Sweet wines are sweet because they contain residual sugar. Residual sugar is natural grape sugar that either remains unfermented at the end of the fermentation process or is introduced to the wine after fermentation via a sweetening agent. Dry wines typically have only 0.1–0.2% residual sugar. By comparison semisweet wines usually have 1–3% residual sugar, and sweet wines such as Eiswein and late harvest wines can have as much as 30% residual sugar. A variety of techniques are used to produce sweet wines. Here is a quick rundown of the ones that use natural grape sugar…
The most common way for a winemaker to make a slightly sweet wine is to stop fermentation before all the naturally present grape sugar has been converted to alcohol. Riesling is produced using this approach.
Reserved Juice or Süssreserve
Reserved juice, or süssreserve, is a technique developed in Germany to control the level of unfermented sugar in wine. Winemakers set aside a portion of the pressed grape juice before fermentation and sterilize it or centrifuge it so it will not ferment. After the rest of the juice has fully fermented, the reserved juice is added back to the wine to enhance its sweetness and fruitiness.
Straw Wine or Raisin Wine
This technique dates back to pre-Roman times and is still used today. Freshly picked grapes are laid on mats or hung in bunches so that they can partially dry (either in the sun or a cool ventilated room), thus concentrating the grape’s sugar and flavor. The grapes are then crushed and the liquid is fermented. Wines produced using this approach are usually fairly sweet with more concentrated flavor, more viscosity, and aromas of dried fruit. Amarone is produced using this method.
When grapes are left on the vine beyond regular harvest time, the sugar in the grapes becomes more concentrated and the grapes develop richer flavor—they may even begin to shrivel on the vine, concentrating the sugar even more. Wines made from late harvest grapes are often intensely sweet and more viscous with aromas of dried fruit.
Icewine or Eiswein
Icewine, or Eiswein, is produced by picking grapes when they are frozen on the vine and then pressing them before they thaw. Often the grapes are conveyed to the winery in insulated carriers. Because the majority of the water in the grape is frozen when the grapes are pressed, the resulting juice has a high concentration of sugar and acidity. The solution is fermented, and the resulting wine is sweet, but not necessarily high in alcohol. Icewine can be made by harvesting the grapes and then freezing them (cryoextraction), but this is prohibited by regulations in Germany, Austria and Canada.
Noble Rot (Botrytis cinerea)
This technique relies upon a naturally occurring fungal spore called Botrytis cinerea. Once Botrytis has settled on a grape, it pierces the skin with tiny filaments, and the grape dehydrates on the vine as water evaporates through the microscopic holes in its skin. As a result, the grape has high levels of sugar, acids and flavors. Botrytis is only beneficial when it grows on fully ripe grapes. Furthermore, climactic conditions must be such that the growth of the fungus is controlled. If it is too damp, Botrytis can spread quickly, destroying the fruit. In this case, it is called Gray Rot instead of Noble Rot. Wines made from grapes affected by Noble Rot are very sweet and viscous. Tokaji is produced using this technique.