## 2 Mechanisms of Acidogenic Fermentation
### Key Microbial Communities Involved in VFA Production  
Apart from the species mentioned in the previous section, such as _Clostridium_, _Selenomonas_, and _Bacteroides_, there are numerous other microorganisms that play essential roles in acidogenic fermentation. The specific VFA produced largely depends on the microbial species present, as each group of microorganisms favors the production of a particular type of acid [29]. These communities interact and adapt to substrate availability and environmental conditions, ensuring that the dominant microorganisms guide the process toward the generation of acetic, butyric, propionic acids, or other VFAs [30]. Some examples of key communities in specific VFA production are gathered in Table 2.  
Acetic acid is one of the most commonly produced VFA during acidogenic fermentation, driven by various bacteria, especially in carbohydrate-rich environments. _Acetobacter aceti_ plays a central role in industrial acetic acid production due to its ability to metabolize a wide range of sugars, including glucose, ribose, and xylose [31]. This versatility makes it ideal for continuous fermentation processes, often applied in large-scale production using organic waste substrates such as date extract [32]. Furthermore, the use of _Acetobacter_ in processes utilizing cheese whey as a substrate has led to particularly high yields of acetic acid. Under optimized conditions, _A. aceti_ has been used to achieve efficient results, producing 84.1 g/L of acetic acid with a yield of 97.4% and a productivity of 7.2 g/(L-h) [33]. In addition, _Moorella thermoacetica_ and _Aurnatiochytrium limacinium_ are highly effective at fermenting lignocellulosic sugars derived from agricultural residues, such as switchgrass, forest residues, wheat straw and sugarcane straw, making it crucial in waste valorization strategies aimed at producing acetic acid from renewable sources. When adapted to these waste-derived hydrolysates, these microbial strains can achieve up to 80% of theoretical yields for bio-acetic acid production, demonstrating the efficiency of using agricultural residues as feedstock. This strategy not only enhances acetic acid yields but also leads to a 75% reduction in media costs compared to traditional organic fermentation mediums, further highlighting the economic and environmental benefits of using waste materials in fermentation processes [34]. Another important microorganism, _Clostridium lentocellum_, is capable of converting cellulose-based materials like paddy straw and cotton straw into significant amounts of acetic acid, demonstrating its potential in cellulose-rich. In fact, this species can efficiently ferment various crystalline cellululosic materials, achieving a maximum acetic acid yield of 0.67 g acetic acid per gram of substrate [35]. Mixed cultures, like those involving Saccharomyces cerevisiae and _Acetobacter pasteurianus_, are also important for maximizing acetic acid yields. In such systems, Saccharomyces converts glucose into ethanol, which is then oxidized to acetic acid by _Acetobacter_. This co-culture has been shown to produce high concentrations of acetic acid, up to 89.3 g/L in fed-batch fermentation processes, demonstrating the benefits of microbial synergy in industrial setups [36]. Propionic acid is primarily produced through the fermentation of various substrates by species of the _Propionibacterium_ genus, including _P. acidipropionici_, _P. freudnerichii_, _P. shermanii_, and _P. thoenii_, which effectively convert hexoses and pentoses into propionate [37]. _P. acidipropionici_ plays a key role in producing propionic acid from renewable substrates. For instance, with Agave bagasse pretreated through steam explosion and enzymatic hydrolysis,