D-xylose fermentation by Fusarium oxysporum and other fungi: Dissertation

Xylose fermentation was studied in order to provide the basis for an economically feasible ethanol production process using sugars derived from biomass.During the study 67 different yeast strains and 27 Fusarium strains were tested.The best strain for xylose fermentation was Fusarium oxysporum VTT-D-80134, which was selected for more extensive experiments.It was found that xylose metabolism in F. oxysporum starts by reduction of xylose to xylitol followed by oxidation to xylulose, yielding ultimately ethanol and carbon dioxide.Yields almost as high as the theoretical maximum (0.44 _ 0.50 gg-l) were obtained from xylose (50 gl~l) in semianaerobic shake flask fermentations in six days.In optimized conditions, involving strict control of oxygen supply, the fermentation time was shortened to four days.The ethanol yield was 0.43 gg-l based on added xylose.The maximal specific ethanol production rate was 0.14 gg-lh-l~ F. oxysporum was able to ferment all the sugars present in birchwood hydrolyzates, except L-a rabinose and L-rhamnose, which were used only for aerobic growth.Xylan was not fermented directly to ethanol due to the inability of the fungus to hydrolyze the Polymer to xylose. The main sugars, D-glucose and D-xylose, were fermented successively without diauxia.Sugar concentrations up to 200 gl~ caused only a slight decrease in the fermentation rate.Ethanol concentrations below 45 gl~l did not inhibit the fermentation.Inhibition was, however, caused by some compounds present in wood hydrolyzates.Continuous fermentation by immobilized F. oxysporum cells was unsuccessful due to the growth of mycelium out from the beads. however, a continuous process with free cells appeared more promising.