Molecular Gastronomy: Exploring the Science of Flavor (38 page)

bottle must be brought up from the cellar more than an hour before dinner is

served. This is not a detail to be left to the last minute.

Bottles Versus Glasses

Given these measurements, can we deduce the time needed to bring wine

up to room temperature if it is summertime and the temperature inside is 27°c

(81°f)? Alas, no; further measurements are needed, together with the equa-

tions of thermodynamics. But for our purposes it will suffice to consider cer-

tain orders of magnitude. If the temperature of the bottle to begin with is 12°c

(54°f), it will take about eighty minutes for it to reach 27°c (81°f). But if room

temperature is only 19°c (66°f), it will take more than two and a half hours,

starting from the same initial temperature. It also turns out that the upper part

of the bottle warms up more quickly than the lower part. The difference may be

as much as 4°c (7°f), which produces significant differences in taste between

the first and last sips of wine because the higher the temperature, the more

quickly its aromas evaporate.

Finally the guests are seated. Once the wine, warmed to 16°c (61°f), has been

poured into their glasses, how fast will it heat up when the ambient tempera-

ture is 23°c (73°f)? This time, the temperature in a traditional glass increases

on average by 0.2°c (0.36°f) per minute, which ought to be enough to stop us

from pouring the wine until our friends are ready to hoist their glasses.

256 | investigations a nd mod el s

77

Champagne and Its Foam

Proteins give champagne its distinctive zz.

w h e n w e h e a r t h e u n m i s t a k a b l e s o u n d of the cork popping off

a bottle of champagne, we stop talking and look closely at what happens as it

is poured into our glass. If the foam subsides slowly, if the frill of bubbles is

delicate and persistent, and if the liquid is effervescent, the wine is considered

to be of good quality. Conversely, a rapid fall in the level of the fizz, the absence

of a collar, and the presence of large bubbles are taken to be signs of inferior

quality, even though the taste of the beverage may otherwise be satisfactory.

Champagne makers therefore strive to produce wine that has a delicate and

stable foam.

Researchers at Moët et Chandon recently obtained European Union funding

to study the physical chemistry of the foam of champagne. In particular they

wanted to know which molecules are responsible for its stability and which

physical mechanisms account for changes in its structure. At the beginning of

the project, evaluation was conducted solely by a panel of judges, a labor-inten-

sive and time-consuming procedure. In short order two devices for obtaining

reliable measurements were put into operation. In one, known as a Mosalux, a

gas was injected into a fritted glass at the base of a cylinder containing the wine

in order to measure dilation and the average duration of the foam. The other,

a video system equipped with pattern recognition software, was used to track

changes in the structure of the foam in actual champagne glasses.

| 257

These devices were first used in connection with filtration studies. Wine

growers filter their products to give them clarity and to reduce the concentra-

tion of colloids before precipitating the tartaric acid because colloids limit the

crystallization of the acid—this despite the fact, as they are well aware, that

filtering hurts the quality of their wines, causing them to lose their roundness

in the mouth.

How does filtering change the foam of champagne? It is generally thought

to be harmful because it eliminates proteins, which are tensioactive molecules.

These molecules are composed of hydrophilic parts that dissolve easily in water

and hydrophobic parts that avoid water, preferring to be in contact with the air

inside the bubbles. Proteins help stabilize egg white foams and the bubbles in

champagne: By coating the bubbles they impede the formation of new bubbles

and prevent existing bubbles from fusing with their neighbors.

Brewers of beer had observed that filtering did not harm its head, but this

is because proteins are much more plentiful in beer than in wine. Research

has also shown that proteins with a molecular mass higher than 5000 create

a stable head.

In 1990, Alain Maujean and his colleagues at the University of Rheims

noted a relationship between the concentration of proteins and foaming ca-

pacity in thirty-one wines chosen at random. Yet they were unable to deter-

mine the conditions for producing a stable foam. Three researchers at Moët

et Chandon, Joël Malvy, Bertrand Robillard, and Bruno Duteurtre, used the

Mosalux to study this problem. By subjecting still wines to ultrafiltration, they

were able to separate out a part rich in macromolecules (notably proteins) and

another part poor in such molecules. Then, by mixing them in different pro-

portions with the base wine, they obtained wines containing various protein

concentrations.

Foam Measured

On insufflating these wines with carbon dioxide, the researchers observed

the same pattern: The foam initially accumulated and rose in the cylinder of

the Mosalux, then subsided a bit, reaching an equilibrium for a time before

finally dropping as the gas ceased to be injected. Wines whose protein concen-

tration was reduced by 20–100% exhibited similar foaming behavior, but the

higher the content of proteins having a molecular mass greater than 10,000,

258 | investigations a nd mod el s

the greater the measured quantity of foam. Although the bubbling action of a

wine during the first seconds was independent of its macromolecular concen-

tration, these macromolecules substantially retarded the foam’s decline and

the accompanying dilation of its bubbles. As expected, filtering greatly harmed

the foam. The disappearance of only a milligram of protein per liter of wine

(out of about ten milligrams normally) reduced its ability to foam by half; re-

moval of 2 milligrams of proteins shortened the average duration of the foam

by half as well.

These first studies were supplemented by measurements of the foaming

power of wines from which colloids and particles had been removed through

successive filtrations. The dilation and average duration of the foam diminish

by more than half when a wine is filtered by membranes whose pores have a

diameter of 0.2 micrometers. With wines that have been aged for more than a

year, the consequences of filtering them through pores of the same diameter

are still worse, whereas wines filtered through pores 0.45 or 0.65 micrometers

in diameter have a more stable foam. This shows that the macromolecules and

particles do not act in the same fashion on the bubbles.

How, then, do they act? In the first seconds after injection of the gas, the

films separating the bubbles are stabilized by the tensioactive macromol-

ecules. Once the quantity of these macromolecules at the liquid–gas interface

exceeds a certain minimum threshold, the foam rapidly coalesces because the

bubbles, which contain only carbonic gas, are not in equilibrium with the

ambient atmosphere. It may be that other macromolecules in addition to pro-

teins—sugars, for example—act on the bubbles as well. But what is clear—as

clear as filtered champagne—is that filtering interferes with foaming and so

must be done with care.

Champagne and Its Foam
| 259

78

Champagne in a Flute

Champagne bubbles are more stable in glasses that have been cleaned with-

out a dishwashing detergent.

s p a r k l i n g w i n e s a r e j u d g e d f i r s t by the uniformity of their color

and by their intensity, clarity, and effervescence. The palate has ample reason

to react unfavorably when the eye perceives a bit of tartar or other loose foreign

particles. The wines of Champagne nonetheless are measured against a dif-

ferent yardstick. The bubbles that rise up from the bottom of the glass are the

most obvious index of quality in the popular mind, together with the accumula-

tion of a fine foam at the top of the glass. The foam should be a few millimeters

high, but not more, and the bubbles should be small.

But is the absence of foam a sign of an inferior champagne? Many gour-

mets are convinced it is. Producers, equally convinced of the quality of their

wines, have tended to point the finger at glassmakers, suspecting that the prob-

lem arises from the variable surfaces of the glassware into which champagne

is poured. Patrick Lehuédé and his colleagues in the research department of

the Saint-Gobain group have studied the effect of glasses on the foam of spar-

kling wines. Their experiments showed that champagne ought not be unfavor-

ably judged by the absence of a foam, which in any case results not from the

quality of champagne glasses but from the way in which these glasses are

washed and stored.

In theory the formation of the bubbles is simple. Champagne is not in

stable equilibrium when it is first opened and comes into contact with the air

outside the bottle. The gas escapes from the liquid, forming bubbles either

260 |

on flaws present in the body of the glass—scratches, for example—or, more

commonly, on the surface of the glass. The size of the bubbles depends on the

energy of the surface coated by the liquid, which is to say the ease with which

a surface accepts contact with other materials, whether liquid or gaseous. Once

formed the bubbles grow in size because the pressure of the gas in the liquid is

greater than the pressure inside the bubbles, with the result that the gas mol-

ecules diffuse toward the bubbles. Eventually the Archimedean thrust exerted

on the bubbles exceeds their force of adhesion, and the bubbles detach from

the sides of the glass and slowly begin to rise.

This rough description can be refined. The adhesive energy of a bubble

clinging to the glass is proportional to the surface area of contact between

the bubble and the glass, which in turn depends on the surface energy of the

glass. Surface energy is conventionally measured with respect to the angle of

contact of a water droplet with the glass. When this energy is great the surface

is well moistened by the liquid, the contact area is small, and the bubble is

quasispherical. The bubble then becomes detached when its diameter reach-

es a few tenths of a millimeter. This is what happens with most ordinary

sodium–calcium glasses when they are clean. By contrast, when the surface

energy is low, which is to say when the liquid does a poor job of moistening

the solid, the bubble detaches itself only once it has become large (more than

a millimeter in the case of certain plastic materials). In other words, gour-

mets who refuse to drink from plastic flutes are right: Champagne bubbles

are smaller in a glass flute.

Theory Tested by Bubbles

The Saint-Gobain physicists first studied the formation of bubbles using

various microscopic methods. Contrary to a common belief, bubbles do not

form on flaws in the glass itself, for scratches are rare. Microscopy demon-

strated that they appear mainly on limestone and tartar deposits and on cellu-

lose fibers left on the surface of the glass by towels used in drying. The proof?

Glasses free from any deposit, prepared in a clean room and immediately filled

with champagne, do not allow a single bubble to form.

Nonetheless, Lehuédé and his colleagues showed that high-energy surfaces

are rapidly contaminated (within a few hours in the case of an ordinary glass)

by organic molecules that are present in the air (these abound in kitchens,

Champagne in a Flute
| 261

as you can see by running your finger along the ceiling above the stove). The

inherent advantage of glass and crystal over plastic therefore is reduced when

flutes have gotten dirty from sitting for a long time in an inappropriate envi-

ronment, such as on wooden shelves, which release organic essences (as you

can readily tell by smelling the wood).

The bubbles that detach themselves from the surface of traditional cham-

pagne glasses once they have become sufficiently large rise and feed the collar

of foam, whose height depends not only on the number and dimensions of the

bubbles but also on their stability. Certain compounds are well known for their

power to modify this stability, such as the antifoaming agents typically found in

red lipstick (so that frothy substances do not adhere to the lips), which explains

why people wearing lipstick have less foam in their champagne glasses after

the first sip. The effect is spectacular, as you can see for yourself by touching

the foam in a glass of champagne with the tip of a lipstick.

Researchers at Moët et Chandon discovered that detergents frequently

used in dishwashers have the same effect. Adsorbed on the surface of a flute,