Innovative methods for extraction of essential oils from medicinal plants

: Essential oils are concentrated liquids of intricate combinations of volatile substances, extracted from various plant parts. Several bioactive substances with antibacterial and antioxidant activities are abundant in essential oils. Some essential oils have also been employed in medicine. Due to the risk associated with employing synthetic preservatives, the use of essential oils as natural additives for extending the shelf life of food products has also drawn considerable attention. They are used in the pharmaceutical, cosmetic, and food industries for their functional properties. There are various methods for extraction, but both the quality and the percentage yield of essential oil never remain the same. So, innovative and non-conventional techniques of essential oils extraction from medicinal plants were evoled to get quantitative and qualitative yield. In the present article, we searched and reviewed innovative techniques used for the extraction of essential oils from medicinal and aromatic plants through electronic searches of PubMed, Medline, Wiley, Scopus, and Google Scholar. For the extraction of essential oils, several innovative/non-conventional techniques have been reported in literature. Extraction of essential oil by using innovative techniques retards the risk of losing the essential components of plants, maintains the quality, reduces chemical risk, extraction time, acts eco-friendly, and increases the percentage yield of the essential oils. This paper presents the success story of innovative extraction methods of essential oils in accordance with sustainable development and environmental protection.


INTRODUCTION
Essential oils (volatile oils) are aromatic liquids that have been extracted from various plant parts, including leaves, flowers, seeds, and bark, when introduced to a solvent that dissolves volatile oil.They are good resources of numerous biologically active constituents which possess many pharmacological activities and have a significant role in the growth and development of aromatic herbs.Volatile oils confer fragrance to reproductive organs and fruits that attract pollination animals.To save plants, they resist phytophagous organisms, including viruses and phytoplasma vectors (Iriti & Faoro, 2009).They also contain different carbon-and hydrogen-based compounds called terpene hydrocarbons.A basic hydrocarbon found in essential oils is isoprene.Mono-, sesqui-, and diterpenes are the result of combination of two, three and four isoprenes joined together respectively.The constituents of essential oils may be broadly classified as volatile and non-volatile fractions.Volatile oils are categorized into mono-/ sesquiterpene components and oxygenated derivatives along with esters, aliphatic aldehydes, and alcohols.The oxygenated derivatives of hydrocarbons are derived from isoprenoid pathways.These chemical moieties help determine the physicochemical properties, including solubility, nature, appearance, stereochemistry, and biological activities such as antimicrobial, antiviral, antioxidant, hepatoprotective, spasmolytic, analgesic, antidiabetic, and protect against cardiovascular diseases including atherosclerosis, thrombosis, and carminative (Edris, 2007;Reichling et al., 2009).Many preclinical studies of essential oils have been published on various cell and animal models.The conventional methods employed to extract volatile oils are being used worldwide owing to the fact that the essential oils are widely consumed.Selection of extraction method plays a critical role in the percentage yield of essential oils; it may affect the physiochemical properties as well.Some extraction methods are best suited to particular plant types and parts.Modern technologies have constantly been developed to overcome the limitation of traditional methods and enhance extraction efficacy, such as steam distillation, hydrodistillation, solvent extraction, maceration, carbon dioxide extraction, cold-press extraction, enfleurage, etc.However, innovations are required in conventional techniques to get the quantitative and qualitative yield of essential oils from medicinal plants.The comparison of conventional and non-conventional extraction methods of essential oils is shown in Figure 1.

INNOVATIVE TECHNIQUES
Innovative/ non-conventional techniques are "green" in concept.Various innovative extraction methods have been proposed (Figure 2) to overcome the limitations of traditional methods.These are less time-consuming, environmentally friendly, improve yields and quality of essential oils with less utilization of solvents and energy (Figure 3), and help avoid toxic chemicals.The conventional technology involves the transfer of heat energy via conduction and convection mechanisms which ultimately may cause variations in the temperature gradient within the product.Therefore, several novel heating techniques that contain more efficiency have been developed as summarized in Table 1 (Neetoo & Chen, 2014).Table 1 also entails the source, plant parts used, secondary metabolites of volatile oils, and their pharmaceutical importance.

Supercritical Fluid Extraction
Supercritical fluid extraction (SFE) is a sophisticated method of isolating one component from a combination utilizing a supercritical fluid as an extracting solvent composed of gases or liquids.Their dissolving power is controlled by temperature and/or pressure.However, it is employed to isolate those chemical classes of volatile oils, which cannot be advisable to extract through the steam distillation method.The schematic representation of SFE system is shown in Figure 4.The most commonly used solvent is carbon dioxide (CO2) for several practical reasons like low critical pressure (73.8 bar) and temperature (31℃), nonflammable, noncorrosive, safe, cheap, and availability in high purity.Using the press release, it may be easily extracted from plant material (Rozzi et al., 2002).One drawback is that CO2 is nonpolar, so it cannot extract polar analytes (Pourmortazavi & Hajimirsadeghi, 2007) alone but can be used as a supercritical fluid with co-solvents such as dimethyl ether, ethane, ethylene ethanol, freons, methanol, nitrous oxide, propane, ethylene, etc.The SFE method of essential oils from various plants has been listed in Table 1.Additional benefits of this method include providing a high-quality range with advanced biological and functional properties compared to other products obtained (Cappuzzo et al., 2013) by using the hydrodistillation method.This method is environmentally friendly because the nontoxic solvents used in the extraction process leave no harmful residue.The extracting solvent can be quickly recovered from the extract because of its high volatile nature.The components with high boiling points get easily extracted at low temperatures considered suitable for thermolabile components.The principal limitation of this method is the complexity of the system, which increases the cost of the equipment.Another disadvantage is that elevated pressure requires a more considerable upfront capital expenditure.Stashenko et al., 2004;Braga et al., 2005;Lima Juiz et al., 2015;Santos et al., 2016;Saroj et al.,  Antioxidant and antimicrobial activity (Patel et al., 2006;Janet et al., 2015;Baptista et al., 2018;Dzamic et al., 2009) Eugenia caryophyllata Thunb; Myrtaceae (Buds, leaves, and stems) Clove Eugenol, eugenol acetate, trans-caryophyllene Antioxidant, antimicrobial and anti-inflammatory activity (Sohlait, 2015;Wenqiang et al., 2007;Ivanovic et al., 2011;Mahboubi and Mahboubi, 2015;Sohilait et al., 2018;Öztürk and Özbek, 2005).

Subcritical Liquid Extraction
SLE takes place when liquid reaches a pressure greater than Pc (critical pressure) but less than Tc (critical temperature).This condition is said to be in the subcritical stage.The schematic representation of the SLE system is shown in Figure 5.The solvents used to extract essential oils in this process are H2O and CO2.The fluid's subcritical condition has several advantages: decreased density, lower viscosity, and improved gas-liquid diffusivity.Because this extraction procedure is performed at a moderate working temperature, it is regarded as the best alternative strategy for thermolabile components.Compared to other traditional ways, this process takes 15 minutes to complete (Khmelinskii & Woodcock, 2020).The subcritical liquid extraction method of essential oils from various plants is listed in Table 2.The advantages of this method include their being simple, less time-consuming, cost-efficient, and environmentally friendly.
It is a powerful alternative to the essential oils extraction technique as it enables a fast essential oil isolation process (Shirsat et al., 2004).The major disadvantage of this extraction technique is that it requires high pressure to maintain the solvent in the subcritical state, which increases the operating costs.Antimicrobial, anti-inflammatory, and antioxidant activity (Reverchon et al., 1995;Zhi-linga et al., 2011;Akgu et al., 2000;Lakušić et al., 2014;Danh et al., 2012;Wells et al., 2018) Lavandula hybrida; Lamiaceae (Flowers)

Microwave-Assisted Hydrodistillation
This microwave-assisted hydrodistillation (MWHD) technique is used to heat the solvent in place of regular electric heating.This approach works by changing the polarity of water and then heating it with microwaves.Electromagnetic energy is converted into heat energy utilizing microwave energy directly generated through molecular interaction between aromatic plants (materials) and the electromagnetic field (Eskilsson & Björklund, 2000;Routray & Orsat, 2012;Thostenson & Chou, 1999).These waves may cause some structural alterations within the plant cells.The schematic representation of the MWHD system is shown in Figure 6.Heat and mass transfer occur in the same direction, i.e., from the inner cells to the outside.The factors affecting the efficiency of this technique include time, temperature, physicochemical properties of the extracted compounds, dielectric properties of the sample mixture, and solvent type.The MWHD extraction method of essential oils from various plants has been reported in the literature (Table 3).This method is used in industry, and it provides an excellent versatile tool that covers a wide range of plant materials under suitable conditions.This method shows speedy extraction performance with low solvent consumption.It is an ecofriendly method due to the less CO2 emission from the atmosphere (Lucchesi et al., 2004;Moradi et al., 2018;Ferhat et al., 2006).This method offers protection from thermolabile compounds, and the efficiency is firmly based on the dielectric constant of plant material and water, respectively (Brachet et al., 2002).There is only one disadvantage: the microwave technique can lead to changes in the stereochemistry of compounds and can convert them from one isomer to another (Fadel et al., 2011;Norfatirah et al., 2013;Jeyaratnam et al., 2016).

Ultrasound-Assisted Extraction
The UAE method permits highly selective and escalation of volatile oils to get separated from plant material.The principle behind this method is that it develops cavitation of some tiny bubbles within the solvent system due to the passage flow of ultrasound waves which usually allow a more significant percentage of a solvent system within the plant material that enhances the surface area (Garcı´a-Pe´rez, 2006).Plant raw materials are immersed in water or another solvent (such as methanol or ethanol) and subjected to ultrasound (Figure 7) (Assami &Pingret, 2012).This technique involves extracting essential oil components from leaves, seeds, and flowers (Sereshti et al., 2012).Factors that significantly affect the percentage yield and quality of the essential oils are ultrasonic frequency, duration of ultrasound treatment, immersion time, extraction temperature, the duty cycle of ultrasound, features, and size of plant materials (Sun et al., 2019).The ultrasound-assisted extraction method of essential oils from various plants has been listed in Table 4.This technique is advantageous for heat-sensitive combinations due to mean temperature and saving energy.It is a suitable method of extraction to get high valuable volatile oils.The efficiency of extraction can be enhanced using this technique which may cause disruption of plant cell walls and improved mass transfer through the formation of cavitation babble effects (Entezari, 2004).The disadvantages of this method include poor purity, low efficiency, and lengthy process (Lu et al., 2012;Mura et al., 2015).Antimicrobial, antioxidant, antiinflammatory and anxiolytic activity (Arafat et al., 2020;Mahmud et al., 2009)

Solvent-Free Microwave Extraction
Dry distillation and microwave heating energy in combination are used in this process (Chemat et al., 2003;Chemat et al., 2004).In this method, the moisture present in the plant material is used as a solvent (Lucchesi et al., 2007).Before undergoing the extraction process, the herbal materials are moistened with water for approximately 2 hrs.The moistened materials are microwaved, and a condenser is used to collect the essential oils.The thermal stress and pressure generated within the plant tissues being treated in the case of microwave heating may cause their disruption more rapidly when compared with the traditional methods.The panel embedded within the instrument controlled the temperature, pressure, and irradiation power (Figure 8).The essential oil is dried in the desiccator and stored in a dark place.This method is used to quickly isolate volatile oils from herbs, spices, and seeds, as listed in Table 5.The main advantage of this method is the single-stage isolation and concentration of essential oil (Lucchesi et al., 2004;Bayramoglu et al., 2008).Other benefits of this green technique include efficiency, selectivity, and shorter time (Boubia et al., 2009;Lopez-Avila et al., 1994).Antimicrobial, anti-vomiting, anti-asthma and anti-spasm activity Wang et al., 2006;Wannera et al., 2010;Esmaeili, 2015) Zanthoxylum bungeanum; Rutaceae (Fruit)

Microwave Hydro-diffusion and Gravity
Microwave hydro-diffusion and gravity (MHG) is one of the novel green methods of extracting volatile oils.This technology utilizes microwaves and earth gravity to harvest and extract volatile oils that hydro diffuse from the inner cell areas to the exterior of the plant material.The schematic representation of the MHG extraction system is shown in Figure 9.It is typically carried out at atmospheric pressure with no solvent added.It was designed for small-scale experimentation and processing (Vian et al., 2008).This technique is used for expeditious isolation of volatile oils, as listed in Table 6.The advantage of this method is that it is economical, requires less energy, is highly efficient and does not require any solvent/water (Lucchesi, 2005).The extraction time is in minutes compared to hydrodistillation, which takes hours (Vian et al., 2008).Antimicrobial, anti-vomiting, anti-asthma and anti-spasm activity (Wannera et al., 2010;Esmaeili, 2015;Benmoussa et al., 2018)

Ohmic Heated Water Distillation
Ohmic heated water distillation (OHWD) is a revolutionary process for isolating essential oils that use ohmic or Joules' heating (Shirsat et al., 2004), and it requires less power (per mL) (Gavahian et al., 2012).Controlling treatment homogeneity necessitates the most accurate modeling inputs.The schematic representation of the OHWD extraction system is shown in Figure 10.The heating rate is proportional to the square of the electric field strength and the conductivity of the medium.This technique is used for expeditious isolation of various volatile oils, as listed in Table 7. Ohmic heating is a highly energy-efficient and safe technology.It gives rapid and relatively homogenous heating.The quality of essential oils extracted by this method is good.The main disadvantage of ohmic heating is the high cost, the corrosion in the electrodes, and constant cleaning (Zareifard et al., 2003;Goullieux & Pain, 2005).Antioxidant, antifungal and antimicrobial activity (Tunç and Koca, 2019;Selles et al., 2020;Kaur et al., 2019) 2.8.Solar Distillation Solar energy serves in the agriculture field by saving money and reducing the environmental pollution.New technology has been developed to improve the efficiency of the distillation process by utilizing renewable energy sources such as sunlight.This method is used about the same amount of heat energy per unit weight of plant material (Garg & Prakash, 2006).A Scheffler fixed steam receiver, condenser, focus concentrator, oil separator, distillation still, and other components are used in solar distillation (Figure 11).The amount of energy available for the distillation process is determined by the sun intensity and the solar distillery's thermal and optical efficiency.It is a low-cost method for extracting essential oils from medicinal plants.Essential oils from different plant sources are extracted using a solar distillation system like eucalyptus leaves, peppermint leaves, clove buds, fennel seeds, basil, lavender, cumin, cardamom, orange, lemon, rosemary, citrus, Cymbopogon, etc. (Table 8) (Al-Hilphy et al., 2022;Radwan et al., 2020;Afzal et al., 2017;Yen and Lin, 2017).Antioxidant and antimicrobial activity (Afzal et al., 2017;Schmidt et al., 2009;Mimica-Dukić et al., 2003) Pinus (Roxburghii) α-Pinene (Afzal et al., 2017) Lavandula angustifolia; Lamiaceae (Flowers) Lavender β-Pinene, β-phellandrene, borneol, camphor, linalool, terpineol, linalyl acetate, terpineol-4-ol, cymene and 1,8-cineole Antimicrobial, anti-inflammatory, and antioxidant activity (Radwan et al., 2020;Lakušić et al., 2014;Danh et al., 2012;Wells et al., 2018) Cymbopogon citratus (DC) Stapf..; Poaceae (Leaves) Lemon grass, oil grass Geranial (citral a), neral (citral b), myrcene Antioxidant, antileishmanial, antispasmodic, analgesic, antiinflammatory, anti-pyretic, diuretic, anticonvulsant, and sedative activity (Yen and Lin, 2017, Hanaa et al., 2012, Santin et al., 2009, Blanco et al., 2009) Syzygium  (Al-Hilphy et al., 2022, Alam et al., 2021;Noumi et al., 2018) 3. CONCLUSION Various essential oils can be utilized as natural food additives and emanate from various sources.Many researchers have proven the effectiveness of innovations in traditional methods to extract volatile oils from various plant materials.The creation, improvement, and scale-up of these advances from the laboratory to pilot and industrial-scale all need modeling the experimental data.Reverchon et al., 1999 andSovová, 2005 provided some of the most successful ways.The objective of research scientists in the twenty-first century is to stimulate advances in traditional essential oil extraction processes that increase the yield and quality of volatile oils for aromatherapy and pharmaceutical uses.Essential oils' mechanisms of action must be understood to determine their efficacy as phytotherapeutic agents.On the other hand, innovative approaches, including microwave, ultrasound, ohmic heat, and solar energy-assisted extraction, can be coupled with conventional extraction methods for efficacious production of essential oils.Further research is necessary to find essential oils with novel bioactivities or functionalities.

Figure 1 .
Figure 1.Comparison of conventional and non-conventional extraction methods of essential oils.

Figure 2 .
Figure 2. Non-conventional and conventional extraction methods of essential oils.

Figure 3 .
Figure 3. Advantages of innovative extraction techniques of essential oils.

Figure 9 .
Figure 9. Schematic representation of the microwave hydro-diffusion and gravity (MHG) extraction system.

Table 1 .
Supercritical fluid extraction (SFE) of essential oils from various sources.

Table 2 .
The subcritical liquid extraction (SLE) of essential oils from various sources.

Table 3 .
The microwave-assisted hydrodistillation (MWHD) based extraction of essential oils from various sources.

Table 4 .
Ultrasound-assisted extraction (UAE) of essential oils from various plant sources.

Table 5 .
Solvent-free microwave extraction (SFME) of essential oils from various sources.

Table 6 .
Microwave hydro-diffusion and gravity (MHG) method of extraction of essential oils from various sources.

Table 7 .
Ohmic heated water distillation (OHWD) method of extraction of essential oils from various sources.

Table 8 .
Solar distillation (SD) method of extraction of essential oils from various sources.