The must weight already reveals something about the expected quality of the wine. It is measured in degrees Oechsle (abbreviated ° Oe); named after its inventor, the Württemberg pharmacist and mechanic Ferdinand Oechsle. Other terms for comparable measurements include Balling, Baumé in France, Brix in the New World and Klosterneuburger Mostwaage (KMW) in Austria.
The must weight denotes the weight ratio of one liter of must to one liter of water at 20 ° C, i.e. the density or the specific weight of a must. For example, if the weight ratio is 1.076, it is a must with 76 degrees Oechsle. The must weight primarily results from the sugar content, from which the later alcohol content in the wine can be derived. For example, a must with 90 ° Oe contains 209 grams of sugar per liter, which after fermentation gives 12 to 13 % alcohol by volume in the wine.
In ripe berries that have been perforated by the ‘noble rot’ fungus, Botrytis cinerea, the water evaporates when sunny. This further concentrates the existing sugar content. In Beerenauslese musts with 120 ° Oe or higher, sugar concentrations of over 300 grams per liter are achieved.
As the grapes ripen, the sugar content in the berries increases, and with it the must weight in the juice. The higher the must weight, the higher the quality or Prädikat level of the late-ripening wine.
Geological and climatic factors influence growing conditions for grapes, and furthermore, individual grape varieties ripen earlier or later and develop different amounts of sugar. As a result, the minimum starting must weights to ensure a comparable standard for quality wine varies according to region and grape variety. Quality-oriented wine-growers tend to prefer higher starting must weights than required by law.