Open access peer-reviewed chapter

Extractions Methods and Biological Applications of Essential Oils

Written By

Sonu Kumar Mahawer, Himani, Sushila Arya, Ravendra Kumar and Om Prakash

Submitted: 16 January 2022 Reviewed: 31 January 2022 Published: 27 May 2022

DOI: 10.5772/intechopen.102955

From the Edited Volume

Essential Oils - Advances in Extractions and Biological Applications

Edited by Mozaniel Santana de Oliveira and Eloisa Helena de Aguiar Andrade

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Abstract

Plants produce secondary metabolites for defense and based on the biosynthetic pathway, these chemical compounds are broadly divided into three categories namely nitrogen-containing compounds, phenolic compounds, and terpenes. Essential oils and other such compounds are known for their biological activities. The extraction of essential oils is a challenging aspect for researchers in the field of natural products. Hydrodistillation is a time-consuming and very tedious method. Nowadays, accelerated solvent extraction, supercritical fluid extraction, subcritical water extraction, microwave hydrodiffusion are promising alternatives for conventional methods with several advantages. Essential oils have several biological activities in the field of pharmacological, ethnopharmacological, pesticidal, etc.

Keywords

  • essential oils
  • secondary metabolites
  • hydrodistillation
  • biological activities
  • accelerated solvent extraction

1. Introduction

Essential oils are the highly concentrated, fragrant oil of plant origin that are obtained by steam distillation, dry distillation, hydrodiffusion, or other suitable mechanical methods without heating. These are also denoted as plant “essences” in aromatherapy literature and the method of extraction is critical to categorizing an aromatic constituent as an essential oil [1]. Chemically, these are the mixture of several terpenes or terpenoids which are the polymers of isoprene units. Essential oils are synthesized in the cytoplasm and are usually present in the form of minute droplets between cells. These are insoluble in water, lipophilic and soluble in organic solvents, volatile and aromatic in nature. Almost all plant parts, such as leaves barks, flowers, rhizomes or roots, peels, seeds buds, are reported as the source of essential oils, and several techniques are also known to obtain the essentials from different plant parts [2].

The plant families encompassing species known for most economically significant essential oils are not limited to one taxonomic group only but these are found in all plant classes—gymnosperms such as Pinaceae and Cupressaceae families; angiosperms such as Magnoliopsida, Liliopsida, and Rosopsida. The most important plant families among dicots are Apiaceae, Compositae, Germiniaceae, Illiciaceae, Lamiaceae, Lauraceae, Myristicaceae, Myrtaceae, Oleaceae, Rosaceae, Santalaceae, etc. whereas, among monocots Zingiberaceae, Poaceae and Acoraceae are the important families [3].

The variations in chemical properties of essential oils vary with their chemical composition and their stereochemical structures, the chemical composition may vary in respect to types of chemical components and their stereochemical nature with the extraction methods used along with the plant type, age, climatic conditions, growth stage, altitude, etc. [4]. Essential oils are the plant secondary metabolites synthesized in the plant cell via metabolic pathways derived from the primary metabolic pathways the synthesis of these metabolites in the plants is often under stress (abiotic and/or biotic) conditions, primarily intervened by different signaling molecules [5] and have been reported for several biological activities which depend upon the chemical composition and stereochemical nature of constituent compounds in essential oils.

In this chapter, we are focusing on the basic information of essential oils, their extraction methods available, and their biological activities in the pharmacological, antimicrobial, and crop protection in agents in agricultural fields.

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2. Essential oils and their chemical constituents

Essential oils are complex mixtures composed of terpenoids and nonterpenoid volatile hydrocarbons. The basic building unit of essential oils is called an isoprene unit (C5H8; 2-methyl-1,3-butadiene) and these are arranged following the isoprene rule in a head-to-tail fashion. There are some functional groups also attached which contribute to the biological activities of the essential oils. These groups are mainly alcohols, aldehydes, esters, ethers, ketones, and phenols [6]. Among terpenes, there are subclasses as monoterpenes, sesquiterpenes, diterpenes in the essential oils. Mono terpenes are the results of the combination of two isoprene units, similarly, sesquiterpenes have resulted from three and diterpenes are from four isoprene units. Alcohols, ketones, and carboxylic acids are the functional groups found in the oxygenated derivatives of terpenes, which are jointly known as terpenoids. Apart from terpenes, alcohols, ketones, esters are also found in essential oils as a single component or in combination with terpenes. The basic classification of terpenes is given in Table 1.

S. No.TerpenesNumber of isoprene unit(s)Number of carbon atomsExample(s)
1Monoterpenes210pinene, myrcene, limonene, thujene
2Sesquiterpenes315bisabolene, zingiberene, germacrene, caryophyllene
3Diterpenes420retinol, taxol, and phytol
4Triterpenes630squalene, hopane
5Tetraterpenes840carotene, lycopene, and bixin
6polyterpenes>8>40natural rubber

Table 1.

Classification of terpenes.

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3. Extraction methods

Various parts of aromatic plants can be extracted to obtain the essential oils. Choice of extraction method depends upon the characteristics and components needed for the purposes. In some circumstances, improper and unsuitable extraction techniques can destruct and alter the biological activity of chemical compounds present in essential oils, for example, loss of active compounds, staining, off flavor, and in some cases physical changes in essential oils. For effective extraction with high efficiency, low cost and less tedious methods are required to obtain the high-quality essential oils with high production yield. There are numerous methods that are available for the extraction of essential oils from different parts of plants.

These methods can be grouped into two categories; conventional methods and advanced methods.

3.1 Convention extraction methods

3.1.1 Hydrodistillation

Hydrodistillation is the oldest and most basic oils extraction method which was discovered by Avicenna. The process of extracting essential plant oils by hydrodistillation begins with the plant materials being immersed directly in water inside the vessel and then boiling the entire combination. The devices consist of a heating source, vessel (Alembic), a condenser to convert vapor into liquid, and a decanter to collect the condensate and to separate essential oils with water [7]. This extraction process is a one-of-a-kind way to extract plant materials, such as wood or flowers, and it is commonly employed for extractions requiring hydrophobic natural plant material with a high boiling point. Because the oils are surrounded by water, this process allows essential oils to be extracted at a controlled temperature without overheating. The extraction principle is based on isotropic distillation. Water or other solvents, as well as oil molecules, are present under atmospheric pressure and during the extraction process (heating). The capacity to isolate plant components below 100°C is the fundamental benefit of this extraction approach [8].

3.1.2 Steam distillation

Steam distillation is a form of distillation or separation technique for temperature-sensitive compounds that are insoluble in water and may break down at their boiling points, such as oils, resins, and hydrocarbons. The basic principle of steam distillation is that it allows a mixture of compounds to be distilled at a temperature that is significantly lower than the individual constituent’s boiling point. These compounds, on the other hand, are volatilized at a temperature close to 100°C under atmospheric pressure in the presence of steam or boiling water, by heating plant materials with steam generated by a steam generator. Heat is the primary determinant of how well plant material structures degrade and rupture, releasing aromatic components or essential oils in vapor form [9]. The steam condenses into water when it cools. The film on the water surface (distillate/hydrosol) is then decanted from the top to separate the essential oil from it. At its most basic level, steam distillation has the advantage of being a reasonably inexpensive process to operate, and the qualities of the oils produced by this approach are well known [10]. Masango designed a novel steam distillation extraction technique to enhance separated essential oil yields while reducing wastewater production during the extraction process. A packed sheet of plant samples is put above the steam source in this setup. Boiling water is not allowed to combine with the botanical components, and only steam is allowed to travel through the plants. As a result, less steam is required in the process, and the amount of water in the distillate can be lowered [11]. In another study, by adopting the steam distillation extraction procedure, Yildirim et al. reported a component 2,2- diphenyl-1-picryl hydrazyl (DPPH) that was utilized to evaluate the antioxidant activities of essential oils [12]. It was shown to have a higher yield of antioxidant components than hydrodistillation-extracted oils.

3.1.3 Cold expression

In the cold expression method, oil is expeller-pressed at low temperatures and pressure. This method ensures that the resultant oil is 100% pure and keeps all of the plant characteristics. It is a mechanical extraction method in which heat is lowered and minimized throughout the raw material batching process. This process is mostly used to extract essential oils from plants, flowers, seeds, and citrus oils, such as lemon and tangerine [13]. In this process, scrubbing is used to remove the outer layer of the plants that contain the oil. The entire plant is then crushed to extract the substance from the pulp and the essential oil from the vesicles. By centrifugation, the essential oil rises to the surface of the substance and is separated from it [14]. The oils derived in this manner have a short shelf life. According to reports, oil produced in this manner contains more of the fruit aromatic character than oil made any other way.

3.1.4 Destructive distillation

Only birch (Betula lenta or Betula alba) and cade trees (Juniperus oxycedrus) are used to extract using this approach. After enduring a destructive process under tremendous heat, the hardest components of these woods (e.g., barks, boughs, and roots) are subjected to dry distillation through tar. After condensation, decantation, and separation, a characteristic leathery and empyreumatic oil is formed [15].

3.1.5 Hydrodiffusion

Contrasting to steam distillation, the steam in this technique is fed from the top to the bottom of the alembic. Through a perforated tray, the vapor mixture, including Eos, is directly condensed underneath the plant support. Separating EOs is done in the same way as previous distillation processes. In comparison to steam distillation, this approach can minimize steam usage and distillation time while also providing a higher yield [15].

3.2 Advanced extraction methods

Considering the concepts of economically sound, eco-friendly, high efficiency, and quality production, the efforts were made with respect to the extraction techniques for essential oils from plants.

3.2.1 Microwave-assisted extraction (MAE)

A microwave is a contactless heat source that can attain more effective and selective heating. Microwaves can complete the distillation in minutes instead of several hours in the conventional distillation method. In this method, plant materials are subjected to a microwave reactor with or without organic solvents or water under different levels of microwave treatments, according to the required protocols [15]. Nowadays, this technique is of high attention among researchers because of its unique heating mechanism (based on friction), cost-effectiveness, high efficacy under normal conditions, higher extraction yield, less extraction times, and high selectivity. Several studies have been reported on the extraction of essential oils using MAE. Recently in 2020, Drinić et al. performed the microwave-assisted hydrodistillation (MAHD) of Origanum vulgare L. spp. hirtum essential oil and compared with conventional hydrodistillation (HD) using Clevenger-type apparatus [16]. For MAHD, they used an apparatus consisting of a microwave oven connected with a Clevenger-type apparatus. The water to plant ratio was kept similar for both HD and MAHD, 20:1 (w/w). MAHD was accomplished at three different power levels (180, 360, and 600 W) till no more essential oil was obtained. MAHD was found to be a method with several pros over conventional HD. MAHD was found to have less extraction time (24–45 min) as compared to HD (136 min), higher yields of essential oil (2.55–7.10%) as compared to HD (5.81%), higher oxygenated compounds percentage (78.89–85.15%) as compared to (76.82), and it was proven to be a more eco-friendly method (in terms of electrical consumption (0.135–0.240 kW h) as compared to HD (1.360 kW h). Several authors have been published regarding the optimization of extraction procedure using MAE techniques, for instance, successive microwave-assisted extraction optimization for essential oil from lemon peels waste [17] and microwave extraction of essential oils from Eucalyptus globulus leaves [18], and many more. A schematic diagram of the microwave-assisted extraction setup is depicted in Figure 1.

Figure 1.

Microwave-assisted extraction setup.

3.2.2 Ultrasound-assisted extraction

Similar to MAE, ultrasound-assisted extraction has also been developed to enhance the efficacy along with the reduction in extraction time. The collapse of cavitation bubbles generated through ultrasonication mass transfer and release rate of essential oils get increased by the breakdown of cavitation bubble generated and this cavitation effect is largely depending upon different parameters, such as frequency and intensity of ultrasound, incubation time, temperature in UAE, there are less chances of thermal breakdown, and quality and flavor remain better of essential oils [19, 20].

In a study, ultrasound-assisted hydrodistillation was performed to increase the yield of essential oil from cinnamon bark [21]. They optimized several parameters, and the method developed was compared with conventional hydrodistillation. They found an enhanced yield of essential oils along with a significant reduction in extraction time. Moreover, the scrutiny of electricity utilization and CO2 production demonstrates the eco-friendly and economically soundness of ultrasound-assisted hydrodistillation procedure over conventional hydrodistillation. A schematic diagram of the ultrasound-assisted extraction setup is shown in Figure 2.

Figure 2.

Schematic diagram of ultrasound-assisted extraction method of essential oils from plant materials.

3.2.3 Supercritical fluid extraction (SFE)

When temperature and pressure are increased over critical points for a given liquid or gas, a supercritical fluid (SF) occurs. The boundary between liquid and gas vanishes in the supercritical zone, and a homogenous fluid arises. Supercritical fluids have a diffusivity and density that distinguishes them from liquids and gases. In contrast to liquids, the density of SFs varies when pressure and temperature values vary, hence a little rise in pressure can result in a massive increase in fluid density, followed by a change in the SF’s solvating power. This phenomenon allows for the extraction of specific components from a multicomponent mixture. As a result, supercritical fluid extraction’s key benefit is selectivity. The use of this technique may help in the extraction of natural products which have a chance to be degraded at high temperatures. Along with high extraction yield and less extraction time required, this method also allows to recover the solvent used because of the SF’s volatile nature, which makes it an economic and environmentally sound extraction method for essential oils and other natural products [22]. Presently, more than 90% of SFE activities are carried out by using CO2, for a variety of uses. CO2 is abstemiously nonflammable, nonexplosive, nontoxic, accessible at cheap cost and high purity, and readily removed from extracts, in addition to having a relatively low critical temperature (32°C) and pressure (7.4 MPa). CO2 also has low surface tension and viscosity but has a diffusivity that is two or three times that of other fluids [23]. CO2 has a polarity similar to pentane in the supercritical zone, making it appropriate for lipophilic compounds extraction. CO2’s fundamental flaw is that it lacks the polarity needed to remove polar compounds [24]. Regarding practical application, in a study, the supercritical CO2 extraction was optimized for the extraction of flower essential oil of the tea (Camellia sinensis L.) plants. As per the results showed, the optimum conditions were observed as—pressure of 30 MPa, temperature of 50°C, static time of 10 min, and dynamic time of 90 min for successful extraction of essential oils from the flowers of the tea plant in the sufficient amount [25]. A schematic presentation of the supercritical fluid extraction technique is shown in Figure 3.

Figure 3.

Schematic diagram of supercritical fluid extraction.

3.2.4 Subcritical water extraction

Subcritical water is defined as water with a temperature above boiling point to a critical point (100–374°C) and a pressure high enough to keep the liquid condition. A phase diagram of water is shown in Figure 4. At ambient temperature and pressure, the dielectric constant of water remains highest among all the nonmetallic liquids, which is reduced significantly in the range of organic solvents, such as acetonitrile, methanol, ethanol, and acetone, and water acts as organic solvent at this temperature and pressure conditions and the plant compounds can be extracted efficiently using subcritical water (Figure 5). The extraction by using subcritical water occurs in the following steps; (1) rapid entry into matrix pores, (2) desorption of solutes from active sites of the matrix, (3) dissolution of solutes in the aqueous fluid, (4) diffusion of solutes through static aqueous fluid in porous materials, (5) diffusion of solutes through the layer of stagnant fluid outside particles and finally, (6) elution of solutes by the flowing of the bulk of aqueous fluid [26]. Currently, SCWE is getting importance in the extraction of essential oils also from different plant parts. For example, SCWE is used to extract essential oils from Alpinia malaccensis leaves. The optimum conditions for extraction were found as; the temperature of 156°C, extraction time of 25 minutes. They also reported the interaction of temperature and reaction time parameters using regression analysis. They also conducted kinetics modeling and reported that second-order kinetics was followed by SCWE [27].

Figure 4.

Phase diagram of water.

Figure 5.

Simple laboratory setup for SCWE.

3.2.5 Turbo distillation

The turbo distillation method is similar to conventional water distillation; however, in turbo distillation, the mixture is agitated constantly at a suitable speed using a stainless steel stirrer (Figure 6). This approach works well with coarse raw materials and difficult-to-extract substances (spices, woods). When compared to aqueous distillation, turbo distillation reduces distillation durations and energy consumption while also preventing volatile components from degradation. In actuality, it is a type of water distillation-based green extraction [28]. Essential oils can be extracted from difficult-to-extract parts from plants and others using the turbo distillation extraction method. In a study conducted by Mnayer et al., the essential oils and flavonoids were extracted simultaneously using turbo extraction-distillation [29].

Figure 6.

Turbo hydrodistillation apparatus.

3.2.6 Simultaneous distillation extraction

Likens and Nickerson established simultaneous distillation–extraction (SDE) in 1964, and it has been effectively used to extract essential oils, aromatic compounds, and other volatile products from a variety of matrices. Steam distillation, in combination with continuous extraction with solvent of the mixture of solvents, becomes superior as compared to conventional solvent extraction or extraction with a mixture of solvents. This is a single-step isolation-concentration technique which reduces extraction time significantly, along with the reduction in the solvents used because of continuous recycling and there is no need for clean up after this approach as the extracts obtained in this technique are devoid of no-volatile components, such as cuticular waxes and chlorophylls [30, 31]. Ribeiro and his coworkers in 2021 performed SDE for the extraction of essential oils from Rosmarinus officinalis L [32]. In this study, they assessed the effect of the solvent nature and the optimum time required for extraction. Pentane solvent was found to be best for the performance of SDE for 1 h extraction time.

3.2.7 Pulsed electric field-assisted extraction (PEFAE)

Pulsed electric field (PEF) reduces negative impacts of traditional heating approaches and is a capable substitute to other extraction methods, such as boiling and ultrasound-assisted or microwave-assisted extraction. Moderate to the high strength of the electric field is used on. The PEFAE technique uses moderate to high electric field strength (EFS) ranging from 100 to 300 V/cm and 20 to 80 kV/cm in batch mode and continuous mode extraction, simultaneously. In the electroporation or electropermeabilization (mechanism involved in PEFAE) external electric force is used to augment the cell membrane permeability [33]. The material of interest is kept in between the electrodes and a high-strength electric field in terms of voltage which punctures the cell membrane by the formation of hydrophilic pores and the membrane, its physical functionality and the extraction takes place [34]. PEFAE is of two types viz. batch PEFAE and continuous PEFAE. In Batch PEFAE, the simple is firstly treated with a little solvent between two electrodes and then treated samples removed from the pretreatment unit and stirred at different intensities with the help of a magnetic stirrer to check the solvent evaporation. Apart from the promising results, this process increases operational time because of the low capacity of the system. Therefore, continuous PEFAE is mostly in use at the place of the batch process. In continuous PEFAE, the mixture of solvents is pumped into the treatment chamber by a peristaltic pump at a constant fluid velocity [35]. This technique is getting popularized for the intensification of essential oil extraction [36]. In a study, the PEF was explored for the intensification of essential oil extraction from Marrubium vulgare. In this study, PEF pretreatment was done for the purpose of improvement in the permeabilization of the biological membranes. The results revealed a significant enhancement in the extraction rate of essential oils.

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4. Biological applications of essential oils

4.1 Pharmacological applications

4.1.1 Anticancer mechanism of essential oils

In most cancer chemotherapies, highly cytotoxic drugs are used that target proliferating cell populations. The nondiscriminatory nature of these drugs leads to severe side effects in normal cells. Natural essential oils and their constituents play a significant role in cancer prevention and treatment. Various mechanisms are responsible for the chemopreventive properties of essential oils, such as antioxidant, antiproliferative, and antimutagenic, enhancing detoxification and synergistic action of their constituents. There is a direct relationship between the production of reactive oxygen species to the origin of oxidation and inflammation that can lead to cancer. Mitochondrial DNA damage can result from oxidative stress which leads to an increase in the mutation rate within cells and thus promoting oncogenic transformation. Besides this, reactive oxygen species (ROs) specifically activate signaling pathways and promote tumor development through the regulation of cellular proliferation, angiogenesis, and metastasis [37]. EOs components react with ROs and form reactive phenoxy radicals which can then react with further ROs to prevent further [38]. EOs also induces the expression of antioxidant enzymes, such as catalase, superoxide dismutase, glutathione peroxidase, and glutathione, which leads to an increase in intracellular antioxidant activity, subsequently leading to a significant reduction in tumor mass (Figure 7). Several studies have shown the anticancer activity of EOs against various cancers, some are summarized in Table 2.

Figure 7.

Antioxidant mechanism responsible for chemo preventive mechanism.

CancerSpeciesMajor essential oil constituentsCancer cell lines usedReference
LungMorinda citrifoliaL-scopoletin, nordamnacanthal, β-morindone, α-copaene, 9-H-pyrido[3,4-b]indole, β-thujeneA549[39]
BreastBoswelia sacraα-pinene, α-thujene, myrcene, boswelic acidT47D, MCF7, MDA-MB-231[40]
ColonCitrus limettioidesd-limonene, triacontane, sabinene, β-myrceneSW480[41]
OvaryNepeta ucrainicagermacrene D, bicyclogermacrene, β-bourbonene, β-elemene, spathulenol, cubenolA2780[42]
LiverThymus citriodorusborneol, thymol, 3, 7-dimethyl-1, 6-octadiene-3-ol, 1-methyl-4-[alpha-hydroxy-isopropyl] cyclohexene, camphorHepG2[43]
Uterus and CervixCasearia sylvestrisα-zingiberene, E-caryophyllene,α-acoradiene, α-muurolol, viridifloreneSiha[44]
OralSolanium spirale Roxb.E-phytol, n-hexadecanoic acid, β-selinene, α-selineneKB[45]
PancreasAngelica archangelicaβ-phellandrene, α-pinene, bicyclogermacrene, sabinene, bicycloelemene,PANC-1[46]
LeukemiaMalus domesticaeucalyptol, phytol, α-farnesene, pentacosaneTHP-1[47]
SkinSchefflera heptaphyllaβ-pinene, 4-methyl-1-(1-methylethyl)-3-cyclohexene, 3,7-dimethyl-1,6-octadien, β-caryophylleneA375[48]

Table 2.

Anticancer activity of EOs against various cancer cell lines.

4.1.2 Essential oil as an antioxidant agent

Free radicals and other reactive oxygen species cause oxidation of biomolecules which ultimately leads to molecular alterations, including chronic disorders associated with the aging process, arteriosclerosis and cancer [49], Alzheimer’s disease [50], Parkinson’s disease, diabetes, and asthma. Essential oils also exhibit remarkable antioxidant activity/free radical scavenging activity which has often been confirmed by physicochemical methods (Table 3). The essential oils of some medicinally important plants, such as basil, cinnamon, clove, nutmeg, oregano, and thyme, have proven radical-scavenging and antioxidant properties [63]. The antioxidant properties are mainly dependent on the chemical constituents, such as in Thymus the antioxidant activity is mainly attributed to the presence of thymol and carvacrol content (36.5 and 29.8%).

PlantFamilyMajor essential oil constituentsAntioxidant assayReference
Limnophilla indicaPlantaginaceaeepi-cyclocolorenone, α-gurjunene, 5-hydroxy-cis-calamenene, α-gurjunene, β-caryophyllene,DPPH radical scavenging, Reducing power, Metal chelating of Fe2+[51]
Zanthoxylum armatumRutaceaeα-pinene, germacrene-D, E-caryophyllene, α-cadinol, 2-undecanoneDPPH radical scavenging, Reducing power, Metal chelating of Fe2+[52]
Premna mucronataLamiaceaeethyl hexanol, 1-octen-3-ol, linalool, methyl salicylate, E-caryophylleneDPPH radical scavenging, Reducing power, Metal chelating of Fe2+[53]
Salvia reflexaLamiaceaepalmitic acid, phytol, E-caryophyllene caryophyllene oxideDPPH radical scavenging, Reducing power, Metal chelating of Fe2+[54]
Coleus barbatusLamiaceaebornyl acetate, n-decanal, sesquisabinene, β-bisabolene, δ-cadineneDPPH radical scavenging, Reducing power, Metal chelating of Fe2+, H2O2 radical scavenging, Nitric oxide radical scavenging, Superoxide radical scavenging[55]
Globba sessilifloraZingiberaceaemyrcene, β-caryophyllene, selin-11-en-4α-ol, β-longipinene, manool, germacrene D and β-eudesmolDPPH radical scavenging, Reducing power, Metal chelating of Fe2+[56]
Caryopteris foetidaverbenaceaeδ-cadinene, farnesene, β-caryophyllene, γ-cadinene,spathulenol, τ-muurololDPPH radical scavenging, Reducing power, Metal chelating of Fe2+[57]
Nepeta catariaLamiaceaecis-nepetalactone, bicyclo [3.1.0] hexane-2-undecanoic acid, methyl ester, (cis-,trans) nepetalactone,DPPH radical scavenging, Reducing power, Metal chelating of Fe2+[58]
Cotinus coggygriaAnacardiaceamyrcene, α-pinene, α-terpineol, cymene, sabineneDPPH radical scavenging, Metal chelating of Fe2+[59]
Foeniculum vulgareApiaceaelimonene, estragole, trans-anethole, fenchone, eucalyptolDPPH free radical scavenging, ferric reducing power assay, thiobarbituric acid reactive species assay, ferrous ion-chelating[60]
Origanum vulgareLamiaceaecarvacrol, thymol, p-cymene, caryophyllene, 3-careneDPPH radical scavenging, Reducing power[61]
Ocimum basilicumLamiaceaelinalool, methyl chavicol, eucalyptol, eugenol, trans-α-bergamoteneDPPH radical scavenging, β-carotene bleaching assay, TBHQ inhibition[62]

Table 3.

Reported antioxidant activities of essential oils of different plant families.

4.1.3 Essential oil as an antidiabetic agent

Hyperglycemia is a condition of diabetes that arises as a result of the inability to either produce insulin or use it to regulate normal glucose levels in the blood. Inhibition of α-glucosidase and α-amylase is an important factor to control postprandial hyperglycemia in the management of type 2 diabetes mellitus as both enzymes are involved in the digestion of carbohydrates. α-amylase is involved in the break down of long-chain carbohydrates into disaccharides while α-glucosidase breaks down starch and disaccharides to glucose or monosaccharides. Thus, by inhibiting the enzyme, carbohydrate breakdown can be delayed, and ultimately absorption of glucose in the bloodstream is reduced [64]. Essential oils bind to the active site of the enzyme (α-amylase or α-glucosidase) and act as an inhibitor to form an enzyme-inhibitor complex thus inhibiting the enzyme activities (Figure 8).

Figure 8.

Mechanism of enzyme (α-amylase and α-glucosidase) inhibition by essential oil [65].

Several essential oils and their constituents have been analyzed for their antidiabetic potential such as essential of plant Syzygium aromaticum, Cuminum cyminum [66], Nepeta hindostana [57], Oliveria decumbens, Thymus kotschyanus, Trachyspermum ammi, Zataria multiflora [67], and Carthamus tinctorius [68].

4.2 Antimicrobial application

EOs and their constituents play a vital role in possessing antimicrobial activities. The antimicrobial activity of essential oil mainly depends on three characteristics—the nature of EO (hydrophilic or hydrophobic), its chemical constituents, and the targeted organism [69, 70]. Due to their hydrophobic nature, EOs passes across the cell wall and cytoplasmic membrane and disrupt the cell wall structure and make them more permeable. The membrane permeability leads to leakage of macromolecules and other cellular materials leading to cell death [71]. In bacteria, the permeabilization of the membranes is associated with loss of ions and reduction of membrane potential, the collapse of the proton pump, and depletion of the ATP pool. EOs can also damage lipids and proteins by coagulating the cytoplasm. Figure 9 represents different kinds of the mechanism of action of essential oils on microorganisms. The antimicrobial activity of EO’s is mostly due to the presence of phenols, aldehydes, and alcohols. Terpenoids are one of the major constituents present in EOs and have oxygen atoms or methyl groups, which are localized or removed from specific enzymes by which they show greater activities. Generally, it has been observed that EOs are more active in gram-positive bacteria than gram-negative bacteria due to the presence of peptidoglycan layer which lies outside the outer membrane. Whereas, in gram-negative bacteria, the outer membrane is composed of a double layer of phospholipids and it is linked with the inner membrane by lipopolysaccharides thus hydrophobic macromolecules, such as essential oils constituents are unable to penetrate the membrane which is responsible for the resistance of the gram-negative bacteria to EOs. Aflatoxins, which are toxic secondary metabolites produced by common fungi, such as Aspergillus flavus and A. parasiticus, cause contamination of many food products. These aflatoxins are teratogenic, carcinogenic, and mutagenic. Some essential oils not only inhibit the growth of such fungi but can also stop the production of aflatoxins. EOs are effective against a wide range of plants and human pathogenic bacteria, fungi, and viruses by using different assays, as summarized inTable 4.

Figure 9.

Mechanism of essential oil action on micro-organisms.

Essential oilActionTarget microorganismReference
Cinnamomum osmophloeumAntibacterialEscherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Salmonella sp, Vibrio parahemolyticus[72]
Mentha piperita, Ocimum basilicum LAntibacterialE. coli O157:H7 and Salmonella typhimurium[73]
Thymus vulgaris, Pimpinella anisum seedsAntibacterialS. aureus, Bacillus cereus, E. coli, Proteus vulgaris, Proteus mirabilis, Salmonella typhi, S. typhimurium, K. pneumoniae and P. aeruginosa[74]
Allium sativum, Artemisia Judaica, Satureja hortensis, Rosmarinus officinalis Cedrus libani, Chenopodium ambrosioidesAntifungalAspergillus niger, Aspergillus parasiticus, Aspergillus flavus, Aspergillus fumigatus,[75]
R. officinalis, Foeniculum vulgare, Artemisia Judaica, Artemisia absinthium, Artemisia biennisAntifungalBotrytis cinerea; Botrytis fabae[75]
Artemisia judaica, A. absinthium, A. biennisAntifungalPythium debaryanum, Trichophyton rubrum; T. mentagrophytes; T. roseum[76]
A. sativum, Artemisia judaica, A. absinthium, A. biennisAntifungalPenicillium cyclopium; Fusarium oxysporum;[76, 77, 78]
Citrus essential oilAntiviralSARS-CoV2[79]
Illicium verumAntiviralHerpes simplex virus[80]
Allium cepa, A. sativum, Cuminum cyminum, Corriandrum sativum, Petroselinum sativum, O. basilicumAntiviralHerpes simplex virus[81]

Table 4.

Antimicrobial activities of essential oils.

4.3 Pesticidal applications of essential oils

Pesticides include a wide range of compounds with very different actions (such as herbicides, insecticides, nematicides, rodenticides, avicides, algicides, fungicides, bactericides, and others) [82]. Due to the high toxicity, environmental pollution, high cost, and many more disadvantages of chemical pesticides, researches are intended toward finding novel solutions with lower toxicity, fewer damaging behavior toward the environment, and a better specificity of action. In this regard, a number of botanicals have historically been used for the control of storage pests, particularly in the Mediterranean region and Southern Asia; however, the importance of essential oils arose in the 1990s following the discovery of their fumigant and contact insecticidal activities against a wide range of pests [83].

Essential oil plays a significant role in the plant’s defense against bacteria, viruses, insects, fungi, and herbs. Essential oils are a complex and distinctive mixture of compounds that can be considered for next-generation pesticides. In the case of insecticidal actions, some oils appear to interact with the neuromodulator octopamine, while others appear to interfere with GABA-gated chloride channels, indicating that they have a neurotoxic mechanism of action. With the evidence of the potential of essential oils in pest control, these are considered as new approaches in pest control viz. essential oil-based pesticides [84]. There are several applications of essential oils in plant protection, such as insecticidal, herbicidal, nematicidal, and fungicidal. Some such recent studies have been enlisted in Table 5.

S. No.Botanical source of essential oil (species)Pesticidal Action asActivity againstReference(s)
1.Limnophila indica, Cotinus coggygria and Hedychium spicatumAntifeedantlarvae of Spilosoma obliqua[51, 59, 85]
2.Coleus barbatusAntifeedantthird instar larvae of Spilosoma oblique[55]
3.Cinnamomum camphoraInsecticidal and insect repellentAphis gossypii Glover[86]
4.L. indica, C. coggygria and Hedychium spicatumHerbicideRaphanus raphanistrum sub sp. sativus[59, 85, 87]
5.C. barbatusHerbicideR. raphanistrum[55]
6.Cinnamomum zeylanicumAcaricideDermatophagoides spp. and Tyrophagus putrescentiae mites[88]
7.Ocimum L.FungicideRhizoctonia solani[89]
8.Alpinia allughas and Alpinia malaccensisFungicideColletotricum falcatum, Rhizoctonia solani, Sclerotinia sclerotium and Sclerotium rolfsii[90, 91]

Table 5.

Pesticidal applications of essential oils from different plant species.

4.4 Other biological activities of EOs

Several pharmaceutical and biological activities, such as antibacterial, antifungal, anticancer, antiviral, antidiabetic, antimutagenic, antiprotozoal, anti-inflammatory, antipyretic, analgesic, hepatoprotective, antidiarrheal, antihyperlipidemic, diuretic, neuroprotective, and pesticidal activities, have been reported in various medicinal and aromatic plants bearing essential oil (Figure 10) [75]. Essential oils of different plant families of angiospermic plants possess various therapeutic qualities like medicinal and antimicrobial properties (constipation, dysentery, malaria, measles, stomach pain, yellow fever, and dental care).

Figure 10.

Various biological activities of essential oils.

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5. Conclusion

Essential oils are the important secondary metabolites of plants and are found in almost all parts of the plants. There are several extraction techniques are available. Conventional methods, such as hydrodistillation, steam distillation, cold expression, have several disadvantages. To overcome such cons, researchers have been developed several advanced extraction techniques for essential oils viz. microwave-assisted extraction, supercritical fluid extraction, ultrasound-assisted extraction, subcritical water extraction, etc. These advanced methods also have some cons and there is a need to research for the cheap, easy, eco-friendly methods to be developed. In this chapter, various biological applications, such as pharmacological, antimicrobial, pesticidal activities, have been discussed. There are several aromatic plant species that are remained unexploited for their essential oils and their potential in biological application. There is a need to explore newer species of aromatic plants in this regard and further research is needed in the future in respect to the extraction techniques and biological application of essential oils.

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Written By

Sonu Kumar Mahawer, Himani, Sushila Arya, Ravendra Kumar and Om Prakash

Submitted: 16 January 2022 Reviewed: 31 January 2022 Published: 27 May 2022