The bubbles are considered an important indication of the quality of a sparkling wine; the process of foaming/pearling is called moussing in German. The finer the rising bubbles and the more balanced the ring of foam that forms on the surface (la fine collerette de mousse à la surface), the more delicate and sparkling it tastes. During the sensory testing of a sparkling wine, the sparkling quality is also examined. 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 the rising bubbles lasts (also called "cordon"), the higher the quality of a sparkling wine is judged to be. Strictly speaking, the bubbles are formed from the dissolved carbon dioxide only during the sudden drop in pressure when the bottle is opened. They form on small particles in the wine as well as on microscopic irregularities on the glass, which act as germs. Tests with perfectly smooth glasses have shown that champagne hardly bubbles at all. Essentially, the "quality"of the bubbles depends on the way the sparkling wine is made. The longer the wine is stored on the yeast after fermentation in the bottle, the finer the bubbles. Large bubbles in sparkling wines, on the other hand, are pejoratively referred to as "oeil (yeux) de crapauds" (toad eyes).
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Egon Mark
Diplom-Sommelier, Weinakademiker und Weinberater, Volders (Österreich)