# Harnessing Agri-Food Waste as a Source of Biopolymers for Agriculture
## 2 Sources of Biopolymers
### Polyhydroxyalkanonates (PHAs): A Versatile and Biodegradable Bioplastic  
Chemically, PHAs are polyesters consisting of hydroxyalkanoate monomers linked by ester bonds. The general chemical structure of PHAs can be seen in Figure 6. PHAs are biopolymers naturally produced by several bacteria and archaea and stored under the form of granules inside the cell as a reserve carbon source. The specific chemical structure of PHAs can vary widely depending on the microbial species producing them, the carbon source provided during cultivation, and the environmental conditions. To synthetize PHAs,  
Figure 5: Chitin and chitosan chemical structure. these microorganisms usually use glucose or fatty acids as a substrate: AFW can be a good and cheap source of these compounds, thus representing a good strategy for cutting costs, given that PHA is 15 times more expensive than petroleum-based plastics [100]. [PERSON] et al. [101] (Table 6, Entry 1) utilized a multi-stage process in which they selected PHA-storing microorganisms under least and famine (FF) conditions in a sequencing batch reactor (SBR) and found out that the PHA production is widely influenced by the selection of PHA producers, which is in turn influenced by the organic loading rate (ORL). [PERSON] et al. (Table 6, Entry 2) [102] used corn straw saccharified with the aid of microwave technology and obtained a greater yield during scaling up in a stirred tank 3L bioreactor, compared to a shake flask. Moreover, the authors were also able to determine the specific PHA that was being produced, which was poly(3-hydroxybutyrate-_co_-3-hydroxyvalerate) (PHBV). Similarly, [PERSON] et al. (Table 6, Entry 6) [106] saccharified a waste biomass to accumulate PHA, however they obtained a lower final titer. [PERSON] et al. [103] (Table 6, Entry 3) determined that the type of PHA monomer is influenced by the selection of the substrate, as _C.necator_ usually produces short-chain length PHAs, but with the mixture that they used as a substrate (without the addition  
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Figure 6: Polyhydroxyalkanoates chemical structure. of a nitrogen source), they were able to produce medium-chain length PHAs. In Entry 4 [104], _P. monteilii_ was grown on sugarcane bagasse black liquor through a two-phase fermentation: (i) optimization of cell growth in batch configuration, (ii) pulse-feeding in fed-batch configuration, with the C:N ratio finely tuned to maximize the production of PHA. During the growth phase, the percentage of PHA in the total cell concentration was less than 1%; however, in the accumulation phase this percentage was increased to 12.8%. [PERSON] et al. [63] (Table 6, Entry 5) proposed a three-step valorization of fruit waste on a pilot scale: the first step aimed to convert the biomass into soluble fermentation products (SFP) through fermentation, then the SPF media, rich in butyrate, was fed to the second and third steps. The second step was aimed at microbial culture selection to improve PHA yield, and the third step involved PHA accumulation. Overall, the process was able to afford a remarkable maximum PHA content of 80.5% and a storage yield of 0.98 gCOD-gCOD-1; the overall process yielded 0.45 gCOD-gCOD-1. A more complex fermentation set-up, comprising the separation of SPF production step and the PHA creation, and the accumulation step can dramatically improve the PHA yield. The usage of PHAs in agriculture has been explored mainly as much films [107; 108]; for this application, PHA is usually blended with other biodegradable components to improve its processability. For example, the addition of medium-chain length PHA increased the processability of PLA [109] and slowed down its biodegradation rate [108]. It was found that the addition of a small percentage of UV stabilizer drastically slowed down the degradation rate of polybutylene adipate terephthalate (PBAT)/PHA mulching films when subjected to accelerated aging test (AAT).