The Complete Guide to Botrytis

There are many ways of making sweet wines, but the most famous examples use a potentially deadly method. The fungus Botrytis cinerea attacks grapes (and occasionally other parts of the vine) in humid conditions. It can develop into devastating gray rot but, in the right conditions, a desirable Botrytis infection occurs, producing what is called noble rot. Many of the world’s greatest sweet wines are crafted from the furry, mold-covered shriveled grapes that result.

In the vineyard

Botrytis helps make some of the world’s greatest sweet wines.
© Wikipedia

Many of the great sweet wine regions tend to be sited near a body of water where mists are created, such as the confluence of a faster moving, colder tributary with a slower, warmer, main-stem river (as with the Bodrog and Tisza in Tokaj and the Ciron and Garonne at Sauternes). There needs to be sufficient sunshine to dissipate the mist as the day progresses, limiting the growth of the fungus, and enough warmth to allow grapes to fully ripen.
More generally, noble rot can occur when a period of rain near harvest time is followed by dry weather. In New Zealand, for example, Marlborough Sauvignon Blanc growers usually rush to pick the bulk of their crops if rain threatens at harvest time. But a parcel may be left out in the hope that subsequent sunshine will create the required microclimate for noble rot.

Certain grape varieties are more susceptible to rot as they have soft skins and closed bunches, which do not allow airflow around the berries. Semillon is prized because of its thin skin and rich, oily character. The best-known other examples among white grapes are Sauvignon Blanc (a minor partner in most Sauternes, but a valuable contributor of acidity to a blend with Semillon), Chenin Blanc, Riesling, Gewürztraminer and Tokaj’s Furmint. Merlot and Pinot Noir are among the most rot-susceptible of the major dark-skinned varieties, though botrytized red wines are seldom attempted as noble rot damages pigments producing brownish colors, and off aromas can derive from the phenolics in the skin.

Botrytis cinerea is present throughout the year in most vineyards, but will only develop and affect fruit when conditions suit. It can live through the winter as mycelium (small cylindrical filaments) or sclerotium (fruiting bodies) within dormant buds, vine bark or on plant debris on the ground. Old cluster stems are thought to be a particularly common site of concentration.

This necessitates prompt removal of prunings in vineyards prone to infection. Controlling botrytis is dependent on a range of practices, including allowing air movement through the vineyard and not allowing canopies to become too dense – which not only blocks airflow but also makes it harder to spray fungicides if needed. However, in modern, sustainable vineyards, systemic preventative chemical sprays are frowned upon; in any case botrytis is able to build up a resistance to most fungicides.

Humid nights and early mornings late in the season (around late September in the Northern Hemisphere) allow germination and the infection to take hold, then warm, dryer days are required to limit the growth of the fungus. Destructive gray rot develops when the botrytis causes the grape to split rather than shrivel, allowing other fungal and bacterial infections to take hold. This can occur earlier in the season if warm humid conditions occur while the grapes are unripe; the time around veraison, when the grape begins to soften and change color, is the most risky. Continued wet weather will also cause the fungus to spread through the bunch as gray rot.

The mycelium and sclerotium structures in the vine wait for sufficient water (around 15 constant hours of rain, mist, dew or irrigation) and nutrients (especially sugars) in maturing fruit for the spores to germinate, forming millions of conidiophores – structures that closely resemble grape clusters. These then release conidia (the individual “grapes” on the cluster), which land on the fruit. The development of rot proceeds for about a month before harvest. The conidia (spores) develop hyphae (filaments) that sink through the skin into the berry pulp. In less humid conditions these filaments draw water away through the berry skin to evaporate.

The fungus also releases enzymes that break down the pectin component of the cell walls of the berry. The sugars in the berry are concentrated and the botrytis begins to alter the grape’s acidity. Tartaric acid appears to undergo significant metabolic degradation. Thus, while the grape needs to be mature before the mold strikes, producers of dessert wines do not want the grapes to be super ripe, as they want a backbone of acidity to carry through the process to ensure balance and age-worthiness. In turn this gives another reason why the timing of the harvest is absolutely crucial.
Meanwhile, glucose is oxidized into gluconic acid, which is responsible for the apricot character in botrytized wines. The degree to which this oxidation happens varies between different grape varieties. Malic acid levels seem to be relatively unaltered, aside from being concentrated along with all other components apart from water.

Under a microscope, botrytis looks uncannily like a bunch of grapes.
© UC Davis

The berry shrivels and dehydrates but must not split if it is to be used for a dessert wine. By the end of the process it will only release a couple of drops of golden liquid. This drying of the grape and concentration of the juice appears to modify the metabolism of the botrytis mold, stabilizing its development. In addition botrycine and other antiobiotics are produced, which prevent other bacteria and fungi such as Penicillium and Aspergillus from creating phenolic off flavors.

The microclimate around the bunch varies, so individual berries within the bunch can display different levels of rot. In turn there can be considerable variation in levels of botrytis across bunches in the vineyard. For top producers in Bordeaux, the Loire and elsewhere, this necessitates multiple passes through the vineyard, allowing some bunches more time to develop rot, thus achieving some measure of consistency.
Few producers can charge Sauternes Grand Cru prices to pay for such labor-intensive hand harvesting, so they will pick once and sort the affected berries from the bunches in the winery. The heavily reduced yield (10 to 20 percent of a standard crop) and risks of leaving fruit on the vine also have cost implications. Attacks from hungry birds either eating whole berries or pecking holes in fruit – encouraging the spread of gray rot – increase costs by requiring the purchase, deployment and removal of netting.

The best vineyards deliver the necessary conditions for botrytis development with sufficient regularity to make them economically viable to concentrate on sweet wine production. Nevertheless great Sauternes Crus are priced to allow for around one year in four where no dessert wine is made and the wine is sold cheaply (usually in bulk) as Bordeaux Blanc. Between the two extremes are years where dessert wines are made but with less pronounced botrytis characters.

In Germany, great botrytis-infected Trockenbeerenauslese wines can be made, but not with enough regularity to devote vineyards to the category. Many wineries here will make a range of styles from a single site, leaving a few rows from each vineyard parcel on the vine to develop botrytis or become ice wines. German wine classification is based on sugar levels in the grapes, not the finished wine; Kabinett, Spätlese and Auslese grapes can be fermented to dryness, and particularly in the first two cases, noble rot is uncommon. Grapes for Beerenauslese (selected overripe berries from late harvest fruit) and Trockenbeerenauslese (selected shrivelled berries) categories must contain more sugar than can be fermented out and therefore signify sweet wines, which almost always contain botrytis. Versions from sun-dried grapes are legally possible, if very rare due to the climate.

Botrytized wines have been made for centuries in various parts of Europe where conditions are favorable, though early versions would not have been intentional. Historical documentation suggests Tokaji in Hungary was probably first; Laczkó Máté Szepsi (ancestor of leading modern producer Istvan Szepsy) is credited with first writing down the Aszú process in 1630, though an inventory document listing Aszú wines dating back to around 1571 has been found (Aszú translates as dessicated, though it was associated with noble rot from this time). Tokaji wine was first sent to by the Prince of Transylvania to Louis XIV of France in 1703; the latter referred to the wine as Vinum Regum, Rex Vinorum (Wine of Kings, King of Wines), thus providing the impetus for the production of botrytized wines in Sauternes, which was certainly well organized by the mid 19th Century.

In the Loire valley, fragrant rich botrytized wines are made from Chenin Blanc, notably in Bonnezeaux, Chaume and Quarts de Chaume. In Alsace, Sélection des Grains Nobles wines are made from Pinot Gris, Gewürztraminer Riesling and Muscat. Riesling is responsible for the most prestigious wines in Germany and Austria, but other grapes such as Scheurebe and Silvaner may be used in the former. The Austrian town of Rust near the Neusiedlersee is known for its Ausbruch (the Austrian equivalent of Aszú) dessert wines from a range of varieties including Chardonnay, Muskateller, Sauvignon Blanc and Welschriesling. Furmint was once more dominant here, reflecting the historical link between Rust and Tokaj, and is now making a minor comeback. Pink dessert wines have even been made from Pinot Noir.

Most of these regions producing botrytis styles, in both hemispheres, are relatively cool, humid mescoclimates. Riverina in central New South Wales, Australia is somewhat unusual, with its hot summers and low annual rainfall. But even this region gets late autumn rains and high humidity at the end of the growing season enabling botrytis to develop.

In warmer regions, like much of California, only a few vineyards have suitably humid microclimates, and even then production volumes vary wildly each year. Far Niente’s Sauternes-inspired Dolce operation in Napa tried various methods to make output more consistent, spraying blocks with cultured botrytis, only to see untreated blocks performing better. They now rely on natural conditions to shrivel their Sémillon and Sauvignon Blanc, and sometimes only 5 percent of fruit grown makes the final selection – with a sizable proportion often eaten by wasps.

In the winery

The pressing of grapes affected by noble rot is a tricky business. Gentle pressing with no crushing beforehand may to be used to further guard against the ingress into the wine of undesirable bacterial or fungal infections spoiling the final wine. In contrast to dry white winemaking, later pressings tend to be of higher quality as they contain more sugar and botrytis-derived flavor compounds. The most shriveled grapes may need several pressings to release any juice at all.

Classic Sauternes winemaking involves a very slow fermentation over two or more months; the yeasts – even though they are specially selected for the job – struggle in a must which is so high in sugar. Botrytized musts also tend to contain antibiotics such as botrycine and be low in nutrients enjoyed by yeasts such as ammonia and thiamine. Therefore stuck (halted) fermentations are fairly common.

The development of botrytis cinerea also produces another key group of polysaccharides (sugars). These are mainly polymers of mannose and galactose with some glucose and rhamnose with antifungal properties which inhibit fermentation and increase acetic acid (volatile acidity) and glycerol during fermentation. Acetobacter on the grapes can produce high levels of volatile acidity (acetic acid) and ethyl acetate; if balanced by other components in the wine they add complexity, but if they dominate they become a fault.

Care must be taken to avoid re-fermentation, as many of these sweet wines will have enough residual sugar to further raise their alcohol by volume another 5 or 6 percent. Once fermentation has reached the desired balance of alcohol and residual sugar, fermentation is stopped by adding sulfur dioxide.
At the end of fermentation the wine is cloudy and hard to clarify due to the presence, interspersed in strand-like structures through other substances, of large-molecule glucane colloids, which do not settle, resist fining, and require cold settling over several weeks to make the particles even larger if conventional filtering or centrifuging is to be attempted without rapid clogging of equipment. Glucanase enzyme additions, which break down glucans and speed up the process, are permitted in EU winemaking regulations.

Cold stabilization is used alongside sulfur dioxide to ensure that the fermentation stops. Nevertheless, as well as the challenges to clarify the wine, Sauternes and similar wines are prone to throw crystal deposits. This because botrytis produces saccharic and mucic acids, which then form insoluble calcium salts after the wine is bottled. These are harmless to consumers, if disconcerting.

While malic acid remains present in a botrytized wine, malolactic fermentation is rare, due to the sulfur dioxide additions used to halt the alcoholic fermentation. Otherwise, high sugar and glycerol levels would probably assist the process.

A top Sauternes might be bottled after 16 months in new oak barriques, an Australian Botrytis Semillon might spend three years in barrel, while wines from the Loire and Germany tend to be bottled the spring after harvest. Tokaji is traditionally aged oxidatively in partially filled casks; the risk of microbial spoilage is reduced by high sugar and alcohol levels.

During both vinification and bottling, higher-than-average levels of sulfur dioxide additions are needed, as the enzyme laccase – a product of botrytis – increases risks of oxidation, and will stand up to standard sulfur additions. It is the key factor determining the golden color of dessert wines. In addition, gluconic acid is synthesized by any acetic acid bacteria that enter the wine, forming lactones that bind sulphur dioxide. These two factors mean botrytized wines are permitted higher additions under European Union regulations.

In the glass
Botrytis modifies many of the relevant chemical compounds that shape flavor and aroma (including terpenes, which may be where medicinal notes in botrytized wine derive), and synthesizes new ones such as sotonol, which is usually associated with honeyed aromas. That said, some grapes, like Muscat, lose more varietal character than others, such as Riesling and Semillon.

Some of the descriptors associated with noble rot wines, like honey, flowers, ripe apricots, dried fruit and marmalade might also be applied to dessert wines made from other methods, such as ströhwein, or vin de paille. Picking a botrytis wine in a blind tasting usually involves searching for a combination of the above characteristics with some more medicinal aromas, with notes of wet absorbent cotton or the smell of wet sticking plaster in the shower.

If you’d like to discover some of the flavors and characteristics of a botrytized wine for your self, give the Covenant “Zahav” Muscat Canelli or the Semillons by Stony Hill and Chateau Les Justice a try! And, as always, don’t hesitate to reach out to us at 415-337-4083 or by emailing frank@goldengatewinecellars.com!