# 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  
A VFA solution rich in valeric and propionic acids plays a crucial role in the production of PHBV, and the substrate used, together with the anaerobic fermentation conditions, are fundamental to obtaining a high proportion of these acids. The substrate to be valorized, in this case, agri-food waste, can come from different sources, some of which are described below:  
* **Vegetable waste**. Agriculture, food processing, and surplus sales generate sugar-rich wastes that can be anaerobically fermented to produce VFAs. Their high carbohydrate content favors the formation of acetic, propionic, and valeric acids, depending on the fermentation conditions. [PERSON] et al. reported in 2020 that these residues can produce a high yield of VFAs, especially when an appropriate carbon-to-nitrogen (C/N) ratio is achieved [37].
* **Vegetable oil industry wastes**. Residues from oil extraction, such as palm and soybean waste, are rich in lipids and carbohydrates. [PERSON] et al. found in 2020 that these wastes are highly valorized for VFA production, especially when co-fermented with other wastes containing higher amounts of carbohydrates [38].
* **Whey**. A by-product of the dairy industry that is an excellent source of carbohydrates and proteins. [PERSON] et al. already found in 2015 that bacteria such as _Propionibacterium_ can enhance propionic acid production when the pH and retention time conditions are suitable [39].  
Figure 1: Schematic overview of the dark fermentation process for VFA synthesis. Adapted from [PERSON] et al. [35], [PERSON] et al. [36].  
* **Fish industry waste.** The valorization of fish waste, such as tuna, for the production of biochemical compounds has been studied. In 2017, [PERSON] et al. published a study in which they improved the efficiency of the process by adjusting the operating conditions of the bioreactor. [40].
* **Industrial wastewater from agri-food transformation sector.** This type of waste is rich in organic compounds that are suitable for anaerobic fermentation to produce VFAs [41]. According to [PERSON] et al. (2011), by adjusting the C/N ratio and maintaining a suitable pH between 5 and 6.5, it is possible to obtain a VFA profile with high levels of valeric and propionic acids. Recent studies have shown that the co-fermentation of agro-industrial residues can maximize the production of VFAs with these compounds [41]. This type of waste includes **vinasses**, a liquid by-product of alcohol distillation in beverage production. They contain carbohydrates, organic acids, and other fermentable compounds that can be converted into VFAs, such as acetic, propionic, and valeric acids, depending on the microorganisms and conditions used in the process. Subjecting vinasse to anaerobic digestion can result in the production of VFAs where the main component is acetic acid, with lower concentrations of propionic and valeric acids, as demonstrated by [PERSON] and [PERSON] in their study published in 2011 [42].
* **Low value-added starches**. Derived from agricultural by-products, these residues can be pre-treated, for example, by enzymatic hydrolysis, to break them down into simple sugars that can be anaerobically fermented to produce VFAs. An example of low-value starch is **brewer's spent grain**, which is rich in fiber and carbohydrates and can also be valorized by anaerobic fermentation to produce VFAs, including valeric and propionic acids, as shown by [PERSON] et al. in their study (2021) [43]. Finally, [PERSON] et al. (2019) demonstrated that **potato processing residues**, such as peel or pulp, are also suitable for VFA production. Pretreatment and co-fermentation, as well as the adjustment of pH conditions and the inoculum/substrate ratio, can optimize valeric acid production [44].  
A number of studies investigating the conditions for the production of VFAs, in particular valeric and propionic fatty acids, from some of the above-mentioned agri-food wastes are described below.