Sweetness in wine is called residual sugar and is usually measured in grams per litre (g/L). Basically, residual sugar or ‘RS’ is the sugar from the grapes that’s left over after fermentation. The more residual sugar remaining in a wine, the sweeter the wine is. It is certainly not due to the addition of sugar to a wine as my picture may suggest!
When the winemaking process begins, yeast consumes the natural sugars (glucose and fructose) present in the grape juice, producing alcohol, as well as a number of other compounds contributing to a wine’s aroma and flavor, such as carbon dioxide, various flavor compounds, and other compounds, such as glycerol, that soften the mouthfeel of a wine when tasted.
It is impossible to get residual sugar levels down to absolute zero at the end of fermentation, and there will be small amounts of both glucose and fructose remaining in a dry wine. The residual sugar remaining in a dry wine is usually around 1 to 2 g/L.
Winemakers can also choose to stop yeast fermentation at any point, resulting in a sweeter tasting wine, with natural sugar levels above 2 g/L. For example, the juice from grapes picked at a sugar ripeness level of 13 baumé will contain a total of 250 g/L of sugars. If fermented dry, the alcohol level will be close to 13%, and the RS will be 1 to 2 g/L. If the winemaker chooses to stop the fermentation when the alcohol reaches 11%, the resulting wine will then contain around 40 g/L of RS. If the fermentation is stopped earlier again, say when the wine has 8% alcohol, there may be up to 100 g/L of RS.
Winemakers in Australia can also sweeten a wine by adding grape concentrate, which is certainly legal and perfectly OK to do. (Grape concentrate is a concentrated version of grape juice made by extracting water out of the juice).
There is a clear distinction that has to be made between ‘commercial’ or ‘commodity’ wines made in large volumes and sold at relatively low prices through supermarkets in Australia and overseas, and the higher priced, ‘premium’ wines. The RS levels of many of these cheaper wines (both white and red wines) are often between 4–10 g/L. Most people buying these wines would be unaware that part of the flavour and the softer mouthfeel that is appealing in these wines, comes from the fact that the wines are sweeter. In comparison, the RS levels of many of the higher priced barrel fermented white wines and premium quality red wines are 1 – 2 g/L.
To sum up the varying levels of RS in a wine, and how it is ranked in regards to it’s sweetness level:
Dry 1 – 2 g/L
Off Dry 5 – 10 g/L
Semi Sweet 11 – 30 g/L
Sweet Greater than 30 g/L
This has to be one of the best, graphical presentations that I have found on RS levels in wine. Thank-you to Wine Folly! (www.winefolly.com)