# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 2 Production of Valeric- and Propionic-Rich VFA Streams from Agri-Food Wastes
### Agri-Food Waste Typically Used for VFA Production
#### 2.2.5 Production of VFAs Rich in Valeric Acid and Propionic Acid from Non-Agri-Food Waste  
In this case, two main studies, dating from 2009 and 2020, promoting the production of these acids from non-agri-food waste, are reviewed below. They are notable for their strategies for the generation of VFAs, which are rich in the acids of interest and could be exported to dark fermentation with the agri-food waste that could be valorized. In the context of valeric acid generation, research has been conducted by [PERSON] et al. [109] to assess the potential of incorporating riboflavin into the fermentation process as a means of producing valeric acid from waste-activated sludge. Furthermore, the potential for solids reduction was explored. The results indicated that the addition of riboflavin (0.5 mM) led to a significant increase in VFA production, with the most abundant acid being valeric. An increase in riboflavin doses from 0.05 to 5 mM resulted in a notable rise in the DOC fraction of valeric acid in relation to total VFAs, reaching 62.8%. This is a higher proportion than that observed with other chemical supplements. Furthermore, the addition of riboflavin resulted in an enhanced reduction in mixed liquor volatile suspended solids (MLVSSs). The application of a riboflavin dose of 0.2 mM resulted in the maximum reduction in MLVSS, which reached 47.4% compared to the control without riboflavin, which demonstrated a reduction of 33.9%. Riboflavin may, therefore, be an option as a cost-effective chemical agent for the enhancement of fermentative valeric acid production from waste-activated sludge, in conjunction with solid reduction. In the case of propionic acid, a number of strategies have been investigated with a view to increasing its production. Two examples are provided below for reference:  
A number of studies have demonstrated that the selection of the appropriate microorganism is a critical factor in the formation of propionic acid. The utilization of bacteria belonging to the genus _Propionibacterium_, such as _Propionibacterium_ _acidipropionici_, is of paramount importance due to their exceptional efficiency in propionic acid production [110; 111; 112; 113; 114]. In the literature, the transformation of an anaerobic activated sludge (WAS) to VFAs was enhanced by the addition of alkylpolyglucose (APG) [115]. The APG significantly enhanced the production of short-chain fatty acids (SCFAs), particularly propionic acid. This was achieved by accelerating solubilization and hydrolysis, improving acidification, inhibiting methanogenesis and balancing the C/N ratio of the substrate. Furthermore, the operational lifespan was diminished. The results demonstrated that the yield of SCFA with 0.3 g of APG per gram of total suspended solids (TSSs) in 4 days was 2988 \(\pm\) 60 milligrams of COD per liter, a significantly higher value than those obtained with WAS alone or WAS alone plus APG alone. The corresponding propionic acid yield was 1312 \(\pm\) 25 mg COD/L, which is 7.9 times higher than that of WAS alone. The investigation demonstrated that during anaerobic treatment of WAS in the presence of APG, solubilization was accelerated, while hydrolysis and acidification were enhanced, and methanogenesis was inhibited. Furthermore, the activities of pivotal enzymes engaged in WAS hydrolysis and acidification were augmented by the presence of APG. It is of greater significance that by maintaining equilibrium in the C/N mass ratio of the substrate, the production of propionic acid was enhanced. A comparative analysis of the activities of pivotal enzymes and the prevalence of anaerobic instrumental microorganisms in sludge hydrolysis and SCFA production, with and without APG, revealed that the stimulation of SCFA accumulation, particularly propionic acid, was predominantly attributable to biological processes. Table 1 summarizes the different results reported in Section 2, where the more suitable agri-food waste for their valorization to specific fatty acids are reviewed, and the conditions for their transformation.