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Cork taster

gosto a rolha (PO)
sapore di tappo (I)
goût de bouchon, bouchonné (F)
sabor a corcho (ES)
cork taint, cork taste, corky taste (GB)

Designation (also corker, cork taste, corkcton) for a dreaded wine defect. In Austria this is also colloquially described as "the wine stops" (stubble = cork). It manifests itself by a musty, musty and chemical smell of wet, rotting wood or leather. The smell is sometimes also described as earthy, in which the alcohol type geosmin is involved. The taste is unpleasantly bitter and astringent. Often, however, the fault can only be perceived by smell. A typical characteristic is a lack of fruitiness or a partially or completely covered varietal typicity of the wine.

Korkschmecker - Glas mit Korken und TCA-Molekül

The unpleasant aftertaste lasts for a long time. At a higher wine temperature the sympthoms become even more apparent. With red wine the perception threshold is somewhat higher due to the covering tannins, here the fault is possibly not perceived as strongly. The main cause of the "real cork taster" is the chemical substance trichloroanisole (TCA), the exact chemical name is 2,4,6-trichloroanisole. This was first proven in 1981 by Prof. H. Tanner at the Swiss Federal Research Institute in Wädenswil (Switzerland). This substance is produced by microbial methylation of trichlorophenol (TCP). This means that TCP is converted into TCA by microorganisms such as moulds. In Australia, the substance methoxy-dimethylpyrazine was isolated in 2004, which is regarded as the second cause.

Causes

Although TCA usually enters the wine via the cork, it is by no means cork specific. Instead, the starting material TCP can come from many sources, which makes it difficult to determine the cause. Unfortunately, this is also the reason why the cork problem is trivialised or, in the worst case, regarded as insignificant. In the 1990s it was still common practice in cork production to bleach and sterilize the bark with chlorine-containing substances. This was changed by most cork producing countries, today hydrogen peroxide is used. Another reason is the inadmissible preservation of corks and barrels with the toxic wood preservative PCP (pentachlorophenol)

Similarly, chlorine-containing substances can be caused by polluted rainwater (for example, caused by bush or forest fires and industrial air pollution), by the use of pesticides in the vineyard and by the use of chlorine-containing substances in winemaking. In Europe, however, pesticides that could serve as precursors to TCA formation may no longer be used. In France, many Châteaux are currently renovating roof trusses and wooden beams in general that have been impregnated with wood preservatives (TCP, other chlorophenols). These can evaporate and then enter the cellar atmosphere and from there into the wine. It is also possible to sum up all sources of chlorine.

Perception

One phenomenon is that not all people possess the ability to perceive, or at least have a diminished ability to do so. A trained and sensitive taster already perceives a few billionths of a gram (1 nanogram = 0.000000001 g), but for less trained people this can be above 30 ng. Cork tasters can appear very clearly or even very hidden, so that they are hardly noticeable. In this case they are aptly called "Fruit scalping". In white wine 1 ng, in red wine 5 ng TCA per litre is considered the olfactory perception threshold. In water this is considerably higher, the perception threshold can be 200 ng per litre. But the problem is that there are a number of similar chemical compounds besides trichloroanisole which cause a similar smell or taste in wine. One of them is tribromanisole (TBA).

The Wädenswil Institute in Switzerland has defined cadins, geosmines, methyls, octanes and pyrazines in this respect, with ten or more sub-variants of some of them. The human sense of smell is generally not very selective, so that additive effects are possible in the presence of two or more musty compounds and are even very likely in the presence of untrained people. The two wine faults mould taste and barrel taste also smell quite similar. However, these faults are rather rare nowadays. As a doctrine is considered that the substance TCA is the main cause of the "real cork taster". Whether TCA is always detected 100% sensorially is the question. This is only possible without doubt by chemical analysis.

Verification procedure

By means of a newly developed sensor, the molecule 2,4,6-trichloroaniosol, which is responsible for the cork taste, can be quickly and clearly detected in wine. The test procedure was developed in Switzerland at the University of Fribourg in collaboration with the University of Bordeaux. By means of a sponge-like, porous supramolecular network, the cork taster molecules can be "captured". As soon as a TCA substance is present and has settled in the pore of the sensor, there is an optical signal. The sensor can be regenerated and is then ready for new measurements.

Occurrence (quantity of wine concerned)

Considerable amounts of the worldwide wine production are at least negatively affected by the cork taster, and in the worst case, completely inedible. However, estimates of the error rate fluctuate considerably. Most sources agree that at least three percent of bottles are faulty. However, quantities of up to 10% are often spoken of and some experts estimate the proportion to be as high as 15% and even higher. Even with "only" three percent, however, this would be an unbelievable amount of about one billion (that is a thousand million) bottles per year that are inedible due to this wine defect.

Solution approaches

In the meantime, there are some approaches to solutions in cork production. Either one kills the mould fungus germs by radiation or heating the cork discs with microwaves (see under dolphin), or one inactivates the metabolic products of the moulds (anisoles and phenols) by the enzyme suberase. This method was developed at the Geisenheim Institute and has been used in Portugal since 1999. A hundred percent solution does not seem to have been found yet. In the meantime, a worldwide practice is the use of alternative closures such as screw caps or glass corks.

Picture left: from Adriano Gadini on Pixabay
Picture right: From Quasar Jarosz in Wikipedia, CC BY-SA 3.0, Link
Source detection method: The winegrower 06/2019

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