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Agroresidue-based biorefineries 249

11.2.2 Biopolymer

11.2.2.1 Poly-3-hydroxybutyrate
Poly-3-hydroxybutyrate (PHB) serves as an eco-friendly biopolymer with structural
properties similar to polypropylene. It is biodegradable, biocompatible, and can be pro-
duced from renewable carbon sources. The main limitation for the commercialization
of biopolymer is the cost contributed by carbon substrate, which accounts for about
50% of the total production cost (Albuquerque et al., 2007).
Microwave-assisted acid-pretreated chili postharvest residue (MWAAP CPHR)
serves as an ideal substrate for the production of PHB by Bacillus firmus NII 0830
(Sindhu et al., 2018). Under optimized conditions the strain produced 61.23% and
67.83% of PHB, respectively, with MWAAP CPHR and MWASP CPHR. PHB pro-
duction using a combined pretreatment and hydrolysis of rice straw was reported by
Sindhu et al. (2016). The fermentation of the nondetoxified rice straw hydrolyzate
without any nutrient supplementation by Comamonas sp. yielded 35.86% of PHB.
Acid-pretreated hydrolyzate obtained from rice straw serves as an excellent car-
bon source for the production of PHB by B. firmus NII 0830 (Sindhu et al., 2013).
The study revealed that B. firmus NII 0830 can grow and produce PHB in nonde-
toxified acid pretreated hydrolyzate of rice straw. Under optimized conditions the
strain produced 1.9 g/L of biomass with 1.697 g/L of PHB. PHB production from
alkali pretreatment hydrolyzate of sugarcane tops by Comamonas sp. was reported
by Prabisha et al. (2014). The hydrolyzate obtained after enzymatic saccharification
of alkali-pretreated sugarcane tops serves as a medium for the production of PHB.
Under optimized conditions the strain produced 55.85% of PHB with a productivity
of 0.195 g/L. The optimum conditions were pH of 7.0, incubation time for 96 h,
reducing sugar concentration of 1.25%, and KH 2 PO 4 concentration of 1.05%.

11.2.2.2 Poly-γ-glutamic acid
Poly-γ-glutamic acid (PGA) is an extracellular polymer produced by different
Bacillus sp. It is biodegradable and nontoxic hence finds applications in various
fields, such as cosmetics, wastewater treatment, food, oil recovery, and biosorbent
of heavy metals. One of the main limitations for commercialization is the cost con-
tributed by the carbon source. Several research and developmental activities are
going on for the cost-effective production of PGA using lignocellulosic biomass or
other waste streams as the sole carbon source.
Anju et al. (2017) evaluated different lignocellulosic biomass for the production
of PGA by Bacillus sp. The study revealed that among the different lignocellulosic
biomass evaluated for PGA production, such as sugarcane trash, sugarcane bagasse,
rice straw, sorghum stover, and cotton stalk, rice straw gave better results with
82.97 g/L. This indicates the potential of rice straw hydrolyzate as a cost-effective
substrate for PGA production by Bacillus amyloliquefaciens. Zhu et al. (2014)
developed a strategy for cost-effective production of PGA using xylose and corncob
fiber hydrolyzate in Bacillus subtilis HB1. The study revealed that corncob hydroly-
zate can be used as a low-cost substrate for the production of PGA by B. subtilis