Lignocellulosic biomass that is obtained as agriculture by-products and/or industrial residues is a nonfood stuff competitive feedstock for the sustainable production of important liquid fuels such as bioethanol or biobutanol and chemical products through the biorefinery processes.

Cellulose, hemicellulose, and lignin are the main components of lignocellulosic biomass. From the biochemical point of view, high amounts of sugars present in cellulose and hemicellulose can be chemically produced, using acid as the catalyst, or enzymatically hydrolyzed and converted into biofuels by a fermentation process.

Pretreatment technologies based on biological approaches are interesting to improve the efficiency of the bioconversion processes and to overcome barriers in the scale-up and commercialization of renewable biorefineries.

In this chapter, after a brief introduction, a preliminary analysis of suitable strains and their productivity is done in the first part, with reference to processes involved both in bioethanol and biobutanol production.

During the bioethanol production from lignocellulosic biomass, pretreated lignocelluloses are converted to simple sugars, in hydrolysis reactors, by catabolic enzymes, and consequently, enzymatic hydrolysis of lignocellulosic biomass is then described, under different points of view. Operating conditions, methodologies, and biochemical aspects of enzymatic hydrolysis are described with economic and energetic considerations in the second part. Apart from that, traditional downstream purification and membrane-assisted enzymatic hydrolysis as innovative methods are compared too in this chapter.

The hydrolyzate is fermented to ethanol by ethanologenic yeasts, separate enzymatic hydrolysis and fermentation (SHF), which is still the main process configuration for the biofuels production from lignocellulose. In order to overcome the SHF limitations, integrated conversion technologies have been developed, including simultaneous saccharification and fermentation, simultaneous saccharification and co-fermentation, and consolidated bioprocessing.

All these processes are described in the third part of the chapter.

Several different pretreatment methods (physical, chemical, biological, electrical, or a combination of these) promote the lignocellulose breakdown, reducing recalcitrance biomass and facilitating enzymes to access their substrates. However, pretreatment processes of lignocellulosic materials cause the formation of numerous lignocellulose-derived by-products that can inhibit microbial growth and fermentation yields. A detailed description of the effect of fermentation inhibitors such as furan derivatives, weak acid (acetic, formic, and levulinic acids), phenols, and other inhibitors is done in the concluding part of the chapter. Some strategies are also suggested for minimizing inhibitor effects.