## 4 Discussion  
### Correlation between Dominant Acid-Producing Microorganisms and Organic Acid Contents  
Ethyl caproate, ethyl acetate, and ethyl butyrate were the main volatile compounds in the SFB. The precursors of caproic acid, acetic acid, and butyric acid are mainly produced by acid-producing functional bacteria in PM and _Zapei_. Simultaneously, caproic acid and butyric acid are mainly produced by PM microorganisms [46]. The caproic-acid-producing bacteria in PM previously reported in the literature include _Caproiciproducens galaticfluorans_[47], _C. amylolyticum_[14], _Ruminococcaceae bacterium_ CPB6 [48], _Caproiciproducens fermentans_[49], _C. fermenticellae_[16], _Ruminococcaceae bacterium_ BL-4, _Clostridium_ sp. BL-3 [20], and _Clostridium kluyveri_ DSM 555 [50]. These caproic acid-producing bacteria can utilize a variety of precursors to synthesize caproic acid through reverse beta-oxidation using ethanol or lactic acid as precursors in the form of acetyl-CoA [36, 51]. In particular, _R. bacterium_ CPB6 was shown to synthesize caproic acid from lactic acid [48]. It was demonstrated that _C. galacitolivorans_ BS-1 produces n-caproic acid through chain elongation [22, 52]. In addition, a large number of species within the genus _Clostridium_, often found in PM and fermented grains, can decompose starchy raw materials or metabolize glucose to produce mixed acids, such as butyric acid and acetic acid, under anaerobic conditions. Based on the findings of the present study, as well as the information on the newly reported species _Clostridium acidigenes_, it was shown that the EFB mainly contained _C. guangxiense_[53], _C. algliaceis_[54], _C. kogasense_[55], _Clostridium autoethanologenum_[56], _Clostridium chromiireducens_[57], _Clostridium butyricum_, _Clostridium diolis_[58], _Clostridium liquoris_[59], _Clostridium luticellari_[60], and others. Moreover, it was demonstrated herein that the enrichment rounds led to an enrichment of _C. fermenticellae_ (with ASV101 as a representative), _C. amylolyticum_ (with ASV152 and ASV125 as representatives), _C. guangxiense_ (with ASV95 as a representative), _C. algliaceis_ (with ASV109 as a representative), and _C. publibutyricum_ (with ASV136 as a representative). In addition, the enrichment stage in which the highest diversity of acid-producing bacteria and the highest total acid content were found was the main fermentation stage. Therefore, the main fermentation stage is the best for pure strain isolation, and it is also considered the best stage for harvesting the products of mixed bacterial fermentation. Subsequently, we aimed to explore the relationship between the acid-producing functional microorganisms and the metabolites in the PM-enriched samples. To achieve this, we further obtained new combinations by grouping 29 OTUs with similar acid-producing functions based on previously reported data to perform a canonical correspondence analysis and construct a Spearman-correlation heat map between the composition of the functional microbial community and the metabolites identified in the samples (Figure 4). It was revealed that the EFB samples with positive correlations with the volatile acids were obtained during rounds 2 to 4 of the enrichment, while the samples obtained in the later enrichment stages showed negative correlations with the volatile acids (Figure 4A). Moreover, the Spearman correlation thermogram (Figure 4B) showed that the presence of microbial groups (C_a) with caproic-acid-producing ability in the EFB samples was positively correlated with the caproic acid contents in the samples. Among these microbial groups, _C. fermenticellae_ (with ASV101 as a representative) and _C. amylolyticum_ (with ASV152 and ASV125 as representatives) were important constituents of the microbial community for caproic acid accumulation. In addition, microorganisms (AB_a) such as _C. guangxiense_ (mainly ASV95), _C. algliaceis_ (mainly ASV109), _C. kogasense_ (mainly ASV104), and _C.