## 2 Mechanisms of Acidogenic Fermentation  
### Biological Processes Involved in Acidogenic Fermentation  
Acidogenic fermentation is a crucial phase within the broader anaerobic digestion process. During this stage, soluble organic compounds derived from the hydrolysis of complex organic materials, such as proteins, carbohydrates, and lipids, are fermented by specific microorganisms into VFA, hydrogen, and carbon dioxide. The acidogenic phase is primarily responsible for the production of short-chain fatty acids, including acetic, butyric, propionic, and valeric acids, which serve as key intermediates in various biotechnological processes such as bioplastic production. This fermentation step is carried out by a wide range of facultative and obligate anaerobic bacteria. The process typically begins with the breakdown of soluble organic substrates, as carbohydrates, proteins and lipids, into simpler molecules such as monosaccharides, amino acids, and long-chain fatty acids, which are then subjected to fermentation following different metabolic pathways (Figure 1). The microbial activity during this phase is highly dependent on the availability of substrates, the composition of the microbial community, and environmental factors like pH and temperature [19]. One of the key microbial genera involved in acidogenic fermentation is _Clostidium_, a strict anaerobe that plays a significant role in the fermentation of carbohydrates [20]. _Clostidium_ species are known for their ability to produce VFAs, particularly butyric acid, through the fermentation of sugars [21]. This process typically involves the conversion of glucose into pyruvate via the Embden-Meyerhof-Parnas pathway, followed by the reduction in pyruvate into acetyl-CoA, which serves as a precursor for various VFAs. Acetyl-CoA can be further converted into acetic acid, while butyric acid is produced through a series of enzymatic reactions involving butyryl-CoA dehydrogenase [22]. Protein-rich substrates undergo a different set of reactions, facilitated by specific proteolytic microorganisms. Amino acids, derived from the hydrolysis of proteins, are fermented through pathways such as oxidative deamination or Stickland reactions [23]. In oxidative deamination, amino acids are converted into their corresponding keto acids, which are further fermented into VFAs, such as acetic and propionic acids. Stickland reactions, on the other hand, involve coupled oxidation and reduction in amino acids, producing VFAs, ammonia, and carbon dioxide as by-products. These reactions are carried out by species such as _Selenomonas_ and  
Figure 1: VFA production from organic macromolecules (adapted from Duong and Nga [11]). _Bacteroides_, which are important for maintaining the balance of VFAs in the fermentation environment [24]. The fermentation of lipids is another critical pathway in acidogenesis. Lipids, which are hydrolyzed into glycerol and long-chain fatty acids, are broken down by lipolytic bacteria. Glycerol is fermented into pyruvate, which is subsequently converted into acetic acid, while long-chain fatty acids are subjected to \(\beta\)-oxidation. This process generates acetyl-CoA, which can be funneled into the VFA production pathways, yielding acetic acid and hydrogen as primary products. The fermentation of lipids is particularly important in industrial applications where lipid-rich waste streams are used as feedstock, as they can contribute significantly to the overall VFA yield [25]. A key feature of acidogenic fermentation is its dependency on the microbial consortia present in the reactor. These microbial communities are highly adaptable and can switch between different metabolic pathways depending on the substrates available and the environmental conditions. For instance, when methanogens are inhibited by maintaining a low pH (below 6), acidogenic bacteria dominate the fermentation process, ensuring that VFA are produced rather than being consumed and converted into methane [26].