## 2 Mechanisms of Acidogenic Fermentation
### Key Microbial Communities Involved in VFA Production
This enhancement is attributed to the added nutrients and pH-buffering properties of the sweet sorghum bagasse hydrolysate. Co-fermentation with glycerol, a byproduct of biodiesel production, further increased yield to 0.59 g/g, with higher propionic acid concentrations achieved in bioreactor setups, demonstrating _P. freudeneiichi_'s high tolerance to product inhibition and suitability for high cell density fermentation [41]. Additionally, apple pomace extract and potato wastewater were effectively used as substrate components, resulting in a propionic acid yield of 0.42 g/g and a concentration of 12.71 g/L. These waste-derived media showed nearly equivalent production levels to traditional laboratory media, underscoring the potential for cost reduction in fermentation. Furthermore, this medium promoted additional metabolite production, such as trehalose, highlighting _P. freudeneiichi_'s capacity to contribute to a more sustainable propionic acid production process [42]. Finally, for protein-rich substrates, _Peptostreptococcus_ plays a crucial role. These bacteria thrive in nitrogen-rich environments and break down amino acids into VFA such as acetic and propionic acids, contributing to the acidogenic process. Their activity is particularly important in systems where protein degradation is a key part of the substrate composition, ensuring that nitrogenous compounds are efficiently converted into useful VFAs [43]. The adaptability of _Peptostreptococcus_ and other proteolytic bacteria makes them essential in optimizing the conversion of complex substrates into VFAs, especially in heterogeneous waste streams where protein content may vary significantly. In the case of butyric acid, production is mainly driven by _Clostridium_ species, particularly _C. butyricum_ and _C. tyrobutyricum_, which efficiently utilize sugars like glucose and xylose to produce high levels of butyric acid [44]. For example, _C. tyrobutyricum_ can generate butyric acid yields of up to 86.9 g/L when fermentation conditions, such as pH, are carefully controlled [45]. This strain is particularly effective in systems that use immobilized cells, such as fibrous-bed bioreactors, which enhance productivity in industrial applications by maintaining cell stability and maximizing butyric acid output. Additionally, the ability of _C. tyrobutyricum_ to thrive on a variety of substrates, including acne molasses and wheat straw hydrolysate, makes it versatile for different industrial waste streams [46]. Furthermore, _C. tyrobutyricum_ can utilize both glucose and xylose for butyric acid production, with yields influenced by sugar concentration and ratio. Higher glucose levels (up to 75 g/L) resulted in greater butyric acid yields, reaching 27.0 g/L, while at lower concentrations (5-15 g/L), xylose was more efficient. In mixed sugar media, _C. tyrobutyricum_ preferentially consumed glucose before xylose, and higher xylose-to-glucose ratios (4:1 or 3:2) improved butyric acid production when total sugar was maintained at 30 g/L [47]. In addition, mixed cultures involving _Bacillus_ and _C. tyrobutyricum_ have been used to produce butyric acid from sucrose, reaching concentrations of over 20 g/L. Notably, _C. tyrobutyricum_ is a hyper-butyrate-producing bacterium that can re-assimilate acetate into butyrate via butyryl-CoA/acetate CoA transferase instead of the typical PTB-BK pathway (Phosphotransbutylrylase- Butyrate kinase) found in other clostridia and microbial species. This unique metabolic route allows _C. tyrobutyricum_ to achieve high yields of butyrate and, through metabolic engineering, also high titers of n-butanol, enhancing its potential for industrial applications in producing C4 chemicals from various substrates, including sucrose [48]. Other strains like _C. acetobutylicum_ have also been engineered for butyric acid production, achieving up to 2889 mgCOD/L by modifying key metabolic pathways and using bioaugmentation techniques. For example, the bioaugmentation of _C.