Exclusion of Saccharomyces kudriavzevii from a wine model system mediated by Saccharomyces cerevisiae.

This study investigated the competition and potential hybrid generation between the species Saccharomyces cerevisiae and S. kudriavzevii in a wine-model environment. Our main goal was to understand why S. kudriavzevii has not been found in wine fermentations whilst their hybrids are present. Auxotrophic mutants (Ura(-) and Lys(-)) were used to favour the selection of hybrids and to specifically differentiate the two species in mixed fermentations carried out at different temperatures (17 °C, 24 °C and 31 °C). Both yeasts showed a reduction in their maximum specific growth rates in mixed fermentations, indicating a clear antagonistic effect between the two microorganisms. Temperature played an important role in this competition. In this way, S. kudriavzevii was less affected at 17 °C, but S. cerevisiae was clearly the best competitor at 31 °C, preventing the growth of S. kudriavzevii. Population levels of S. kudriavzevii always significantly decreased in the presence of S. cerevisiae. Ethanol was measured throughout the fermentations and in all cases S. kudriavzevii growth was arrested when ethanol levels were < 5 g/l, indicating that this compound did not influence the competitive exclusion of S. kudriavzevii. Killer factors were also discarded due to the K(-) R(-) phenotype of both strains. Finally, no prototrophic interspecific hybrids were isolated in small-scale fermentations at any temperature assayed. Our results show that the lack of competitiveness exhibited by S. kudriavzevii, especially at high temperatures, explains the absence of this species in wine fermentations, suggesting that natural S. cerevisiae × S. kudriavzevii hybrids most likely originated in wild environments rather than in industrial fermentations.

Modulation of the glycerol and ethanol syntheses in the yeast Saccharomyces kudriavzevii differs from that exhibited by Saccharomyces cerevisiae and their hybrid.

In the last years there is an increasing demand to produce wines with higher glycerol levels and lower ethanol contents. The production of these compounds by yeasts is influenced by many environmental variables, and could be controlled by the choice of optimized cultivation conditions. The present work studies, in a wine model system, the effects of temperature, pH and sugar concentration on the glycerol and ethanol syntheses by yeasts Saccharomyces cerevisiae T73, the type strain of Saccharomyces kudriavzevii IFO 1802(T), and an interspecific hybrid between both species (W27), which was accomplished by the application of response surface methodology based in a central composite circumscribed design. Results show that carbon flux could be especially directed towards glycerol synthesis instead of ethanol at low pH, high sugar concentrations and low temperatures. In general, the non-wine yeast S. kudriavzevii produced higher glycerol levels and lower ethanol content than wine strains S. cerevisiae T73 and the hybrid W27, with specific and different glycerol production profiles as a function of temperature and pH. These results were congruent with the higher glycerol-3-phosphate dehydrogenase activities estimated for this species, chiefly at low temperatures (14 degrees C), which could explain why S. kudriavzevii is a cryotolerant yeast compared to S. cerevisiae.

Application of a substrate inhibition model to estimate the effect of fructose concentration on the growth of diverse Saccharomyces cerevisiae strains.

In this study, we performed an analysis of the ability of four Saccharomyces cerevisiae and one S. bayanus var. uvarum strains, isolated from different industrial processes, to ferment increasing amounts of fructose (from 0 to 70%, w/v). Overall yeast growth was estimated by integration of the area under optical density vs. time curves. Subsequently, this parameter was modeled by means of a substrate inhibition model. All strains showed a similar behavior against fructose concentration in spite of their different origins, but with slight differences. The optimum fructose concentrations to stimulate yeast growth were obtained between 4.33 and 6.05%, while the maximum concentrations above which growth was completely inhibited were attained between 59.56 and 63.85%. Statistically, model parameters calculated for wine yeast strains were significantly different than those obtained for yeasts from Agave and table olive fermentations, except for the maximum inhibitory concentration. The methodology used in this work could be useful for the industry in general as a first procedure to select yeast strains with higher fructose preferences or tolerances, and especially for winemakers, where the risk of spoilage increases by the presence of a marked residual fructose concentration in the finished wine.

Effects of temperature, pH and sugar concentration on the growth parameters of Saccharomyces cerevisiae, S. kudriavzevii and their interspecific hybrid.

The effects of temperature, pH and sugar concentration (50% glucose+50% fructose) on the growth parameters of Saccharomyces cerevisiae T73, S. kudriavzevii IFO 1802(T) and the hybrid strain S. cerevisiae x S. kudriavzevii W27 were studied by means of response surface methodology based in a central composite circumscribed design. Lag phase could not be properly modelled in the wine model system, where yeasts started the fermentation in few hours after inoculation. In the case of the maximum specific growth rate (micro(max)), the temperature was the most important variable for three yeasts, although the effects of sugar concentration (in T73 and W27) and pH (W27 and 1802) were also significant (p<0.05). The only retained interaction was between the variables temperature and pH for yeast 1802. The polynomial equations built for micro(max) were used both to assess the behaviour of yeasts as a function of the factors and to predict their growth. In the case of temperature, the profiles obtained by the equations showed that response of the hybrid W27 was similar to T73 and different to 1802. When pH was the factor under study, the response of the hybrid W27 was closer to 1802 than yeast T73. For sugar concentration, the response of the hybrid W27 was similar to T73 but different to 1802. To the best of our knowledge, this is the first time that predictive models are used to assess and compare the response of a hybrid strain with respect to its parental species. The information obtained could also be useful to estimate the possible effect of climatic change on yeast growth.