# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 3 Production of Bioplastics from Volatile Fatty Acids  
As mentioned above, VFA can be produced by dark fermentation from various sources of agro-industrial waste. Once produced, they serve as a substrate for various microorganisms in the synthesis of bioplastics. VFAs can be assimilated by a range of microorganisms [116; 117; 118]: bacteria, heterotrophic microalgae, and yeasts, as evidenced by Figure 3 and this allows for the production of a variety of bioproducts. As seen in Figure 3, VFAs can be transformed into more than just PHAs by utilizing different bacteria, microalgae, and yeast. Depending on the type of microorganism employed, Omega-3-rich oil or single-cell oil can also be obtained. The following paragraphs briefly describe VFA transformation by microalgae and yeast. Microalgae are considered photoautotrophic organisms; however, several species have heterotrophic metabolism [119]. Oleaginous microorganisms can thrive in heterotrophic cultures using VFAs as a carbon source, accumulating significant amounts of valuable products like omega-3 fatty acids [120]. Acetic acid, an intermediate in anaerobic digestion, frequently accumulates \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Waste** & **pH** & **T** (\({}^{\circ}\)O)** & **OLR** & **SRT** (days = d)** & **Additives and Other** & **VFA** & **Valeric and** **Propionic Acid** & **Valeric and** **Propionic Acid** \\  & & & & **(days = d)** & **Considerations** & & **Yield (\%)** \\ \hline \hline \multirow{4}{*}{
\begin{tabular}{c} Whey \\ [77] \\ \end{tabular} } & 6 (just for \\  & propionic & 35 & Low: 3 g \\  & acid & & COD/(L-d) & STR = 10 & - & - & - \\  & & & & & & -Valeric: 25 (pH = 5, \\  & & & & & & & STR = 10 d) \\  & & & & & & -Propionic: 180 mg \\  & & & & & COD/gVS\({}_{\text{Fed}}\) & \\  & & & & & -Acetic: 289.5 mg & \\  & & & & & COD/gVS\({}_{\text{Fed}}\). & \\  & & & & acid) & & -Butyric: 157.2 mg & \\  & & & & & COD/gVS\({}_{\text{Fed}}\) & \\  & & & & & & Hexanoic: 89.4 mg & \\  & & & & & COD/gVS\({}_{\text{Fed}}\) & \\  & & & & & & COD/gVS\({}_{\text{Fed}}\) & \\  & & & & & & -Propionic: 1,2\% & \\  & & & & & & COD basis & \\  & & & & & hydrolysis to BSG & - & -Valeric: 2\% COD \\  & & & & & & basis \\  & & & & & & -Propionic: 3.85 g & \\  & & & & & & COD/L (d 48). & \\  & & & & & & -Acetic: 10.56 g & \\  & & & & 48 d & Recirculation of & COD/L (d 48). & \\  & & & & (propionic & CO\({}_{2}\) and H\({}_{2}\) & -Butyric: 20.42 g & \\  & & & & & & -Acetic: 10.56 g & \\  & & & & 48 d & Recirculation of & COD/L (d 48). & \\  & & & & (propionic & CO\({}_{2}\) and H\({}_{2}\) & -Butyric: 20.42 g & \\  & & & & & & generated that & COD/L (d 56). & \\  & & & & & & increased & -Valeric: 3.77 g & \\  & & & & & & -Acetic: 3.02 g & \\  & & & & & & -Acetic: 13.02 g & \\  & & & & & & -Acetic: 10.56 g & \\  & & & & & & -with CO\({}_{2}\) & \\  & & & & & & recirculation. & \\ Activated & & & & & & & Valeric acid: 62.8\% \\  & & & & & & & with a riboflavin & \\  & & & & & & & dose of 5 mM \\  & & & & & & & \\ Activated & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\  & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the operating conditions (pH, T\({}^{\circ}\), OLR, STR) and other considerations for the production of a VFA-rich solution in valeric and propionic acids by dark fermentation of the different wastes mentioned above. during dark fermentation processes and can be converted by microalgae into acetyl-CoA, the primary precursor for lipid synthesis [121]. There are two pathways for the metabolic oxidation of acetyl-CoA:  
1. In the glycosylomes, the glyoxylate cycle converts acetyl-CoA to malate. 2. In the mitochondria, the tricarboxylic acid (TCA) cycle converts acetyl-CoA to citrate [122,123].