Champagne bubbles are more stable in glasses that have been cleaned without a dishwashing detergent.
SPARKLING WINES ARE JUDGED FIRST 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 different 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 accumulation 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 gourmets are convinced it is. Producers, equally convinced of the quality of their wines, have tended to point the finger at glassmakers, suspecting that the problem 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 sparkling wines. Their experiments showed that champagne ought not be unfavorably 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 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 molecules 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 reaches 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, gourmets 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 demonstrated that they appear mainly on limestone and tartar deposits and on cellulose 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, 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 environment, 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 champagne 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, these compounds are dissolved when it is filled with champagne. Although they do not disturb the wine’s effervescence, they do affect the stability of the foam, causing the bubbles to burst when they reach the surface. Dishwashing liquid, though also harmful, is less of a problem because rinsing eliminates most of it.