Sweet Wine Production Methods

Botrytis is a fungus that can grow on ripe grapes and cause noble rot, WSET explains. It makes tiny holes in the grape skin, which causes the water inside the grape to evaporate and concentrates the grape's acids, sugars and flavors. The same fungus can easily destroy grapes, so the right conditions are needed: damp, misty mornings allow the fungus to grow, and warm, dry afternoons prevent it from destroying the grapes. Bordeaux.com describes grey rot, or bunch rot, as the destructive counterpart, caused by excessive moisture throughout the day in the vineyard. The AWRI notes that Botrytis can oxidise glucose to produce gluconic acid and can also produce glycerol. WSET names Sauternes, where Sémillon is the most important variety because it is particularly susceptible to botrytis, and Tokaji Aszú from Hungary, made principally from Furmint, as botrytised sweet wines. Under German wine law, Beerenauslese may use only nobly rotten or at least overripe berries, and Trockenbeerenauslese only largely shrivelled nobly rotten berries, and both must be picked by hand. In Alsace, Vins d'Alsace states that Sélection de Grains Nobles comes from harvesting grapes with noble rot, keeping the driest berries.

• Sauternes: morning mists form where the cooler Ciron flows into the warmer Garonne, followed by sunny afternoons (Bordeaux.com)
• Noble rot does not spread through the cluster in a uniform fashion, so harvesting requires hand-picking and several passes through the vineyard over a two-month period (Bordeaux.com)
• Bordeaux.com states that one entire vine makes only one glass of botrytised wine, and that Sauternes residual sugar is usually between 130 g/L and 150 g/L, varying with the level of noble rot
• Tokaj: WSET credits rivers like the Bodrog and Tisza with generating the humid mornings in which noble rot develops
• Botrytis produces laccase, an oxidative enzyme that in the presence of oxygen can cause oxidative spoilage and rapid browning of must, so winemakers minimise or eliminate exposure to oxygen (AWRI)

Ice wine concentrates sugar by freezing rather than by rot. WSET describes the process: healthy grapes are left on the vine into winter; when the temperature drops sufficiently, the water in the grapes freezes, and the grapes are picked and pressed while frozen. Ice crystals and skins are separated from the small amount of unfrozen liquid in the press, and that juice contains high concentrations of acids, sugars and flavors. The OIV defines icewine as wine made exclusively from fresh grapes that underwent cryoselection in the vineyard without recourse to physical procedures, frozen during the harvest and pressed in that state. German wine law requires Eiswein grapes to be frozen at harvest and pressing, and WSET notes that botrytis plays no part in Eiswein. In British Columbia, VQA Icewine may use only Vitis vinifera varieties or Vidal Blanc, frozen naturally on the vine while the air temperature is -8°C or lower and pressed in a continuous process while still frozen.

• OIV prescriptions for icewine: harvesting and pressing at a recommended -7°C or lower; minimum 15% potential alcohol in the must, which cannot be increased; minimum 5.5% acquired alcohol; volatile acidity no higher than 2.1 g/L
• British Columbia Icewine: juice of at least 32° Brix after each pressing and an average of at least 35° Brix; no sweet reserve and no chaptalization
• British Columbia rules out any artificial method of concentrating sugar for Icewine, including artificial refrigeration below -4°C
• The OIV lists cryoselection by physical procedures, partially freezing grapes in a cold room before pressing, as a separate practice from cryoselection in the vineyard
• WSET: Eiswein requires the same amount of sugar in the grapes as Beerenauslese, concentrated in a different way

Drying grapes is another way to concentrate them. The OIV recognises natural raisining: exposing grape bunches on straw, racks and other supports, or suspending them, in the sun or in the open or a naturally ventilated enclosure, until the desired sugar content is reached. In Italy's Veneto, WSET describes the appassimento method as picking the grapes and drying them indoors. For Recioto della Valpolicella, which has Corvina at its heart, the grapes lose weight as their water content evaporates; winemakers traditionally dried them on straw mats or bamboo racks, or by hanging bunches from wooden beams. Decanter reports that Valpolicella's Consorzio says 50 to 60 days of drying can be enough for Recioto, but sometimes more than 100 days are needed. Recioto keeps sugar at the end of fermentation, starting at around 50 g/L, while Amarone is fermented to a drier style. In the Jura, Vins du Jura states that Vin de Paille grapes dry for at least six weeks on a bed of straw, on racks or hanging, and a slow fermentation that ends naturally gives a naturally sweet wine of 14.5 to 17 degrees of alcohol.

• Recioto della Valpolicella: grapes must be dried until at least 1 December following harvest, and fermentation may not begin until January or February (Decanter)
• Vin Santo del Chianti Classico: minimum 60% Malvasia and/or Trebbiano Toscano; grapes dried on or off the vine to at least 270 g/L of sugar; a minimum of about three years of ageing, including 24 months in small barrels called caratelli (Italian Wine Central)
• Vin de Paille is pressed between Christmas and the end of February, giving 15 to 18 liters of must per 100 kilos of grapes, and then ages three years in small casks (Vins du Jura)
• The OIV also lists raisining by physical treatment: forced artificial ventilation of grapes at 35-40°C

Killing the yeast is most commonly done by adding high-strength distilled alcohol to the fermenting grape juice, WSET explains; this raises alcohol to 15-20% abv, which kills the yeast before they have fermented all the sugars. The OIV's liqueur wine category covers wines of 15% to 22% acquired alcohol made from grape musts, including partially fermented musts, and/or wine, to which distillates, spirits and alcohol of vitivinicultural origin are added. For Port, WSET describes only a short period of fermentation before the fortifying spirit is added, killing the yeast and leaving a high level of residual sugar; the spirit, aguardente, is a local spirit of 77% abv. For Rasteau's Vins Doux Naturels in the Rhône, Vins Rhône states that alcohol is added to the fermenting musts to stop fermentation, that mutage is mandatory using neutral grape-based alcohol of at least 96% proof, and that the final wine must have at least 15% abv and at least 45 g/L of residual sugar. In Jerez, the Consejo Regulador describes naturally sweet sherries made from over-ripe or sunned grapes, generally Pedro Ximénez or Moscatel, whose must is only partially fermented, with wine alcohol added once fermentation is under way to conserve most of its original sweetness.

• Port must be made from grapes grown in the Douro Valley, and its short fermentation calls for techniques that quickly extract color, tannin and flavor, including robotic paddles and foot treading (WSET)
• Rasteau Vin Doux Naturel: Grenache noir, gris and blanc make up at least 90% of the blend; the final proportion of added alcohol is between 5% and 10% (Vins Rhône)
• Pedro Ximénez sherry: the grapes are dried in the sun ('asoleo'), which concentrates the sugars and also the natural acidity; the fermentation is stopped by adding wine alcohol, and ageing is exclusively oxidative (Consejo Regulador)

Late harvest leaves grapes on the vine past the point when they would normally be harvested, which WSET notes is possible only if the weather is dry and suitably warm. The grapes accumulate more sugar and develop dried fruit flavors, and after a longer period they start to turn into raisins as their water evaporates. WSET adds that late-harvest wines can be dry, off-dry, medium or sweet. German wine law allows only fully ripe grapes picked in a late harvest for Spätlese, and only fully ripe or nobly rotten grapes for Auslese; WSET says Spätlese Riesling grapes are typically picked around two weeks later than Kabinett grapes. In Alsace, Vendanges Tardives, officially recognised by a ministerial decree of March 1, 1984, are sweet wines from grapes picked when over-ripe; they must have a particularly high sugar content at harvest and can only be sold after ageing for 18 months (Vins d'Alsace). Süssreserve works differently. WSET explains that off-dry or medium Riesling is usually made by interrupting the fermentation by removing the yeast or by adding unfermented grape juice (Süssreserve), and that adding sweetness to a dry wine lets winemakers precisely control the level of sweetness, using sugar that comes from grapes: unfermented or concentrated grape juice, or a sweet wine.

• Juice from extra-ripe, botrytis-affected and frozen grapes can be so concentrated that the yeast cannot eat all the sugar before fermentation stops (WSET)
• Filters can remove yeast before fermentation is complete, giving a lower-alcohol wine with residual sugar (WSET)
• British Columbia's regulation defines sweet reserve as single strength grape juice or grape must added to wine as a sweetener, and limits it to 15% or less of the finished wine's volume
• Alsace's Sélection de Grains Nobles, recognised by the same 1984 decree, also needs a particularly high sugar content at harvest and 18 months of ageing before sale (Vins d'Alsace)

Primary sources (WSET, the OIV, the AWRI, German wine law, British Columbia's Wines of Marked Quality Regulation, Bordeaux.com, Vins d'Alsace, Vins du Jura, Vins Rhône, and the Consejo Regulador of Jerez): https://www.wsetglobal.com/media/13022/wset_l2wines_sessionplans_en_may2023_issue2.pdf, https://www.wsetglobal.com/knowledge-centre/blog/2025/decoding-german-riesling-a-guide-to-quality-and-styles, https://www.wsetglobal.com/knowledge-centre/blog/2022/december/13/understanding-wines-port, https://www.wsetglobal.com/knowledge-centre/blog/2024/tokaji-decoded-the-science-behind-hungary-s-iconic-sweet-wines, https://www.oiv.int/standards/international-code-of-oenological-practices/part-i-definitions/special-wines, https://www.oiv.int/standards/international-code-of-oenological-practices/part-i-definitions/special-wines/icewine-eiswein, https://www.oiv.int/standards/international-code-of-oenological-practices/part-i-definitions/special-wines/liqueur-wine, https://www.oiv.int/standards/international-code-of-oenological-practices/part-ii-oenological-treatments-and-practices/grapes/control-of-the-sugar-content-of-the-harvested-grapes, https://www.awri.com.au/industry_support/winemaking_resources/climate-wine-production/managing-botrytis/, https://www.gesetze-im-internet.de/weing_1994/__20.html, https://www.bclaws.gov.bc.ca/civix/document/id/complete/statreg/168_2018, https://www.bordeaux.com/en/mag/blog/what-is-noble-rot/, https://www.vinsalsace.com/en/gouts-et-couleurs/aoc/vendanges-tardives/, https://www.vinsalsace.com/en/gouts-et-couleurs/aoc/selection-de-grains-nobles/, https://www.jura-vins.com/le-voluptueux-vin-de-paille.htm, https://www.vins-rhone.com/en/aoc-vin-doux-naturel-rasteau, https://www.sherry.wine/sherry-wine/naturally-sweet-sherry-wine, https://www.sherry.wine/sherry-wine/naturally-sweet-sherry-wine/pedro-ximenez. Secondary sources: https://www.decanter.com/learn/advice/what-s-the-difference-between-amarone-and-recioto-della-valpolicella-51636/, https://italianwinecentral.com/denomination/vin-santo-del-chianti-classico-doc/. Last verified 2026-09-28