Essential oils (EOs) are hydrophobic liquids containing various volatile compounds. In the 16
th century, the word ‘essential oil’ was defined as ‘Quinta Essential’ by Paracelsus von Hoenheim
(Pichersky et al., 2006). EOs are produced in large quantities in oil sacs/oil glands of medicinal and aromatic plants and can be extracted from leaves, bark, seeds and flowers (
Tongnuanchan and Benjakul, 2014;
Mahato et al., 2019). They are a complex mix of over 300 different compounds which are mostly organic and volatile in nature
(Vainstein et al., 2001; Pophof et al., 2005; Sell, 2006). Since medieval ages essential oils had been used in various cultures, due to their medicinal properties. Their various promising properties, like being stimulants, anti-depressants and anti-bacterial have helped them in gaining popularity in recent years. They have found applications in the field of therapeutics, being natural, safe and cost effective
(Herman et al., 2019). Similarly their hydrophobic nature enable them to infuse into lipids of the cell membranes of microbes, resulting in disruption of the cell structure
(Sikkema et al., 1994). Also different phenolic compounds present in them contribute to their anti-bacterial activity
(Lambert et al., 2001; Dorman et al., 2000). In addition, EOs being lipophilic in nature, disrupt microbial cell homeostasis by making cells more permeable and less organized
(Chaudhari et al., 2020).
In spite of their several advantages, the use of EOs is limited due to various intrinsic factors like high volatility, photo-sensitivity, hydrophobicity and low stability
(Kumar et al., 2020). Therefore, in order to overcome these limitations associated with the use of essential oils there is a need for development of innovative technologies. Nanotechnology based techniques like nano-encapsulation has been developed as one such technology that has significant potential in addressing this problem. It involves the use of delivery vessels, also referred to as nano carriers to encapsulate bioactive molecules. This protect these molecules from various environmental factors such as oxygen, pH, light
etc. It also offers different advantages to EOs such as protection from degradation, enhanced bioactivity targeted delivery and controlled release
(Singh et al., 2020; Bastos et al., 2020). Currently, a wide range of coating materials are being extensively used namely starch, cellulose, chitosan, guar gum
etc., as encapsulating materials which enhance the bioactive potential of EOs
(Kumar et al., 2020). Moreover encapsulation of EOs improves the bioactive properties of essential oils and can be used in food and pharmaceutical industries
(Kapustova et al., 2021). Nano carriers can be utilised for encapsulating essential oils with enhanced antifungal activity
(Kapustova et al., 2021; Napoli et al., 2020). Furthermore, nano based essential oil capsules show controlled mycotoxin and fungal contamination in agri based food systems
(Chaudhari et al., 2021). This review illustrates various techniques/methods for encapsulation of essential oils.
Chemical nature of essential oils
EOs are complex mixture of compounds which are mostly alcohols, ethers, aldehydes, ketones, esters, amines, phenols and terpenes
(Dihifi et al., 2016; Sell, 2006). The terpene family occupies the major composition of EOs. Thousands of compounds belonging to terpenes have been characterized in essential oils namely, alcohol derivatives (geraniol), ketones (menthone), aldehydes (citronellal) and phenols (thymol)
(Modzelweska et al., 2005). They are rich in mono-terpenes, which include geraniol, terpineol (lilacs), limonene (citrus), myrcene (hops), linalool (lavender) and pinene (pine) and sequiterpenes like chamazulene (German chamomile) (
Breitmaier, 2006). Non-terpene compounds such as eugenol, saffrole
etc., are also present in EOs. Some esters are also found in EOs such as linalyl acetate and geraniol acetate in lavender and sweet marjoram respectively
(Safayhi et al., 1994; Arumugam et al., 2016). Essential oils containing ketones include rosemary, clary and sage
(Nazzaro et al., 2013). Some selected essential oils and their major chemical compounds are illustrated in (Table 1).
Various volatile components present in essential oils possess biocidal activity. These volatile compounds have a profound demand in wide spanning industries like food, pharmaceutical and pesticide. Chemical compounds present in EOs and their biocidal activities are depicted in Table 2.
Different techniques of essential oil nanoencapsulation
Essential oils can undergo chemical alterations because of their volatility and decomposition when exposed to light, heat and oxygen (
Scott, 2005). Nanoencapsulation of EOs, involve two terminologies namely core materials and wall materials. The material which gets encapsulated is known as the core material, active agent or internal phase, whereas, the encapsulating substances are called as wall materials, matrices, carrier agent or external phase (
Zuidam and Shimoni, 2010;
Pandit et al., 2016). Encapsulation of essential oils leads to their sustained release in a controlled manner, can penetrate deep inside the tissues and are readily taken up by the cells because of their miniscule size (
Ravi Kumar, 2000). Thus, nanoencapsulation of essential oils leads to their increased bioavailability, enhanced controlled release and precision targeting of the bioactive compounds
(Mozafari et al., 2006).
Nanoencapsulation techniques
Nanoencapsulation of bioactive compounds either involve top-down or bottom-up approaches. In top-down approach, precise tools are applied to reduce the size and shape to achieve desirable applicability of the nanomaterials that are created. On the other hand, the bottom-up approach constructs materials through self-organization and self-assembly of molecules and is influenced by several factors such as temperature, pH, ionic strength and concentration (
Augustin and Sanguansri, 2009).
Classical approaches of essential oil nano-encapsulation
Emulsification
This technique has been used to encapsulate bioactive compounds in aqueous solutions by producing nanoemulsions which are colloidal dispersions consisting of two immiscible liquids, one being dispersed into the other having droplet sizes in the range of 50 to 1,000 nm (
Sanguansri and Augustin, 2006). Drying techniques like spray drying and freeze drying after emulsification can be used to produce nanoemulsions that can either be used directly in the liquid state or in a dried powder form. Nanoemulsions exhibit high kinetic stability because of their extremely small emulsion droplet size and this plays a critical role in the retention of surface oil content of the product
(Solans et al., 2005; Sonneville-Aubrun et al., 2004; Jafari et al., 2008). Being a non-equilibrium system, spontaneous formation of nanoemulsions is not possible and consequently requires energy input. Hence production of nanoemulsions is generally attained through high-energy emulsification methods. Nanoemulsions provide significant possibility for the encapsulation of bioactive food supplements or oil-soluble nutraceuticals that can be utilized in food stuffs
(Silva et al., 2012).
Coacervation
The technique of coacervation involves phase separation of single or mixture of polyelectrolytes from a solution with later deposition of these around essential oil or bioactive components resulting in the formation of coacervates. When cross linking agents like glutaraldehyde and transglutaminase are added to coacervates, the coacervate becomes more robust
(Tiwari et al., 2020). Significant increase in the antimicrobial and antioxidant potential of
Pimentadioca essential oil after encapsulation in chitosan/carrageenan using the method of complex coacervation had been reported
(Dima et al., 2014).
Inclusion complexation
This technique, in general refers to the encapsulation of a supra-molecular association of a ligand into a cavity-bearing substrate
i.e., shell material by Vander Waals force. This technique is chiefly utilized to encapsulate volatile organic molecules like essential oils and vitamins. Essential oils are nanoencapsulated by entrapping them inside a polymer cavity by utilizing hydrogen bond and Vander Waals forces. Cyclodextrins have been widely used polymers to nanoencapsulate essential oils using this technique
(Tiwari et al., 2020).
Trending approaches in essential oil nanoencapsulation
Electro spinning
It is a process of producing nanofibers utilizing high electric voltage. Its principle is based on the processing of bio-polymers by exposing to high electric impulses. The resulting materials are nano structures showing better performances over bulk materials
(Rostamabadi et al., 2020). Nanofibers prepared by electrospinning have excellent characteristics, such as controllable fiber diameter, high porosity and large specific surface area. Furthermore, various functional active substances can be added to the spinning solution to prepare nanofibers with a wide range of functional properties
(Yao et al., 2021). Various variables/parameters governing the production of nano fibres using this technique are:-
· Parameters governing the process (electric field intensity).
· Solution characteristics (viscosity, surface tension).
· Electrospinning environment (temperature)
(Jaiturong et al., 2018; Rostamabadi et al., 2020; Ding et al., 2019).
The process of electrospinning can be categorized into coaxial electrospinning, single nozzle electrospinning (Dev and Hemamalini, 2018) and emulsion electrospinning
(Garia-Moreno et al., 2016; Feng et al., 2019). Electrospin based nano structures can be efficiently utilised for encapsulating bioactive moleculess
(Rostamabadi et al., 2020). In addition to various advantages of electrospinning technique, some limitations also accompany this process. These include:
· Industrial up scaling in an eco-friendly way.
· Inadequate
in vivo studies.
· Precise solvent evaporation control rate.
Therefore, further research in this approach is necessary in order to circumvent these limitations.
Electro-spraying
Electro-spraying is a promising approach as it is versatile and possesses properties like no use of organic solvents and high temperature
(Wang et al., 2020; Jawarok et al., 2008; Zhu et al., 2012). It is an emerging area of research for encapsulating bio active compounds. Here polymer liquids are exposed to high electric field resulting in fine liquid droplets. Nano sized particles are the final processed products when the solvents (in liquid particles) are evaporated. Various challenges such as low throughput hinder its large scale commercialization. Moreover electrosprayed products are subjected to aggregation and need appropriate wall materials
(Wang et al., 2020).
Supercritical fluid technique
Conventional encapsulation techniques employ the use of high temperatures/evaporation which limits or deteriorates the structures of volatile oils. Techniques such as supercritical fluid encapsulation can be used as an alternative. It involves non-pre/post thermal processing
(Akolade et al., 2020). It is broadly utilized due to its low critical temperature requirement as well as minimal utilization of organic solvents
(Ezhilarasi et al., 2013). The SCF technique can be categorized according to the function of SCF in the encapsulation process, as solvent, antisolvent, solute or cosolvent, nebulization compound, extractor and antisolvent techniques
(Keven et al., 2014). There are various techniques that involve supercritical fluids and these include supercritical anti-solvent process (SAS) and its various modifications, rapid expansion of supercritical solutions (RESS), gas antisolvent process (GAS), super-critical fuid extraction of emulsions (SFEE), aerosol solvent extraction system (ASES), precipitation with compressed fluid antisolvent (PCA)
etc. However, the SAS has recently received an enormous attention more than other methods because of its feasibility of application
(Nerome et al., 2013; Esfandiari and Ghoreishi 2015).
Nanoprecipitation/solvent displacement
Its principle relies on the precipitation of polymer from organic phase on addition of an aqueous phase
(Singh et al., 2020). It is an effective method to produce nanocapsules in the size range of 100 nm and below which exhibit properties like, good stability against degradation, sustained release, higher encapsulation efficiency and enhanced bioavailability during
in vivo studies along with displaying enhanced uptake by cells
(Ezhilarasi et al., 2013). Appropriate solvent and non solvent phase need to be selected, which may vary for each bioactive components and the polymer and solvent need to be of food grade. Since it is a fast and economic method, it has been found to be most suitable for encapsulating hydrophobic substance than hydrophilic core materials (
Ladj-Minost, 2012). Table 3 summarizes the techniques used for the encapsulation of essential oils.
Augmented antimicrobial activities of essential oil
Essential oils are sensitive volatile liquids which can be readily degraded when exposed to environmental factors (
Sebesan and Caraban, 2008). Therefore to protect them from these intrinsic factors, essential oil formulations came in light which involves dispersing them in special carrier materials such as nanogels and nanoemulsions which has high loading capacity, high stability and significant release properties
(Rasoli et al., 2008). Thus, encapsulation is one of the most efficient methods for the formulation of bioactive oils and various approaches have been developed in this direction. EOs exhibit potential antimicrobial activities against a wide spectrum of micro flora. The interest in the use of essential oils as natural antimicrobials and preservatives in the food industry has geared up in the last years due to growing consumer demand for natural and safe preservatives with good organoleptic properties. Various encapsulated oils and their antimicrobial activity with improved efficacy has been shown in (Table 4).
