Industrial Lignocellulosic Waste Biomasses: Enormous Substrates for Harnessing Enzymes and Bioethanol Productions

Lignocellulosic waste, consisting mainly of cellulose, hemicellulose and lignin, is the most abundant global biomass. Industries generate millions of tonnes of this waste biomass every year. This includes: sugarcane bagasse, corncob, rice straw and husk, oil palm fibres and empty fruit bunches, wheat straw, brewer’s spent grains, among others. These wastes could be valorized to produce high-value industrial lignocellulase enzymes, such as cellulases, hemicellulases and ligninases, by several microorganisms, including bacteria and fungi. Furthermore, the crude or purified lignocellulase enzymes or the microbes producing these enzymes could be used to hydrolyze the lignocellulosic wastes into simple sugars. The sugars are then fermented to bioethanol using simultaneous saccharification and fermentation, or a separate hydrolysis and fermentation technique. However, an initial pretreatment of lignocellulosic biomass greatly facilitates delignification and deconstruction of the lignocellulose complex structure, making the cellulose and/or hemicellulose components more accessible for enzymolysis. This review presents a holistic compilation of the different industrial lignocellulosic waste biomasses. It also highlights the microbial enzymes that can hydrolyze the lignocellulosic biomasses for industrial applications, including the production of fermentable sugars and bioethanol, as well as the different microorganisms producing the enzymes. The processes in the production of these metabolites, and the challenges encountered with the valorization of these wastes, as well as the possible solutions, are also adequately discussed.