German wine - wine basics
As soon as the newly harvested grapes arrive at the winery, they are pressed. The must, or juice, is then placed in barrels or stainless steel tanks and fermentation begins.
Fermentation is a natural process; grapes have yeast on their skins. When the yeast comes into contact with the natural sugar in the must, it reproduces rapidly and converts the sugar into carbon dioxide (CO2), which escapes, and alcohol.
Riper grapes with higher amounts of natural sugar therefore have the potential to produce a wine with more alcohol. On the other hand, if there is not enough natural sugar in the must, the alcohol will be too low and the wine will have little staying power and will taste out of balance.
When fermentation is completed, the wine is usually completely dry (trocken), because the yeast has converted all of the sugar into CO2 and alcohol. By volume the yeast is "exhausted" and no longer able to convert sugar into CO2 and alcohol.
But yeast has its limits if the must had a very high sugar content (very ripe or overripe grapes), once the amount of alcohol produced reaches 12-15%. Some of the original, natural sugar will remain in the wine - this is called residual sugar.