Science
Sugar in wine: from grape to residual sweetness
Where grape sugar comes from, what happens to it in fermentation, and why residual sugar and perceived sweetness are two different things.
The short answer
Grapes accumulate glucose and fructose during ripening. Yeast consume most of it during fermentation, and whatever remains when fermentation stops or is stopped is residual sugar. How sweet that tastes depends heavily on the acidity, tannin and alcohol alongside it.
Overview
Sugar is the raw material of the whole process. Alcohol content is essentially a record of how much sugar was in the grapes and how much of it the yeast got through.
A dry wine is not a wine with no sugar. Fermentation typically leaves a couple of grams per litre that yeast cannot metabolise, and that is well below what anyone would notice.
Sweetness is also the element readers most often misidentify, because intensely fruity dry wines are regularly described as sweet.
How it works
The mechanism, at the scope it has actually been established.
Ripening grapes accumulate roughly equal quantities of glucose and fructose. Yeast prefer glucose, so the sugar remaining late in a fermentation is disproportionately fructose — which is relevant because fructose tastes sweeter than glucose at the same concentration.
Fermentation stops for one of several reasons: the sugar runs out; the alcohol reaches a level the yeast cannot tolerate; the temperature is dropped and the yeast filtered out; or spirit is added, as in Port.
Any of the deliberate interventions leaves residual sugar. So does starting with so much sugar — from botrytis, drying or freezing — that the yeast cannot finish it.
Perceived sweetness is then modified by everything else in the glass. Acidity suppresses it strongly, which is why a Mosel Kabinett with 40 g/L of sugar can taste merely off-dry. Tannin and bitterness suppress it too; alcohol and fruit intensity can enhance the impression of it.
Why it matters in the glass
It is why the sweetness on a label and the sweetness in the mouth are different questions, and why WineHQ’s sweetness tools work from balance rather than from a sugar figure alone.
It is also why "I do not like sweet wine" is so often really "I do not like low-acid wine".
What people get wrong
2 widely repeated claims this page corrects.
Not true
“Chaptalisation is adding sugar, so chaptalised wine is sweeter.”
Sugar added before or during fermentation is consumed by the yeast and converted to alcohol. It raises the strength, not the sweetness.
Not true
“Riesling is a sweet wine.”
Most Riesling made today is dry. Germany’s own production is majority dry, and Alsace, Austria and Australia make it dry almost exclusively.
Perceived sweetness is not predictable from residual sugar alone. Acidity, tannin, alcohol and carbon dioxide all modify it, and no published formula converts a sugar figure into what a taster will report.
Go shallower or deeper
The beginner page and the chemistry page are both correct, and neither substitutes for the other.
Sources
Sources consulted
- The Australian Wine Research Institute — AWRI
- International Code of Oenological Practices — OIV
- Research on sweetness, acidity and cross-modal suppression — Peer-reviewed sensory science literature
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