Browsing by Author "Thevelein, J. M."

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    Correlation between glucose/fructose discrepancy and hexokinase kinetic properties in different Saccharomyces cerevisiae wine yeast strains
    (2008) Berthels, N. J.; Cordero Otero, R. R.; Bauer, Florian; Pretorius, I. S.; Thevelein, J. M.
    Grape juice contains about equal amounts of glucose and fructose, but wine strains of Saccharomyces cerevisiae ferment glucose slightly faster than fructose, leading to fructose concentrations that exceed glucose concentrations in the fermenting must. A high fructose/glucose ratio may contribute to sluggish and stuck fermentations, a major problem in the global wine industry. We evaluated wine yeast strains with different glucose and fructose consumption rates to show that a lower glucose preference correlates with a higher fructose/glucose phosphorylation ratio in cell extracts and a lower K m for both sugars. Hxk1 has a threefold higher V max with fructose than with glucose, whereas Hxk2 has only a slightly higher V max with glucose than with fructose. Overexpression of HXK1 in a laboratory strain of S. cerevisiae (W303-1A) accelerated fructose consumption more than glucose consumption, but overexpression in a wine yeast strain (VIN13) reduced fructose consumption less than glucose consumption. Results with laboratory strains expressing a single kinase showed that total hexokinase activity is inversely correlated with the glucose/fructose (G/F) discrepancy. The latter has been defined as the difference between the rate of glucose and fructose fermentation. We conclude that the G/F discrepancy in wine yeast strains correlates with the kinetic properties of hexokinase-mediated sugar phosphorylation. A higher fructose/glucose phosphorylation ratio and a lower K m might serve as markers in selection and breeding of wine yeast strains with a lower tendency for sluggish fructose fermentation. © 2007 Springer-Verlag.
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    Discrepancy in glucose and fructose utilisation during fermentation by Saccharomyces cerevisiae wine yeast strains
    (2004) Berthels, N. J.; Cordero Otero, R. R.; Bauer, Florian; Thevelein, J. M.; Pretorius, I. S.
    While unfermented grape must contains approximately equal amounts of the two hexoses glucose and fructose, wine producers worldwide often have to contend with high residual fructose levels (>2 gl-1) that may account for undesirable sweetness in finished dry wine. Here, we investigate the fermentation kinetics of glucose and fructose and the influence of certain environmental parameters on hexose utilisation by wine yeast. Seventeen Saccharomyces cerevisiae strains, including commercial wine yeast strains, were evaluated in laboratory-scale wine fermentations using natural Colombard grape must that contained similar amounts of glucose and fructose (approximately 110 gl-1 each). All strains showed preference for glucose, but to varying degrees. The discrepancy between glucose and fructose utilisation increased during the course of fermentation in a strain-dependent manner. We ranked the S. cerevisiae strains according to their rate of increase in GF discrepancy and we showed that this rate of increase is not correlated with the fermentation capacity of the strains. We also investigated the effect of ethanol and nitrogen addition on hexose utilisation during wine fermentation in both natural and synthetic grape must. Addition of ethanol had a stronger inhibitory effect on fructose than on glucose utilisation. Supplementation of must with assimilable nitrogen stimulated fructose utilisation more than glucose utilisation. These results show that the discrepancy between glucose and fructose utilisation during fermentation is not a fixed parameter but is dependent on the inherent properties of the yeast strain and on the external conditions. © 2004 Published by Elsevier B.V. on behalf of the Federation of European Microbiological Societies.