How wine corks are made. From Portuguese cork oaks to the neck of the bottle.

Where cork comes from

About 80% of the world's cork comes from Portugal and Spain, where the cork oak thrives on poor soils with little water. The film reaches back into history: the Romans sealed amphoras with cork, and Portugal's King Sancho I passed an early law to protect cork trees. Natural cork became the stopper of choice for wine in the 19th century, and the industry grew up around Porto, close to its best customers in the Port trade.

Cork is the outer bark of the tree. It is light, elastic and compressible, and its pH is close to that of wine. It is also renewable: the bark is stripped, the tree lives on, and the cork forests support a rich ecosystem.

Harvesting by hand every nine years

The harvest is a skilled job. Workers use an axe to cut the bark without hurting the trunk underneath, between May and July, when the tree is growing a fragile new layer of cells that lets the bark come away cleanly. Each tree is harvested only every nine years, the time needed for the bark to grow thick enough to punch a stopper. Look for the number painted on each trunk: it shows the last digit of the harvest year.

Buyers visit and sample the forests in winter, long before harvest, to judge how much of the bark will be good enough for natural corks.

Inside the factory, step by step

The bark reaches the factory in Montijo within a day or so and is sorted at once. The foot of the plank, which grew near the soil, is cut away because moisture and organic matter there can feed fungi that create TCA, the compound behind cork taint. The planks then season for at least six months, as required by law, and are boiled for an hour to flatten them and restore moisture.

After another sorting, the best planks are punched into natural corks, thinner wood goes to Champagne corks and the rest to technical corks. Corks are cut about six millimeters wider than the bottle neck so they seal tightly. They are washed, disinfected and treated with steam distillation, much like the perfume industry extracts scents, to remove unwanted aromas. Finally they are graded by eye for porosity and color.

Hunting for TCA, cork by cork

The film spends time on quality control, and it is fascinating. The human nose can detect TCA at parts per trillion. The company tests samples soaked in a wine-like liquid with gas chromatography, and you see how individual corks are sniffed by people, a method the team developed about 15 years ago and has since paired with machines that screen for the molecule. The film also points out that TCA is not unique to cork; coffee beans can be affected too.

Technical corks are made from the leftover cork, cut, dried and milled into granules, then molded. By adjusting granule size and density, makers tune each closure, including the Champagne-style stopper with natural cork discs.

What winemakers want from a cork

In the Alentejo, the team at Herdade do Mouchão, which grows 700 hectares of cork oak, explain why their top reds stay under natural cork and how an independent auditor checks their suppliers. Another producer shows how they chose a long cork first, then found a bottle to fit it. At Mouchão, wines spend three to five years in large old casks before bottling, and the top reds can need up to 25 years in bottle. The other producer ferments in lagares, where treading brings in oxygen that helps the yeast work slowly and steadily.

Find out why both believe a slow trickle of oxygen through the cork is key to graceful aging. You will also hear why the cork makers had to invent their own machines, since no ready-made technology existed for their industry.