BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 424

Although S. cerevisiae cannot utilize xylose, the genes encoding the key enzymes
in the pathway for xylose utilization are present in its genome; however, their expres-
sion levels are too low to allow for xylose utilization. In fact, only very slow growth on
xylose has been observed when those genes are overexpressed. There are naturally
occurring microorganisms able to ferment xylose into ethanol under oxygen-limited
conditions (e.g., the yeast Pichia stipitis ). However, the rate and yield of ethanol pro-
duction from xylose in these strains are very low. Due to a redox imbalance in the cell,
these strains also excrete xylitol as by-product, thereby reducing the yield of ethanol
on substrate (see Figure 13.10).
Given all these aspects, a vast amount of research is currently being devoted to the
engineering of microorganisms for hexose and pentose fermentation to ethanol.
Rapid advances are being made, especially with S. cerevisiae as host microorganism.
The engineering focuses on three aspects: (i) transport of the sugar across the cell
membrane, (ii) coupling of the sugar metabolism to the main glycolytic pathway,
and (iii) maintenance of a closed redox balance. For the latter, e.g., the introduction
of a pathway for xylose utilization that does not lead to xylitol excretion (see
Figure 13.10) has succeeded in providing a under academic

research and has now been taken up by industry for further development and
implementation.

“ proof of principle ”

13.3.5 Process Configurations for Hydrolysis and Fermentation

Most process concepts for the production of bioethanol from lignocellulosic feed-
stocks start with pretreatment, followed by enzymatic hydrolysis of the cellulose part
and a yeast-based fermentation of the resulting sugars. Lignin, the main by-product in
the process, can be directly used as a solid fuel or as a source for higher added-value
biorefinery products. This configuration is often referred to as sequential or separate
hydrolysis and fermentation. In such a configuration, each process step can be
optimized independently, resulting in different operating conditions (e.g., tempera-
ture, pH, and residence time). In addition, it is possible to have a separate C 5 and
C 6 fermentation. The main advantage of this configuration is its flexibility. However,
during hydrolysis, high levels of glucose accumulating in the reactor inhibit the
activity of the enzyme.
An alternative that is receiving much attention is that of simultaneous saccharifi-
cation and fermentation. In this configuration, cellulose hydrolysis and ethanol
fermentation take place in the same processing step, resulting in less pieces of equip-
ment. One of the main advantages of this configuration is that the glucose produced
during hydrolysis is immediately consumed during fermentation, avoiding product
inhibition on the enzymes and minimizing contamination by other microorganisms.
A disadvantage is that the optimum temperature for enzymatic hydrolysis is typically
higher than that of fermentation, and hence, a compromise in operating conditions is
required. In order to overcome this, research is currently being done on ethanol
fermentation at higher temperatures (Blanch, 2012).