Page 54 - Advances in bioenergy (2016)

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Hydrolysis and Fermentation for Cellulosic Ethanol

Production

2*
1*
Charilaos Xiros, Evangelos Topakas, and Paul Christakopoulos, 3*
1 Industrial Biotechnology, Department of Chemical and Biological Engineering, Chalmers
University of Technology, Gothenburg, Sweden
2 BIOtechMASS Unit, School of Chemical Engineering, National Technical University of

Athens, Zografou Campus, Athens, Greece
3 Biochemical and Chemical Process Engineering, Division of Sustainable Process
Engineering, Department of Civil, Environmental and Natural Resources Engineering,
Luleå University of Technology, SE-97187, Luleå, Sweden

INTRODUCTION

The depletion of fossil fuels together with the global economic and environmental
consequences encourage the search for renewable energy sources, such as biofuels. One of the
most promising feedstocks for biofuels production is plant biomass that contains large amounts
1
of sugar polymers, such as cellulose and hemicelluloses. When subjected to enzymatic
hydrolysis, these polysaccharides are transformed into glucose and other fermentable pentoses,
which might further be converted to liquid fuels such as bioethanol. The last few years, the

processes involved in the conversion of cellulose and hemicellulose have been closely related
to the word ‘recalcitrance’, to emphasize obstacles that can impede the conversion. 1
Recalcitrance is linked to each step in biomass conversion, which can be costly, driving the
production costs to exceed those of the transportation fuel competitors derived from fossil
fuels or those derived from starch, sucrose, and vegetable oils (e.g., first-generation bioethanol
2
and biodiesel). The strong glycosidic bonds of cellulose together with its associated crystal
structure (Figure 2.1) prescribe application of either harsh physicochemical conditions or use
of a consortium of different cellulose acting enzymes. The applied harsh conditions and
cellulolytic enzymes add considerable expense to the construction of saccharification reactors
and downstream processing. However, the protein production efficiency of cellulases has been
increased more than 10-fold nowadays resulting in a serious decrease in their price, rendering

enzymatic saccharification more economical than physicochemical methods. 3

At present, recombinant strains of Saccharomyces cerevisiae, and in a lesser extent of
Escherichia coli and Zymomonas mobilis strains, are considered as the most successful
microorganisms for biofuel production in industrial scale. Bottlenecks and obstacles, such as
the narrow range of fermentable sugars for some microorganisms, the imbalanced anaerobic
metabolism for some others, or the low uptake rates for some sugars, are challenges which
have to be nearly solved or overpassed. Although several improvements are yet to be done,