Page 104 - Lignocellulosic Biomass to Liquid Biofuels

80 Lignocellulosic Biomass to Liquid Biofuels

responsible for the synthesis of butanol into E. coli [153 159] Moreover,
different engineered strains were developed via mutagenesis based on
S. cerevisiae [156,160,161], Ralstonia eutropha [162], Lactococcus lactis, and
Lactobacillus buchneri [163].
Finally, even if currently biobutanol is mainly produced by Clostridia,
they cannot directly utilize lignocellulose. Therefore metabolic construc-
tion or isolation of novel cellulolytic/hemicellulolytic and solventogenic
bacteria to achieve direct butanol production from lignocellulose offers a
promising alternative [164]. The present research efforts are focused on
developing CBP strategies for biofuels production wherein microorgan-
isms are used to hydrolyze and ferment inexpensive lignocellulosic materi-
als directly into desired products without additional enzymes [85].
C. thermocellum, C. cellulolyticum, and Clostridium thermopapyrolyticum can
directly utilize lignocellulosic biomass [165]. Several strains of Clostridium
cellulyticum, including metabolically engineered and wild-type strains, have
been reported to generate value-added products directly from cellulose,
but few studies investigated wild-type or metabolically engineered strains
to produce butanol directly from cellulose or xylan, which constitutes the
principal hemicellulosic component of plant wastes [164,166 170].

3.3 Enzymatic hydrolysis
3.3.1 Enzymatic hydrolysis of lignocellulosic biomass
Enzymatic hydrolysis is an essential stage in the transformation of cellu-
lose, from pretreated biomass, to glucose. The bioconversion of cellulose
to glucose is being carried out by using cellulase enzymes under mild
operating conditions of temperature (in the range 40°C 50°C) as well as
pH ranges between 4.5 and 5.0, in order to negate the corrosion problems
[171].
The efficacy of the hydrolysis process is governed by the degree of
pretreatment of the biomass in terms of lignin removal and hemicelluloses
solubilization including hydrolysis, and enzyme loading. Lignin and the
acetyl group that are present in the hemicelluloses could produce binding
with cellulase not really productive and consequently could limit the
hydrolysis process [172]. The efficacy of the hydrolysis process was found
to be enhanced by the addition of nonionic surfactants by which it
changes the cellulose surface properties and lowering enzyme loading.
Surfactants, such as polyethylene glycol (PEG), were found to intensify
the enzymatic conversion of the lignocellulosic biomass from 42% to 78%