Three distinct steps make up secondary fermentation stage of the champagne production cycle:
- Bottling (Tirage)
- Secondary fermentation (in the bottle), also known as Prise de mousse or ‘capturing the sparkle’
- Aging on the Lees
The blended wine or vins clairs are drawn into bottles, in a process called tirage. To start secondary fermentation in the bottle the winemaker adds a liqueur de tirage, which is made from:
a) Still champagne vins clairs
b) A small quantity sugar (often rock candy), that corresponds to 24g of sugar per litre
c) Liquid cultures of active wine yeast strains called ferments de tirage.
Liqueur de tirage
There is a real art in developing the liqueur de tirage. Firstly, the dry yeast is activated in a mixture of vins clairs, water and diammonium phosphate. The strain of yeast significantly effects the flavour and aroma of the final champagne, and winemakers chose the yeast stain based on their house ‘style’ and the type of champagne they want to produce.
The still wine provides food for the yeast, while the small amount of water prevents the final alcohol content raising above maximum the permitted level of 12°.
Sugar provides the element Carbon which is necessary for fermentation. The diammonium phosphate provides Nitrates necessary for cell growth.
The purpose of adding the liqueur de tirage is to kick-start bottle fermentation (prise de mousse).
This process is designed to raise the pressure in the bottle to 6 kg/cm2 by the end of secondary fermentation.
Starting Fermentation
Fermentation is initiated in high-tech vats that have computer controlled temperature control, oxygen injection and mixing paddles. In the first stage of secondary fermentation, the yeast culture acclimatizes and adapts to its new environment. This process takes at least 4 days and can last several weeks. Yeast cells multiply and the fermentation process begins.
Bottling
As stated by the laws of champagne production bottling of champagne can only begin in the January following the harvest.
Shortly before bottling, a riddling agent called adjuvant de remuage e.g. bentonite, may be added. This agent is inactive and has no effect on the taste of the wine. Once bottle fermentation is completed, the yeast cells die and fall to the bottom of the bottle. These deposits are called lees. The riddling agent reacts with the lees causing it to clump together as well as preventing it sticking to the bottom of the bottle. Lees that have clumped together slip more easily slip down the neck of the bottle during the riddling process.
Once the riddling agent has been mixed with the fermenting wine, the mixture is bottled in a high-speed bottling machine or plant. The equipment used depends on the size of the producer. A 5cl space is left at the top of each bottle before capping. During bottling a thin plastic tube is inserted into the neck of the bottle. This aids the collection of lees at the end of riddling. The bottle is then sealed with a crown metal cap called a bidule. The cap is just like the one used on a bottle of beer.
After bottling the filled champagne bottles are stacked (in a process called called entreillage), onto wooden pallets or into metal crates. These are then transported to the cellars so that secondary fermentation can take place in a controlled environment.
Storage in cellars
The bottles are aged in dark cellars where the temperature range is kept at 9-12°C. This is the optimum temperature range necessary for secondary fermentation. The bottles are stacked in the cellar on wooden slats called lattes which separate each row of champagne bottles. Lattes ensure that if a bottle overpressures and explodes, the whole row doesn’t collapse.
Secondary Fermentation
The prise de mousse occurs when the wine starts to bubble. For a period of 6 – 8 weeks after bottling the yeasts consume the sugar added at the bottling stage. CO2 is released along with other compounds that all contribute to the wine’s sensory profile.
Because the champagne bottles are sealed, the CO2 gas cannot escape from the bottle and the gas dissolves in the wine.
The pressure inside the bottle increases to 6 atmospheres. It is interesting to know that champagne bottles have been designed to withstand up to 12 atmospheres pressure.
When the champagne bottle is finally opened by a consumer, the pressure inside the bottle is released and the CO2 dissolves out of the liquid to create effervescence.
There is a big difference in the length of aging between non-vintage and vintage champagne. Non-vintage champagne must spend a minimum of 15 months ageing in the cellars. Vintage must age for a minimum of 36 months. Most producers age their champagne for much longer than the minimum requirements. Different grape varieties also age at different rates, for example, Pinot Meunier ages quicker than Chardonnay.
After bottling and secondary fermentation, the base wine has been transformed into champagne. The stored champagne is left to stand and rest in the cellar. After many months of rest, the dead yeasts (lees) left after secondary fermentation completes settle in the bottom of the bottle.
The lees are important. They contribute significantly to the development of champagne’s aromatic profile by adding for example amino acids and proteins. The stored champagne is now ready for the next stage in the process, riddling.