During wine fermentation, different strains of Saccharomyces cerevisiae compete in the same fermenting
must and dominance takes place when one strain overcomes all the others. The purpose of this study was
to investigate this phenomenon by identifying S. cerevisiae strains endowed with this feature and to test them
in laboratory fermentations. First, autochthonous S. cerevisiae from Nebbiolo fermentations were isolated, molecularly
identified and characterized. Genetically diverse S. cerevisiae strains were subsequently subjected to physiological
characterization and to micro-scale fermentation, the weight loss kinetics was measured and HPLC
analysis was performed at the end of the fermentation. Then, the strains that presented good fermentation characteristics
were chosen for further analysis and to determine the dominance feature. For this purpose, couples of
strains were co-inoculated in Nebbiolo must and the fermentations were monitored by microbiological and
chemical analysis. Two different inoculation approaches were used: co-fermentations in flasks with mixed
cells and reactor co-fermentations, in which the cells from the two different strains were kept separate by
means of a 0.45 μm filter membrane, which allowed the fermenting must to move freely between the two compartments.
During the flask co-fermentations, a minisatellite PCR protocol was applied, in order to differentiate
the two strains and determine which one was able to dominate. The protocol included a culture-dependent approach
and an independent one. In the first case, DNA extraction was performed on all the colonies scraped off
the plates after sampling. In the second case, DNA extraction was performed directly on the fermenting must.
The strains that were able to dominate were tested against several S. cerevisiae in order to confirm this dominance
behavior. Dominance was observed in the early stages of fermentation, as early as 3 days. Combinations
of dominant and not-dominant strains were subjected to further tests in a co-fermentation reactor system, in
order to perform single-strain analysis so as to obtain a better understanding of the dominance behavior. Surprisingly,
the results obtained in the flask co-fermentations were not confirmed. In fact, the two strains, one which
was hypothesized to be dominant and the other not-dominant, coexisted throughout the fermentation period.
The results of this study suggest that the dominant behavior of S. cerevisiae is only expressed when they sense
other yeasts in the same environment.