regulate the release of food additives (e.g., antioxidants, dyes, and flavors).
-
Biodegradable films require at least one polymeric component, based on lipids (e.g., waxes, fatty acids, and acylglycerol), proteins (e.g., caseins, whey protein, and zeins), polysaccharides (e.g., cellulose, starch, and gums), or their mixtures
[9][8]. In addition, additives (such as plasticizers and surfactants) to modify the intermolecular bonds between polymers can be incorporated, or even bioactive compounds with fungicide, herbicide, and antioxidant capacity (e.g., natural extracts) to inhibit the development and spread of pathogenic microorganisms
[63,64,65,66][28][29][30][31]. Accordingly, the thickening, gelling, and emulsifying properties of the dispersion, the texture of the matrix during polymerization, the cohesion and assembly when dehydrated, and the organoleptic and mechanical characteristics in the final film are improved
[55,67,68,69,70,71][20][32][33][34][35][36]. However, not all polymers have the intrinsic properties suitable for use in the production of biodegradable films
[72][37].
Polysaccharides are the most used polymers in the formulation of packaging materials for food preservation, due to their low cost and accessibility; however, the choice of polysaccharides in the formulation of biodegradable films is preferred due to the easy modification of nature hydrophilic with additives, which is reflected in its mechanical and barrier properties
[9][8].
The efficiency of barrier properties such as water vapor permeability (WVP), ethylene content, and oxygen level maintain control of the transfer of ambient moisture and volatile components (e.g., aromas and flavors)
[73][38], which influence the food preservation
[74][39], dehydration of fresh products, hydration of dry products, or oxidation of polyunsaturated fats in food
[9][8]. On the other hand, mechanical properties such as tensile strength (TS), elongation at break (EB), and young modulus (E) are related to the integrity and brittleness of films during handling and storage
[75][40]. Some investigations reported that mechanical properties improved with the incorporation of hydrophobic additives (e.g., glycerol or sorbitol)
[76][41]. Physicochemical characteristics such as color, opacity, luminosity, morphology, and roughness are directly related to the type and concentration of polysaccharides, as well as the film-making method
[77,78][42][43].
4. Biodegradable Films Based on Polysaccharides
4.1. Biodegradable Films Based on Starches
Starch-based biodegradable films are the most studied because they are considered isotropic, colorless, tasteless, odorless, nontoxic, and biologically degradable
[162][44]; however, the hydrophilic nature of starch requires proper formulation to generate functional films (i.e., mechanical strength) with control over water content and WVP
[15][45]. However, the source of production has an influence on the mechanical and barrier properties, due to the fact that, by their nature, starch films have good WVP, but mechanical properties depend on the crystallinity of the starch used
[9][8].
4.2. Biodegradable Films Based on Celluloses and Derivates
Celluloses are used in various food packaging materials because they are versatile, low-cost, and nontoxic, and they have excellent mechanical properties. For example, cellophane is the most widely used cellulose derivative in packaging; however, its formulation with synthetic materials classifies it as a highly polluting material
[176][46]. There are other natural derivatives of cellulose that are used in the formulation of biodegradable films, such as carboxymethyl cellulose, cellulose regenerate, and some cellulose esters (e.g., cellulose acetate, palmitate, and octanoate). Films generated with cellulose or their derivatives are flexible and moderately resistant, with efficient WVP; however, these properties vary depending on the hydrophobicity and crystallinity modification caused by the incorporated additives
[9][8].
4.3. Biodegradable Films Based on Pectins
Pectin-based biodegradable films are the natural material proposals that have presented the most disadvantages, due to their fragile nature and poor WVP and mechanical properties; however, these films have a positive influence with the addition of plasticizing agents
[100][47]. For example, Shafie et al.
[98][48] and Gouveia et al.
[100][47] generated biodegradable pectin films; in the first study, no plasticizer was used, while, in the second study. glycerol was evaluated. The films with glycerol showed an evident increase in ST, but a decrease in EB. Such an effect is contrary to what happens to cellulose films with the addition of glycerol
(Section 4.2); however, it is not a recurrent result in pectin films. In previous studies
[180][49], the effect of increasing glycerol concentration in pectin films was evaluated, and the results were consistent with a decrease in TS and an increase in EB (as in cellulose-based biodegradable films).
4.4. Biodegradable Films Based on Gums
Gums are defined as hydrocolloids due to their properties as emulsifiers, stabilizers, texturizers, thickeners, coating agents, and film generators; however, their potential application depends on the type of gum and its source
[183][50]. The gums most used as film-forming agents are guar, gellan, xanthan, Persian, konjac glucomannan, and Arabic gums; however, the study carried out by Pedreiro et al.
[184][51] established that their main application is as a coating applied directly (by immersion or dipping) on the surface of foods (e.g., tomatoes, guavas, mangoes, and mushrooms) because they are GRAS, they can perfectly incorporate bioactive compounds (e.g., extracts or antimicrobial agents) and extend the shelf life of products
[184][51].
4.5. Biodegradable Films Based on Agars
Agar is easy to extract, safe, and cheap, making it a profitable polysaccharide for biodegradable film formation
[187][52]. The formation of the agar matrix results from the intermolecular interaction of hydrogens of agarose with water molecules; in addition, its properties allow it to interact with bioactive substances such as antimicrobials
[188][53]. According to Mostafavi and Zaeim
[189][54], the greatest limitations of biodegradable agar films are their fragility, low elasticity, high solubility, low thermal stability, and high permeability; therefore, one solution is to incorporate other polysaccharides and plasticizing agents. However, in general, the mechanical (i.e., TS and EB) and barrier properties are relatively low (compared to starch films). Furthermore, the incorporation of plasticizers does not significantly increase the values.