Detoxification of Lignocellulose Hydrolysates: Biochemical and Metabolic Engineering Toward White Biotechnology

Chemical hydrolysis of lignocellulosic biomass (LB) produces a number of inhibitors in addition to sugars. These inhibitors include lignin-derived phenolics, carbohydrate-derived furans, and weak acids that have shown a marked effect on the productivities of various metabolites and the growth of biocatalysts in the fermentative reaction. In the past, a number of physicochemical and biological approaches have been proposed to overcome these fermentation inhibitors, including modified fermentative strategies. Additionally, the timely intervention of genetic engineering has provided an impetus to develop suitable technologies for the detoxification of lignocellulosics in biorefineries. However, the improvements in detoxification strategies for lignocellulose hydrolysates have resulted in significant loss of sugars after detoxification. Hydrolysis of myco-LB (LB after fungal pretreatment) has been recognized as a promising approach to avoid fermentation inhibitors and improve total sugar recovery. Biotechnological inventions have also made it possible to widen the range of suitable biocatalysts for biorefineries by microbial-routed induction of enzymatic expression for the elimination of inhibitors, eventually improving ethanol production from acid hydrolysates. This article aims to highlight the strategies that have been adopted to detoxify lignocellulosic hydrolysates and their effects on the chemical composition of the hydrolysates to improve the fermentability of lignocellulosics. In addition, genetic manipulation could widen the availability and variety of substrates and modify the metabolic routes to produce bioethanol or other value-added compounds in an efficient manner.