## 2 Mechanisms of Acidogenic Fermentation
### Biotechnological Production of Volatile Fatty Acids  
Although some VFAs such as propionic and butyric acids are currently produced at industrial scale via pure culture fermentation using refined substrates, no consolidated industrial process yet exists for their production from organic waste through mixed culture acidogenic fermentation. Most developments in this area remain at the pilot or demonstration stage. Industrial production typically relies on species such as _Propionibacterium acidipropionici_ or _C. _butyricum_, operated under sterile conditions with defined substrates like glucose, lactose, or glycerol. These systems offer high selectivity and productivity but depend on food-grade feedstocks and tight process control (e.g., pH 6.5, long fermentation times, anaerbiosis), which limits their sustainability and scalability [62].  
The biotechnological production of VFAs from organic waste streams involves a series of interconnected stages that include the preparation of the microbial inoculum, the formulation of the fermentation medium, the selection of operational parameters, and the configuration of suitable bioreactors. These steps are critical for directing microbial metabolism towards acidogenesis while suppressing competing processes such as methanogenesis. Inoculum preparation typically relies on anaerobic mixed microbial cultures due to their resilience and broad metabolic capabilities. Common sources include sewage sludge, digestate from anaerobic reactors, or animal manure. To inhibit methanogenic activity and favor acid accumulation, the inoculum is often subjected to pretreatments such as heat shock (e.g., 85 \({}^{\circ}\)C for 1 h), acidification (pH below 4.0 for 24 h), or chemical inhibition using agents like 2-bromoethanesulfonate [63]. These treatments selectively suppress methanogens while preserving fermentative bacteria. Following pretreatment, the inoculum is sometimes acclimated to the substrate over several batch cycles to enhance performance [64]. The fermentation medium is usually composed of selected organic substrate (e.g., food waste, lignocellulosic hydrolysates, glycerol, or agro-industrial by-products) and is sometimes supplemented with macro- and micronutrients. Fermentation is generally conducted under anaerobic conditions in mesophilic (30-37 \({}^{\circ}\)C) or thermophilic (50-55 \({}^{\circ}\)C) temperature ranges, depending on the desired acid profile and the microbial community involved. Acidogenesis is favored at mildly acidic pH values (typically 5.0-6.5), which suppress methanogenic pathways and encourage the accumulation of VFA [65]. Figure 2 schematically illustrates the main stages of VFA production from organic waste while Figure 3 shows a pilot-scale facility for VFA production from waste streams, where the excess fermentation stream is valorized through methane generation.  
Various bioreactor configurations have been employed for VFA production. For continuous production, systems such as continuous stirred tank reactors (CSTRs; such as those shown in Figure 3), sequencing batch reactors (SBRs), up-flow anaerobic sludge blanket reactors (UASBs), and anaerobic membrane bioreactors (AnMBRs) are employed. These configurations enable better control over solid and hydraulic retention times (HRTs) and improve process stability. However, industrial implementations increasingly favor membrane-based systems, which allow the decoupling of biomass retention and HRT, thereby increasing volumetric productivity [13].  
Figure 2: Biotechnological production of VFAs from organic waste.