## 3 Results and Discussion
### Effect of Fermentation on Phenolic Content  
In fermented products, the glycoside form of polyphenols can be transformed into the aglycone form by microorganisms to improve their bioavailability in the intestine and perform their beneficial functions better. Changes in total phenolic content (TPC), bound phenolic content (BPC), and free phenolic content (FPC) during oat fermentation by LAB are depicted in Figure 3a-c, respectively. The results indicate that a strain has strict specificity in phenolic acid metabolism/degradation/hydrolysis as reported in the literature [31]. For example, as shown in the figure, the TPCs of some strains were distinctly different at the given fermentation time (\(p<0.05\)). Compared with the unfermented samples, the TPCs of all fermented samples showed a decreasing trend at the later stage of fermentation, particularly after 12 h, except for O-Lc. However, after 48 h of fermentation, only the TPC of O-Lc increased by 3.14%. Similar results were found by [PERSON] et al. [32], who fermented jujube juice with \(L\). _plantarum_ and \(L\). _casei_ to increase TPC. Other results showed that fermentation could also reduce phenolic content. Moreover, extractability was reduced by the self-polymerization of phenolic compounds and/or the interaction with other macromolecules (such as amino acids and starch) [33]. In addition, compounds are also be transformed and degraded into other healthy monomers [8].  
Figure 2: Changes of \(\beta\)-glucan content in oats fermented by different strains at different times. Different letters indicate significant differences among different fermentation time a given fermentation strain (\(p<0.05\)). 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 \(L\). _bulgaricus_; O-St, sample fermented with \(S\). _thermophilus_. h, hours.  
Figure 3: Contents of total polyphenols (**a**), bound polyphenols (**b**) and free polyphenols (**c**) in samples of different strains at different fermentation times. 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. bulgarcicus_; O-St, sample fermented with _S. thermophilus_. h, hours.  
Figure 3b,c show that the FPCs of the five strains were higher than that of the control sample within 24 h of fermentation, whereas the BPCs had the opposite trend. Additionally, an increase in the FPC of each sample was often accompanied by a decrease in BPC. This phenomenon can be explained by the transformation of BPC into FPC due to the metabolic activities of microorganisms. LAB grow rapidly in the early stage of fermentation and can use sugar and protein to partially release BPC. In addition, esterase, decarboxylase, and \(\beta\)-glucanase are produced in the proliferation process. Among them, \(\beta\)-glucanase hydrolyzes the \(\beta\)-glycosidic bond of conjugated phenolic compounds [34], resulting in the release of conjugated phenolic compounds and an increase in FPC [35]. Ferulic acid esterase also releases BPC from the grain cellulose matrix into a free form. At different fermentation times, the FPC of O-Lc was always higher than those of other samples (\(p<0.05\)), especially at 12 h, when it could reach 73.70 mg GAE/L, which was 1.32 times higher than that of the control samples. The BPC in O-Lc was remarkably lower than those of O-St, O-Lc, and O-Lb. In addition to glucanase activity, the ability of bacterial strains to degrade phenol esters or tannins, different induced phenol decarboxylase activities, and different substrate acidity may also contribute to this result [36]. Overall, fermentation can change the phenolic contents in the samples. However, whether the compositions of free and bound monomeric phenolic compounds change needs further verification.