Chitosan is among the most studied polysaccharides for the development of film/coating packaging. This is justified by its versatility, good physical-chemical and biological properties. Mechanical properties are affected by the chitosan DA, solvent, pH, concentration, viscosity, and molecular weight ^[1][4]. Moreover, chitosan is generally less expensive and commercially available when compared to other biopolymers ^[13][25]. The first reports of chitosan films are from the mid-1930s, when George W. Rigby patented it ^[14][24]. Since then, the employment of chitosan as films, coatings, or composites, to different products has been extensive. Recent works on the sustainable application of chitosan-based films for food packaging were reviewed by Haghighi et al. ^[3][22] and Mujtaba et al. ^[1][4].

As with any other packaging, edible films and coatings must protect the food integrity, maintaining quality, controlling mass and volatile losses, extending its shelf life, preventing surface contamination, offering mechanical protection, and improving its sensory properties (Figure 16) ^{[3][15][16][17]}[22,86,87,88]. Although, it is not expected that edible films can completely replace all conventional packaging, they can be used to significantly reduce the use of petroleum-based plastics, decrease food losses, and reduce the environment pollution in the long-term. Edible films and coatings may have in their composition two main based materials, a biomacromolecule-based matrix, and additives, such as plasticizers, cross-linkers, other reinforcements substances and functional ingredients ^[17][88]. These materials can be used alone or blended. As edible packaging is composed of natural-based materials, it can be classified as a subgroup of biobased and biodegradable packaging ^[16][87]. Generally, polysaccharides, proteins, lipids, and their mixtures are used for the development of edible films and coatings ^[17][88].

Films based on chitosan are environmental-friendly materials and can be degraded by microorganisms ^[18][89]. Leceta et al. ^[19][90] have studied the environmental assessment between two different food packaging systems, polypropylene, and chitosan films, during different life cycle stages. For them, from about 1 kg of chitosan-based film waste, 20% can be considered as compost and the remaining 80% degraded as organic matter by microorganisms. Furthermore, these authors found that the highest positive impact of chitosan-based films was related to the end-of-life scenarios, especially associated with composting and carcinogens, when compared to polypropylene films, which can provide a reduction of environmental pollution generated by the food packaging industry.

In another work, the biodegradability of chitosan-based films on three different soils, industrial compost, commercial garden soil, and soil from a vineyard, was evaluated. The authors found that the properties of the chitosan-based film deteriorated in less than 3 days and its biodegradation occurred in all tested soils after 14 days ^[20][91]. Moreover, when these films were incorporated with Quercus polyphenol extract its active properties in compost and garden soil occurred in 6 days, whereas the total biodegradation process was not completed in the vineyard soil during the 14 days. These authors also found that adding water to the soil decreased the rate of biodegradation from the chitosan film on the terrestrial environment. These findings are relevant to confirm the biodegradability of chitosan films when using it as a sustainable alternative food packaging.

Pure chitosan films can only be prepared via solvent casting. In this process, chitosan is dissolved using enough solvent, mainly, acidified water, placed on a flat surface and let to dry until constant weight. Generally, these pure films are reported to exhibit a smooth, continuous, and compact surface, but are brittle and fragile possessing low mechanical properties ^[21][22][23][26,33,92]. To overcome the low mechanical performance and high sensitivity to water, the addition of plasticizers and other different approaches have been indicated. Some of these use crosslinking, complexation, graft copolymerization, surface coating, filler incorporation and others ^[3][22]. Blending chitosan with other polymers by solution or extrusion blending to form composite films could also be a strategy. Binary or ternary blends, e.g., CS-gelatin, CS-alginate, CS-gelatin-caprolactone, have been developed for food and biomedical applications ^[22][33]. The formation of a cohesive film-forming material resistant to rupture between CS and cellulose in the paper industry have also been reported, as the wet strength of paper can be improved by CS addition ^[22][33]. In another study, CS was used as a surface coating agent on printing paper, improving its mechanical properties, such as its strength, and inhibited the growth of bacteria, such as E. coli ^[24][93].

Regarding the mechanical properties, tensile strength (TS) and elongation at break (EB) are two of the most important characteristics for film applications ^[13][25]. TS is related to the maximum tensile stress that a film can hold up ^[25][94]. Moreover, the TS of chitosan films with high and low molecular weight was described as dependent on storage time due to the conformational changes of chitosan molecules by the free volume reduction ^[26][95]. The type of solvent acid used to prepare chitosan-starch composite films also influenced TS according to the following order: lactic acid < malic < acetic acid ^[27][96]. In another work, chitosan film displayed a significantly lower TS, 18.252 MPa, but a higher EB, ~40%, than gelatin and composite films ^[28][97]. In the development of chitosan-zein composite films, Sun et al. ^[25][94] found that TS was influenced by different plasticizers, sorbitol < glycerol and PEG-400; this result was explained by the number of hydroxyl groups in the plasticizer. Sorbitol has the capacity of binding a higher number of water molecules. The TS values may also decrease with the increase in pH because of a lower chitosan dissociation ^[6][57]. Furthermore, in chitosan composite films with quinoa protein, it was observed that TS and EB were influenced by the presence of protein in the films ^[29][98]. Some application examples of chitosan-based films are listed in Table 12.

Water vapor and the oxygen barrier are relevant factors to consider in the development of food packaging since both can influence the food quality and shelf life. The evaluation of water vapor permeability (WVP) of polysaccharide-based films provides information of diffusion and solubility of water molecules through the polymeric matrix and will be influenced by its composition. In this context, the WVP of high and low molecular weight chitosan films was investigated by Kerch and Korkhov ^[26][95]. They found that the high molecular weight chitosan films exhibited higher permeability. The addition of a natural phenolic antioxidant, protocatechuic acid, significantly decreased the WVP of chitosan composite films ^[43][112]. Furthermore, the addition of vegetable olive and corn oils displayed a significant effect decreasing the WVP by reducing the hydrophilic content of the film ^[34][103]. Different mechanical and barrier properties of chitosan films have been reported. These properties are affected by chitosan molecular weight, solvent, plasticizer content, film composition, and pH, which makes the comparison between them more challenging.

In the development of biobased films, the hydrophilic character of their natural macromolecules matrix should be considered since water molecules may be a problem for the shelf life of the product as well as the physical stability of the packaging. To overcome this issue, hydrophobic or less hydrophilic substances can be incorporated to the films modifying the physical properties. These substances are denominated plasticizers and are generally of low molecular weight. Plasticizers are responsible for increasing the matrix free volume and molecular mobility to amorphous biopolymers, competing chain-to-chain H-bonding along the polymer chains ^[44][45][6,7]. They are often used to reduce the brittleness by reducing the polymer interchain interactions by putting themselves between polymer molecules, although they do not chemically bind to it ^[17][88].

The most commonly used plasticizers are glycerol, sorbitol, polyethylene glycols (PEG), lipids and their derivatives, such as fatty acids, phospholipids, lecithin, oils, and waxes, low molecular weight sugars, e.g., fructose-glucose syrups and honey, and others ^[44][17][23][6,88,92]. Crosslinkers and micro/nano-reinforcements can be incorporated into film packaging aiming superior tensile, gas and water barrier properties. The improvement of coatings cohesive strength can be achieved by application of other physical treatments, i.e., irradiation, by crosslinking formation ^[44][6]. More recently, films’ plasticization can be performed using deep eutectic solvents (DES), as sustainable solvent systems ^[46][47][48][8,9,10].

DESs are obtained by the complexation of a hydrogen bond donor (HBD) with a quaternary ammonium salt (hydrogen bond acceptor—HBA), producing liquids with physical and solvent properties analogue to ionic liquids (ILs) ^[49][50][11,12]. Ionic liquids are liquid solvents with a melting point below 100 °C and are formed from systems composed primarily of one type of discrete anion and cation. Initially, DESs were extensively used to decrease the temperature of molten salt. Nowadays, they have been used for a wide range of applications, from lubrication of steel to synthesizing drugs ^[50][51][12,113].

DESs contain large asymmetric ions of the HBA that have low lattice energy and so, low melting points, caused by hydrogen bonding formation. A wide range of mixtures of substances can form DES, which was critically reviewed by Smith et al. ^[50][12]. For plasticizing film packaging the most common DES used are the mixture of choline chloride-citric acid (ChCl-CA), choline chloride-urea (ChCl-Urea) and choline chloride-glycerol (ChCl-Gly). The latter has been used mainly for chitosan-based coatings ^[46][47][52][8,9,13]. Particularly, the eutectic mixtures of ChCl:Gly at a molar ratio of 1:2 were found to efficiently plasticize starch and starch/zein blends, and chitosan and chitosan/micro-crystalline cellulose/curcumin composites ^[47][48][9,10].

Many advantages can be pointed out from using DES as they are much cheaper, safer and easier to manufacture than ionic liquids. Moreover, DES are chemically and thermally stable, non-flammable, possessing high dissolution ability, lack of flammability, low volatility, low melting point and tailorability, as well as being water neutral, and low- or non-toxic. The DES toxicity, including cytotoxicity and phytotoxicity, is highly dependent on the testing species used on the mixture and their responses, as well as their physicochemical properties, concentration, and salts counteranion species ^[52][13]. In the work of Mbous et al. ^[53][114], the toxicity of some DES was tested, and they found that pure ChCl exhibited lower cytotoxic values than its aqueous solutions (EC50 30 ≥ ChClaq ≤ 40 mM), suggesting that ChCl did not dissociate after crossing the cellular membrane, implying a nontoxic profile of ChCl-based DES, although more in vivo or in vitro studies could be further investigated ^[53][114]. As they are often biodegradable, they can be considered green solvents. DESs are appropriate for biobased packaging processing and application. For the preparation of polysaccharide films, the DES mixture should be separately prepared and incorporated to the matrix materials or can be formed in situ after heat homogenization with other components ^[52][13].