Also known as residual sweetness (RS), the amount of sugar in the wine that is retained by a natural end of fermentation or by deliberate stopping. The latter can be achieved in various ways such as cooling or by adding sulphur or alcohol. The residual sugar consists mainly of fructose (fruit sugar), because the glucose (grape sugar) is converted faster into alcohol and carbon dioxide, and to a small extent of non-fermentable sugars(pentoses). The remaining sugar content (dry to sweet) can be optionally stated on the label, as regulated by wine law. The phenomenon that the subjective perception of sweetness in wine, especially at high acidity levels, can differ relatively strongly from the actual analytical values is described under sweet.
For sparkling wines, different residual sugar values and, in some cases, different terms apply to sparkling wines than to still wines due to their carbon dioxide content. As a rule, a wine contains between 4 and 50 g/l of residual sugar, depending on the vinification or wine type. For certain sweet wines this can go up to 250 g/l (for Tokaj Eszencia even up to an unbelievable 450 g/l). By the way, the residual sugar is not broken down during the ageing or bottle maturing of a wine. Under ideal fermentation conditions (warm fermentation) a wine can ferment to a residual sugar content of 0.7 g/l. A complete fermentation to 0 g/l is not possible under natural conditions, as there are always residues of non-fermentable sugars remaining. Therefore, even a dry wine still contains a small amount of sugar. Wines with a content of less than 4 g/l are considered "completely fermented" (very dry). The content can also be increased by sweetening (addition of sugar in various forms) according to legal regulations.
Residual sugar can trigger unwanted bottle fermentation or form undesirable acetic acid and carbon dioxide. This is prevented by prior filtration (removal of yeast fungi) or pasteurisation. In a wine analysis the residual sugar in g/l is determined as reducing sugar. The sweetening of wine regulated by wine law is called residual sweetening. In contrast, the addition of sugar to increase the alcohol content is called enrichment. The procedures for determining the residual sugar content are listed under sugar content.
Regarding the other ingredients in wine, see under total extract. Complete lists of the many vinification measures or cellar techniques, as well as the various types of wine, sparkling wine and distillate regulated by wine law are included under the keyword vinification. Comprehensive information on wine law can be found under wine law.