The presence of residual carbon dioxide (CO2) in sparkling wines is a result of fermentation. It's important to note that the artificial introduction of CO2 into wine is largely prohibited, and products subjected to such forced carbonation are labeled as "carbonated wines."
Injecting CO2 into wines typically produces large, poorly integrated bubbles with a short-lived existence. To accurately control the CO2 levels, winemakers employ a meticulous process involving the addition of measured sugar to a dry wine, initiating a second alcoholic fermentation in a sealed container to capture the CO2 (known as "prise de mousse"). The quantity of CO2 is directly correlated to the final pressure, governed by the ideal gas law.
This process forms the basis for two successful sparkling wine production methods: the bottle-fermented/traditional method and the Charmat/cuve close method.
Before the prise de mousse, the winemaking process for sparkling wines follows the typical production of white wines, referred to as the "base wine." Grapes ideal for sparkling wine are harvested early to maintain low sugar content and high acidity. These base wines undergo fermentation, may undergo malolactic fermentation (MLF), are aged, and stabilized before the prise de mousse.
"Bottle fermentation," specifically in the traditional method, proves to be the most effective technique for prolonged contact (or "en tirage") with dead yeast after the prise de mousse. This yeast contact, a process known as autolysis, imparts richness, aromatics (such as toasted bread and nutty notes), and ensures small bubble sizes.
The final stages of the sparkling wine production process involve removing the yeast sediment and balancing the wine's acidity with sugar (dosage). In bottle-fermented wines, the riddling/shaking process collects the yeast deposit in the bottle neck. Subsequently, the neck is frozen to trap the yeast deposit in an ice cube, which is then expelled (disgorgement). For Charmat-made wines, a counter pressure filter is used to filter out the yeast deposit while retaining the CO2.
The addition of a liquid sugar solution (dosage) allows for a range of residual sugar levels in the final product and helps harmonize the high acidity characteristic of sparkling wines.
As an alternative to a precisely measured second alcoholic fermentation, another approach involves completing the first fermentation in a bottle or Charmat tank before all sugars are consumed. Wines resulting from this process, particularly when finished in a bottle, are labeled as "methode ancestrale" or "petillant naturel." This method is more straightforward when executed in a Charmat tank, as seen in the production of Otra Maximus Prosecco.
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