Residual Sugar

How residual sugar is measured

Residual sugar is measured chemically, usually by a laboratory test that detects the unfermented glucose and fructose in the wine. The standard unit is grams per litre (g/l). A wine with less than 4 g/l reads as bone dry to most tasters. A wine at 8 to 12 g/l reads as dry but with a soft, almost imperceptible sweetness. Above 20 g/l the wine starts to taste noticeably sweet, and above 50 g/l it is firmly in dessert wine territory.

The same wine can be finished at different sugar levels by the winemaker's choice of fermentation endpoint, by blending with a sweeter component, or by dosage. A sparkling wine, for example, has its sugar level set at the dosage stage, just before the final cork is inserted.

RS and perceived sweetness

Perceived sweetness depends on more than just the sugar number. Acidity cuts sweetness, so a 10 g/l wine with high acidity can taste drier than a 6 g/l wine with low acidity. Tannin, alcohol and carbonation all modify the perception, which is why a fizzy wine with the same sugar as a still wine often tastes drier. Body and texture also matter: a thicker, more glycerol-rich wine can mask sugar more effectively than a thin, light wine.

Residual sugar in the UK

English still wines are usually finished dry, in the 0 to 4 g/l range. English sparkling wines are usually in the Brut to Extra Brut range, at 0 to 12 g/l, with a growing band of Brut Nature at less than 3 g/l. The UK consumer has shifted firmly towards drier styles over the last decade, and the major English houses have responded.

The exceptions are English dessert wines, where the residual sugar is the whole point. Botrytised wines from Chapel Down, Nyetimber, Three Choirs and a handful of others sit at 80 to 200 g/l, in the Sauternes or Tokaji range. Late-harvest wines and the small ice wine experiments at a few estates can go even higher.

Sugar on the label

UK and EU rules require the dryness descriptor on sparkling wine labels, which is effectively a sugar category. Still wines do not have to declare residual sugar, but the technical sheet that follows the wine through the supply chain will usually list it. Consumers can find the RS for most wines on the producer's website or by request.

For sparkling wines, the rules require the descriptor to match the actual sugar level. A wine labelled Brut must contain less than 12 g/l. A wine labelled Brut Nature must contain less than 3 g/l. Mis-labelling is a compliance issue and a trading standards matter.

Sugar in the cellar

The winemaker controls residual sugar in several ways. The most common is to stop the fermentation before all the sugar is gone, by chilling the wine and sulphuring it. This is the standard approach for white wines with a touch of residual sugar. Another approach is to ferment completely dry and then back-blend with a small amount of unfermented or partially fermented juice, called süssreserve in German.

For sparkling wine, the sugar is set at the dosage, by adding a measured amount of liqueur d'expédition. For very sweet wines, the sugar is concentrated in the grape, either by drying the grapes on straw mats (the appassimento method used for Recioto) or by allowing noble rot to desiccate them (the Sauternes and Tokaji method). In the UK, the most common technique is to stop the fermentation with a touch of sulphur and let the natural sugar remain.