Page 66 - Advances in bioenergy (2016)

30% of the above calculated value). Continuous and fed batch processes are often adopted to
cope with the inhibitory effects of compounds present in lignocellulosic hydrolysates.
Continuous systems are most often used with immobilized cells because bacterial
contamination with Lactobacilli is a very often problem in stirred tank reactors using free
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yeast cells. The results obtained by Hoyer et al. who studied the effects of different enzyme
feeding strategies on ethanol yield from spruce under high gravity (HG) conditions, show that
fed batch or continuous fermentation strategies not only for the enzymes addition, but also for
the cells or the raw material could result in significant process improvements.

Consolidated Bioconversion Processes (CBP)

Lignocellulose bioconversion technology has exhibited a trend toward increasing
consolidation over the last decades. In CBP, the saccharification and the fermentation of
biomass are carried out in a single stage by the same microorganism. The major advantage of
CBP technology is the avoidance of the cost of dedicated cellulase production which is
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associated with significant potential energy savings. Therefore, the majority of scientific
efforts dealing with this issue have tried to develop ‘CBP microorganisms’ based on two
strategies: (category I) engineering a cellulase producer to be ethanologenic and (category II)
engineering an ethanologen to be cellulolytic.

Significant part of the research done so far focused on Thermocellum cellulolyticum (category
I) and S. cerevisiae (category II). On building S. cerevisiae acquiring cellulolytic activity,
several approaches have been tested: expressing heterologous cellulases, mimicking the

cellulosome system of complexed enzymes bound to the cell surface, or codisplaying different
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fungal cellulolytic activities. Fujita et al. described the efficient direct fermentation of
amorphous cellulose to ethanol, which was achieved by developing a yeast strain codisplaying
three types of cellulolytic enzymes. H. jecorina endoglucanase II and cellobiohydrolase II
(CBH-II) and Aspergillus aculeatus β-glucosidase were simultaneously codisplayed as
individual fusion proteins in yeast. It was shown that codisplay of CBH-II significantly
affected cellulose hydrolysis, suggesting that combination of cellulases with various functions
is effective in producing efficient degradation of cellulose. Cell-surface engineering systems
established using the yeast S. cerevisiae are expected to be used as paradigm for many more
applications. The surface-displayed proteins are self-immobilized on the cell surface, with this

feature being passed on to daughter cells as long as the genes are retained by the cells. The
ability to display enzymes should further enhance the status of S. cerevisiae as a novel and
attractive microorganism able to act as a whole-cell biocatalyst. This is particularly important
when target substrates cannot be taken up by the cells. 68

Biorefinery

With emerging research trends in the production of bio-based products, the concept of
biorefinery will continue to gain momentum. The definition of the term ‘biorefinery’ has been
subject to debate, but the overall goal of the biorefinery production approach is the generation
of a variety of goods from different biomass feedstocks, within a particular biorefinery,