Bioprocess intensification for xylitol production from renewable resources

Julen Ordeñana Manso*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

This thesis revolves around the utilization of corn fiber as a feedstock for the development of a xylitol production platform. Xylitol is a five-carbon polyol with numerous applications in different industries and with an ever expanding market. It is currently produced by a very energy intensive chemical hydrogenation process, which makes xylitol not only expensive but also unsustainable. Thus, this thesis aims to evaluate xylitol production from a biorefinery perspective.
Corn fiber, a byproduct of the corn wet-milling process, is a lignocellulosic biomass with a great potential for use in bioprocesses due to its low lignin and high starch content. The hemicellulosic fraction is however known for having a complex glucuronoarabinoxylan structure, which makes it quite resistant to enzymatic hydrolysis processes. For this reason, this thesis started evaluating the use of three commercial hydrolytic enzymes (Cellic® CTec2, Pentopan® Mono BG, and Termamyl® 300L) for the development of an enzymatic cocktail that acts synergistically for the degradation of the different fractions of corn fiber (cellulose, hemicellulose and starch, respectively). The obtained optimal combination however did not completely hydrolyze corn fiber, so, in order to reduce the recalcitrance, a set of hydrothermal pretreatments were used. The hydrothermal pretreatment at 175 °C allowed to reduce the enzymatic load used and diminished the recalcitrance of the corn fiber, yielding an almost complete release of glucose and more than half of the xylose.
The new hydrolysis process was used in Simultaneous Saccharification and Fermentation (SSF) together with Kluyveromyces marxianus. The operating temperature was reduced and the liquid phase from the pretreatment was discarded in order to improve cell viability. Although this made the yeast able to grow, the measures did not yield a high xylitol production, primarily because of the still relatively high stressor level and the presence of glucose in the medium. In order to solve the latter, gene editing tools for the manipulation of the metabolism of K. marxianus were developed. Even if the obtained tools demonstrated the ability to perform double strand breaks in the targeted loci, it was not possible to integrate exogenous DNA and perform changes to the genome of the yeast. Manipulation of the physicochemical conditions of the fermentation (oxygen transfer rate and fermentation pH) did however successfully increase the xylitol production yield to a maximum of 0.36 grams of xylitol per gram of xylose.
This thesis ends with a study of the downstream xylitol purification process. Crystallization is the most common process for xylitol recovery, thus, a model based on theoretical data was made using AspenPlus V14. The results of the model were later compared to empirical crystallization assays. The effect that xylose and acetic acid have on the crystal yield, purity and xylitol recovery was also empirically assessed.
On the whole, this thesis has advanced the knowledge in the biomanufacturing of xylitol by improving fermentation conditions and assessing the effect of “contaminants” in the crystallization process. Furthermore, this thesis describes one of the most effective procedures for the saccharification of corn fiber, which allowed for ethanol and xylitol production in an SSF process.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Bioengineering
Number of pages115
Publication statusPublished - 2024

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