The yeast cells turn delicious, sweet grape juice into wine. They eat sugar and produce alcohol as a by-product. Many vineyards like to use indigenous yeasts—those found on the grape skins themselves—as each vintage can bring a different combination of yeast strains, which may allow for more distinctive and complex wines. Most wineries, though, use cultivated, or commercial yeasts.
There is a distinction, however: As some wineries find that a specific yeast that was indigenous to their vines produced a better than average result, the winery may begin to cultivate the yeast. Most wineries that use truly commercial yeasts use them because they are efficient and tolerant of high-alcohol environments (aka the Achilles' heel of many indigenous yeasts; it can lead to what are known as stuck, or incomplete fermentations.)
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