What constitutes the world's finest wine? Would it be the red wines of Bordeaux or Burgundy? The cult wines centered in America? The beloved Champagne? Or perhaps the trendy natural wines of today?
In Germany, the legally classified highest-grade wine is TBA (Trockenbeerenauslese), namely, noble rot wine. Even without such legal classification backing, the fact that TBA consistently records the highest prices at annual wine auctions proves that this wine is indeed the finest in both name and reality.
Reference Data: The highest price at the 2019 Nahe region auction held annually by VDP was 18,000 euros for Dönnhoff's 2015 Hermannshöhle Riesling TBA Magnum. The highest price at the 2018 auction was 6,500 euros for Keller's 2015 Pettenthal Riesling TBA Goldkapsel Magnum.
Incidentally, the highest price at the 2019 Mosel region auction was 6,600 euros for J.J. Prüm's 2013 Graacher Himmelreich TBA Magnum.
The Entity Known as Botrytis
There is an absolutely essential element in creating wines that command such astonishing prices. This is a type of mold called Botrytis cinerea (hereinafter referred to as Botrytis), also known as noble rot fungus.
The 2019 harvest season in Germany was characterized by prolonged rainfall, as if all the rain that hadn't fallen during the summer was falling at once. Combined with slightly higher temperatures, this caused the health condition of grapes to deteriorate rapidly (note: some regions did not experience rain).
Botrytis, which had been completely absent until just before harvest, suddenly increased dramatically in occurrence during this period.
Was this mold proliferation occurring just before harvest truly a blessing from heaven that could be called noble rot? This article provides a detailed explanation of Botrytis.
Characteristics of Noble Rot Wine
First, let's confirm what kind of wine noble rot wine is.
There are three extremely famous noble rot wines in the world. These three great noble rot wines are:
- Sauternes (France)
- Tokaj Wine (Hungary)
- Trockenbeerenauslese (Germany)
All of these are made from grapes infected with the mold called Botrytis and are characterized by being extremely sweet wines.
The residual sugar content in noble rot wines is often over 200g/L, with some exceeding 300g/L. Regular red Coca-Cola contains approximately 100g/L of sugar, so roughly speaking, these wines are two to three times sweeter than Coca-Cola.
They are also known for having a distinctive aroma called "noble rot aroma" caused by the influence of Botrytis.
On the other hand, noble rot wine can broadly be called any wine made from grapes infected with Botrytis, so it is made worldwide outside of the above countries. The nomenclature varies, but some label it as TBA in imitation of Germany, while others label it as Botrytis.
In Germany, the definition of TBA is established by wine law, so even if grapes are infected with Botrytis, they cannot be called TBA if they do not meet the requirements.
Essential Requirements for Noble Rot Wine
As we have seen, the requirement that defines noble rot wine is that the grapes used as raw material are infected with Botrytis.
Occasionally, we see definitions like "wine made from noble rot grapes," but there is no grape variety called "noble rot grapes." This means grapes infected with Botrytis, which is called noble rot fungus.
The True Nature of Botrytis
Botrytis is a type of mold.
Because it is a fungus necessary for making special wines like TBA, one might think it is some kind of special fungus. However, in reality, Botrytis is a resident fungus, meaning it is an extremely common fungal species that generally exists in the air.
Its characteristics include the ability to survive and be active across a very wide temperature range, and having a very fast proliferation cycle from germination. The fungus itself can survive as long as temperatures exceed 0 degrees Celsius, but like common molds, it tends to prefer somewhat higher temperatures, with an optimal activity temperature of 20-23°C.
It also prefers high humidity environments, entering a danger zone when atmospheric humidity exceeds 90% due to rainfall.
The image is not far from common molds that frequently occur during Japan's rainy season.
Botrytis Infection Route
Botrytis spores are extremely fine, so they easily float in the atmosphere via wind and rain. Using their size, they can attach to even small wounds and begin activity.
This is precisely the infection route of Botrytis.
In vineyards, grape berries are constantly exposed to various threats. Insects, animals, hail, disease, and workers often create minute, micrometer-sized wounds on their surface. Sometimes berries are damaged by overlapping with each other within clusters.
Botrytis enters through these wounds.
Since the situation progresses completely beyond human visual detection, it is almost impossible to completely prevent this infection.
From Botrytis Infection to Sweet Wine
Botrytis that attaches to grape wounds germinates there and extends roots toward the interior of the grape berry.
What is characteristic here is that Botrytis can self-generate multiple enzymes, and through the action of these enzymes, it can dissolve the waxy layer on the skin surface and the cell walls beneath it.
Incidentally, what affects this process is the thickness and characteristics of the skin for each grape variety. Varieties with physically thick skins or those with skin portions that change to develop resistance to enzymatic dissolution have high resistance to Botrytis.
Some of the enzymes produced by this mold are the same as those used in general wine production, so there are no particular adverse effects on human health.
Explanations commonly found on the internet state that water from inside the fruit evaporates through holes that Botrytis creates in the skin, causing concentration of sugar and other components, but this is not the whole story.
Sugar Concentration Through Glucose Consumption
Botrytis itself consumes glucose contained within the grape.
Normally, one might think that consuming the sugar within grapes would leave no sweetness in the remaining fruit.
However, in reality, the sugar contained in grape juice is sucrose, which is glucose and fructose combined in a 1:1 ratio. Therefore, even if glucose is consumed, fructose remains in the fruit. Fructose has extremely strong sweetness among sugars, and compared to sucrose with a sweetness degree of 1, glucose has 0.6-0.7 while fructose has 1.2-1.5 (1.7-1.8 according to some sources).
Relative Sweetness Increase Through Acid Reduction
Furthermore, Botrytis is known to decompose organic acids present in grape juice, particularly tartaric acid. The degree of tartaric acid reduction varies by grape variety and Botrytis infection level, but according to one study, reductions of nearly 50% have been reported.
In other words, in grapes infected with Botrytis, evaporation of water that diluted sugar content occurs simultaneously with metabolic removal of glucose that chemically diluted sweetness, resulting in sweet concentration, plus relative sweetness increase due to reduced acidity.
Reference: Yeast also utilizes glucose during fermentation. Fructose is also used, but it goes through a step of first being converted to glucose, so glucose has higher consumption priority. This is one reason why noble rot wines generally have low alcohol content.
Botrytis Infection is Not All Benefits
So far, we have explained the mechanism by which sweetness is concentrated in grapes infected with Botrytis. Looking at this alone, one might think that when grapes are infected with Botrytis, they become sweet and only good things happen. However, the actual situation is not that simple.
Handling Botrytis involves various difficulties.
Disadvantages brought by Botrytis include:
- Grape rot
- Secondary disease infection
- Increased handling difficulty due to by-products
- Gray mold disease occurrence
- Color loss
Infected Grapes Must Dry
First, for grapes infected with Botrytis to properly sweeten, moisture inside the fruit must evaporate.
This requires daytime weather to be dry. Moreover, not for just a day or two, but for the entire period needed for all infected berries to dry. If rainfall increases during this drying period and high atmospheric humidity continues for an extended time, grapes infected with Botrytis become merely rotten grapes rather than noble rot grapes.
Generally, regions suitable for noble rot wine are characterized by warm days and cold nights - diurnal temperature variation. Areas along Germany's Rhine River are typical examples, where river fog from the Rhine in cold mornings raises atmospheric humidity to promote Botrytis development in vineyards, while warm sunny periods during the day advance drying of infected grapes, creating ideal noble rot wine.
Indeed, if river fog causes Botrytis development, such favorable environmental cycles function well.
However, when Botrytis occurs due to humidity increases from prolonged harvest-time rainfall, the situation is very different. If the rain is temporary, it might be acceptable, but with prolonged rain like in 2019, mold-infected grapes have no chance to dry. In such cases, the longer mold-infected grapes remain, the more they simply rot away.
Risk of Other Diseases Also Increases
Botrytis opens holes in the skin and enters the grape interior through them. This means the grape interior becomes completely open to the outside world. As already explained, fruit moisture evaporates through these holes, but simultaneously, juice spills out.
Juice has high sugar content, and various things are attracted to this sugar. These include insects and other microorganisms like different molds. These become secondary damage attacking the grapes.
Very commonly seen in clusters with Botrytis is another mold called Penicillium. This Penicillium mold has high toxicity to humans, so mixing it into pressing must be avoided as much as possible.
Additionally, other molds may cover the grape surface.
Even Successful Drying Doesn't Make Noble Rot Wine Simple
Suppose grapes infected with Botrytis at good timing are harvested in a well-dried state. Even then, various difficulties exist.
First, pressing to obtain juice is not simple.
Fruit in good condition suitable for noble rot wine production is dried fruit. In appearance, it differs little from raisins. Naturally, pressing lightly cannot extract juice from such fruit. Therefore, widely used membrane presses are useless, and presses capable of applying very large loads, like traditional basket presses, are necessary.
Moreover, juice obtained after pressing cannot achieve high activity due to reasons such as extremely high sugar content exceeding yeast tolerance. Therefore, fermentation becomes very slow. In some cases, fermentation may take several years.
In juice pressed from Botrytis-infected grapes, the activity of Laccase, an oxidizing enzyme, becomes very high. Additionally, the content of polyhydric alcohols including glycerin, acetaldehyde, and ketone compounds increases.
As a result, binding with sulfurous acid increases, requiring addition of larger amounts of sulfur dioxide to secure free SO2 levels. Furthermore, filter clogging risks increase, causing various other effects.
Gray Mold Disease Risk
The explanation so far covered cases where Botrytis attaches to grapes that have already increased in ripeness. However, Botrytis infection actually occurs regardless of grape ripeness and does not necessarily occur only on fruit. Botrytis damage in grape cultivation is actually greater when infecting unripe fruit. Also, while not very frequent, stem infection also increases damage.
A commonly heard topic when discussing Botrytis is gray mold disease. This refers to the condition where Botrytis infects still-unripe fruit or flower clusters before fruiting.
Fruit and flower clusters affected by gray mold disease experience poor fruit set and increased rot susceptibility, likely leading to significant yield reductions. Additionally, if mixed into pressing, it causes bitterness and cloudy flavors.
The causative agent of gray mold disease and noble rot fungus are distinguished only by the fruit's ripeness at the time of grape attachment.
Botrytis is Red Wine's Great Enemy
Noble rot wine is basically made from white wine grape varieties. This is because producing red wine's red color becomes very difficult with Botrytis-infected grapes.
The reason red wine grape varieties lose their red color when infected with Botrytis is the aforementioned oxidizing enzyme Laccase. Laccase particularly oxidizes phenolic compounds. Anthocyanin, which is why red wine is red, is classified as a phenolic substance. In other words, anthocyanin is also oxidized by Laccase.
Phenolic substances undergo oxidation processes, become long-chained, increase molecular weight, and precipitate. Precipitated phenols cannot influence wine color, so color becomes lighter. In white wine, this enzyme's influence advances oxidation color, meaning brown coloration.
Among grape-derived oxidizing enzymes, there is Tyrosinase, but the difference between this enzyme and Laccase is that Laccase can oxidize almost all types of phenolic substances. Laccase has extremely low inactivation effect from sulfur dioxide, making it one of the troublesome enzymes.
Today's Summary: Rot Does Not Necessarily Become a Blessing Even if Noble
Botrytis, when infected under ideal conditions, certainly becomes the driving force for creating very valuable wine worthy of being called "noble." However, if even one condition goes wrong, what remains is significant yield reduction and the very troubling problem of how to make wine from low-quality grapes.
Moreover, even under ideal conditions, Botrytis is not necessarily desirable for producers. This is because once Botrytis attaches, those grapes can only be made into wine for that purpose. In other words, the wine the producer really wanted to make may not be possible.
Creating wines with high monetary value and creating one's ideal wine do not necessarily coincide.
Of course, creating excellent noble rot wine is not easy even with ideally harvested grapes, and being able to create this is proof of outstanding skill as a winemaker. However, that is that, and this is this.
Grapes infected with noble rot fungus inevitably acquire distinctive aromas and flavors that cannot be avoided. Whether this is taken as a blessing or a major obstacle to achieving one's ideals depends entirely on what the producer thinks and seeks. If what they seek is not there, they need to take concentrated measures to prevent this mold's occurrence at the cultivation stage.
