Читать книгу Nanotechnology-Enhanced Food Packaging - Группа авторов - Страница 44

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 used and the metabolism of the microorganism [138]. Another important factor to be considered is the extraction of these biopolymers, which may influence their properties and applicability [139]. Figure 2.2 presents the steps to obtain PHA, its structure, and the most common PHA derivatives. Additionally, polymerization through the opening of β-butyrolactone chains using zinc or aluminum can be performed to produce PHA derivatives [140].

Figure 2.2 Polyhydroxyalkanoate (PHA) extraction and its general structure, where R is variable, and n can be from 1 to 13.

Food packaging materials based on PHA are the most common application of this biopolymer. PHA has interesting physicochemical due to the monomeric composition of its copolymers. The PHA-based materials properties vary from fragile and crystalline thermoplastics (PHB) to elastomeric ones (PHV, PHBV) [141]. PHA is considered biodegradable and biocompatible [140–142]. In addition, hydrophobicity property makes PHA-based materials insoluble in water and provides good water barrier properties. PHA is a semi-crystalline material with crystallinity degree between 30% and 80%, and melting temperature oscillating between 50 and 180 °C [133, 138]. Beyond PHAs, PHB is the most studied one regarding food packaging applications. PHB has similar properties to those found in conventional plastics; hence PHB can be extruded and molded to manufacture films at industrial scale. However, since this biopolymer has high crystallinity, the films and coatings become rigid and brittle [143]; in this sense, the incorporation of 3-hydroxyvalerate, producing PHBV, or other monomers has been used to overcome its shortcoming. The blend with other biopolymers or incorporation of other types of materials to produce composites is also studied in the food packaging area [133]. Table 2.7 shows an overview of studies that applied PHA and its derivatives as food packaging materials, highlighting the main results obtained by the authors.

The PHA derivatives have been used as food packaging materials; most studies have focused on the application of this biopolymer such as film for direct or indirect food contact, as well as barrier to coat paper-based packaging (Table 2.7).

Table 2.7 Films and coatings based on polyhydroxyalkanoates for food packaging applications.

Components	Production approach	Main results	References
PHAa)/long alkyl chain quaternary/graphene oxide Nanocomposite	Casting	Films with improved mechanical, thermal, and oxygen barrier properties. Films with antimicrobial activity against S. aureus and E. coli	[144]
PHBb)/PEGc)/organobentonite or organovermiculite	Casting	Increase on the processability of the PHBb)with the improvement of the thermal resistance and crystallinity degree	[145]
PHBVd)/PHBb)/PLAe)/catechin	Electrospun/thermo-pressing molding	The addition of PHBb)increased the PLAe)-based fibers crystallinity. The film presented antioxidant activity related to catechin release on fatty food model. The incorporation of PHBb)/PLAe)layer improved the mechanical properties	[146]
PHAe)/apple extract/cellulose	Casting	The PHAa)increased the hydrophobicity and transparency of the films whereas the tensile strength was reduced. The apple extract gives an antioxidant property to the coating	[147]
PHBVd)/cellulose/TECf)or PEGc)	Extrusion	A sufficient adhesion (cohesion break) between paper and PHBVd)layer was obtained. The use of TECf) or PEGc) produced PHBV layers with lower defects and with increase of grease barrier property	[148]
PHBb)/bacterial cellulose/zinc oxide nanoparticle	Thermo-pressing molding/plasma	Films with better mechanical properties and antimicrobial activity against E. coli and S. aureus	[149]
PHBb)/starch/montmorillonite/eugenol	Extrusion	Reinforced films with antimicrobial properties manufactured at industrial scale	[150]
PLAe)/PHBb)/cinnamaldehyde	Casting	Films with better mechanical properties and slower release of the active compound. These films were used to extend the shelf life of salmon dices	[151]
PHBb)/palladium nanoparticles	Electrospinning/thermo-pressing molding	Films with oxygen scavenging capability and good barrier properties	[152]
PHBb)/silver nanoparticles	Thermo-pressing molding	Films with antimicrobial activity against S. enterica and L. monocytogenes. In addition, nanoparticles did not influence the biodegradability of films	[153]
PHBVd)/PEGc)or PHBb)/zein	Electrospinning/thermos-pressing molding	Films with acceptable mechanical and thermal properties, as well as with antimicrobial activity against L. monocytogenes	[154]

a) PHA: polyhydroxyalkanoate.

b) PHB: poly(3-hydroxybutyrate).

c) PEG: poly(ethylene glycol).

d) PHBV: poly(3-hydroxybutyrate-co-3-hydroxyvalerate).

e) PLA: poly(lactic acid).

f) TEC: triethyl citrate.