Production and characterization of microbial poly-γ-glutamic acid from renewable resources

Abstract

Poly-γ-glutamic acid (PGA) is an anionic, naturally occurring, biodegradable, edible and water soluble polyamide that has widespread applications <i>viz</i>., thickener, cryoprotectant, drug carrier, biological adhesive, biopolymer flocculant, heavy metal absorber, bitterness relieving agent, etc. Here, we evaluated effectiveness of renewable resources for production of γ-PGA using an isolated <i>Bacillus sp</i>. Lignocellulosic biomass such as rice straw, sugarcane trash, sugarcane bagasse, cotton stalk and sorghum stover were evaluated for production of γ-PGA. Pretreatment was carried out using 1% (w/w) H₂SO₄ for rice straw and 2.5% (w/w) H₂SO₄ for rest of the biomass, followed by enzymatic hydrolysis using cellulase enzyme. Comparative evaluation of sugar yield from various biomass were done and highest percentage of reducing sugar was obtained from rice straw hydrolysate. The hydrolysate was concentrated to obtain 1.01 g/mL of reducing sugar and was added to the production media for the fermentative production of γ-PGA. The maximum PGA obtained was 82.97 g/L by supplementing rice straw hydrolysate (reducing sugar concentration: 4%) by <i>Bacillus amyloliquefaciens</i>. The γ-PGA was characterized by NMR, FTIR, gel permeation chromatography and amino acid analyzer. Present work shows the feasibility of using lignocellulosic biomass as a cheap and economically efficient carbon source for the fermentative production of microbial γ-PGA.