Residual Sugar in Wine: Sweetness Explained
Understand what residual sugar means in wine, how it affects sweetness and body, and how to identify dry, off-dry, and sweet wines using RS measurements.
Residual sugar is the unfermented grape sugar left in wine after fermentation stops. It determines a wine's sweetness level, from bone-dry (under 4 g/L) to sweet (over 45 g/L).
One of the most common misconceptions about wine is that “dry” means the opposite of “fruity.” In reality, a wine can taste intensely fruity yet be bone dry, or it can taste barely fruity yet contain significant sweetness. The key to understanding this distinction is residual sugar, a concept that clarifies how sweetness actually works in wine.
What Is Residual Sugar?
Residual sugar, commonly abbreviated as RS, refers to the natural grape sugars that remain in a finished wine after fermentation. During fermentation, yeast consumes grape sugar and converts it into alcohol and carbon dioxide. If fermentation runs to completion, meaning the yeast consumes all available sugar, the resulting wine will be dry with little to no residual sugar. If fermentation is stopped before all the sugar is consumed, the remaining sugar stays in the wine, making it taste sweet.
Residual sugar is measured in grams per liter (g/L). A bone-dry wine typically contains less than 4 g/L of residual sugar. An off-dry wine might have 4 to 12 g/L. A medium-sweet wine falls in the 12 to 45 g/L range. A sweet dessert wine can contain anywhere from 45 to over 400 g/L, as in the case of some botrytized wines like Sauternes or Tokaji Aszu.
Why Residual Sugar Matters
Understanding residual sugar helps you choose wines that match your taste preferences. Many people who say they do not like sweet wine actually enjoy wines with moderate residual sugar, they just do not realize it. Conversely, people who claim to only drink dry wine may be surprised to learn that some of their favorite wines contain measurable amounts of residual sugar.
This is because our perception of sweetness is strongly influenced by other factors, particularly acidity. A wine with 10 g/L of residual sugar and high acidity might taste nearly dry because the acid counterbalances the sugar. The same amount of sugar in a low-acid wine would taste noticeably sweet. This interplay between sugar and acid is one of the most important balance equations in winemaking.
Residual sugar also affects body and mouthfeel. Sugar adds viscosity and weight, making wines feel fuller and rounder on the palate. This is why even wines perceived as dry sometimes contain a small amount of residual sugar, typically 2 to 4 g/L, that adds body without creating a noticeable sweet taste.
For wine professionals and WSET students, assessing sweetness is a fundamental component of the Systematic Approach to Tasting. You need to identify whether a wine is dry, off-dry, medium-dry, medium-sweet, or sweet, and understand how the sweetness level relates to the wine’s overall balance and quality.
Residual Sugar in Practice
Winemakers control residual sugar primarily by managing fermentation. To make a dry wine, they allow fermentation to continue until the yeast has consumed all the sugar. To make a sweet wine, they stop fermentation early, either by chilling the wine to halt yeast activity, adding sulfur dioxide to kill the yeast, or filtering out the yeast. Some sweet wine producers use grape must (unfermented juice) that is so sugar-concentrated that the yeast dies naturally once the alcohol level reaches about 15 percent, leaving significant unfermented sugar behind.
Natural concentration methods also create high-sugar musts. Late harvest wines are made from grapes left on the vine past normal ripeness, allowing water to evaporate and sugar to concentrate. Noble rot (botrytis cinerea) shrivels grapes on the vine, creating intensely concentrated juice used for Sauternes, Tokaji, and some German Beerenauslese and Trockenbeerenauslese wines. Ice wine is made from grapes naturally frozen on the vine, where pressing the frozen fruit concentrates the sugars.
Riesling is perhaps the grape most associated with the full spectrum of residual sugar levels. German Riesling ranges from trocken (dry) through feinherb (off-dry) to Spatlese, Auslese, Beerenauslese, and Trockenbeerenauslese, each category representing progressively higher sugar levels at harvest. What makes Riesling special is its remarkably high acidity, which allows it to carry significant residual sugar without tasting cloyingly sweet.
Chenin Blanc from the Loire Valley demonstrates a similar range, from bone-dry Savennieres to lusciously sweet Coteaux du Layon and Bonnezeaux. South African Chenin Blanc also spans the sweetness spectrum, with many value-oriented bottlings containing subtle off-dry character.
When tasting, you can detect residual sugar by paying attention to a coating or syrupy texture on your palate after swallowing, and a sweet taste concentrated on the tip of your tongue. Compare this against the acidity to determine the overall balance. A wine where the sweetness and acidity are in harmony will feel balanced and refreshing regardless of its sugar level.
Explore with Sommo
Understanding residual sugar and sweetness levels is a core wine tasting skill. Sommo’s WSET study tools and structured tasting notes help you practice identifying sweetness, and scanning any bottle reveals its sweetness profile so you can always choose wines that match your preferences.
