# 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
#### 2.2.2 Production of VFAs Rich in Valeric Acid and Propionic Acid from Low-Value-Added Starches
[102] at the different pH levels tested were the following:  
* Acetic acid was the main product treated at pH 7.0 and 11.0, representing 46-77% and 74-92% of the total VFAs, respectively, while butyric acid was the main product at pH 5.0 and uncontrolled pH, representing 11-75% and 38-82% of the VFA mixture, respectively. * In the pH 7.0 treatment, the percentage of propionic acid increased as the fermentation progressed, while the acetic acid content decreased. The highest propionic acid production was obtained at pH 7.0. At this level, a favorable balance is achieved between the activity of acidogenic microorganisms and the production of propionates, while the production of other VFAs, which could predominate at lower or higher pH levels, is reduced. * In particular, the percentage of butyric acid decreased with increasing pH. When treated at pH 5.0 and uncontrolled pH, the hexanoic acid content was about 20% of the total VFA at the end of fermentation. In the same study, paying attention to the SRT, a prolonged SRT allows acidogenic bacteria to adapt and maximize fermentation, increasing the proportion of VFAs, particularly at near-neutral pH conditions. This provides time for microbial metabolism to favor the production of odd-chain acids such as propionic acid. In terms of organic loading, propionic acid production increases with moderate organic loading, as an excess can lead to acidification of the medium, inhibiting the production of certain VFA and favoring other acids such as butyric acid. Moderate levels allow efficient conversion of substrates to propionic acid without excessive accumulation of by-products [102]. Beer industry waste  
As for another type of low-value starch, beer bagasse, according to the recent publication of [PERSON] et al. [105], the production of VFAs from beer bagasse is maximized by a synergistic pretreatment of ball milling followed by enzymatic hydrolysis. This process facilitates the breakdown of the lignocellulosic structure of beer bagasse, increasing the solubilization of the available carbohydrates, which enhances the production of VFAs. Although using beer bagasse as a substrate in dark fermentation, valeric and propionic acids are not the main fatty acids obtained in a high proportion: [PERSON] et al. [106] recently found that it can be deduced that in order to maximize the production of valeric and propionic acids from beer bagasse, the conditions to be adjusted are again pH and organic load. The results showed that operating at a higher pH (around 6.0) and a high organic load (OL of 40 g COD/L) maximizes the diversity and concentration of organic acids. However, proportionally, valeric and propionic acids are more abundant compared to the remaining acids at 10 g COD/L compared to 40 g COD/L. These acids are produced as part of a mixture of volatile fatty acids, which favors the proportion of odd-chain acids under these conditions. 2.3 Production of VFAs Rich in Valeric Acid and Propionic Acid from Low Value-Added Starches and Cheese Whey: Co-Fermentation  
[PERSON] et al. [107] investigated the co-fermentation of beer bagasse and cheese whey. Despite the fact that a complete fermentation process was employed, rather than a dark fermentation process whose primary objective is not the production of VFAs, an increase in their synthesis was observed as a result of the methodology utilized. The study was published in 2022, and it was carried out in two phases (P-I and P-II), and three different conditions where the brewery spent grains fed (BSG) was set at 35 g VS and three different cheese whey loads of 20, 30, and 40 g COD/L were established. In the second phase (P-II), the biogas (H\({}_{2}\) + CO\({}_{2}\)) produced during the acidogenic fermentation (P-I) was recirculated to provide a source of inorganic carbon (CO\({}_{2}\)) and an electron donor (H\({}_{2}\)) for the homoacetogens in the mixed culture. Propionic acid was synthesized in a lower concentration than the other acids and was only detected from the 16 th day onward.