## 5 Targeted Acidogenic Fermentation for Bioplastics Production
Current research emphasizes the strategic manipulation of substrate composition and operational parameters, such as pH, temperature, and retention time, to direct VFA production toward profiles that maximize their utility, particularly in the context of bioplastic synthesis [28]. As highlighted earlier, the VFA profile is strongly influenced by the composition of the feedstock, while operational parameters in the bioreactor significantly shape the microbial community and its metabolic outcomes. These parameters play a critical role in steering the fermentation process, as they determine which microbial groups thrive and which metabolic pathways dominate. For instance, a high OLR combined with a short HRT favors the proliferation of hydrolytic and acidogenic bacteria, while inhibiting slow-growing methanogens, thereby minimizing VFA degradation. Similarly, factors such as pH and temperature uniquely impact the activity and functionality of different microbial groups. To optimize VFA production, the careful adjustment of these conditions is essential. To visually summarize the overall process from biomass to application, Figure 5 illustrates the main stages and influencing factors in the conversion of organic waste into VFAs, including feedstock types, pretreatment strategies, microbial fermentation conditions, and end-use applications. The pH of the acidogenic fermentation process is a critical factor that directly influences microbial activity, substrate degradation, and the resulting VFA profile. Maintaining an optimal pH range is essential for maximizing VFA production and steering the fermentation process toward desired outcomes. The ideal pH for acidogenesis typically lies within the acidic range of 4.5-6.0, where methanogenic activity is effectively suppressed, enabling the accumulation of VFA. In this range, acetic acid is usually the dominant product, followed by propionic and butyric acids, depending on the substrate composition [130]. Maintaining a pH below 6.0 prevents methanogens from consuming VFAs, resulting in acidification yields as high as 60-80% of the soluble COD [131; 132]. However, if the pH drops below 4.5, the acidogenic pathways can be disrupted, leading to the production of undesired byproducts like ethanol or lactic acid due to shifts in microbial activity [133]. As the pH increases beyond 6.0 and approaches neutral or alkaline conditions (7.0-9.0), the microbial dynamics and metabolic pathways change significantly. Alkaline pH conditions enhance the hydrolysis of protein-rich substrates and support the production of butyric and valeric acids [15]. However, extremely high pH levels (10-12) may promote substrate hydrolysis but significantly inhibit acidogenic activity, resulting in lower VFA accumulation [130]. The specific pH also affects the VFA profile, with acetic and propionic acids dominating under acidic conditions, while butyric and valeric acids are more prevalent in alkaline environments. The VFA profile generated under different pH conditions plays a pivotal role in determining the type of bioplastic that can be synthesized. Acetic acid, produced predominantly at acidic pH, serves as a key precursor for polyhydroxybutyrate, a bioplastic valued for its high crystallinity and biodegradability, ideal for packaging and agricultural applica  
Figure 5: Schematic representation of the biomass-to-VFA pathway. tions [134]. Propionic acid, also favored in acidic environments, supports the production of polyhydroxyvalerate, which can be copolymerized with polyhydroxybutyrate to form poly(3-hydroxybutyrate-co-3-hydroxyvalerate). This copolymer offers enhanced flexibility and toughness, expanding its potential uses [135]. In contrast, alkaline conditions promote the production of butyric acid, which is a precursor for medium-chain-length polyhydroxyalkanoates (mcl-PHA). These mcl-PHA have elastomeric properties, making them suitable for applications requiring flexibility, such as medical devices and adhesives [136].