually separated from one another. This method allows for the extraction of the essential oil and the formation of floral water [hydrosol].
Hydrodistillation is effective on most plant parts; however, not all plants can undergo this process. Plants that are high in esters should not be water-distilled since it will impact the chemical composition of the final product.
Cold Pressing
Cold pressing is a method that allows for the extraction of essential oils from citrus fruits. The fruit is placed in the machine, which uses spikes to puncture the oil pockets within the peel [rind] of the fruit. After that, the whole fruit is pressed to squeeze the juice and oils out of it. Since the oil has a different density than the rest of the juice and pulp, it can be separated with the use of a centrifuge.
Cold pressing is performed on fruits such as oranges, mandarins, tangerins, lemons, limes, grapefruits, and bergamots. The part of the plant used to obtain the essence is the rind [peel] of the fruit.
Solvent extraction
Solvent extractions describe the processes of essential oil isolation that use various types of solvents, such as volatile [food-grade] solvents, oils, fats, and carbon dioxide. This category of extraction methods includes CO2 extraction, enfleurage, maceration, and food-grade solvent extraction.
CO2 extraction
CO2 extraction, also referred to as supercritical carbon dioxide extraction, is a process where carbon dioxide is pressurized until it reaches a state where it isn't either a gas or a liquid. In this form, it is then used to extract the oil from the botanical elements. After that, once the pressure is removed, CO2 evaporates, leaving an essential oil available to collect.
CO2 extraction is a preferred option for plants that yield too little oil when steam-distilled or when steam distillation is impossible, e.g., rose hip seed or calendula. They are also marketed not simply as essential oils but as CO2 essences.
Enfleurage
Enfleurage is a rarely used method of essential oil extraction. It utilizes warm fat as a solvent that helps obtain the plant's essence. Botanical elements are placed on sheets of warm fat, which absorbs the aromatic constituents of the plant. This process is repeated as many times as necessary until the fat is completely infused with the essential oil. After that, another solvent, alcohol, is used to separate the essential oil from the fat. Once this process is complete, the remaining fragrant fat is used to make soaps.
Enfleurage is used on plants that naturally contain very little oil. Among such plants are jasmine and tuberose.
Food-grade solvent extraction
This extraction method utilizes food-grade solvents such as ethanol, benzene, dimethyl, or hexane to isolate essential oils. The plant material is soaked in the solvent and left until it completely dissolves. After that, the extract that was obtained evaporates, leaving the plant oil behind. Such a highly concentrated form of essential oil is called an "absolute."
Absolutes are created when steam distillation is not a viable extraction method for a particular plant. Very delicate plants, such as jasmine or rose, do not do well in heat or under CO2 pressure. Solvent extraction allows for a more appropriate isolation of the essence, making it very potent and highly concentrated. This is why absolutes are not exactly the same as essential oils.
Maceration
Maceration is the oldest and most traditional extraction method. It does not produce pure essential oils but rather infused oils. This technique utilizes vegetable oil as a solvent. The plant material, which is ground to a powder, is soaked in heated oil, and after that, it is strained. Such infused oil can be subjected to further extraction with the use of another solvent, such as alcohol, which leads to the creation of thick plant extracts and resins.
Maceration produces extracts that carry a lot of fragrance but also maintain the original qualities of the plant.
The contamination of essential oils
To prove the quality of essential oils, companies that manufacture them use various laboratory tests that check their purity and detect possible contaminants. They have quite a wide variety of tests at their disposal: refractive index, optical rotation, specific gravity, gas chromatography (GC), and inductively coupled mass spectrometry (ICP-MS). Some of these tests serve as tools for identifying the chemical components of each essential oil and, therefore, helping establish its quality. Others, on the other hand, help detect possible contaminants such as pesticides, heavy metals, plasticizers, and other toxins. However, before it even comes to testing a finished product, some essential oil extraction methods can help diminish the risk of contamination on a much greater scale than others.
Pesticides are commonly found in many plants, plant components, and herbs. Steam distillation is known to diminish the amount of pesticides in essential oils. Pesticides are heavy molecules that do not evaporate into the steam. Therefore, this extraction method allows for a better reduction of potential agricultural contamination with pesticides in the essential oils. It is more likely that essential oils obtained through cold pressing or CO2 extraction will contain pesticides. Cold-pressing is used to extract essences from the peel of the fruit, and the chances are that it will also release pesticide residue into the oil. Supercritical CO2 extraction also allows for greater concentrations of pesticides[5] in the extracted oils compared with steam distillation.
Tip: Always choose oils that come from organic cultivation. Additionally, some manufacturers offer steam-distilled citrus essences, even though cold pressing is still the primary method of extracting citrus essential oils. They are believed to have less vibrant and intense aromas, though.
Solvents utilized to extract essential oils from botanical materials can also contaminate the finished product. Hexane, one of the most popular solvents used to isolate fragrances, is a chemical compound composed of n-hexane, which often contains other substances such as benzane. According to studies, the metabolites of n-hexane can be linked with several health ailments, including nervous and reproductive system impairments, endocrine disruption, and even Parkinson's disease[6]. Food-grade solvents, even though marketed as safe if consumed within limits, can cause damage to our health when accumulated. The residue they leave behind is tiny, but if you ingest solvent-extracted essential oils, you need to be aware of the risk they carry. The supercritical carbon dioxide extraction technique, on the other hand, does not leave any solvent residue in the finished product. CO2 evaporates completely and can't be traced in the essential oils.
Tip: Avoid consuming essential oils that were extracted using solvents. The solvent contamination could eventually harm you. Although CO2 extraction produces substances free of solvent, the essence is so potent that it should not be ingested either.
Heavy metal contamination is related to the environment in which the plants used for the extraction of essential oils are cultivated. If the soil, water, and air of the region are highly polluted, the botanical material is highly likely to carry some part of this pollution with it into the manufacturing facilities. However, similar to pesticide contamination, steam distillation is able to greatly diminish the amount of heavy metals in the essential oil. One study looked at the heavy metal content of the Centella asiatica plant. It was found that even though the plant had heavy metals in its leaves, after steam distillation, the essential oil that was obtained was heavy metal-free.
"This is because steam distillation causes the volatile constituents to vaporise and the heavy metals in the leaves are too heavy to be vaporised."[7]
Tip: A lot depends on the region where the plants used to manufacture essential oils come from. Since, for us as customers, it is very difficult to monitor and know the kind of pollution the botanical material is exposed to prior to harvesting, we are left with the choice of opting for essential oils that come from organic cultivation and are steam-distilled.
Overall, steam distillation of essential oils has a major advantage over other extraction methods in diminishing the risk of contamination with pesticides, heavy metals, and solvents. The CO2 extraction technique also offers solvent-free oils. However, it does not reduce the heavy metal and pesticide load. Additionally, CO2-extracted oils are much more potent and concentrated than stem-distilled essential oils; therefore, they might carry potentially more health risks if they are not pure.
The shelf life of essential oils
Essential oils have a shelf life, just like every other product on the market. They do not spoil or grow mold, but they oxidize, change their chemical structure, and alter aromatic properties. It has been measured that, on average:
steam-distilled essential oils can last for up to 2 years if stored under proper conditions. Most commonly, they last in unchaged condition for an average of 18 months.
CO2 extracts have a longer shelf life and better oxidative stability than essential oils obtained through steam distillation. They last for 2 years, and some of these oils can go even beyond that timeframe.
cold-pressed essential oils, on the other hand, have a shorter shelf life—typically, 9 to 12 months.
absolutes [solvent-extracted oils] have the longest shelf life. They can last for 5 years, and some even beyond that.
It is crucial to define what "proper conditions" entail. Essential oils, absolutes, and extracts store best in dark glass jars rather than plastic ones. They ought to be kept out of direct sunlight, in a constant, cool environment, and with the container securely closed.
Final Thoughts
Today's essential oil industry makes the most use of steam distillation as its main extraction procedure. Both from a production and a consumer standpoint, this process has many benefits. While we will skip over its business advantages, it is crucial for us to know that it provides high-quality oils that are free of most environmental toxins and maintain the majority of the original characteristics of the plant they were extracted from. Some of the fragrances, however, can be less vibrant and aromatic than oils obtained through different means. CO2 essences and absolutes offer much more fragrant extracts, but their concentrations are so high that they are not safe to be ingested. Additionally, these extraction techniques do not remove many of the toxins that are present in the plants. One thing needs to be kept in mind above all others when choosing the oils we want to purchase: organically grown plants and herbs, regardless of the manufacturing procedure, produce safer essential oils.
Takeaway Points
There are three main categories of essential oil extraction methods: azeotropic extractions, cold-pressing, and solvent extractions.
Azeotropic extractions include steam distillation, hydrodistillation, and hydrodiffusion.
Solvent extractions include CO2 extraction, enfleurage, maceration, and food-grade solvent extraction.
Steam distillation is the most widely applied extraction method in the essential oil industry.
Steam distillation can eliminate the most toxins and pollutants from the botanical elements during manufacturing.
Organically grown plants and herbs make the safest essential oils.
The extraction process and storage conditions affect the essential oils' shelf life.
References
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