A Word About Cork

Natural wine corks are one of the most remarkable products of nature, and have played a critical role in the development of the modern premium wine world.

The incredible properties of natural cork are primarily the result of cork's unique structure. The bark of the cork tree is composed of tiny cells, each a 14-sided polyhedron, with the intercell space entirely filled with air. There are approximately 40 million of these cells in a single cubic centimeter of cork bark.

Cork is the bark of the cork oak tree (Quercus Suber) which grows widely in specific regions of the Western Mediterranean. Traditionally, cork forests have been wild - left to themselves and harvested when appropriate. But recent investments in the industry have led to developments in the forests - including genetic selection of better trees, and advanced pruning techniques.

Cork trees grow in a forest, require a great deal of sun, and an unusual combination of low rainfall and high humidity. The quality and thickness of the bark varies according to these growing conditions.

The fact that corks are made of the bark harvested from living trees has lead European environmentalists to encourage the use of cork over other, less natural, alternatives. Cork forests not only reduce the need for other polluting industries, they also help clean the air of the pollutants caused by these industries.

In fact, cork forests are among the most regulated crops in the world. The forests are considered national treasures, and are regulated by the government.

The largest cork producing country is Portugal, followed by other Mediterranean countries such as Spain, Algeria, Tunisia, Morocco and Italy. Portugal produces nearly 50% of the corks in the world, and the quality of its corks makes the country a leader in both quantity and quality.

Cork trees are harvested in the summer, every nine or ten years, after they reach 25 centimeters in diameter.

Harvesting a cork oak requires very skilled labor. A special axe is used to cut into the bark and then used as a lever to gently pry the cork off the tree.

The cork is peeled off in large panels from the main sections of the trunk, including the large branches. About a third of the bark can be harvested from the tree at one time.

Great care is taken to avoid cutting into the cambium layer of the living tree.

After the harvest, the trees will be left alone to re-grow their bark, which takes about 9 years.

There is a significant difference between the first harvest, or virgin harvest, and the third harvest. The virgin cork is rough, crumbly, and can only be used for cork board, insulation, gaskets, shoe soles, etc. By the third harvest, the cork is a smooth, clear bark that will make great wine corks.

After the cork is harvested, it is carefully stacked in the forest so that rainfall will gently wash through it. The cork is left to age for a period of months in the forest to allow the moisture content to stabilize.

While the cork is in the forest, cork suppliers inspect it and select their lots.

Harvesting a cork oak requires very skilled labor. A special axe is used to cut into the bark and then used as a lever to gently pry the cork off the tree.

The cork is peeled off in large panels from the main sections of the trunk, including the large branches. About a third of the bark can be harvested from the tree at one time.

Great care is taken to avoid cutting into the cambium layer of the living tree.

After the harvest, the trees will be left alone to re-grow their bark, which takes about 9 years.

While the cork is in the forest, cork suppliers inspect it and select their lots.

All cork must be boiled before it is worked to make it more pliable, and to fully expand the lenticels. The cork cells are collapsed and wrinkled before boiling, but after boiling, the air in the cells expands and creates a very tight, uniform cell structure that looks like a honeycomb.

The corkwood is brought in from the field and stacked at the factory for its final aging. Once the producers have determined the corkwood has stabilized, it is stacked tightly together on pallets.

Then the corkwood is taken by forklift to huge vats where it is boiled for an hour. After the boiling, the expanded, flat cork is lifted from the vat and left to dry. When it has dried to 20% moisture content, it's ready to be worked.

Demand for top quality cork by winemakers has encouraged our focus on quality through the entire process. To achieve this quality, the corkwood must have very few defects and be consistent in color, texture and density. Most of the sorting is done by hand to ensure that only the finest bark gets used.

Corkwood is sorted by quality, thickness and size. After this initial sorting, the cork takes another trip to the boilers.

The second boiling is performed to moisten the corkwood so that it can be punched, and to remove any remaining particulate matter from the bark.

The corkwood is inspected and graded every step of the way. This is done to ensure that only the best quality corkwood becomes a Cork Supply USA cork. Rejected cork material will be used for gaskets, cork flooring - even insulation on NASA’s space shuttle.

Following the corkwood sorting process, it is sliced into narrow strips as wide as a wine cork is long, from which the corks will be punched. Each cork shows the growth rings of the tree running vertically through it.

The punching is very technical work. While lower grade corks can be punched by automated machines, premium corks require hand punching by skilled craftsmen. They ensure that imperfections in the corkwood do not turn up in the final cork.

Corks have traditionally been washed in chlorine. They were placed in net bags and manually dunked in the chlorine solution, rinsed in water and neutralized in a vat of oxalic acid.

Today's methods have greatly improved this. Modern washing systems utilize a combination of stainless steel equipment, computerized control systems and mild chemical solutions. The corks are rinsed in clean water, washed in the chemical solutions and neutralized to remove residual oxidants.

There are now a variety of washes-hydrogen peroxide (Alpha TM), potassium metabisulfite (Natura TM) and traditional chlorine wash.

Moisture control following the washing is a critical element in this effort. Today, top producers are using a combination of techniques to quickly dry the corks. First the corks are centrifuged to remove surface moisture. Then a temperature and humidity controlled room dries them further.

The moisture is lowered to 8% where mold growth is inhibited.

After the corks are washed and dried, they are visually inspected and graded yet again.

One procedure is to use a team of very highly trained workers, who individually inspect each cork to determine its quality.

The latest system involves optical sensors programmed by computer to select corks on the basis of a number of factors. And while these are very fast and efficient, most cork producers will tell you that they still prefer to use people for the final check - because they are more accurate than computers.

Corks are available in many different grades, with overall wood quality being the difference. The quality relates directly to density, porosity, major and minor defects (cracks, holes, etc.), and cosmetic appearance. High quality premium corks will have an improved appearance as well as a lower defect level.

One of the most complicated subjects in the cork industry is the concept of grading.

Because they are natural products, it is impossible to perfectly classify corks - some people may prefer one type, while others look for something else. At Cork Supply USA we have several different types of premium corks to fit any need.

Dedusting: The corks are tumbled in a vacuum to remove dust and particles.

Moisturizing: Corks have been maintained as dry as possible to inhibit mold growth. Optimum bottling conditions require cork moisture between 5 and 8%.

Treatment: To improve sealability and ease of insertion/extraction, the corks are coated with customized levels of paraffin and silicone.

Packaging: Corks are automatically counted, sanitized with sulphur dioxide gas and sealed in gas barrier bags. Depending upon size/winery capacity, the corks are shipped in boxes of 5,000 or a reusable Eco-Box with a 30,000 cork capacity.