Enhancement of Poly-γ-glutamic Acid Formation and Antioxidant Activity in Cheonggukjang Fermented with Bacillus velezensis CH7P29

This study aimed to isolate Bacillus strains producing poly-γ-glutamic acid (γ-PGA), a natural antioxidant biopolymer, from Cheonggukjang, optimize fermentation conditions, improve γ-PGA production in the food by applying the selected strain, and investigate its potential association with antioxidant activity. Among 157 isolates, Bacillus velezensis CH7P29 demonstrated the highest γ-PGA production (602.80 ± 6.55 μg/mL) and was selected for fermentation. Optimal in vitro conditions for γ-PGA production were determined as 40 °C and 0–2% NaCl. In Cheonggukjang fermentation under optimal conditions, the CH7P29-inoculated group exhibited a marked increase in γ-PGA content (16.02 mg/g on day 4), which was significantly higher than the uninoculated control (not detected) and reference strain groups (12.25 and 11.38 mg/g, respectively). Simultaneously, its DPPH radical scavenging activity exceeded 60%, and γ-PGA content strongly correlated with antioxidant activity (R = 0.95). Conversely, total polyphenol content remained stable, indicating that γ-PGA and microbial metabolites contributed primarily to antioxidant activity. Further analysis supported this by showing that γ-PGA accounted for 37.40% of the total DPPH radical scavenging activity in the CH7P29-inoculated group at the end of fermentation. These data emphasize that B. velezensis CH7P29 is a promising functional starter culture for enhancing the biofunctional properties of Cheonggukjang and fermented soybean food.