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Noble rot (French:pourriture noble;German:Edelfäule;Italian:Muffa nobile;Hungarian:Aszúsodás) is the beneficial form of a greyfungus,Botrytis cinerea, affectingwinegrapes.[1] Infestation byBotrytis requires warm and humid conditions, typically around 20 °C (68 °F) and above 80% humidity.[2] If the weather stays wet, the undesirable form, "bunch rot" or "grey rot", adversely affects winemaking by disrupting fermentation and changing the taste, aroma, and appearance of the final wine.[1][2][3] Grapes typically become infected withcinerea when they are ripe. If they are then exposed to drier conditions and become partiallyraisined, this form of infection is known as noble rot. Grapes picked at a certain point during infestation can produce particularly fine and concentrated sweet wine. Wines produced by this method are known asbotrytized wines,[4] and are considered a distinct category ofdessert wines.[3]
The primary distinction between botrytized wines and other naturally sweet, non-fortified sweet wines, such as late-harvest wines,ice wines, orstraw/raisin wines, lies in the range and richness of aroma compounds generated byBotrytis cinerea.[3] Significant differences are also evident in other components, like sugar alcohol and acid composition, due to microbial activity.[3]Descriptors frequently used for these wines highlight flavours of peach, apricot, pear, quince, raisin, and honey, along with unique "botrytis" or roti characteristics.[3][5] Additionally, a notable feature of botrytized wines is their high acid content, which prevents them from tasting cloying, even with sugar levels often exceeding 200 g/L.[3]
WhenBotrytis cinerea infects grapes, it initiates several physiological changes that are crucial for the development of noble rot wines. The fungus penetrates the grape skins while feeding on the grapes, causing microscopic wounds that lead to the evaporation of water from the berries.[5] This dehydration concentrates the remaining sugars, acids, and flavour compounds within the grapes, resulting in their higher density. The mass of the grapes can decrease by up to 60%, leaving behind a shrivelled, sugar-rich berry.[2]
The fungus primarily consumestartaric acid, which is more prevalent thanmalic orcitric acid in healthy grapes, with remaining tartaric acid transformed intogluconic acid andglycerol.[5] As observed in Furmint grapes used forTokaji Aszú, the infection process changes the ratio of tartaric to malic acid, from 2:1 in healthy berries to 1:3 in botrytised berries. The increased levels of citric acid and the formation of higher levels of sugar alcohols (such as glycerol,arabitol,mannitol,sorbitol, andinositol) contribute to the complexity and mouthfeel of botrytized wines.[1][2] The grapes' metabolic stress response also leads to the production and concentration of variousaroma compounds. The berries concentrate precursors for citrus aromas like lemon and grapefruit, and also generate compounds for stone fruit aromas such as apricot and peach in a direct metabolic response to the fungal attack. Some of these aromatic characteristics, including the distinct honey aroma, are directly attributable to the botrytis fungus itself.[1][2]
The stress response of grapes to botrytis infection is similar to their response to drought. The breakdown of grape skin cells by the fungus facilitates water evaporation, intensifying the dehydration process. This stress response allows the grapes to adapt metabolically, slowing down the infection rate compared to grey rot, which spreads too quickly for the plant to respond effectively.[2]
According to Hungarian legend, the firstaszú (a wine using botrytised grapes) was made by Laczkó Máté Szepsi in 1630. However, mention of wine made from botrytised grapes appears before this in theNomenklatura of Fabricius Balázs Sziksai, which was completed in 1576.[6] When vineyard classification began in 1730 in theTokaj region, one of the gradings given to the variousterroirs centered on their potential to developBotrytis cinerea.
There is a popular story that the practice originated independently inGermany, where theRiesling producers atSchloss Johannisberg (Geisenheim, in theRheingau region) traditionally awaited the say-so of the estate owner,Heinrich von Bibra, Bishop ofFulda, before cutting their grapes. In 1775, the story claims the abbey messenger was robbed en route to delivering the order, delaying cutting by three weeks, time enough for thebotrytis to take hold. The grapes were presumed worthless and given to local peasants,[7] who produced a surprisingly good, sweet wine which became known asSpätlese, orlate harvest wine. In the following few years, several different classes of increasingmust weight were introduced, and the originalSpätlese was further elaborated, first intoAuslese in 1787[8] and laterEiswein in 1858 (although Eiswein is usually made from grapes not affected byBotrytis).[9]
In some cases, inoculation occurs whenspores of the fungus are sprayed over the grapes, while some vineyards depend on natural inoculation from spores present in the environment.
Botrytized wines can be made using many varieties of grape. Due to anthocyanin oxidation, a majority of varieties are white. The ideal varieties mature late in the season in order for ripening to coincide with the best environmental conditions for noble rot to occur. Thick skinned cultivars can also be used to make harvesting infected grapes less difficult.Riesling andSémillon are the primary cultivars used in the production of botrytized wines. The Hungarian variety,Furmint, is the predominant variety used in the production of TokajiAszú. Other varieties, such asPicolit,Gewürztraminer,Chenin blanc, andPinot blanc, are used, depending on tradition and adaptation to local conditions.Jackson, Ronald (2008).Wine Science Principles and Applications. Oxford, UK: Elsevier Inc. p. 525.ISBN 978-0-12-373646-8.
Some of the finest botrytized wines are picked berry by berry in successivetris (French for "selections").
Internationally renowned botrytized wines include theaszú ofTokaj-Hegyalja inHungary (commonly called Tokaji or Tokay),[5]Sauternes fromFrance[5] – where the process is known aspourriture orpourriture noble, andBeerenauslese orTrockenbeerenauslese wines fromGermany andAustria.[5] Other wines of this type include theRomanianGrasă de Cotnari, FrenchCoteaux du Layon, FrenchMonbazillac, AustrianAusbruch,[5] South African Noble Late Harvest and Croatian wine maker Vinarija Mihalj fromKutjevo (producing Mačevo brdo-Graševina). Depending on conditions, the grapes may be only minimally botrytized. Botrytized wines are also produced byCalifornian andAustralian winemakers.[5]
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