Essential Oils-Loaded Electrospun Biopolymers: A Future Perspective for Active Food Packaging

,e growth of global food demand combined with the increased appeal to access different foods from every corner of the globe is forcing the food industry to look for alternative technologies to increase the shelf life. Essential oils (EOs) as naturally occurring functional ingredients have shown great prospects in active food packaging. EOs can inhibit the growth of superficial food pathogens, modify nutritious values without affecting the sensory qualities of food, and prolong the shelf life when used in food packaging as an active ingredient. Since 2016, various reports have demonstrated that combinations of electrospun fibers and encapsulated EOs could offer promising results when used as food packaging. Such electrospun platforms have encapsulated either pure EOs or their complexation with other antibacterial agents to prolong the shelf life of food products through sustained release of active ingredients. ,is paper presents a comprehensive review of the essential oil-loaded electrospun fibers that have been applied as active food packaging material.


Introduction
For many decades, synthetic polymers have dominated food packaging industry due to their favourable properties such as softness, lightness, and transparency. So far, petrochemical-based plastics such as polyethylene terephthalate (PET), polyvinylchloride (PVC), polyethylene (PE) [1], polypropylene (PP) [2], polystyrene (PS) [3], and polyamide (PA) [4] have been extensively used as packaging materials as a result of their low cost of synthesis, decent mechanical performance, lack of permeability to oxygen, carbon dioxide, anhydride, and aromatic compounds as well as heat sealability, all of which contribute to food preservation and waste reduction. Despite several desirable properties, the usage of conventional plastics needs to be restricted since their long degradation cycles pose serious ecological threats to the environment. erefore, it is increasingly important to employ novel biodegradable raw materials. Although complete replacement of conventional plastics with eco-friendly plastics is impossible to achieve, at least for specific applications such as food packaging, the use of biobased active food packaging seems to be a realistic solution. Active food packaging is a new method to prolong the shelf life of food products and to maintain of their safety, quality, and integrity. According to the European regulation (EC) No. 450/2009, active packaging consists of systems that interact with the food as they would absorb substances such as moisture, carbon dioxide, or odour from packaged food or release desired materials such as antimicrobial, antioxidant compounds or flavours into the packaged food (European Commission 2009) [5].
Despite the importance of active food packaging, there are limitations associated with the existing polymeric materials to serve as optimal active packaging and modifications are necessary. Such modifications involve addition of other additives such as antimicrobial and antioxidant agents which are required. EOs derived from different parts of aromatic plants have been extensively researched for being a natural product and their nutritional health benefits. e main interest in EOs lies in their various therapeutic properties, namely, antioxidant, antimicrobial, antitumour, analgesic, insecticidal, antidiabetic, and anti-inflammatory [6][7][8][9]. As of 2019, the number of published papers regarding essential oils in food industry is almost 800 papers, indicating the use of EOs as biopreservatives in all types of foods [10].
EOs hold great potential for active food packaging applications as they can be directly added to the food products or incorporated into food packaging for gradual release during transportation and storage to improve shelf life and preservation [11]. Although EOs have been demonstrated as an alternative to chemical preservatives in active food packaging systems, they are associated with certain limitations that need to be resolved for successful incorporation in active food packaging systems. High volatility, low aqueous solubility, and intense smell are the major shortcomings that have limited the usage of EOs in food industry. erefore, the encapsulation of oils has been considered as a key solution in food packaging. Recent advances in encapsulation technology have improved the stability of sensitive components during production, through a reduction in evaporation and degradation of volatile compounds as well as firmer control over the capsules' dimensions, shape, and morphology during the encapsulation process [12,13]. ese methods carry out chemical encapsulation through ionic gelation, simple and complex coacervation, cocrystallization, interfacial polymerization, molecular inclusion, entrapment in liposomes, and ionic gelation plus electrostatic interactions [14].
Among the various approaches for nanoencapsulation, electrospinning is a versatile, easy to operate method for continuous fabrication of nanostructures [15]. Electrospun membranes exhibit a fibrous morphology with large surface area to volume ratio, high porosity, and fiber diameters in the range of nano to micron, all of which are favourable properties for the sustained release of active ingredients from the packaging membrane to the surface of the food [16]. While electrospinning is widely applied in the fields of tissue engineering [17], wound dressing [18], enzyme immobilization [19], and electrode materials [20], its application as food packaging is only recently explored [21]. ese recent advances in applications of loaded electrospun membranes in active food packaging call for a review on this topic. Although there are several reviews on relevant topics, the subject has the potential to be reviewed on its own merits.
Fernández-López and Viuda-Martos studied application of EOs in food systems [10]. Ribeiro-Santos et al. reviewed application of EOs incorporated into films and coatings in food packaging. Films containing EOs are usually produced via casting method [22]. In another study, São Pedro et al. reviewed nanoencapsulation of EOs into lipid carriers such as solid lipid nanoparticles, liposomes, and nanoemulsions for drug delivery systems. ey concluded significant improvement in antimicrobial activity of EOs [23]. In the above-mentioned reviews, the authors have emphasized the potential of EOs to be used as a part of packaging materials or their direct incorporation into the food matrix. However, this review focuses on EOs, as effective protective antimicrobials, and their incorporation in active food packaging through electrospinning. e properties of the polymeric matrices, main active components of Eos, and encapsulation through electrospinning are reviewed. e main objective of this review is to provide a broad insight into the potential applications of electrospun nanofibers encapsulating EOs as active food packaging materials. To the best knowledge of the authors, there is no review available on application of encapsulated EOs in food packaging using electrospinning technique.

Essentials Oils (EOs) for Active Food Packaging
EOs are produced by angiospermic plants and have found various usages in different industries [24]. Among all the plant species, only aromatic plants are sources of EOs.
Aromatic plants form about 10% of plant species (over 17,000) and are well distributed around world [25]. EOs are secondary metabolites which could be derived from different plant organs including flowers (jasmine, rose, chamomile, violet, and lavender), buds (clove), leaves (thyme, eucalyptus, salvia, and rosemary), fruits (star anise), twigs (Luma chequen), bark (cinnamon), seeds (cardamom), wood (sandal), rhizome, and roots (ginger), all of which have the potential to be applied in food packaging as antimicrobial and antioxidant agents [26][27][28][29][30]. e chemical composition and quality of EOs depend on characteristics of the source plant such as growth conditions, variety, geographical origin, age, season, and condition of the plant when harvested. Extraction method, analysis conditions, and processing chemicals can also affect their properties [31][32][33][34]. eir extraction yield is usually very low (about 1%) which makes them valuable rare substances. EOs consist of concentrated lipophilic volatile aroma compounds including terpenes, terpenoids, and phenol-derived aromatic and aliphatic components. e phenolic compounds in EOs can diminish or almost eliminate the presence of microorganisms and minimize lipid oxidation [35]. e natural extracts of EOs are classified as Generally Recognized as Safe (GRAS) by the US Food and Drug Administration (FDA) and received approval for safety and effectiveness [36]. erefore, in food related application, they are more suitable alternatives to synthetic antioxidants such as butylated hydroxytoluene (BHT) or butylated hydroxyanisole (BHA) which might have a carcinogenic effect [36]. ese oils are substances accountable for the active function of packaging with the flexibility to be settled in a different container or be directly added to the packaging material. In either of the cases, the release of the oils during transportation and storage leads to increased shelf life.
Electrospun-loaded EOs could be the answer to market demands as they allow foods to reach the consumers with 2 Advances in Polymer Technology their original or enhanced organoleptic properties, increased shelf life, and improved safety [37][38][39][40]. e packaging materials produced in these systems can contain active ingredients designed for sustained release during storage or transportation to delay food deterioration. Table 1 summarizes the essential oils that have been successfully used in food packaging for improved efficiency.

Electrospinning and Nanoencapsulation in Active Food Packaging
Electrospinning which was first patented by Cooley and Morton in 1902 is a straightforward method for producing continuous micro-and nanofibers [62]. Unlike the conventional methods of fiber production which are based on application of mechanical force, electrospinning uses highvoltage electrostatic repulsive force for drawing and fiber stretching. erefore, via electrospinning, it is possible to obtain fibers with diameters in the range of nanometres [63]. A typical electrospinning device is made up of a high voltage power source, a pump with tuneable feeding rate, a capillary as spinneret, and a collector. High voltage power source oppositely charges the capillary (commonly more than 10 kV) and the collector creating the pulling force for whipping instability [63]. When the charges building up within the polymer solution accumulate to a critical amount, a jet starts initiation from the capillary tip. e jet undergoes uniaxial stretching and thinning, while moving toward the oppositely charged collector and solvent evaporation is happening in the meantime. Once the jet reaches the collector, all the solvent should be evaporated, and formation of micro-or nanofibrous morphologies should have taken place. ere are two sets of parameters that affect the morphology of the electrospinning products: intrinsic parameters such as solution viscosity, solvent evaporation rate, and conductivity of the polymer solution and processing parameters which include applied voltage, feeding rate, collector shape and texture, and collection distance [64]. Figure 1 shows the various aspects involved in production of encapsulated electrospun fibers including EO extraction (Figure 1(a)), complexation and solution preparation (Figure 1(b)), and a typical electrospinning system (Figure 1(c)). Several other techniques such as phase separation, bicomponent extrusion, template synthesis, drawing, centrifugal spinning, and melt blowing have been reported to produce polymeric micro/nanofibers [65][66][67][68][69]. However, for food applications, most of these methods suffer from various shortcomings such as difficulty of control, lack of applicability to a wide range of materials, and exposure to chemicals affecting the stability of sensitive nutrients and prohibiting their usage due to toxicity concerns [70][71][72]. Due to their size and high specific surface area, electrospun nanofibers have an edge in encapsulation efficiencies and demonstrate major potential for applications requiring controlled release of active ingredients, while exhibiting appropriate levels of biodegradability and biocompatibility [73]. e electrospun antimicrobial membranes are fabricated by adding antimicrobial agents into the polymer solution followed by electrospinning. Although the incorporation of EOs into nanofibers was demonstrated earlier, there was no report on the application of electrospun-loaded EOs for antimicrobial packaging materials until 2016 [74]. Biopolymers such as polyvinyl alcohol (PVA) [75], chitosan (CS) [74], polycarbonate (PC) [76], and polyethylene oxide (PEO) [77] have been popular options for encapsulation of EOs through electrospinning because of their excellent properties in generating nanofibers and membranes [21].
Within the last few years, nanoencapsulation of biological compounds in carrier materials has found its way to food technology as a simple yet effective technique for modification of the quality of food and preservation [78]. Encapsulation is effective in enhancing the stability and solubility of EOs as well as masking their strong flavour. Due to the hydrophobicity, instability, and volatility of EOs, they usually cannot be directly added to electrospinning solution and a carrier substance such as cyclodextrins (CDs), liposomes (LO), and CS is usually required [79,80].

Encapsulation of Cinnamon Essential Oil (CEO).
CEO is a plant-derived antimicrobial substance of which FDA has categorized it as GRAS. Its protective qualities do not disturb the physicochemical or nutritious properties of the food. CEO exhibits a broad spectrum of antimicrobial activity against a variety of microorganisms through prevention of cell wall biosynthesis, functions of membrane, and specific enzyme activities [81]. Encapsulation of CEO protects it from harsh environmental conditions, extends the shelf life, and allows controlled release of the active compound [82].
Conn et al. reported successful use of CEO as an antimicrobial agent against common microorganisms [83,84]. CEO was encapsulated into β-cyclodextrin (β-CD) before incorporation in electrospun PVA fibers. CDs are cyclic oligosaccharides produced through enzymatic conversion of starch and exhibit nontoxic and biodegradable characteristics. Several studies have reported the use of CDs for foodrelated applications [85][86][87]. CDs are composed of α-1,4linked glucopyranose units and form a shortened cone-like structure cavity. As a result of their unique chemical structure, they form noncovalent host-guest inclusion complexes with EOs, CEO/β-CD inclusion complex (CEO/ β-CD-IC), enhancing their solubility, chemical stability, and bioavailability and protecting them from oxidation [88]. e mechanism behind the complexation is the displacement of the high-enthalpy water molecules occupying the cavity, with a guest molecule of proper polarity and dimensions [89,90]. erefore, they are often used to encapsulate various types of food additives and essential oils [89].
β-CD is the most commonly used member of CDs which is a hydrophobic molecule capable of entrapping EOs within its inner cavity [91]. A novel antimicrobial membrane electrospun based on PVA, CEO, indicated molecular interactions amongst PVA, CEO, and β-CD, resulting in enhancement in thermal stability of CEO and masking its special flavour. ermogravimetric analysis of PVA/CEO/ β-CD nanofibers indicated a shift of the second weight loss peak to higher temperature (110-160°C) due to possible formation of chemical or hydrogen bonds between PVA and CEO. e inhibition zone of nanofibers based on PVA/CEO/ Advances in Polymer Technology β-CD was reported to be wider than that of PVA/CEO. e entrapment of CEO into the cavities of β-CD improved the solubility of CEO and led to a more effective release into the agar medium [92]. Moreover, water contact angle results indicated that addition of CEO/β-CD reduced the hydrophobicity of the nanofibrous membrane. Perhaps that explains the excellent antimicrobial properties of the PVA/ CEO/β-CD nanofibrous film against both Gram-positive and Gram-negative bacteria. According to the report, it effectively extended the shelf life of strawberries and showed potential for active food packaging applications. Furthermore, the electrospun membranes are managed to incorporate greater amount of CEO compared to casted film which resulted in enhanced antimicrobial activity.
Rieger and Schiffmann investigated the antimicrobial property of electrospun CS/cinnamaldehyde/PEO nanofibers against E. coli [74]. It was demonstrated that the inherent antibacterial properties of chitosan combined with the quick release of cinnamaldehyde (CA) achieved elevated inactivation rates against Escherichia coli and Pseudomonas aeruginosa [74].
Wen et al. for the first time reported the use of polylactic acid (PLA) and CEO as antimicrobial food packaging material. CEO/β-CD-IC was successfully produced through coprecipitation method and modified the thermal stability of CEO. PLA was the selected polymer matrix since it is an FDA approved bioplastic as food-contact material [83,93]. e CEO/β-CD-IC was encapsulated into PLA nanofibers by electrospinning [94]. e electrospun biodegradable PLA/ CEO/β-CD nanofilm demonstrated better antimicrobial activity, against Escherichia coli and Staphylococcus aureus, compared to PLA/CEO nanofilm. e results indicated that PLA/CEO/β-CD fiber efficiently increased the shelf life of pork, suggesting a potential application in active food packaging [94].
Liu et al. studied the effect CEO contents (1%, 1.5%, 2%, and 2.5% v/v) on the efficiency of encapsulation, nanoparticle dimensions, and antibacterial activity of CS nanoparticles loaded in PLA. CS nanoparticles as the carriers of CEO were added to the PLA solution followed by electrospinning. e obtained electrospun fibers were capable of sustained release of CEO [95].
e study suggested that addition of CEO could enhance the antibacterial activity of the PLA/CS-CEO fibers. e optimal composition was reported to be PLA/CS-CEO-1.5, which showed the greatest antibacterial activity against E. coli and S. aureus (99.3% and Table 1: Essential oils incorporated in food packaging applications.

Essential oil combinations
Properties Food product Applied film material Rosemary [41] Antimicrobial Chicken Cellulose acetate Cinnamon clove [42] Antimicrobial Bakery Cassava starch Lemon, thyme, and cinnamon [43] Antibacterial NA Chitosan Cinnamon, winter savory, and oregano [44] Antimicrobial Bologna and ham Alginate Bergamot [45] Antifungal and antibacterial NA Chitosan Garlic, rosemary, and oregano [46] Antimicrobial NA Whey protein isolate (WPI) Oregano [47] Antimicrobial Advances in Polymer Technology 98.4%, resp.) during the incubation period. e observation was a result of high crystallinity of CEO and its strong interactions with CS which lowered the solubility of PLA and allowed the CEO to exhibit a similar antimicrobial activity even at slower release rate [96]. Lin et al. reported encapsulation of CEO/β-CD/proteoliposomes into polyethylene oxide (PEO) nanofibers to study their antimicrobial behavior against Bacillus cereus (B. cereus) [97]. B. cereus is one of the major sources of contamination and spoilage in meat products [98]. B. cereus shows high adaptability to extreme environments (acidic, alkaline, and high temperature) [99]. Nanoantibacterial liposomes, artificial lipid vesicles, are microscopic morphologies made up of a central section enclosed by concentric phospholipid bilayers embedding aqueous cores. ey are capable of encapsulating both hydrophilic substances in the inner aqueous section and hydrophobic matters within lipid bilayers and amphiphilic molecules at the lipid/water interface [88]. Proteoliposomes tend to agglomerate and shed when they come into a direct contact with food surface, as a result reducing the bioactivity of encapsulated substances. In addition, the hydrophobic nature of nanoliposomes has a negative effect on their encapsulation efficiency (EE) of EOs. erefore, for further enhancement of the EE and stability of CEO in proteoliposomes, β-cyclodextrin (β-CD) is added to the system. e physicochemical stability of CEO proteoliposomes was significantly enhanced by introduction of β-CD. Taking advantage of bacterial protease secreted from B. cereus, the controlled release of CEO from proteoliposomes was achieved via proteolysis of protein in proteoliposomes. Additionally, the antibacterial efficiency of CEO/β-CD proteoliposomes against B. cereus was improved as a result of their stability by encapsulation in nanofibers. e nanofibrous combination showed satisfactory antibacterial efficiency as active food packaging for beef against B. cereus without any impact on sensory quality while extending the shelf life.  [75]. LYS is a natural antimicrobial enzyme classified as GRAS which is usually derived from chicken egg white [100,101]. LYS exhibits antimicrobial action toward Grampositive bacteria through splitting the bonds between N-acetylmuramic acid and N-acetylglucosamine of the peptidoglycan in the cell wall [102]. In this study, CEO was selected for its antimicrobial activity against bacteria (L. monocytogenes and S. enteritidis) and molds (A. niger (ATCC1015) and Penicillium (CICC41489)). e choice of PVA as the electrospinning matrix was because of its solubility in water soluble and biocompatibility as it has been extensively used in food preservation [103]. Furthermore, PVA could be applied without usage of organic solvents, thereby preventing the denaturation of LYS [104]. e aim of their study was to reduce the dosage of CEO in the packaging material without affecting antimicrobial action. e study suggested that the concentration of antimicrobial agent consisting 2% CEO and 0.25% LYS (w/w) in PVA matrix has a decent performance as food packaging material. Shao et al. fabricated ultrafine PVA/permutite/CEO membranes via electrospinning [105]. PVA doped with permutite powder was applied as polymer matrix and CEO was encapsulated as the antibacterial component. Permutite is a nontoxic highly stable aluminosilicate which can be engaged in reversible interactions with various types substances including gas molecules [106]. e fabricated membranes were characterized for physicochemical properties such as morphology, porosity, surface area, tensile, and chemical interaction. Furthermore, the antibacterial action of the as-prepared films was explored for fruit packaging by using fresh cut strawberries. Fresh cut fruits hold a large share of the market since they only require partial processing without any additional preparation [107]. However, being susceptible to microorganisms such as mold, yeast, and fungi remains to be the major problem for the fresh cut fruit [108]. e release behavior in electrospun 6 Advances in Polymer Technology membranes is diffusion controlled as it is physically adsorbed by polymeric matrix and very low activation energy is required for the release process [109]. Such property finds importance at lower temperatures (4-6°C), where the release of EOs at low temperatures will hardly achieve the minimum concentration required for preservation [110]. In the study, differential scanning calorimetry and pore distribution results suggested weak physical interactions between the CEO and the fibers as a result of mesoporous adsorption (15.77 J/g and 37.7 J/g) which can benefit the release of CEO at low temperature. e authors concluded that PVA/permutite/CEO fibrous films have the potential to delay the rapid corruption of strawberries during storage.

Encapsulation of Eugenol Essential
Oil. Eugenol (4-allyl-2-methoxyphenol), most abundantly found component in clove oil, nutmeg oil, and cinnamon oil, has been successfully applied in food preservation, cosmetics manufacturing, and traditional medicine. Several researches have confirmed antibacterial, antioxidant, anti-inflammatory, and local anesthetic properties of eugenol [111]. Presence of high contents of phenolic compounds such as eugenol in volatile oils exhibits strong antioxidant properties [112]. Eugenol is a yellow oily liquid which exhibits lipid peroxidation induced by reactive oxidase system (ROS) because of its radical scavenging activity [35,36,113]. Like other EOs, eugenol also suffers from high volatility and poor water solubility. erefore, encapsulation of eugenol is necessary to increase its effectiveness and longer shelf life [114]. Kayaci et al. in a joint research demonstrated that the EE of the volatile active agents such as menthol, vanillin, eugenol, geraniol, and allyl isothiocyanate was only effective when used with CD inclusion complexes (CD-IC) [15,[115][116][117][118]. One of the challenges in encapsulation is to incorporate higher amounts of the active ingredient in the polymeric matrix [115,119]. When electrospun polymers appear as carrier matrix, the amount of the active ingredients in CD system is often limited to 5% of the fabricated membrane's weight. is is due to difficulty of electrospinning uniform nanofibers from polymeric solution containing higher amounts of CD. is research group recently focused on nanofibrous webs encapsulating much higher amounts of active agents (loading of ∼10% (w/w) or more with respect to fibrous CD matrix). ey reported that production of polymer-free electrospinning from only CD systems was spun [120][121][122][123].
Celebioglu et al. prepared excessively concentrated aqueous CD solutions (160% (w/v)) [124]. ey prepared inclusion complexes between eugenol (guest molecule) and cyclodextrins (host molecules) at a molar ratio of 1 : 1 CD : eugenol followed by electrospinning to obtain nanofibrous webs (eugenol/CD). Since dimensions and form of the CD cavity are a significant parameter for formation of effective inclusion complexation, three different CD derivatives were applied in this work. e three modified CDs, namely, hydroxypropyl-beta-cyclodextrin (HP-β-CD), hydroxypropyl-gamma-cyclodextrin (HP-c-CD), and methyl-betacyclodextrin (M-β-CD), were used to create inclusion complexes with eugenol. e resulting electrospun eugenol/ CD samples showed self-standing and flexible characteristics as a mat web and displayed rapid solubility in water. Figure 3 demonstrates the chemical structure of modified CDs, eugenol, and schematic illustration of the inclusion complexation formation between CD and eugenol. Moreover, thermal stability of eugenol was enhanced for eugenol/CD/ IC (up to ∼310°C) in comparison with pure eugenol (up to ∼200°C). Furthermore, eugenol/CD exhibited effective antioxidant activities.
e strongest interactions within the complex were observed between M-β-CD and eugenol compared to the other two host CD molecules (HP-β-CD and HP-c-CD) for eugenol/CD samples. e authors suggested potentials for their membranes in food related and oral-care applications.
Liposomal encapsulation of eugenol is an alternative way to decrease the damage to eugenol at processing and storage phase. Liposome containing eugenol is a natural and effective antioxidant that can encapsulate hydrophobic and lipophilic drugs [125]. However, liposomal instability as a major drawback can affect their efficiency as an antioxidant. Furthermore, conventional liposomes often have low EE and instability of vesicle aggregation, fusion, or rupture, all of which pose serious challenges to the commercial exploitation of eugenol liposomes as an industrial antioxidant. erefore, the stability of eugenol liposome can affect the performance of the final product as active food packaging material. e stability of liposomes is being improved through various strategies such as polymer-coated liposomes, hydrogel-liposome composites, and nanoparticlestabilized liposomes [126][127][128].
Cui et al. reported an undemanding method to form an antioxidant based on novel complexes of liposome encapsulated SiO 2 -eugenol. SiO 2 -eugenol forms a supramolecular assembly with a core of colloidal particles covered by a lipid shell [127]. SiO 2 nanoparticles in food industry serve as an additive which can provide stiff support for the lipid bilayer thin film to enhance physical stability of liposome [129]. Besides being porous, the hydrophilic property of SiO 2 nanoparticles facilitates absorption when volatilization occurs [130,131]. ere are few reports of immobilization of liposome on electrospun nanofibers for food preservation [132,133]. While SiO 2 -eugenol liposomes could not be electrospun on their own, their combination with PEO could be electrospun and showed great potential as food packaging material [103,134]. Cui et al. reported production of a novel electrospun membrane based on PEO and SiO 2 -eugenol liposomes [135]. Figure 4 shows schematic illustration of liposomes-encapsulated SiO 2 -eugenol.
e study explored application of SiO 2 as an architectural template for liposomes which could bring promising prospects to application of eugenol-based antioxidant. As a proof of concept, their electrospun membranes were exposed in contact with beef over a period of 60 days and exhibited excellent antioxidant activity.
Munteanu et al. investigated the biodegradation and tensile properties of PLA membranes coated with antimicrobial and antioxidant CS-EO complexes [136]. Chitosan has exhibited significant antibacterial action against various pathogens such as Klebsiella pneumoniae, Escherichia coli,

Advances in Polymer Technology
Staphylococcus aureus, and Pseudomonas aeruginosa [137]; therefore, it has been a popular subject of studies in antibacterial coatings [138]. It also can encapsulate other antibacterial agents such as nisin [139], which could further enhance the antimicrobial action of the packaging coatings [140]. PLA electrospun membranes were coated with bioformulations containing the essential and vegetable oils, that is, clove and argan oils loaded into CS. 99% of the argan oil is composed of acylglycerols, while the antioxidant constituents are tocopherols, squalene, sterols, and phenols [141,142]. Eugenol as the major volatile constituent of clove oil (about 80%) is accountable for its antioxidant and antimicrobial properties [143]. e study involved coaxial electrospraying of encapsulated CS over the electrospun PLA membrane which was placed on the metallic collector. Based on the report when high molecular weight CS was used, the coaxial electrospinning produced beaded CS nanofibers with the oil encapsulated and distributed along the beaded fibers. On the other hand, use of lower molecular weight CS led to electrospraying loaded chitosan nanoparticles and providing smoother surface. e PLA films coated with CS-EO formulations exhibited higher antibacterial activity compared to the films coated only with CS. e clove oil's antibacterial activity was higher than argan oil due to its higher phenolic content. e beaded fibers showed better antibacterial activity compared to nanoparticles perhaps due to the higher specific surface area of the rougher nanofibrous morphology of the coating layer.
Melendez-Rodriguez et al. encapsulated eugenol within the pores of mesoporous silica nanoparticles (MCM-41) by vapor adsorption [144]. Subsequently, for the first time, electrospinning technique was applied to include MCM-41 particles containing eugenol into poly(3-hydroxybutyrateco-3-hydroxyvalerate) (PHBV). To fabricate continuous films, the produced electrospun fibers were thermally treated at 155°C. e resultant PHBV films filled with eugenolloaded mesoporous silica nanoparticles revealed increased mechanical strength, adequate thermal resistance, and good barrier properties to water vapor and limonene. According to their results, they suggested that the films can be used in the form of coatings or interlayers for active food packaging applications.
Cui et al. evaluate the antibacterial activity of clove oilloaded chitosan nanoparticles (CO@CNPs) and gelatin electrospun nanofibers against E. coli O157:H7 biofilms on cucumbers [145]. Once CO@CNPs were used at 30% (w/v), E. coli O157:H7 population reduced to approximately 99.98% and high antibacterial activity was achieved after 8 hours. Following this, electrospinning was applied to incorporate the prepared CO@CNPs into gelatin nanofibers. After applying 9 mg/mL gelatin/CO@CNPs treatment for 24 h, the population of E. coli O157:H7 biofilm decreased to almost 99.99% in vitro. e results confirmed that treatment of the gelatin/CO@CNPs nanofibers could maintain the flavour and color of cucumber for more than 4 days. Based   [146]. However, TEO suffers from several shortcomings such as volatility, hydrophobicity, and special flavour which highlight the need for encapsulation which is usually achieved through a coprecipitation of TEO into ß-CD [91]. THY is also the major volatile component of essential oils derived from plants belonging to the Lamiaceae family. It is a monoterpene usually found in oregano and thyme; however, its preservation and delivery applications remain to be a challenge due to its hydrophobic and volatile nature. It is registered in European flavouring list and classified as GRAS by FDA [147]. THY exhibits antimicrobial action against a wide range of microorganisms such as bacteria, fungi, and yeasts [148][149][150] through its capacity to disturb the lipid bilayer of the cell membrane and could increase membrane permeability [151]. THY is typically used for food preservation and control of postharvest decay of fresh produce [152,153].
Lin et al. reported production of gelatin nanofibers containing TEO/β-cyclodextrin ε-polylysine nanoparticles (TCPNs) for controlling the propagation of Campylobacter jejuni (C. jejuni) [154]. C. jejuni is the major source of contamination on poultry surface which poses huge threats to human health. Moreover, C. jejuni is a zoonotic pathogen that lives as a symbiotic microorganism in the digestive tract of the poultry [113]. During poultry slaughter, C. jejuni can leak and rupture through external channel and cause meat contamination. e C. jejuni contaminated meat is known to be the main cause of human campylobacteriosis which accounts for 8.4% of the diarrheal diseases [155].
C. jejuni meat contaminations may not be completely inactivated as it shows high resistance against some frequently used antibiotics such as cephalosporins, quinolones, and gentamicin [156] and the demand for an efficient natural antibacterial agent to replace chemical antibiotics frequently used in meat industry. Since C. jejuni primarily contaminates meat surface, an antimicrobial packaging has a great potential to inhibit its reproduction.
A simple TEO/β-CD-IC is not effective against C. jejuni as β-CD has an electroneutral nature and its absorption onto negatively charged bacterial cell wall is limited. To address this issue ionic gelation was applied to adsorb cationic biopolymers onto the surface of TEO/ β-CD-IC [157]. In the study, ε-polylysine (ε-PLY), a biodegradable cationic biological metabolite with outstanding antibacterial properties, was selected to prepare TEO/β-CD ε-polylysine nanoparticles (TCPNs) [158]. e presence of -NH 2 along the ε-PLY chains improves nanoparticle binding onto negatively charged bacterial cell wall leading to an acceleration in the apoptosis process [159]. Antibacterial nanofibers were obtained by adding TCPNs into the polymer matrix via electrospinning. e prepared membranes showed outstanding antimicrobial action against C. jejuni, through membranolysis. e results confirmed that damaged cell membrane and proteins leakage of C. jejuni were results of antimicrobial activity of nanoparticles. Chicken samples packed in the antimicrobial membranes possessed lower aerobic bacterial count and thiobarbituric acid (TBA). Total volatile basic nitrogen (TVBN) and pH values were lower as well without any adverse effect on color, texture, and sensory evaluation, signaling bright prospects for the membranes in poultry preservation.
Aytac et al. reported encapsulation of THY/c-CD-IC into in electrospun zein nanofibrous web for food packaging application [160]. Two different molar ratios THY/c-CD (1 : 1 and 2 : 1, resp.) were prepared and encapsulated into electrospun zein nanofibers. Figure 5 shows the chemical structure of THY, schematic representation of c-CD, and THY/c-CD-IC formation. e choice of c-CD was due to its lack of adverse effects on nutrient absorption in food products and nutraceutical applications [161]. Successful formation of complex between THY and c-CD at both molar ratios was reported. Cyclodextrin inclusion complexes of thymol (THY/CD-IC) have been demonstrated to be applicable in pork meat systems to prevent oxidation and enhance meat stability at high relative humidity (up to 75%) for long storage periods [162][163][164][165].
However, it is worth mentioning that the release of THY was higher than that from zein-THY/c-CD-IC nanofibrous membranes (2 : 1). Similarly, zein-THY/c-CD-IC nanofibrous membranes (2 : 1) had stronger antibacterial activity against E. coli and S. aureus. In brief, zein-THY/c-CD-IC nanofibrous membranes were most efficient at decreasing the bacterial count in meat stored over a 5-day period at 4°C. us, these membranes exhibit great potential as antibacterial food packaging material.
Zhang et al. encapsulated THY in poly(lactide-co-glycolide), PLGA fiber through core-shell coaxial electrospinning [166]. PLGA which is known for its biodegradability and controlled delivery properties [167] has the capability of encapsulating hydrophobic substances and had shown great potential in enhancing the efficiency of delivery in food systems [168,169]. e study indicated that nanofibers with good core-shell structure were formed and volatile THY was encapsulated successfully. e results demonstrated that PLGA can efficiently suppress the volatilization of THY, so the encapsulated thymol gradually evaporates into the fruits and vegetables storage environment. In their work, the antibacterial and fruit preservation ability of the nanofiber films were tested by strawberry. e results suggested that the produced membranes efficiently inhibit bacterial, fungal, and yeast growth to extend the shelf life of fruits. is novel biocompatible antibacterial packaging material would have a huge application prospect for food preservation.

Encapsulation of Tea Tree Oil (TTO), Peppermint Oil (PO), Chamomile Oil (CO), Chrysanthemum Essential Oil (CHEO), and Moringa Oil (MO).
TTO is a natural essential oil which is composed of several organic substances such as terpene hydrocarbons, terpene alcohols, and terpene phenols. ere are several reports regarding antimicrobial and anticancer properties of TTO [170,171]. However, like other EOs, the application of TTO remains limited due to its chemical instability in exposure to air and high temperatures [172]. erefore, forming noncovalent inclusion complex with ß-CD as an antibacterial agent paves the way toward stability [173].
Cui et al. studied the incorporation of TTO/β-CD-IC, as antibacterial agent into electrospun PEO, to fabricate antibacterial packaging material [174]. As mentioned earlier, TTO is not completely soluble in aqueous PEO solution and the volatilization of TTO would intensify during the electrospinning which might create further problems regarding quick release during storage [175]. erefore, in the study, the ß-CD and TTO were used as a host guest to create water-soluble inclusion complex. e membranes were subjected to plasma treatment, and after the treatment the efficiency of antibacterial agent release from PEO nanofibers was improved appropriately. e plasma treated electrospun membranes exhibited the highest antibacterial activity against E. coli O157:H7 on the beef for a period of 7 days, with inhibition efficiently of 99.99% at either 4°C or 12°C. According to their results, the plasma-treated PEO nanofibers containing TTO/β-CD prolong the shelf life of beef and sustain its sensory quality which suggests a bright prospect in food preservation. Tang et al. fabricated gelatin nanofibers incorporating two kinds of EOs, that is, peppermint essential oil (PO) and chamomile essential oil (CO), for potential edible packaging application [176]. PO has excellent antimicrobial property, which made it the subject of several studies in food preservation, pharmaceuticals, and wound dressing [177][178][179]. CO on the other hand is mostly used in medicinal tea, cosmetics, perfumery, and food industry due to its calming, antibacterial, and antioxidant properties [180][181][182][183]. Gelatin nanofibers loaded with PO and CO were successfully produced with morphological homogeneity and smoothness. All the gelatin nanofibers containing PO, CO, or their combination showed enhanced antibacterial action proportionate to EO content against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) as well as certain antioxidant property. Incorporation of CO resulted in improved antioxidant activity, while the antibacteria activity against E. coli and S. aureus was better for the nanofibers containing EO. e mixture of PO and CO in gelatin nanofiber showed overall better bioactivities compared to the samples that contained only one of them. Addition of EOs enhanced the surface hydrophobicity of nanofibers based on water contact angle results. is is important since the high hydrophilicity and sensitivity to moisture of gelatin are one of its major drawbacks as packaging material [184]. erefore, it is not surprising that researches have been focused on blending gelatin with other hydrophobic materials to eliminate its drawbacks for food packaging applications [185,186]. Along with hydrophobic polymers, EOs could be an alternative to be incorporated in gelatinbased packaging for improvement of bioactivities as well as the required physical properties [187]. e high surface area to volume ratio and the nanostructure of electrospun nanofibers were shown to improve the sustained release of bioactive ingredients to food surface and intensify the bioactive actions [16,84]. Furthermore, the addition of EO is reported to enhance mechanical flexibility of electrospun membranes and to control tensile modulus and strength.
e mentioned phenomena are achieved through affecting morphology and diameter of nanofibers by inducing rearrangement of protein network and cross-links between the polymer chains and some constituents of EOs [5]. Finally, the cytotoxicity test demonstrated the noncytotoxicity of gelatin nanofibers incorporated with PO and CO, thus indicating the potential of the gelatin/EOs nanofibers as prospective food packaging.
Chrysanthemum essential oil (CHEO) is an aromatic oil extracted from chrysanthemum which has antioxidant and anti-inflammatory properties [188]. Chrysanthemum has cardiovascular protective functions and can prevent a variety of diseases such as hypertension, atherosclerosis, and coronary heart disease [189,190].
Since CHEO had a negative effect on phosphofructokinase activity, hexokinase, and pyruvate kinase in L. monocytogenes cells, antibacterial properties of CHEO against L. monocytogenes increased. e slow release of CHEO from CHEO/CS/NF effectively prolonged the antibacterial action. e antibacterial application of the CHEO nanofibers against L. monocytogenes was tested on beef. Beef parameters like thiobarbituric acid reactive substances (TBARS), pH values, and texture at different storage temperatures (4°C, 12°C, and 25°C) were evaluated. Due to the presence of antioxidant components in CHEO released from CHEO/CS/NF, the beef parameters like thiobarbituric acid reactive substances (TBARS) value in treated beef were lower (0.135 MDA/) comparing with the untreated sample. Moreover, pH value for beef sample packed with CHEO/CS/NF showed 6.43 (after 10 days of storage) that was lower than pH value of unpacked sample (7.05) at 4°C. ey suggested a potential application in food packaging.
Moringa oil (MO), an essential oil derived from Moringa oleifera, is resistant to autooxidation. Moringa oleifera grows in South and Central America, Africa, Southeast Asia, and the Indian subcontinent [192]. Lin et al. fabricated MOloaded chitosan nanoparticles (MO@CNPs) and loaded them into gelatin nanofibers for biocontrol of L. monocytogenes and S. aureus on cheese [193]. Beside the excellent physicochemical properties of nanofibers, they had outstanding antibacterial activity against S. aureus and L. monocytogenes on cheese at 4°C and 25°C and negligible impact on the surface color and sensory quality of cheese within 4 days of storage. Hence, the MO@CNPs loaded gelatin nanofiber demonstrated excellent antibacterial action as a candidate for further food application studies.
To summarize, Table 2 demonstrates successful encapsulation of the different EOs into various carriers and electrospun matrices with their food related properties. e relevant electrospinning parameters could be found in Table S1 in Supplementary Materials.

Conclusions
Growing consumer demand for safe and chemical-free natural products has improved the quality and safety of the products against pathogenic deterioration and lipid oxidation within their shelf life. Especial attention has been given to EOs as natural additives and antimicrobial and antioxidant agents classified as GRAS. In this regard, the study of EOs has great importance, since it has been proven that their antimicrobial activity can replace artificial preservatives. However, their low solubility in water, oxidation susceptibility, and volatilization have limited their use. e encapsulation technique is a beneficial way in improving the stability of EOs and thus their efficiency as antibacterial/ antioxidant agents. e combination of electrospinning and nanoencapsulation EOs has allowed successful development of new antimicrobial packages for food preservation. ese new nanoencapsulation techniques have attracted global interest in food packaging and research area due to the positive protection and environmental friendliness. As presented in the article, these new combinations take advantage of encapsulation and slow release properties of electrospun polymers as a potential new platform for stabilization of natural oils for food packaging.
While the initial results of loaded electrospun fibers for food packaging are promising, it should not be forgotten that the composite nanofibers hugely depend on electrospinning process. Electrospinning despite various advantages has serious problems as well. Low production rate especially for aqueous polymeric solutions remains a major problem, along with limitations to selection of highly volatile organic solvents as well. Particularly, solvents must not have any adverse effect on the antimicrobial properties of the active ingredients or bring the risk of future side effects. Technical advancements in electrospinning in terms of process control and production rate within the last three decades have provided an opportunity to use nanofibers for more applications. However, electrospinning still needs lots of further innovations in order to upscale production rate and quality for food packaging application at industrial scale. e current review demonstrated that electrospinning technology is a potential new platform for enhanced stabilization by encapsulation of natural oils in nanofibers. Whilst the initial results suggest great prospects for loaded nanofibers as food packaging, to produce a feasible product at industrial scale, loaded electrospun fibers require extensive processing.

Conflicts of Interest
e authors declare no conflicts of interest.