Secondary fermentation in the bottle

Secondary fermentation in the bottle is most famously used in champagne production, although it is also common in other sparkling wines and beer production. After blending the different wines which will form the champagne, and prior to, or during, bottling a small amount of 'liqueur de tirage' is added. This is a mixture of wine, sugar and yeast. The bottle is sealed with a temporary crown cork and laid to rest in horizontal position in one of Champagne's miles of chalky tunnels.

When the yeast starts acting on the sugar, a second fermentation starts, which increases the alcohol content of the wine and produces carbon dioxide. This gas is unable to escape and so dissolves into the wine, creating champagne's fizz.
This second fermentation in the bottle also produces dead yeast cells which eventually break down and interact with the wine, adding a distinctive flavour. These dead yeast cells are known as 'lees' and the length of time a champagne has spent on its lees greatly affects the quality of the final product.