This spongy material which comes from the bark of the cork tree has traditionally been used to seal wine bottles. The best cork is said to come from Portugal. Currently, there is a movement to turn to man-made materials to replace cork due to the shortage of cork trees.
A Brief History of Corks in Bottles
Corks have been used as bottle stoppers for as long as we have had wine. We know that the Greeks in the 5th century BCE sometimes used corks to close wine jugs. Following in their footsteps, as usual, the Romans also used the cork as a stopper and also coated corks with pitch to seal the closure.
Corks, however, were not the closure of choice in those ancient days: the most common closures for wine jugs and amphora were a coating of pitch or gypsum over the opening of a vessel or a film of olive oil floating on the surface of the wine. The use of corks was apparently completely given up for some reason in the medieval times. At any rate, paintings from that era depict twists of cloth or leather used to stop the jug or bottle, sometimes with sealing wax to make a secure closure.
We find corks beginning to be mentioned again at the end of the 16th century. By the time Shakespeare wrote “As You Like It” (Between 1598 and 1600) they were well enough known for Rosalind to say impatiently to her cousin Celia: “I pray thee take thy cork our of thy mouth, that I may drink thy tidings.”
The marriage of cork and bottle, at least in England, took place by degrees over the first half of the 17th Century. The alternative closure of the time, stoppers of ground glass made individually to fit the bottle neck, held their own for a remarkably long time. Worlidge’s “Treatise of Cider”, published in 1676, declares that great care is needed in choosing good corks, “much liquor being absolutely spoiled through the only defect of the cork. Therefore are glass stoppels to be preferred….” at the cost of no small trouble since each one had to be ground to fit a particular bottle, using emery powder and oil. The “stoppel” was then tied to the bottle by a piece of packthread around a button on top. As late as 1825, the ultimate luxury stoppers were abandoned because they were usually impossible to extract without breaking the bottle.
The nature of cork is such that its success as a closure depends upon its fitting snuggly into an opening with a relatively uniform diameter. This, it was not until the 17th century, when glass bottles were first made with more or less uniform openings, that the cork truly came into its own.
Many wine historians have linked the development of the glass bottle and its cork stopper as two necessary prerequisities for the modern international wine trade. Wine no longer need be shipped in bulky, awkward clay vessels or wooden barrels. The economies of space enabled ships to carry more wine and the wine was much less subject to spoilage in the shipment.
Because cork stoppers prevented oxygen from spoiling the wine, both in shipment and in subsequent storage, it became evident that wine benefited from its maturing time in the bottle. The desirable properties of aged wine made it more valuable, and collecting and cellaring wines from many different regions became both feasible and profitable.
One thing remained to be invented before the cork closure became truly practical: a corkscrew so that the cork could be driven right in, not left half-out like a stopper. The first mention in print of a corkscrew is in 1681. It was described (by one N. Grew) as a “steel worm used for the drawing out of corks out of bottles.” They had been in use for at least half a century for drawing bullets and wadding from firearms. But the word corkscrew was not coined until 1720. It was originally called a bottlescrew.
Why Do We Use Cork Today?
In the almost three hundred years intervening between the renewed usage of cork and the end of the 20th century, cork has been the overwhelming closure of preference for fine wines around the world. The very sound of a cork’s firm, round “plop” from the mouth of a bottle sets taste buds salivating in anticipation of a delicious treat. There are several very sound reasons for cork’s continuing popularity: cork’s component materials and structure give it a unique set of physical and mechanical properties that make it ideal as a bottle closure. They are:
Lightness: cork is very light in weight and low in density.
Impermiability: cork is very resistant to moisture penetration (Why do you think it is used in life jackets?)
Compressability: cork is capable of compression to half its dimension with no loss of its flexibility. And it is the only solid which may be compressed in diameter without expanding its length.
Flexibility: when removed from compression, cork will recover about 85% of its initial volume immediately and more than 98% after 24 hours.
Adherence: the slicing of the surface cells in forming a cork stopper produces an extraordinary cupping effect. Millions of cells are opened and function as suction cups. This provides an exceptional power of adhesion to wet, smooth surfaces.
Temperature and age stability: cork retains its properties at both high and low extremes of temperature and will age almost indefinitely without deterioration.
Cork is the only material known that tolerates imperfections in glass and compensates for them.
Cork is bio-degradable.
How Does Cork Seal The Bottle?
Cork’s natural vegetable tissue is composed of closed air cells arranged with polyhedric geometry which allow pliability and do not leak. The cells are banded together in a perfectly regular manner at a rate of about 40 million per cubic centimeter. An average wine cork therefore contains almost 800 millions cells, which act as suction cups to prevent seepage of wine from the inside of the bottle. Suberin, a complex fatty acid substance, gives cork its basic composition. It is the tissue that makes cork unique and gives it its particular elastic characteristics.
The Source of Cork and Its Manufacture
Quercus Suber (thus, suberin) is the botanical name for a kind of slow growing, evergreen oak that grows well and prolifically throughout specific regions of the Western Mediterranean – and only in these regions. It requires a great deal of sunlight and a highly unusual combination of low rainfall and somewhat high humidity. (Experiments in growing cork trees in North and South America, Russia and Japan have so far proved disappointing.) The quality and thickness of the bark will vary according to its specific growing conditions. It has evolved the spongy substance of cork as protection and insulation for itself, particularly against fire.
Most trees will die if their bark is removed, because the bark helps to carry the sap that is essential to the life of the tree. The cork oak, however, has two layers of bark. The inner layer is alive and it is the base on which a new inner layer grows each year. As the old layers move outward and die, they serve as insulation, protecting the tree from the hot arid winds in the growing areas. The dead outer layer can be stripped away without injuring the tree, but care must be taken not to penetrate the inner living bark.
If you are planning to grow cork trees for a living, be prepared to wait to least 25 years until the first harvest of cork is mature. Cork from the first harvest, however, is irregular in size and density, and not suitable for wine stoppers. It will probably be used for floor tiles or sound insulating materials. Nine more years must pass before the tree can be harvested again. Even this second harvest of cork is not good enough for wine bottle stoppers. It is not until the third harvest, or when the tree is 52 years old, that the regularity of size and density of cells renders it acceptable for wine bottle usage. A cork tree will yield between 13 and 18 useful harvests in its lifetime.
The cork is tripped by hand with the aid of small sharp axes, and the resulting cork strips are then stacked and weathered. The tree itself is carefully marked and numbered, so that future harvesters will know that particular tree needs nine years before it is harvested again.
The cork manufacturers then inspect the weathered stacks of cork to determine if they will buy and at what price. They truck the cork north from the cork forests to Portugal’s cork factories, and stack the cork for an additional 3 months or more to let it weather and dry. The weathering process is designed to arrive at the optimal amount of moisture in the cork. The proper moisture content is crucial for the elasticity and compressibility of the cork. In addition, the layer of cork closest to the interior of the tree has considerably higher moisture content, so the weathering ensures consistency of moisture level throughout the sheet of cork.
After all this drying, the cork is immersed in boiling water for at least 90 minutes to sterilize it and to enable it to flatten from its original curved tree trunk shape. After boiling, the corks “ripen” for 3 to 4 weeks in order to achieve the desired moisture level. Next the cork is trimmed into strips and holes are punched into it for the correct size and shape for the bottle cork. The width of the bark strip forms the diameter of the cork, not the length of the cork. Thus, growth rings of the tree are to be found imbedded longitudinally in the cork. This stage of the manufacturing process requires a keen eye as the hole punchers maneuver the strips for maximum quality. This is one of many quality control steps in the manufacture of the cork. Next the cork heads are polished so that the cork will have a specific, uniform length and the body is polished so that it will have a specific, uniform diameter.
The corks are then washed and dried. Most are bleached in either chlorine or hydrogen peroxide in order to disinfect the cork of any remaining impurities, and some are just rinsed without bleach, depending on the specific winery’s request. Corks are graded for quality and then each cork of comparable quality is branded with the name of the winery that has ordered that particular batch.
A final surface treatment, either silicone and / or paraffin or a resin, is sprayed or tumbled onto the surface of the cork. This treatment eases insertion of the cork into the bottle and improves the seal against the glass. The corks are bagged in plastic bags and shipped to their final destination, the winery that ordered them.
What about alternative closures? The two front runners are a synthetic, plastic substance trademarked as Cellukork, and the screw cap. Cellukork is made of ethylene vinyl acetate, and looks and feels similar to real cork. You use a corkscrew to remove it from the bottle. It has two obvious drawbacks: one is that it often fits so tightly in the bottle that it is very difficult to remove (a problem that will no doubt be resolved through research); the other problem is only a potential problem – but a very serious one if it materializes. That is, is the synthetic material truly non-reactive and inert over long periods of time? Will it impart no flavors of its own to the wine? This is worthy of consideration for any wine that will be cellared for a number of years before it is drunk. Naturally, wineries using these plastic corks are deliberately aging wines to see what happens, but it is too soon by several years to know the outcome.
Aside from the quality issue, the image of a screw cap is firmly lodged in many peoples’ minds as the “epitome of cheap wine.” Many fine wine producers, sensitive to the fine wine market, hesitate to switch to a screw cap because they do not want their wine to be perceived as of inferior quality.
The best part for the consumer is that screw caps are very user friendly – no more struggles if you forget to bring the corkscrew to the picnic! Many wine marketers believe that the down-home nature of screw caps can only benefit the industry by making wine more accessible – figuratively as well as literally – as we erase the elitist image associated with the ritual of opening wine.