Cork or No Cork?

The Science of Sealing Wine Bottles

Screwcaps and synthetic cork are becoming more popular for sealing wine bottles. But how do they affect the chemistry of the wine?


By TIMOTHY WOGAN
Story from Firstscience.com


When you uncork a bottle of wine this holiday season, you may not necessarily be removing a piece of cork. In recent years, winemakers have been opting for other materials to seal up wine bottles, such as synthetic cork and screwcaps. While it may seem much less romantic to unscrew a bottle or pop out a piece of plastic, research has found that these new materials do have advantages over natural cork.

For a winemaker, a corked bottle of wine is one of the biggest nightmares. Contrary to popular belief, being 'corked' has nothing to do with bits of cork floating in the wine. In fact, it means the cork is contaminated with a chemical taint called TCA (2,4,6-trichloroanisole). Although harmless to health, this is amazingly potent stuff, detectable to the palate at concentrations of six parts per trillion: a tablespoon of TCA could ruin the entire U.S. output this year. Found randomly in the bark of cork trees (which is what, after treatment, goes into the bottle), TCA imparts a musty, slightly moldy quality to the wine and suppresses fruit aromas.

Most of us couldn't identify a corked bottle of wine if it bit us on the nose, which is what concerns winemakers. Californian producer Carol Shelton says she would rather get a bottle returned than have the customer say to their friends, 'I don't much care for it.' "I suspect a lot of people decide they don't like our wine, when really, they don't like the taste of cork taint," she says. With an estimated 5-8% of bottles tainted, it's hardly surprising that winemakers are frustrated.

Some cork companies are working on ways of either removing TCA from corks or stopping the TCA moving from cork to wine (such as the membrane covered ProCork), but many winemakers are taking a more radical approach and abandoning the ubiquitous cylinder of tree bark altogether.

Alternatives to cork

Currently, synthetic cork and screwcaps are the two alternatives to cork that are widely-used. Synthetic cork is a squidgy bit of plastic that replaces the spongy bit of cork. It eliminates TCA contamination, but it's not for wines intended for long aging, since synthetics don't seal as well as natural cork, and wines under synthetic cork oxidize within a few years. If wine is oxidized, it means that oxygen has reacted with the ethanol (alcohol) in the wine resulting in ethanal - which makes the wine taste flat and dull. Around 90% of wine is consumed within 24 hours of purchase, however, so synthetic corks have a viable future, even if they will never replace natural cork entirely.

But an increasing number of wines are now being sealed with screwcaps. Although there are problems with consumer acceptance, many winemakers are seriously excited. Stelvin, who make the majority of screwcaps for wine, are adamant that their closures are not just a solution to the problem of cork taint, but that they are actually superior to cork, "a true necessity to answer to the best aging." But this is also where the controversy begins. Battle lines are drawn between the evangelists and the skeptics, and scientists have lined up on both sides.

One uncontroversial advantage is that the screwcap doesn't absorb, or "scalp" chemicals from the wine. The white grape Riesling, for example, produces a chemical called 2,4,6-trimethyldihydronaphthalene as it ages, which smells and tastes rather like petrol. This is apparently more pleasant than it sounds, and is regarded as a crucial signature of Riesling by aficionados. Natural cork absorbs 40% of this chemical, whereas screwcaps absorb none. Perhaps this explains the wide acceptance of screwcaps among producers of Riesling: 100% of Riesling bottled in 2000 in Australia's Clare Valley used screwcaps.

Oxygen under the screrwcap

As with synthetic corks, the controversial subject is oxygen transmission. Ironically, however, the complaint here is that the seal may be too good. Screwcaps provide, on average, a much tighter seal than cork.

For many years, wine scientists have argued that all the reactions that take place inside a wine bottle - and which cause wine to mature - are reductive, - the opposite of oxidation reactions. This implies that a bottle with a tighter seal (i.e. a screwcap) is ideal since it doesn't allow as much oxygen to penetrate inside.

Supporters of screwcaps argue that, although slight oxidation may flatten some of the harsher notes of a young wine, it does nothing to develop the depth or complexity that bottle aging is supposed to produce. They insist that, if a wine takes longer to reach its peak under a screwcap, that peak will be higher.

Peter Godden of the Australian Wine Research Institute says, "While there is no evidence that any oxygen is required for the reactions we think of as 'wine development', and many such reactions clearly need no oxygen, the presence of oxygen may speed up 'development'." Stelvin, bizarrely claims that "everybody knows" wines mature more quickly with more oxygen (everybody, apparently, aside from screwcap cheerleaders such as the New Zealand Screwcap Initiative, who repeat ad nauseam that oxygen is not involved in wine maturation.

The smell of aging

Some experts argue that the greater (but still tiny) amount of oxygen transmitted by natural cork is needed to prevent reduction problems. Wine can contain sulfides, and with extremely low oxygen concentration, like when a screwcap is used, these sulfides can reduce to compounds called thiols, some of which smell rubbery or flinty. In the presence of slightly more oxygen, this does not happen.

The response from the screwcap lobby is that the presence of thiol precursors (chemicals that can turn into thiols) in wine is a fault, since they should be removed using copper sulphate before bottling. Winemakers have lined up on both sides - some insisting they have followed all the correct procedures and still encountered problems, while others proudly state that their screwcap experiments have been great successes. Californian winemaker Randall Grahm says, "My sense is that for any competent winemaker this is no biggie."

Some experts predict that "designer" screwcaps will eventually become the norm for premium wines, allowing the winemakers to specify the level of oxygen transmission allowed by the closure post-bottling, just as they can control the winemaking process before the wine is bottled.


(Published: 10.05.2008.)

Send your comments