Residual sugar (or RS) refers to the sugars left unfermented in a finished wine. It is measured by grams of sugar per litre (g/l).
The amount of residual sugar affects a wine’s sweetness and, in the EU, the RS level is linked to specific labelling terms. For example, a wine containing more than 45 g/l is a ‘sweet wine’.
It’s rare for a wine to drop below 1 g/l, because some types of sugar simply cannot be consumed by the yeast.
On the other hand, at least 150g/l of residual sugar is required for a ‘six puttonyos’ Tokaji sweet wine.
And, while it isn’t the fashion today, some Champagnes from the 19th century have been found to contain 150g/l of residual sugar.
These days, labelling terms for still wines in the EU include:
Up to 4g/l Dry / Sec
4 g/l – 12 g/l Medium dry / demi-sec
12 g/l – 45 g/l Medium (Medium sweet)
More than 45 g/l Sweet / Doux
For sparkling wines, the labelling terms are regulated as follows:
Up to 3 g/l Brut Nature
Up to 6 g/l Extra Brut
Up to 12 g/l Brut
12 g/l–17 g/l Extra Dry / Extra Sec
17 g/l –32 g/l Dry / Sec
32 g/l –50 g/l Demi-sec
More than 50 g/l Sweet / Doux
It’s worth noting that besides residual sugar, other elements of wine could also affect people’s perception of sweetness.
For instance, high acidity gives freshness and a sense of lightness, even if the wine has a considerable amount of residual sugar.
In fact, the upper limit of residual sugar for ‘dry’ and ‘medium dry’ still wines can be raised to 9g/l and 18 g/l, if there is the appropriate level of acidity to balance.
Other factors, such as oak, fruit flavours, alcohol levels and serving temperature, can also have an impact on the perceived sweetness of a wine.
Sometimes they can trick drinkers by bringing the perception of sweetness to a wine with a very low RS level.
Making sweet wines
There are many reasons why extra sugars are ‘left over’ in wine without being converted into alcohol.
For example, sometimes the grape juice has such high levels of sugar that it cannot be fully turned into alcohol. This is mainly because the accumulated alcohol can eventually impede the activities of the yeast.
Typical sweet wines made using juice with a high sugar concentration include:
- Sauternes (noble rot)
- Icewine (frozen grapes)
- Late harvest sweet wines, like Recioto della Valpolicella, produced from dried grapes.
Winemakers can also choose to terminate the fermentation before the sugar has been used up.
This can be done by cooling down the ferment then filtering out the yeast, such as for Moscato d’Asti, or by adding grape spirits or sulphites to kill the yeast, such as for Ruby Port and ‘vin doux naturels’.
At a certain level, residual sugar may help a sharp, acidic wine to taste mellower and a plain wine more flavoursome.
Some everyday wines are made by simply adding sweeteners, such as preserved grape juice, to dry wines before bottling to make them more palatable.
However, residual sugar can also be a dangerous enemy to the stability of a still wine, because it may trigger re-fermentation in the bottle. Microbes may feed on the sugars left in the wine and generate unwanted flavours and gas.
Therefore, it is essential for producers to either completely eliminate the fermentable sugar in the wine, or get rid of the yeast via sterile filtration at the point of bottling.