Page 122 - Lignocellulosic Biomass to Liquid Biofuels

96 Lignocellulosic Biomass to Liquid Biofuels

Hemicellulose
hydrolizate
and pretreated Solid residue:
lignocellulosic Direct microbial lignine
biomass conversion (DMC) microorganism biomass
Consolidated
Separation
Bioprocessing
bioreactor Ethanol
with community of
microorganisms
Hexose and
pentose
fermenting
and cellulolytic
producing
microorganism

Figure 3.4 Simplified process for CBP. CBP, Consolidated bioprocessing.

T. mathranii, Thermoanaerobium brockii, and C. thermosaccharolyticum strain.
The advantage to use thermophilic cellulolytic anaerobic bacteria com-
pared to conventional yeasts for producing bioethanol lies in their capacity
to directly employ a wide range of inexpensive biomass feedstocks and to
tolerate extreme temperatures. However, one of the major drawbacks for
the industrial exploitation of thermophilic cellulolytic anaerobic bacteria is
their low bioethanol tolerance (,2%, v/v) [227]. Currently, several stud-
ies are focused on the development of CBP yeast for the efficient manu-
facture of bioethanol and on the identification and exploitation of mixed
cultures able to carried out effectively the hydrolysis of lignocellulosic
biomass simultaneously with fermentation step [201].

3.5 Effect of fermentation inhibitors

The rigid structure of lignocellulosic biomass hinders the enzymatic
hydrolysis of biomass polysaccharides to fermentable sugars, thus prevent-
ing the biomass conversion into bioalcohols, as ethanol, and biochemicals
[228]. Several different pretreatment methods (chemical, physical, electri-
cal, biological, or a combination of those means) promote the lignocellu-
lose breakdown, reducing recalcitrance biomass and facilitating enzymes
to access their substrates [229]. However, pretreatment processes of