Dear Susan:

Sweet wines, but not dry wines, have what is called residual sugar—natural sugar from inside the grapes that was never fermented (by yeasts) into alcohol. So it remains in the wine and makes that wine sweet. Residual sugar does not go away exactly. But over a long time—15 or more years—the sugars begin to transform molecularly. The result is that older sweet wines begin to seem less sweet as they age. Some very old sweet German Rieslings, or very old Sauternes, for example, do not taste really sweet at all.