Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 37

designed to dissolve cellulose or lignin from lignocellulosic biomass and deconstruct
the crystalline structure of cellulose molecules for enzymatic hydrolysis. Thus, IL
pretreatment has been extensively investigated recently. Meanwhile, almost all ILs
can be recovered, which not only reduces their usage, but also makes them more
environmentally friendly [ 42 ]. However, there are still many challenges for ILs to be
practical in the pretreatment of lignocellulosic biomass for the production of bulk
commodities like ethanol, and regeneration of ILs is one of them.

3.4 Biological Pretreatment

Compared with physical and chemical pretreatments in which expensive equip-
ment, chemicals and intensive energy consumption are needed, biological pre-
treatment by solid fermentation employs microorganisms that degrade
lignocellulosic biomass at mild conditions without special requirements for
equipment [ 43 ]. Both bacteria and fungi have been explored, but rot fungi asso-
ciated with wood decay are the predominant species in lignocellulose degradation
for the purpose of biofuel production, particularly white-rot fungi due to their
abundant ligninolytic enzymes, including lignin peroxidase, manganese peroxi-
dase, laccases and other enzymes, and better selectivity in lignin degradation [ 44 ].
Although biological pretreatment is energy-saving and environmentally
friendly, its disadvantages are apparent. Firstly, the extremely low degradation rate
requires times as long as weeks for a significant change in the structure of the
lignocellulosic biomass, making the process mismatched with the subsequent
hydrolysis of cellulose and fermentation of sugars. Secondly, significant biomass
is lost during the process, not only the lignin which is mineralized into low-
molecular-weight fragments that might be further catabolized into the useless final
product CO 2 [ 45 ], but also sugars released from hemicelluloses and even cellulose
by the hydrolytic enzymes (simultaneous decay with lignin degradation) as a
carbon source to support the growth of the microorganisms [ 46 ]. Finally, the
control of microbial growth and metabolism under open and solid fermentation
conditions with mixture species is unreliable, which inevitably affects the
subsequent processes such as cellulose hydrolysis and ethanol fermentation.
Therefore, biological pretreatment is less attractive from the viewpoint of com-
mercial application.

4 Enzymatic Hydrolysis of Cellulose and Co-Fermentation
of C5 and C6 Sugars

Following pretreatment, enzymatic hydrolysis is needed to further depolymerize
the cellulose component to glucose, which can be used for ethanol fermentation
together

with sugars released from the hydrolysis of hemicelluloses during