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December 17, 2008 > TechKnow Talk: Put a Cork in It!

TechKnow Talk: Put a Cork in It!

The enjoyment of a bottle of good wine begins with the ritual of pulling the cork. The gentle "pop" releases a hint of the aroma within, soon followed by the visual appeal of the wine as it is poured. Three of the senses have been stimulated before the glass is even taken in hand.

Natural cork has been the wine bottle closure of choice for hundreds of years. If cork is so well suited for this task, why have competing products such as synthetic materials and screw caps become popular in recent years? Are they better or just less expensive?

Cork grows within the bark of trees. Commercial cork comes from the cork oak, which produces a very thick layer of this material. Cork oak grows primarily in the western Mediterranean. Portugal and Spain produce more than 80 percent of the world's cork with Italy, Algeria, and Morocco contributing about five percent each.

Harvested in long strips using a knife or hatchet, cork is then boiled to soften it and allow the tough outer bark layer to be removed. Bottle stoppers are punched out of these strips and packaged for sale. Some will be branded or stamped with winery logos. The leftover material is ground up and used for cork flooring and other products. The cork oak will re-grow its cork and outer bark, and can be harvested every 8-12 years.

Cork contains a waxy substance called suberin, which prevents rot and renders it unappetizing to insects. The waxy nature of the suberin also creates a seal with the bottle, locking out air and moisture. These qualities, combined with its natural elasticity (springiness) and light weight make cork an ideal bottle stopper.

There are various qualities of cork, based partially on the degree of porosity. Less porous corks provide a better seal, but are more expensive. (A high quality cork may cost $1; a poor quality cork can be had for 20 cents or less.) Bottles sealed with cork must be stored on their sides or upside down to prevent the cork from drying out and shrinking.

The seal provided by cork, while very good, is not perfect. Some air does penetrate the cork, allowing a very limited amount of oxygen to reach the wine and interact with it. This oxidation process is called bottle aging, and softens the intensity of tannins in red wines, highlighting more complex and subtle flavors. This is in part why some wines improve with age.

This is very good for a heavily tannic red wine, but white wines (which contain no tannin) or fruity red wines intended to be consumed in their youth may not fare as well when subjected to lengthy storage in a corked bottle. The oxidation process will begin to degrade the bright flavor of these wines after only a very few years.

Wine is also subject to a condition known as cork taint. This is caused by the presence of a chemical called trichloranisole (TCA). Though harmless, TCA produces a musty odor that corrupts the flavor of the wine. It can range from a very subtle effect detectable only by an expert to a strong odor rendering the wine undrinkable. TCA tainted wine is said to be "corked."

The process leading to cork taint is not fully understood, but may involve the interaction of naturally occurring fungi with chemicals used to sterilize the cork. Various studies have indicated some degree of cork taint in 1-7% of aged, corked bottles, though recent changes in processing methods and chemicals are probably reducing this percentage.

Partially in an effort to address the problem of cork taint, some wineries have begun using synthetic corks, made of a rubbery plastic material that has a similar appearance to cork. These may expand upon extraction and are thus not ideal for resealing a partially consumed bottle. Synthetic corks are inexpensive - less than 10 cents each - but are more permeable to air than natural cork, allowing the wine to oxidize more quickly. They will not dry out, so the bottle can be stored in any orientation.

Another approach is the much-maligned screw cap, an increasingly popular aluminum closure that threads directly onto the bottle. Among its many advantages is the ease of removal and replacement, requiring no special tools. It also provides a very tight air seal, much better than natural or synthetic cork, and is very inexpensive.

Most New Zealand wineries and many elsewhere, including California, now use screw caps for at least some of their wines. The most significant negative is consumer perception. Several surveys have indicated wine drinkers prefer cork to other closure methods and are especially cold to the screw cap. Again, the tradition and ritual of pulling the cork is, for many, a valuable aspect of the experience.

Glass stoppers have become popular with some European wineries in recent years. These are marketed under the names Vino-Seal or Vino-Lok. They provide an excellent seal, possibly even better than screw caps, and are handy for opening and resealing the bottle. They are relatively expensive however, comparable in cost to high quality cork.

What type of closure is best? It depends on the wine and the intent of the consumer. Most wines sold in the U.S. are consumed within a few weeks. Few people have either the interest or facility to properly cellar wine for long periods. Wine makers understand this and produce many wines intended for immediate consumption, featuring soft tannins and ripe fruit flavors.

Screw caps or glass stoppers, because they provide the best seal, are ideal for holding these ready-to-drink wines unchanged over time, even for many years if stored in cool, dark conditions. These wines begin to lose their bright, fresh flavors behind cork or synthetic closures if not drunk within a year or two.

Synthetic corks work well for wine that will be consumed right away but are fine too for wines that will benefit from a little aging. They certainly solve the problem of cork taint with a product that looks and "pops" like real cork. However, because they allow a more rapid oxidation rate, the wines age relatively quickly and should not be stored long term.

For robust red wines with strong tannic character, intended to mature over many years, there is no substitute for natural cork and careful aging over a period of years to decades. While the serious wine lover may lose a few bottles to cork taint, he or she will be more than compensated by the graceful, gradual development of quality wines over the years.

Fifth-generation winemaker Steven Kent Mirassou, of Steven Kent and La Rochelle wineries in Livermore, has shifted to a screw cap for his signature chardonnay to best preserve its fresh and fruity character, but continues to use cork for his red wines. He explains, "The chief advantage of cork is its ability to let air pass . . . The wine's dance with oxygen is one of the many chemical reactions that occur in wine, and it is very important in the wine's long-term maturation. . . . So, for those wines (such as our bigger cabernets) that benefit by slow aging, we'll stick with the humble bark plug."

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