— ABOUT WINE —
N° 4 / Winter 2009
CHAMPAGNE
A delicate balance BY STEVE CHARTERS MW 02 — Champagne's credibility gap BY TOM STEVENSON 09 — Single vineyard
champagnes BY ESSI AVELLAN MW 18 — Autolysis BY HERVÉ ALEXANDRE 28 — The science behind the bubbles BY GÉRARD
LIGER-BELAIR, RÉGIS GOUGEON & PHILIPPE SCHMITT-KOPPLIN 36 — A quest for the best BY PASCAL AGRAPART 44
TONG N° 4
THE
SCIENCE
BEHIND
THE
BUBBLES
— BY GÉRARD LIGER-BELAIR, RÉGIS GOUGEON & PHILIPPE SCHMITT-KOPPLIN, FRANCE & GERMANY —
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TONG N° 4
EVER SINCE THE DAYS OF BENEDICTINE MONK DOM PIERRE PÉRIGNON (1638-1715),
ITS ELEGANT EFFERVESCENCE HAS MADE IT A CELEBRATORY WINE. IN CHAMPAGNE
WINES AS IN ALL SPARKLING WINES, EXCESS CARBON DIOXIDE MOLECULES FORM AS
A BY-PRODUCT OF ALCOHOLIC FERMENTATION. AS SOON AS YOU UNCORK A BOTTLE
OF CHAMPAGNE, THE PROGRESSIVE RELEASE OF CO 2-DISSOLVED GAS MOLECULES
CAUSES BUBBLE NUCLEATION, KNOWN AS THE “EFFERVESCENCE PROCESS.”
Gérard Liger-Belair is professor of Chemical
Physics at Reims University in France. He also
heads the university’s “Bubble team”, exploring the
science of thin films, bubbles and foams and their
interdisciplinary applications. Among other prizes,
he has won the Association of American Publishers’
2004 award for his book “Uncorked, the Science of
Champagne”, published by Princeton University
Press. He carries out his study of champagne bubbles with Dr Régis Gougeon, associate professor of
Physical Chemistry at the University of Burgundy,
and Dr Philippe Schmitt-Kopplin of the Institute
of Ecological Chemistry in Munich.
Approximately 5 litres of CO2 escape from a typical 0.75 litre champagne bottle. For an idea of
how many bubbles are involved in the degassing
process, divide the volume of CO2 to be released
by the average volume of a typical bubble - 0.5 mm
diameter. The resulting number is close to 108.
Critics judge the quality of a particular champagne by the way its bubbles behave, among other
things. Small bubbles that rise slowly through the
liquid are usually considered much more desirable
than larger bubbles. The aspect of the foam ring
on the liquid surface, the so-called collerette, is
caused by the bubbles in the glass – another
important feature of champagne. And yet it is
only quite recently that the tools of physical
chemistry have been used to identify the physicochemical mechanisms behind nucleation, and the
rise and collapse of bubbles in champagne and
other sparkling wines.
UNCORKING THE BOTTLE
Have you ever considered the velocity reached
by an uncontrolled champagne cork popping
out of a bottle? Measurements conducted in our
laboratory in Reims showed typical velocities as
between 50 and 60 km/h.
When opening a bottle of champagne (or any
carbonated beverage), you will notice the small
cloud of fog that forms right above the bottleneck (wonderfully illustrated – Figure 1 – by
high-speed photographer Jacques Honvault.)
Contrary to popular belief, this cloud is due to a
significant drop in temperature in the headspace
below the champagne surface, itself caused by
a sudden expansion of gas when the bottle is
uncorked. The rapid temperature drop causes the
instantaneous condensation of water vapour in
the form of this characteristic cloud.
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TONG N° 4
jectories in the champagne bulk.) Fliers are a significant source of bubbles in glasses poured with champagne. The photograph of a typical flute filled with
champagne displayed in Figure 3a shows a detail
in Figure 3b, in which you can identify some fliers.
Figure 1. Champagne cork popping out of a bottle; the cloud of fog
above the bottleneck appears clearly (© Jacques Honvault).
BUBBLE NUCLEATION
a.
A close study of glasses that have just been filled
with champagne reveals that most of the bubble
nucleation sites are located on preexisting gas cavities. These cavities are trapped inside hollow,
more or less cylindrical cellulose-fibre-made structures measuring some 100 µm long with a cavity
mouth of several micrometres. Figure 2 shows a
typical fibre acting as a bubble nucleation1 site.
b.
Figure 3. Photograph of a typical flute poured with champagne (a), and
close-up of particles acting as bubble nucleation sites floating freely in
the bulk of the flute (called fliers), thus creating those charming bubble
trains in motion in the champagne bulk (b) (© Alain Cornu/Collection CIVC).
BUBBLE RISE
After their birth on cellulose debris, it is buoyancy that brings the bubbles to the liquid surface. As
they rise, they go on developing by continuously
absorbing the carbon dioxide molecules dissolved
in the liquid “matrix”. Bubbles thus steadily accelerate along their way through the champagne.
High-speed photographs show this acceleration in
the steadily increasing space between the bubbles of a particular bubble train (see Figure 4.)
Tasters of champagne and sparkling wine are
traditionally concerned with the size of the bubbles (there is a saying that the smaller the bubbles, the better the wine), which explains why
so much attention is devoted to modelling the
average size of ascending bubbles. Recent calculations have shown that the final average size of
ascending bubbles is however the result of a hugely complex interplay between several parameters.
Figure 2. A typical cellulose fibre absorbed on the wall of a glass poured
with champagne; clearly visible is the gas pocket trapped inside the fibre’s
cavity that causes bubble formation (© Gérard Liger-Belair/Cédric Voisin).
Flutes that have been towel-dried just before serving display an excess of bubble nucleation sites,
and thus an excess of effervescence. Some of the
particles that act as bubble nucleation sites (most
of them including cellulose fibres) may detach
themselves from the glass wall to eventually
immerse themselves into the rest of the champagne. Yet these particles remain active (in terms
of bubbling capacity), provided a gas pocket with
a radius of curvature larger than the critical radius
has been trapped within them. These particles
immersed in the champagne bulk produce those
easily-recognisable bubble trains, which seem to
dance erratically inside the glass while you taste the
champagne. These suspended particles are called
fliers (due to their often complex and circling tra-
Figure 4. Characteristic bubble train promoted by the repetitive bubble
formation process from a single cellulose fibre; bubbles are clearly
seen developing as they rise (© Gérard Liger-Belair).
1. “bubble nucleation” is the scientific term for “bubble birth”.
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G. LIGER-BELAIR, R. GOUGEON & PH. SCHMITT-KOPPLIN
Let’s look at a few of them:
1. The longer the travelled distance h, the
larger the bubble. This dependence of bubble
size on the distance it has travelled through the
liquid means that, in a champagne tasting, the
average bubble size at the champagne surface
varies from one glass to the next. In a narrow
flute, for example, the champagne is poured to
about three times the level of that in a typical
coupe (which has a shallower bowl and a much
wider aperture.) Therefore, the diameter of average bubbles in a flute will be larger than those in
a coupe, as shown in the photograph in Figure 5.
2. Bubble size is also strongly dependent on
atmospheric pressure. Were you to enjoy a glass of
champagne at the top of Mount Everest, where the
overall pressure is about 30% of that at sea level,
Figure 6. A few seconds after pouring and the collapse of the foamy head,
the surface of a champagne flute is covered with a layer of bubbles
approximately arranged in a hexagonal pattern that strikingly resembles beeswax (© Gérard Liger-Belair).
free surface of a glass filled with champagne also
reveals an unexpected and lovely phenomenon. A
few seconds after pouring and the collapse of the
foamy head, the inside surface of a champagne
flute covers itself with a layer of bubbles – like a
bubble raft, where each bubble is characteristically surrounded by six neighbouring bubbles (see
Figure 6.) In scientific terms, bubbles arrange
themselves approximately in an hexagonal pattern
that strikingly resembles that of beeswax. While
snapping pictures of the bubble raft that appears
after pouring, we accidentally took pictures of
bubbles collapsing close to one another in the raft.
When the bubble-cap of a bubble ruptures and
leaves an open cavity at the free surface, adjacent
bubble-caps are sucked towards this empty cavity
and create unexpected and short-lived flowershaped structures that are unfortunately invisible
to the naked eye (see Figure 7.)
Figure 5. Bubble size distribution at the free surface of champagne
glasses 30 seconds after pouring, whether champagne is served in
a coupe (left) or a flute (right) (© Gérard Liger-Belair).
the bubbles would increase in volume almost by
a factor of four! This is basically the same phenomenon that causes gas embolism in the blood vessels
of divers who resurface too quickly after having
breathed high-pressure air under water.
3. Bubble size also strongly depends on the
concentration of CO2 dissolved in champagne.
The lower the CO2 content, the smaller the bubbles. In fact, cork is a porous material which is
far from being completely hermetic with regard
to gas exchange. CO2 molecules are therefore able
to slowly diffuse through the cork along the ageing process. This is the reason why old champagnes systematically have small bubbles.
4. The gravity acceleration that drives the bubble rise (through buoyancy) also plays a relatively
important role in the final bubbles’ size. On the
moon, where gravity is about 1/6 of that on earth,
average bubble volume would increase by a factor
of almost three (we don’t yet have photographs
to illustrate this phenomenon…)
Figure 7. Flower-shaped structure found during the collapse of bubbles
in the bubble raft at the free surface of a flute poured with champagne
(bar = 1 cm) (© Gérard Liger-Belair).
A PATERNOSTER OF AROMAS
From the consumer’s point of view, bubbles are
essential to champagne, sparkling wines and in
fact any carbonated beverage. Without bubbles,
BUBBLES LIKE FLOWERS
The close observation of bubbles collapsing at the
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TONG N° 4
ing to the fruity aroma of several grape varieties
(see Figure 9.) Each dot in Figure 9 represents the
concentration factor of a given compound found
in the aerosols (ie the ratio of its concentration in
the aerosols to its concentration in the bulk below
the champagne surface.) These compounds, mostly including saturated and unsaturated fatty acids,
act as surfactants (ie as a double-ended compound
with one end attracted to the liquid phase and the
other repulsing it.) It has been suggested that
champagne bubbles pull these compounds out
of the liquid bulk, with one end attracted to the
bubbles’ airy interior and the other to the liquid
outside. The bubbles then rise to the surface of the
glass where they pop, releasing the compounds in
the form of aerosols. A scheme of this mechanism
Figure 8. The myriad of bubbles that collapse at the surface of a glass
of champagne radiate a cloud of tiny droplets that are characteristic of
sparkling wines and that complement the sensual experience of the
taster (© Alain Cornu/Collection CIVC).
champagne wouldn’t be champagne, and sparkling
wines and beers would be flat. And yet the role of
effervescence is far more than simply aesthetic.
When champagne or sparkling wine is poured
into a glass, the myriad of ascending bubbles collapse and radiate a multitude of tiny droplets
above the free surface, in the characteristic form
of refreshing aerosols (see Figure 8.) Based on a
phenomenological analogy between the fizz of
the ocean and the fizz in champagne wines, the
hypothesis was put forward a few years ago that
aerosols found in the headspace above a glass
poured with champagne could considerably
enhance the drink’s fragrance release. It does this
by bringing chemical compounds to the taster’s
nostrils, demonstrating both surface activity and
organoleptic interest. Very recently, ultra highresolution mass spectrometry was used to analyse
the aerosols released by champagne bubbles.
Figure 10. Scheme of the “bubble bursting” mechanism responsible for
the ejection of champagne aerosols over-concentrated with compounds
showing both surface activity and aromatic properties. These compounds
appear as red dots. Below are high-speed photographs of a bubble
collapse, leading to the projection of a liquid jet which quickly breaks
up into tiny droplets.
is shown in Figure 10, together with high-speed
photographs of the popping process. This recent
discovery supports the idea that rising and collapsing bubbles are a continuous paternoster lift
for aromas in every glass of champagne. Aerosols
were thus shown to hold the organoleptic “essence”
of champagne.
FURTHER READING
GOUGEON, R., LUCIO, M., FROMMBERGER, M.,
PEYRON, D., CHASSAGNE, D., ALEXANDRE, H.,
FEUILLAT, F., VOILLEY, A., CAYOT, P., GEBEFÜGI, I.,
HERTKORN, N., SCHMITT-KOPPLIN, PH. (2009)
The chemo-diversity of wines can reveal a metabologeography
expression of cooperage oak wood. Proceedings of the National
Academy of Sciences USA, vol. 106:9174-9179.
LIGER-BELAIR, G. (2009) Le Champagne: effervescence!
La science du champagne, Odile Jacob.
LIGER-BELAIR, G., CILINDRE, C., GOUGEON, R.,
LUCIO, M., GEBEFÜGI, I., JEANDET, P., SCHMITTKOPPLIN, PH. (2009) Unraveling different chemical fingerprints
between a champagne wine and its aerosols. Proceedings of the
National Academy of Sciences USA, vol. 106:16545-16549.
Figure 9. Concentration factor analysis of all masses present in the
mass spectra of champagne aerosols and bulk, respectively.
Compared with the liquid bulk in the glass itself,
the aerosols contained an over-concentration of
compounds known to be aromatic or precursors
of aromas, such as isomers of dihydrovomofiliol
or Blumenol B and Annuionone G, contribut— 42 —