## 7 Concluding Remarks and Future Perspective  
Even though the chemical properties of lignin and its derivatives, its biodegradation, and related enzymes have been studied intensively, their application in biorefinery-derived platform chemicals and materials is still in the proof-of-concept phase. To date, the individual processes or techniques for lignin utilization have been studied at the tube-, flask-, and bench-scales. Nevertheless, emerging technologies for the scale-up and industrial application face bottlenecks of bio-based processes for lignin valorization from the raw lignin resource to final products for chemicals or materials. The following prospective could be considered for developing the techniques for lignin valorization in the production of platform chemicals and materials within a biorefinery:  
(1) Lignin is a macromolecule consisting of heterogeneously aromatic polymer compounds, containing syringyl-, gualacryl-, and hydroxyphenyl-subunits. Although the predominant subunit composition ratios are different depending on the plant species, these aromatic compounds are potential substances for renewable chemicals, which can be obtained through lignin depolymerization with thermal or catalytic deconstruction. Depolymerized lignin subunits could fractionate based on their chemical properties for further manipulation processes. In addition, these depolymerization processes would need high-energy consumption steps such as the pretreatment of lignocellulosic. Mushroom ligninolytic enzymes could be potential biocatalysts for the initial depolymerization step, combined with mild condition extraction by organosolv or ionic liquid from lignocellulosic [164; 165]. Mushroom ligninolytic enzymes are secreted proteins including lacases, LiPs, MnPs, VPs, and other oxidases, in the solid-state fermentation [37]. The enzymatic processes with secreted oxidase proteomes for lignin depolymerization from the solid phase of lignocellulosic biomass could reduce energy consumption and byproducts. Moreover, the residual biomass after these treatments could be also utilized further for biorefinery products. (2) Although biocatalyst processes are an eco-friendly technology, the production of ligninolytic enzymes incurs higher financial costs in the scale-up process in lignin valorization rather than chemical catalysts. Since effective and powerful oxidases are produced from higher eukaryotic microorganisms, such as fungi, most researchers have developed a yeast or fungus expression system, such as _P. pastoris, K. lactis, S. cerevisiae, A. niger_, and _A. oryzae_, for recombinant protein production considering the secretion and glycosylation of an active enzyme [47; 49; 57; 62; 71; 75; 81]. These heterologous expression systems for the recombinant ligninolytic enzymes should enhance protein productivity to reduce the production costs of renewable chemicals. In addition, the secreted proteins in the solid-state fermentation after mushroom harvesting could be useful resources to obtain native enzymes through further formulation processes. This topic should be further studied to identify proteomes for ligninolysis and establish a novel protocol for the formulation of crude proteins extracted from the solid media. (3) The bioconversion or biocatalytic process for lignin valorization faces many challenges due to the characteristics of biological molecules such as instability, toxicity, and solubility of lignin-derivative molecules, as well as the strict reaction conditions. Most aromatic compounds derived from depolymerized and fractionated lignin subunits are also hydrophobic, insoluble, and toxic properties for microorganisms. Thus, the high concentration of aromatic derivatives cannot be transformed into products by biological processes. Nevertheless, bioprocesses would still be attractive methods for eco-friendly sophisticated reactions that are simple, low-energy consumption processes without byproducts that can convert lignin monomers to usable chemicals and materials.