Definition - What does Yeast mean?
In the context of wine making, yeast are the single-celled fungus that are the catalyst for turning grape juice into wine through the process of fermentation, as they convert sugar in grape juice into alcohol and carbon dioxide. Saccharomyces cerevisiae is the most common yeast used to make wine, as it is tolerant of high alcohol levels and is reliable and active during fermentation.
WineFrog explains Yeast
Without yeast, there would be no wine. Yeasts from the strains Candida Genera and Kloeckera are found naturally on grapes and are often airborne as well. Yeast trigger fermentation and feed on the natural sugar in grape juice, converting it to alcohol and carbon dioxide. The natural yeasts found on wine grapes often trigger fermentation during harvest and as the grapes are crushed, but these natural yeasts are often not able to survive when the alcohol content goes up to over 5 % or in the presence of sulfur dioxide, which kills harmful bacteria and yeasts that can spoil the wine.
Wine makers use different strains of yeast that can tolerate both higher alcohol content and the presence of sulfur dioxide. Fermentation stops when the yeast have converted all the sugar in the wine, however winemakers often stop fermentation before that happens to leave residual sugar and sweetness in the wine. The yeast can be stopped through filtration or by dropping the temperature of the wine.