Figure 1.7 Cellulose derivatives categorized based on their pH‐responsive behavior and chemistry.
1.3.11.1 Gelatin
Gelatin is a water‐soluble protein that is prepared by thermal denaturation of collagen in the presence of dilute acid (gelatin type A) or alkali (gelatin type B). Collagen is found in animal skins and bones such as connective tissues, skin, and bones (see Figure 1.8) [117]. Its structure is triple helix, being stabilized by the formation of hydrogen bridges between the chains through amino and carboxyl groups. When collagen is denatured, the triple helix breaks and the polypeptide chains adopt a random configuration, forming gelatin composed mainly of glycine, proline, and 4‐hydroxyproline residues [118–120]. Then, gelatin results in a heterogeneous mixture of single or multi‐stranded polypeptides, each with extended left‐handed proline helix conformations and containing between 300 and 4000 amino acids. Gelatin is primarily used as a gelling agent forming transparent elastic thermoreversible gels on cooling below 35 °C. It can be used as a valuable biopolymer in tissue engineering applications. Moreover, the gelification properties of gelatin, its capacity of forming and stabilizing emulsions, and its adhesive properties and dissolution behavior make this biopolymer a potential polymer for the manufacture of bio‐based films [121]. However, its poor mechanical properties, especially in the wet conditions, limit its application as a packaging material, [122, 123]. Many techniques, including vapor cross‐linking, orientation, and use of fillers such as hydroxyapatite nanoparticles (nHAs) and tricalcium phosphate (TCP), have been developed to reinforce gelatin‐based films [124].
1.3.11.2 Wheat Gluten
WG is a protein complex having two major components, known as glutenin and gliadin. It is composed of proteins containing water‐insoluble and ethanol‐soluble prolamins and water‐ and ethanol‐insoluble glutelins (70–80 wt%) in combination with small amounts of wheat oils, starch, and insoluble hemicellulose [125]. The gliadins are mainly monomeric single chain polypeptides, whereas the glutenins are polymeric and disulfide linked polymeric chains [126]. These components are responsible for the physical and chemical properties of WG and confer it with higher viscoelastic properties compared with other plant proteins [127, 128]. WG has been used for the production of sustainable food packaging because it is a renewable raw material, available in large quantities, fully biodegradable, and inexpensive. It has properties suitable for applications in edible films and biodegradable packaging. The methods used for processing WG‐based bioplastics are casting, extrusion, and compression molding [129, 130]. WG‐based films have exceptional oxygen and carbon dioxide barrier properties. However, they have lower water vapor barrier, mechanical strength, and thermal properties in comparison with conventional plastic films [131, 132]. Lignocellulosic reinforcement fillers have been used to improve the properties of WG since they can interact with proteins and lead to the formation of protein–polyphenol complexes, allowing a higher flexural strength, tensile strength, and modulus [133, 134]. Also, WG/montmorillonite (MMT) films were prepared by melt mixing and thermoforming [135]. The introduction of up to 5 wt% of MMT increased the mechanical properties in ways that were not possible by just the variation of the glycerol content and the processing temperature.
Figure 1.8 Scheme of the gelatin manufacturing from denaturation of collagen for film formation.
1.3.11.3 Soy Protein
Soy protein (SP) is an inexpensive renewable resource, sustainable, abundant, and functional, constituted by different globulins with mainly polar amino acids including acidic and basic amino acids and nonpolar amino acids fractions such as 2S, 7S, 11S, and 15S. The major components of SP are β‐conglycinin (7S, nearly 35%) and glycinin (11S, nearly 52%) [136]. The protein with higher 11S fractions produces stronger films with lower water uptake than those richer in 7S, which is attributed to the presence of different sets of amino acids in 7S and 11S fractions [137]. Likewise, different chemical treatments and plasticizers have been explored to improve the intrinsic brittleness and low water resistance of SP‐based films. Among them, glycerol, ethylene glycol, and propylene glycol have been found to outperform 1,3‐propanediol. Glycerol and water can significantly increase the flexibility of films made of SP, but greatly decrease the tensile strength [138].
1.3.11.4 Corn Zein
Zein is a group of alcohol‐soluble prolamin proteins found in the endosperm of corn. It is constituted by hydrophobic and neutral amino acids as well as some sulfur‐containing amino acid [139]. Corn zein has a Mw ranging from 18 to 45 kDa and it is soluble in ethanol solutions in water at 60–70 wt% [140, 141]. Indeed, corn zein is produced commercially by extraction with aqueous alcohol and dried to a granular powder. The high proportion of nonpolar amino acid residues confers highly hydrophobic properties and solubility characteristics to zein, allowing its use in food packaging materials [142]. Zein films are formed by dissolving the protein into aqueous ethanol or isopropanol, heating to 65–85 °C, cooling down to 40–50 °C, and casting them by solvent evaporation. The resultant films are, however, translucid and present an intense yellow color (see Figure 1.9). Glycerol is often used to reduce the film brittleness though it tends to easily migrate through the film matrix because of the weak interaction between protein and plasticizer molecules. Migration of glycerol results in loss of flexibility in the film. Zein films have good oxygen barrier properties and are greaseproof, which have been attributed to the helical conformation of the protein, but their mechanical properties and water resistance are low, similar to those of other protein films [143]. In order to overcome these deficiencies, blended films of zein with other biodegradable biopolymers have been widely studied [144–146].
Figure 1.9 A zein film obtained from corn.
1.3.11.5 Milk Proteins
Biodegradable films can also be formed from milk proteins. The two most important types in the packaging field are casein and whey protein [147]. Casein, comprising 80% of total milk protein, consists of three main components, α, β, and γ with Mws in the 19–25 kDa range. It forms colloidal micelles in milk and is stabilized by calcium phosphate bridging. Casein precipitates when milk is acidified to the isoelectric point (pH = 4.6). Acidified casein is converted to functional soluble caseinates, that is, sodium and calcium caseinates, by neutralization through addition of alkali [148]. Biodegradable films based on caseinates can be obtained by solubilization in water followed by casting and drying. Film formation in water is feasible due to its emulsification capability [149]. Films from caseinates are transparent, with good mechanical and oxygen barrier properties but poor water vapor permeability in the range of WG‐ and SP‐based films [150].
Whey protein is the milk protein that remains soluble in milk serum after casein is coagulated during cheese
