Ethanol from lignocellulosic materials: Pretreatment, acid and enzymatic hydrolyses, and fermentation

Ethanol production from lignocellulosic materials is a major global task in producing liquid fuel by sustainable processes. The structure of the lignocellulose is usually opened by dilute-acid hydrolysis or steam explosion in a pretreatment step, while the resulting cellulose and hemicelluloses can be cleaved to the monomers (sugars) by acid or enzymatic hydrolyses. The hydrolyzates are then fermented to ethanol by using baker's yeast or other microorganisms. The acid hydrolysis suffers from a number of inhibitory by-products including furans, phenolic compounds and carboxylic acids, whereas the enzymatic one is still expensive and slow. Very good progress has been made within the last two decades on development of pretreatments, hydrolyses, fermentation techniques and recombinant microorganisms. These advances are briefly reviewed here.