Valorisation of Xylose to Renewable Fuels and Chemicals, an Essential Step in Augmenting the Commercial Viability of Lignocellulosic Biorefineries

Biologists and engineers are making tremendous efforts in contributing to a sustainable and green society. To this end, there is an increase in interest towards waste management cum valorisation. Lignocellulosic biomass (LCB) is most abundant material on earth and an inevitable waste predominantly originating from agricultural residues, forest biomass and municipal solid waste streams. LCB serve as a renewable feedstock for clean and sustainable processes and products. Cellulose and hemicellulose represent the structural carbohydrates in LCB, which on depolymerisation create sugar platform by liberating glucose and xylose, respectively. The preferential utilization of glucose and/or absence of xylose metabolic pathway in microbial systems alienate and discard xylose valorisation, a bottleneck in commercial viability of LCB-based biorefineries. Xylose is the second most abundant sugar in LCB, but a non-conventional industrial substrate unlike glucose. The current review sought to summarize the recent developments in biological conversion of xylose into a myriad of sustainable products and associated challenges. The review discusses the microbiology, genetics and biochemistry of xylose metabolism with hurdles requiring debottlenecking for efficient xylose assimilation. It describes the microbial cell factories naturally consuming xylose along with product accumulation formation with further justification for rewiring of metabolic networks to ameliorate xylose-based bioproduction. A case study that provides an argument on the suitable pathway for biomass and SA production from xylose through elementary flux mode analysis has been provided. Finally, a product portfolio from xylose bioconversion has been evaluated along with significant process in enzyme, metabolic and process engineering approaches eventually to maximize the product titers and yield empowering the LCB-based biorefineries. Towards the end, the review is wrapped up with current challenges, concluding remarks, and prospects with argument for an intense future research into xylose-based biorefineries.