Portugal makes 40 million cork wine stoppers a day. We visited a cork forest to see how they're made.

• Cork has been used as a wine stopper for centuries and is still the stopper of choice for a large majority of top-quality wines. 
• These corks come from the outer layer of the oak tree, which eventually grows back.
• Portugal is a primary producer of the material. It has the largest cork oak forest by area in the world.
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The process of harvesting the cork oak takes precision, years of practice, and a good axe.

In the Alentejo region of Portugal, workers spend their summers delicately removing the outer layers of the trees by hand before sending them to be processed into something more recognizable: cork. 

Natural cork has been used in winemaking for centuries and is still the stopper of choice for 89% of Wine Spectator's top 100 wines.

Portugal has the largest cork oak forest by area in the world.

While it takes around 15 years for a cork oak tree to grow its first layer of cork, it is harvested in cycles of nine years. And only from the third harvesting on - or 27 years later - the raw material called amadia cork is ready for processing. 

Cork worker Simão Fortio started harvesting cork when he was 15 years old. At 45, he has been harvesting cork for over 30 years. He explained that he and other workers harvest the cork during the summer with axes. Oftentimes, men work in pairs to help each other, "very carefully not to harm the tree." 

"It's very difficult. It takes a lot of knowledge to remove the cork without hurting the tree," Fortio said.

Portugal is the world leader not only in cork extraction, harvesting 100,000 tons of it a year, but also in selling cork products.

 "2018 was an historical year for the cork industry," said João Rui Ferreira, President of the Portuguese Cork Association. For the first time, Portugal exported more than 1 billion euros' worth of cork, he added. 

Wine stoppers make up the majority of the market, representing 70% of total exports.

"Knowing that 40 million cork stoppers are produced daily in Portugal, it's very likely that when you're opening a wine bottle, the cork stopper is coming from Portugal," Ferreira said.

After a period of rest, the cork planks are ready for the first stage of the industrial process: the boiling.

Raw cork is boiled for at least one hour to reduce its humidity, making it softer. The planks are trimmed to size and then punched to form the natural cork stoppers.

Seven out of 10 bottles of wine in the world use cork as its choice of stopper material, Ferreira said. 

"[Cork] exports have been growing in average of 4.5% over the last 10 years, which is a growth that is bigger than the wine industry's," Ferreira added. "It shows that [the Portuguese Cork Association is] not only taking part of the growth of the wine business [but] taking market share back to cork out from some competitors as artificial closures."

Wine tourism manager for Companhia das Lezírias, Olena Cherkashyna, said cork has benefits beyond aging a wine and maintaining its quality; it's also more ecological than other kind of stoppers, like plastic or metal.

Ferreira said that the Portuguese Cork Association tries to not waste any of the cork produced. "Even the smallest residues, the powder we have from the retrification from small parts of our production, we use to produce energy." 

APCOR sees the battle against single-use plastic as an opportunity for cork.

"All of the product that we extract from the tree, is used no only in cork stoppers, but also in other applications from construction to automotive industry, to aerospace, sports, fashion, clothes," Ferreira said.