# 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
#### 2.2.1 Production of VFA, Rich in Valeric Acid and Propionic Acid, from Cheese Whey
Nevertheless, the type of valorization that is of most interest for the purposes of this review is the production of VFAs. Research studies dated from 2020, for example, that of [PERSON] et al. [77], conclude that whey is a suitable substrate for VFA production due to its high content of fermentable organic matter. These studies analyze how pH, solid retention time, and organic loading rate affect VFA production. [PERSON] et al. [78] analyzed, in 2024, the conversion of organic matter from whey to VFAs and the effect of the pH control on the process using a new method based on gas permeable membranes. The outcomes of this study demonstrate that up to 54% of bioconversion was achieved under acidic conditions with sequential pH control. In contrast, under alkaline conditions, the pH control led to a decrease in bioconversion to VFAs. This finding indicates the feasibility of recovering and concentrating VFAs through the utilization of the gas-permeable membrane-based method. Another recent study is that of [PERSON] et al. [79], where the production of VFAs from whey is analyzed using controlled temperature and pH conditions. In this study, a multiscale analysis is used to optimize the dark fermentation process. The study also concludes that the rescaled range fractal methodology can be used for indirect monitoring of VFA production under different operating conditions. The studies mentioned above are only examples of the wide range of variables and study parameters that exist throughout the production of VFAs from whey. This review addresses the different points of optimization, focusing on the production of the VFAs of interest: valeric and propionic. Based on two main studies [77, 80] the variables and ferentation process conditions to be maintained in order to maximize the production of valeric and propionic acids from whey are detailed below:  
* Substrate. A high Kjeldahl nitrogen content has been shown to increase the propionic acid yield to 78% of the total VFA production. Therefore, the characteristics and composition of the substrate are some of the most important factors influencing VFA production and composition [80]. * Retention times. In an anaerobic digestion process, a distinction must be made between the SRT--the average time the microorganisms are in the digester--and the HRT--the time the wastewater or sludge is in the digester. Both times coincide with suspended growth digesters where there is no recirculation. [PERSON] et al. [77] studied three different HRTs in dark fermentation: four, six, and ten days at three different pH levels. They used a sequential bioreactor (SBR) type anaerobic reactor with a working volume of 2 L, with two daily cycles of 12 h, each with four stages: loading, reaction, sedimentation, and liquid extraction. It was operated at a temperature of 30 \({}^{\circ}\)C. Under these operating conditions, they concluded that the conditions that maximized the production of valeric and propionic acids were an increase in the SRT, up to 10 days, regardless of the pH used. When the SRT was increased from 4 to 10 at pH 5, propionic acid increased from 13% to 24% of the final VFA composition. Similarly, at pH 5.5 and 6, propionic acid increased from 17% to 24% and 24% to 36%, respectively. In the case of valeric acid, its concentration increased with increasing SRT independently of pH, from 12% to 25%, from 10% to 22%, and from 9% to 15% for pH 5, 5.5, and 6, respectively. Thus, increasing the SRT from 4 to 10 days tended to stimulate the formation of odd VFA (propionic and valeric) to the detriment of even VFA such as acetic and butyric. * pH. A less acidic pH, a change from 5 to 6, stimulates the production of propionic acid and reduces that of valeric acid, regardless of the SRT used [77]. Other studies [81,82] use a different type of reactor, in this case, a continuous stirred tank reactor (CSTR) with an SRT of 48 h, also conclude that increasing the pH from 5.25 to 6 favors the production of propionic acid to the detriment of butyric acid.