There are three main methods of sparkling wine production. The first is simple injection of carbon dioxide (CO2), the process used in soft drinks, but this produces big bubbles that dissipate quickly in the glass.
The second is the Metodo Italiano, or the Charmat process, in which the wine undergoes a secondary fermentation in bulk tanks, and is bottled under pressure. This method is used for Prosecco and Asti in particular, and produces smaller, longer-lasting bubbles.
The third method is the traditional method or Méthode Champenoise. With this method the bubbles for more complex wines are produced by secondary fermentation in the bottle. As the name suggests, this is used for the production of champagne and other quality sparkling wines, but is slightly more expensive than the Charmat process.
More economical sparkling wine is made by simple injection of CO2 from a carbonator.
Metodo Italiano (Charmat Process)
The Charmat process is known as Metodo Charmat-Martinotti (or Metodo Italiano) in Italy, where it was invented and is most used. The wine undergoes secondary fermentation in stainless steel tanks rather than individual bottles, and is bottled under pressure in a continuous process. Many grape varieties, including Prosecco, are best suited for fermentation in tanks. Charmat method sparkling wines can be produced at a slightly lower cost than Méthode Champenoise wines.
The traditional method is the process used in the Champagne region of France to produce the sparkling wine known as Champagne. It used to be known as the Méthode Champenoise but the Champagne producers have successfully lobbied the European Union to restrict that term to wines from their region. Thus wines from elsewhere may not use “Méthode Champenoise” when sold, and most times will use traditional method, Méthode Traditionnelle or the local language equivalent.
After primary fermentation and bottling, a second alcoholic fermentation occurs in the bottle. This second fermentation is induced by adding several grams of yeast (usually Saccharomyces Cerevisiae although each brand has its own secret recipe) and several grams of rock sugar.
At this time the champagne bottle is capped with a crown cap. The bottle is then riddled (see below), so that the lees settles in the neck of the wine bottle. The neck is then frozen, and the cap removed. The pressure in the bottle forces out the lees, and the bottle is quickly corked to maintain the carbon dioxide in solution.
The larger Champagne producers have a number of press houses situated throughout the region. Grapes used for Champagne are generally picked earlier, when sugar levels are lower and acid levels higher. Except for pink or rosé Champagnes, the juice of harvested grapes is pressed off quickly, to keep the wine white.
The first fermentation begins in the same way as any wine, converting the natural sugar in the grapes into alcohol while the resultant carbon dioxide is allowed to escape. This produces the base wine. This wine is not very pleasant by itself, being too acidic. At this point the blend, known as the cuvée is assembled, using wines from various vineyards, and, in the case of non-vintage Champagne, various years.
The blended wine is put in bottles along with yeast and a small amount of sugar, called the liqueur de tirage, and stored in a wine cellar horizontally, for a second fermentation. Champagne requires a minimum of 1.5 years under the Appellation d'Origine Contrôlée to completely develop the entire flavor. For years where the harvest is exceptional, a vintage (millesimé) is declared and the wine has to mature for at least 3 years.
During the secondary fermentation the carbon dioxide is trapped in the bottle, keeping it dissolved in the wine. The amount of added sugar will determine the pressure of the bottle. To reach the standard value of 6 bars (600 kPa) inside the bottle, it is necessary to have 18 grams of sugar, and the amount of yeast, Saccharomyces cerevisiae, is regulated by the European Commission to be 0.3 gram per bottle. The liqueur de tirage is then a mixture of sugar, yeast and still champagne wine.
Aging on lees
Wines from Champagne cannot legally be sold until it has aged on the lees in the bottle for at least 15 months in the case of non-vintage Champagne. Champagne's AOC regulations further require that vintage Champagnes are aged in cellars for three years or more before disgorgement, but most top producers exceed this minimum requirement, holding bottles on the lees for 6 to 8 years before disgorgement.
After aging (a minimum from one and a half to three years), the sediment (lees) must be consolidated for removal. The bottles undergo a process known as riddling (remuage in French). In this stage the bottles are placed on special racks called pupitres. This places the bottles at a 45º with the cork pointed down. Every few days the bottles are given a slight shake and turn and dropped back into the pupitres (eventually the angle is increased). The drop back into the rack causes a slight tap, pushing sediments toward the neck of the bottle. In about 6 to 8 weeks the position of the bottle is pointed straight down with sediment in the neck of the bottle.
This manual way of riddling sparkling wine is still used for Prestige Cuvées in Champagne, but has otherwise been largely abandoned because of the high labor costs. Mechanised riddling equipment called gyropalettes are used instead.
The removal process is called disgorging (dégorgement in French), and was a skilled manual process, where the cork and the lees were removed without losing large quantities of the liquid, and a dosage (a varying amount of additional sugar) is added. Until this process was invented Champagne was cloudy, a style still seen occasionally today under the label Méthode Ancestrale. Modern disgorgement is automated by freezing a small amount of the liquid in the neck and removing this plug of ice containing the lees.
Immediately after disgorging but before corking, the liquid level is topped up with liqueur de expedition. At this time, it is common to add a little sugar, a practice which is known as dosage, which means that the liqueur de expedition consists of a mixture of base wine and sucrose. The amount of sugar added will determine the sweetness of the Champagne, since the sugar previously present in the wine was consumed in the second fermentation. Most of the time, sugar is added to balance the high acidity of the Champagne rather than to actually achieve a sweet-tasting wine. Brut Champagne will only have a little sugar added, but only Champagne called nature or zéro dosage will have no sugar added at all. A cork is then inserted with a capsule and wire cage securing it in place.
Even experts disagree about the effects of aging on Champagne after disgorgement. Some prefer the freshness and vitality of young, recently disgorged Champagne, and others prefer the baked apple and caramel flavors that develop from a year or more of bottle aging.