A Life in the Bubble

They welcome you to Christmas parties and pop their way in the New Year: the bubbles. They rise, dance and burst with the most style in Champagne, the golden socialite in the family of wines. Once again during the holiday, thousands, hundreds of thousands of pops will sound as bottles get uncorked over the world. The famous sparkling wine was born when the wineries of the Champagne region endeavoured to compete with the neighbouring duchy of Burgundy. To balance the acidity and lightness of their wines, they added yeast and sugar to the bottles after the initial fermentation. As the bottles are sealed during this second fermentation, the CO₂ produced stay in solution in the wine. Also, the starving yeasts cannibalize their own cell walls in the last phases of the process, releasing additional molecules. The bottles are kept upside down so that the lees fall in the neck, to be eventually. According to local tradition, the monk Dom Perignon developed the process, although it has been described some years before by a C. Merrett in the Royal Society’s proceedings. Ironically, Perignon had been hired in the first place to work on the wine production and reduce the Champagne wines’ effervescence, seen as a defect at the time. The pressure from the gas wrecked havoc in the cellars, causing the bottles to explode over time. Bubbles lovers can be happy that, if he did set new rules for the production of the Champagne wines, he was never able to eliminate its fizz.

Champagne grew to be seen as an affordable luxury, the drink of celebration for the upper and middle classes, thanks to a clever publicity effort. The label is strictly regulated: obviously, only wines form the region, produced according to the traditional méthode champenoise can bear it. Moreover, only nine historical varieties of grapes are allowed for the production, but only three are commonly used: Chardonnay, Pinot Noir and Pinot Meunier.

In the end, it is its bubbles that crown champagne king of festivities. What seems a simple empty sphere in the liquid hides a surprising complexity. Suddenly free from its glass prison, the gas escapes by any air/liquid interface available. At the bottleneck, the adiabatic gas release causes a sudden and noticeable drop of temperature: try noticing the cloud of condensed humidity next time you pop a bottle open. In the liquid’s body, the slightest air pocket can serve as a nucleation site for the CO2 to accumulate. Once a critical amount of CO₂ is attained in the pocket, a bubble breaks free and rise to the surface, leaving a vacuum behind that again, quickly get filled, starting a new cycle. If any fibber left from a dishcloth, any dust spec in the glass can serve as a nucleation site, some glass manufacturers engrave a pattern at the bottom of the flute to enhance the production of bubbles.

Born on a spec or in an engraving, the bubbles begin their lives of travel. Their mass movement creates an important convection effect in the glass. How the liquid is mixed by its bubble depends in the end of the glass’ shape. Flutes are more thoroughly agitated than coupes, affecting the outgassing process and creating different convection fluxes of the aromas. Bubbles can thus cause different “taste profiles” of a champagne served in different glass shapes. As it rises on its life’s journey, the bubble keeps growing. Given a champagne of typical density, the ideal gas constant, an average temperature of service of champagne and the expected pressure inside the bubble, a champagne bubble will expand at a rate of 430 mm/s (comparatively to 150 mm/s for a beer bubble). Some chemists specialized in champagne physic-chemistry – yes, such people exist – the final average size of a champagne bubble is expressed as

R ≈ 2.7 · 10^-3 · q^5/9 · (1/(r · g))^2/9 · (((c_L – k_H) · P₀)/P₀)^1/3 · h^1/3

Assuming the diffusion coefficient obtained by the Stokes-Einstein equation, and where q is the liquid’s temperature, r, the liquid’s density, g, the gravitational acceleration, c_L, the CO₂ content, k_H, Henry’s law constant, P₀, the ambient pressure and h is the distance traveled. In other words, the bubbles will tend to be larger when you start drinking, as there is more CO₂ in solution. On the other hand, the heat from your hand might compensate to a certain extent the effect. The glass will also affect the bubble sizes, as bubbles have a longer trajectory on average in the higher flutes. If you were to drink you champagne on the moon or on Mars, the bubbles would also be larger...

After their birth at the frontier of water and air and their journey through an amber world the bubbles arrive at their cemetery: the surface. A bubble will survive for a time assembled in rafts of up to six members, expanding slowly to an approximate critical radius of 100 nm, surviving then for a time varying between 10 and 100 ms. The life of a bubble might be short, but they go with a swan song. As they burst out of existence, their envelope of liquid is projected in the air. This envelope was enriched in surfactant molecule. The aerosols over the glass are thus enriched as well. It has been shown by mass spectrometry of champagne’s headspace contains ethyl esters of fatty acid such a myristoleic, palmitoleic, and oleic acids, that not only contributes to the aroma of a wine but also to its foaming capacity. Notably, the decanoic and dodecanoic acid esters can be enriched, giving respectively toasty and dry aromas. Higher concentrations of norisoprenoids have also been noted in the aerosols, these being responsible of notable fruity aromas. Bubbles are thus not only part of the ‘tactile’ experience of champagne, nor simple mixing agents, they create an entire experience for the taster before the liquid even touches its lips.

Beware of the consequence of champagne though: some lightweights say that sparkling wines make then tipsy faster: the CO₂ somehow enhancing the effect of ethanol. Although a study attempted to address the question (Roberts, 2007), the jury is still out. The presence of CO₂ in the ethanol solutions only amplified individual variations in alcohol absorption. But driven by the hypothetical synergy of CO₂ and C₂H₅OH or not, be careful when you pop open the bottle. As A Galloway notes in an article in the Lancet. “Champagne-cork injury to the eye” (yes, that is the actual title), the cork can easily attain speeds of 15 m/s, reaching the eye in a hundred us (an average blink takes 300-400 us). Another article (“Bottle cork injury and cap injury to the eye”) underlines in the most serious tone that “Bottle cork injury of the eye can cause severe damage to the globe with secondary loss of visual function that can be permanent as we observed in the literature and in the series we report. The dangerous effect of pressurized fluid, even under normal circumstances, is well known, as shown in various studies.”

Further reading :

Aguie-Beghin V. Adriansen Y. Peron N. Valade M. Rouxhet P. Douillard R., 2009, Structure and Chemical Composition of Layers Adsorbed at Interfaces with Champagne, J. Agric. Food Chem. 57, 10399–407

Archer D, Galloway NR 1967, Champagne-cork injury to the eye. Lancet 2, 487–9

Cavallini G.M., Martini A. Campi L., Forlini M. 2009, Bottle cork and cap injury to the eye: a review of 34 cases, Graefes Arch Clin Exp Ophthalmol, 247, 445-50

Gallart M. Lopez-Tamanes E. Suberbiola G. Buxaderas S. 2002, Influence of Fatty Acids on Wine Foaming, J. Agric. Food Chem., 50, 7042-5

Liger-Belair G. , Polidorib G., Jeandet P. 2008, Recent advances in the science of champagne bubbles, Chem. Soc. Rev., 37(11), 2490–511

Liger-Belair G. Vuillaume S. Cilindre C. Polidori G. Jeandet P. 2009, CO2 Volume Fluxes Outgassing from Champagne Glasses in Tasting Conditions: Flute versus Coupe, J. Agric. Food Chem. 57, 4939-47

Liger-Belair G. Cilindrea C. Gougeon R. Lucioc M. Gebefu I. Jeandet P. Schmitt-Kopplinc P. 2009, Unraveling different chemical fingerprints between a champagne wine and its aerosols, Proc. Natl. Aca. Sci. 106 (39), 16545–9

Roberts C. Robinson S.P. 2007, Alcohol concentration and carbonation of drinks: The Effect On Blood Alcohol Levels, J. Forens. Leg. Med., 14, 398–405