# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 1 Introduction
Economically, waste vaporization not only helps to reduce the costs associated with waste management but also creates new sources of income for agri-food producers and related industries. This fosters a sustainable business model where waste becomes part of a new value chain, leading to the creation of green jobs and promoting technological innovation within the context of the circular economy. Therefore, the valorization of agri-food waste is a sustainable solution to the problem of waste generation. Various technologies and methods for waste conversion have been studied for decades to mitigate the impact of the agri-food industry on the environment and ecosystems. Among them, the following can be highlighted: * **Aerobic fermentation for bioplastics production:** * **Polyhydroxyalkanoates (PHAs). ** PHAs are biopolymers produced by microorganisms, such as bacteria, under specific microbial stress conditions: excess carbon and lack of nutrients (e.g., nitrogen or phosphorus). Their properties, similar to those of conventional plastics, together with their biodegradability, make them of particular interest in the search for sustainable alternatives to petroleum-based products [15]. These biopolymers can be produced from any agri-food waste that can be previously transformed into a VFA solution. * **Polylactic acid (PLA). ** From whey, lactic acid can be produced by fermentation, which will be used as a raw material for the synthesis of PLA, a biodegradable bioplastic [16]. * **Polybutylene succinate (PBS). ** A biodegradable biopolymer derived from renewable raw materials or agricultural waste, such as agri-food by-products. It stands out for its biodegradability and is a flexible, resistant material used in applications such as packaging and single-use products, especially in the food and agricultural sector [17]. * **Composting. ** Compost improves soil structure by increasing water-holding capacity and promoting biological activity. It is produced by the aerobic decomposition of organic matter in by-products such as fruit peels, vegetable waste, manure, etc. It is one of the most common forms of waste valorization [18]. * **Vermicomposting. ** This is a variation in the previous process in which worms (e.g., _Eisenia fetida_) are used to speed up the process. The result is a biofertilizer with properties similar to or better than conventional compost because it contains a higher number of beneficial microorganisms and readily available nutrients [19]. * **Biochar. ** Biochar is a solid by-product from the pyrolysis of agricultural and food waste that can be used as a soil amendment. It improves the soil's ability to retain nutrients and water, while increasing the stability of organic matter in the soil [20]. * **Anaerobic digestion**. Agri-food waste is transformed in the absence of oxygen by microorganisms. The product is a gaseous mixture (biogas), and a stabilized residue or sludge (digestate) containing microorganisms responsible for the degradation of organic matter. The process is characterized by four distinct metabolic stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These stages are mediated by different groups of specialized microorganisms [21]. In terms of valorization, digestate is used as a biofertilizer. It may or may not undergo additional stabilization processes [22]. Furthermore, the gas phase is predominantly composed of CO\({}_{2}\) and CH\({}_{4}\), with the latter being a viable energy source [23]. * **Dark fermentation. ** It is an anaerobic fermentation stopped at the acidogenesis step where agri-food waste produces a VFA solution. The VFA can be used to produce biofuels or high-value-added compounds such as bioplastics. The gas phase is rich in biohydrogen, which can be valorized [24]. As shown, each one of these valorization strategies contributes with different bioproducts to the circular economy model. The combination of dark fermentation followed by aerobic fermentation results in the biotransformation of agri-food waste into bioplastics.