Chitosan-based composites play an important role in food packaging applications and can be used either as protective films or as edible coatings. Due to their costs and performance (i.e., a barrier against water vapor, thermal, and mechanical properties) when compared to traditional petroleum-based plastics, their use on a large scale is still limited. But several new approaches in the packaging industry are emerging to overcome some of these issues. This review presents the current trends toward the production and application of chitosan composites in the food packaging and coating industry.

Chitosan is a flexible biopolymer that can be used to fabricate non-toxic, biodegradable, biocompatible, and antimicrobial food packaging and edible food coatings. It can be an alternative to petroleum-based synthetic non-degradable plastic-based food packaging materials. With this, chitosan films and coatings have been studied extensively as they are renewable, biocompatible, biodegradable, and non-toxic. Moreover, this natural bioactive polymer has an inherent antimicrobial activity as a film or coating for food preservation [1]. Several approaches towards chitin/chitosan composite films and coatings in the packaging industry are emerging namely, the incorporation of nanofillers and bioactive agents or as blends or bilayers with other biopolymers. Novel and more sustainable processes for chitosan production are also being implemented through the use of novel plasticizers to obtain thermoplastic chitosan films.

Coatings are defined as coherent layers formed from coating materials to a substrate, which can be either directly applied onto the surface of foods, as edible coatings, or onto the surface of packaging materials to functionalize them [3,4].  As an edible coating, chitosan has been extensively studied to extend the shelf-life of food products, especially fruits and vegetables (Table 1). Good reviews on this subject are available [5–8]. The coatings can be applied directly onto food products by the addition of a liquid film-forming dispersion (with a paintbrush, fluidizing, spraying, or dipping) or of molten compounds [9]. In fruits and vegetables, chitosan-based coatings have demonstrated the ability to retard ripening and water loss and reduce decay [10], while in meat products they can improve their quality by delaying moisture loss, enhancing product appearance, reducing lipid oxidation and discoloration, and also as a carrier of food additives [11]. Moreover, chitosan coatings possess good oxygen and carbon dioxide barrier properties [12] along with their intrinsic antimicrobial properties which can also impede microorganisms’ development and synergistically extend the shelf life of the coated food [9–11].

Current trends in coatings are the incorporation of preservatives into the polymeric matrices, aiming to preserve the food and increase its shelf life. This type of coating (antimicrobial and/or antioxidant coatings) is an alternative to the conventional coatings for food, which protect only from water loss or damage [13]. In the case of chitosan coatings, the active compound may enhance the intrinsic antimicrobial properties of this polysaccharide, thus, its preservative ability. Recently, the incorporation of active compounds of natural origin in biodegradable films or edible coatings is playing a significant role in more environmentally friendly packaging [14]. The active compounds, such as natural extracts from plants, rich in phenolic compounds, or essential oils, are capable of enhancing either antimicrobial and/or antioxidant properties of chitosan, thus increasing the preservative properties of the coating and its ability to extend the shelf life of foods [15–17]. In addition, this type of packaging is attractive to the growing number of consumers looking for greener packaging options.

With the advancement of nanotechnology, new concepts, such as nanocoatings, which consist of ultra-thin nanoscale layers (less than 100 nm) built-up onto surfaces, are also being explored. This type of coating has the advantage of not modifying the surface topography of the material while adding physical and chemical functions to the surface, such as altering gas barrier properties, surface hydrophobicity, or conductive properties, to name a few [3,18].

Altering the surface of packaging materials can be done via several methods and techniques, which depend on the purpose of the material to be developed and can be divided into two groups: migratory or non-migratory active packaging. Examples of the former are through embedding, non-covalent immobilization, or layer-by-layer deposition, and the latter with photografting or covalent immobilization [3].

Improvements in mechanical properties, better performance in terms of water vapor permeability, and lower water solubility have been reported for combinations of chitosan with other polysaccharides, such as starch, pectin, or alginate [35–37], microbial polysaccharides [38,39] and proteins, like gelatin [40] and whey proteins [41,42], compared to chitosan stand-alone films. This fact is attributed to the formation of polyelectrolyte complexes through electrostatic interactions between the protonated amino groups of chitosan and the negatively-charged side-chain groups in the other biopolymer at the operating pH [35,43]. Some authors reported difficulties in the total solubilization of one of the polymers in specific conditions and the formation of insoluble complexes between polymers in blend preparation [41]. Bilayer systems can overcome this constraint, and reports show that these systems have better water vapor barrier properties than blend films [40,42].

The incorporation of nanoscale reinforcements (e.g., montmorillonite, nanocellulose, metal oxide nanoparticles) in chitosan films, that can interact chemically or physically with the polymeric chain, is an approach that seeks to rectify the intrinsic flaws, like low water resistance, poor mechanical and barrier properties, that are attributed to the hydrophilic nature of chitosan [17,44].

Montmorillonite (MMT) is a layered silicate mineral clay, naturally present in volcanic rocks (bentonites), which is being pointed as a reinforcement material to bioplastics due to its wide availability, swelling, and plasticizer ability, mechanical resistance, and low cost, just to mention a few characteristics [45].

Recently, several studies with nanocomposites based on chitosan and MMT have been investigated, and, in general, an enhancement in the mechanical and barrier properties is observed when MMT is incorporated into the chitosan film [16,17,46–50]. Beigzadeh Ghelejlu et al. (2016) [46], Giannakas et al., (2016) [51], and Nouri et al. (2018) [49], noticed that low amounts of nanoclay in the bio-based films it is possible not only to improve strength, stiffness, and elongation at break but also increase water and oxygen barrier. In terms of optical properties, Souza et al., (2018) [45] showed that chitosan films with MMT exhibited a remarkable light block barrier, especially at UV wavelength, acting as extra protection against oxidation processes. Further, was also demonstrated that this kind of nanocomposites have an increased antimicrobial activity [49,51]. Inspired by this Pires et al., (2018) [48] and Souza et al., (2018) [52] tested the nanocomposites in a perishable food matrix, demonstrating their potential to be used as primary packaging material, being capable of retarding deterioration process by antimicrobial and antioxidant mechanisms and extending its shelf life.

Cellulosic fibers in the nanoscale, namely cellulose nanofiber (CNFs) and cellulose nanocrystals (CNCs), are an appealing reinforcement in chitosan towards the production of environmentally friendly composite films with refined physical properties due to their high compatibility with chitosan. The high interaction due to the electrostatic association and hydrogen bonds between nanocellulose with large length-diameter ratios and chitosan molecules causes the formation of an interactive network structure providing an increment in the film’s crystallinity [53,54]. Thereby, chitosan/nanocellulose composites have a large spectrum of applicability and potential in the field of biomedical, packaging, and water treatment [55–57]. In two different studies, nanocrystalline cellulose was incorporated as a reinforcing agent in chitosan-guar gum [58], starch-chitosan, and gelatin-chitosan composites [59]. Both works accomplished a transparent and thermally stable biopolymer-based nanocomposite with improved mechanical and barrier properties. This new type of safe, non-toxic, renewable, and biodegradable chitosan/nanocellulose films, as a novel food packaging material, may one day replace petroleum-based polymers.

Nanoscale metal oxides, such as ZnO, SiO₂, TiO₂, or MgO, add more value to chitosan due to their synergetic properties, including antimicrobial, UV blocking, and magnetic properties, in addition to their reinforcing ability [60–63]. Among the metal oxides, zinc oxide (ZnO) is one of the most broadly applied materials in several fields due to its notable antimicrobial and photocatalytic properties. ZnO nanoparticles in parallel with other metal oxide nanoparticles are regarded as safe materials for human beings and have been used as food additives, packaging materials, and in water purification [61]. In Youssef et al., (2015) [60] work, films loaded with ZnO nanoparticles showed antibacterial activity against Staphylococcus aureus, Escherichia coli, S. typhimurium, Bacillus cereus, and Listeria monocytogenes. Recently, Al-Naamani et al. (2016) [61] obtained successful results, showing that chitosan/ZnO coating on polyethylene films provided an effective antimicrobial defense against S. enterica, E. coli, and S. aureus, with fully-inhibited growth of the pathogens after 24 h incubation. Titanium dioxide (TiO₂) is also an attractive inorganic nanomaterial that has lifted great interest in environmental and energy fields because of its low cost, high photocatalytic performance, high chemical stability, and biocompatibility [62]. The addition of TiO₂ nanopowder has been reported to enhance the mechanical properties of chitosan-based nanocomposite films [64,65]. The chitosan/TiO₂ film produced by Zhang et al. (2017) [62] showed efficient antimicrobial activity against four tested strains, Escherichia coli, Staphylococcus aureus, Candida albicans, and Aspergillus niger with 100% sterilization in 12 h. Moreover, it induced the leakage of cellular substances through the damaged membranes. Furthermore, the work of Silva et al., (2017) [66] highlighted that tensile strength and elastic modulus of chitosan nanocomposites with 5 (w/w%) MgO improved by 86% and 38%, respectively, compared to pristine chitosan. Chitosan nanocomposites with MgO nanoparticles also showed superior UV-shielding and moisture barrier properties. Therefore, the fabricated chitosan/metal oxide nanocomposite films with enhanced physicochemical properties can be used as a potential food packaging material.

Microbiological growth and oxidative processes are two mechanisms responsible for food quality deterioration leading to important changes such as loss of nutritional values, texture modifications, and the development of undesirable compounds such as off-flavors, colored, and even toxic substances to humans [4]. Thus, active packaging plays an important role in the food industry, preventing waste and promoting an increment in product shelf life [16]. In order to potentialize the innate characteristics of chitosan films, bioactive compounds like antimicrobial and antioxidant agents, gas scavengers, moisture absorbents, and nutraceutical compounds, can be added. Due to the health concerns of the consumers, current research in active packaging has focused on developing natural preservative systems such as those based on nisin, lysozyme, essential oils, or fruit and plant extracts which exhibit antioxidant or antimicrobial properties which can be an alternative to those based on artificial additives and chemical preservatives [16,67,68]. However, the use of these natural compounds in food preservation is frequently limited because of their application costs and other disadvantages like their intense aroma and potential toxicity [69]. Thus, the design of an active package where there is no contact between the substance and the food constitutes an amazing opportunity with some advantages like no taste transfer, reduced organoleptic changes, and even distribution of the active compounds in the headspace [4].

Scientific research in the field of chitosan active packaging has been focused on the identification of the active bio compounds that confer better antioxidant and antimicrobial capacities to the edible films [15,68,70]. Moreover, further studies have been published to understand to what extent the introduction of these natural compounds affects the mechanical properties of the films [4,71]. Recently the films have been brought into contact with different food matrices in order to study their influence on the organoleptic properties of the food over the shelf life [16,17,72,73]. Lekjing (2016) [73] investigated the effects on the quality and shelf life of cooked pork sausages coated with chitosan/clove oil, demonstrating that the combination of these two components inhibited microbial growth, retarded lipid oxidation, and extended the shelf life of cooked pork sausages for more than six days. However, there were some initially negative impacts on odor and taste attributes, at the start of storage time. In similar works, supplementation with ginger and rosemary essential oils also reduced the poultry meat oxidative processes [17], and Souza et al., (2019) [16] showed that in the in vitro assays, chitosan films with rosemary demonstrated good antimicrobial activity against Bacillus cereus (reduction of 7.2 log) and Salmonella enterica (reduction of 5.3 log). Briefly, the bioactive agents incorporated in chitosan films showed great promise for their application in extending the shelf-life and maintaining the quality of food products and controlling postharvest fungi and foodborne bacteria in food systems. Extra work needs to be done to understand the interactions between chitosan and bioactive compounds in order to optimize the effectiveness of the bioactive incorporated agents. Moreover, most of the studies use the casting method for the production of the polymers, which is a technique not readily applicable to the packaging industry, unlike compression molding or extrusion, in which the material is submitted to high temperatures. Thus, new tests and studies should be conducted to overcome the challenge of how to keep the antimicrobial/antioxidant activity of essential oil/extracts in the films during the high temperature of the plastic production processes [74].

Adding a wide range of lipid components, natural waxes, resins, fatty acids, and vegetable oils, to films will also confer hydrophobicity to the film and reduce moisture [75]. A decline in water solubility has been reported for chitosan films with beeswax [76] and a decrease in water vapor permeability was described for films with oleic acid [77], neem-oil [78], and cinnamon essential oil [79], among others.

The intrinsic reactive groups of chitosan, namely, OH and –NH₂, allow the chemical modification of chitosan, enhancing its application potential. The reaction between chitosan’s amino groups and carbonyl compounds via imine functionalization results in chitosan-based Schiff bases, which are of importance for certain food packaging applications (Figure 1). Chitosan-based Schiff bases have shown antimicrobial activities as powders/whiskers/films/membranes, and, interestingly, exhibit better antimicrobial properties than bare chitosan [80]. Moreover, the antimicrobial action of chitosan-based Schiff bases can be augmented by loading metal ions or metal nanoparticles through the covalent coordination bond. Some chitosan-based Schiff bases have also shown antioxidant activities improving this way the functional properties of bare chitosan. Some examples are the Schiff bases obtained from the reaction of chitosan with D-fructose, quercetin o-quinones, eugenol aldehyde, or carvacrol aldehyde [80].

The thermomechanical kneading approach was used to test different plasticizers in the production of chitosan film [105]. Different non-volatile polyol plasticizers (glycerol, xylitol, and sorbitol) were studied, and the thermomechanical treatment was done in an internal mixer in the presence of water, acetic acid, and polyol investigated. Sorbitol (the highest molecular weight polyol tested) resulted in plasticized chitosan with the highest thermal, mechanical, and rheological properties, while the films produced with glycerol (the lowest molecular weight polyol) had the lowest thermal, mechanical and rheological properties, but the highest amorphous phase content, which made its processability easier, despite its poorer properties [105].

More recently, chitosan was plasticized using a one-step extrusion process in the presence of glycerol and acetic acid solution, and mixed with polyethylene to produce blends with different content of plasticized chitosan [106]. The resulting films presented a brown color and increasing haze with chitosan plasticized content, and the mechanical and oxygen barrier properties of the polyethylene films were nearly unaffected by the presence of plasticized chitosan, while the water vapor permeability increased with the amount of the incorporated carbohydrate [106].

Similar results are reported for biodegradable blends of thermoplastic starch with plasticized chitosan obtained by thermocompression [107], blown extrusion [108,109], and melt extrusion [110]. In this regard, the extrusion processes allow the preparation of plasticized chitosan-based materials on an industrial scale, which may overcome the scale-up problem of producing chitosan films.

Chitosan can be also processed using DES. Galvis-Sánchez et al. (2016) [111] prepared thermocompression molded films with chitosan (deacetylation degree 90%), CC, and citric acid (CA) (molar ratio 1:1). CC and CA were added separately to chitosan (not as a liquid mixture), and this three components system was heated for 30 min at 70 ◦C and then 3% acetic acid solution was added, and the formed paste was hot-pressed at 120 ◦C. In comparison to chitosan/CA films, chitosan/CC/CA ones exhibited higher water sorption ability. Moreover, incorporation of CC into chitosan/CA matrix resulted in tensile strength decrease, and slight elongation at break increase. Similar results were obtained by Almeida et al., (2018) [112] where CC/lactic acid was used as a plasticizer for chitosan films with curcumin. Chitosan/microcrystalline cellulose films (plasticized with CC/G in the presence of curcumin) can be applied as pH-responsive materials [113].

Natural deep eutectic solvents (NADES) prepared from cheap raw materials were tested to produce thermoplastic chitosan films [104]. Four types of NADESs based on choline chloride prepared with malic acid (MA), lactic acid (LA), citric acid, and glycerol were used as hydrogen bond donors, and as a polymeric matrix, two chitosan with different deacetylation degrees (DD) (DD = 76 and 81). Transparent thin chitosan films were produced by thermo-compression molding, and film properties (mechanical and water resistance) varied depending on their composition/structure. A more homogenous surface, compact with lower water permeability and stronger resistance, was obtained for chitosan with lower DD and with the NADES CC/CA and CC/MA, while films produced with CC/glycerol resulted in a material with weaker properties [104].

Therefore, DES and NADES are suitable green solvent materials to be used as plasticizers to produce chitosan thermo-compressed films with tailored properties at a large scale.

Recently chitosan films and coatings have been extensively studied for food preservation since they are bioactive, biocompatible, biodegradable, and antimicrobial. However, their performance, in terms of thermal, mechanical, and water barrier properties, needs to be improved in order to be produced on a large scale and at a low cost. Blends and bilayers with other biopolymers and various nanocomposites have been developed to improve the mechanical and barrier properties of chitosan films and coatings.

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Victor G. L. Souza 1,* , João R. A. Pires 1 , Carolina Rodrigues 1, Isabel M. Coelhoso 2,* and Ana Luísa Fernando 1
1 MEtRICs, Departamento de Ciências e Tecnologia da Biomassa, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal; jr.pires@campus.fct.unl.pt (J.R.A.P.); cpe.rodrigues@campus.fct.unl.pt (C.R.); ala@fct.unl.pt (A.L.F.)
2 LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de
Lisboa, Campus de Caparica, 2829-516 Caparica, Portugal
* Correspondence: v.souza@campus.fct.unl.pt (V.G.L.S.); imrc@fct.unl.pt (I.M.C.)

Disclaimer: This article was edited by ChitoLytic and its purpose is for information only.  ChitoLytic makes no representation that the information contained within this article or video is original to ChitoLytic. 

Author Contributions: The review paper was planned and written with the contributions of all the authors. All authors have read and agreed to the published version of the manuscript.
Funding: This work has been supported by FCT—Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UID/EMS/04077/2019 and UIDP/04077/2020. This work has also been supported by the Associate Laboratory for Green Chemistry—LAQV which is financed by national funds from FCT/MCTES (UID/QUI/50006/2019 and UIDB/50006/2020).
Conflicts of Interest: The authors declare no conflict of interest.