# Harnessing Agri-Food Waste as a Source of Biopolymers for Agriculture
## 2 Sources of Biopolymers
### Polylactic Acid (PLA): A Non-Toxic Alternative for Mulching Films and Food Packaging  
PLA is a biopolymer with properties like those of conventional plastics deriving from fossil fuels; the PLA chemical structure can be seen in Figure 7. Since the properties of PLA are better when the polymer consists exclusively of the stereoisomer L- (L-LA) [112], the cheaper way is trough fermentation. Lactic acid (LA) can derive from the fermentation catalysed by a wide range of microorganisms that can, consequently, grow on and exploit a wide range of substrates; for this reason, a broad spectrum of AFW finds its usage in this stream of production. The optimization of the use of AFW as fermentation substrates to produce LA is crucial for both a waste valorization approach and for the cost reduction of the process [113]. Before the fermentation step, the biomass typically undergoes a pre-treatment, which may be physical, biological, chemical, or a combination of these methods: this is because agricultural by-products, which are usually lignocellulosic, need to be delignified and saccharified for the microorganism to use them as substrates [114]. Alternatively, the process needs to employ a microorganism with both the capacity to degrade complex lignocellulosic biomasses and to produce LA; this approach is called consolidated bioprocessing (CBP) [115]. In Table 7, some of the agricultural waste biomasses that have been investigated as substrates to produce LA are reported. In this case, different microorganisms can afford different yields of LA. Figure 7: Polylactic acid chemical structure. An example of this was the study elaborated by [PERSON] et al. [116] (Table 7, Entry 1) where a maximum theoretical yield of LA from glucose was obtained; however, the LA yield calculated on the starting biomass was affected by a lower yield in the previous step (38.1%, enzymatic saccharification). Entries 2 and 3 [117] report a non-sterile \(B\). _coagulants_ DSM2314 fermentation that led to satisfying yields when fermenting xylose-rich hydrolyses; in fact, they were very similar to the one obtained when fermenting pure xylose (0.77 g LA/g Xyl); moreover, the lack of sterility is an advantage as it greatly reduces operation costs. [PERSON] et al. [119] (Table 7, Entry 5) followed a different path when treating corncob residue, as they applied an alkali deligrification followed by a whole-cell enzymatic saccharification with \(C\). _thermocellum_\(\Delta\)pyrF: p2638-BGL, an engineered strain with enhanced \(\beta\)-glucosidase activity [120]. The saccharified fraction was then fermented by \(G\). _stearothermophilus_ 2H-3, a contaminant strain found in corncob residue that can produce high quantities of LA with very little substrate competition by other fermentation products. Another advantage of this fermentation method is that saccharification and fermentation can be carried out in a one-pot successive fermentation. Despite the advantage of not requiring additional pre-treatments, CBP with \(R\). _oryzae_[118] (Table 7, Entry 4) afforded a much lower LA final titer and yield on biomass, but it is worthy to note that this bioprocess mainly aimed to produce bioethanol, which was obtained with a final titer of 18.83 g/L. Overall, to enhance the competitiveness of PLA compared to fossil-derived plastics, a thorough optimization process is required, encompassing biomass pre-treatment, fermentation parameters, and, ultimately, the selection of bacterial and fungal strains [121]. PLA, being both biodegradable and non-toxic, is a particularly useful biopolymer in agricultural applications. To be specific, PLA has been widely investigated in its role in the formation of mulch films [122] and food packaging [123] and has been proposed for the construction of shade nets and greenhouse covers [124].