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Improve Xylose Utilization
1.The significance of improving xylose utilization:
The commercialization of second-generation bioethanol has
not been realized due to several factors, including poor biomass
utilization and high production cost. It is generally accepted that
the most important parameters in reducing the production cost
are the ethanol yield and the ethanol concentration in the
fermentation broth.
Ethanol is recovered by distillation, which becomes
economically feasible when the concentration exceeds 40 g/L
(Zacchi and Axelsson, 1989 ). This concentration therefore serves
as benchmark when different process alternatives are evaluated.
Hence, almost complete conversion of both hexoses and
pentoses must be achieved in SSF at a high solid content.
2.Why xylose utilization is so difficult ?
The probable main reason for this is that xylose and glucose
compete for the same transport systems, and the affinity for xylose is
approximately two orders of magnitude lower than for glucose.
Therefore, the glucose concentration must be low to obtain efficient
xylose uptake. However, glucose in fact enhances xylose utilization at
low but nonzero concentrations. This can be attributed to induction of
suitable xylose transporter systems, improved cofactor generation and
induction of glycolytic enzymes.
A low but nonzero glucose concentration– giving a high xylose to
glucose ratio – is therefore desirable in a process aiming at co fermentation of glucose and xylose.
3.How to improve xylose utilization:
Method 1: A Fed-Batch SSCF
Instead of a batch SSCF process , one may instead use a
fed-batch SSCF process. In this way the following advantages
are gained : 1) The viscosity of the medium can be maintained
low due to a gradual feeding of new material to the reactor,
in which the viscosity decreases due to enzymatic
degradation 2) the effect of toxicity of the hydrolyzate can
be decreased as a result of both adaptation of the yeast and
gradual biological detoxification ; 3)there may be a beneficial
effect on the xylose uptake from a changed concentration
ratio of xylose to glucose in the medium.
--------Olofsson K, Rudolf A, Lidén G: Designing simultaneous saccharification
and fermentation for improved xylose conversion by a recombinant strain
of Saccharomyces cerevisiae. J Biotechnol 2008, 134: 112-120.
------Olofsson K, Rudolf A, Lidén G: Designing simultaneous
saccharification and fermentation for improved xylose conversion by a
recombinant strain of Saccharomyces cerevisiae. J Biotechnol 2008,
134: 112-120.
Material : wheat straw
Pretreatment method : acid treatment
Microorganism: S. cerevisiae TMB3400
Method: Fed-batch SSCF with 4% WIS initially, 4 additions during 12 h to a final
WIS-content of 7%.
Fed-batch SSCF with 4% WIS initially, 8 additions during 24 h to a final WIScontent of 7%.
Fed-batch SSCF with 6% WIS initially, 4 additions during 12 h to a final WIScontent of 9%.
Fed-batch SSCF with 6% WIS initially, 8 additions during 24 h to a final WIScontent of 9%.
Results:
Method 2: Enzyme feeding
---------Olofsson
K, Wiman M, Lidén G: Controlled feeding of cellulases
improves conversion of xylose in simultaneous saccharification and cofermentation for bioethanol production. J Biotechnol 2010,145 :168-175
Material : spruce
Pretreatment method : acid treatment
Microorganism: S. cerevisiae TMB3400
Method and Results:
Method 3: Both enzyme and substrate feeding
--------Olofsson K, Palmqvist B, Liden G: Improving simultaneous saccharification and cofermentation of pretreated wheat straw using both enzyme and substrate feeding.
Biotechnol Biofuels 2010,3:17
Material : wheat straw
Pretreatment method : acid treatment
Microorganism: S. cerevisiae TMB3400
Method:
Results:
Method 4:Prefermentation
----Bertilsson M, Olofsson K, Lidén G: Prefermentation improves xylose utilization in
simultaneous saccharification and co-fermentation of pretreated spruce. Biotechnol
Biofuels 2009,2:8.
Prefermentation: the concept of prefermentation, that is, the
fermentation of initially available free glucose in the liquid before
addition of enzymes, was tested. The idea was to reduce the
competitive inhibition on xylose uptake by hexoses in this way.
Material : Wood chips from spruce
Pretreatment method : steam-pretreated
Microorganism: S. cerevisiae TMB3400
Results:
Method 5: Integration of first-generation and secondgeneration processes
--Borbála Erdei, Balázs Frankó ,Mats Galbe and Guido Zacchi: Separate hydrolysis
and co-fermentation for improved xylose utilization in integrated ethanol production
from wheat meal and wheat straw. Biotechnology for Biofuels 2012, 5:12
Material : wheat straw and wheat meal
Pretreatment method : steam pretreated
Microorganism: S. cerevisiae TMB3400
Method:
Results:
Method 6: Two-step SSCF
-----Jin, M.J., Lau, M.W., Balan, V., Dale, B.E., 2010. Two-step SSCF to convert
AFEX-treated switchgrass to ethanol using commercial enzymes and Saccharomyces
cerevisiae 424A(LNH-ST). Bioresour. Technol. 101, 8171–8178
Material : switchgrass
Pretreatment method : AFEX-treated
Microorganism: S. cerevisiae 424A(LNH-ST)
Method:
For two-step SSCF(Fig. 1B), xylanase and pectinase were added first to perform prehydrolysis for 8 h. Yeast cells and a small percentage of Spezyme CP and Novozyme 188
were then added to the hydrolyzate to carry out SSCF (step 1) for 60 h. After 60 h, the
rest of Spezyme CP and Novozyme 188 enzymes were fed to re-lease glucose from glucan,
which was subsequently fermented to ethanol by the yeast (SSCF step 2).
Results:
Method 7: extending the pre-hydrolysis time
---- Mingjie Jin, Christa Gunawan, Venkatesh Balan, Ming W. Lau, Bruce E.
Dale:Simultaneous saccharification and co-fermentation (SSCF) of AFEX pretreated corn
stover for ethanol production using commercial enzymes and Saccharomyce cerevisiae
424A(LNH-ST). Bioresource Technology 110 (2012) 587–594
Material : corn stover
Pretreatment method : AFEX-treated
Microorganism: S. cerevisiae 424A(LNH-ST)
Method and Results: