Cereal and legume protein edible films: a sustainable alternative to conventional food packaging

ABSTRACT

The food industry faces significant challenges in generating biodegradable materials for packaging food. Studies on the production of edible films and coatings based on macromolecules such as carbohydrates, proteins, and lipids (and their combination) from cereal grains or legumes have provided helpful information about component concentrations, interactions, and the optimal conditions to elaborate films or coatings. However, the final application of edible films and coatings can depend on the compatibility between polymer matrix materials and their mechanical and barrier properties. This paper reviews the utilization of proteins from cereal and legumes in the development of edible films and the processing conditions that potentially modify the functional properties of the films, including the combination with additives to improve their properties enhancing food handling, transportation, storage, and preservation, without affecting the environment. In addition, the present research addresses the main methods to elaborate edible films and the use of novel technologies in film formulation.

KEYWORDS:

• Edible films and coatings
• Proteins
• Cereal seed
• Legumes

Introduction

The high demand for processed foods raises the necessity of the industry for packaging that facilitates food handling, transportation, storage, and preservation^[1]. However, these containers are typically made with petroleum-derived polymers such as polyethylene, polypropylene, and polystyrene,^[2,3] which are nonrenewable or biodegradable plastics with recycling rates below 20%, therefore contribute to environmental contamination due to the generation of microplastics.^[4] In addition, synthetic derivatives cause adverse effects on food and consumer health, because compounds from packaging materials can migrate into food.^[5,6] For instance, printing inks, such as molybdate orange^[7] and adhesives, such as toluene, ethylene acetate, and hexane.^[8,9] Furthermore, other plastic additives, such as lubricants, plasticizers, and heat and light stabilizers also have toxicity potential.^[1,5] Consequently, packaging plastics faces significant challenges in guaranteeing food safety and in generating the most negligible environmental impact.

Accordingly, the food industry has sought natural, renewable, and biodegradable sources to develop containers to preserve food quality, including proteins, carbohydrates, lipids, or a combination of them, which have allowed the production of suitable materials for formulating edible films and coatings.^[10]

Edible films and coatings are thin layers made from biopolymers that function as a protective barrier between food and its surrounding environment.^[11–13] Edible films are formed separately and then used as a wrapper on the food, while coatings are applied directly in liquid form on the food surface.^[14] As both are in direct contact with food, they must not have toxic components nor alter the food’s sensory quality. On the other hand, they must have mechanical efficiency, reduced permeability to water vapor, physicochemical stability, and microbiological safety in the food supply chain. Moreover, they should be environmentally and economically sustainable.^[12,15,16]

Generally, when manufacturing edible films and coatings, materials must be dispersed and dissolved in water, alcohol, water/alcohol mixtures, or other solvents that are safe for human consumption.^[17,18] Dispersion can be facilitated through pH adjustment and heating. Then, the film-forming dispersion is cast and dried at a desired temperature and relative humidity to obtain the film.^[15,17] Additionally, additives such as plasticizing and stabilizer materials, antimicrobial and antifungal agents (for example essential oils, organic acids, peptides, and enzymes), and nanoparticles can improve the properties or provide new functional properties of active and intelligent packaging.^[19]

Likewise, current research in the production of edible films and coatings includes the incorporation of probiotic microorganisms and compounds with nutraceutical and antioxidant properties (for example Omega-3-rich fish oil and phenolic compounds, among others). Due to these trends, the field of study of these polymeric materials extends not only to their mechanical or barrier properties, but also to the viability of these compounds within the polymeric matrix, their controlled release, their stability under different conditions (temperature, humidity, light), their interaction with food, and their final sensory characteristics.^[20]

The development of films and coatings, through new methodologies and new ingredients, must contemplate the study of their main physical and chemical characteristics, which is why some determinations are generally included in the characterization, such as optical tests (color, opacity, transparency), mechanical properties (tensile strength, elongation at break, and Young’s modulus), hydrophilicity of the films, (water contact angle and water absorption), Barrier properties (water vapor and gases Permeation), washability and solubility tests, and biosafety evaluation^[19,21]

This paper reviews the utilization of proteins from cereal and legumes in the development of edible films and the processing conditions that potentially modify the thickness, tensile strength (TS), and water vapor permeability (WVP) of the films, including the combination with additives to improve their properties, enhancing food handling, transportation, storage, and preservation, without affecting the environment.

Films from plant proteins

Production of edible films from plant proteins has advantages over other types of edible materials since they usually have excellent film-forming and gas-barrier capacities, in addition to providing mechanical properties such as tensile strength and elongation, which can be improved by pH modification, crosslinking, chemical or enzymatic hydrolysis, or irradiation processes.^[22–24]

The choice of the proteins is based on the physicochemical properties they provide (e.g., thickness, water vapor permeability, and tensile strength) and the packaged product’s characteristics. Water vapor permeability is one of the most studied parameters for food packaging. It depends on structural factors, such as the type and composition of the matrix and the hydrophilic-hydrophobic balance.^[25,26]

Research has focused on the use of plant proteins from cereals and legumes, such as corn zein (prolamin), wheat gluten (glutelin and prolamin), and isolated proteins (globulins, the major fraction in legumes) from beans, peas, soybeans, chickpeas, and lentils.^[27] Although on a smaller scale, proteins from pseudocereals (globulins) such as quinoa, chia, and amaranth have also been examined because they typically do not generate undesirable taste and have a lower economic cost compared to animal proteins.^[28–31]

Furthermore, the use of cereal and legume proteins shows a series of advantages compared to films made with polysaccharides (Table 1); among them, vegetable protein films have lower values of water solubility, moisture content, and water vapor permeability.^[38] However, the structural and mechanical properties of vegetable protein films are more dependent on the type of material used and the manufacturing conditions than polysaccharide films.

Table 1. Advantages and disadvantages of films made with cereal and legume proteins.

Vegetal protein vs Polysaccharide	Advantage	Disadvantage	Reference
Zein vs. Alginate	↓ Solubility ↓ Opacity ↓ Moisture content ↓ WVP	↑ Thickness ↓ Tensile strength ↓ Elongation at break	^[Citation32]
Zein vs. Modified starch	↓ WVP ↑Young´s Modulus ↑Uniform Surface	↓Transparency ↓ TS ↓ Elongation at break	^[Citation33]
Wheat Gluten vs. Pectin	↓ Moisture content ↓ WVP ↑ Elongation at break	↑ thickness ↓ TS ↓ Young´s Modulus	^[Citation3]
Wheat Gluten vs. Starch	↓ Thickness ↓ WVP	↑ Water absorption capacity ↓ Thermal stability	^[Citation34]
Oat protein vs. Pullulan	↑ TS	↓ Elongation at break ↑ WVP ↑ Moisture content	^[Citation35]
Bean protein vs. Chitosan	↓ WVP	↑ Thickness ↓ Elastic Modulus ↓ TS ↓ Elongation at break ↑ Opacity	^[Citation39]
Lentil protein vs. Starch	↓ Moisture content ↓ Solubility ↓ WVP ↑ Young´s Modulus ↑ Strength at break	Non-uniform Surface	^[Citation40]
Chickpea protein vs. Pullulan	Morphological integrity ↑ TS ↓ Solubility ↓ Moisture content ↓WVP	↑ Thickness ↓ Elongation at break	^[Citation36]
Mung bean protein vs. Pullulan	↑ TS ↓ solubility ↓WVP	↓ Homogeneity ↑ Thickness ↓ Elongation at break ↑ Opacity	^[Citation37]

TS, tensile strength, WVP, water vapor permeability.

In recent studies, to reduce some of the disadvantages of films made with cereal and legume proteins, different strategies have been implemented to develop more stable edible films with superior mechanical and barrier properties (Figure 1). Among these strategies are denaturation of the protein and addition of other macromolecule types, additives, and bioactive compounds, among others.^[41]

Figure 1. Strategies for modifying biofunctional properties of edible films from vegetal proteins.

For example, due to the globular structure of some plant proteins (in the native state), compact structures are present, so inducing an unfolding of the protein through thermal or acidic denaturation and enzymatic hydrolysis,^[42,43] some functional groups can be exposed on the surface of the protein. Figure 2 represents the interactions of proteins with other macromolecule types present in the polymer matrix; for example, the addition of polysaccharides generates interactions such as Van der Waals bonds, electrostatic, hydrophobic, hydrogen bonds, or through chemical bonds as in the Maillard reaction.^[44] On the other hand, lipids can interact with proteins through hydrophobic interactions; in this way, the mechanical and barrier properties will be influenced by the structure of the films and protein-polysaccharide or protein-lipid interactions.^[45] Plasticizers help reduce molecular interactions and increase mobility between protein chains, considerably improving mechanical properties.^[46]

Figure 2. Interactions of proteins with other macromolecule type.

Finally, the presence of functional groups on the surface of the proteins also allows interactions between the protein with bioactive compounds and nanomaterials, which helps to enhance some biofunctional properties of the polymer matrix.^[47,48] Some of these strategies will be discussed later in this review.

Cereal grains proteins

Protein content represents 7.5 to 16.9% of the cereal grain.^[49,50] Corn, wheat, and rice proteins have been used to form films because of the relationship between hydrophobic-hydrophilic amino acids content,^[29,51] these films have shown quality physical properties such as thickness, water vapor permeability (WVP), and tensile strength (TS) in edible films (Table 2) which allows cereal proteins to be a viable option for producing suitable materials for formulating edible films and coatings.

Table 2. Thickness, water vapor permeability (WVP), and tensile strength (TS) in edible films from cereal grains protein.

Protein	Additive	Method	Thickness (mm)	WVP (g m/m^2 s Pa) x10^−11	TS (MPa)	Reference
Corn(zein) Prolamins 80% (α-zein, β-zein, γ-zein, and δ-zein) (Higher hydrophobic AA content)						^[Citation52]
92% protein mass fraction	+ chitosan	Casting	0.06	2.88	34.09	^[Citation53]
	+ chitosan, glycerol	Casting	0.09	4.38	16.71
75% protein	+ alginate, glycerol	Casting	0.05	9	-	^[Citation29]
	+ chitosan	Casting	0.20	81.1	-	^[Citation26]
	+ chitosan, anise oil	Casting	0.24	33.6	-
	+ chitosan, orange oil	Casting	0.21	45.0	-
	+ chitosan, cinnamon oil	Casting	0.23	42.5	-
95% protein	+ glycerol, oleic acid	Compression molding	-	-	1.70	^[Citation54]
Wheat (Gluten) Glutelins and Prolamins (Glutenins and Gliadins= Gluten) 75–85%						^[Citation55]
86% purity	+ glycerol, PLA nanofibers	Casting	0.46	~7x10−6	4.05	^[Citation56]
7.73% protein content	+ glycerol	Casting	0.10	2.00	2.08	^[Citation57]
	+ glycerol	Casting	0.09	6.0	~2.50	^[Citation3]
Pure	+ glycerol	Extrusion	1.70	-	-	^[Citation58]
Rice Glutelins 75–83%						^[Citation59
Concentrate	+ pullulan, glycerol, propyleneglycol alginate, polyethylene glycol	Casting	0.11	1.2	12.00	^[Citation60]
Modified protein	+ ammonium shellac, glycerol	Casting	0.11	9.86	7.64	^[Citation61]

Corn zeins

Zeins belong to the prolamins group, representing approximately 80% of the corn’s total protein. They contain four main fractions: α-zein (MW = 21–25 kDa), β-zein (MW = 17–18 kDa), γ-zein (MW = 27 kDa), and δ-zein (MW = 9–10 kDa).^[52,62,63]

Zeins have film-forming properties due to their hydrophobic characteristics related to their amino acid profile, for example, Leucine, Proline, and Alanine^[52,64]; zein films are produced by developing hydrophobic bonds, hydrogen bonds, and limited disulfide bonds between zein chains.^[65] Their advantages comprise high impermeability to oxygen, aromatic components, and moisture. Nevertheless, rigidity and brittleness constitute the main disadvantages of zein films.^[24,66,67] Plasticizers such as glycerol, polyethylene glycol (PEG 400), and sorbitol are used to improve their physicochemical characteristics by increasing thickness and water vapor permeability.^[29,53] However, a high concentration of plasticizers decreases tensile strength (Table 2).

Comparably, polysaccharides and essential oils can enhance the characteristics of films by providing antimicrobial effects.^[68] For example, Escamilla-García et al.^[26] manufactured chitosan and zein films combined with essential oils of anise, orange, and cinnamon. As a result, the film’s thickness increased with the addition of essential oils due to the film-forming solution’s higher density than the control sample (Table 2). Furthermore, the chitosan-zein films with essential oils exhibited enhanced impermeability to water vapor and hardness, and growth inhibition of Penicillium sp. and Rhizopus sp., because of the reduction of inter and intramolecular interactions of film components.

Wheat gluten protein

Wheat gluten proteins are divided into two groups: gliadins (prolamins) and glutenins (glutelins),^[69] which represent about 80% of the total wheat proteins, highlighting a high content of the amino acids Leucine, Threonine, Methionine. Hydrated wheat gluten is a suitable biopolymer for film production because it forms a viscoelastic protein network caused by hydrogen bonding and hydrophobic interactions.^[56,69] In addition, disulfide bonds between two polypeptide chains improve the wheat gluten film’s mechanical properties.^[70] Wheat gluten films also present excellent gas-barrier properties (Table 2) due to their selective permeability (the relationship between carbon dioxide and oxygen permeability).^[57]

Different strategies have been tested to favor the development of films from gluten. For example, Rocca-Smith et al.^[71] observed that the solubility of gluten was increased at pH 4, allowing the formation of transparent films, while at pH higher than 8, brown and opaque films were produced. However, Sartori et al.^[3] made gluten film blends with absolute ethanol, glycerol, and distilled water at pH 10, which were centrifuged to eliminate the insoluble part of gluten, thus achieving transparent films.

The protein amount, pH, and ethanol content influence the mechanical and barrier properties of gluten films, which can be strengthened by adding binding or reinforcement agents, such as polysaccharides, phenols, or lipids.^[56,57] Chavoshizadeh et al.^[72] demonstrated that chlorophyll and polypyrrole increased tensile strength, possibly by forming bonds between polypyrrole and gluten. In addition, polypyrrole showed an antimicrobial effect against Escherichia coli.

Gluten-based films have also been produced using glycerol as a plasticizer^[73] reinforced by polylactic acid (PLA) nanofibers, thus achieving improved mechanical properties.^[56] Similarly, mixtures of banana fibers and wheat gluten cross-linked with citric acid, have displayed an optimization of the final properties of the films.^[74] Despite these promising results, the biggest drawback of gluten films is that individuals with an intolerance to gluten proteins cannot consume them.

Rice protein

Glutelins are the primary protein fraction in brown and milled rice (75–81% and 79–83%, respectively) (Amagliani et al. 2017).^[126] Pioneering research on rice-based films by Shih^[60] indicated that in films of rice protein concentrate with pullulan, the higher the protein concentration, the more surface tension increased. Likewise, the results evidenced that an alkaline pH decreased permeability and solubility. The author suggested that films with equal proportions of protein concentrate, and pullulan achieve a satisfactory balance of tensile strength and resistance to water vapor.

A further example of films produced from rice protein using glycerol as a plasticizer described that adding malic acid improves the barrier properties to water vapor without affecting the film thickness.^[69] Likewise, films from rice bran protein hydrolyzates showed a non-homogeneous mixture and lower moisture content when combined with gelatin/Eudragit® NE 30D.^[75]

Other authors evaluated the effect of ultrasonication on films from self-crosslinking rice protein hydrolysates and chitosan, observing that ultrasound treatments did not significantly modify thickness values but decreased surface tension values and improved resistance to breaking and oxygen permeability.^[76] Other strategies have been proposed to enhance the characteristics of the films, such as modification of the type and concentration of the plasticizers, pH adjustment of the film-forming solution, and the application of heat treatments.^[77,78]

Legume and beans

Legume protein

After cereal grains, legumes are the second most important crop for the human diet due to their protein content, which ranges between 17 and 40%, depending on the species.^[79–82] Furthermore, the characteristics of their proteins have aroused interest in edible film production (Table 3). The features of the proteins of the most common legumes are described below.^[93]

Table 3. Physical Characteristics in edible films from legume proteins.

Protein source	Additive	Method	Thickness (mm)	WVP (g m/m^2 s Pa) x10^−11	TS (MPa)	Reference
Bean Globulins 50–70% (11S Legumin and 7S Phaseolin)						^[Citation83]
Isolate	+ Glycerol	Casting	0.07	33.2	6.2	^[Citation84]
Isolate	+ Chitosan, glycerol	Casting	0.07	4.9	4.2
Concentrate	+ Glycerol	Casting	0.05–0.04	2.6–26.9	-	^[Citation82]
Pea Globulins 65–80% (11S Legumin and 7S vicilin)						^[Citation85]
Isolate	+ acetylated cassava starch, glycerol	Extrusion	-	2.73x10−9	.65	^[Citation86]
Isolate	+ Sorbitol, anhydrous milk fat + Sorbitol, candelilla wax	Casting Casting	-	1.62 1.15 1.11	~6.3 ~4.5 ~5.4	^[Citation87]
Lentil Globulins 70% (11S Legumin and 7S vicilin)						^[Citation88]
Concentrate	+ Glycerin	Casting	0.15	3100	4.24	^[Citation89]
Concentrate	+ Glycerol (50%)	Casting	0.17	25.0	~4.0	^[Citation90]
Concentrate	+ Glycerol (100%)		0.24	67.2	~1.0
Isolate	+ Cassava starch, glycerol, water	Extrusion/thermo-compression	0.00029	1.4	2.4	^[Citation40]
Faba bean Globulins 80% (11S legumin and 7S vicilin/convicilin)						^[Citation91]
Concentrate (pH 7)	+ Glycerol	Casting	0.07	34.2	-	^[Citation92]
Concentrate (pH 10)	+ Glycerol	Casting	0.07	5.0	-
Concentrate	+ Glycerol (50%)	Casting	~0.1	13.9	~10	^[Citation90]
Concentrate	+ Glycerol (100%)	Casting	~0.16	38.9	~2

TS, tensile strength. WVP, water vapor permeability

Bean protein

Globulins are the main proteins in the bean seed (Phaseolus vulgaris L.). Vicilin (7S) or Phaseolin, represents 50% of total protein.^[82,94] Wen et al.^[84] used bean protein and chitosan at different proportions at acidic pH (3.0) to formulate edible films, reporting that tensile strength decreased when using a higher ratio of bean protein isolate, while water vapor permeability increased in the composite films (Table 3). The authors discussed that the low interaction of these two biopolymers may have formed a less compact microstructure, therefore contributing to the increase in water vapor permeability.

Analogously, Tang et al.^[95] made films with protein isolates from three types of bean cultivars: Phaseolus vulgaris, P. angularis, and P. aureus, obtaining better resistance in the films with the highest concentration of 11S globulin compared to 7S globulin because the former tends to form disulfide bonds. They also observed that when applying heat treatment (85°C/16 h), the tensile strength values increased due to the high interactions between protein components in the film network. Conversely, heating reduced surface hydrophobicity of the film attributed to heat-induced protein unfolding, which can lead to poor water vapor barrier properties.

Another experiment using isolated proteins from different bean varieties combined with glycerol developed thin films with low water vapor permeability (Table 3). Neither adjusting the pH at 7.0, 8.0, or 9.0 nor heating affected the film thickness.^[82] However, heat treatment increased the tensile strength values of the films at all adjusted pH solutions, which was attributed to the possible displacement of the proteins and exposure with subsequent bonding of their hydrophobic and sulfhydryl groups during the formation of the film.^[96] Unfortunately, research on bean proteins is scarce since it has focused on starch-based films.^[97]

Pea protein

Pea (Pisum sativum L.) proteins are mainly globulins (11S legumin and 7S vicilin), accounting for 65–80%, and albumins, representing about 10–20% of total protein.^[98] Therefore, it presents a balance of hydrophilic (Arginine) and hydrophobic (Valine and Methionine) AA content. These stand out for their techno-functional properties: emulsifying, foaming, and gelling capacities. Although their application in the food industry is limited due to their low solubility and thermal stability, they have presented promising results in the preparation of edible films.^[99,100] Kowalczyk et al.^[101] reported better tensile strength performance of pea protein films with sorbitol than glycerol, possibly due to the increase in OH groups, which would intensify the polyol-protein interaction.

On the other hand, lipids have been shown to boost barrier properties against the water vapor of protein-based films. For instance, Kowalczyk et al.^[87] tested pea protein isolate emulsion films with sorbitol, anhydrous milk fat, candelilla wax, lecithin, and oleic acid. Results showed that the permeability value decreased in films with anhydrous milk fat and candelilla wax (Table 3). Polysaccharides such as pullulan and cassava starch have also been blended with pea protein to generate films using extrusion and electrospinning methods.^[86,102] Finally, ultrasound and high-pressure homogenization have also been applied to generate structural changes in the pea protein, modifying the water vapor permeability values and increasing the surface tension values in films.^[103,104]

Lentil protein

Lentil (Lens culinaris) proteins, as in other legumes, are found mainly in the cotyledons.^[105–107] About 70% of the protein content in lentils are globulins (7S vicilin and 11S legumin). All fractions of the lentil protein are glycosylated, especially the vicilin.^[108] Lentil proteins are rich in the amino acid Arginine, Ac. aspartic, Ac. Glutamic, Leucine, and Lysine.^[88] Strong and flexible films with acceptable mechanical properties for packaging have been obtained from these proteins. In addition, films based on lentil protein have shown better permeability than films made with corn zein or wheat gluten.^[89]

Hopkins et al.^[90] developed films from lentil protein concentrate with different glycerol concentrations, observing that thickness increased significantly with a higher plasticizer concentration, while tensile strength values decreased (Table 3). Moreover, they detected that the protein’s purity, molecular weight, structure, and charge could affect the protein-protein interactions, modifying the film’s mechanical properties, particularly tensile strength. Other studies have assessed the effect of lentil protein concentration in cassava starch films with glycerol obtained by casting and extrusion-thermocompression processes. In both methods, the presence of the protein in the polymer matrix increased Young’s modulus and breaking stress values. However, the water vapor permeability values decreased in films prepared by casting. At the same time, no significant changes in this parameter were reported when utilizing extrusion-thermocompression due to the high temperatures used during this process, where the available water molecules evaporate, decreasing the effective diffusion of water vapor in the polymer matrix.^[40] On the other hand, reduced tensile strength values and Young’s modulus were reported in films formed by mixing lentil protein isolate and corn zein.^[109]

Faba bean protein

Faba beans (Vicia faba L.) proteins comprise 80% globulins composed of 11S legumin and 7S vicilin/convicilin.^[91,110] Their applications for film manufacturing have under-researched. For example, Montalvo-Paquini et al.^[92] evaluated the effect of pH (7, 8.5, and 10) on films from bean protein concentrate with glycerol, describing that changes in pH did not affect thickness. Nonetheless, at an alkaline pH, water vapor permeability diminished (Table 3). In a similar study, Saremnezhad et al.^[111] showed that the tensile strength increased at pH 12 in films based on faba bean protein with 40% glycerol, while water vapor permeability declined at these same conditions.

Faba bean proteins presented a lower film thickness (Table 3) than other legumes (pea, lupinus, lentil, and soybean). Nevertheless, the addition of glycerol improved tensile strength and water vapor impermeability.^[90] In addition, films based on faba bean protein isolate with carrageenan achieved better compatibility between the macromolecules through pH modification and denaturation temperature of the protein allowing its reactive sites to interact with carrageenan and forming stable structures.^[112]

Correspondingly, cellulose nanocrystals (CNC) have been added to improve the mechanical properties of films based on faba bean protein. The results showed that as the concentration of the CNC increased, the tensile strength was magnified. On the other hand, the lower the CNC content, the lesser the decrease in water vapor transmission rate. This was attributed to a reduction of films’ hydrophilicity and a rise in tortuosity (the net increase of the length of the diffusion path). Moreover, incorporating CNC into the faba bean protein films enhanced the intramolecular interactions between the hydroxyl groups of the CNC and the amino and carboxyl groups of the proteins.^[113]

Trends in edible films

Casting, extrusion, and other molding methods have been used in most studies on preparing edible films from vegetable proteins, some of the most common application methods of edible coatings on products are shown in Figure 3, for example, dipping, spraying, fluidized bed and panning.^[114] The casting method is the most widely used for the preparation of edible films from cereal and legume proteins due to its ease of manufacture. However, a drawback is the uniformity of the film, which is influenced by the amount of sample that is added to the mold, therefore it must be defined depending on the area of the mold used.^[114]

Figure 3. Main development methods, application, and improvement strategies of edible films and coatings.

Extrusion is the second most used method for the preparation of edible films at a commercial level, where pressures between 0–500 bar and temperatures of 70–500°C are reached, which favors the interaction between the macro components, obtaining uniform films with less permeability than the casting method.^[40,114] In the same way, with the compression molding method, films with greater mechanical resistance are obtained,^[115] however, as they require more sophisticated equipment, they require greater energy consumption and maintenance.

On the other hand, it is important to consider that films made from materials from natural sources often show inferior properties compared to synthetic films. In the case of water vapor permeability, films made with proteins present higher values and lower mechanical properties compared to synthetic films, which is why continuous research has focused on different strategies to improve the physical properties of films of proteins. As documented in this review, some of these strategies include the type and concentration of the plasticizer, the modification of the protein structure by denaturation (chemical, enzymatic or thermal), and the addition of other macromolecules such as lipids or polysaccharides. In addition, other trends in crafting include electrospinning, high-pressure, or non-ionizing radiation from ultraviolet light (UV) techniques.^{[12,102,104,116]} High-speed homogenization and ultrasonication have also been shown to boost the homogenization of film-forming solutions in order to improve the interactions between the components, resulting in films with better physical properties.^[87,103]

Other investigations have implemented dielectric barrier discharges to generate cold plasma, modifying functional properties such as impermeability, elongation, and tensile strength (G.^[33] Additionally, 3D printing technology has achieved highly customizable film properties.^[117] Correspondingly, nanoparticles addition in film-forming solutions has increased the bio-functional properties of films^[118,119] and PLA nanofibers, thus achieving improved techno-functional properties.^[56] Lastly, drying methods (microwave, vacuum, and superheated steam) have been examined to reduce the production times of edible films to transpose them to the industrial level.^[115]

In addition to these previously discussed strategies, the inclusion of nanostructures, bioactive compounds, and cross-linking agents has been shown to significantly improve the biofunctional, mechanical, and barrier properties of films made with proteins of animal origin.^[120] Also, the modification of some plant-origin proteins has been investigated using deep eutectic solvents (DESs) and organic acids, however, the addition of DESs to polymeric matrices of cereal and legume proteins remains scarce compared to the matrix prepared from polysaccharides thus, it is interesting to integrate them and document their effect on the different polymeric matrices based on plant proteins.^[121]

In this work, we explain the convenience of adding bioactive compounds that will contribute to improve the functionality of films as packaging material. However, for vegetal proteins it has been reported that subjecting them to enzymatic hydrolysis could generate peptides with biological activity. Extensive hydrolysis (>10%) increases the bioactive properties (antioxidant, anti-inflammatory, etcetera),^[122] whereas a limited hydrolysis (<10%) improves the techno-functional properties (foaming, emulsion, and film-forming capacity),^[42] and the interaction with polysaccharides is also increased, for example with galactomannan gums.^[123] Another way of improving the techno-functional and biofunctional properties of proteins is through germination, which can be explained by the hydrolysis of the reserve proteins or the generation of new proteins of low molecular weight. Amaranth has been reported to be able to reduce body weight, cholesterol, LDL, and glucose levels.^[124] Hence, modification of proteins through the mentioned alternatives can provide the peptides, polypeptides, and reduced molecular weight of proteins, which will improve the functionality of the vegetal proteins to form edible films.

Finally, cereal and legume proteins are compatible materials for the encapsulation, protection, and administration of bioactive compounds (minerals, vitamins, nutraceuticals, probiotics, antimicrobials, and antioxidants).^[125] Therefore, these proteins of vegetal origin could be studied in more detail when adding them to the polymeric matrix, as well as their stability or feasibility under the different elaboration or storage conditions, even their role as bioactive materials in other areas not related to the preservation of food quality.

Conclusion

Proteins from cereal grains and legumes are a viable alternative to petroleum-derived polymers in producing food packing. The interaction of proteins with additives such as plasticizers, polysaccharides, and essential oils in film-forming solutions strengthens the barrier and mechanical properties of edible films as an increases water vapor permeability, diminishes tensile strength, modifies the barrier properties, and lower water vapor permeability. Most of the studies obtained the films by casting. However, extrusion and molding are promising processes that promote interactions between the film-forming solutions components due to high temperatures and elevated pressures. On the other hand, further research should focus on the impact of protein modification through hydrolysis or the use of ultrasound or high pressure – aside from alterations through temperature and pH adjustments – on the interactions of the components on the film-forming solution. Those improvements contribute to successfully implementing proteins from cereal grains and legumes in edible film production, reducing pollution and health problems generated by the excessive use of synthetic packaging.

Author contribution

Alejandra Linares-Castañeda, Xariss M. Sánchez-Chino, and Luis J. Corzo-Ríos: conceptualization, writing – original draft, investigation, and analysis. Yolanda M. Gómez y Gómez, Jorge Martínez-Herrera and María S. Cid-Gallegos: formal analysis, and visualization. Cristian Jimenez-Martínez and Luis J. Corzo-Ríos: formal analysis, writing – review and funding acquisition.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data sharing is not applicable to this article.

Additional information

Funding

This work was financially supported by the Instituto Politécnico Nacional, Mexico, through Secretaría de Investigación y Posgrado projects 20220525 and 20220550.