## 4 Discussion
Table 4 also shows that increasing the plant material content at the selected fermentation time (from 9 to 10 days) results in a significant increase in TPC from about 3.76 mmol GA/L \(\pm\) 0.04 (plant material content 0.32%) to about 8.62 mmol GA/L \(\pm\) 0.03 (plant material content 3.20%) (Table 4). Fermentation of black soybeans in the presence of _Bacillus subtilis_ BCRC 14715 and using such solvents as water, methanol, ethanol, and acetone increased the total phenolic and flavonoid content, as well as the antioxidant activity of the obtained extracts. Phenols of different classes are converted into compounds that are more bioactive than the parent compounds, leading to higher contents of TPC and AA [49]. The fermented acetone extract had the highest total phenolic content of 40.42 \(\pm\) 0.48 mg GA/g and the highest DPPH free radical scavenging effect (EC\({}_{50}\) = 0.65 \(\pm\) 0.02 mg/mL), while the fermented methanol extract showed the highest Fe\({}^{2+}\) ion chelating capacity (EC\({}_{50}\) = 2.17 \(\pm\) 0.19 mg/mL). The active substance associated with the DPPH radical scavenging effect was most efficiently extracted from black soybeans using water as a solvent. Water effectively extracted iron (II) ion chelating principles from unfermented (yield = 44.20% \(\pm\) 0.21) and fermented (yield = 50.79% \(\pm\) 1.98) black soybeans, respectively [63]. Fermentation of cheonggukiang soybeans using _Bacillus subtilis_ CS90 probiotics increased total polyphenol content (from 253 to 9414 mg/kg) and DPPH radical scavenging activity (from 53.6 to 93.9%), while total flavonoid gallate content decreased by about 70% over the 60-h fermentation period. As fermentation time increased, significant differences in total phenolic acids were observed: 311.01 mg/kg (time = 0 h), 400.45 mg/kg (time = 12 h), 508.48 mg/kg (time = 24 h), 733.59 mg/kg (time = 36 h), 890.05 mg/kg (time = 48 h), and 1066.92 mg/kg (time = 60 h). In particular, the level of phenolic acids increased significantly with a decrease in flavonoid gallates during the fermentation run. Total phenolic content (TPC) is higher after the fermentation of plant raw materials in the presence of LAB microorganisms, which can be explained by the generation of metabolites and the release of phenols from the plant matrix. However, in some cases, TPC reduction after fermentation is more common due to enzymatic or non-enzymatic oxidation, and diffusion of soluble phenolic compounds into fermentation secretions (exudates). In addition, the total phenolic content may increase or decrease depending on the strains of microorganisms [15; 52; 53]. During the ongoing fermentation of plant raw materials, it has been observed that gallates are converted into flavanols (epigallocatechin gallate into epigallocatechin). In addition, flavanols (catechin, epicatechin, epigallocatechin) exhibit antiviral, antimutagenic, anticancer, anti-obesity, and hypocholesterolemic effects. Flavanols are also considered to be effective antioxidants, and they work by scavenging oxygen radicals and chelating metal ions (for example, Fe\({}^{2+}\) ions). Phenolic acids, such as gallic acid and its derivatives, also have antioxidant, antimutagenic, and anticancer effects. Gallic and rosmary acids were the most potent antioxidants among simple phenolic and hydroxycinamic acids. A review of the literature indicates that phenolic acid content correlates with the antioxidant capacity of FPEs [64; 65]. Degradation of anthocyanin glucosides to their aglycone forms and free phenolic acids is also observed during fermentation in the presence of lactic acid bacteria. Fermentation of jussar pulp by _L. deubruekii_, assisted by the action of \(\beta\)-glucosidase, led to the conversion of cyanidin 3-O-glucoside and cyanidin 3-O-rutinoside to the main product, protocatechic acid, which is a phenolic compound.