Lignocellulose Conversion, p. 111-130
The second generation bioethanol represents a main challenge in global efforts to utilize renewable resources rather than fossil fuels. However, the close association of cellulose and hemicelluloses to lignin in the plant cell wall makes it difficult to degrade lignocellulose into fermentable sugars. Consequently, pretreatments are necessary to make the polysaccharides more accessible to the enzymes, but the high temperature and extreme pH conditions required give rise to problems when using conventional enzymes in the saccharification step (Galbe and Zacchi 2002). Microorganisms thriving in habitats characterized by harsh conditions, and the enzymes derived therein, represent a helpful tool in the development of bioethanol production processes. In fact, they allow bioconversions at non-conventional conditions under which common biocatalysts are denatured. The use of high operational temperatures allows energy savings by reducing the cooling cost after high temperature pretreatments, and, in ethanol production, thermophilic conditions permit ethanol evaporation allowing harvest during fermentation.