# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 1 Introduction
Because of that, this review will focus on the production of VFA from agri-food wastes for the subsequent production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) or PHBV. The liquid fraction resulting from the dark fermentation is rich in VFAs. These VFAs are short-chain organic compounds formed from carboxyl groups, including low molecular weight acids such as acetic, propionic, butyric, valeric, and isovaleric acids. They are mainly formed during the anaerobic fermentation of organic matter, as an intermediate product of the acidogenesis step. Their production is influenced by several factors, such as:  
* Type of substrate. This influences the type and quantity of VFAs. Waste rich in carbohydrates generally favors the production of acetic acid, whereas waste rich in fat can increase the production of butyric acid. * Operating conditions. Factors such as pH, solids concentration, and temperature also influence the type and amount of VFA produced. * Microorganisms. The microorganisms present in the fermentation medium are also important factors in an anaerobic fermentation process. Genera such as _Clostridium, Lactobacillus_, and _Bacteroides_ play an important role in VFA production [25]. In the context of VFA production from agri-food waste, several projects in Europe have demonstrated the success of this type of waste valorization. One example is the NoAW (No Agro-Waste) project. Its aim is to use agricultural waste to produce high-value bioproducts, including VFAs. Wine industry residues, cereals, and dairy products have been used to produce biofuels, making a significant contribution to waste reduction and promoting the circular bioeconomy and sustainable development [26]. Currently, there is an increasing number of studies investigating the mechanisms and technologies to optimize the production of VFAs from agri-food waste. Anaerobic fermentation is the most commonly used process and has improved thanks to ongoing research and development of more efficient bioreactors and microorganisms for the degradation of organic waste [27]. However, the variability in the composition of agri-food wastes is the main challenge for the effective performance of the fermentation process. In this sense, advances in waste pretreatment and the use of co-substrates have helped to improve the production rate of VFAs, allowing their application in various industries, such as [27]:  
* Biofuel production. VFAs are used as precursors in biogas production, where they are converted to CH\({}_{4}\) by methanogenesis during anaerobic digestion. * Food industry. VFAs are used as preservatives, acidity regulators, and food additives due to their antimicrobial properties. * Bioplastic production. They are used as feedstocks for aerobic fermentation in the production of polyhydroxyalkanoates (PHAs), biodegradable plastics used as alternatives to petroleum-based products. In this work, to accurately review the state-of-the-art in agri-food waste valorization to volatile fatty acids (VFAs), special attention is given to maximizing the production of valeric and propionic acids. This involves explaining the dark fermentation process used for VFA production. Additionally, the biomasses with the highest potential for VFA biotransformation will be examined, with a particular focus on detailed studies of select biomasses such as whey and low-value-added starches. The Section 3 will review the transformation of these VFAs into valuable bioplastics, with an emphasis on their further transformation into poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). This includes a review of the different routes of VFA transformations, the mechanisms of VFA transformation into polyhydroxyalkanoates (PHAs), and the most common microorganisms employed. Furthermore, fermentation conditions for the PHA transformation of VFAs, as well as the physico-chemical properties of PHAs and other bioplastics, will be reviewed.