Designing simultaneous saccharification and co-fermentation of lignocellulose for improved xylose conversion

Kim Olofsson

Research output: ThesisDoctoral Thesis (compilation)

4 Downloads (Pure)

Abstract

Fuel ethanol from lignocellulose is one sustainable alternative to the fossil
fuels of today. All sugars in the material must be utilized in order to achieve
high overall ethanol yields. Baker’s yeast, Saccharomyces cerevisiae, has been
engineered to ferment the pentose sugar xylose from lignocellulose to
ethanol. However, ethanol production from xylose is slow and often incomplete.
In this work simultaneous saccharification and co-fermentation
(SSCF) of xylose and glucose has been investigated with the purpose of
improving xylose conversion and ethanol yields in non-detoxified lignocellulosic
hydrolyzates.

There are two main approaches to improve xylose conversion, which
both have been investigated in this work. One way is to enhance the performance
of the yeast by designing the process. The other way is to improve
the yeast itself by genetic and/or evolutionary engineering.

It was found that through careful design of different feeding strategies
the xylose fermentation in SSCF could be significantly increased. Fed-batch,
prefermentation, controlled enzyme feeding as well as combined enzyme and
substrate feeding, had all positive effects on the xylose conversion. Depending
on feed strategy and process conditions, this also resulted in a significant increase
of the ethanol yield. Moreover, by designing the SSCF process, it was
possible to increase the final solids content in the SSCF and still obtain a
relatively high ethanol yield on total sugars, which is crucial for the process
economy in commercial scale.

The effect of improved xylose transport capacity in the yeast was investigated
by expression of the glucose/xylose facilitator Gxf1 from Candida
intermedia, in strains of S. cerevisiae which were assessed in SSCF. The
improved transport proved to increase xylose uptake, but had only a minor
effect on the ethanol yield. The enzyme xylose reductase (XR) was implicated
to control xylose fermentation in SSCF of pretreated lignocellulose. When a
mutated XR (mXR), with higher activity and altered co-factor preference, was
integrated in S. cerevisiae the xylose uptake was significantly improved,
resulting in a higher ethanol yield in comparison to when the native Pichia
stipitis XR was used. Gxf1 had, however, only little influence when the
combined effect of Gxf1 and mXR was studied in SSCF, which indicated that
the initial xylose catabolism was still rate limiting.
Original languageEnglish
QualificationDoctor
Awarding Institution
  • Department of Chemical Engineering
Supervisors/Advisors
  • Lidén, Gunnar, Supervisor
Award date2011 May 13
Print ISBNs978-91-7422-270-8
Publication statusPublished - 2011

Bibliographical note

Defence details

Date: 2011-05-13
Time: 10:30
Place: Lecture hall K:B, Kemicentrum, Getingevägen 60, Lund University Faculty of Engineering

External reviewer(s)

Name: van Zyl, Willem H.
Title: Prof.
Affiliation: Department of Microbiology, University of Stellenbosch, South Africa

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Subject classification (UKÄ)

  • Chemical Engineering

Keywords

  • bioethanol
  • lignocellulose
  • simultaneous saccharification and co-fermentation
  • xylose fermentation

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