What are wine corks made of?

The bark of the cork oak, Quercus suber. Natural corks are punched from a single piece of bark; agglomerated and technical corks are made from cork granules bound with a food-safe binder, sometimes with natural cork discs on the ends.

Does harvesting cork hurt the tree?

Not when it is done properly. APCOR, the Portuguese cork association, says the tree grows a new layer of cork after stripping, and that the work is done by hand by skilled workers to avoid damaging the tree. Harvests must be at least nine years apart.

How old is a cork oak before its bark is used for wine corks?

Usually more than 40 years. The first harvest comes at about 25 years and the second nine years later, but APCOR says neither is good enough for stoppers. Wine-quality cork comes from the third harvest onward.

Where does most cork come from?

Portugal. APCOR's figures put Portugal at about 85,000 tonnes a year, 46% of world production, followed by Spain at 33%.

Are screw caps better than corks?

Neither is better for every wine. The Australian Wine Research Institute says screw caps let in very little oxygen, very consistently, which keeps fruit fresh, while natural corks vary much more depending on the grade of cork. The best choice depends on the wine.

How Are Corks Made? Harvest, Cork Types & Screw Caps

Wine corks are made from the bark of the cork oak, Quercus suber, which is stripped by hand in summer and grows back. The bark is left to rest, boiled, dried and then punched into stoppers, or ground into granules for cheaper corks. A tree has to be more than 40 years old before its bark is good enough for wine corks, and it can then be harvested every nine years for around two centuries. Here is how corks are made, step by step, the main types of cork, and how they compare with screw caps.

Most of the facts below come from APCOR, the Portuguese cork association, and the Natural Cork Council, a nonprofit of cork suppliers based in Napa, California, which was called the Cork Quality Council until January 2024.

Where cork comes from

The cork oak grows around the western Mediterranean. APCOR names the producing countries as Portugal, Spain, Morocco, Algeria, Tunisia, Italy and France, and its production table puts Portugal first, with about 85,000 tonnes a year, or 46% of the world total, followed by Spain with 33%. The tree has been protected by law in Portugal since the Middle Ages and was named the country’s National Tree in 2011. A cork oak may only be cut down if it is dead or diseased, with written permission.

What makes cork possible is the tree’s bark. APCOR explains that the layer that produces cork, the phellogen, stays active after the bark is stripped and forms a new layer. That is why cork can be harvested again and again without felling the tree. In our view, it is one of the few forest products that keeps the forest standing, and one more reason Portugal matters to wine beyond its own bottles (see our guide to Portuguese wine).

How cork is harvested

Harvest	Tree age (approx.)	What it’s used for
First (desbóia)	About 25 years	Virgin cork, too irregular for stoppers; used for other products
Second	About 34 years (9 years later)	Secondary cork, still not good enough for stoppers
Third and later (amadia)	About 43 years on, every 9 years	Reproduction cork, suitable for quality wine stoppers

The figures are APCOR’s. Before the first harvest, the trunk must measure at least 70 cm (27.6 inches) around, 1.3 m (4.3 feet) above the ground. Harvests are then at least nine years apart, for an average of about 15 harvests over the tree’s life and good-quality cork for around two centuries.

Stripping happens in the tree’s most active growth period, from about mid-May or early June to mid or late August of the year. It is done with an axe by skilled workers called descortiçadores. APCOR describes six stages: opening a vertical cut along a deep crack, a horizontal cut to set the size of the plank, prying the plank loose, twisting it off, tapping away leftover pieces at the base, and finally marking the tree with the last digit of the harvest year, so everyone knows when it is due again.

How natural corks are made, step by step

Resting. The cork planks are stacked outdoors for no less than six months to stabilize.
Boiling. The planks are boiled in clean water for at least an hour. APCOR says this cleans the cork, removes water-soluble substances, makes it softer and more elastic, increases its volume by around 20% and greatly reduces microbes.
Stabilizing. The boiled cork rests for two to three weeks so the planks flatten and reach a moisture content of around 14%.
Grading. The edges are trimmed and the planks are sorted by hand into quality classes by thickness, porosity and appearance.
Cutting strips. The planks are cut into strips a little wider than the length of a stopper.
Punching. Natural stoppers are punched out of a single piece of cork.

The leftovers aren’t wasted: the Natural Cork Council notes that the granules used in agglomerated corks are by-products of natural stopper production.

Types of cork stopper

Type	How it’s made	Typical use
Natural cork	Punched from one piece of bark	Wines meant to age; the Natural Cork Council says a high-quality cork can give a perfect seal for decades
Colmated	Natural cork whose surface pores are filled with cork powder fixed with a binder	The Natural Cork Council says filling the pores improves appearance and performance
Multi-piece	Two or more pieces of natural cork glued with a food-safe glue	Not specified by APCOR
Technical (1+1 or 2+0)	Agglomerated cork body with natural cork discs glued to both ends (1+1) or two discs at one end (2+0)	Wines to drink within two or three years, per the Natural Cork Council
Agglomerated	Cork granules bound with a food-safe binder, molded or extruded	Low-cost option that generally seals for about 24 months
Sparkling wine cork	Technical-style body with one to three natural cork discs, wider than a still-wine cork	Champagne and sparkling wine; the extra width holds the pressure
Bar-top (capsule)	Natural or agglomerated cork with a glued-on top of wood, plastic, metal or glass	Fortified wines and spirits, such as Port, Madeira and whisky, per the Natural Cork Council

The descriptions come from APCOR and the Natural Cork Council. The Council also counts a stopper as a cork stopper only if it contains at least 75% natural cork material, and notes that some agglomerated stoppers that don’t meet its rules add plastic micro-spheres.

Tip: The cork in your hand tells you something. A single smooth piece suggests a natural cork; a speckled, even-looking body with a disc on the end is a technical cork; and a uniformly grainy one is agglomerated. In our view, a technical or agglomerated cork usually signals a wine made to drink young.

Cork taint and how cork suppliers test for it

Suppliers now test for it routinely. The Natural Cork Council says its members analyze soaks of cork samples by GC/MS (gas chromatography/mass spectrometry), a method that detects TCA down to 1 part per trillion. If any of the soaks from a lot reaches 1 ng/L, the whole lot is held back.

According to the Council’s audit page, 99.4% of incoming cork shipments audited from June 2023 to the end of 2025 had releasable TCA below 1 part per trillion, fewer than 1% of natural cork lots were rejected, and TCA levels are 99% lower than when its records began in 2001. These are the industry’s own figures, so treat them as such. Our corked wine guide lists independent estimates of how often taint still turns up.

Screw caps, synthetics and other closures

Cork still dominates. The Australian Wine Research Institute (AWRI) says cork-based closures, natural and agglomerated, hold about 70% of the global wine market, with screw caps and synthetic closures splitting the remaining 30% roughly evenly. Australia is the exception: there, screw caps seal about 90% of bottled wine.

The real difference between closures is how much oxygen they let in, measured as the oxygen transmission rate (OTR). The AWRI’s fact sheet compares them:

Closure	Oxygen let in (AWRI)	Consistency
Screw cap	Very low to low	High
Technical/agglomerated cork	Low to moderate	Less variable than natural cork
Natural cork	Varies with the grade of cork	Highly variable
Synthetic closure	Moderate to high	Varies widely between products

Why it matters, according to the AWRI’s closure trials: low oxygen keeps fresh fruit characters; moderate oxygen can bring cooked, stewed fruit notes; high oxygen leads to oxidized wine quickly; and very low oxygen can lead to “reductive” aromas in some wines. In our view, that is why screw caps suit fresh whites like Sauvignon Blanc and Riesling so well, and why the best closure depends on the wine. Too much air is the route to oxidized wine.

Note: A screw cap is not a sign of a cheap wine. It is a choice about how much air the winemaker wants in the bottle.

What this means for you

Don’t judge a wine by its cork. Smell the wine, not the stopper. A good-looking cork can still carry taint, and a technical cork on a weeknight wine is perfectly normal.
Opening tricky corks.

Our pick for a simple takeaway: a natural cork is a piece of tree bark that took more than 40 years to grow, nine more to regrow, and months to prepare, which is a good reason to recycle it rather than throw it away.