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There is no simple definition of a dessert wine. In the UK, a dessert wine is considered to be any sweet wine drunk with a meal, as opposed to the white fortified wines (fino and amontillado sherry) drunk before the meal, and the red fortified wines (port and madeira) drunk after it. Thus, most fortified wines are regarded as distinct from dessert wines, but some of the less strong fortified white wines, such as Pedro Ximénez sherry and Muscat de Beaumes-de-Venise, are regarded as honorary dessert wines. In the United States, by contrast, a dessert wine is legally defined as any wine over 14% alcohol by volume, which includes all fortified wines – and is taxed more highly as a result. This dates back to a time when the US wine industry only made dessert wines by fortification, but such a classification is outdated now that modern yeast and viticulture can produce dry wines over 15% without fortification, yet German dessert wines can contain half that amount of alcohol. Examples include Sauternes and Tokaji Aszú.
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In the absence of other techniques, makers of dessert wine have to produce their sugar in the vineyard. Some grape varieties, such as Muscat, Ortega and Huxelrebe, naturally produce a lot more sugar than others. Environmental conditions have a big effect on ultimate sugar levels – the vigneron can help by leaving the grapes on the vine until they are fully ripe, and by green harvesting and pruning to expose the young grapes to the sun. Green harvesting reduces the number of bunches on a vine early in the summer, so that the sugar production of the leaves is divided between fewer bunches. Unfortunately the vigneron cannot control the sun, but a sunny year can help sugar levels a lot. The semi-sweet Auslese wines in the German wine classification are probably the best example of this approach; most modern winemakers perceive that their customers want either fully dry or ‘properly’ sweet dessert wines, so ‘leave it to nature’ is currently out of fashion. But most of the Muscats of ancient times were probably made this way, including the famous Constantia of South Africa.