Bubbles in Champagne (and other sparkling wines) are caused by dissolved CO2 in the wine reacting with imperfections in the glass. The CO2 is a byproduct of yeast fermentation, which produces both alcohol and CO2, which, in turn, is dissolved in the wine because of the pressure in the bottle.
Bubbles appear when the dissolved CO2 touches a nucleation site, which is an imperfection in the glass, such as dust, traces of detergent, etc. The bubbles generally start at the bottom because that’s where most imperfections reside.
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