Mold on Wine Bottles: What It Means for Your Collection

Mold on wine bottle capsules and labels can be a sign of appropriate cellar conditions. It still demands careful assessment to distinguish harmless external growth from potential wine faults. External mold on the bottle or capsule typically indicates high humidity, which is often beneficial for cork integrity. Internal mold, or "cork taint", can severely impact wine quality. The Australian Wine Research Institute (AWRI) notes that 2,4,6-trichloroanisole (TCA) is the main compound responsible for cork taint. According to the AWRI, it imparts a distinct musty, mouldy aroma. Experts can detect TCA at levels as low as 1-2 ng/L. The AWRI found that its presence at 1 ng/L suppressed the ratings for overall aroma intensity and positive fruit-derived characters of a particular Semillon wine. So surface mold isn't inherently bad. Its presence still calls for a closer look at storage conditions and the wine's potential for fault, especially when you weigh its future value at auction.

For those looking to buy wine at auction, understanding these nuances is key.

What does mold on a wine bottle indicate?

Mold on a wine bottle typically indicates high humidity in your storage environment. This is often a sign of good conditions for long-term wine aging. Some degree of humidity is required to keep cork enclosures from drying out. If a cork dries out, it can allow oxygen to enter the bottle, potentially causing the wine to spoil or oxidize. Wine experts like Jancis Robinson often cite 75% humidity as ideal. She also notes "very little significant research to definitively establish an optimal range,". However, relative humidity above 80% has been found to increase the risk of mildew formation on the outer surface of cork, according to Chanut et al. (2023).

Excessive humidity is good for corks but can damage wine labels. That may hinder identification or hurt potential resale value, as we point out. Maintaining stable, cool temperatures is also crucial for wine storage. Jancis Robinson, recommends constant temperatures between 10 and 15°C. Tom Stevenson speculates that 11°C may be the most ideal temperature for storage and aging. Karen MacNeil recommends keeping wine intended for aging in a cool area with a constant temperature around 13°C. Temperature fluctuations can cause corks to expand and contract, leading to oxidation of the wine.

This external mold on the bottle or capsule itself is generally distinct from internal wine faults. External condition affects auction grades. Our guide on label and capsule mould against auction condition grades works through the detail.

Can mold on the cork or capsule affect the wine inside?

Mold on the cork or capsule can affect the wine inside if it leads to "cork taint," a broad term for an off-odor and off-flavor wine fault. The primary culprit is 2,4,6-trichloroanisole (TCA), which the AWRI describes as having a distinct musty, mouldy aroma. The AWRI also states that TCA is generally formed as a result of molds growing on cork and coming into contact with trichlorophenol. Trichlorophenol and related chlorophenols can arise in the cork by aerial contamination. They can also arise by the chlorination of phenol, a natural component of cork, according to the AWRI.

Human sensitivity to TCA varies. Some experts can detect it at levels as low as 1-2 ng/L. The AWRI found the aroma threshold of TCA in a Pinot Noir wine to be 1.4 ng/L