WHAT'S THAT STUFF?

BETHANY HALFORD

If you're anything like me, your New Year's Eve celebration was punctuated by the distinct pop of a champagne bottle being uncorked. Of course, if you're anything like me, half a glass later and 20 minutes into 2004 you were tipsily embarrassing yourself, giggly and grateful that you didn't have to drive home.

	PHOTODISC
	PHOTO BY GÉRARD LIGER-BELAIR
	POSEY OF POP A "flower-shaped structure" appears for less than 100 microseconds when the collapse of an exploding champagne bubble deforms its neighbors [Langmuir, 19, 5771, (2003)].

Champagne may be the elixir of celebration, but for something so closely tied to joyous events, champagne is mercilessly encumbered by laws. The Appellation d'Origine Contrôlée outlines 35 of these rules conceived to uphold the quality of Champagne wines. It also defines the region of Champagne, an 84,000-acre area about an hour's drive east of Paris. This demarcation separates champagne from other sparkling wines made with the "méthode champenoise." According to the appellation, if the grapes weren't grown in Champagne, the wine isn't champagne.

From a chemical perspective, though, champagne and all other sparkling wines must conform to just one law, Henry's law: The amount of gas dissolved in a fluid is proportional to the pressure of the gas with which it is in equilibrium. This dissolved gas--carbon dioxide in the case of champagne--gives the wine its characteristic effervescence. In an unopened bottle, CO₂ gas dissolved in the wine is in equilibrium with gas in the space between the cork and the liquid. Uncorking the bottle releases this headspace gas and disrupts the equilibrium. Following Henry's law, the dissolved CO₂ leaves the wine via bubbles, reestablishing the equilibrium through effervescence.

Champagne makes its gas naturally during fermentation. Yeast turns glucose from grape juice into CO₂ and ethanol. The same fermentation process occurs in all wines, but valves on the casks let winemakers release the CO₂ so that it doesn't build up.

Champagne gets its characteristic bubbles by trapping CO₂ gas while in its bottle, where it ferments a second time. The happy accident of this second fermentation was discovered in the mid-1600s, before which all Champagne wines were still--that is, unbubbly. The chilly onset of winter in northern France stopped the fermentation process before the yeast had consumed all the wine's sugar. Although the French drank this young, still wine in the winter, warm spring weather triggered a second fermentation that added effervescence to the wine that had been shipped abroad and bottled upon arrival.

With the popularity of this prized sparkling wine, Champagne's cellarmasters tried to control the phenomenon, with varying degrees of success. Some bottles of the wine would have no bubbles at all, while others would explode under unbearable pressures of CO₂ gas.

Today, champagne houses employ chemists and oenologists to control this second fermentation and champagne's unique flavors in a more scientific manner. According to Frederic Panaiotis, a winemaker with Veuve Clicquot, the early stages are essentially the same for making all white wines. "Routine analyses include monitoring sugar, acidity, and pH in grapes during the ripening process in order to harvest at the optimum time and checking residual sugars at the end of alcoholic fermentation to make sure all sugars have been metabolized, as sugar could be metabolized by bacteria and turned into volatile acidity--a major flaw in wines."

During bottling, the winemakers add sugar and yeast to the wine for the requisite second fermentation. "Chemical analyses are used to follow the process," says Bertrand Robillard, an organic chemist who is the project manager for Moët & Chandon's R&D department. "We do SO₂ analysis and monitor glycerol and gluconic acid. For other parameters, like sugar, we use FTIR [Fourier transform infrared spectroscopy]."

Panaiotis says that no less than 600 different chemical compounds can be found in a bottle of champagne, each lending its own unique quality. But he adds that the wine's final flavors ultimately depend as much on art as they do on science. "Often chemical analysis can help make a decision on various aspects of the process," Panaiotis says, "but for tasting, the only performing tool is the taster, namely his nose, his palate, and his tasting ability and memory."

Even with careful flavor finessing, champagne would be just another fine white wine were it not for those delicate little bubbles. As they ascend the length of the glass in tiny trains, they drag along molecules of flavor and aroma until they explode at the surface of the liquid, tickling the nose and stimulating the senses.

Connoisseurs believe the smaller the bubble, the finer the champagne. Gérard Liger-Belair, a professor of chemical physics at the University of Reims Champagne-Ardenne, in France, says that if the bubbles are smaller, there will be more of them to release flavor and aroma.

With the ultimate goal of making champagne with smaller bubbles, Liger-Belair has undertaken several studies of the effervescent phenomenon [Sci. Am., 288, 81, (2003)]. He recently showed that, contrary to expectation, both champagne and a low-quality sparkling wine have the same diffusion coefficients, even though their bubbles have different sizes [J. Agric. Food Chem., 51, 7560 (2003)].

"Today, it is not yet possible to control the bubble size," Liger-Belair says, "except by diminishing CO₂ content, which is obviously not legal." But he thinks that careful study of chemicals dispersed in the wine--salts, carbohydrates, and minerals--may provide keys to controlling bubble formation.

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