# Harnessing Agri-Food Waste as a Source of Biopolymers for Agriculture
## 2 Sources of Biopolymers  
### Lignin: Extraction Methods and Applications in Agriculture  
L Lignin is a high molecular weight aromatic biopolymer (Figure 1) that can be found mainly in the woody stems of plants, but it is also a component of plant cell walls [16]; this characteristic makes lignin one of the most abundant polymers on earth. This polymer is mainly composed of the monomeric moieties of sinapyl alcohol, coniferly alcohol, and \(p\)-coumaryl alcohol. Valorization of lignin, a complex organic polymer found in the cell walls of plants, refers to the processes of converting it into higher-value products. Traditionally, lignin has been considered a by-product of the paper and pulp industry [17], often burnt for energy or used as a low-value filler in various applications. Nonetheless, with increasing interest in sustainable and renewable resources (pruning waste, straw, stalks, and bagasse), there has been growing attention towards finding more valuable uses for lignin. Overall, lignin valorization holds great promise for reducing the environmental impact of industries relying on lignocellulosic biomass, creating new revenue streams and contributing to the development of a more sustainable bioeconomy. Biorefineries mainly using food industry waste are gaining importance in lignin production. Indeed, the only way to make a biorefinery economically viable encompasses the valorization of all the waste components, including lignin. However, developing cost-effective and scalable processes aimed at the creation of a market for lignin-derived products is still challenging. Lignin has been explored for different applications in the medical, pharmaceutical, and electrochemical fields and as a plasticizer [18]. Moreover, depolymerized lignin is a platform chemical that can potentially substitute aromatic chemicals deriving from the fossil-fuel industry [18; 19]. Delignification usually aims to remove lignin from a lignocellulosic biomass, obtaining a relatively pure holocellulose fraction for further purposes and lignin as a by-product. The literature displays a lot of different delignification methods, such as alkaline, acid delignification, organosolv process, autohydrolysis, or enzymatic methods [20]. The lignin pyrolysis method has also been explored to obtain lignin biochar. Although biochar may have agricultural applications, it is important to note that lignin biochar is usually exploited as a filler/additive for biocomposites, rather than biopolymer itself. Additionally, the roasting method is not effective in reducing GHG emissions [21]. The possibility to further valorize the lignin for different applications paved the way for research aiming to improve delignification processes through combination with green technologies such as microwave, ultrasound (US), or steam explosion, affording higher lignin yields with reduced extraction time. Recently, solvents such as ionic liquids, natural deep eutectic solvents (NaDESs), and dioxane have also been investigated for lignin extraction [20]. Delignification techniques employed in recent studies are shown in Table 1, where the best yields were found coupling alkaline delignification with other different pretreatments. Figure 1: Lignin chemical structure. Indeed, [PERSON] et al. (Entry 1) [22] observed that hydrothermal prehydrolysis promoted the depolymerization of lignin macromolecules, facilitating the delignification process. It was also observed that lignin obtained by alkaline delignification has a lower molecular weight and a higher content of hydroxyl groups, making it an interesting source of value-added functional bio-based materials. In addition, [PERSON] et al. (Entry 4) [25] successfully removed 93% of lignin from corncobs by coupling autohydrolysis with alkaline delignification, which was also more selective in lignin removal compared to dioxane delignification.