There are several ways of producing sweet wines; some techniques are easy winemaking decisions, others are more complex and require a little luck.
Stopping fermentation results in a lower alcohol sweet wine as the yeast is prevented from converting the sugars present into alcohol. Fermentation can be halted by adding Sulphur Dioxide or chilling the fermenting wine. Fortification will also stop fermentation as the yeast cannot survive in such high levels of alcohol.
The sweetness is sometimes added in the form of unfermented grape juice or rectified concentrated grape must. These methods are inexpensive and mainly used in high-volume production.
Premium wines tend to have the grapes naturally sweetened. The below techniques will concentrate the sugars, acid and flavours in the wine.
Under specific growing conditions, the fungus Botrytis Cinerea can develop. Also known as Noble Rot, this fungus will puncture the skins of grapes just enough for the water within the grape to evaporate on warm sunny afternoons.
Drying the grapes and late harvest wines. Grapes can either be dried on the vine or after picking. As the grapes begin to dehydrate, the flavours and sugars become concentrated. Winemakers have to be careful to avoid the development of grey rot on these berries.
Grapes are left on the vine at the end of the growing season and allowed to freeze. When these frozen grapes are pressed, the ice remains in the press, meaning the resulting juices are concentrated. This is known as Ice Wine or Eiswein.
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