## 3 Results and Discussion
### Effect of Fermentation on Flavonoid Content  
Flavonoids are important natural organic compounds that exist widely in nature. Most flavonoids have strong biological activities, which has aroused research interest. Flavonoids occupy an increasingly important position in daily diet and disease treatment owing to their extensive pharmacological effects and low toxicity. The content change of flavonoids during oat fermentation is shown in Figure 4. The content of total flavonoids in oats fermented by different strains showed a downward trend within 48 h as shown in Figure 4a. This result may be related to the large reduction of conjugated flavonoids. According to previous reports, flavonoids can be transformed into free forms in the fermentation process of soybean and tea, but a decrease in the total amount may not indicate a decrease in biological activity. Bound and free flavonoids decreased during the fermentation of corn and other grains [37]. Researchers attributed this to the metabolism of these compounds during microbial fermentation, such as the degradation/polymerization of flavonoids into dihydroxy flavonoids analogues and the activity changes of enzymes, including glycosidase, glycosyltransferase, tannase, esterase, and hydrolase [34]. In addition, some reactions may induce flavonoids to transform into other metabolites through flavonoid methylation, glycosylation, flavonoid deglycosylation, and flavonoid-sulfuric acid conjugation.  
Figure 4b,c shows that after 4 h of fermentation, the free flavonoids in the samples of the other four bacteria except O-Lp were increased, and the bound flavonoids were decreased. O-Lp had the opposite trends initially but gradually had the same trends in the later fermentation time. This outcome could be related to the depolymerization of bound flavonoids and the formation of soluble free flavonoids. Interestingly, we found that at each fermentation time point, the free flavonoid content of O-Lc was distinctly higher than in those of other strains (\(p<0.05\)), reaching 202.60 mg RE/L at 48 h, an increase of 20.47% compared with the unfermented sample. Flavonoid glycosides may be consumed during fermentation and released in the form of aglycones. LAB can convert flavonoid glycosides in Cudrania tricuspidate leaves into flavonoids, quercetin, and kaempfer [38].  
Current studies on the biotransformation of flavonoids in food fermentation processes are few. [PERSON] et al. [39] fermented milk containing _Scutellaria bicalensis_ with _Lactobacillus brevis_ and found that bacialin and wogonoside were converted into their aglycone forms, baicalein and wogonin, respectively, which have higher biological activities. The biotransformation mechanism of flavonoids in LAB-fermented food remains to be further studied.  
Figure 4: Contents of total flavonoids (**a**), bound flavonoids (**b**) and free flavonoids (**c**) in samples of different strains at different fermentation time. Different letters indicate significant differences among different strains a given fermentation time (\(p<0.05\)). O-Lp, sample fermented with _L. plantarum_; O-La, sample fermented with _L. acidophilus_; O-Lc, sample fermented with _L. casei_; O-Lb, sample fermented with _L. bulgarcus_; O-St, sample fermented with _S. thermophilus_. H, hours.