## 3 VFA as Bioplastic Precursors  
The economic feasibility of bioplastics production remains a significant barrier to its widespread adoption, with costs being 5 to 10 times higher than those of conventional plastics [19; 82]. A substantial portion of these costs--up to 50%--is attributed to the carbon source used in production [83]. This makes VFAs, derived from organic waste streams, a highly attractive alternative. VFAs are critical intermediates in the production of polyhydroxyalkanoates (PHAs), a class of biodegradable polyesters synthesized by bacteria. As direct metabolic precursors, VFAs provide the carbon building blocks for PHAs, making them a key component in sustainable bioplastic production. Their compatibility with both pure and mixed microbial cultures (MMCs) enhances the feasibility of integrating VFA production into bioplastic manufacturing processes [84; 85]. In particular, MMCs provide a cost-effective alternative by eliminating the sterilization requirements associated with pure cultures, thereby significantly reducing production expenses [86]. This approach takes advantage of the metabolic flexibility of various bacterial genera, such as Bacillus megaterium, Alcaligenes eutrophus, and Pseudomonas oleovarans, which can efficiently utilize VFAs as carbon sources for PHA synthesis, further emphasizing their adaptability and industrial potential [87]. The composition of VFAs is a decisive factor in determining the monomeric structure and resulting properties of PHAs. Even-chain VFAs, such as acetic and butyric acids,predominantly promote the production of 3-hydroxybutyrate, which is associated with high crystallinity and thermal stability, making them suitable for rigid applications like packaging materials and agricultural films. Conversely, odd-chain VFAs, including propionic and valeric acids, facilitate the synthesis of 3-hydroxyvalerate, which enhances flexibility and elasticity, key for medical applications such as drug delivery systems, tissue engineering scaffolds, and surgical sutures [88,89]. By tailoring the ratio of even-chain VFAs to odd-chain VFAs during acidogenic fermentation, PHA can be customized to meet specific performance requirements across diverse applications. The synthesis of PHAs relies heavily on achieving optimal concentrations of 3-hydroxybutyrate and 3-hydroxyvalerate, as these monomers are pivotal for efficient polymer formation [28]. Strategies to refine the fatty acid composition for enhanced production include the use of nano zero-valent iron, which has shown promise in promoting 3-hydroxybutyrate synthesis [90,91], and the addition of sodium dodecylbenzene sulfonate to facilitate 3-hydroxyvalerate production [92]. The further diversification of PHA properties can be achieved by introducing additional monomers, such as 3-hydroxyhexanoate and 4-hydroxybutyrate. These components enable the development of high-performance bioplastics for demanding applications, including automotive parts and electronic casings, where enhanced mechanical and thermal properties are essential [93]. This ability to fine-tune PHA characteristics underscores the importance of precise control over VFA composition during fermentation, aligning with the needs of various industries. Predicting the composition of VFAs produced during acidogenic fermentation is challenging due to the interplay of numerous factors, including substrate type, pH, temperature, OLR, HTR, and the composition of the microbial community [1]. For instance, carbohydrates typically favor the production of butyric acid, while protein substrates are more likely to yield acetic and propionic acids. However, these relationships are highly variable and depend on specific experimental conditions, making precise predictions difficult [94]. This complexity is further compounded by interactions between substrates in mixed waste streams, which can lead to unpredictable VFA profiles [16]. To address these challenges, mathematical modeling has emerged as a valuable tool for predicting VFA composition.