The amount of sugar that has developed in the grapes' juice, or must, is an important indication of ripeness. This is referred to as «must weight» and it is measured on a hydrometer, a thermometer-like device which has a graduated scale that compares the specific gravity of the must to the specific gravity of water at 20o C/68o F. This measurement is expressed in degrees Oechsle in Germany (comparable measurements in other countries include Babo, Balling, Baumé and Brix). From this rating, the potential alcohol level of the finished wine can be determined - which in turn, is an important criterion for quality category classification, i.e. to qualify for a specific quality category, grapes have to reach a minimum starting must weight. The table below shows the range of minimum starting must weights correlated to the various quality categories (= ripeness levels).
Geological and climatic factors influence growing conditions for grapes, and furthermore, among individual grape varieties, there are differences as to how early or late they generally ripen as well as how much sugar they generally tend to develop. As a result, the minimum starting must weights vary according to region and grape variety in order to ensure an overall, comparable standard of quality regardless of these natural differences. Quality-oriented wine-growers strive to achieve higher starting must weights than required by law.
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