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Dear Dr. Vinny,
Why is it that late-harvest or dessert wines often don't contain as much alcohol as a California Viognier or Australian Shiraz? Is it because the residual sugar is not fermented into alcohol? And if so, why doesn't that sugar convert rather than sticking with the wine?
—Jacob J., Boston
You've pretty much answered your own question: the residual sugar doesn't convert into alcohol, which is a conscious decision by a winemaker.
While yeasts are known for gobbling all the sugar in sight (the byproduct, as you know, is alcohol), this doesn't happen indefinitely. The yeast will eventually slow down, poop out and die. Once the alcohol gets over 15 or 16 percent, things start looking dicey for the yeast.
But when making dessert wine, a winemaker will stop the process before all the sugar is gone. Some ways to halt fermentation include chilling, filtering, or adding sulphur to a fermenting wine.
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