**3. Essential oil incorporation technology in active packaging**

The EOs, which are known for their biological properties, are increasingly being employed as natural preservatives in food packaging. This approach tends to limit the usage of synthetic additives, increasing consumer acceptance of safe products [19]. Indeed, the EOs compounds can be progressively released at a suitable rate from the active packaging into the atmosphere surrounding the food product, exerting their positive antibacterial and antioxidant effects and, therefore, increasing the shelf life of the food product [20].

However, the EOs are challenging to efficiently include in active food packaging due to their volatilization, insolubilization in water, and chemical instability. As a result, these active agents must be integrated into matrices to sustain their biological characteristics during packaging manufacturing and then during various stages of food transportation and storage [18, 21]. Furthermore, to enhance the EOs biological effects in active packaging, optimal retention, and sustained, controlled release are

required. This latter feature is particularly essential in extending the shelf life of the foods by prolonging the release period of EO compounds for keeping a continuous biological activity [22].

Several approaches have been taken to develop controlled-release active packaging. All of them are based on the use of biodegradable polymers or copolymers with filmogenic properties, such as polysaccharides (cellulose, starch, and chitin), proteins (gelatin, zein, gluten, and casein), and lipids [23]. To these promising vehicle polymers, plasticizers, crosslinking, and reinforcing agents can be added to enhance the mechanical properties of film packaging [24].

The EOs can be included in polymer matrices by simply blending film ingredients and casting methods to film formation or by employing several encapsulation technologies for EOs incorporation in active packaging film matrices. The casting method is widely used in the production of film packaging [25–30]. It simply entails spreading over a flat surface a prepared film-forming solution containing an active ingredient such as EOs and a filomgenic polymer, both dissolved in a solvent. The solvent is then removed by drying. A plasticizer that changes three-dimensional organization, lowers attractive intermolecular interactions, and increases free volume and chain mobility is typically added to the basic film recipe. Glycerol is the most common plasticizer used for its stability and compatibility with hydrophilic biopolymers. As a result, the film has greater extensibility and flexibility, both of which are important in film design [31]. However, the direct incorporation of EO in active films via the blending and casting methods has several limitations. It has been stated that microencapsulated oregano EO in soy protein concentrate films provides emulsion-based products with better mechanical properties as well as antibacterial action against food pathogens compared to films containing free EO [32]. Therefore, EOs encapsulation, applied to active films, constitutes an interesting alternative for preserving the active agents. It consists of forming a physical barrier between the active agent and the surrounding environment, providing the created capsules physical and chemical stability as well as enhanced biological (antibacterial and antioxidant) and functional qualities (better handling) [33].

Encapsulation is the process of entrapping active agents (core materials as EOs) by another substance that serves as the wall material, resulting in nanometer, micrometer, or millimiter capsules [18, 34]. Furthermore, encapsulating EOs before forming active films is more efficient because it increases EO stability and bioavailability and enables controlled release to the external medium around the encapsulated particles by the diffusion process. This is the primary role of active film packaging in preserving food products [35]. Zhang and his collaborators [22] emphasized the necessity of gradual release of EO components and a prolonged sustained release time of EO to ensure the efficiency of antibacterial activity throughout the shelf life of food goods.

Microcapsules or nanocapsules can be generated depending on the encapsulation technology specificity. Several investigations on active agent nanoencapsulation have recently been published [33, 36, 37]. Kapustova and his collaborators [38] stated that the nano-range (10<sup>−</sup><sup>9</sup> ) of nanocapsules, which are a thousand times smaller than microcapsules, increases the surface-to-volume area for better efficiency in the delivery of EOs to targeted locations with greater stability and dispersibility.

Encapsulating technologies were classified into two categories: those that use chemical processes such as complex coacervation, liposomes, solid nanoparticles, and ionic gelation and those that rely on physical processes such as spray drying, extrusion, and solvent removal [39]. In general, more than one of the above technologies is often used to produce the desired microcapsules [40].

Furthermore, emulsifying the EOs compounds is usually required prior to encapsulation; it is considered as a preencapsulation step [41]. To stabilize the emulsion, high shear [42], or high pressure [43] or ultrasonication [44] must be used to homogenize the wall-core material. Nevertheless, droplets (oil in water) have such a loose structure they cannot effectively protect active substances; therefore, they must be immobilized in a solid matrix [45].

#### **3.1 Encapsulation methods based on physical processes**

Spray drying is commonly used to encapsulate EOs compounds [43, 46, 47]. It is primarily based on a three-step process: (1) wall-core material dispersion preparation, (2) wall-core material dispersion homogenization, and (3) dispersion atomization and drying [18]. The wall material must be carefully selected to improve EO component retention while also preventing oxidative alterations and volatilization [18]. Whey protein is commonly used as a wall material to facilitate emulsion formation and interfacial stabilization. Other additives, such as maltodextrins, can be employed to aid the encapsulation process, resulting in a bigger crust surrounding the drops and adequate oxidation protection [48].

Talon and his collaborators [43] examined whey protein and lecithin as wall materials for spray drying encapsulation of eugenol (7%). Both have been found to be effective against antioxidant and antibacterial properties when tested on *Escherichia coli* and *Listeria innocua*. Zhang and his collaborators [47] used spray drying technology to encapsulate a mixture of three EOs in gelatin-chitosan: cinnamon (*Cinnamomum cassia*), peppermint (*Mentha haplocalyx*), and lemon (*Citrus limon*). The EO combination exhibited a synergistic antibacterial activity based on the cooperation of different EOs.

Extrusion is often used to encapsulate active agents [34]. It works by forcing a substance through an orifice of varying width and shape according on the desired capsules [49]. Five extrusion technologies are used based on extruder specificity and other parameters: (1) hot-melt extrusion, (2) melt injection, (3) centrifugal/ co-extrusion, (4) electrostatic/electrospinning, and (5) particle from gas-saturated solution. In hot-melt extrusion, the wall material is first introduced to the extruder, after plasticized, the active agent is applied to promote the interaction of the wall and core materials. In melt-injection, the active agent is directly dispersed in the heated wall material (80–140°C), then pressed through orifices into a bath of cold solvent to allow capsule solidification. Both extrusion processes required polymers with high flow characteristics and active agents that could endure high temperatures.

In co-extrusion technology, the wall and core materials are introduced separately through several nozzles located on the extruder's exterior surface. The wall material and active agent come into contact at the interface due to centrifugal forces, resulting in a polymerization reaction and the formation of microcapsules [34]. Because it requires less energy for encapsulation, this co-extrusion method is particularly suited for EOs compounds and probiotic bacteria as active agents.

Electrostatic extrusion, known as electrospinning, is a one-step process for producing micro and nanocapsules [50]. The introduction of an electric field between a charged needle (containing the microcapsules) and the collecting solution causes the microcapsules to be unable to stand at the mouth of the needle, resulting in the formation of a charge stream of small drop [34]. Since, electrospinning operates at ambient temperature and atmospheric strain, it is particularly suited to the encapsulation of EOs compounds [51].

### **3.2 Encapsulation methods based on chemical processes**

Complex coacervation is largely employed for active agent encapsulation such as EOs [44, 52, 53]. It is mostly achieved by electrostatic forces of attraction between at least two polymers with opposite charges in aqueous fluids, with small contributions from hydrogen bonding, van der Waals forces, and hydrophobic interactions. As a result, the colloidal system separates into two liquid phases: one polymer-enriched precipitate phase and one polymer-depleted precipitate phase [54].

Polymers involved in coacervation are proteins and polysaccharides as wall material for the encapsulation of the active agent. This one is incorporated through emulsification in wall material to provide stability and protection. Finally, the capsules are separated using a physicochemical environment destabilization (pH and temperature) [18].

Ban and his collaborators [44] used coacervation to encapsulate ginger EOs in a mixture of chitosan (CH) and sodium carboxymethyl cellulose (CMC). The microcapsules with the same CH/CMC ratio have a crosslinking structure that may bind EOs, resulting in a high encapsulation efficiency (88.5%) and retention rate of volatile EO release to extend jujube shelf life.

Cyclodextrins (CDs) have been regarded as one of the preferred encapsulating polymers in the pharmaceutical industry and, recently, in the food industry [55]. CDs are a distinct family of molecules that are produced naturally through the degradation of starchy compounds. They are classified into three types: α-, β-, and γ-cyclodextrins and made up of D-glucose units linked together by glycocidic bonds between α-(1,4) carbon atoms. The toroidal structure of these compounds provides a hydrophobic interior cylindrical cavity and hydrophilic sides. As a result, the central cavity can form a stable combination with a guest molecule [37, 56]. EOs are well suited to being encapsulated in cyclodextrins, and so remaining protected while being released from the inclusion complex at a controlled rate, which is ideal for active packaging applications.

Silva and his collaborators [55] recently developed CD polymers such as CD nanosponges (CD-NS). These nanosponges are innovative crosslinked cyclodextrin polymers nanostructured within a three-dimensional network. They provide better stability and formulation flexibility with sustained release.

The technology of ionic gelation encapsulation has received a lot of attention in recent years because of its high adaptability to many types of active agents and low-cost approach [32]. This technology is particularly useful for encapsulating EO compounds to protect them from environmental deterioration [57, 58]. It is based on the ionic crosslinking of a polymer solution containing the active substance to encapsulate in the presence of multivalent cations [59]. As a result, complexation between oppositely charged species occurs under continual agitation [39]. Alginate and chitosan are the two most common coating materials. Both are nontoxic, highly biocompatible polymers with good mechanical resistance, making them appropriate for active food packaging applications [23].

Solid lipid nanoparticles (SLNs) are gaining popularity as attractive carriers for bioactive agents, particularly those with a lipophilic character such as EOs. SLNs are nanometer-sized colloidal particles formed from oil-in-water emulsions containing lipids that are solidified at room temperature and stabilized by the addition of a surfactant. The advantages of SLNs over other encapsulation methods include their biodegradability, gradual degradation, sustained release of active agent, and



#### **Table 1.**

*Technological trends of active film packaging containing EO.*

greater encapsulation effectiveness for lipophilic substances [60]. **Table 1** reports technological trends of active films packaging containing EO in the last 6 years (ScienceDirect—Elsevier).
