Extraction, Characterisation, Biological Properties and Applications of Essential Oils: A Review

Nikita Raghuvanshi^a,b, Bhanushree Gupta^a

^aCenter for Basic Sciences, Pt. Ravishankar Shukla University, Raipur, India-492010

^bSchool of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur, India-492010

Abstract

In recent decades, essential oils have emerged as natural supplements to synthetic substances in medicine, agriculture, food industries etc. The modern techniques for extraction of essential oils have significantly reduced the time consumption and increased the yield in comparison to the conventional techniques that have been in use for so long. Advanced characterisation techniques like Gas chromatography (GC), Liquid Chromatography (LC), Mass Spectrometry (MS) etc., provide high accuracy in characterisation depending upon the nature of essential oils or other major phytoconstituents. Studies have shown essential oils to possess biologically significant activities like antibacterial, antifungal, anti-inflammatory, antioxidant, antihistamine, anticholinesterase, anti-cancer, antiaging etc. These activities of essential oils have made them eligible for their application in food preservation, medicines, industries, agriculture etc. Thus, the traditional knowledge of plants and extraction of essential oils from their different parts of significance can contribute to a healthy society if efforts are made towards enhancing their natural properties for maximum utilisation. The present review discusses different sources and compositions of essential oils, common extraction and characterisation techniques, some biological properties of essential oils and their applications in various industries.

Keywords: Essential oils, Mass spectrometry, antifungal, anticholinesterase, phytoconstituents.

1. Introduction

Essential oils are volatile odorous oils extracted from various parts (leaves, bark, roots, etc.) of plants. The essential oils extracted from different aromatic plants (like spices or medicinal plants) differ in their odour and flavour owing to the variety in the type and amount of constituents present in them. Some common plants and their parts used for essential oil extraction along with their major chemical constituent(s) are listed in Table 1. The organoleptic compounds, responsible for the aroma and flavour are present at varied concentrations in different parts of the plant depending on the part of the plant chosen for extraction and also on some growth parameters like climate and soil characteristics. Their molecular weights are usually less than 300 and they have some characteristic properties in common. These properties are optical activity, high refractive index, immiscibility with water, sufficient solubility to impart aroma to water, and solubility in most organic solvents such as alcohol and ether. Several methods can be utilized for the extraction of essential oils e.g., effleurage, expression, hydro distillation, steam distillation etc. However, steam distillation is the most used technique for commercial-scale production in related industries. Essential oils are considered secondary metabolites functional in plant defence against microbes(Tajkarimi et al., 2010). The essential oils and their phytoconstituents that have been investigated are known to possess several biological properties including antioxidant(Tit & Bungau, 2023), antimicrobial (Garzoli, 2023), antiparasitic (AlGabbani et al., 2023), antimutagenic(Rasgele & Altin, 2023), anticancer(Sharma et al., 2022a), anti-inflammatory (Zhao et al., 2023), anti-ageing s(Raina et al., 2023), anticholinesterase(Raina et al., 2023)etc.

Table 1. List of common plants and their parts with major component(s) of essential oil

Common name	Scientific name	Part of plant	Major compound(s)	References
Basil	Ocimum basilicum L	Flowers, leaves, stem	Linalool, estragole, eugenol, methyl chavicol	(da Silva et al., 2021)
Turmeric	Curcuma longa L	Leaves, rhizomes	Turmerone, phellandrene, curcumin	(Ray et al., 2022)
Clove	Syzygium aromaticum L.	Buds	Eugenol, caryophyllene	(Abadi et al., 2022)
Peppermint	Mentha piperita	Leaves	Methanol, methanone	(Pérez-Vázquez et al., 2022)
Ginger	Zingiber officinale	Rhizomes	Citral, zingiberene	(Kalhoro et al., 2022)
Bay leaf	Laurus nobilis L.	Leaves, flowers	1,8-cineole, linalool, methyleugenol	(Ordoudi et al., 2022)
Cinnamon	Cinnamomum zeylanicum	Leaves, bark	Cinnamaldehyde, eugenol	(Stevens & Allred, 2022)
Tea tree	Melaleuca alternifolia	Leaves, bark	Terpin-4-ol, terpene, 1,8-cineole, p-cymene	(Borotová et al., 2022)
Thyme	Thymus vulgaris L.	Leaves	Thymol, p-cymene, Terpinene	(Ghafarifarsani et al., 2022)
Orange	Citrus aurantium var.	Fruit	D-limonene, β-myrcene	(Radünz et al., 2021)
Nutmeg	Myristica fragrans Houtt.	Seed	Sabinene, limonene, methyl eugenol, myristicin	(Nikolic et al., 2021)
Black pepper	Piper nigrum L.	Leaves, seeds	β-caryophyllene, limonene	(Ashokkumar, Murugan, et al., 2021)
Lavender	Lavanda angustifolia L	Flowers, leaves	Linalool, Linalyl acetate, β-Caryophyllene	(Ciocarlan et al., 2021)
Ajowain	Trachyspermum ammi L.	Seeds, fruits	Thymol, p-cymene, γ-terpinene, carvacrol	(Mazzara et al., 2021)
Fenugreek	Trigonella foenum-graecum L.	Seeds	Linoleic acid, palmitic acid	(Akbari et al., 2019)
Fennel	Foeniculum vulgare Mill.	Seeds	trans-anethole, estragole, limonene, and fenchone	(Sabzi Nojadeh et al., 2021)
Cumin	Cuminum cyminum	Seeds	Cuminaldehyde, γ-terpinene, β-pinene	(Padilla-Camberos et al., 2022)
Allspice	Pimenta dioica	Berries	Eugenol, 1,8-cineole	(Padilla-Camberos et al., 2022)
Coriander	Coriandrum sativum L.	Aerial parts, seeds	Linalool, 2-decenal	(Raveau et al., 2021)
Clary Sage	Salvia sclarea L.	Aerial parts	Linalool, linalyl actetate, Germacrene-D	(Raveau et al., 2021)
Cardamom	Elettaria cardamomum L.	Seeds	α-terpinyl acetate,1,8-cineole	(Vellaikumar, et al., 2021)
Mint	Mentha piperita L.	Leaves	Piperitenone oxide, 1,8-cineole	(Ilić et al., 2022)
Oregano	Origanum vulgare, L	Leaves	Thymol, bergamol, terpineol	(Radünz et al., 2021)
Sandalwood	Santalum album L.	Bark	α-and β-santalenes	(Raghavendra & Mahesh, 2022)
Rosewood	Aniba rosaeodora	Bark	Linalool, α-terpineol	(Teles et al., 2020)
Cedarwood	Cedar atlantica	Bark	δ-cadinene, β-farnesene	(Kačániová et al., 2022)
Parsley	Petroselinum crispum	Seed	Myristicin, sabinene, β-myrcene	(Foudah et al., 2022)
Star Anise	Illicium verum	Fruit	Trans-anethole, limonene, estragole	(Yu et al., 2021)

 

Conventionally, the extraction of essential oil was done through expression methods like enfleurage, effleurage and defleurage, hot maceration process, pelatrice method, cold press, dry press method, etc. but they carried some limitations as well. Modern technologies have been developed over the years to overcome these limitations and to enhance the efficiency of extraction. The modern methods are mostly based on the distillation process and solvent extraction. Hydro-distillation, hydro-diffusion and steam distillation methods are collectively called azeotropic distillation.

It is crucial to perform chemical profiling of essential oils to determine their composition and variation in concentration of different constituents present in essential oils extracted from different plants or different plant parts of the same plant or plant parts of different varieties of the same species. This distinction helps in determining the phytoconstituents responsible for the biological activities of essential oils. The characterization of essential oils for their composition is done through chromatographic techniques like gas chromatography (GC), Liquid Chromatography (LC) etc. coupled with a detection technique like mass spectrometry (MS), flame ionization detection (FID) etc.

The biological properties of essential oils have generated a wide range of applications. These implications are relevant to both industries and the medicinal fields. The use of synthetic food preservatives has laid roots for skin allergies, cancer, intoxication, and other degenerative conditions. Essential oils have been known to possess antioxidant and antimicrobial activities, and their ability to protect food from pathogenic and spoilage microorganisms raises their eligibility to be used as natural additives in foods and food products. These can also be used as active compounds in packaging materials, by improving their water vapor barrier property associated with their hydrophobic nature. Essential oils are being sought as an alternative to these non-natural products in food preservation(Hussain et al., 2021).

Apart from food preservation, essential oils have also been applied in the field of therapeutics and medicine. Cosmetics and aromatherapy are the leading heads in utilizing the benefits of essential oils. Owing to their insecticidal and plant growth-enhancing properties essential oils have been utilized in the field of agriculture.

2. Sources and Composition

Several plants can be utilized for the extraction of essential oil. However, the part of a plant which acts as the major source of essential oil can always be different. Also, the quantity of different components present in essential oils extracted from different parts of the same plant may vary. The essential oils are mainly composed of low molecular weight (<1000 Da) volatile components, around 85-99%. Essential oils contain over 300 compounds. The chemical composition of essential oils is mostly contributed by secondary metabolites (like terpenes, terpenoids, flavonoids, alkaloids, polyphenols, indigenous pigments, etc.), other aromatic compounds and aliphatic constituents. Terpenes and terpenoids have a structural backbone made up of isoprene units. The structural representation of common terpenes and terpenoids is presented in Table 2. The major compounds found in essential oils are mainly divided into two classes: Terpene hydrocarbons and oxygenated hydrocarbons.

Table 2. Structural representation of common terpenes, terpenoids and flavonoids found in various essential oils

Terpenes
Terpenoids
Flavonoids

 

2.1 Terpene Hydrocarbons

Terpenes are a class of aromatic compounds that have a general formula of (C₅H₈) _n and a basic structure formed from 5-carbon-based isoprene (C₅H₈) units. Based on the number of C-units present in the terpenes molecules, they are divided into six classes: hemiterpenes (C₅), monoterpenes (C₁₀), sesquiterpenes (C₁₅), diterpenes (C₂₀), triterpenes (C₃₀) and tetraterpenes (C₄₀). Further higher classes of terpenes are known as carotenoids. Monoterpenes are made up of two isoprene units and are the major components (around 90%) in essential oils.

2.2 Oxygenated Hydrocarbons

These compounds are derived from terpenes and are termed terpenoids (or isoprenoids). Terpenes are modified by the addition of functional groups like alcohol, aldehyde, ketones etc. to form terpenoids. Some examples of terpenoids are represented in Table 3.

Table 3. Examples of Terpenoids

Class	Examples
Phenols	thymol, eugenol, carvacrol, chavicol
Alcohols	borneol, isopulegol, nerolidol, α-santalol, lavanduol, α-terpineol, santalol
Aldehydes	citral, myrtenal, cumin aldehyde, citronellal, cinnamaldehyde, benzaldehyde
Ketones	carvone, menthone, pulegone, fenchone, camphor, thujone, verbenone
Esters	bornyl acetate, linalyl acetate, citronellyl acetate, geranyl acetate
Ethers	1,8-cineole, anethole, elemicin, myristicin
Oxides	1,8-cineole, bisabolone oxide, linalool oxide, sclareol oxide
Lactones	bergaptene, nepetalactone, psoralen, aesculatine, citroptene

3. Extraction of Essential Oils

The process of extraction is crucial in case studies based on essential oils as it defines the quality and quantity of the yield. The extraction method is selected based on the type, state, and form of the plant material. Inappropriate selection of extraction methods may result in loss of bioactivity, natural characteristics, or physical properties. Discolouration, off-odour/flavour, increased viscosity, etc. might also take place. In the worst cases, even alteration of the chemical signature of essential oil may occur. Almost any part of a suitable plant can act as a source and essential oil can be extracted from it to be utilized in applications like food preservation and others. The modern methods are mostly based on the distillation process and solvent extraction.

3.1 Steam Distillation

Figure 1. Schematic diagram of steam distillation process.

This method is a common and efficient choice for the extraction of essential oils. The process of steam distillation involves passing steam through crushed or chopped plant material in upward direction. The vapours flowing through the plant material carry the volatile components along with them. The heat carried by steam bursts and breaks the cell structure of plant material and causes the release of phytochemicals. The temperature of steam must always be sufficient for this rupture. The vaporized mixture is then condensed and collected, where the aqueous and non-aqueous components get separated based on their lipophilicity. Figure 1 shows a diagrammatical representation of the steam distillation process. The steam distillation method is efficient in extracting 93% of essential oil and the remaining 7% can be extracted by further processing(Masango, 2005). This method can be coupled with hydrodistillation for better yields(El Kharraf et al., 2021).

 

3.2 Hydro-distillation

Hydro-distillation method for extraction of essential oils involves boiling plant materials completely immersed in water. This method is suitable for capturing the hydrophobic phytochemicals having a high boiling point. The water surrounding plant materials protects them from overheating, which might cause damage to the desired outcome. Upon boiling the essential oil vapors move along with steam and the mixture is allowed to condense. The separation of aqueous and non-aqueous phases takes place upon condensing. Figure 2 shows a diagrammatical representation of the hydro-distillation process. Hydro-distillation method is utilized after coupling it with heating techniques in modern extraction procedures. Microwave-assisted Hydrodistillation(MAHD)(Elyemni et al., 2019), Ultrasonic-assisted hydro-distillation (UAHD)(Sneha et al., 2022) and ohmic-assisted hydrodistillation(Sharifi et al., 2022) are three such cases.

Figure 2. Diagrammatic representation of hydro-distillation.

3.3 Hydro-diffusion

The hydro-diffusion method is similar to the steam-distillation method, as it involves steam as the carrier of heat. In this method, steam is passed from the top of the plant material. This process also protects the plant materials from the damage caused by boiling. Hydro-diffusion method is advantageous over the hydro-distillation method as the processing time is lesser and the yield obtained for a given amount of steam is higher. The traditional hydro-diffusion method has been modified to perform hydro-diffusion and gravity methods(Bousbia et al., 2009).

3.4 Solvent extraction using an organic solvent

Organic solvents like methanol, petroleum ether, ethanol, dimethyl sulphoxide etc. can be utilized for the extraction of essential oils in place of water. Because of the non-aqueous nature of essential oils, this method provides a better yield. This procedure involves mixing plant material with the organic solvent and heating the mixture. This method can be operated efficiently at low temperatures. The mixture is then filtered and the solvent is separated from essential oil by evaporation.

3.5 Solvent extraction using supercritical carbon dioxide

Evaporation of the solvent in the case of organic solvent may lead to the loss of volatile components from essential oil. If not evaporated then the solvent residue may affect the activity of essential oils. Carbon dioxide (CO₂) is capable of forming supercritical fluid at high-pressure conditions. As soon as the room temperature is attained, CO₂ vaporizes leaving no solvent residue. Thus, using supercritical CO₂ is a better option when compared to organic solvents

4. Characterisation Techniques

The modern analytical techniques for the characterisation of essential oils are based on chromatography principles. The essential oil contains both volatile and non-volatile components. The volatile components are analyzed by Gas Chromatography (GC), while non-volatile components are analyzed by Liquid Chromatography (LC).

In both techniques, the components are eluted by the mobile phase and are separated based on their affinity with the stationary phase. In GC, the mobile phase is a carrier gas containing vaporized analytes, while in LC, the mobile phase is a solvent or mixture of solvents. The separated analytes are recorded, and a chromatogram (signal v/s time) is generated following a Gaussian distribution curve scheme. The chromatogram delivers both qualitative and quantitative information. The peak area and height determine the amount of analyte present; the peak width determines the band spreading, and the solute is identified by characteristic retention time, which is also a function of the nature of the solvent.

4.1 Gas Chromatography-Mass Spectrometry (GC-MS)

Mass spectrometry involves the ionization of analytes to generate the gaseous ions, with or without fragmentation. The ions are then analyzed for their mass-to-charge ratios and relative abundances(Todd, 1995). The analytes can be ionized by exposing them to electric fields or energetic species (like electrons, ions, or photons) or thermal methods.  Although destructive, this technique is susceptible, requires a small sample size, is lower in expense, simple in design, and caters to fast data acquisition rates.

Gas chromatography coupled with mass spectrometry has great potential in determining the volatile compounds, which hold a significant share in the chemical constitution of essential oils. The mass spectrum of unknown compounds acquired from the GC-MS hyphenated technique is compared against the MS reference library created with standardized protocols of compound analysis. The incorporation of retention indices with MS libraries enhances the accuracy in the identification of compounds(Costa et al., 2007).

4.2Fast Gas Chromatography

Compared to traditional GC, fast GC provides sufficient resolving power in less time by combining appropriate columns and instrumentation. With improved run conditions, analysis times can be reduced by 3–10 times(Korytár et al., 2002). This technique is more analytically sensitive and efficient in terms of speed. The objective of Fast GC is accomplished by altering some analytical parameters like length and internal diameter (ID) of the column, carrier gas, linear velocity, stationary phase, film thickness, oven temperature, and ramp rate. This method necessitates instruments equipped with high split ratio injection systems to aid smaller sample column capacities, increased inlet pressures, rapid oven heating rates, and fast electronics for detection and data collection.

4.3 GC – Olfactometry

Fuller et al. first modified the gas chromatography to determine the volatile odour activity. The standard GC is incorporated with an olfactory port along with or in place of other detectors.  GC-O is utilized in addition to a flame ionization detector (FID), thermal conductivity detector (TCD), mass spectrometer, or photoionization detector.

4.4 Enantioselective GC:

The primary objective of Es-GC is to characterize the enantiomeric excess (ee) and enantiomeric ratio (ER) in chiral compounds. This technique requires a small sample size and provides high separation efficiency and selectivity along with high precision and reproducibility. The resultant information is crucial in characterizing essential oils and is considered equal to ‘fingerprint.’ Es-GC can be hyphenated to MS for more efficiency.

4.5Liquid Chromatography-Mass Spectrometry (LC-MS)

Although non-volatile components of essential oils hold a small share in their chemical constitution, they are significant when analyzing samples like citrus essential oil obtained by cold pressing methods. Thus, information gathered from GC techniques is not sufficient. Such non-volatile compounds are analyzed using LC, referred to as High-performance LC (HPLC). In normal phase (NP-HPLC) applications, the slightly polar analytes are separated based on their affinities towards an opposite stationary phase and a non-polar mobile phase, and the result is obtained in terms of elution time of analyte, which is highly influenced by the degree of adsorption of the analyte on the stationary phase. In reversed-phase (RP-HPLC) applications, a non-polar stationary phase and a moderately polar aqueous mobile phase are involved. The purified fractions obtained from HPLC or LC techniques are analyzed by coupled mass spectrometry. UV detection and spectrofluorimetric detections have been engaged as analyzing tools.  

5. Biological Activities

Essential oils are known to possess various biological activities that can prove to be a boon to humankind if utilized properly.

5.1 Antioxidant activity

The production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) as byproducts of various biological processes occurring within the human body is harmful and deteriorating in nature. Studies have bridged the relationship between the oxidative damages caused by ROS and multiple diseases that include ageing (Head, 2008), cancer(Paz-Elizur et al., 2008), diabetes(Jain, 2006), and Parkinson’s disease(Blesa et al., 2015) among many others. To counter these ROS, our body requires antioxidants. Antioxidants are defined as compounds capable of inhibiting or de-escalating an oxidation process. Natural antioxidants, like Vitamin C, Vitamin E, polyphenols/flavonoids, etc., are molecules capable of preventing oxidation of a substrate even when it is present in a lower concentration than the substrate. Studies have reported their effectiveness in preventing the above-mentioned diseases. The antioxidant activities of essential oils can be evaluated through various Hydrogen Atom Transfer (HAT) and Electron Transfer (ET) methods. Some antioxidant assays are categorically listed in Table 4. A schematic representation of some popular assays has been depicted in Figure 3.

Table 4. List of antioxidant assays

Category	List of antioxidant assays
Hydrogen Atom Transfer methods (HAT)	Oxygen radical absorbance capacity (ORAC) method
	Lipid peroxidation inhibition capacity (LPIC) assay
	Total radical trapping antioxidant parameter (TRAP)
	Inhibited oxygen uptake (IOC)
	Crocin bleaching nitric oxide radical inhibition activity
	Scavenging of H2O2 radical
	1,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical scavenging method
	Scavenging of superoxide radical formation by alkaline
Electron Transfer methods (ET)	Trolox equivalent antioxidant capacity (TEAC)
	Ferric reducing antioxidant power (FRAP)
	2,2-diphenylpicrylhydrazyl (DPPH) free radical scavenging assay
	Copper (II) reduction capacity
	N,N-dimethyl-p-Phenylenediamine (DMPD) assay
Other assays	Total oxidant scavenging capacity (TOSC)
	Inhibition of Briggs-Rauscher oscillation reaction
	Chemiluminescence
	Electrochemiluminescence
	Fluorometric Analysis
	Enhanced chemiluminescence
	TLC bioautography
	Cellular antioxidant (CAA) assay
	Dye-substrate oxidation method

 

(a) ABTS radical scavenging method

(b) DPPH free radical scavenging assay

 

(c) FRAP method

Figure 3. Mechanism of some antioxidant assays (a) ABTS radical scavenging methods, (b) DPPH free radical scavenging assay and (c) FRAP method.

5.2 Anticholinesterase activity

Alzheimer's disease (AD) is a slowly progressive neurodegenerative disease. It is a disorder that causes degeneration of brain cells and is the leading cause of dementia(Cipriani et al., 2011). AD is characterized by neurotic plaques and neurofibrillary tangles, that result from the accumulation of amyloid-beta (Aβ) peptide in affected areas of the brain, the medial temporal lobe, and neocortical structures(Selkoe, 2001). Disorders like Alzheimer’s disease may cause a progressive loss of cognitive functions, which may further result in reduced oxygen supply to the brain, tumours, vitamin B₁₂ deficiency, other nutritional deficiencies, and so on(Nakaizumi et al., 2018). There is currently no widely effective treatment that can stop or slow the progression of Alzheimer's disease. Natural ingredients are expected to play an important role in the emergence of potentially neurodegenerative disorder therapeutic avenues. The utilization of secondary metabolites is beneficial(Sweeney et al., 2018). The inhibition of cholinesterase by essential oils has been investigated using Ellman’s colourimetric method(Kamli et al., 2022). The mechanism followed in Ellman’s method has been depicted in Figure 4.

Figure 4. Mechanism of Ellman's method.

5.3 Antimicrobial activity

The bioactive components present in essential oils cause disruption in the cell wall of pathogens. Because of their hydrophobic nature, components of essential oil move rapidly across the lipids of bacterial cell membranes, disrupting cell wall structures and making them more permeable (Figure 5). Essential oils extracted from the plant parts of turmeric(Joshi et al., 2021), pepper(Le et al., 2022), clove(Yoo et al., 2021) etc. have been investigated for their antimicrobial activities.

Figure 5. Schematic representation of the antimicrobial action of essential oils(Wang et al., 2020).

 

5.4 Anti-inflammatory activity

Inflammation is a defense process of human body involving the recognition and removal of foreign stimuli by the immune system. Immune-responsive compounds, cytokines and interleukins are produced by macrophages, keratinocytes and lymphocytes in the human body(Jacob et al., 2013). The components of essential oils like thyme, chamomile, eucalyptus, lavender etc. modulate the transcription of the pro-inflammatory cytokines to reduce inflammation(Pandur et al., 2021).

6. Applications

Essential oils have a wide range of applicability. Different sectors where essential oils have been applied include the Food and beverage industries, paper and printing industries, paint and textiles industries, medical sector, agriculture sector etc. These have also been applied to adhesives, cosmetics and toiletries. The major applications of essential oils are discussed below.

6.1 Food Preservation

Owing to their antimicrobial activity against common food-borne bacteria and fungi, essential oils have been studied and employed for food preservation and increasing their shelf life(Tongnuanchan & Benjakul, 2014). The food industry has utilized several essential oils in the form of flavouring agents and they also have shown potential as food-grade preservatives(Angane et al., 2022). Essential oils have been modified in the form of capsules(Yang et al., 2023), bioactive films(Mohamad et al., 2022), edible coatings(Ju et al., 2019), chitosan-based membranes(Maleki et al., 2022), food packaging(Mukurumbira et al., 2022) etc. to enhance their role as preservatives. Essential oils extracted from herbs and spices have proved to be better than synthetic chemical additives. Essential oils have been applied for the preservation of meat and meat products(Smaoui et al., 2022), bread(Rahman et al., 2022), dairy products(Badola et al., 2023), aquatic food(Shahidi & Hossain, 2022), fruits and vegetables(Pandey et al., 2022). Active packaging of food products using essential oils is highly advantageous as essential oils possess antioxidant and antimicrobial properties that help in shelf-life improvement. Also, the food waste in the case of such packaging can be reused as a source of essential oil. However, the usage of essential oils in food preservation is accompanied by some limitations due to their high volatility, low lipophilicity and easy degradation. These limitations can be resolved by encapsulating essential oils(Carpena et al., 2021).

6.2 Medicines and therapeutics

Essential oils are being studied for their biological properties and have shown results significant to the field of medicine and health care. Aromatherapy is a traditional and most popular application of essential oils in this field and utilizes them to treat several diseases. Aromatherapy utilizes the antiseptic and skin permeability properties of essential oils. Some plants whose essential oils are used in aromatherapy include clary sage, eucalyptus, lavender, lemon, peppermint, rosemary, tea tree etc. The utilization of essential oils in aromatherapy has been reviewed(Ali et al., 2015). The effect of clove essential oil on memory function has also been studied through aromatherapy(Ansariniaki et al., 2022).  The application of essential oil to treating skin anomalies(Lee et al., 2022) and dermatological hair problems(Abelan et al., 2022) has been studied recently. Anticancer(Sharma et al., 2022b), anti-inflammatory(Jaradat et al., 2022), antiaging(Lohani & Verma, 2022), and neuroprotective(Rashed et al., 2021) potential of several essential oils have been investigated and they can be employed in the formulation of drugs to counter the aforesaid human-related problems.

 

6.3 Agriculture

Essential oils can prove to be beneficial in the field of sustainable agriculture. They have shown significant activities against plant pathogens, weeds and a broad spectrum of microorganisms in different in vitroand in planta studies carried out(Raveau et al., 2020). Due to their remarkable phytotoxic activities,essential oils are suitable candidates for the development of novel bio-herbicides(Wan & Rengasamy, 2022). They also have a potential role as pesticides to play in integrated pest management and organic farming as they are environment-friendly. The biological activities of essential oils have been applied to control plant pests and diseases(Basaid et al., 2021). The insecticidal properties of essential oils have also been studied(Bravim dos Santos et al., 2021). Essential oils have a potential role in extending fruit shelf life by fighting against postharvest pathogens(El Khetabi et al., 2022).

6.4 Cosmetics and Toiletries

Essential oils have emerged as natural ingredients in cosmetics and toiletries due to their odorous character and beneficial biological properties like antioxidant, anti-inflammatory, antimicrobial etc. They have been utilized in the manufacturing of fragrances and perfumes. These oils are used as active ingredients or preservatives in various skin and hair care products like moisturizers, lotions, cleansers, conditioners etc. The application of essential oils and their components in cosmetic products have been properly reviewed recently(Guzmán & Lucia, 2021).

7. Conclusion

Essential oils can be extracted from different parts of a variety of plants. Aromatic plants like spices, flowers, herbs, etc. possessing medicinal properties are chosen for the purpose. The extraction process can be properly chosen to maximize the yield. These oils can be utilized in their natural form or modified into capsules, bio-active films, etc. for their applicability in food preservation. Essential oils and their active agents can act as natural medicine or an alternative to commercially available medicines in the treatment of diseases associated with pathogens and metabolism. If studied properly the essential oils may prove to have the potential to deliver a synergistic effect with the drugs used in the treatment of different diseases. If properly explored to their full potential, essential oils can be a boon to humankind.

8. Future prospects

Owing to the global developments in recent years, the antiviral properties of essential oils can be peculiarly studied and applied for prevention and treatment. The use of natural aromatics for inhalation and their interaction with the central nervous system is an interesting field and can be further explored. Work can still be done to maximize their already existing potential in various fields of food preservation, medicine etc. by enhancing their activities through molecular size-modification, structural rearrangement of components etc. Despite considerable applications, essential oils also showcase some limitations. Firstly, being lipophilic they show less to no interaction with the polar moieties. Secondly, due to their high volatility and instability, their effects are acute. Another important aspect of essential oils is their chemical variability. Being majorly composed of secondary metabolites they are considerably affected by external factors which may degrade their quality over time. Recent studies have suggested the applicability of nanotechnology in the field of essential oils. Preparation of nano formulations of essential oils like nano emulsions, and nano-hydrogels not only promote hydrophilicity but also have the potential to mould essential oils into the desired frame of applications with enhanced stability and bio interaction. Essential oils can also be encapsulated using nanocontainers and studied for their kinetics and release mechanism. Such methods and studies would intensify their biological applications.

Acknowledgement

The financial support from the Chhattisgarh Council of Science and Technology (CCOST), Raipur (C.G.) is highly acknowledged (1258/CCOST/MRP/2021).

 

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