## 3 Plastics Used for Food Packaging
### Selected Plant-derived Polymers  
#### 3.3.1 Lignocellulosic Biomass and Lignin  
Lignocellulosic biomass is the major structural component of plants, mainly consisting of cellulose (40-60%), hemicellulose (10-40%), and lignin (15-30%), whereby the latter one is the most complex constituent [139]. Lignin is a randomly crosslinked macromolecule composed of the three monolignols \(p\)-coumaryl alcohol, coniferly alcohol, and sinapyl alcohol, which form the residues \(p\)-hydroxyphenyl (H), guiaacyl (G), and syringyl (S), respectively, as shown in Figure 4[140]. It can be obtained from woody biomass (e.g., pine, poplar, birch), annual plants (e.g., wheat straw, miscanthus, switchgrass), or agricultural residues (e.g., sugarcane bagasse) by various extraction processes. The molecular structure of lignins strongly depends on the botanical origin but also on the growing site, season, and isolation process [141; 142]. There are different types of technical lignins that can either be classified as sulfur-containing or sulfur-free. The most common ones are lignocellulosic, kraft lignin, organosolv lignin, and soda lignin [139]. Most industrial lignins originate from the pulp and paper industry with up to 90 million tons of kraft lignin released per year worldwide, though only 2% of it are used commercially for value-added products [143]. One reason for that might be the deficient quality or missing specifications of technical lignins, as they are rather undefined products with a complex composition and impurities from the pulping process (such as remaining sugars or thiol groups). So far, this restricts their industrial exploitation, as resulting products have varying properties, which are inferior to fossil-based products. Nevertheless, in consideration of the global energy crisis and the depletion of fossil fuels and petrochemicals, the potential of lignin has been a key topic in biorefinery research [139; 144; 145; 146]. As polyphenols, lignins possess numerous interesting functional properties, such as antioxidant activity, and thus, they are investigated as active packaging materials for the protection of light- or oxygen-sensitive goods [119]. Antioxidant polymers are a field of great interest, as the use of macromolecular antioxidants is related to the possibility to produce materials with long-term stability. Due to its cross-linked 3D structure and enzymatic resilience, lignin possesses a higher thermal and biological stability compared to low molecular weight compounds and thus could be used in special fields where the exploitation of low-molecular antioxidant substances would be inefficient due to their higher diffusion rates. Moreover, carcinogenic effects have been observed for synthetic antioxidants: butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), for example, are able to cause cytotoxicity and carcinogenesis as shown in in vivo studies [147]. [PERSON] et al. have already shown that lignin has the potential to serve as raw material for antioxidants like guaiacol and 4-vinylguaiacol, whereby their antioxidant activity is comparable to that of commercial antioxidants [148]. Consequently, lignin has gained increasing interest as environmentally benign antioxidant and its ability to improve mechanical, thermal, and barrier properties when incorporated in conventional packaging films. In general, a reduction in WVP of lignin-based films is explained by the hydrophobicity of lignin. It acts as a barrier in the polymeric matrix and increases the path for the diffusion of water vapor, resulting in lower permeation of water molecules through the polymer film [149]. Next to the utilization of lignin as additive/blend in different polymer matrices, it could also be used as raw material for the development of polymeric packaging materials, e.g., as polyol substitute in polyurethanes or polyesters or as phenol substitute in resins. [PERSON] et al. investigated softwood lignin esterified with tall oil fatty acid as coating on paper board.