Physicochemical properties, production, and biological functionality of poly-γ-d-glutamic acid with constant molecular weight from halotolerant Bacillus sp. SJ-10

Poly-γ-glutamic acid (γ-PGA) is an unusual anionic homopolyamide that is biodegradable, edible, and nontoxic. It has a wide variety of industrial applications depending on its combined cations, and molecular weight. In this study, extracellular viscous γ-PGA produced by halotolerant Bacillus sp. SJ-10 isolated from a traditional Korean salted-fermented seafood was purified and characterized. The physicochemical analysis indicated that the γ-PGA produced by Bacillus sp. SJ-10 consists primarily of d-glutamic acid residues combined with sodium cations. When batch fermentation was performed with 8% NaCl for 3 d, Bacillus sp. SJ-10 produced approximately 24.7 g/L γ-PGA with a molecular weight of approximately 400 kilodaltons (kDa). Under fermentation conditions with 6% NaCl, the maximum yield was 26.2 g/L regardless of the molecular weight. The γ-PGA sodium salt with a molecular weight of 400 kDa exhibited antioxidant activity by scavenging 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) radicals and reducing molybdenum, with maximal scavenging activity at 0.5 mg/mL and reducing activity at 1 mg/mL (20 μg ascorbic acid-equivalent), respectively. These results suggest the potential use of γ-PGA in the food, cosmetic, and biomedical industries for its antioxidant qualities. Our results also provide an economical method for controlling the molecular weight of the γ-PGA produced.