Why does wine have a cork?

Cork works as a closure because the bark of the cork oak consists almost entirely of gas-filled cells and springs back almost completely after being compressed.

Helena Pereira describes the structure in a review article in BioResources: 40 to 70 million cells per cubic centimetre, cell walls 1 to 1.5 micrometres thick, and solid material that takes up only about 10 percent of the volume. The rest is gas. Air-dry cork therefore weighs only 150 to 160 kilograms per cubic metre, against 1,250 kilograms for the cell wall material itself.

If you press the cork in to half, the permanent deformation stays limited to 3 to 9 percent, and the material hardly expands in width. A cork pressed into the neck therefore keeps pushing against the glass. Suberin, on average 53 percent of the cell wall, makes the whole barely permeable to liquid.

Cork is stripped from the living tree, and that is only allowed once the trunk is thick enough: 70 centimetres in circumference at 1.30 metres height, usually around the 25th year.

After that, Portugal applies a legal cycle of at least nine years between two harvests, laid down in Decreto-Lei 169/2001. The first two strippings yield coarse cork that is no good for stoppers. Only from the third stripping, when the tree is older than 40 years, does cork of stopper quality come from the tree.

A cork oak lives to be 250 to 300 years old and yields 12 to 14 harvests in its lifetime, after which extraction stops at around the age of 150 to 200 years. About 67,000 stoppers come from one tonne of cork sheets. The tree remains standing through all of this, because only bark is removed.

Portugal is by far the largest producer: of the annual world production of 186,573 tonnes, 85,145 tonnes come from Portugal, or 46 percent.

• Portugal: 85,145 tonnes, 46 percent
• Spain: 61,504 tonnes, 33 percent
• Morocco: 11,686 tonnes, 6 percent
• Algeria: 9,915 tonnes, 5 percent
• Tunisia: 6,962 tonnes, 4 percent
• Italy: 6,161 tonnes, 3 percent
• France: 5,200 tonnes, 3 percent

The cork oak forest covers 2,139,942 hectares worldwide, as calculated in Wood Research in 2020. Portugal has about 720,000 hectares of that, which according to the sixth Portuguese forest inventory accounts for 22.3 percent of the Portuguese forest area. Seven countries around the western Mediterranean together supply almost the entire world production.

Yes, but very little, and how much differs greatly per type of closure.

The Cork Quality Council measured in 2017, over 36 months, how much oxygen came through different closures. Natural cork of the highest quality let through 1.5 to 2.5 milliliters, natural cork of first quality 2.1 to 2.5 milliliters, and technical cork less than 0.8 milliliters. Screw caps ranged from 0.66 to 55 milliliters, depending on the liner material. A synthetic closure was already at 2.6 milliliters after ten months and then went beyond the measuring range.

Cork itself is barely permeable to liquid. The diffusion coefficient of water in cork lies, according to BioResources, between roughly 10 to the power minus 10 and 10 to the power minus 13 square metres per second at 25 degrees. What comes in is therefore gas exchange in very small amounts and not seepage.

Cork oak forest is not felled for the harvest, so the forest remains standing and the same trees can keep supplying cork for centuries.

APCOR, the Portuguese trade association for cork, calculates 112 grams of captured CO2 per natural cork stopper, including the forest that goes with it. By the same calculation, a plastic closure emits ten times as much CO2 as a cork stopper, and an aluminium capsule twenty-four times as much.

The cork oak is the national tree of Portugal and covers 22.3 percent of the Portuguese forest. Because the bark regrows every nine years and only becomes usable for stoppers after 40 years, cork is a raw material that demands patience: one tree yields no more than 12 to 14 harvests in two centuries.