Secondary fermentation is a second round of yeast fermentation, most importantly the one that makes sparkling wine: a base wine is bottled (or tanked) with added sugar and yeast, and the resulting fermentation traps carbon dioxide as bubbles. In the traditional method it happens in the bottle the wine is sold in; in the tank (Charmat) method, in a pressurised tank. (Malolactic conversion is sometimes loosely called secondary fermentation too.)
WHY IT MATTERS FOR COLLECTORS
How secondary fermentation is done separates Champagne's bottle-fermented finesse from Prosecco's tank-method freshness. It is the defining step of sparkling wine and a major driver of style and price.
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