Designation for unfermented grape must that has been sterilised by sulphur dioxide or filtration. In cooler wine-growing areas, this is added to the already fermented wine in order to reduce the acidity and increase the residual sugar content. At best, the must is the same as that from which the wine was produced. For this reason, a portion is drawn off before fermentation begins, which must be sterile or completely free of yeast to prevent secondary fermentation. Before being added to the wine, it must be desulphurised.
However, the term sweet reserve is sometimes also used for the process of sweetening itself. The required quantity can be calculated using the so-called Cramer cross or (much simpler) using sweetening tables. In Austria, sweetening in any form (including by means of a sweet reserve) is prohibited at the Kabinett quality level and for all Prädikat wines. In Germany, the addition of sweet reserve is also permitted for Prädikat wines, but is rarely used.