a) SSPS: soybean soluble polysaccharide.
b) SPC: soybean protein concentrate.
c) SPI: soybean protein isolate.
2.2.6 Starch and Derivatives
Starch is a carbohydrate, considered one of the most abundant polysaccharides in the world. This macromolecule is in the form of granules that are synthesized after the polymerization of glucose in plant [118]. Starch is composed of amylose and amylopectin [119]. Amylose is formed by glucose units linked by α-(1,4), containing sparing α-(1,6) branch points (∼1%). This macromolecule has a linear structure, and it contributes mainly to the amorphous phase of the starch granule, whereas amylopectin is a branched biopolymer formed by α-(1,4) glucose, highly branched by means of α-(1,6) glucose links (∼5%). This macromolecule is responsible by the crystalline structure in starch granules [120].
Starch can be isolated from different botanical sources including cereals (corn, wheat, rice, and sorghum), vegetables (pea, chickpeas, and lentils), and tubers (cassava, potato, sweet potato, and yam). Due to the overall structure of starch, botanical sources, climate conditions, geographic location for cultivation, and soil type, the ratio of amylose to amylopectin is variable [118].
Films and coatings based on starch are extensively studied due to its abundance, low cost, and excellent film-forming properties. Furthermore, starch-based films are transparent, odorless, tasteless, and good gas barrier. Hence, films and coatings based on starch have been used to extend the shelf life of fruits (Table 2.6) [120, 127]. The physical properties of starch-based materials are impacted by the starch source, manufacture time and temperature, type of plasticizer, co-biopolymers, and storage conditions [120].
The molecular weight, amylopectin/amylose ratio, and crystalline fraction (15–45%) depend on the botanical source of starch, modifying the physicochemical properties of films and coatings based on this macromolecule [128]. A higher proportion of amylopectin in the starch granules leads to manufacturing films with lower water solubility. Unfortunately, starch-based materials have poor mechanical and gas barrier properties. Aiming to improve the mechanical and barrier properties of starch-based materials, some researchers have studied the addition of different plasticizers, solvents, and macromolecules. Also, other natural compounds such as fatty acids, natural plant extracts (green tea, pomegranate), organic compounds, and essential oils have been investigated, aiming the manufacture of starch-based materials with antimicrobial activity [120, 129]. Another alternative is to blend the starch with other biopolymers such as gelatin, guar gum, and chitosan (Table 2.6) [121, 130, 131]. Thus, the interactions among biopolymer improve the mechanical and barrier properties in starch films [120]. Finally, fatty acids can be added in starch-based materials to reduce the water sensibility in these materials; however, fatty acids could induce phase separation and reduce the mechanical properties in films and coatings [132].
2.3 Main Polymers Obtained by Microbial Production
The main polymers obtained by microbial production can be classified as: polyhydroxyalkanoates (PHAs), poly(hydroxy-butyrate), and poly(hydroxy-butyrate-co-hydroxyvalerate) [11]. PHAs are thermoplastic polyesters composed of hydroxyalkanoic acid as the monomeric unit. This polymer is generally classified according to the number of carbon atoms in the side chain. In this sense, side chains having between 3 and 5, 6 and 14, and more than 14 carbon atoms can be classified as short-, medium-, and high-chain-length PHAs, respectively. Depending on the R group linked to the main chain, different derivatives of PHA were found in the consulted literature [133]. In general, 150 different units of homo or combination of copolyesters of PHA can be found [134]. Beyond the PHAs, the short-chain-length PHAs such as poly(3-hydroxybutyrate) (PHB), poly(3-hydroxyvalerate) (PHV), and their copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) are the most popular and commercially exploited [135].
Table 2.6 Films and coatings based on starch for food packaging applications.
| Components | Production approach | Main results | References |
|---|---|---|---|
| Pea starch/guar gum/shellac and oleic acid | Layer-by-layer | Coatings were used to reduce the weight loss and preserve the firmness in citrus fruits during storage | [121] |
| Cassava starch/casein/gelatin/sorbitol | Dip coating | Coatings showed desirable optical properties and water vapor transmission rate, and these materials delayed the ripening of guava fruits in two days | [122] |
| Corn starch/gelatin/guabiroba pulp | Casting | Guabiroba pulp improved the mechanical properties of films | [123] |
| Mango kernel starch/glycerol/sorbitol | Dip coating | Coatings were used to reduce the weight loss and maintain the sensory attributes of tomatoes at 20 °C | [124] |
| Potato starch/glycerol/extracts: white and green tea | Casting | Films reduced the weight loss and darkening in apple slides during storage | [125] |
| Cassava starch/glycerol/rosemary extract | Casting | Hydrophobic films with better barrier properties against UV light with promissory applications in food packaging | [126] |
PHA can be produced by means of microbial routes using bacteria and cyanobacteria. Bacillus, Cupriavidus, Haloferax, Halomonas, and Escherichia have been the most used microorganism to synthetize PHA [136, 137]. Beyond the cyanobacteria, Synechococcus, Spirulina, Nostoc, Phormidium, and Synechocystis genus can be highlighted as producers of PHA and its derivatives [135, 138]. The microorganisms produce PHA as a carbon and energy store, and they encountered intracellular cytoplasmic inclusions when some stress culture condition is applied as nitrogen, phosphorus, or oxygen limitation [138, 139]. Various factors affect the PHA produced: the microorganism itself, the substrate source, the stress condition applied, the activity of the enzymes, etc. [133]. The R side chain of the PHA can vary from hydrogen to methyl tridecyl group, and this is dependent on the substrate