## 5 Targeted Acidogenic Fermentation for Bioplastics Production
Mesophilic processes operate at temperatures between 20 and 43 \({}^{\circ}\)C, with 35-37 \({}^{\circ}\)C typically considered optimal. These conditions have been found to be effective for feedstocks such as cow manure, maize silage, or the organic fraction of municipal solid waste, as both hydrolytic and acidogenic activities are enhanced. For instance, a study on the anaerobic fermentation of cow manure and maize silage reported that mesophilic conditions favored the production of acetic and butyric acids. The optimal condition was identified as an organic loading rate of 17.9 g TVS m\({}^{-3}\) with a 4-day HRT at 37 \({}^{\circ}\)C, achieving an acidification yield of 183.2 g COD kg VS\({}^{-1}\)[137]. These VFAs were essential precursors for PHAs synthesis. Thermophilic processes, conducted at 50-60 \({}^{\circ}\)C, are often utilized for the anaerobic fermentation of recalcitrant feedstocks, such as protein-rich substrates [144]. Although thermophilic processes involve higher energy demands, they can achieve high bioconversion efficiencies with a specific VFA profile, simplifying downstream processes [145]. [PERSON] et al. [85] investigated the acidogenic fermentation of cheese whey under thermophilic conditions to produce VFAs for subsequent PHA biosynthesis. The study found that thermophilic conditions favored the production of butyric acid. Other works [146; 147] indicate that thermophilic conditions can lead to increased concentrations of longer-chain VFAs, such as caproic and valeric acids, which are valuable for producing mcl-PHAs. Both mesophilic and thermophilic conditions can promote the growth of methanogenic archaea, leading to the conversion of VFAs into methane and reducing fermentation efficiency. To mitigate this, high temperatures should be combined with operational parameters like short hydraulic retention times and acidic pH to suppress methanogenesis [28]. Conversely, psychrophilic processes operate at temperatures below 20 \({}^{\circ}\)C and can be effective for feedstocks characterized by a high content of readily biodegradable carbohydrates [148]. Low temperatures significantly limit the growth rate of methanogens, promoting VFA accumulation. Studies have shown that psychrophilic conditions favor the production of acetic and propionic acids [149], which are key precursors for synthesizing polyhydroxybutyrate and polyhydroxyalerate, respectively. In summary, temperature control is crucial in directing the VFA profile during anaerobic fermentation, thereby determining the specific types of bioplastics that can be synthesized. Mesophilic conditions tend to favor the production of shorter-chain VFAs like acetic and butyric acids, suitable for PHB production, while thermophilic conditions can enhance the production of longer-chain VFAs, beneficial for mcl-PHA synthesis. Psychrophilic conditions, by suppressing methanogenesis, can also be tailored to produce specific VFAs for targeted bioplastic production. The composition of the substrate is another factor that significantly influences the profile of VFAs produced during anaerobic fermentation. The relative proportions of proteins, carbohydrates, and lipids in the feedstock determine the metabolic pathways activated by the microbial community, leading to the formation of specific VFAs [43; 142]. For instance, carbohydrate-rich substrates typically result in higher production of acetic and butyric acids [150], while protein-rich substrates tend to increase the formation of propionic and valeric acids [151]. The microbial community's composition adapts to the substrate, with certain bacterial phyla playing pivotal roles in VFA production. Notably, bacteria from the Firmicutes phylum are instrumental in hydrolyzing complex polymers and fermenting them into VFAs [152]. Within this phylum, genera such as _Clostridium_ and _Butyrivivrio_ are recognized for their butyric-producing capabilities [153].