Mould on Wine Cork: Harmless Growth or Cork Taint?

Mould on wine corks can signal two very different scenarios for your wine: a superficial, often harmless external growth, or a deeper contamination that can lead to cork taint. External mould, typically found on the capsule or cork top, is usually a sign of high humidity in your storage environment. While it can damage labels and reduce resale value, it rarely affects the wine itself. The real concern for wine quality arises from internal mould, which can produce aroma-intense compounds like 2,4,6-trichloroanisole (TCA). The Australian Wine Research Institute (AWRI) states that TCA is "generally formed as a result of moulds growing on cork and coming into contact with trichlorophenol," imparting a "musty, mouldy aroma" to the wine. Understanding the distinction is crucial for assessing your bottle's condition.

What causes mould on the outside of a wine cork?

Mould on the outer surface of a wine cork is primarily caused by high relative humidity in the storage environment. A study by Chanut et al. (2023) indicates that "relative humidity above 80% has been found to increase risk of mildew formation on the outer surface of cork." Some humidity is necessary to prevent corks from drying out and allowing oxygen ingress. Excessive humidity can lead to mould growth. that "Too moist, and mould and contamination may occur." This external mould can also "pose the risk of damaging wine labels," which may "hinder identification or hurt potential resale value,". Maintaining appropriate humidity levels is key to preserving both the wine and the bottle's external condition. For more on ideal storage, see our guide on how to store wine.

Does external cork mould affect the wine?

External cork mould typically does not directly affect the wine's quality. It does indicate storage conditions that could lead to other issues. The mould growing on the outside of the cork or capsule is superficial. It usually does not penetrate the cork deeply enough to taint the wine itself. However, the presence of external mould is a strong indicator of high humidity. While high humidity can lead to mould, it is distinct from cellar mould versus active seepage on old Bordeaux, where active seepage suggests a compromised seal.

The conditions that foster external mould, particularly the presence of certain moulds, can contribute to the formation of cork taint. That happens if those moulds come into contact with chlorophenols. The AWRI states that 2,4,6-trichloroanisole (TCA), the primary cause of cork taint, is "generally formed as a result of moulds growing on cork and coming into contact with trichlorophenol." External mould itself might be benign. It still serves as a warning sign that your storage environment could be conducive to the development of more serious, wine-affecting faults.

What is cork taint, and how does it relate to mould?

Cork taint is a significant wine fault characterized by undesirable aromas and flavors, often described as "musty," "mouldy," "earthy," or "mushroom,". This fault arises from aroma-intense compounds that are transferred into wine after bottling. The main compound responsible for cork taint is 2,4,6-trichloroanisole (TCA), which we estimate accounts for 80-85% of all cork taints.

The connection to mould is direct: the AWRI explains that TCA is "generally formed as a result of moulds growing on cork and coming into contact with trichlorophenol." Trichlorophenol and related chlorophenols can be present in cork due to aerial contamination or chlorination of natural phenol in cork. Some moulds detoxify them by adding a methyl group, forming TCA.

Other compounds can also contribute to cork taint:

• 2,4,6-Tribromoanisole (TBA): Another compound responsible for cork taint, behaving similarly to TCA and having a "similar distinct musty, mouldy aroma," according to the AWRI. Chatonnet et al. (2004) reported TBA perceptible at 4 ng/L in wine.
• 2-Methoxy-3,5-dimethyl pyrazine (fungal must): The AWRI identifies this as "possibly the second most important form of cork taint after TCA," with a threshold of 2.1 ng/L in a neutral white wine.

The sensory thresholds for these compounds are remarkably low. The AWRI reports a TCA aroma threshold in a Pinot Noir wine as 1.4 ng/L (Duerr, 1985), with some AWRI tasters detecting it below 1 ng/L. Prescott et al. (2005) found a consumer rejection threshold of 3.1 ng/L. that some experts can detect TCA at levels as low as 1-2 ng/L, with a specially trained group reporting a threshold of 0.3 ng/L. Individual sensitivity can vary significantly, and some assessors only detect the problem at mg/L levels.

It is important to note that cork taint can also be "systemic." "Systemic TCA tainting occurs when TCA has infiltrated a winery via means other than cork and can affect the entire production of wine instead of just a few bottles." This can involve contaminated wine barrels, drain pipes, wooden beams in cellars, or rubber hoses. While unrelated to mould on the cork itself, it highlights the pervasive nature of this fault. For issues related to external bottle condition, such as corrosion, consult our capsule corrosion page.

How do storage conditions influence cork integrity and wine quality?

The integrity of a wine cork and, consequently, the quality of the wine it protects, are profoundly influenced by storage conditions: temperature, humidity, and bottle orientation. These factors dictate the rate of oxygen ingress, which is critical for wine aging.

A comprehensive study by Chanut et al. (2023) on microagglomerated cork stoppers revealed key insights into oxygen transfer. The study found that the oxygen diffusion coefficient of the cork stopper alone was "not modified, whatever the storage conditions" over 24 months. However, the total oxygen transfer includes the crucial glass-cork interface. Temperature and the presence of wine significantly changed it.

Temperature's Impact

Temperature plays a critical role in the long-term barrier properties of the bottleneck-cork system. Chanut et al. (2023) observed the following:

Storage Temperature	Duration of Stable Barrier Properties (D_total)	Observation After This Period
20°C	24 months	Remained unchanged
35°C	Up to 9 months	Significant transfer at glass-cork interface started to occur
50°C	Within the first 3 months	Tremendous oxygen transfer at glass-cork interface, approaching air diffusion after 6 months

At higher temperatures (35°C and 50°C), the study attributed the increased oxygen transfer at the glass-cork interface to a "partial melting of the surface treatment agent" (composed of paraffin and silicone) or a "modification of the mechanical properties of the cork." For instance, at 50°C, the liquid:solid ratio of the coating increased to 41%, compared to 6% at 20°C. This suggests that even temperatures easily reached during bottle shipping (35°C) can compromise the cork's seal over time. We recommend keeping wine at constant temperatures between 10 and 15°C (50 and 59°F) for slow, complex development.

Humidity and Wine Contact

The presence of wine itself influences the cork's barrier properties. Chanut et al. (2023) found that in the presence of model wine, the total oxygen diffusion coefficient for the bottleneck-stopper system nearly doubled after 3 months of storage at 20°C, increasing from 2.3 × 10−11 to 4.7 × 10−11 m2 s−1. This increase was attributed to the sorption of water and ethanol in the cork. That can "plasticize the material" and reduce the force the stopper applies to the glass, thereby favoring oxygen transfer at the glass-cork interface.

Storage guidance states that "Some degree of humidity is required in order to keep wines with cork enclosures from drying out." It cites "75% humidity is often cited as ideal" but also acknowledges "very little significant research to definitively establish an optimal range."

Bottle Orientation

The traditional advice for storing wine bottles is on their side to keep the cork moist. However, research presents varied findings. Chanut et al. (2023) specifically investigated the effect of storage position (vertical, cork in contact with vapor phase; horizontal, cork in contact with liquid phase) on microagglomerated corks at 20°C. Their study concluded that the storage position of the bottle had no significant influence on the oxygen transfer, neither through the cork... nor through the glass-cork interface over 24 months.

Conversely, we mention that "Most wine racks are designed to allow a wine to be stored on its side" to keep the cork moist. It also references research suggesting a slight angle as ideal, allowing the cork to stay damp while keeping the ullage air bubble at the top for slower, more gradual oxidation. For Champagne and other sparkling wines that the Comité Interprofessionnel du Vin de Champagne (CIVC) recommends storing them on their side, stating they will, however, keep well for several years if stored on its side in a cool, dark, draft-free place.

Understanding these dynamics is crucial for preserving your collection and for evaluating wine ullage levels explained when buying or selling. The impact of these factors can be seen in the live fine wine market index, where condition plays a role in value.

How common is cork taint?

Cork taint affects a notable percentage of bottled wines, though estimates of its prevalence vary. We cite aa study showing a 0.7: 1.2% taint rate.a 2005 study of 2800 bottles tasted at the Wine Spectator blind-tasting facilities in Napa, California, 7% of"7% of the bottles were found to be tainted."

Efforts to reduce cork taint have shown progress. The Cork Quality Council, in a 2013 report, indicated a "sharp reduction in TCA levels, of around 81 percent" compared to eight years prior. This report stated that 90 percent of samples of natural cork stopper shipments showed values of under 1.0 ppt and only 7 percent showed results of 1.0: 2.0 ppt. While the incidence has decreased, cork taint remains a persistent concern for collectors and investors. Understanding the risks associated with cork taint is an important aspect of fine wine investment.

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