Emerging Strategies for Modifying Lignin Chemistry to Enhance Biological Lignin Valorization

Biological lignin valorization represents a promising approach contributing to sustainable and economic biorefineries. The low level of valuable lignin-derived products remains a major challenge hindering the implementation of microbial lignin conversion. Lignin's properties play a significant role in determining the efficiency of lignin bioconversion. To date, despite significant progress in the development of biomass pretreatment, lignin fractionation, and fermentation over the last few decades, little efforts have gone into identifying the ideal lignin substrates for an efficient microbial metabolism. In this Minireview, emerging and state-of-the-art strategies for biomass pretreatment and lignin fractionation are summarized to elaborate their roles in modifying lignin structure for bioconversion. Fermentation strategies aimed at enhancing lignin depolymerization for microbial utilization are systematically reviewed as well. With an improved understanding of the ideal lignin structure elucidated by comprehensive metabolic pathways and/or big data analysis, modifying lignin chemistry could be more directional and effective. Ultimately, together with the progress of fermentation process optimization, biological lignin valorization will become more competitive in biorefineries.