Abstract:Yeasts involved in veil formation during biological ageing of Sherry wines are mainly Saccharomyces cerevisiae, and they have traditionally been divided into four races or varieties: beticus, cheresiensis, montuliensis and rouxii. Recent progress in molecular biology has led to the development of several techniques for yeast identification, based on similarity or dissimilarity of DNA, RNA or proteins. In view of the latest yeast taxonomy, there are no more races. However, molecular techniques are not enough to understand the real veil-forming yeast diversity and dynamics in Sherry wines. We propose a reliable method, using a microtiter reader, to evaluate the fermentation and assimilation of carbon and nitrogen sources, the osmotolerance and the antibiotic resistance, using 18 S. cerevisiae and 5 non-Saccharomyces yeast strains, to allow correct identification and classification of the yeast strains present in the velum of flor complex.
Brewery worts of 16, 18, and 20° P were fermented with two yeast strains (J-3015 and J-2036), at three temperature programs (initial temperature of 10°C and maximum temperatures of 16, 18, and 20° C). The attenuation curves were normal with the three wort gravities. The pitching rate had to be increased proportionally to the increase in gravity. The percent of dead cells in the yeast collected after each fermentation was proportional to the gravity of the wort, and this effect was more pronounced when the yeasts were in contact with beers with an alcohol content above 4% by weight at high temperatures (18-20°C). The collected yeast was used in subsequent fermentations, adjusting the pitching rate to compensate for the percent of dead cells. The total vicinal diketone (VDK) curves were different, with strain J-3015 producing higher levels of VDK precursors-during fermentation when a maximum temperature of 16° C was used. Both strains showed a tendency to produce lower levels of total VDK as the maximum temperature of fermentation was raised to 20° C.
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