The winemaker kick-starts the effervescence by adding a sweet solution known as the ‘liqueur de tirage’ – still Champagne mixed with cane or beet sugar (20-24 grams/litre, for a rise in pressure by the end of fermentation of 5-6 atm, or 60 to 90 pounds per square inch) plus selected, acclimatized yeast cultures and additives that assist the ‘remuage’ process (riddling). These consist of bentonite or bentonite-alginate that make the sediment heavier, encouraging it to slide down the neck of the up-turned bottle and collect against the cork.
The rules of the Champagne appellation forbid ‘transvasage’: the transferring of the newly effervescent wine from one bottle to another (from a half-bottle to a jeroboam, for instance). All Champagne wines must be sold in the bottle in which they underwent their second fermentation. The bottles used must be made of strong glass, in accordance with strict specifications relating to pressure resistance and general durability. They must be capable of withstanding high pressure and repeated handling.
Once filled, the bottles are hermetically sealed with a polyethylene stopper known as a ‘bidule’, held in place by a wire cage/metal cap. A few producers still use cork for the ‘tirage’ (bottling) stopper. The bottles are then transferred to the cellar and stacked ‘sur lattes’: horizontally, row upon row, these days mostly in steel crates on a palette.
Inside the bottle, the wine undergoes a second fermentation that continues for 6-8 weeks. The yeasts consume the sugar, releasing alcohol and carbon dioxide, along with esters and other superior alcohols that contribute to the wine’s sensory profile.