34.7 Packaging Materials and Bioplastics

529

34.7.3

Polyhydroxyalkanoates (PHAs)

The term PHAs is used to describe various aliphatic polyesters, such as

poly-3-hydroxybutyrate

(P3HB),

poly(3-hydroxy

butyrate-co-3-hydroxyvalerate

(PHBV), poly-4-hydroxyvalerate (PHV), polyhydroxy hexanoate (PHH) and

polyhydroxy octanoate (PHO) naturally produced by bacterial fermentation of

AFW carbohydrates. The PHA granules are recovered by disrupting the cells

of natural molecules [103]. The bacteria (Cupriavidus nector, Methylobacterium

rhodesianum, or Bacillus megaterium) produce polyhydroxybutyrate (PHB) under

nutrient-deficient conditions. It is considered that the bacteria use the product

as energy storage molecule and metabolize in the absence of any other sources.

The microbial synthesis of PHB involves condensation between two molecules of

acetyl-CoA to form acetoacetyl-CoA that is further reduced to hydroxybutyryl-CoA.

The later compound acts as a monomer for polymerization to form P3HB [104].

The heat resistance and gas barrier properties of PHA/P3HB are similar to

polyvinyl chloride and polyethylene terephthalate (PET) thermoplastics. However,

the stiff and thermally unstable nature of BPM vis a vis synthetic plastics pose

limitation to their widespread use. The incorporation of another PHA monomer

3-hydroxyvalerate (HV) into the P3HB polymer to synthesize the copolymer PHBV

improved P3HB properties to a certain extent [105]. The high production cost of

PHA is another challenge and is the impetus for the development of cost-effective

process using AFW.

34.7.4

Reinforcement in Bioplastic Properties

The modulation in bioplastic properties was envisaged to enhance its packaging

applications in food sector [106]. The approaches involved the addition of plasticizer

such as glycerol, natural fiber such as almond/walnut shell [107], hull of soyabean

[108], laminating with paper, bilayer of PLA and P3HB, copolymerization, com-

posite formation, etc. The plasticization involves the use of glycerol and other food

grade polymers in bioplastics. The cross-linking and compatibility between PLA and

poly(glycerol succinate-co-maleate) (PGSMA) is known to improve the toughness of

BPM [109]. The high tensile strength and elongation of pectin cellulose (obtained

from orange waste) thin film was achieved using glycerol as plasticizer and maleic

anhydride as compatibilizer [110].

34.7.4.1

Natural Extract

The addition of natural extracts from plant parts in the BPM not only improves the

gas barrier property but also extends the shelf life of food by preventing it from

spoilage. The olive leaf and propolis extract were incorporated in PLA matrix with

the aim to release antimicrobial/antioxidant agents in the bio-based food packag-

ing systems [111]. Del Nobile et al. [112] incorporated lemon extract in packaging

based on PLA, polycaprolactone (PCL) and LDPE. Mesquita et al. [85] extracted

carotenoids from the Bactris gasipaes fruit waste and incorporated into chitosan film

to enhance its mechanical property and antioxidant activity.