Essential Oil Extraction Process

Abstract

Essential oils can be extracted using various methods. Process choice significantly impacts yield and quality, leading to the development of processes aiming for maximum essential oil (EO) yields in a chemical state close to their native structure. In this chapter, various extraction techniques, including conventional ones and their intensification, are discussed along with their respective pros and cons. Additionally, new eco-friendly extraction methods have been introduced to enhance the conventional production of essential oils. The most traditional, straightforward, and widely utilized extraction techniques are hydrodistillation and steam extraction. In actuality, steam extraction techniques are used to extract 93% of all essential oils. Other common extraction techniques include enfleurage (particularly used with roses), cold pressing (just for citrus peel), and organic solvent extraction. The low yield, loss of volatile chemicals, lengthy extraction durations, and hazardous solvent residues of these procedures are its drawbacks. Microwave-assisted extraction and supercritical fluid extraction are two of the latest essential oil extraction techniques that have received considerable interest.

1. Introduction

Essential oil (EO) is a secondary metabolite synthesized by medicinal and aromatic plants. It represents less than 5% of the total plant composition. Over 3000 types of EO have been identified, but only 300 were found to be economical [1, 2].

EO is volatile, generally colorless, and liquid at room temperature. It is highly soluble in organic solvents, alcohol, and fixed oils but sparingly soluble in water. It has very high optical activity, a variable refractive index, and sometimes a distinctive taste. In addition, essential oils have a characteristic odor, that is responsible for the fragrance specific to the aromatic plant. Chemically, EO components can be divided into terpene compounds and aromatic compounds. It is a mixture of bi-active chemical components such as terpenoids, terpenes, and phenolic compounds. They are made up of terpene compounds, acids, alcohols, esters, aldehydes, ketone epoxides, sulfides, and amines [3, 4].

They are synthesized by the majority of plant organs, in particular flowers, buds, leaves, seeds, stems, and fruits. These EOs can be stored in epidermal cells, cavities, and the secretory cells of glandular trichomes.

EOs are known for their biological activity, notably their antioxidant, antifungal, antimicrobial, antiviral, antiparasitic, antimycotic, and insecticidal properties [5, 6].

Several extraction techniques are used; Hydrodistillation and steam extraction are the oldest, simplest, and most commonly used methods. Other extraction methods can also be used: cold pressing, especially applied to rose.

The EO extraction method generally depends on the botanical material used. It is one of the main factors determining EO quality. An inappropriate extraction procedure can damage or alter the chemical composition of EO. This results in a loss of bio-activity and natural characteristics. In the most serious cases, this is accompanied by discoloration, an unpleasant odor or flavor, and physical changes such as increased viscosity [7].

The principle of EO extraction is relatively straightforward. However, the process chosen can have a significant effect on the yield and quality of the distillate obtained [8]. Various processes have therefore been developed to obtain maximum yields of EO with a chemical state as close as possible to their native structure.

According to the European Pharmacopeia, an essential oil can be obtained by steam distillation, distillation, or a mechanical process. Other processes include supercritical CO₂ extraction, volatile organic solvent extraction, microwave extraction, and ultrasonic extraction. The aim of this chapter is to present an overview of the various extraction methods.

2. Location and yield of essential oil

Plants have the natural ability to produce volatile compounds in trace amounts. However, only a small percentage of plant species, around 10%, are considered “aromatic”. This property of accumulating essential oils is found in specific plant families distributed throughout the plant kingdom, including Pinacea (pine and fir), Cupressaceae (cedarwood), and angiosperms. The most significant families are dicotyledons such as Asteracea (chamomile), Apiaceae (coriander), Geraniaceae (geranium), Lamiaceae (mint), Illiciaceae (anise), Lauraceae (cinnamon), Rosacea (rose), Sandatalacea (sandalwood), Myrtaceae (eucalyptus), Myristicaceae (walnut), Oleacea (jasmine), and Rutacea (lemon). Monocotyledons are mainly represented by the families Zingiberaceae (ginger), and Poacea (vetiver) [9, 10].

EOs are natural secretions produced by cells and found in plant parts such as flowers (rose), leaves (lemongrass), flowering tops (lavender), bark (cinnamon), roots (iris), bulbs (garlic), fruits (vanilla), seeds (nutmeg), or rhizomes (ginger). Essential oils are extracted from specific parts of plants, such as sage or lavender. The most concentrated or secretory parts of the plant are harvested at the optimum yield period, which varies depending on the plant. For example, mints are harvested before flowering, lavenders during flowering, and seed plants after flowering or after morning dew for fragile flowers. It is important to note that plant growth conditions can also affect yield and essential oil content. The collection period and drying methods can also impact the yield. Therefore, it is crucial to choose the right harvesting time and drying and extraction methods to obtain the maximum yield and quality of essential oils [8].

This text reviews both traditional and “green” extraction techniques, comparing their performance with conventional methods and emphasizing the benefits of “green” technology in plant extraction research.

3. Conventional extraction methods

Conventional extraction methods can have some drawbacks, such as the degradation of unsaturated compounds and loss of certain components. It is great to hear that there are ongoing efforts to improve and optimize extraction techniques and that these techniques are carefully chosen based on the plant organ and desired product quality. It is also important to note that the analytical composition of EOs can vary depending on the extraction technique used and that factors such as distillation duration, temperature, operating pressure, and raw plant material quality can all influence EO yield [11].

3.1 Steam extraction

Steam extraction (Figure 1) is a widely used and official method for extracting essential oils from plants. This method accounts for 93% of essential oil extractions and can take anywhere from 1 to 10 hours depending on factors such as extraction time, temperature, pressure, and type of material [12].

Figure 1.

A schematic representation of steam extraction of essential oils.

In this extraction system, plant material is exposed to a stream of steam without prior maceration. The heat applied breaks down the cells of the plant material, releasing the essential oil. The steam, saturated with volatile compounds, is then condensed, and the essential oil is recovered by decanting the water/oil mixture [13, 14].

One of the advantages of steam extraction is that the absence of direct contact between water and plant material, and then between water and aromatic molecules, prevents hydrolysis or degradation of essential oil [15]. The “head” fractions, which contain the most volatile molecules, can be collected in as little as half an hour, with 95% of the volatile molecules being collected [16].

The technique works by ensuring that the combined vapor pressure equals the ambient pressure at about 100°C, allowing volatile components with boiling points ranging from 150 to 300°C to be evaporated at a temperature close to that of water. It is also interesting to note that this technique can be carried out under pressure depending on the extraction difficulty of the EOs [17].

3.2 Hydrodistillation

Hydrodistillation (HD) (Figure 2) is a standard EO extraction method. It enables the extraction of water-insoluble natural products with a high boiling point. The process involves complete immersion of the plant material in water, followed by boiling. This operation is generally carried out under atmospheric pressure. The steam formed is condensed by the refrigeration system at a water flow rate.

Figure 2.

A schematic representation of hydrodistillation of essential oils.

This method protects the extracted oils from overheating. The advantage of this technique is that the required material can be distilled at temperatures below 100°C.

Distillation may seem like a simple process for extracting essential oils, but it comes with several drawbacks. In developed countries, its use has become outdated due to the overheating of plant material and the production of burned-smelling oils. However, this method is still effective for powders and hard materials. It is important to note that exposure to boiling water for extended periods can cause weathering reactions and hydrolysis of esters into alcohols and acids, which can have serious consequences for oils with high ester levels. Rectification is often necessary to remove unwanted impurities or constituents responsible for unacceptable odor. Distillation time varies depending on the type of plant material, with woody plant organs requiring longer distillation times than herbaceous plants [18].

3.3 Hydrodiffusion

Hydrodiffusion is another method conventional method for extracting essential oils from plant materials. It involves the use of steam and water to extract the oils. The plant material is placed on a grid above water in a distillation vessel, and steam is injected into the bottom of the vessel. The steam then passes through the plant material, carrying the essential oils with it. The steam and oil mixture then condenses on a cooled surface, with the oil and water separating into two layers. This method is particularly useful for extracting essential oils from delicate plant materials, as it uses lower temperatures and less pressure than other methods [19, 20].

3.4 Cold pressing

One of the oldest extraction methods for essential oils from citrus peels such as lemon, orange, bergamot, and grapefruit is cold pressing. This technique mechanically tears the peels by simply pressing them to extract the volatile essences contained in the citrus pericarps. Until the early twentieth century, cold-pressed citrus oils were produced manually. The process produces an aqueous emulsion, which is then centrifuged to separate the essential oil. This method is preferred for citrus peel essential oil extraction because it avoids thermal alteration of the aldehydes. This process results in the production of an aqueous emulsion, which is then centrifuged to separate the EO [21].

3.5 Enfleurage

Enfleurage is another conventional extraction method that dates back to antiquity. It is based on the affinity of fragrances for fats, and concerns plants that retain their fragrance after being picked (such as jasmine or tuberose). The flowers are spread out on frames coated with odorless grease. The flowers’ fragrance is absorbed by the grease until saturation. The flowers are changed regularly (e.g., every 24 hours for jasmine). When the fat is saturated by the flowers, the operation is complete. Saturation can last up to a month. The resulting pomade is then melted. After decanting, the mixture is cold-treated with alcohol. The alcohol draws out the fragrance on its own, without taking on the fats. This extraction technique is virtually dying out due to its high cost, and the extracted oils have no applications in the food industry [14, 22, 23].

3.6 Organic solvent extraction

Solvent extraction is commonly employed to extract EOs that exhibit thermal labile properties, such as those extracted from flowers. The plant material is placed in a solvent bath. Successive washings charge the solvent with aromatic molecules. After separation by filtration, the emulsion is distilled to extract the EO.

Solvent extraction has been used for fragile or delicate floral materials, which cannot withstand the temperature of distillation. Various solvents, including hexane, acetone, petroleum ether, ethanol, or methanol, can be used for extraction [7].

Solvent extraction is relatively fast and inexpensive. The chosen solvent must be permissible, inert, and stable to heat, light, or oxygen. Its boiling temperature should preferably be low to facilitate elimination.

The produced EO contains a small amount of solvent residue, making it unsuitable for food applications. However, if alcohol is used as the solvent, it is considered “food-grade” and safe for consumption. This method is commonly used in the perfume industry [24].

In practice, the solvent is mixed with the plant material, heated to extract the EO, and then filtered. The filtrate is then concentrated through solvent evaporation. It is later mixed with pure alcohol to extract the oil and distilled at low temperature.

However, this method is relatively time-consuming, making the oils more expensive than other methods. Additionally, solvent residues in the final product can cause allergies, toxicity, and affect the immune system [25].

The limited use of this extraction method is justified by its cost, toxicity and safety issues, and environmental protection regulations. However, HE yields are generally higher than with distillation. What is more, this technique avoids the hydrolyzing action of water vapor.

4. New “green” extraction methods

4.1 Microwave-assisted extraction

Since 1986, microwave energy has been widely used in chemistry laboratories. Researchers have studied the potential of this unconventional energy source for synthetic, analytical, and processing applications. Currently, there are over 3000 articles documenting the use of dielectric heating in synthesis and over 1000 articles documenting its use in extraction.

Microwave-assisted extraction is a revolutionary technology that has garnered a lot of interest. It has a distinctive friction-based heating mechanism. It is inexpensive, and performs well under atmospheric conditions.

Microwave-assisted extraction achieves higher extraction yields, shorter extraction times, and improved selectivity as compared to traditional extraction techniques. This process is also less complicated and expensive than supercritical fluid extraction. However, it usually requires for more organic solvent, which makes it less environmentally friendly [26].

Recent methods of microwave-assisted extraction include microwave-assisted vacuum hydrodistillation, compressed air distillation, and microwave-assisted accelerated steam distillation [27].

4.1.1 Dielectric heating and fundamentals of microwave extraction

Microwave irradiation utilizes a specific electromagnetic field frequency, similar to activated photochemical reactions. The frequency range is vast, extending from 300 MHz to 300 GHz, but only certain frequencies are authorized for industrial, scientific, and medical use. These include frequencies of 0.915 and 2.45 GHz. The magnetron, found in domestic and laboratory microwave furnaces, is a typical microwave generator for such frequencies. Industrial magnetrons can reach powers of several tens of kilowatts, while laboratory devices generally have powers of less than 1 kW. Solid-state generators have recently been introduced, which narrow the microwave generator’s emission band, allowing the user to vary the system’s frequency within the range of authorized industrial, scientific, and medical frequencies. This variation can play a crucial role in chemical synthesis, particularly with regard to selectivity and efficiency. However, solid-state generators operating at 2.45 GHz typically have a power rating of 100 W, which is also frequently used in medical applications [28].

Microwave-assisted extraction (MAE) is a process that removes solutes from a solid matrix into a solvent. The process involves complex phenomena such as heat transfer electromagnetic transfer, mass transfer, and momentum transfer [29].

4.1.2 Microwave solvent-assisted extraction

Microwave solvent-assisted extraction (Figure 3) have revolutionized the field of bioactive compound extraction. This technique has significantly reduced extraction times, minimized organic solvent consumption, and resulted in energy and cost savings [30].

Figure 3.

Experimental set-up for microwave solvent-assisted extraction.

Moreover, microwave solvent-assisted extraction is an environmentally friendly and sustainable method that contributes to the development of “green” procedures.

A new and efficient method for extracting essential oils from Angelica sinensis root has been developed using a microwave-assisted deep eutectic natural solvent (NADES). The study found that NADES based on choline chloride and citric acid were more effective in extracting essential oils [31].

4.1.3 Compressed air microwave distillation (CAMD)

This method (Figure 4) uses the principle of steam entrainment, with compressed air instead of steam, to extract the essential oil. The extraction process consists of a compressor, a microwave oven and a refrigeration system. Compressed air is injected into the reactor, where the matrix is heated by microwaves and immersed in water. The steam, saturated with volatile molecules, is directed to a recovery container located outside the microwave oven and cooled by a refrigeration system. In just a few minutes, the water and aromatic molecules are condensed and recovered [30, 32].

Figure 4.

A schematic representation compressed air microwave distillation.

A similar method using a condenser to cool the extraction gas (temperatures ranging from −20 to −15°C) has also been patented [33]. This extraction method is environmentally friendly, as no organic solvents or artificial chemical compounds are added.

4.1.4 Microwave hydrodistillation (MWHD)

The MWHD (Figure 5) was developed by Stashenko et al., in 2004. It is based on the classic hydrodistillation principle. The process consists of a hydrodistillation unit placed inside a domestic microwave oven with a side port, through which an external glass condenser is connected to the round filter containing the matrix and water [34].

Figure 5.

Microwave hydrodistillation.

Microwave hydrodistillation is a widely used technique for extracting essential oils from various aromatic plants and spices, with examples such as Thymus vulgaris L., Zataria multiflora Boiss., and Satureja montana.

An improved version of this technique was developed in 2007, which involves introducing a microwave coaxial antenna insulated inside a glass flask containing the matrix and water [35]. This in situ microwave heating offers advantages in terms of time and energy savings and can be useful for industrial applications.

Microwave Steam Distillation (MSD) (Figure 6) is another innovative technique that was developed. It is based on the conventional steam distillation principle and has been successfully used for the extraction of essential oil from Lavender flowers [30].

Figure 6.

Microwave steam distillation (MSD).

4.1.5 Solvent-free microwave extraction (SFME)

This is one of the most recent techniques for the microwave-assisted extraction of essential oils, without solvents and using water at atmospheric pressure. The SFME process consists mainly of four parts: a reactor where the matrix to be treated is placed, a microwave oven, a cooling system, and an essential oil container where the oil is collected (Figure 7).

Figure 7.

Improved solvent-free microwave extraction (improved SFME).

The process is based on a relatively simple principle, described as microwave-assisted dry distillation; the fresh matrix is placed in a microwave reactor without the addition of water or organic solvent. Heating the raw material with water breaks down the glands containing the essential oil. This phase releases the essential oil, which is then carried away by the steam produced by the water in the matrix. A cooling system located outside the microwave oven enables continuous condensation of the distillate, composed of water and essential oil, and the return of excess water to the bottle, thus maintaining the appropriate moisture content of the matrix. For example, Milestone’s “DryDist” laboratory microwave oven makes it easy and efficient to extract high-quality essential oils.

Wang et al. in 2006 proposed an improved SFME extraction method. The method is based on the addition and mixing of carbonylated iron powder with the dry matrix. Spherical particles of carbonylated iron are capable of absorbing part of the energy emitted by microwaves and returning it to the medium in the form of heat. In this way, the matrix can be heated by simple conduction without any auxiliary energy. Various types of materials such as activated carbon, graphite powders, and ionic liquid (1-hexyl-3-methylimidazolium hexafluorophosphate) can absorb microwave radiation [36].

4.1.6 Microwave hydrodiffusion and gravity (MHG)

The Microwave Hydrodiffusion and Gravity (MHG) process was invented by a team of researchers led by Dr. Farid Chemat at the University of Avignon in France [37]. The team developed the MHG process as an alternative to traditional methods of essential oil extraction, which can be time-consuming and require large amounts of energy. The MHG process was first introduced in 2004 and has since gained popularity in the essential oil industry due to its efficiency and effectiveness.

The process of Microwave Hydrodiffusion and Gravity (MHG) involves the following steps:

• Preparation: The plant material, such as herbs or flowers, is first cleaned and dried to remove any impurities.

• Loading: The dried plant material is placed in a vessel that is suitable for microwave heating, such as a glass container or a microwave-safe bag.

• Microwave Heating: The vessel containing the plant material is exposed to microwave radiation. The microwaves generate heat, causing the essential oil compounds within the plant material to vaporize.

• Hydrodiffusion: As the plant material is heated, water molecules present in the plant cells also vaporize. This creates a hydrodiffusion effect, where the vaporized water carries the essential oil compounds with it.

• Condensation: The vapor containing the essential oil compounds and water is then cooled down, causing it to condense. The condensation occurs in a separate container or condenser unit.

• Separation: The condensed mixture of essential oil and water is then separated. This can be done using techniques such as decantation or using a separating funnel.

• Collection: The essential oil, which is lighter than water, floats on top and can be collected from the separated mixture.

• Analysis and Storage: The collected essential oil can be analyzed for quality and stored in suitable containers to preserve its aroma and therapeutic properties.

4.2 Supercritical fluid extraction

Supercritical fluid extraction (SFE) is a separation technique that utilizes supercritical fluids as the extracting solvent. A supercritical fluid is a substance that is above its critical temperature and pressure, which results in unique properties that make it an effective solvent for extraction.

The principle of SFE is based on the fact that the solubility of a substance in a supercritical fluid increases with pressure, while the density of the fluid increases with pressure and temperature. By adjusting the temperature and pressure, the solubility of the substance can be controlled and optimized for extraction.

In SFE, the supercritical fluid is pumped into a vessel containing the sample to be extracted. As the fluid passes through the sample, it dissolves the target compounds, which are then carried out of the vessel and into a collection vessel by depressurization or by lowering the temperature. The extracted compounds can then be separated from the supercritical fluid by condensation or by other means.

SFE has several advantages over traditional extraction methods, including reduced solvent use, shorter extraction times, and higher yields of target compounds.

Supercritical fluid extraction (SFE) can be performed in a variety of ways: batch, semi-batch, or continuous. Plant material is placed in a container and supercritical fluid is added at a specific flow rate until the appropriate extraction conditions are reached. Compared with conventional solvent extraction methods, supercritical fluid extraction offers several advantages, including a lower temperature suitable for thermosensitive compounds and a solvation power that can be controlled by modifying pressure and/or temperature, enabling high selectivity. Supercritical fluids are more effective than liquid solvents in penetrating porous materials and extracting compounds, resulting in faster extraction and a more environmentally friendly process. CO₂ and small amounts of organic solvents can be used as nontoxic fluids, and this method can be used on an industrial scale [38].

However, high pressures should be avoided when extracting essential oils to prevent the extraction of undesirable compounds.

To ensure the success of EFS, various factors need to be taken into account, such as sample type, preparation, fluid type, delivery method, and extraction conditions. CO₂ is commonly used due to its low critical temperature, cost-effectiveness, nontoxicity, absence of odor and taste, and ease of disposal. Adjusting the process conditions makes it possible to selectively extract the desired components. Compared with steam distillation, EFS has shorter extraction times, lower energy costs, and greater selectivity. The EFS method also makes it easier to manipulate oil composition by modifying extraction parameters [39].

5. Conclusion

In conclusion, there are multiple methods for extracting essential oils, and the process chosen can greatly affect the amount and quality of the oil produced. To maximize yields and maintain the natural structure of the oils, extraction processes have been developed. This chapter explores different extraction techniques, both conventional and intensified, highlighting their advantages and disadvantages. It is improved that new techniques have been proven to produce higher quality extracts in a shorter time compared to traditional techniques. However, regulatory standards do not list these extracts derived from innovative techniques as essential oils due to the narrow definition of essential oils based solely on conventional extraction methods. Furthermore, new environmentally friendly methods have been introduced to improve traditional essential oil production. Therefore, it is becoming increasingly crucial to modify or re-establish industry standards to encompass a broader range of extraction techniques.