Sweet white wines range from table wines with just a hint of sweetness to intensely sweet dessert wines. Knowing which white wine to choose can be a challenge, because many grape varieties produce both sweet and dry wines. The best method, of course, is to sample as many as possible until you find the right level of sweetness, but there are still a few general rules.
The Science of Sweetness
The heart of winemaking is fermentation, the process by which microorganisms turn the sugar from grapes into alcohol. In some wines, the yeast turns all the sugar into alcohol, producing dry wine. In other cases, the winemaker stops fermentation before it is complete, leaving some residual sugar in the wine. These wines are sweeter and typically -- although not always -- lower in alcohol content.
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Semisweet White Wines
Many semisweet or lightly sweet white wines come from grapes that also produce dry wines. For example, Chenin Blanc can be either dry or lightly sweet; most South African Chenin Blanc wines are drier. German riesling wines are usually slightly sweet; French or American wines made with the same grapes will often tend toward being dry. Other grapes such as Gewürztraminer or Moscato can produce wines that contain only small amounts of sugar but still taste sweet because of their fruity aromas and low levels of acidity.
Sweet White Wines
Extreme sweetness usually comes not from a particular type of grape but from the method of harvesting. For example, late harvest whites come from grapes left on the vine for a long period of time to build up sugars. One of the most famous sweet whites is Tokaji -- pronounced "tokay" -- a Hungarian white wine made from grapes affected by "noble rot." This microorganism attacks the grapes, dehydrating them; the resulting grapes have a much higher ratio of sugar to water. Other methods of dehydrating grapes include freezing: When water freezes out of grapes, a higher concentration of sugar ensues; the resulting wine is called ice wine. Drying grapes after harvesting to reduce their water content produces sweet dessert wines like vin santo.
Some very sweet wines are fortified -- the winemaker adds brandy or another distilled spirit to the wine to stop fermentation. The result is wine that retains much of its sugar but also contains a very high level of alcohol compared with other sweet wines. Examples of white fortified wines include sherry and white port.