Page 118 - Lignocellulosic Biomass to Liquid Biofuels

92 Lignocellulosic Biomass to Liquid Biofuels

Pretreated
lignocellulosic
biomass Solid residue:
Simultaneous
saccharification lignine
fermentation
Hydrolytic Enzyme hydrolysis Separation
enzymes and Ethanol
hexose
fermentation
Hexose reactor
fermenting Fermentation
microorganisms broth rich in
(yeast) unreacted xylose
Pentose
(xylose) Separation
fermentation
Pentose Ethanol
fermenting reactor
microorganisms
SSF yeast
Figure 3.2 Simplified process for SSF. SSF, Simultaneous saccharification and
fermentation.

time, operating costs, inhibitors, and increasing the hydrolysis rate [209].
However, Gauss et al. suggested to carry out the enzymatic hydrolysis and
fermentation simultaneously, as early as 1976 [210]. The authors noted
the low yields of glucose achieved by the fungus T. reesei in a traditional
separate enzymatic hydrolysis, probably as a result of end product inhibi-
tion of the hydrolysis by glucose and cellobiose. Therefore they suggested
that a higher overall ethanol yield was achieved during simultaneous pro-
cess of enzymatic hydrolysis and fermentation, attributable to the deletion
of glucose and cellobiose by the fermentation, and the consequential
release of end product inhibition [211]. The main advantage to carry out
saccharification and fermentation simultaneously in a single reactor
(Fig. 3.2) is the possibility to rapidly convert the sugars newly formed
(mainly glucose), produced by hydrolyzing enzymes, into ethanol,
decreasing their buildup in the medium and alleviating feedback inhibi-
tion of cellulase [212].
A way of preventing end product inhibition by sugars cellobiose is to
use commercial cellulase preparations with extra-BGL, otherwise yeasts
able of fermenting cellobiose, as Brettanomyces claussenii [213] or recombi-
nant K. oxytoca [214]. Furthermore, the several compounds contained in
pretreatment hydrolyzates, which act as inhibitor of hydrolytic enzymes,