Quantifying the Effects of Ethanol and Temperature on the Fitness Advantage of Predominant Saccharomyces cerevisiae Strains Occurring in Spontaneous Wine Fermentations

Abstract

Different Saccharomyces cerevisiae strains are simultaneously or in succession involved in spontaneous wine fermentations. In general, few strains occur at percentages higher than 50% of the total yeast isolates (predominant strains), while a variable number of other strains are present at percentages much lower (secondary strains). Since S. cerevisiae strains participating in alcoholic fermentations may differently affect the chemical and sensory qualities of resulting wines, it is of great importance to assess whether the predominant strains possess a "dominant character." Therefore, the aim of this study was to investigate whether the predominance of some S. cerevisiae strains results from a better adaptation capability (fitness advantage) to the main stress factors of oenological interest: ethanol and temperature. Predominant and secondary S. cerevisiae strains from different wineries were used to evaluate the individual effect of increasing ethanol concentrations (0-3-5 and 7% v/v) as well as the combined effects of different ethanol concentrations (0-3-5 and 7% v/v) at different temperature (25-30 and 35°C) on yeast growth. For all the assays, the lag phase period, the maximum specific growth rate (μ_max) and the maximum cell densities were estimated. In addition, the fitness advantage between the predominant and secondary strains was calculated. The findings pointed out that all the predominant strains showed significantly higher μ_max and/or lower lag phase values at all tested conditions. Hence, S. cerevisiae strains that occur at higher percentages in spontaneous alcoholic fermentations are more competitive, possibly because of their higher capability to fit the progressively changing environmental conditions in terms of ethanol concentrations and temperature.

Keywords: Saccharomyces cerevisiae strains; ethanol; fitness advantage; spontaneous wine fermentation; temperature.

Figure 1

Dendrogram from UPGMA clustering analysis, based on Dice coefficient of mt-DNA RsaI restriction patterns of the S. cerevisiae isolates from 32 spontaneous wine fermentations carried out in six different wineries (A, B, C, D, E, and F) in Tuscany (Italy). S1-S6 indicate commercial starter cultures. Arabic numerals at 60% similarity indicate the different clusters.

Figure 2

Isolation frequencies of one “high frequency”(HF)-S. cerevisiae strain and one “low frequency ” (LF) S. cerevisiae strain, representative of each winery (A, B, C, D, E and F) after 24 h and 10 days in co-fermentations in synthetic must at 28°C. The “HF” and “LF” strains were inoculated at the same cell concentration (10⁴ cell/mL).

Figure 3

Fitness advantage at different ethanol concentrations calculated for each pair of HF/LF-S. cerevisiae strains from the six different wineries (A, B, C, D, E, and F) considering the average growth rate calculated between 0 and 8 h in synthetic medium at 28°C.

Figure 4

Fitness advantage at different concentrations of ethanol and temperatures calculated for each pair of HF/LF-S. cerevisiae strains from the six different wineries (A, B, C, D, E, and F), considering the average growth rate calculated between 0 and 8 h in synthetic medium.

Figure 5

Theoretical time required by HF-S. cerevisiae strains to dominate on LF-S. cerevisiae strains in the six wineries studied (A, B, C, D, E, and F).