Residual sugar

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Residual sugar (or RS) is the measure of the amount of sugars that remain unfermented in the finished wine.

Residual sugar is usually measured in grams of sugar per litre of wine. Even among the driest wines, it is rare to find wines with a level of less than 1 g/L, due to the unfermentability of certain types of sugars, such as pentose. By contrast, any wine with over 45 g/L would be considered sweet, though many of the great sweet wines have levels much higher than this. For example, the great vintages of Château d'Yquem contain between 100 and 150 g/L of residual sugar. The sweetest form of the Hungarian Tokaji wine - Eszencia - contains over 450 g/L, with exceptional vintages registering 900 g/L. Such wines only avoid the cloying taste associated with such elevated levels of sugar by carefully developed use of acidity. This means that the finest sweet wines are made with grape varieties that keep their acidity even at very high ripeness levels, such as Riesling and Chenin Blanc.

Residual sugar typically refers to the sugar remaining after fermentation stops, but it can also result from the addition of unfermented must (a technique practiced in Germany and known as süssreserve) or ordinary table sugar. The latter technique is generally used on low quality, often non-grape wines, and in many winemaking regions is banned altogether.