The first step in producing sweet wine is obtaining grapes with high sugar levels. One way to do this is to simply leave the grapes on the vine until the sugar reaches the desired concentration. This method can be referred to as late harvest, vendange tardive, spätlese, auslese, etc., depending on where the wine is produced.
Another common method in harvesting grapes with high sugar levels is to allow a fungus called botrytis cinerea, or "noble rot," to attack the grapes. This fungus pokes holes in the skin of grapes, which allows water to evaporate and concentrates the sugars inside the berries. Botrytis adds a distinctive aroma and flavor to the wine and increases its viscosity. Botrytis is common in the production of Sauternes and the sweet Chenin Blanc wines of the Loire (Bonnezeaux, Quarts de Chaume, Vouvray Moelleux) amongst others.
A third way to concentrate the amount of sugar is to harvest after the grapes have frozen when temperatures in the vineyard drop below freezing. When the bunches are pressed, the sugar concentration in the must is quite high because all of the water in the grapes is frozen. This method is used in the production of Eiswein. This effect can be reproduced by freezing grapes, a process called cryoextraction.
For wines like Amarone, Vin Santo and Vin de Paille, grapes are slowly dried in order to concentrate sugars.
And finally, some winemakers raise sugar levels by simply adding sugar to the fermenting must (chaptalization), or through the use of technology such as reverse osmosis, vacuum evaporation or spinning cone technology. These solutions are typically used for the least expensive sweet wines.
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