Optimized production of poly (γ‐glutamic acid) (γ‐PGA) using Bacillus licheniformis and its application as cryoprotectant for probiotics

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Bacteria produce poly (γ-glutamic acid) (γ-PGA), a polymer of l- or d-glutamic acid, as a defense response and have gained importance due to their applications in food and pharmaceutical industry. In the present investigation, production of γ-PGA using cost-effective carbon substrate, characterization of the produced polymer, and its application as cryoprotectant for selected freeze-dried probiotic bacteria were investigated. Central composite rotatable design of response surface methodology was used to study the main and the interactive effects of medium components: rice bran and casein peptone concentration. Rice bran at 35% (w/v) and casein peptone at 7.5% (w/v) were found to be optimal at an initial pH of 7.5, and incubation temperature of 37°C for 48 H produced 8.2 g/L γ-PGA on dry weight basis. The thermal properties such as melting temperature, heat of fusion, and thermal stability were also studied. Ten percent (w/v) of γ-PGA with 10 percent of sodium alginate (w/v) protected viability of Bifidiobacterium bifidum NCDC 235 and B. adolescentis NCDC 236 during freeze drying at -80 ˚C for 48 H. The γ-PGA synthesized by the reported bacterium with GRAS status is suitable for food and biomedical applications.