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yeast - A single-celled organism, part of the plant kingdom, that plays a critical role in the making of wine by processing sugar into alcohol during fermentation. When fermentation is complete, the yeast typically dies and settles to the bottom of the fermentation vessel as part of the lees. Some strains of yeast die when the alcohol content reaches a certain level, terminating fermentation. Along with alcohol the yeast produce carbon dioxide, which for most wines is allowed to escape during fermentation, but which is trapped in the bottle during the secondary fermentation that produces fine sparkling wine. Saccharomyces cerevisiae is the botanical name of the strain of yeast most often used for wine and beer making, and it wasn't until 1857 that it was discovered by French botanist Louis Pasteur that yeast was responsible for fermentation. Though wild strains of yeast are prevalent -- their microscopic spores float in the air -- winemakers now frequently use yeast specifically cultured for winemaking. See also autolysis, brettanomyces, flor and stuck fermentation.

yeast autolysis - The breakdown of yeast during aging on the lees, in which compounds are released that heighten the sensory qualities of the wine and increase its complexity.

yeasty - A descriptive term for a wine with the bready aroma of yeast, usually the result of having been aged on the lees, (see sur lie). It is not an undesirable quality in these wines, but can be a flaw in other wines.

yema - The first must juice, obtained by straining not pressing, without any pressure at all.

yield - The measurement of how productive a vineyard is.

young wine - Wine which has not undergone an ageing process and which is usually marketed within its first year.