## 5 Targeted Acidogenic Fermentation for Bioplastics Production
Understanding the relationship between substrate composition and VFA profiles is crucial for optimizing the production of specific acids, which serve as precursors for various bioplastics, as previously indicated. Proteinaceous substrates, such as sewage sludge or microalgae, play a significant role in anaerobic fermentation due to their ability to release amino acids during degradation. These amino acids are subsequently converted into carboxylic acids through two primary metabolic pathways. The first pathway, Stickland fermentation, involves the utilization of amino acids as electron acceptors, resulting in the production of VFAs with the same carbon chain length as the originating amino acid [154]. The second pathway, amino acid reduction via the Wood-Ljungdahl pathway, relies on hydrogen-utilizing bacteria for successful conversion [155]. Studies have demonstrated that protein-rich substrates predominantly lead to the formation of acetic and propionic acids as the major VFA. For example, [PERSON] et al. [156] reported that microalgae biomass subjected to acidogenesis produced VFA profiles dominated by acetic and propionic acids, constituting 50% of the total carboxylic acids. Similarly, [PERSON] et al. [43] observed the dominance of these acids in 60-65% of the VFA profile, while [PERSON] et al. [157] noted an increase to 70-80% when sewage sludge was used as a substrate. The microbial community in protein-rich anaerobic fermentation processes is typically dominated by members of the phylum Firmicutes, accounting for 65-85% of the total community [154]. Specific genera, such as _Peptostreptococcus_ and _Sporoanaerobacter_, are responsible for producing propionic and acetic acids, respectively, while _Clostridium_, known for its capability to perform Stickland fermentation, facilitates further VFA production under anaerobic conditions [28]. The coexistence of these microbial mechanisms highlights the intricate interplay between Stickland fermentation and the WoodLjungdahl pathway in shaping the VFA profile during anaerobic fermentation of proteinaceous substrates. In contrast, carbohydrate-rich substrates yield distinct VFA profiles, with acetic and butyric acids emerging as the predominant carboxylic acids [158]. Carbohydrate degradation produces a variety of sugars, such as fructose, glucose, and galactose, which are metabolized into VFAs. Butyrate-type fermentation and acetogenic pathways dominate this process under mild operational conditions. Studies show variability based on substrate composition; for example, [PERSON] et al. [159] found that anaerobic fermentation of potato peels resulted in 40% acetic acid dominance, while [PERSON] et al. [132] reported butyric acid concentrations of 41.1-43.5% from cucumber and tomato residues. Increased carbohydrate content in substrates has been correlated with higher butyric acid yields. The microbial community involved in carbohydrate degradation includes bacteria from the orders _Lactobacillus_ and _Clostridiales_. _Lactobacillus_ encompass metabolically diverse organisms capable of producing lactic acid, which can be further converted into VFA. Key species such as C. _tyrobutyricum_ and _Aectobacterium woodii_ drive butyrate and acetate production, respectively [159], while _Ruminococcus_ plays a critical role in converting carbohydrates into acetic and butyric acids [133]. It is worth noting that the bioconversion efficiency of organic matter into VFAs depends on the substrate's composition and biodegradability. Typically, 10-50% of the COD in feedstocks is converted into VFAs [160]. However, substrates containing recalcitrant compounds, such as lignin in lignocellulosic biomass, require pretreatment to improve microbial access. Common pretreatment methods, including enzymatic hydrolysis, steam explosion, and chemical catalysis, enhance substrate bioavailability but add complexity and cost to the process [161].