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  • 1.
    Horváth, I. S.
    et al.
    Dept. of Chem. Reaction Engineering, Chalmers University of Technology.
    Franzén, C. J.
    Dept. of Chem. Reaction Engineering, Chalmers University of Technology.
    Taherzadeh, Mohammad J
    Dept. of Chem. Reaction Engineering, Chalmers University of Technology.
    Niklasson, C.
    Dept. of Chem. Reaction Engineering, Chalmers University of Technology.
    Lidén, G.
    Department of Chemical Engineering, Lund Institute of Technology.
    Effects of furfural on the respiratory metabolism of Saccharomyces cerevisiae in glucose-limited chemostats2003Ingår i: Applied and Environmental Microbiology, ISSN 0099-2240, E-ISSN 1098-5336, Vol. 69, nr 7, s. 4076-4086Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Effects of furfural on the aerobic metabolism of the yeast Saccharomyces cerevisiae were studied by performing chemostat experiments, and the kinetics of furfural conversion was analyzed by performing dynamic experiments. Furfural, an important inhibitor present in lignocellulosic hydrolysates, was shown to have an inhibitory effect on yeast cells growing respiratively which was much greater than the inhibitory effect previously observed for anaerobically growing yeast cells. The residual furfural concentration in the bioreactor was close to zero at all steady states obtained, and it was found that furfural was exclusively converted to furoic acid during respiratory growth. A metabolic flux analysis showed that furfural affected fluxes involved in energy metabolism. There was a 50% increase in the specific respiratory activity at the highest steady-state furfural conversion rate. Higher furfural conversion rates, obtained during pulse additions of furfural, resulted in respirofermentative metabolism, a decrease in the biomass yield, and formation of furfuryl alcohol in addition to furoic acid. Under anaerobic conditions, reduction of furfural partially replaced glycerol formation as a way to regenerate NAD+. At concentrations above the inlet concentration of furfural, which resulted in complete replacement of glycerol formation by furfuryl alcohol production, washout occurred. Similarly, when the maximum rate of oxidative conversion of furfural to furoic acid was exceeded aerobically, washout occurred. Thus, during both aerobic growth and anaerobic growth, the ability to tolerate furfural appears to be directly coupled to the ability to convert furfural to less inhibitory compounds.

  • 2.
    Westman, J
    et al.
    Högskolan i Borås, Institutionen Ingenjörshögskolan.
    Mapelli, V.
    Taherzadeh, M.J.
    Högskolan i Borås, Institutionen Ingenjörshögskolan.
    Franzen, C.J.
    Flocculation causes inhibitor tolerance in Saccharomyces cerevisiae for 2nd generation bioethanol production2014Ingår i: Applied and Environmental Microbiology, ISSN 0099-2240, E-ISSN 1098-5336, Vol. 80, nr 22Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Yeast has long been considered the microorganism of choice for second generation bioethanol production due to its fermentative capacity and ethanol tolerance. However, tolerance towards inhibitors derived from lignocellulosic materials is still an issue. Flocculating yeast strains often perform relatively well in inhibitory media, but inhibitor tolerance has never been clearly linked to the actual flocculation ability per se. In this study, variants of the flocculation gene FLO1 were transformed into the genome of the otherwise non-flocculating laboratory yeast strain Saccharomyces cerevisiae CEN.PK 113-7D. Three mutants with distinct differences in flocculation properties were isolated and characterised. The degree of flocculation and hydrophobicity of the cells were correlated to the length of the gene variant. The effect of different strength of flocculation on the fermentation performance of the strains was studied in defined medium with and without fermentation inhibitors as well as in media based on dilute acid spruce hydrolysate. Strong flocculation aided against the readily convertible inhibitor furfural, but not against less convertible inhibitors, such as carboxylic acids. During fermentation of dilute acid spruce hydrolysate, the most strongly flocculating mutant with dense cell flocs showed significantly faster sugar consumption. The modified strain with the weakest flocculation showed a hexose consumption profile similar to the non-transformed strain. These findings may explain why flocculation has evolved as a stress response, and can find application in fermentation-based biorefinery processes on lignocellulosic raw materials.

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