Studies using xylose fermenting yeast for lignocellulosic fermentation

Examensarbete för masterexamen
Master Thesis
Raju Duraiswamy, Varuni
Lignocellulosic hydrolysates such as spruce and Arundo donax were used in Separate Hydrolysis and Fermentation (SHF) process to analyse the fermentative performance of each recombinant Saccharomyces cerevisiae strain: GS1.11-26 and VTT C-10883, engineered for xylose utilization. In shake flask experiments, GS1.11-26, an industrial strain showed better fermentative performance and thereby it completely consumed xylose present in spruce compared to the other laboratory strain, VTT C-10883. The complete consumption of xylose by the industrial strain was not observed in Arundo. Furthermore, analysis of both strains in bioreactors showed that GS1.11-26 has higher ethanol production rate and ethanol yield on consumed sugars in both spruce (0.42 g/g) and Arundo (0.38 g/g). In spruce material, in addition to sugars, inhibitors such as furaldehydes [furfural and hydroxymethyl furfural (5-HMF)] are generally present in high amounts. The compounds are generated during the pretreatment and saccharification process of lignocellulosic biomass to ethanol. Most yeast strains has limited tolerance ability towards these inhibitors and the current research focuses on the development of stress tolerant strains to efficiently convert lignocellulose derived raw material into fuels and chemicals. In the second part of the project, pulse addition experiments were carried out using HMF and furfural to analyse the stress response of VTT C- 10883 towards inhibitors. The addition of 3.9 g/L HMF and 1.2 g/L furfural to the exponential growth phase of VTT C-10883 resulted in decreased specific growth rate [0.07 (±0.006) h-1] compared with the control [0.28 (±0.02) h-1]. The amount of ethanol produced decreased for 2-3 hours after pulse addition, however, at the end of fermentation, approximately 20 g/L ethanol was produced in both pulse and control samples. Also, an increase in the average glycerol yield was observed in high concentration inhibitor pulsed samples. The average yield of acetate increased and xylitol, a major by-product produced during anaerobic xylose metabolism, was found to significantly decrease in the presence of inhibitors. The results of this study showed that the inhibitors furfural and 5-HMF were completely metabolized by VTT C-10883 strain. In the future, more metabolic studies performed using this strain could result in strategies to improve tolerance towards inhibitors.
Energi , Industriell bioteknik , Energy , Industrial Biotechnology
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