Polysaccharide-Based Biodegradable Films: An Alternative in Food Packaging

: Packaging can mitigate the physical, chemical, and microbiological phenomena that affects food products’ quality and acceptability. However, the use of conventional packaging from non-renewable fossil sources generates environmental damage caused by the accumulation of non-biodegradable waste. Biodegradable ﬁlms emerge as alternative biomaterials which are ecologically sustainable and offer protection and increase food product shelf life. This review describes the role of biodegradable ﬁlms as packaging material and their importance regarding food quality. The study emphasizes polysaccharide-based biodegradable ﬁlms and their use in foods with different requirements and the advances and future challenges for developing intelligent biodegradable ﬁlms. In addition, the study explores the importance of the selection of the type of polysaccharide and its combination with other polymers for the generation of biodegradable ﬁlms with functional characteristics. It also discusses additives that cause interactions between components and improve the mechanical and barrier properties of biodegradable ﬁlms. Finally, this compilation of scientiﬁc works shows that biodegradable ﬁlms are an alternative to protecting perishable foods, and studying and understanding them helps bring them closer to replacing commercial synthetic packaging.


Introduction
Packaging is one of the most important issues in the food industry [1] because its functional capacity protects against gases (e.g., oxygen, nitrogen, and carbon dioxide), humidity, and possible mechanical damage [2]. Additionally, it provides the information needed about the product and allows its commercialization and distribution [3][4][5]. Although packaging characteristics depend on the food product they protect, the materials most used are paper, cardboard, metal, glass, and plastic [6]. However, these synthetic materials have been restricted because they consume finite resources and they are not biodegradable, reusable, or recyclable [7]. According to data from 2018, about 102,895 tons of waste is generated per day in Mexico, including cardboard, paper, metals (aluminum), and glass [8], which contributes to the global problem of solid waste distribution in the environment [9]. Furthermore, only 10% of total synthetic materials are recycled [8,10], and the degradation treatment of them is considered dangerous and harmful to human health and economically unprofitable. As a result, synthetic material packaging is used once before being discarded [11].
Recently, food packaging has been directed toward the development of technologies for the generation of packaging with biodegradable materials which can serve as emerging materials or as substitutes for traditional packaging [2]. For example, films with biodegradable characteristics (biodegradable films) are considered a food packaging alternative [12] because they can preserve quality and extend the shelf life of minimally processed products [13,14]. Therefore, its study has been increasing in the last decade, as shown in Figure 1. This review offers an overview of food packaging and its characteristics

Food Packaging
Food packaging is any wrapper whose function is to protect food from physical, chemical, and biological contamination and preserve its quality ( Figure 2) [15]. Physical contamination is any external material (e.g., bumps or pieces of glass, plastic, and wood) that are is part of the food and that is normally associated with unhygienic conditions during the preparation, production, storage, and distribution of food products. In contrast, chemical contamination can occur from the addition of food additives (e.g., flavors, colors, and sweeteners) or other chemicals (e.g., antibiotics, sanitizers, pesticides, and lubricants) during processing, food preparation, and storage. Biological contamination is related to any micro-organisms (e.g., Salmonella sp., Clostridium sp., and Escherichia coli) or harmful fauna (e.g., rats, mice, and cockroaches) that produce toxins (e.g., aflatoxins, citrinin, and alternariol) or cause consumer illness [16,17].

Food Packaging
Food packaging is any wrapper whose function is to protect food from physical, chemical, and biological contamination and preserve its quality ( Figure 2) [15]. Physical contamination is any external material (e.g., bumps or pieces of glass, plastic, and wood) that are is part of the food and that is normally associated with unhygienic conditions during the preparation, production, storage, and distribution of food products. In contrast, chemical contamination can occur from the addition of food additives (e.g., flavors, colors, and sweeteners) or other chemicals (e.g., antibiotics, sanitizers, pesticides, and lubricants) during processing, food preparation, and storage. Biological contamination is related to any micro-organisms (e.g., Salmonella sp., Clostridium sp., and Escherichia coli) or harmful fauna (e.g., rats, mice, and cockroaches) that produce toxins (e.g., aflatoxins, citrinin, and alternariol) or cause consumer illness [16,17]. characteristics and properties and the characteristics that biodegradable films must meet to be considered food packaging materials.

Food Packaging
Food packaging is any wrapper whose function is to protect food from physical, chemical, and biological contamination and preserve its quality ( Figure 2) [15]. Physical contamination is any external material (e.g., bumps or pieces of glass, plastic, and wood) that are is part of the food and that is normally associated with unhygienic conditions during the preparation, production, storage, and distribution of food products. In contrast, chemical contamination can occur from the addition of food additives (e.g., flavors, colors, and sweeteners) or other chemicals (e.g., antibiotics, sanitizers, pesticides, and lubricants) during processing, food preparation, and storage. Biological contamination is related to any micro-organisms (e.g., Salmonella sp., Clostridium sp., and Escherichia coli) or harmful fauna (e.g., rats, mice, and cockroaches) that produce toxins (e.g., aflatoxins, citrinin, and alternariol) or cause consumer illness [16,17].

Characteristics of Food Packaging
Traditional food packaging must be inert to the stored product, which means it must not interact with the product [18]. For this reason, they are considered passive containers because their only function is to create physical protection against mechanical, chemical, and biological damage and a barrier against gases and humidity [2]. However, the study of the adverse effects in food caused by the environment gave guidelines to intentionally incorporate compounds or substances in the container and achieve an active container [19,20]. This type of packaging intentionally modifies the environmental conditions to ensure the sensory and microbiological properties of the food, thus eventually extending its shelf life [18]. Active packaging can be classified into absorbers and emitters. Absorbers capture substances produced by food, such as steam, ethylene, and carbon dioxide, and release them into the environment, while emitters release bioactive substances, such as microbial and antioxidants [18][19][20].
The increase in the demand for food in the market stimulated the innovation of food packaging to improve the characteristics of traditional and active packaging that were used until a few years ago. Subsequently, packaging with new characteristics was conceptualized, or so-called intelligent packaging. This type of packaging is made up of a system influenced by the physicochemical properties, temperature, exposure time, or enzymatic reactions of foods that monitor, generate, and display the information [21]. Intelligent packaging uses chemical sensors or biosensors to monitor food quality, including ripeness, freshness, temperature, oxygen levels, moisture, or other gases [18].
Active packaging is more studied than intelligent packaging since there are many extracts and essential oils extracted from natural sources that have an antioxidant or antimicrobial nature and can be added as a biological additive [22]. Nevertheless, active packaging and intelligent packaging are not mutually exclusive since both can work synergistically together [23], as shown in Figure 3. There are even authors who contradict the idea of using the terms intelligent packaging and smart packaging as synonyms because they establish that smart packaging is active-intelligent packaging in which the characteristics of both are added [18].

Characteristics of Food Packaging
Traditional food packaging must be inert to the stored product, which means it must not interact with the product [18]. For this reason, they are considered passive containers because their only function is to create physical protection against mechanical, chemical, and biological damage and a barrier against gases and humidity [2]. However, the study of the adverse effects in food caused by the environment gave guidelines to intentionally incorporate compounds or substances in the container and achieve an active container [19,20]. This type of packaging intentionally modifies the environmental conditions to ensure the sensory and microbiological properties of the food, thus eventually extending its shelf life [18]. Active packaging can be classified into absorbers and emitters. Absorbers capture substances produced by food, such as steam, ethylene, and carbon dioxide, and release them into the environment, while emitters release bioactive substances, such as microbial and antioxidants [18][19][20].
The increase in the demand for food in the market stimulated the innovation of food packaging to improve the characteristics of traditional and active packaging that were used until a few years ago. Subsequently, packaging with new characteristics was conceptualized, or so-called intelligent packaging. This type of packaging is made up of a system influenced by the physicochemical properties, temperature, exposure time, or enzymatic reactions of foods that monitor, generate, and display the information [21]. Intelligent packaging uses chemical sensors or biosensors to monitor food quality, including ripeness, freshness, temperature, oxygen levels, moisture, or other gases [18].
Active packaging is more studied than intelligent packaging since there are many extracts and essential oils extracted from natural sources that have an antioxidant or antimicrobial nature and can be added as a biological additive [22]. Nevertheless, active packaging and intelligent packaging are not mutually exclusive since both can work synergistically together [23], as shown in Figure 3. There are even authors who contradict the idea of using the terms intelligent packaging and smart packaging as synonyms because they establish that smart packaging is active-intelligent packaging in which the characteristics of both are added [18].

Biodegradable Films
Biodegradable films are solid matrices with a thickness of less than 0.3 mm [24] which are formed with biopolymer material [25,26] using casting, extrusion, or electrospinning techniques [27]. Biodegradable films are considered active packaging as their protective function includes the addition of bioactive compounds [2]. Biodegradable films present greater benefits than conventional packaging since in addition to meeting the main objectives of food passive packaging (i.e., protection against mechanical, chemical, and biological damage), they can also reduce UV light interaction [28], control the absorption of gases (e.g., oxygen, ethylene, carbon dioxide, and water vapor) [28,29], and control the release of bioactive components (e.g., antioxidants, antimicrobials, and flavors) [30][31][32]. These objectives create benefits, including the reduction of lipid oxidation and the increase in the nutritional value of food and its shelf life [33,34].

Polysaccharides in Biodegradable Films Formulations
The formation of biodegradable films is carried out due to the interaction between polymers (i.e., polysaccharides, lipids, and proteins), and polysaccharides are the most widely used biopolymers for biodegradable films due to their accessibility [35]. The main natural sources of polysaccharides are plants (e.g., cellulose, starch, and pectin), animals (e.g., chitin, chitosan, and hyaluronic acid), algae (e.g., alginate, agar, and carrageenan), fungi (e.g., glucans and pullans), and bacteria (e.g., dextran, xanthan, and gellan) [35]. According to the Scopus database, more than 50% of biodegradable film reports are based on plant polysaccharides, with cellulose being the most widely used polymer [36]. This is justifiable because it is the most abundant polysaccharide in nature, it is inexpensive, and it is obtained using simple methods, such as alkaline hydrolysis [37]. Chitosan and chitin (ca. 30%) are the most used animal polysaccharides for the generation of biodegradable films [36], and their extraction is based on the demineralization, deproteination, and distillation of residues from the shells of crustaceans [38]. These animal polysaccharides are appreciated due to their antimicrobial activity caused by their cationic characteristics [35]. In contrast, algae polysaccharides represent around 11% of the natural polymers reported for the generation of biodegradable films [36]. Alginate stands out because its extraction is traditionally carried out from brown algae using hydrolysis acid-alkaline [39]. Less than 10% of the rest of the studies report biodegradable films based on fungi and bacteria [36]. This is because bacteria and fungi need strictly controlled conditions of temperature, substrate, and exposure times so that the production yield is high [40]. However, the production of a polysaccharide by fermentation or by culture cannot be compared with the extraction yield of a polysaccharide from a plant or an animal.

Additives in Biodegradable Films Formulations
Additives, such as plasticizers, crosslinkers, or bioactive components, improve their characteristics and properties [25,26]. In the compilation made by Suderman et al. [41], it was stipulated that the most used plasticizers in biodegradable films are glycerol, sorbitol, xylitol, and fructose because they influence the microstructure and consequently improve mechanical (e.g., brittleness and flexibility), physicochemical (e.g., solubility), thermal (e.g., thermal decomposition), and barrier (e.g., gas absorption) properties. However, the interaction between polymer and plasticizer depends on the nature of both. For example, Sanyang et al. [42] concluded that adding plasticizers (i.e., glycerol and sorbitol) to the formulation of sugar palm starch films reduced brittleness and water absorption and increased solubility and moisture. Kaewprachu [43] reported higher elongation, moisture, and water vapor permeability in fish protein films by adding plasticizers (i.e., glycerol, sorbitol, and polyethylene glycol). The use of lipids in the formulation of biodegradable films has been carried out only in composite films, including some lipids (e.g., oils, waxes, and resins) mixed with polysaccharides and/or proteins, because their hydrophobic nature hinders the formation of cross-linking [44]. For example, Hassan et al. [45] formulated sugar palm starch/chitosan films with olive oil and noted that the lipid acted as a plasticizer that improved the elasticity and brittleness of the biodegradable films and stabilized the thermal and barrier properties. Additionally, Da Silva E Silva et al. [46] noted an increase in mechanical strength and a decrease in water vapor permeability when buriti oil was added to fish gelatin films.
Biodegradable films can use crosslinking agents, such as stimuli (e.g., pH and electrical charges) or components (e.g., ions and enzymes) to generate physical, chemical, or enzymatic changes and improve the interaction between polymeric components [47][48][49][50][51][52]. Chitosan is an acetylated polysaccharide that necessarily needs a cross-linking agent (an acid such as acetic, formic, or lactic acid) [53] to acidify its medium and increase its protonation [20] so it can interact with other molecules and eventually form biodegradable gels and films [54]. In amino polysaccharides or proteins, it is common to use genipin as a crosslinking agent because it produces nucleophilic reactions between amino and carboxylic groups in a neutral-acid medium [55] so they can generate biodegradable films due to intermolecular interactions [56]. Other protein cross-linking agents are enzymes (e.g., transglutaminases) because they can catalyze isopeptide bonds and improve the three-dimensional network of biodegradable films [57,58]. In contrast, the use of radioactive waves (e.g., electron beam, gamma radiation, and ultraviolet light) in starches can cause hydrolysis and linear restructuring of the chains, which then cause an increase in the hydrogen bonds and the crystallinity of the film [59,60].

Biodegradable Films in Food Packaging
Foods are susceptible to spoilage due to physical, enzymatic, chemical, or microbiological effects, which accelerate maturation and senescence and consequently modify product quality [66] (Figure 2). Food quality is based on sensory characteristics and its nutritional and functional properties which allow their acceptability. For direct consumers, sensory characteristics, such as color, appearance, texture, shape, size, odor, and taste demarcate the degree of product acceptability [67]. Therefore, packaging is important as it protects and maintains food quality from post-harvest until it reaches the consumer. Table 1 shows recent reports of the application of biodegradable films in food protection. As can be seen, biodegradable films have been tested on vegetables, fruits, cereals, grains, seeds, dairy and bakery products, meats and sausages, and seafood, and their requirements are varied due to the characteristics of the food product.

Polysaccharide-Based Biodegradable Films for Wet and Fatty Foods
Foods with high moisture content and nutritional composition rich in proteins and lipids, such as meat, are highly perishable [95]. For example, the most consumed meats (i.e., chicken, beef, lamb, pork, and fish) have moisture content between 65-80%, with chicken meat being the one with the highest protein content (~31%) and the least amount of fat (~4%), whereas beef has the highest amount of fat (~8%) and the lowest protein content (~27%) [96]. Some research has proposed the use of biodegradable films for the protection of various types of meat, such as the case of Muppalla and Chawla [84] and Kanatt and Chawla [85] who tested biodegradable films of gum Arabic/polyvinyl alcohol (PVA) and cyclodextrin/gelatin/PVA, respectively, in the protection of chicken breast. Kanatt and Chawla [85] increased the antioxidant and antimicrobial activity against Staphylococcus aureus and fecal coliforms in their films by adding mango skin extract, which prolonged the meat shelf life up to 10 days (at 3 • C). Muppalla and Chawla [84] added the seed cover extract of Zanthoxylum rhetsa to their films, and they noted an increase in bioactive components, antioxidant activity, and antimicrobial activity against Staphylococcal up to 15 days (at 4 • C). The bioactive activity of natural extracts is related to the presence of phenolic components, which unleash oxidation-reduction reactions (redox), donate hydrogens, reduce components, and act as chelating agents [97]. In addition, the redox mechanism affects the proteins of the cell membranes of micro-organisms and causes partial or total damage [98].
Phothisarattana et al. [87] and Leelaphiwat et al. [88] protected pork with starch/poly (butylene adipate terephthalate) (PBAT) films with different additives. Initially, the study by Phothisarattana et al. [87] reported a decrease in lipid oxidation after day 9 and up to day 12 (at 4 • C), attributing the result to the vaporization of oxidized products due to the effect of the three-dimensional conformation of the film. The authors stipulated that the content of zinc oxide (ZnO) nanofillers in the biodegradable films promoted their permeability as increasing the ZnO lowered the formation of oxidized components was lower and increased their release. In addition, the study [87] reported that the increase in ZnO in the films statistically decreased the number of micro-organisms (i.e., total count, lactic acid bacteria, yeast, and mold) in the meat for 12 days (at 4 • C) due to its antimicrobial effect; however, it was remarkable that when using 3-5% of ZnO, the lowest count was obtained, and there was no significant variation between the increase in the additive. The antimicrobial effect of ZnO is related to its ability to penetrate the cell membrane and trigger redox reactions within the micro-organism [99]. Leelaphiwat et al. [88] used nisin and ethylenediaminetetraacetic acid (EDTA) as additives in starch/PBAT films and observed that the antibacterial effect of the additives retarded the proliferation of total bacteria and lactic acid bacteria in meat stored for 12 days (at 4 • C) which was attributed to the concentration and functional release of nisin-EDTA and the exposure of pathogens with them. The combination of nisin and EDTA has already been reported previously due to the synergy of its antimicrobial activities [100]. Nisin has antimicrobial activity due to its ability to enlarge the pores of the membrane and cause egress from cell organelles and inhibit cell wall biosynthesis [101], while EDTA modifies the pH of the medium and denatures the proteins of the cell membrane [102].
Ehsani et al. [93] evaluated two types of polysaccharides (i.e., alginate and chitosan) mixed with glycerol and two additives (sage essential oil and lactoperoxidase) for the generation of biodegradable films to protect fish burgers. The authors [93] report that biodegradable films decreased the concentration of the total viable count, the psychrotrophic bacteria count, Pseudomonas spp., and Shewanella spp. in fish burgers stored for 20 days (at 4 • C); however, the chitosan/lactoperoxidase films presented the most outstanding results. In addition, the combination of chitosan and lactoperoxidase controlled meat oxidation during a longer storage time (20 days at 4 • C). The sensory evaluation showed that the fish burger affected the smell, color, and acceptability; however, the products preserved in the chitosan/lactoperoxidase films maintained their acceptability during the study period (20 days, 4 • C) [93]. The success of the results is closely related to the characteristics and synergy of chitosan and lactoperoxidase. Chitosan naturally has an antimicrobial effect due to the amino groups (-NH 2 ) present in its structure that trigger redox reactions in cell membranes [20]. Lactoperoxidase is a multicomponent system (lactoperoxidase enzyme, hydrogen peroxide, and thiocyanate) that produces hypothiocyanite [103], which crosses the cell membrane through porins and prevents the absorption of substrates (e.g., sugars and minerals) through the release of these to the outside of the cell [104].

Polysaccharide-Based Biodegradable Films for Post-Harvest Foods
The qualities of natural foods, such as fruits, vegetables, cereals, and grains refer to intrinsic characteristics (e.g., color, size, and firmness), sensory parameters (e.g., taste and smell), and nutrients (e.g., vitamins, minerals, fiber, and phytochemicals). These qualities can be negatively affected by physiological (e.g., dehydration, aging, and softening) or microbiological (e.g., bacteria, yeast, or mold) deterioration [105]. For this reason, the application of biodegradable films as a method of post-harvest foods protection has been proposed as an alternative.
The use of natural extracts as natural additives in biodegradable films is a current area of study due to the characteristics that it adds. For example, Sganzerla et al. [73] incorporated feijoa (Acca sellowiana (Berg) Burret) extract into the formulation of starch/pectin films for grape protection over 30 days (at 20 • C). The results of the study [73] show that the firmness of the peel and the pH of the fruits were maintained, and pulp firmness, weight, phenolic content, and antioxidant activity decreased by~40, 47, 24, and 70%, respectively, while the acidity and the soluble solids increased (~19 and 18%, respectively). Indumathi et al. [75] coated grapes with biodegradable films of chitosan and cellulose acetate phthalate for 28 days (at room temperature) and reported that the addition of ZnO nanoparticles in the formulation improved the properties of the films (e.g., tensile strength and elongation). The films allowed the preservation of up to~70% of the weight of the grapes and reduced the bacterial load by~30% compared to fruits without protection, and as a consequence, the shelf life increased [75]. The incorporation of essential oils in biodegradable films has been studied, particularly for their pharmacological effects, such as anti-inflammatory, anticancer, and antioxidant [106]. Motelica et al. [74] combined citronella essential oil with ZnO and silver nanoparticles in chitosan films to pack grapes for 14 days (at 30 • C). The results demonstrated the preservation of the fruits since moisture leakage was reduced and bacterial growth (Candida albicans, Staphylococcus aureus, and Escherichia coli) was controlled [74]. The loss in the production of fruits and vegetables is 25% worldwide because fruit and vegetable shelf life is very low after harvest and their quality standards are very demanding [107]. For this reason, innovation in additives and polysaccharides for the formulation of biodegradable films is considered a viable alternative to reduce the quality and loss of this type of natural food.
Lipid oxidation is the main source responsible for the degradation (i.e., loss of nutrients and bad odors) of fatty foods and foods with a high content of fatty acids, which is a consequence of the enzymatic and non-enzymatic formation of hydroperoxides and propanal [108]. Silva Filipini et al. [77] made biodegradable films of methylcellulose and glycerol to test them in different food matrices, such as soybean oil. The results indicated that the films presented greater resistance to the attractive force, elongation, thermostability, and solubility compared to films of different polymers (i.e., collagen and whey protein), whose characteristics were suitable for the manufacture of container bags to store oil without leaks or disintegration of the biodegradable film [77]. De Farias et al. [81] made biodegradable alginate films with glycerol and norbixin salts to store sunflower oil and evaluate lipid oxidation products. The authors [81] reported an increase in the mechanical properties of tensile strength and elongation with an increase in the concentration of salts. In addition, the quantification of peroxides in the oil showed that on day 6 of storage (at 30 • C), the control (stored in a glass container) presented higher levels than that recommended by the Codex Alimentarius, while the oil stored in the film remained below the norm, and it exceeded what was recommended on day 12. The quantification of conjugated dienes also indicated that on day 15 of storage (at 30 • C), the content increased slightly, which was similar to the control; therefore, the authors [81] considered that biodegradable alginate films with norbixin salts are an alternative for oil conservation. The protection of oils in polysaccharide materials, such as methylcellulose and alginate, depends on their intrinsic characteristics as methylcellulose is highly hydrophobic due to its methyl groups (-CH 3 ) [109], while alginate (anionic polysaccharide) transforms into an amphiphilic component with hydrogel properties by covalently interacting with the salts [110].
Cereal-based food products (e.g., bread, cookies, and muffins) can suffer two types of deterioration (dryness and rancidity) when exposed to high levels of temperature, light, humidity, and oxygen. Dryness occurs when the products lose moisture, which then causes a change in texture as the texture becomes soft, while rancidity occurs when lipids are oxidized or hydrolyzed (by lipases) and phenolic acids are degraded, causing a bitter taste [111,112]. That is why the containers of these products must be able to maintain adequate humidity that can preserve the crispy texture but not provide a medium for the development of micro-organisms (e.g., bacteria and fungi). For example, in some research that uses biodegradable films to preserve bakery products, such as cookies and bread, the use of hydrophobic materials, such as methylcellulose, reportedly prevents the transfer of water or water vapor from inside the packaging to the outside and vice versa. [77]. Other studies have chosen to incorporate natural extracts containing bioactive components with antimicrobial characteristics against pathogens, such as Escherichia coli, Salmonella, and Shigella [73]. In both proposals, the methodologies demonstrate the preservation of the quality of the products and the extension of their shelf life.

Regulatory and Safety Issues of Polysaccharide-Based Biodegradable Films
Food products or products that are in contact with food as packaging must meet various requirements according to the place of production, sale, or marketing. However, each country has national bodies that have basic regulations that can be implemented in a general way. The Food and Drug Administration (FDA) in the United States stipulates that food ingredients and packaging must be Generally Recognized As Safe (GRAS) [113]. The GRAS label can be acquired in various ways; for example, (1) the FDA has a list of products that meet GRAS standards, (2) the characterization of a product by experts outside the FDA, (3) and the publication of a study about a new GRAS food, substance, or packaging [114,115]. All food products marketed must prove the use of food additives (e.g., colors, flavors, or flavor enhancers) only as allowed by national bodies. For example, the national standards of the European Union (European Union Standards) and Mexico (Official Mexican Standards: NOM) require that food products have approved additives and that these be reported on their labeling [34]. Additionally, if products contain allergens, such as proteins, they also must be reported. In the case of biodegradable films based on polysaccharides, the polysaccharides must be classified as GRAS substances. Failing that, they must be synthesized by organisms classified as GRAS. In addition, the additives used in their formulation must individually comply with ingredient standards. If the biodegradable films use proteins or derivatives as part of their ingredients, they must also be reported as part of the allergens, while the biodegradable films under the designation "bioactive" must ensure that added substances, such as antioxidants, antimicrobials, or oils, do not cause toxicity problems in the doses used [34,113]. If classified as packaging, biodegradable films must also comply with regulations for food packaging [116].

Challenges and Perspectives
As seen throughout the sections, the characteristics of polysaccharide-based biodegradable films depend specifically on the manufacturing materials (e.g., polysaccharide type, plasticizers, and additives). However, while applied studies in food have shown that they can decrease pathogens, supply bioactive components, or extend shelf life, their commercial use as food packaging has not been explored enough because most food research has been directed toward synthetic materials [117].
Despite this, some researchers are moving forward and conducting studies on intelligent polysaccharide-based biodegradable films. For example, Maciel et al. [118] generated biodegradable films by immersing a card paper in a film-forming of chitosan and anthocyanin, and their results showed that exposure to temperatures above 40 • C caused an irreversible change from light violet to light yellow due to the sensitivity of anthocyanins. Veiga-Santos et al. [119] generated cassava starch films with spinach and grape extracts to detect the pH change by the color variation; however, their results described a visible color only at extreme pH (0 and 14), especially a yellow color at basic pH attributed to the grape anthocyanins. Similarly, Yoshida et al. [120] made chitosan films with grape anthocyanins, and they noted a reversible color change depending on the pH (i.e., pink at acid pH, blue at neutral pH, and yellow at basic pH). These authors considered the induction of pH change by pathogenic micro-organisms, and their results could be used to offer a future alternative for the quality of food products.
Although the development of intelligent packaging is accelerating, the application of biodegradable films in that area is still under development. However, there is a large number of bioactive components (e.g., metabolites) from natural and biological sources (e.g., micro-organisms) that have a promising future as part of sensors (e.g., gases, firmness, color, and temperature) in polysaccharide-based biodegradable films [23].

Concluding Remarks
Polysaccharides have become the most widely used polymeric material for the formulation of biodegradable films due to their accessibility in terms of costs and sources of production, but even more so because of their ease in modifying characteristics (e.g., solubility, charges, and structure) with variations in pH, temperature, salts, and charged components. Polysaccharide-based biodegradable films are currently being studied, and they have been successfully used in food products. However, the introduction of this type of packaging to the market is still under development, and they cannot compete against the synthetic packaging market. For this reason, researchers are conducting further research to show that soon, polysaccharide-based biodegradable films can be a real alternative as primary packaging but also as intelligent packaging.

Conflicts of Interest:
The author declares no conflict of interest.