Factors increasing poly-γ-glutamic acid content of cheongguk-jang fermented by Bacillus subtilis 168

Cheongguk-jang is a Korean traditional food produced by natural fermentation of boiled soybean. In cheongguk-jang, bacilli are dominant bacteria and produce highly viscous poly-γ-glutamic acid (γ-PGA), which improves human health functions. The purpose of this experiment was to find maximum production condition for the γ-PGA content during fermentation of cheongguk-jang with Bacillus subtilis 168. The most viscous cheongguk-jang was produced when soybean was cooked at 121 °C for 60 min in the presence of 50%(w/w) added water, followed by fermentation at 40 °C for 2 days. Additional conditions for maximum production of γ-PGA were the addition of 0.1%(w/w) FeCl₃·6H₂O, 3.0%(w/w) lactose and 3.0%(w/w) yeast extract as nutrients of inorganic salts, carbon source and nitrogen source, respectively. The three conditions did not show cumulative effect on the γ-PGA production and the addition of iron salt induced the most γ-PGA (0.97 ± 0.05%(w/w)), which corresponded to 2.7 times of the content in control cheongguk-jang.