# 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
#### 2.2.2 Production of VFAs Rich in Valeric Acid and Propionic Acid from Low-Value-Added Starches  
In the literature, different wastes have been used as starch sources. Some of the most relevant are reviewed below. 1. [label=()]
2. Potato industry waste  
Potato peelings can be included in this group of wastes. Potato (_Solanum tuberosum_ L.) is one of the world's most important crops. According to statistics from the Food and Agriculture Organization of the United Nations (FAO), annual worldwide potato production exceeded 350 million tons in 2020 [85]. [PERSON] et al. stated in their study [86] that potato peel contains the following components in dry weight: starch (34%), carbohydrates (63%), protein (17%), lipids (1.2%), and lignin and suberin complex, which are soluble and insoluble acids, and ash (21%). Due to the above composition, by-products of the potato industry are ideal as a carbon source for fermentation [87,88]. They can be used for the production of lactic acid by using mixed culture fermentation [86,87] as a culture medium for the production of bacterial cellulose [89]; as a feedstock for the production of biobutanol or bioethanol [90,91,92]; as a raw material for biodegradable films [93]; as a potential source for the extraction of steroidal alkaloids and starch [94,95]; as a precursor for biochar synthesis [96,97], and for biogas production by anaerobic digestion [86,98]. In addition to the above-mentioned forms of utilization, it is possible to recover potato waste by dark fermentation to produce VFA. In single-phase digesters utilizing this type of waste, the processes of hydrolysis and acidogenesis can occur, leading to the accumulation of VFAs and the inhibition of methanogenic microorganisms [99,100]. In this valorization process, the intermediate products resulting from VFAs can be more valuable than the final products (CH4) [101]. The process of dark fermentation with mixed cultures of this type of waste is notable for its ability to proceed without the necessity for pretreatment, due to the ease with which the waste is degraded, and without the need for additional methanogenic inhibitors to prevent the conversion of VFAs to CH4 [102]. The composition of volatile fatty acids (VFAs) is a pivotal factor, as their regulation exerts a significant influence on the subsequent process [103]. In their study, [PERSON] et al. [84] concluded that when using potato peel waste to produce VFA, pH, organic loading time, and residence time affect the VFA produced. Similar results were obtained by [PERSON] et al. [104]. In their studies, [PERSON] et al. [102] evaluated the influence of pH adjustment as well as the  
Figure 2: Effect of organic loading rate on VFA distribution. HAc = acetic acid, HPr = propionic acid, HBu = butritic acid, and HVal = valeric acid. Modified from [PERSON] et al. [77]. initial organic load on the composition and production of volatile fatty acids (VFAs) during the acidogenesis of potato peel waste. A series of tests were studied within a stirred tank reactor at a temperature of 37 \({}^{\circ}\)C. The working volume was set at 5 L, and the inoculum and residue were mixed at 8% (based on SV content). The operating conditions were as follows: 37 \({}^{\circ}\)C \(\pm\) 0.5 \({}^{\circ}\)C and 300 rpm stirring speed; pH: 5.0, 7.0, and 11.0 during acidogenesis. The uncontrolled pH was used for comparison. In the study carried out by [PERSON] et al. [104], the effect of the initial organic load on the production of VFAs from potato peel waste through acidogenic fermentation was studied. The results show that increasing the initial organic load enhances the concentration of VFAs, particularly butyric acid, but decreases the overall VFA yield. The optimal initial organic load for VFA production was identified as 71.5 g volatile solids/L, which accelerates acidogenesis and reduces hydrolysis time. With regard to the composition of VFAs, the main results to be highlighted by [PERSON] et al.