## 2 Mechanisms of Acidogenic Fermentation
### Key Microbial Communities Involved in VFA Production
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Volatile Fatty Acid** & **Related Microorganisms** & **Key Notes** \\ \hline \multirow{7}{*}{Acetictic} & _Acetobacter acti_ & \\  & _Moorella thermoacetic_ & \\  & _Aurantiolchytrium limacinum_ & Efficient production from carbohydrate-rich \\  & _Clostridium lentocellum_ & substrates like whey and agricultural residues. \\  & _Clostridium ljungdahlii_ & High theoretical yields with agricultural residues. \\  & _Saccharomyces cerevisiae_ & \\  & _Acetobacter pasteurianus_ & \\ \hline \multirow{7}{*}{Propionic} & _Propionibacterium acidipropionici_ & \\  & _Propionibacterium freudenreichii_ & Utilizes glucose, glycerol, and protein-rich \\  & _Peptostreptococcus_ & substrates for high yields of propionic acid. \\  & _Anaertigium propionicum_ & \\ \hline \multirow{7}{*}{Butyric} & _Clostridium butylricum_ & High yields in various substrates; chain elongation \\  & _Clostridium tyrobutyricum_ & in microbial electrosynthesis. Unique metabolic \\  & _Clostridium acetobutylicum_ & pathways allow high titers of n-butanol and \\  & _Megasplaera_ & butyrate, demonstrating versatility in \\  & _Butryriviibrio fibrilsolvens_ & bioaugmentation systems. \\ \hline \multirow{7}{*}{Valeric} & _Clostridium spp._ & \\  & _Propionibacterium freudenreichii_ & Production through chain elongation and robust \\  & _Megasplaera_ & fermentation pathways. \\  & _Lachospiraceae bacterium_ & \\ \hline \multirow{2}{*}{Capric} & _Clostridium kluyveri_ & Produced via reverse \(\beta\)-oxidation with \\  & _Ruminococcaceae bacterium_ & lignocellulosic residues. \\ \hline \multirow{7}{*}{Isobutyric} & _Escherichia coli_ & Engineered pathways for selective production \\  & _Pseudomonas_ sp. & with minimal by-products. Optimized through \\  & _Corynebacterium glutamicum_ & metabolic engineering to achieve higher purity \\ \cline{1-1}  & _Propionibacterium freudenreichii_ & and yield. \\ \cline{1-1}  & _Escherichia coli_ & Production from branched-chain amino acid \\  & _Leuconstoc mesenteroides_ & pathways; useful in dairy fermentations. \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Microorganisms associated with specific VFA production. it achieved a propionic acid yield of 0.44 g/g total carbohydrates and productivity of 0.069 g/L-h. This process, enhanced by pH control, demonstrated efficiency without requiring detoxification steps [38]. In high cell density fermentation, _P. acidiproionici_ utilized glucose and glycerol/glucose mixtures effectively. With 40 g/L glucose over multiple batches, propionic acid yield averaged 18.76 g/L with a conversion rate of 0.59 g/g glucose. Using a glycerol/glucose mix (60 g/L:30 g/L), it reached 35.36 g/L propionic acid per batch, achieving 0.51 g/g carbon source [39]. These methods, especially with cell recycling and biotin supplementation, enhance propionic acid productivity and cell growth, making _P. acidiproionici_ a promising option for sustainable biorefining. _Propionici_ _freudeneiichi_ also plays a crucial role in the sustainable production of propionic acid through various renewable substrates. This bacterium has demonstrated adaptability to a range of by-products and biomass sources, enabling efficient propionic acid production with promising yields. For instance, in the production of propionic acid from whey permeate and corn steep water, _P. freudeneiichi_ achieved a concentration of 13.10 g/L with a yield of 0.335 g/g after 72 h in batch fermentation, while a fed-batch approach allowed for pH control and the further optimization of the process. Additionally, the study employed ion exchange cryogels for purification, achieving selectivity by using taurine-functionalized cryogels, with optimal adsorption at pH 4 for propionic acid, demonstrating the potential of these residual streams as cost-effective carbon sources for propionic acid production [40]. In another study, sweet sorghum bagasse hydrolysate served as a renewable substrate, yielding 0.51 g/g propionic acid, surpassing the yield obtained from glucose (0.44 g/g).