The bubbles are an important indicator of the quality of a sparkling wine; the process of foaming/beading is known as moussing. The finer the rising bubbles and the more balanced the foam that forms on the surface (pearl collerette, French "la fine collerette de mousse à la surface"), the more delicate and sparkling the wine tastes. When a sparkling wine undergoes a sensory test, its effervescence is also analysed. This involves the texture of the bubbles (fine to coarse) and how long the bubbles lasts in the glass.
The finer the bubbles and the longer the play of rising bubbles lasts (also known as the "cordon"), the higher the sparkling wine quality is judged to be. The bubbles from the dissolved carbon dioxide only form when there is a sudden drop in pressure when the bottle is opened. They form on small particles in the wine and on microscopic irregularities on the glass, which act as nuclei. Tests with perfectly smooth glasses have shown that champagne hardly bubbles at all. Essentially, the "quality" of bubbles depends on how the sparkling wine is produced. The longer the wine is left to mature on the lees after bottle fermentation, the finer the bubbles will be.
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Egon Mark
Diplom-Sommelier, Weinakademiker und Weinberater, Volders (Österreich)