Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 27

Saccharomyces cerevisiae, and thus recombinants engineered with the pentose
pathways are needed [ 4 ]. However, relentless efforts for decades and unprece-
dented progress in biotechnology are paving the way to overcome these bottle-
necks leading to a promising harvest [ 5 ].
In this chapter, cutting edge progress in bioethanol production from ligno-
cellulosic biomass is reviewed, with a focus on the characteristics of the feedstock,
leading pretreatment technologies, enzymatic hydrolysis of the pretreated cellulose
component, co-fermentation of the pentose and hexose sugars released from the
hydrolysis of cellulose and hemicelluloses, and process integration and optimi-
zation. Remaining challenges and perspectives for the commercial production of
bioethanol are also highlighted.
2 Understanding Lignocellulosic Biomass
Understanding lignocellulosic biomass, particularly its chemical composition, is a
prerequisite for developing effective pretreatment technologies to deconstruct its
rigid structure, designing enzymes to liberate sugars, particularly cellulase to
release glucose, from recalcitrant cellulose, as well as engineering microorganisms
to convert sugars into ethanol and other bio-based chemicals.
Lignocellulosic biomass is mainly composed of plant cell walls, with the
structural carbohydrates cellulose and hemicellulose and heterogeneous phenolic
polymer lignin as its primary components. However, their contents varies sub-
stantially, depending on the species, variety, climate, soil fertility and fertilization
practice, but on average, for agricultural residues such as corn stover, wheat and
rice straw, the cell walls contain about 40% cellulose, 30% hemicellulose and 15%
ligin on a dry weight basis [ 6 ].
The distinctive feature of plant cell walls is their two-part structure, as
illustrated in Fig. 1 . A primary cell wall is developed with cell division,
and enlarged during cell growth to a fiberglass-like structure, with crystalline
cellulose microfibrils embedded in a matrix of polysaccharides such as hemi-
celluloses. The primary wall of adjacent cells is held together by a sticky layer,
called the middle lamella, composed of pectins, to form the conducting tissue
system arranged in numerous vascular bundles. On the other hand, when cells
cease to grow, a secondary cell wall is gradually deposited between the plasma
membrane and the primary cell wall for better mechanical strength and structural
reinforcement through the incorporation of lignin into xylem fibers, which
accounts for the bulk of lignocellulosic biomass that can be converted to fuels
and chemicals [ 7 ].
The development of the conducting tissue system with the rigid secondary cell
wall is a critical adaptive event in the evolution of land plants, which not only
facilitates the transport of water and nutrients as well as extensive upright growth,
but also raises its recalcitrance to degradation due to the interaction and cross-
linking of cellulose, hemocellulose and lignin [ 3 ].