# Valorization of Agri-Food Waste into PHA and Bioplastics: From Waste Selection to Transformation
## 4 Conclusions  
The generation of agri-food waste is a major global problem with significant environmental and economic impacts. Nowadays, European and international project initiatives play a key role in developing technologies for the valorization of agri-food waste and contribute to the circular economy model. The biotransformation potential of these wastes into volatile fatty acids (VFAs) and bioplastics has been reviewed, with special attention given to poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), a potential polyhydroxyalkanoate (PHA) bioproduct with mechanical properties analogous to those of current fossil-based plastics.  
Waste such as whey, potato peels, and brewery bagasse have demonstrated viability as sources for their valorization into a tailor-made VFA solution rich in valeric and propionic acids. Co-digestion of different wastes can also enhance the production of these specific fatty acids. High transformation yields under controlled fermentation conditions can be achieved, especially for whey, which can reach 36% and 25% yields of propionic and valeric acids, respectively, under common dark fermentation conditions (pH = 6, T = 35 \({}^{\circ}\)C, OLR = 3 g COD/(L-d), and SRT = 10 d). The use of additives, such as riboflavin and alkylpolyglucose, can further improve the concentrations of propionic and valeric acids in the VFA solution. Efforts must, therefore, be made to produce a tailor-made VFA solution as an intermediate carbon source, which is a critical factor in its biopolymerization into PHBV, a biopolymer with improved mechanical and thermal properties.  
Biopolymerization of PHA family polymers can be achieved with various microorganisms. However, one type of PHA can also be produced by different microorganisms. In the case of PHBV, _Cupriavidus necator_ seems to achieve better yields in terms of PHBV accumulation and concentration (1.5 g/L of biomass and 56% PHA accumulation).  
Polyhydroxyalkanoate are a promising alternative to conventional plastics and other bioplastics because they have a unique versatility in terms of mechanical and thermal properties. They can be tuned by polymerization with different co-substrates, allowing PHAs to be tailored to the required application. Furthermore, they are the only biopolymer that can be obtained entirely from the biological transformation of a residual carbon source. This biopolymer already shows better biodegradability, impermeability, and oxygen gas barrier properties than conventional plastics.  
Future work should focus on reducing the cost of bioplastic production by improving process efficiency, developing more accessible technologies, and promoting the integration of these waste vaporization systems into a wider industrial network. Bioplastics require less energy consumption than non-biobased plastics for their production; however, the price of biopolymers ranges between two and five times that of conventional plastics. Administrations may also promote the introduction of bioplastics on the market through their directives. This would reduce the economic constraints associated with fossil-based plastics and make bioplastics more affordable for the market. Overall, this will maximize positive environmental impacts and create new economic opportunities.  
Conceptualization, M.A. and F.J.P.; methodology, M.A. and F.J.P.; software, M.A. and J.d.A.; validation, I.I. and F.J.P.; formal analysis, M.A. and J.V.; investigation, M.A. and F.J.P.; resources, J.M.; data curation, F.J.F.-M.; writing--original draft preparation, M.A.; writing--review and editing, F.J.F.-M., I.I., and F.J.P.; visualization, J.V.; supervision, F.J.F.-M., I.I., and F.J.P.; project administration, J.M.; funding acquisition, J.M. and F.J.P. All authors have read and agreed to the published version of the manuscript.  
The authors would like to thank the European Commission for supporting this work by the Circular Bio-based Joint Undertaking through the project PROMOFER (Grant Agreement Number 101157239).  
The authors declare no conflicts of interest.