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).
2.2.5 Soybean and Derivatives
Soybean is a leguminous composed of proteins (40%), total carbohydrates (insoluble fibers and soluble saccharides) (34%), oil (21%), and minerals (ash) (4%) [93]. The proteins, water-soluble carbohydrates, and fibers are the most used soybean compounds to manufacture films and coatings, aiming food applications.
2.2.5.1 Soy Protein
The commercial soy protein is a by-product obtained from the soy oil extraction, being available as soy protein concentrate (SPC), soy protein isolate (SPI), and soy flour. SPC and SPI differ mainly by protein and carbohydrate contents. According to Koshy et al. [94], full soy flour and SPC showed carbohydrate content (34% and 18%, respectively) and protein content for full soy flour (56%), SPC (65%), and SPI (>90%).The full fat soy flour is produced by grinding soybeans into fine powders, while the SPC is prepared by defatting and removing of water-soluble carbohydrates from soy flour. Finally, the SPI is a highly refined soy flour and free of the most nonprotein components, fats, and carbohydrates [94].
Table 2.4 Films and coatings based on collagen and gelatin for food packaging applications.
| Components | Production approach | Mains results | References |
|---|---|---|---|
| Gelatin/mint essential oil | Dip coating | Coatings with antimicrobial properties used to extend the shelf life of strawberries | [78] |
| Gelatin (PSa))/ starch/acerola powder | Casting | Active films containing acerola compounds | [79] |
| Gelatin (SSb))/TiO2 | Casting | Films with antimicrobial activity against E. coli and S. aureus | [80] |
| Collagen/lysozyme | Dip coating | Coatings with antimicrobial activity against psychrotrophic bacteria used to extend the shelf life of fresh salmon fillets | [81] |
| Collagen (FSc))/sodium alginate | Casting | Biodegradable films with better structural organization | [82] |
| Gelatin (FSc))/cellulose nanofibers/palmitic acid | Casting |
Films with
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