a) PS: porcine skin.
b) SS: shark skin.
c) FS: fish skin.
d) LWs: leather wastes.
e) BBs: bovine bones.
f) BC: bovine cartilage.
The soy protein consists in a mixture of albumin and globulin proteins, which have an isoelectric point from pH 4.5 to 4.8 and are classified in four main categories: 2S, 7S, 11S, and 15S fractions, where S corresponds to Svedberg units [95, 96]. The 7S (β conglycinin) and 11S (glycinin) globulin proteins constitute 35% and 52% of the soy structure, respectively, while the 2S and 15S albumin proteins only correspond to 8% and 5% of the soy structure, respectively [97]. SPI has an ability to form films softer and more transparent than other soy products [96]. Its films also exhibit higher gas barrier than lipids and polysaccharides-based films [94]. However, the complexity of globular proteins decreases the flexibility of chains, which results in the formation of fragile and stiff films and coatings [97].
In order to improve the mechanical properties of soy-based biomaterials, soy proteins have been associated to other materials to form composites since their structure exhibit polar functional groups as hydroxyl, carboxyl, and amine that facilitate its chemical and physical modifications [97] (Table 2.5). These groups are distributed along 18 amino acids composing the soy protein structure including arginine, lysine, leucine, cysteine, phenylalanine, tyrosine, aspartic, and glutamic acid [94, 97].
2.2.5.2 Soybean Soluble Polysaccharide
Soybean soluble polysaccharide (SSPS) is a macromolecule extracted from cell wall material of cotyledons of soybeans [98]. Its structure is similar to pectin, being considered as an acidic carbohydrate composed mainly of rhamnogalacturonan backbone branched by β-1,4-galactan, homogalacturonan, and α-1,3- or α-1,5-arabinan chains [112]. Beyond its ability to form films with excellent mechanical properties and high thermal stability, SSPS films and coatings are extremely sensitive to water molecules (Table 2.5) [113, 114].
2.2.5.3 Soybean Fiber and Derivatives
Soybean fiber is an insoluble fraction obtained from SPC processing and recovered by centrifugation. It is composed of carbohydrates derived from soy cotyledon cell wall as pectin, cellulose, hemicellulose, and lignin [115]. Rhamnose, arabinose, galactose, and galacturonic acids form the pectin structure and are ionically linked to Ca2+ and Mg2+ ions. In the cell wall, pectin fraction is covalently linked to cellulose, phenol groups, and proteins [115, 116]. Lignin is the highest insoluble fraction of the soybean fibers due to the presence of polyphenols, which are produced by polymerization of trans-p-coumaryl, sinapyl alcohols, and coniferyl [117]. The considerable resistance of lignin is caused by the extensive cross-linking on cell wall that characterizes the high crystallinity of its chains. The significant crystallinity of soybean fibers improves oxygen and hydrocarbon barriers of soybean fibers-based materials but limits their flexibility (Table 2.5) [109].
Table 2.5 Films and coatings based on soybean and derivatives for food packaging applications.
| Components | Production approach | Main results | References |
|---|---|---|---|
| SSPSa)/halloysite nanoclay | Casting | Biodegradable and sustainable films for food packaging applications | [98] |
| SSPSa)/ZnO | Casting | Films with antimicrobial activity against P. expansum and C. albicans | [99] |
| SSPSa)/nanorod-rich ZnO | Casting | Films with antimicrobial activity against E. coli and S. aureus | [100] |
| SPCb) | Casting/cold plasma | Films with antioxidant activity for smoked salmon packaging | [101] |
| SPIc)/soy straw | Casting | Films were used to extend the shelf life of fruits and vegetables | [102] |
| SSPSa)/nanoclay Na+ | Casting | Films with improved mechanical and barrier properties and with antimicrobial properties against B. cereus, E. coli, and S. aureus and P. expansum | [103] |
| SPCb) | Thermo-pressing molding | Films with acceptable physicochemical properties manufactured at industrial scale | [104] |
| SSPSa)/cinnamon essential oil nanoemulsion | Casting | Films with antibacterial activity against S. pyogenes and S. aureus | [105] |
| Soybean fibers/acid citric | Casting | Films with improved mechanical and barrier properties | [106] |
| SPIc)/pine (Pinus densiflora) bark extract | Casting | Films with antioxidant activity and improved mechanical and barrier properties | [107] |
| SPIc)/chitosan | Dip coating | Coatings were used to extend the shelf life of apricot fruit (Prunus armeniaca L. “Katy”) | [108] |
| Alginate/soybean fibers (cellulose) |