Although there is some historical evidence suggesting that cork was used as a stopper about 2,000 years ago, its use became more prevalent with the introduction of glass bottles in the 17th century. In recent years, other alternatives such as capsules and plastic stoppers have been introduced as closures for wine bottles. However, cork still remains the principal closure of choice for premium wines. – Writes Rajiv Seth


Cork - an introduction

Cork is essentially a piece of bark from an oak tree known as cork oak, Quercus suber. The cork tree grows naturally in a region bordering the western Mediterranean Sea. The major cork producing countries include Portugal, Spain, France, and Italy in Europe; and Morocco, Algeria, and Tunisia in Africa. Several efforts have been made to grow this species in other parts of the world; but, so far, the results have not been encouraging. Worldwide production of cork is estimated to be 3,075,000 tons. Of this amount, Portugal produces the lion's share (about 55%).

Why use Cork as a Wine closure

Cork is a unique substance and a long proven closure for wines. The cork possesses many remarkable qualities which make it an ideal stopper for wine bottles. Some of the important qualities include: compressibility, resilience, impermeability to liquids, low density, little tendency to rot, and a high coefficient of friction.

Cork is highly compressible. It can be compressed without causing significant lateral expansion. It is also very resilient. After compression, the cork can return to 85% of its original volume in about 20 minutes, and about 95% of its volume after 24 hours. During corking, a cylindrical cork 38 to 40 mm long and 23 to 25 mm in diameter is inserted in a bottle with a bore size of about 18 mm. In this situation, the cork diameter is reduced by 25% (from 24 mm to 18 mm) and the cork volume is reduced by 40 to 50%. Due to its elastic nature, the cork exerts a pressure of 1.5 to 3 kg/cm2 against the glass surface (in the neck of the bottle) and forms an excellent seal. The elastic property of the cork is due to the unique cell structure. As noted earlier, the tiny cork cells are filled with air and thus the tissue can be envisioned as layers of tiny air cushions grouped together. The cork is very light in density (0.12 to 0.25 g/cm3) and a little over 50% of the cork volume is air. When the cork is squeezed (as during corking), the air inside the cells is compressed. The compressed air inside the tiny cells exerts counter pressure which permits the cork tissue to expand and provide an effective seal. With prolonged compression, the gas inside the cells gradually permeates out and the resiliency of the cork is permanently lost; the cork is not as elastic as it was before compression. This phenomenon can be observed when a cork from an old bottle (10 to 15 years storage) is withdrawn.

The elastic property of the cork is influenced by its moisture content. The cork remains fairly elastic for insertion into the bottle within a moisture content range of 5 to 12%. But the moisture content is maintained around the 5 to 7% level in order to discourage microbial growth. Cork is practically impermeable to liquids. This is because the cork tissue is made of tightly packed cells; this leaves practically no room for liquid to pass. One millimeter thick cork tissue may have as much as 30 layers of cells. The waxy and suberous composition of the cell wall makes it even more difficult for liquid or gasses to pass.

Another unique and highly desirable property of cork is its high coefficient of friction. This means that it does not slide easily on smooth surfaces such as glass. The cut surface of the cork consists of broken cells that act like suction cups when they are in contact with the glass. Due to this tendency, the cork adheres tightly to the glass surface (this is what makes it hard to remove the cork from the bottle), and also assumes the shape of the neck of the bottle. In spite of small irregularities in the neck of the bottle, the cork fits well and provides an effective seal.

Cork is a very durable stopper. When in contact with wine, it does not readily degrade. In most cases, very little wine penetrates into the cork and few cork constituents leak into the wine (this assumes good cork). Occasionally, vanillin/woody odors may be imparted to the wine, but overall, it doesn't affect the wine flavor.

How Corks are made

As noted earlier, cork is produced from the bark of a cork oak tree. The cork tree is unique; in that, a careful removal of the bark does not harm the tree and secondly, after stripping the bark, a new bark is regenerated. Although the cork tree is a forest species, it does require some care and attention in order to produce high quality cork bark over a long period of time.

The stripping of the bark begins in summer, usually in July. After the bark is peeled off, the tree produces a reddish fluid which protects the mother bark. The fluid dries to a thin layer and by early autumn, new cork cells begin to form. The first stripping of the bark occurs when a young tree is about 15 to 30 years old and has attained a trunk diameter of 70 cm. The bark that is removed for the first time is known as virgin cork. Its structure is irregular, it is relatively hard (not supple), and is not useful for cork stopper production. This virgin cork is used for producing other cork-based materials.

After a period of about nine years from the first stripping, the bark is removed again. The cork bark removed for the second time is called second bark, and is still not considered good enough to produce cork stoppers. The cork bark that is removed in the third and subsequent stripping is called reproduction cork. Its texture is more uniform and it provides excellent material for producing cork stoppers. Usually the cork quality is considered best when a tree is about 50 years old. The tree will live and produce cork until it is 160 to 180 years old. But after about 100 years, the quality of the cork declines. The stripping of cork is done every nine years. This allows enough time for a tree to grow and produce good sized bark. Starting at 25 years, a tree will yield about 15 stripping.

The stripped bark, also called cork slabs or cork planks, is stacked in piles outdoors. This allows the cork slabs to season or cure. During this period, the sap from the bark dries off and the cork planks undergo weather-induced chemical changes. Following curing, the cork planks are subjected to a boiling process which cleans and disinfects them and also makes the cork planks softer and more flexible. This is important because it allows the semi-cylindrical cork planks to be made into flat slabs called cork board.

In the next step, the cork board is cut into strips. The width of the strip is slightly greater than the length of the cork. The strips are placed on their side and the cork is punched out perpendicular to that of the cork growth. This ensures that the lenticels are positioned on the side of the cork touching the glass when the cork is inserted in the bottle. This reduces the potential leakage of wine through the lenticels.

Cork punching requires great skill in order to maximize the quality and quantity of the corks produced. It is estimated that about 30% of the cork board is used in making cork stoppers. After punching, the corks are polished and the edges are trimmed, if needed. Corks with edges trimmed at 45° are called champerd corks. Some winemakers prefer champerd corks because champering makes it easier to insert the cork during corking. Corks used for fortified wines are made by gluing or binding the plastic top to the end of the cork. Such corks are referred to as T-corks.

After polishing, the corks are rinsed with water to remove dust and treated with a solution of calcium hypochlorite and oxalic acid, followed by a final rinse. The process bleaches and disinfects the cork surface. Since chlorine treatment can contribute to the formation of 2, 4, 6-trichloroanisol, a compound known for causing cork taint, an alternative treatment is preferred.

Instead of chemical treatments, some cork producers sterilize corks by exposing them to irradiation. The process is very effective in eliminating microorganisms such as molds and bacteria. Cobalt-60 isotope (radioactive cobalt) emits gamma rays which can penetrate deep into the cork. This achieves the destruction of harmful microbes both on the surface, as well as, inside the cork.

Following chemical sterilization, the corks are dried to bring the moisture content between 6 to 8%, and then graded. Based on the customer's request, the corks are branded and/or surface treated with silicon or paraffin. Coated corks are often used in high speed bottling. Finally, the corks are packaged in plastic bags containing sulfur dioxide and stored at a temperature of 15 to 20°C and 50 to 70% humidity.

Agglomerate cork or composition cork is another kind of cork used sometimes by the wine industry. It is important to note that the agglomerate cork has a low compressibility and is less elastic;

Champagne corks are a type of composition cork. They are larger than the corks used to stopper still table wines. The upper section of the cork is made of cork particles (like agglomerate cork) and the lower section has two cork disks glued to it. The lower end is in contact with the wine when the cork is inserted into the bottle. Champagne corks must be able to withstand high pressure (about six atmospheres). To achieve this, corks with larger diameters (31 mm instead of 24 mm) are used and they are compressed to a greater degree than table wine corks. For example, the regular corks when compressed are reduced in volume by 45 to 50%. In the case of champagne corks, the volume is reduced by 65%. This provides an effective seal and holds the wine in the bottle under high pressure. Large single piece corks are available for stoppering champagne bottles.

Cork Faults and Quality Control

Cork Defects - A defective cork can cause problems such as leaking. formation of deposits or sediment, and equally important, the development of cork taints. To prevent these problems the vintner should recognize faulty corks and avoid using them. Cork defects can be serious; that is, they can cause leakage and/or make cork insertion difficult. These are defined as critical defects. Other flaws may not be as problematic. They may be related to the appearance of the cork rather than the function. Such defects are considered noncritical. Some of the critical defects are given below.

1. Green wood - This is caused by immature cork cells. The flaw is considered critical if more than 50% of the cork length is made of green wood.

2. Poor cork ends - They are caused by holes, cracks, and/or fissures. If the hole or crack is over one-third the length of the cork, it is a serious flaw. The holes can also be caused by worms. Such corks should not be used.

3. Holes - The presence of any large holes (>2 mm) is considered a critical defect, especially if they are connected together over 50% of the cork length. The holes can be due to insect damage or a large number of lenticels.

4. Belly spots or cuts - These are surface depressions caused by the inner density of the cork or by poor cutting of the cork cylinder. The defect is critical when the spots occur over 50% of the cork's length.

5. Woody corks - This condition results when a cork is cut too close to the bark surface. If over 50% of the cork is woody, it is a serious flaw.

6. Chips, breaks and cracks - This kind of damage usually occurs during the processing of corks. A dry cork is also prone to chipping and cracking. Larger chipped areas or a crack on the cork surface is a serious defect.

7. Poor or improper surface coating - The cork surface is often coated with paraffin, waxes, silicon, and other polymer coatings. The purpose of surface treatment is to make dry corking and cork extraction easier, and also to improve impermeability. When the coating is done improperly, it can cause problems during corking or in forming an effective seal.

8. Dimensions not in accordance with designated size - Incorrect cutting can result in corks that are smaller than the intended size.

Other problems associated with corks are sediment and off odors. Cork dust can sometimes be a source of unsightly deposits. The dust is produced during processing and can be located in large lenticels. Usually coating the cork prevents the dust from getting into the wine.

Cork taint or must and moldy odors can sometimes be imparted to a wine by a defective cork. There are many compounds which are associated with corky (musty and moldy) odors. Most important of these compounds is 2, 4, 6-trichloroanisol. When this compound gets into the wine, it imparts an off odor. It should be noted that this compound can get into the wine from various other sources.

So that's it the Journey of cork immortal from Bark to Bottle, surviving the extreme summers, frequent fires and onslaughts of modern technology, with ever changing scientific perceptions.


Rajiv Seth

Rajiv Seth is a wine educationist, Author and an expert in International Wine Legislation especially European Union. In 1987, he became the first Indian to be awarded a gold medal from WSET, London. He also writes for DelWine. Contact him at royalcellar@yahoo.co.in .