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

Abstract

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.

Keywords: Bacillus subtilis 168; Cheongguk-jang; Fermentation; Poly-γ-glutamic acid.

Fig. 1

Viscosity of cheongguk-jang produced by different cooking conditions. Cooked soybeans for CKJ fermentation was prepared by changing two factors, the amount of added water and the heating time. The fermentation was carried out at 40 °C for 2 days. Values are mean ± SE. *p < 0.05; **p < 0.01; ***p < 0.001 viscosity versus amount of added water within each autoclave time; #p < 0.05; ##p < 0.01; ###p < 0.001 interaction of viscosity and autoclave time; two-way ANOVA with Scheffe post hoc test

Fig. 2

Effect of fermentation temperature and time on viscosity of cheongguk-jang. Fifty mL of water was added to 100 g soybeans and autoclaved at 121 °C for 60 min. Values are mean ± SE. *p < 0.05 viscosity versus fermentation temperature within each fermentation time; ##p < 0.01 interaction of viscosity and fermentation time; two-way ANOVA with Scheffe post hoc test