# Harnessing Agri-Food Waste as a Source of Biopolymers for Agriculture
## 2 Sources of Biopolymers
### Cellulose: A Resourceful Biopolymer in All Its Forms  
Cellulose is the most common polysaccharide in the world. The chemical structure can be seen in Figure 4: traditionally obtained from plants, it is formed by \(\beta\)-D-Glucopyranose molecules [71]. Nowadays, there is a growing interest in the use of bacterial cellulose (BC), which is characterized by the absence of lignin and hemicellulose, allowing for the avoidance of the downstream purification process. Another advantage is that BC is generally endowed with a high crystallinity and water holding capacity. Furthermore, bacteria are a more sustainable source compared to plants because they do not require extensive land use, which disrupts natural habitats. Additionally, bacteria can feed on waste, making them an environmentally friendly option [72]; however, its high production costs are a strong discouragement for its employment [73]. Cellulose is present in the materials market in various derivative forms, such as methyl and ethyl cellulose, carboxymethyl cellulose (CMC), cellulose acetate, cellulose sulphates, amino celluloses, and hydroxypropyl methylcellulose. Cellulose can be present in crystalline, semicrystalline, or amorphous form, in nano or microparticles and in nanofibrils: each of these types of cellulose has its peculiar characteristics and can be used in the production of different materials [74]. As is evident in Table 4, cellulose purification is usually preceded by a delignification step and is often achieved by bleaching the sample or treating it under mild acid/alkaline conditions. Palm leaves (23.5% of lignin) treated with alkaline hemicellulose removal followed by bleaching yielded a final cellulosic solid with a relatively high degree of purity (87.12%) and crystallinity (69.9%) (Table 4, Entry 1) [75]. By carrying out the cascade removal of hemicellulose and lignin, [PERSON] et al. (Table 4, Entry 2) [25] obtained a final solid with a glucan content of 77% after both organosolv and alkaline delignification. The production of CMC from young and mature coconut coir is reported in Entry 3, reporting an improvement in the characteristics directly proportional to the number of bleaching times; it was also documented that mature coconut coir yields more cellulose and more CMC compared to the young ones [76]. Figure 4: Cellulose chemical structure. Cellulose purification can also be done with biological methods involving, for example, the use of a consortium of two ligninolytic fungal strains with very low levels of cellulase activity (Table 4, Entry 4) [64], or with thermosolvent laceas enzymes combined with steam treatment and ethanol (Table 4, Entry 5) [78]. As mentioned before, cellulose can be also obtained in the form of BC, considering that BC-forming microorganisms can grow on AFW. Recent works focusing on BC production from waste are detailed in Table 5, revealing relatively modest yields and final titers. Future research efforts should concentrate on optimizing fermentation conditions to increase BC yield, as feeding waste biomass to bacteria is not enough to make the process economically feasible. In this context, another cost-cutting strategy is the approach adopted by [PERSON] et al. (Table 5, Entry 1) [79], as an alternative to fermentation optimization. This approach was aimed at the full utilization of the biomass, which entailed the extraction of valuable compounds such as essential oils and pectin before the fermentation; using this method, the cost of the entire process is supported by the obtainment of multiple value-added molecules from biomass. Cellulose and its derivatives possess numerous applications in agriculture, as they have been employed for the controlled release of fertilizers [81] and essential micronutrients [82]. Another use of cellulose is as a fruit coating agent for ripening delay [83] or as a hydrogel for drought management [84; 85]. Moreover, mul