The bottles are filled leaving a maximum 5cl headspace. They are then taken down to air-conditioned cellars where they will remain for several years, stocked horizontally away from shocks, light or draughts. The temperature in the cellars is a constant cool 9-12 degrees C, which is the optimal temperature for second fermentation. At a lower temperature it wouldn’t happen at all, and at a higher temperature it would happen too quickly
The Champagne cellars, say some historians, mainly started life as Gallo-Roman or medieval crayères (chalk pits) and were largely ignored until the 18th Century — since when they have been refurbished and enlarged.
1844: Adolphe Jacquesson brings some sun into the cellars
The practice of stacking the bottles horizontally, top to tail, in crates or pallets is known as entreillage. The bottles rest on wooden slats (lattes) that keep the rows separate. That way the entire stack does not collapse in the event that one of the bottles explodes under pressure.
Second fermentation follows on from entreillage. The yeasts added to the as-yet still wine start to act on the also-added sugar, slowly converting the sugar into alcohol and carbon dioxide. This increases the alcohol content of the wine by about 1.2-1.3 degrees. The yeasts then multiply, creating a deposit that will be eliminated from the finished wine. The carbon dioxide meanwhile gradually dissolves in the wine, causing a slow increase in the pressure inside the bottle (6 kg/cm2). Fierce at first, this build-up of pressure tails off within two months. It is the carbon dioxide that creates the pressure under the cork and accounts for that gentle ’pop’ when a bottle of Champagne opened.
Winemaking: Second fermentation
Appendix: And talking of bottles ...
To withstand this pressure, Champagne Houses use high quality bottles of toughened glass free of defects or scratches. It was Englishmen Henry Holden and John Colenet who were the first to manufacture a series of "thick-bodied bottles with long necks" (1662). The French meanwhile waited until the end of the 17th Century to come up with glass bottles of their own: first, black apple-shaped bottles made of thick glass, then tougher pear-shaped bottles, both produced by glass manufacturers in Northern France and the Argonne.
In the 18th and 19th Centuries, pressure was the single worst enemy of sparkling Champagne makers. Repeated explosions destroyed as many as 80% of all bottles, devastating businesses and forcing workmen to wear face masks in the cellars.
Today’s Champagne bottle is a dark green colour and designed to withstand 12 atmospheres of pressure. It weighs 860g and features a large indent in the base, making the bottle more shock resistant and facilitating vertical storage when the bottles are stacked "neck down" (en masse).
The second fermentation phase is crucial to the quality of the future wine. Too rapid and it produces large, flabby bubbles that soon go flat. But taken slowly, at a cool even temperature, it leaves the wine with fine, delicate bubbles that seemingly last forever.