Kinetics of D-glucose and D-fructose conversion during the alcoholic fermentation promoted by Saccharomyces cerevisiae

Although many studies on the different aspects of alcoholic fermentation are available in the literature, it is still difficult to identify the possible causes of the slowing-down or stuck of fermentations, even if the change of some compositional parameters (D-glucose/D-fructose and glycerine produced/hexoses converted ratios) could be assumed as sound signals of a possible deviation from the usual Saccharomyces metabolic pathways. The reason why alcoholic yeasts preferably metabolise D-glucose rather than D-fructose was investigated by a kinetic model based on six functional parameters having a well-defined chemical-physical meaning. The time evolution of different initial concentrations of D-glucose and D-fructose, dissolved in a model solution simulating a must (citrate buffer at pH = 3.4 inoculated by a commercial strain of S. cerevisiae), was investigated adding or not ethanol to the reaction medium. When a reduced amount of ethanol was dissolved in the reaction medium, the time evolution of the fermentation rates of these two sugars did not differ significantly, to diversify rather strongly when the alcoholic concentration increased. The hypothesised mathematical model accounts for this particular kinetic behaviour. In fact, only the sensitivity to ethanol showed by the enzymatic protein involved in the limiting steps of the fermentation process of these two sugars differed significantly, the enzymatic transformation of D-fructose being more sensitive to ethanol than D-glucose. This difference was able to justify the different kinetic behaviours shown by the two sugars when ethanol concentration in the reaction medium increased.