Corks - Past, Present, and Future

Cork is a spongy material which comes from the bark of the cork tree and 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 and problems with tainted corks. This month we will begin a series of articles looking at the history of cork, why it is used and why it’s future may be limited. The following is based on material from Simi Winery, Healdsburg, California.

A Brief History of Corks in Bottles

Corks have been used as bottle stoppers for as long as we have had wine. The Greeks in the 5th century BCE sometimes used corks to close wine jugs. Following in their footsteps, 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 in the medieval times. 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 out 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...” Each stoppel 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. Glass stoppers were finally abandoned around 1825, primarily because they were often impossible to extract without breaking the bottle.

Cork’s success as a closure depends upon its fitting snugly into an opening with a relatively uniform diameter. Thus, 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 prerequisites 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 all the way in, not left half-out like a stopper. The first printed reference to a “bottlescrew” was in 1681 by one N. Grew: “a steel worm used for the drawing out of corks out of bottles.” They had been used for at least 50 years to draw bullets and wadding from fire-arms. The term “corkscrew” was not coined until 1720.

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.
• Impermeability: cork is very resistant to moisture penetration. (It’s used in life jackets!)
• Compressibility: cork can be compressed to half its dimension with no loss of its flexibility. And it can 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 usually lasts 20 years without deterioration.
• Cork is the only material known that compensates for small imperfections in glass.
• Cork is biodegradable.

How Does Cork Seal The Bottle?

Cork’s natural vegetable tissue is composed of closed air cells arranged with polyhedric geometry. This allows pliability without leakage. 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 million 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 slow growing, evergreen oak that grows well and prolifically throughout specific parts of the Western Mediterranean (mostly in Portugal), and only in these regions. It requires a substantial sunlight and a highly unusual com-bination 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. The tree has evolved the spongy substance of cork as protection and insulation, 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. 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 bottle stoppers. It is not until the third harvest, 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 13 to 18 useful harvests in its lifetime.

The cork is stripped 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 weathered stacks of cork are then inspected by the manufacturers 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.

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. The corks then “ripen” for 3 to 4 weeks 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 embedded 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. Next the cork heads are polished so that the cork will have a specific, uniform length and the body is polished for 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, 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.

There's Just One Little Problem.....

With all the beneficial properties of corks as bottle closures, one significant defect rears its ugly head. This is “corkiness,” a condition that exists when wine is tainted by the presence of a chemical compound called 2,4,6-Trichloroanisole - TCA for short. This compound appears to be caused in the cork by the interaction of moisture, chlorine and mold. Corks are often exposed to these elements during their production and TCA can form. (TCA also occurs naturally in the wood and bark of many trees, including the oak family.) Unfortunately, the human nose can detect this “corkiness” at concentrations as low as 4 parts per trillion!

A lightly corked wine may simply smell like cork, while a badly corked wine smells musty, like damp cardboard or old newspapers. The usual rich aroma and flavor of the wine is stripped away by the musty odor. A wine may be more or less corked; that is, there may be just a trace of corkiness that will be detected only if the person smelling the wine knows from prior experience what aromas and flavors a good bottle should have. But a bad case of corkiness is overwhelming and unforgettable! It poses absolutely no health problems - it just doesn't smell or taste good. Ultimately, the problem is that you have spent good money for a bottle of bad wine, and you have absolutely no way of knowing this until you have opened it and poured it.

Should You Worry?

Several years ago, the industry became aware of what seemed to be a growing quality control problem with corks. Portugal, which produces about 52% of the world’s cork supply, had undergone a political revolution in 1974 that changed the management of the cork tree forests and of cork factories. In some cases, unskilled or inexperienced people were managing groves of cork trees. Quality control standards were very poor by today’s standards. Almost 2.5 million acres of cork forests had to be cleaned up, newly planted and reorganized.

Major investment got underway in 1978, and today well-managed forests, stringent quality control in growing, harvesting and processing are combating the problems of 2,4,6-TCA. Peroxide baths instead of chlorine bleaching (as well as “natural” corks with no chemical bath at all) and controlled humidity in shipment are also improving the quality of cork stoppers.

Because world-wide concern with the perceived degradation of cork quality had a tremendous impact on the cork industry, a group of U.S. cork suppliers founded the Cork Quality Council (CQC) in 1992. Its mission was to improve the quality of corks at the source, to develop an educational program to assist wineries and to develop industry standards for cork quality. The CQC has been a major influence in persuading cork producers that it was in their own best interests to monitor cork production in a stringent fashion.

What About Plastic Stoppers and Screwtops?

In 1990, Harvey Steiman of The Wine Spectator estimated that 4.7% of the wines submitted to that magazine were contaminated by corkiness. Craig Goldwyn of The Beverage Testing Institute (1992), estimated that up to 8% of wines are tainted by the time they reach retail stores or restaurants. And Lyn Farmer of The Wine News (1994) estimated that 3% of the wines submitted for review were corked. Personally, I find these estimates quite excessive. In my experience, a 1 - 1.5% corkiness rate seems accurate. But even this is unacceptably high.

Bill Stephens, a respected wine writer for The San Antonio Express-News, is vocally opposed to corks, saying they should go “back to the dark ages where they belong.” The revered tradition of cork as bottle stopper for wine will die hard and slowly, but anyone who has an opinion or preference appears to hold it strongly. At least it makes for lively conversation!

Not all the opposition to corks as wine bottle stoppers is based on quality issues alone. Many people believe that cork usage is an elitist holdover that renders opening a bottle a difficult and esoteric process. And some consumers are intimidated by the cork-sniffing, bottle-opening ritual in restaurants. (You needn’t be! When the waiter presents the cork, just place it on the table and ask to taste the wine. The cork reveals nothing about the wine’s quality.)

What about alternative closures? The two front runners are a synthetic, plastic substance called Cellukork, and the screw cap. Cellukork is made of ethylene vinyl acetate. It looks and feels similar to real cork. A corkscrew is used to remove it from the bottle. It has two 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 the question of whether the synthetic material is truly non-reactive and inert over long periods of time? Will it impart no flavors of its own to the wine? 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. None-the-less, more and more low and mid range producers are switching to Cellukork.

And the screw cap? It provides an excellent air-tight seal, although there is a question as to whether or not it will protect the wine over a very long period of time. Aside from the quality issue, the image of a screw cap is firmly lodged in many 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. However, no fine wine producers are currently marketing their wines with screw caps.