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Resource Pages: Cork Grading
Storing Corks
Corks usually come in plastic bags that have been treated with SO2 gas, as a preservative. Corks should be stored in a dry cool place, about 60-70 degrees F. If the bag has been opened, seal it as tightly as possible. Most manufacturers suggest that corks should be used within 6 months, but we have had no problem in storing corks for a year.

Using Corks
Most manufactures treat corks with a coating of paraffin to ease insertion into the bottle, and suggest that corks are not heated or washed before use. Our advice for corks that are left over from last years bottling is to let them rest for a few minutes in a bucket of SO2-treated water (100PPM) to help rehydrate the cork and kill any bacteria that may have developed. Fill the bottle so that there is about ¾ of an inch of space between the wine and the fully inserted cork. Corks will relax and expand for the next 24 hours, so keep the bottle upright for that time in a 60-70 degree F area. Cold corks will not relax and may not properly seal the bottle. If wine gets in between the cork and the bottle, leakage may occur.

Natural corks: One solid piece of natural cork, cut in the process described above. Natural corks very in price depending on the grade, and are designed to last in bottle aging of at least 10 years. The standard length of natural corks is 1 ¾ inches (44mm) long. Shorter corks are not recommended. Longer corks are available, but are chosen mostly for aesthetic appeal of the loud pop when uncorking, rather than improving the length of bottle aging.

Two-disk corks: Agglomerated cork in the middle with natural cork disks, about 1/8 inch thick, at each end. This style offers lower cost but similar performance of a natural cork. Two-disk corks are increasingly popular with home vintners due to the excellent price and performance. Designed for wine aging of up to 10 years. All Champagne corks are also made in this way.
Cork is a natural and sustainable way of sealing a wine bottle, protecting the wine from oxidation and at the same time providing for micro-oxygenation needed to effectively mature the wine. Time-tested for centuries, cork has the lowest carbon footprint compared to metal or plastic closures. Cork consists of the thick outer layer of bark on a cork tree. Although cork trees grow in many areas, most cork for commercial use comes from Portugal. Harvesting the cork means removing bark from the tree in the spring or summer, at the time the tree is in an active growth phase. In this way, the tree is not damaged, and new cork is quickly regenerated. Today, approximately 85% of wines are sealed with cork.

Cork Production
Cork production starts after the cork bark is cut from the tree by hand, into cark planks. The planks are stacked flat and boiled, which causes the cell structure to expand into a honeycomb structure, and makes the plank more pliable for the manufacturing process. The planks are cut into strips, lengthwise, with each strip as tall as a finished cork, about 1 ¾ inches long. Sharp, cylindrical knives are used to cut the round cork from the strip, sideways into the strip. The grain of the cork is across the cork, not along the length. The newly cut corks are washed, sterilized, and then graded. The remainder of the cork strip is ground up to make agglomerate corks, and a host of other products.

Cork Grading
Every cork vendor has their own classification and naming system for grading corks, with as many as 9 levels. All of them basically separate the corks based on the number and size of holes, cracks, and blemishes. The visual grading scale, reproduced at the bottom of this page, is from the Cork Quality Council, and summarizes the grading process into three broad categories. The Cork Quality Council is a non-profit organization to promote and ensure quality assurance standards. Please visit their site for more information, at www.corkqc.com
There has been a lot of press about TCA, or Trichloroanisole, that can cause cork-taint in wine. TCA is produced by naturally occurring fungus, which can contaminate the cork, and is then transferred to the wine via the alcohol during aging. TCA can be significantly reduced through chemical action of heat and solvent. Most cork manufacturers process their corks before shipping with steam and ethyl alcohol to remove TCA.

Cork Types
All of the corks provided by the reputable vendors can be used for wine closure, but there are three basic styles that offer different price and performance characteristics. Here is a summary.

Micro-granulated corks: Made from granulated natural cork, that is glued together with a food-grade binder. Also called agglomerated corks. Much lower in cost than natural corks, these corks are typically used in wines that are intended to be consumed in 1-3 years.