The content of sugar as the main component in the grapes is expressed by must weight in KMW (Austria) or Oechsle (Germany). In the two countries, the sugar content in the berries is, so to speak, a criterion for the predicate wine grades. The sugar in the grape must is the basis for fermentation, during which a conversion to alcohol and carbon dioxide takes place. Above a certain alcohol content, the yeasts die off and stop fermentation.
The upper limit of the possible alcohol content is about 16 to 18%, when using turbo yeasts during fermentation up to about 20% vol. The associated lower limit of residual sugar in the wine is usually about 0.1 to 0.2 g/l. There is no wine that contains no sugar at all. The phenomenon that the subjective perception of sweetness in wine can differ greatly from the analytical values depending on the amount of other substances is described under the keyword sweet.
Sweetness grades Still wine
In accordance with EU regulations or, in some cases, country-specific wine legislation, the indication of the designation for a certain residual sugar content in the wine on the bottle label is optional. However, Austria has made use of the right to specify this as an obligatory indication. The terms and quantities for still wines in Germany and Austria are shown in the following table, whereby the wine law relevant terms only begin with the line "dry". There is a tolerance limit because there may be uncertainties depending on the laboratory and measurement method. The sugar content may not deviate by more than 1 g/l from the information on the label:
In Austria, extra dry was used as the lowest level for still wine until the country joined the EU in 1995. Despite support from other countries such as Germany, however, the desired EU-wide regulation could not be implemented. However, the term is used for sparkling wine (12 to 17 g/l). In the opinion of many experts, a lowest wine law level up to a maximum of 4 g/l would be sensible, as the range up to 9 g/l is too wide and the complicated definition is confusing for the consumer. In addition, there are the purely colloquially usual, but wine-law irrelevant terms herb, noble sweet, natural sweet, fruit sweet, peck-sweet and sweetish.
Sweetness grades Sparkling wine
For sparkling wine, there are other names and also other values regarding the sugar content. The perception threshold for the recognition of sugar is already given with 3 g/l for still wines, but only with about 15 g/l for sparkling wines. This means that sparkling wine is carbonated, which means that much more sugar has to be added to produce the same degree of sweetness and the quantities must therefore be much higher than for still wine in order to have the same taste sensation (see in detail under Sparkling Wine):
Determination of the residual sugar
The content of unfermented sugar in wine(residual sugar) can only be determined with sufficient accuracy using certain analytical methods or standardised procedures and measuring instruments. Measurement by means of the residual sugar spindle is based on the hydrometric principle(relative density) and provides only inaccurate results. If the Official Test Number (Germany) and the State Test Number (Austria) are assigned to quality wines, this is done during the analytical test. The following methods exist, but some of them are also used to determine other wine ingredients:
- FTIR spectrometer - physical infrared measurement
- Luff-Schorl method (similar to Rebelein) - chemical with citric acid, sodium carbonate
- Rebelein method (Fehling-Rebelein) - chemical with copper sulphate, sodium bicarbonate, Seignette salt
- Reflectometer (e.g. RQflex) - optical instrument for rapid analysis
further information
For the sugar content of grapes or grape must, see must weight and maturation. With regard to wine ingredients, see total extract and analytical testing.
Picture: from PIXABAY