All About Wine Corks

All About Wine Corks

Have you ever wondered about the corks used to seal wine bottles? And why the cork is presented to you in a restaurant? There’s more to this little piece of bark than you knew.

According to the Cork Quality Council, More than 12 billion bottles of wine are closed every year with cork. It is used for more than 70% of wine producers and has been keeping all the qualities of this drink intact for centuries.”

When Dom Pérignon developed his méthode champenoise in the 1600s, the wooden corks previously used for wines didn’t work for champagne. He successfully adopted cork stoppers.

The cork’s natural properties of cork provide lightness (since it consists of approximately 90% air), flexibility, elasticity and compressibility. This enables the cork stopper to regain its initial form and adapt to the neck size and shape of the bottle after insertion, even if the glass undergoes variations in temperature. Because the cork creates a seal, it is impermeable to liquids and gases, and highly resistant to moisture that could lead to oxidation of the wine.

The proper seal depends more upon the diameter of the cork than its length. Ideally, the cork should be at least 6mm wider than the smallest diameter of the bottle’s neck, and compressed no more than 33% of its diameter when inserted.

Where Does Cork Come From?

Cork is derived from the Cork Oak, found mainly in a narrow swath across Western Europe and Northern Africa along the Mediterranean coastline. The main countries producing cork include Portugal, Spain, Morocco, Algeria, Tunisia, Sardinia, Corsica, and France. The cork forest is the dominant ecosystem in its arid coastal regions where it thrives on the poor, sandy soil.

The world’s largest cork tree is the Whistler Tree, found in Portugal’s Alentejo region. At 230 years old, it has been producing the finest wine corks since 1820, yielding recently 825 kg of raw cork. That’s enough to seal 100,000 wine bottles! An average Cork Oak produces enough for approximately 4,000 bottles.

The Cork Oak is also found in the United States as a hearty, evergreen shade tree in USDA Hardiness Zones 8-11. They are most commonly found in southern California, Western Arizona and Florida. Some of the more prominent examples are found on the grounds of UC Davis and Disneyland.

Cork Harvesting

Cork is harvested on a nine-year schedule by removal of the thick outer bark during spring or summer while the tree is in full growth. It breaks away easily, without causing damage to the tree, which quickly regenerates new bark. Since it is a delicate operation, the cork is stripped by hand using traditional tools and methods. The process has never been mechanized.

The first cork harvest is not performed until a cork oak is 25 years old, and every 9-12 years thereafter. The first harvest is called “Virgin Cork,” and is not suitable (neither is the second harvest, called Reproduction Cork”) for stoppers. It is generally used for cork flooring and other products. The third and subsequent harvests produce the “Amadia Cork” used for wine stoppers. A healthy cork oak can be expected to live more than 200 years.

Why Does the Wine Steward Present Me With the Cork?

When your sommelier, steward or server offers you the cork upon opening your bottle of wine, it is for a reason. First, it is to make sure that the cork matches the brand and that no substitutions were made. Second, the quality of the cork tells you how much the winemaker has invested in the wine. You now know the variations in corks and can tell if it is a solid cork or composite.

Look for signs of sediment on the cork. This ensures that the bottle of an aged wine has been stored properly. Check for saturation on the cork. This could mean that the cork dried out, was saturated later, and is thus compromised. But don’t sniff the cork; that will give you no information about the wine.

Now that you know all about wine corks, go enjoy a bottle of your favorite wine.

The Seven Types of Cork Stoppers

There are seven basic types of cork stoppers: natural corks, colmated corks, champagne and sparkling wine stoppers, technical 1+1 corks, agglomerated stoppers, bar top corks, and synthetic.

One-Piece Natural

Natural corks seal wine in a glass container, which aids in the maturation process by sealing in the physical and chemical processes in a very low oxygen-content environment. A perfect seal can be expected for decades.


Colmated stoppers are natural cork with pores that are filled in with the cork powder resulting from the finishing of natural stoppers. Glue from a natural resin and natural rubber is used to affix the dust within the pores to improve its visual appearance and performance. This gives the cork a smooth, unblemished finish.


Champagne stoppers are used to seal high-pressure liquids like champagne, sparkling wines and ciders. They are created from agglomerated cork granules to which up to three discs of natural cork are attached to one end. Their diameter is larger than wine corks to enable them to retain the high internal pressure.

Twin-Top Agglomerated

Technical 1+1 corks were designed to use in bottles of wine meant to be consumed within one to three years. They consist of a dense agglomerated cork body with discs of natural cork glued to the top or both ends. Those with a cork disk at one end are called 1+1 technical stoppers, while those with two discs at one end are called 2+0 technical stoppers. This type of cork is chemically stable and mechanically resistant; submitting well to the bottling and uncorking processes.


Agglomerated Stoppers are those made entirely from cork granules derived from the by-products of the natural stopper production. The granules are bound together with a food-grade substance. These stoppers are for bottles intended for use within a 24-month period, and are economical to use for lower-priced wines and high-turnover products. To be considered a “cork” stopper, it must contain a minimum of 75% natural cork.


Synthetic Corks are closures made from non-biodegradable, but recyclable, plastic compounds designed to look like natural cork and make the familiar “popping” sound when opened. Use of synthetic corks can help protect the wine against “cork taint,” but drawbacks include lack of oxygen to the wine, extraction difficulty, and the inability to use the cork to reseal the wine bottle.


Bar-Top cork stoppers have one end of the cork attached to a cap made of wood, porcelain, metal, PVC, glass or other material. It is generally used with liqueurs, and fortified wines and spirits that are ready to drink. Examples include: port, Calvados, whisky, vodka, cognac, brandy, and clear spirits. They are easily reused, which is important for re-capping bottles not consumed in one sitting.

To assess a cork’s quality, experts look at its density, humidity, surface treatment, extraction force, visual pattern and sampling. Visual grades range between Grade A (no visual flaws) to Grade C.

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