## 5 Targeted Acidogenic Fermentation for Bioplastics Production  
Targeted acidogenic fermentation, understood as the deliberate manipulation of fermentation conditions to produce specific VFA profiles, has emerged as a critical strategy for optimizing bioplastic synthesis [116,123]. The specific VFA--such as acetic, propionic, butyric, or valeric acids--are key determinants of the mechanical, thermal, and degradation properties of bioplastics, enabling the tailoring of these materials for diverse applications. Achieving a desired VFA profile requires a holistic approach, encompassing the selection of appropriate substrates, the application of effective pretreatment methods, the optimization of operational parameters (e.g., pH, temperature, and retention time), and the management of microbial community dynamics. This section examines the latest advancements in operational strategies and process innovations aimed at enhancing the precision and efficiency of VFA production with the focus on bioplastic applications. Targeted acidogenic fermentation enables the efficient conversion of complex organic waste into VFA with tailored profiles, significantly influencing microbial metabolic pathways. For instance, elevated acetic acid concentrations can enhance lipid accumulation in yeast, while higher caproic acid levels have been associated with inhibitory effects on \begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Substrate** & **Operational Setup** & **Yield** & **VFA Composition (\%)** & **Reference** \\ \hline \multirow{3}{*}{Poultry manure} & Batch fermentation: & \multirow{3}{*}{\begin{tabular}{l} Acetic: 80–90, \\ Protein: 10–15 \\ \end{tabular} } & \multirow{3}{*}{[122]} \\  & uncontrolled pH, 37 \({}^{\circ}\)C, S:I & 0.53 g-VFA/g-VS & & \begin{tabular}{l} Acetic: 60, \\ Propionic: 15, \\ Butyric: 20, \\ Others: 5 \\ \end{tabular} & & \\  & & & & \\ \hline \multirow{3}{*}{Waste sludge} & Semi-continuous & \multirow{3}{*}{0.33 g-COD/g-VS} & \multirow{3}{*}{[124]} \\  & bioreactor: S:I ratio 2:1, & & & \\  & 35 \({}^{\circ}\)C, duration 12 days & & & \\  & & & & \\  & & & & \\ \hline \multirow{3}{*}{Waste sludge} & Batch fermentation: pH 9, & \multirow{3}{*}{0.52 g-VFA/g-VS} & \multirow{3}{*}{[125]} \\  & S:I ratio 1:1, 55 \({}^{\circ}\)C, duration 10 days & & & \\ \cline{1-1}  & & & & \\ \cline{1-1}  & & & & \\ \hline \multirow{3}{*}{Waste sludge and apple pulp} & Co-fermentation under & \multirow{3}{*}{0.550 g COD/L; 255.68 mg VFA/g} & \multirow{3}{*}{\begin{tabular}{l} Acetic acid: 60\%, \\ Butyric acid: 30\%, \\ Others: 10\% \\ \end{tabular} } & \multirow{3}{*}{[126]} \\  & thermophilic conditions & & & \\ \cline{1-1}  & (55 \({}^{\circ}\)C), alkaline pH (9), & & & \\ \cline{1-1}  & batch operation & & & \\ \hline \multirow{3}{*}{Sewage sludge and organic waste} & Acidogenic fermentation: & \multirow{3}{*}{0.52 g-VFA/g-VS} & \multirow{3}{*}{\begin{tabular}{l} Acetic acid: 45\%, \\ Propionic acid: 25\%, \\ Butyric acid: 20\%, \\ Others: 10\% \\ \end{tabular} } & \multirow{3}{*}{[127]} \\  & pH 5.5, ambient & & & \\ \cline{1-1}  & temperature, HRT of & 4.5 g/L & & \\ \cline{1-1}  & 5 days & & \\ \hline \multirow{3}{*}{Organic fraction of municipal solid waste} & Anaerobic digestion: pH & \multirow{3}{*}{0.5–0.7 g VFA/g VS} & \multirow{3}{*}{\
\begin{tabular}{l} Acetic acid: 40–50, \\ Propionic: 20–30, \\ Butyric: 15–25 \\ \end{tabular} } & \multirow{3}{*}{[5]} \\  & 6.0; 37 \({}^{\circ}\)C; 15 days & & & \\ \hline \multirow{3}{*}{Organic solid waste leachate} & Upflow anaerobic bioreactors: uncontrolled pH, 186 days of operation & \multirow{3}{*}{
\begin{tabular}{l} Net acidification \\ degree (NAD) of 0.45 \\ \end{tabular} } & \multirow{3}{*}{[128]} \\  & & & \\ \cline{1-1}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._microbial activity [129]. These dynamics underscore the importance of fine-tuning fermentation conditions to achieve the optimal VFA composition for specific applications.