The antioxidative activity and bioactive compounds content of lactic acid fermented green tea (LFG) fermented with an outstanding GABA-producing strain under optimised fermentation conditions were evaluated. Levilactobacillus strain GTL 79 was isolated from green tea leaves and selected based on acid production, growth potential, catechin resistance, and GABA production to be applied to LFG. Through 16S rRNA gene sequence analysis, the strain was identified as Levilactobacillus brevis. The optimised conditions were defined as fermentation at 37 °C with supplementation of 1% fermentation alcohol, 6% glucose, and 1% MSG and was determined to be most effective in increasing the lactic acid, acetic acid, and GABA content in LFG by 522.20%, 238.72% and 232.52% (or 247.58%), respectively. Initial DPPH scavenging activity of LFG fermented under the optimised conditions was 88.96% and rose to 94.38% by day 5. Polyphenols may contribute to the initial DPPH scavenging activity, while GABA and other bioactive compounds may contribute to the activity thereafter. Consequently, as GABA and other bioactive compounds found in green tea have been reported to have health benefits, future studies may prove that optimally fermented LFG by L. brevis GTL 79 could be useful in the food and health industries.

This study investigated the influence of one- and two-step fermentation on bioactive compound production in fermented green tea, i.e., one-step fermented green tea (OFG) and two-step fermented green tea (TFG). One-step fermentation entailed acetic acid fermentation, while two-step fermentation consisted of lactic acid fermentation followed by acetic acid fermentation. Acetobacter pasteurianus PCH 325, isolated from an over-ripened peach, was selected for acetic acid fermentation based on its growth and organic acid production characteristics. Acetic acid fermentation conditions were optimized for one- and two-step fermentation: 3% fermentation alcohol for both processes; 8% and 4% sucrose, respectively; and fermentation at 25 °C for both processes. For lactic acid fermentation of TFG, the inoculum and optimized conditions reported previously were used. Under the optimized conditions, the acetic acid content in OFG and TFG increased 21.20- and 29.51-fold, respectively. Furthermore, through two-step fermentation, γ-aminobutyric acid and lactic acid were produced up to 31.49 ± 1.17 mg/L and 243.44 ± 58.15 mg/L, respectively, which together with acetic acid could contribute to the higher DPPH scavenging activity of TFG. This study suggests that two-step fermentation may be a valuable strategy in industry for raising the amount of acetic acid and/or providing additional bioactive compounds.

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.