Cork vs Screwcap: Understanding Closures for Your Cellar

Understanding the long-term implications of wine closures is crucial for managing your cellar. When considering cork vs screwcap, natural corks offer a traditional seal with a complex oxygen ingress profile that can be beneficial for aging, but they carry a risk of cork taint, primarily from 2,4,6-trichloroanisole (TCA), which can affect an estimated 1-5% of bottles. Screw caps, praised for their consistency and ability to reduce oxidation, largely eliminate TCA risk but may lead to sulphidisation, an aroma taint from reduced oxygen supply. Your decision on whether to hold or open a bottle may depend on the closure type, its inherent risks, and how its oxygen transfer properties interact with your storage environment, particularly temperature, which significantly impacts the integrity of the glass-cork interface over time, as shown by Chanut et al. (PNAS Nexus 2023).

What are the primary risks associated with natural corks?

Natural corks primarily carry the risk of cork taint, a broad term for off-odors and off-flavors transferred from the cork to the wine. The main compound responsible for cork taint is 2,4,6-trichloroanisole, or TCA, which has a distinct musty, mouldy aroma, according to Amon & Simpson (1989) as cited by the AWRI: wine faults and taints. Some tasters at the AWRI can detect TCA at less than 1 ng/L, and its presence at 1 ng/L suppressed the ratings for overall aroma intensity and positive fruit characters in a Semillon wine during sensory evaluation, as noted by the AWRI: wine faults and taints. The consumer rejection threshold for TCA in white wine is between 3.1 and 3.7 ng/L. The cork industry group APCOR cites a study showing a 0.7-1.2% taint rate, while a 2005 Wine Spectator study found 7% of 2800 bottles tasted were tainted.

Beyond TCA, other compounds can cause cork taint. These include 2,4,6-tribromoanisole (TBA), which behaves similarly to TCA with a musty, mouldy aroma perceptible at 4 ng/L in wine, as reported by Chatonnet et al. (2004) and cited by the AWRI: wine faults and taints. TBA can form in wineries from the microbial breakdown of 2,4,6-tribromophenol (TBP), a flame retardant and wood preservative, and has been detected in barrels, plastics, natural corks, and winery atmospheres (AWRI: wine faults and taints). 2-Methoxy-3,5-dimethyl pyrazine, or 'fungal must', is possibly the second most important form of cork taint after TCA, with a threshold of 2.1 ng/L in a neutral white wine, comparable to TCA (AWRI: wine faults and taints).

How do screw caps and synthetic closures compare to corks?

Screw caps and synthetic closures offer distinct advantages and disadvantages compared to natural corks. Screw caps are praised for their consistency and ability to reduce oxidation, largely eliminating the risk of TCA taint. This is a significant benefit given that TCA is responsible for an estimated 80-85% of all cork taints. Synthetic corks are also less subject to cork taint.

However, alternative closures like screw caps and synthetic corks are thought to be prone to another aroma taint: sulphidisation. This can arise from a reduced oxygen supply, which concentrates sulphurous smells from wines with universal preservatives, or from excessive or imbalanced sulphite-based preservatives to begin with. The Court of Master Sommeliers' deductive tasting format lists "Struck Match" as a minor fault, which can be a descriptor for sulphurous notes. The AWRI: wine faults and taints describes various reductive wine faults, including hydrogen sulfide (H2S), which smells like 'rotten egg gas' at 1-2 µg/L, and sulfhydryls (thiols/mercaptans) with aromas like 'cabbage', 'garlic', 'onion', and 'rubber'.

Does storage temperature change what a cork actually does?

It changes the seal, not the cork. Chanut et al. (PNAS Nexus 2023) aged microagglomerated cork stoppers in glass bottlenecks for 24 months and found the oxygen diffusion coefficient of the cork itself never moved, whatever the temperature, the storage position, or the presence of model wine. What moved was the joint between cork and glass. At 20°C the barrier properties of the bottleneck and cork system held for the full 24 months. At 35°C, which Chanut et al. describe as a temperature easily reached during bottle shipping, transfer held for up to 9 months. It then climbed sharply at the glass-cork interface. At 50°C that shift had already happened by 3 months, and by 6 months total transfer approached the diffusion coefficient of oxygen in air, which the authors read as leakage at the interface.

So a hot fortnight in transit is not a cork failure. It is an interface failure. That is why the temperature you store a bottle at sets how long the seal lasts. Chanut et al. note that bottles inside shipping containers can meet fluctuations up to 20°C.

The same 24-month test also settles an argument collectors keep having. Bottles held vertically, cork against the vapour, and bottles held horizontally, cork against the liquid, gave similar total oxygen diffusion at 20°C. On that evidence, whether you lay the bottle on its side had no significant effect on oxygen transfer, at any interval from 3 to 24 months. Chanut et al. note that earlier studies disagreed on the point.

Humidity still matters, though for the cork's mechanics rather than its permeability. Chanut et al. cite work showing that relative humidity above 50% is required for good elasticity of cork-based closures, and that above 80% the risk of mildew forming on the outer surface of the cork rises. That makes cellar humidity a band to sit inside, not a number to minimise.

Closure type sets the risk you carry. Temperature sets how fast it arrives. Our weekly winemaking briefing follows both, so you can decide when to hold and when to open.