Corking vs Screw Cap vs DIAM: Closure Comparison

The closure you choose affects the wine's development in bottle, the risk of cork taint, and how long the wine can age. It's also a marketing decision — some consumers associate screw caps with cheap wine and natural corks with quality, even though the reality is more nuanced. Here's what the science says.

Natural Cork

What it is

Natural cork is harvested from the bark of the cork oak (Quercus suber), primarily grown in Portugal and Spain. The cork is compressed to seal the bottle.

How it works

Natural cork allows very small amounts of oxygen to pass through — approximately 1–3 mg of oxygen per year per closure (highly variable between individual corks). This micro-oxygenation is believed to contribute to the development of aged wines.

The cork taint problem (TCA)

Cork taint — responsible for "corked" wine — is caused by 2,4,6-trichloroanisole (TCA), a compound produced when airborne chlorine compounds react with naturally occurring fungi in the cork. A corked wine smells musty, damp cardboard, or like a wet dog. Even tiny concentrations (5–10 parts per trillion) are detectable by most people.

Industry estimate: 1–5% of wines sealed with natural cork are affected by cork taint to some degree. This is the primary driver for adoption of alternative closures.

When to use natural cork

• Premium wines intended for long ageing (10+ years)
• When the wine style benefits from the micro-oxygenation contribution
• When market positioning requires "traditional" closure

Grades

• Grade 1 (Extra): highest quality, used for premium wines
• Grade 2-3: standard for commercial wine
• Technical corks (Twin-Top, etc.): cork discs on the ends of agglomerated cork body — more consistent than natural cork, less premium
• Synthetic cork: foam or solid plastic. Not recommended for longer ageing — can seal too tightly or allow too much oxygen.

Screw Cap (Stelvin)

What it is

Aluminium screw cap with an inner liner material (typically PVDC or tin-lined). The liner determines the oxygen transmission rate.

The key advantage

Zero cork taint risk. Zero. The closure is sterile and inert.

Oxygen transmission

Screw caps can be chosen for different oxygen transmission rates depending on the liner:

• Tin-lined (Saranex): low oxygen ingress — essentially reductive. Good for fresh white wines and rosés.
• PVDC liner: slightly higher oxygen — closer to natural cork. Suitable for wines needing some development.

This means screw cap is no longer just for early-drinking wines — you can select the liner for the ageing profile you want.

Ageing potential

Long-term aged wines under screw cap often develop in a more reductive, fruit-forward style compared to cork. Many Australian and New Zealand producers have been ageing premium Riesling and Shiraz under screw cap for 15+ years with excellent results.

Consumer perception

In Australia and New Zealand, screw cap has virtually replaced cork for table wine with no quality stigma. In the UK and USA, some consumers still associate screw caps with lower quality — but this perception is declining, particularly in younger drinkers.

When to use screw cap

• White wines, rosés, and aromatic varieties intended for drinking within 5 years
• Any wine where cork taint risk is unacceptable
• Direct-to-consumer sale to educated wine drinkers
• High-volume commercial production

DIAM Cork

What it is

DIAM is a technical cork made from microspheres of cork granules processed with supercritical CO₂ extraction (removes TCA and other taint compounds from the cork material), then bound with food-grade binders.

The advantage

DIAM combines the traditional appearance and insert-and-screw cork mechanism with:

• Negligible TCA contamination risk (supercritical CO₂ extraction removes TCA to below detectable levels)
• Consistent, controlled oxygen transmission rate
• Available in multiple OTR (oxygen transmission rate) grades

DIAM grades

When to use DIAM

• When you want the appearance and tradition of natural cork without the cork taint risk
• Premium wines where consistency is critical
• When you want controllable, predictable OTR for specific ageing profiles

DIAM is now the closure of choice for many premium producers worldwide — including some of the most famous châteaux in Bordeaux.

Comparison Summary

The Home Winemaker's Choice

For home winemakers:

• Screw caps with a simple hand capper are the most practical and consistent choice for wines to be drunk within 3 years
• Natural cork with a hand corker works for medium-term ageing but accept the cork taint risk
• DIAM is increasingly available from home brew suppliers and is the best choice if you want cork-style closure with reliability