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Gelatin and its precursor collagen are biopolymers obtained from protein animal sources. Collagen is considered the most abundant protein of animal kingdom, composed of around 25–35% of total proteins of the animal body and is mostly found in the tissues as bones, cartilage, ligaments, tendons, skins, blood vessels, intervertebral discs, guts, and corneas [70]. Collagen is mainly extracted from bovine, porcine, and fish [71], while gelatin is obtained after partial hydrolysis of the collagen. Gelatin is characterized by its versatility, high digestibility, and gels melting at human body temperature. Both biopolymers show interesting physicochemical and structural properties, especially for food industries [71].

Table 2.2 Films and coatings based on cellulose and derivatives for food packaging applications.

Components	Production approach	Main results	References
WPIa)/CNFb)	Casting	Improvement of structural properties of WPIa)-based films using CNFb)for food packaging applications	[32]
Tara gum/CNCc)/grape skin extract	Casting	Colorimetric pH-sensing films with positive activation test for milk spoilage	[38]
HPMCd)/beeswax	Dip coating	Coatings with antifungal activity against A. alternata on cherry tomato fruit	[39]
CMCe)/sodium montmorillonite/TiO2	Casting	Biodegradable nanocomposite films with improved mechanical and light barrier properties	[40]
PLAf)/rosin modified CNFb)/chitosan	Casting	Films with antimicrobial activity against E. coli and B. subtilis	[41]
Gelatin/CNFb)/chitosan Starch/CNFb)/chitosan	Casting	Films with better structural properties and antimicrobial activity	[42]
Chitosan/gelatin/MCg)/tannic acid	Casting	Films with antimicrobial activity against E. coli and S. aureus	[43]
Cellulose derived from corncob	Thermo-pressing molding	Biodegradable and sustainable films applied in breads	[44]
CMCe)/okra mucilage/ZnO	Casting	Films with antimicrobial activity against E. coli and S. aureus	[45]
κ-Carrageenan/HPMCd)/Prunus maackii juice	Casting	Biodegradable pH-sensitive and antioxidant films for oil and lard packaging application and label for testing pork freshness	[46]
Gelatin/CMCe)/chitin nanofibers/Trachyspermum ammi (ajowan) essential oil	Casting	Films with improved mechanical and barrier properties and antimicrobial activity against E. coli and S. aureus	[47]
ECh)/azo indicators (methyl orange; methyl red)	Casting	Biodegradable material with pH-sensitive properties for food packaging applications	[48]
HECi)/ZnO	Casting	Films with antimicrobial activity against E. coli and S. aureus	[49]
HPMCb)/liposome/CNFb)	Dip coating	Hydrophobic coatings used for hydration of fat food surfaces (slices of almonds and chocolate)	[50]

a) WPI: whey protein isolate.

b) CNF: cellulose nanofibers.

c) CNC: cellulose nanocrystals.

d) HPMC: hydroxypropyl methylcellulose.

e) CMC: carboxymethylcellulose

f) PLA: poly(lactic acid).

g) MC: methylcellulose

h) EC: ethylcellulose.

i) HEC: hydroxyethyl cellulose.

Table 2.3 Films and coatings based on chitosan for food packaging applications.

Components	Production approach	Main results	References
Chitosan/gelatin/starch/sorbitol/tween/geraniol/thymol	Casting	The coatings reduced the weight loss and delayed the physicochemical alterations of strawberries	[59]
Chitosan/black chokeberry extract/acetic acid	Casting	Colorimetric pH indicator films with high resistance to water	[60]
Chitosan/glycerol/sorbitol/acetic acid	Casting	Films with antimicrobial activity against L. monocytogenes	[58]
Chitosan/nisin/potassium sorbate/acetic acid	Casting	Potassium sorbate and nisin reduced the resistance and increased the flexibility and hydrophobicity of chitosan films	[61]
Chitosan/ɛ-polylysine (ɛ-PL)/TPPa)/acetic acid	Self-assembly	Films with low solubility in water and water vapor permeability, as well as with antimicrobial activity against E. coli and S. aureus	[62]
Chitosan/poly (acrylic acid)/sodium chloride/methanol/human plasma/fibronectin/silicone oil	Layer-by-layer	Hydrophobic films highly stable for 28 days in food simulants	[63]
Chitosan/ESsb)/choline chloride based/malic acid/lactic acid/citric acid/glycerol	Thermo-pressing molding	Film with improved mechanical and barrier properties manufactured at industrial scale	[64]
Chitosan/montmorillonite/aromatic aldehydes/ethanol/acetic acid	Self-assembly	Hydrophobic films with good mechanical properties	[65]
Chitosan/carbon/L-(β)-lactic acid/glycerol	Radiofrequency reactive/magnetron sputtering	Films with acceptable barrier properties for food packaging applications	[66]
Chitosan/acetic acid	Casting	Films reduced the growth of mesophilic bacteria in fresh pork loins stored under vacuum, at 4 °C, for 28 days	[67]
Chitosan/sodium alginate/calcium chloride	Layer- by-layer	Chitosan coating layer-by-layer preserved the ascorbic acid content, antioxidant capacity, and firmness and avoid the fungal growth of on fruit bars during storage	[68]
Chitosan nanoparticles/TPPa)/acetic acid	Ionic gelation	Coating was effective to delay the grapes ripening, reducing the weight loss and maintaining the sugar content, soluble solids, the titratable acidity, and sensory characteristics	[69]

a) TPP: tripolyphosphate.

b) ESs: eutectic solvents.

Collagen is produced by connective tissue cells, and it is classified as a superelastic fibrous protein. Analyzing the deconstruction of collagen fibers (Figure 2.1a), their quaternary structure is characterized by a set of collagen fibrils composed of collagen molecules, whose protein structure is tertiary [72]. This super coiling is composed of three identical or nonidentical polypeptide chains twisted together. Each polypeptide chain constitutes the primary structure of the collagen and contains around 1000 units of amino acids, whose glycine (Gly), hydroxyproline (Hyp), and proline (Pro) are in vast majority [72]. The interactions between N—H and C=O (hydrogen bonds) from amino acids are responsible by the α-helical conformation of the collagen secondary structure [73]. On the other hand, collagen tertiary structure is stabilized by means of hydrogen bonds between C–O groups from glycine and O—H groups from hydroxyproline [73]. Finally, the collagen quaternary structure is stabilized by hydrogen bonds, intramolecular van der Waals interactions, and some covalent bonds. Each collagen molecule can have until 300 nm in length and 1.5 nm in diameter [70].

There are at least 28 types of collagen, which differ as to the arrangement of amino acids composing the primary structure. The most abundant collagens are of the types I, II, and III, which manage cell differentiation, proliferation, and migration and provide the scaffolding [70]. Because of the difficult digestion of collagen by the human body, this protein is also commercialized in its complete hydrolyzed form [74].

Gelatin is composed of collagen polypeptide fragments (Figure 2.1b), whose structure is based on α-helical conformation and its combinations (β and γ conformations) [75]. Gelatin functionality depends on raw material, which causes variations of its relative fractions of peptides and molecular mass (95–100 kDa), consequently [70]. The variation of molecular mass of gelatin peptide fractions causes changes in the gelation time (setting time), gel strength (bloom), and viscosity of the biopolymer solution [76]. Gelatin bloom depends on the number of α- and β-chains, which constitute the fractions of the largest peptides, and its viscosity depends on average molecular mass of its peptide chains [70, 77].

During the partial hydrolysis of the collagen, its cross-linking structure is preserved, but some peptide bonds between chains are broken. The cross-linking degree varies as to raw material used to the gelatin fabrication, and its pretreatment determines the type of gelatin that will be produced, type A or type B [75]. Gelatins type A and B are produced by acid and alkaline processes, and they have isoelectric points in a pH range between 6.0 and 9.0, and of around pH 5.0, respectively [77].

Beyond traditional food applications of the gelatin and collagen as emulsifiers, stabilizers, foaming and microencapsulating agents, these biopolymers are also applied as biodegradable films and coatings in order to extend the shelf life of food and as carriers of active agents [72, 78, 79]. The most important applications of films and coatings based on collagen and gelatin are presented at Table 2.4. The versatility of collagen and gelatin to form blends and composites and to carry different active ingredients has been observed, whose main characteristic is the antimicrobial activity.

Figure 2.1 Schematic and chemical structure of collagen (a) and gelatin (b).