Fig. 1.

The steps involved in the bioprocessing to lignocellulose to ethanol. Following chemical pretreatment to break down the tough, recalcitrant material in lignocellulosic biomass, it is more susceptible to enzymatic attack because of the exposure of the underlying carbohydrates (i.e., cellulose and hemicelluloses). Thereafter, there are four possible routes to ethanol production; SHF separate hydrolysis and fermentation, SSF simultaneous saccharification and fermentation, SSCF simultaneous saccharification and co-fermentation, CBP consolidated bioprocessing. Microbial enzymes produced by bacteria, fungi and other micro-organisms are used to convert the exposed cellulose and hemicellulose sugar polymers to simple sugars which can then be efficiently fermented; in all cases except CBP, these enzymes need to be added (the CBP organism(s) both saccharify and ferment the substrate). Microbial fermentation is the final phase in the bioconversion process. By this stage, the hydrolysate contains a mixture of hexose and pentose sugars such as glucose, xylose, mannose, fructose, galactose and arabinose, which are all fermentable by micro-organisms to produce bioethanol