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Corks – From the Tree to the Bottle

by David on June 20, 2009

Wine residue has been found in vessels dating back as much as 8000 years, and those containers were sealed with a variety of materials, including cork. We’re talking about a truly ancient beverage and seal!

Although cork was used in ancient Greece and Egypt, it wasn’t until the 17th century that cork became the most common way to seal bottles. By the end of the 17th Century they were being pushed all the way into the neck, rather than part of the way, introducing the first corkscrews.

The duty of any closure is to protect the wine from oxygen. In that regard, cork has proven to be an excellent closure over all of these centuries. It swells when it comes into contact with moisture and creates a tight seal. Cork comes from the bark of the cork oak tree (Quercus Suber), and the greatest source of cork is Portugal, followed by Spain.

These beautiful trees live as long as 200 years. The first harvest is at 25 years, although the cork won’t be suitable for wine until the tree is over 40 years old. Most trees would die if their bark is stripped, but if it’s harvested correctly, the oak is not harmed, the bark grows back, and it can be harvested again every 9-12 years, depending upon the country of origin and its regulations.

After stripping, the bark is sorted for quality (the cork performs better if it doesn’t have a lot of cracks and holes) and laid out to dry for anywhere from six months to 2 years. The seasoned wood is boiled in distilled water to make it more flexible and to rid it of impurities. It gets another few weeks rest, and then flat strips are cut, evaluated for quality and sorted before the individual corks are mechanically punched out, evaluated and sorted once again. The punched corks are polished to the required length and evenness by an abrasive stone and washed in a hydrogen peroxide solution to eliminate microorganisms that might contribute to off aromas or flavors. Then they’re dried until they’re about 6-8% moisture content and sorted into bales of 10,000 corks.

In spite of all the care in production and cleaning, we still can’t be sure that the cork is free of defects that can cause off aromas and this is a major hurdle. The most common problem associated with cork is that sometimes microscopic fungi, in the presence of moisture, convert naturally occurring, chlorophenols into chloroanisole. This can make the wine smell “corky”-like an old, dank basement or moldy newspapers. The compound that gets into the wine is called 2,4,6 trichloroanisole, or as most prefer to call it, TCA. The good news is that it can’t hurt you. The bad news is that it stinks!

Quality control is changing rapidly. Traditionally, after the bales arrived from Portugal, we teamed up with the supplier’s quality control staff and each selected random samples for independent sensory testing. This consisted of placing the corks in separate containers, soaking them in wine and doing a “smell test”. This method was the best we had for a long time, but it wasn’t terribly effective because of the small sample size and the fallibility of the human senses. As of very recently, enologists have developed far more accurate tests (called “solid-phase microextraction”) for identifying TCA and they can be conducted on the whole bale. Our cork supplier runs tests before the corks leave Portugal, and again when they arrive in California, to assure that they’re below 1 part per trillion TCA (human threshold is believed to be 4 parts per trillion). They’ve also developed a cutting-edge machine that purges the TCA and other off odors from the cork (it deep cleans the corks in a steam distillation of ethyl alcohol). It appears that the TCA issue may not exist in the near future if wineries that prefer natural cork are willing to pay for these added quality control assurances.

With the quality control work accomplished, we finalize our purchase, the corks are printed with our Goosecross logo and a thin film of paraffin and silicone is applied to make them easier to insert and extract. Now the cork is ready to go to work, protecting the wine, until you pull it out of the bottle and it has completed its mission.

Click here for “What about plastic corks and screw caps?”

Pictures are from the Cork Quality Council web-page.

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