Cap or cork, it’s the wine that matters most

SAN FRANCISCO — Don’t judge a wine by its cover. In a survey of the chemistry and flavor of pinot noir and chardonnay, consumers couldn’t discern wines capped with natural corks from screw caps, scientists reported March 25 at the spring meeting of the American Chemical Society. The results suggest that the way its bottle is stopped has little if any effect on a wine’s flavor.

“Wine quality should really be judged by the wine, not the cork,” said Michael Qian of Oregon State University in Corvallis, who led the research. “The right kind of screw cap is just as good as a cork, or even better, because it is more consistent.”

The permeability of a wine’s cork determines the amount of oxygen that enters the wine, and this acts on other compounds that affect flavor. Some traditionalists assert that real corks are the only way to get just the right amount of healthy gas exchange needed for a flavorful wine, while screw caps are suffocating. But Qian’s survey found that wine was just as appealing to taste testers whether it was aged in bottles topped with screw caps or traditional corks. And instead of stifling the wine, one kind of screw cap and the synthetic cork actually did the opposite, allowing the wine to breathe too much.

Working with Qian and colleagues, graduate student Juan He investigated 2006 vintage pinot noir and chardonnay from the Argyle Winery in Oregon. The winery closed 150 bottles each with natural cork, synthetic cork and three screw caps, each with a different lining. Every six months for two years the team uncorked bottles from each of the five groups to test the chemical profile and dissolved oxygen content of the wine under each type of seal. The researchers also had volunteers rate the flavor and aroma of wine from the different kinds of capped and corked bottles.

The chemical analysis revealed that of the five types of seal, the synthetic cork and the cap lined with low-density polyethylene actually let in too much oxygen, running the risk that too many flavorful compounds such as thiols and esters may be oxidized. While consumers weren’t able to detect these differences, if a larger proportion of compounds interacts with oxygen, chemical reactions could kill desirable flavors such as the passion fruit character of some sauvignon blancs, noted Qian.

Screw caps made from a polyvinylidene chloride–tin foil combination (Saran-tin) let in the least oxygen. But the amount of oxygen wasn’t low enough to leave the wine heavy with unreacted sulfur compounds, which can give an “off” quality that is often attributed to too much unreacted methanethiol and was thought to be the bane of screw cap wines. The screw cap lined with polyvinylidene chloride–polyethylene mix (Saranex) had chemical and taste profiles similar to the natural cork, said Qian.

The lack of excessive unreacted sulfur in the screw-capped bottles is “kind of surprising,” said He. “We think the wine itself is more important than the screw cap.”  

Their study suggests that screw caps themselves aren’t all the same; some allow just the right amount of breathability, one parameter of many in getting flavor to bloom.
Sai Prakash Chaturvedula, a chemist with the Coca-Cola Company in Atlanta, says the results are interesting and he’d like to see how the presence and proportion of other compounds in wine that also interact with oxygen, such as polyphenols, are affected by the different caps.

The study continues — the wines will be tested again after three years in the bottle, and the chemical and flavor profiles may change as the wines continue to age, said Qian. Screw caps offer a range of gas permeability, and the choice of a seal should be determined in part by how much oxygen exposure the winemaker wants. So while they don’t offer the flourish of a cork, the trend toward screw caps doesn’t necessarily bode ill for oenophiles.