A distinction must be made between a deliberately induced second fermentation in the bottle in the production of quality sparkling wine and an undesired secondary fermentation in a still wine:
Bottle fermentation in sparkling wine
A second fermentation is initiated in the base wine by adding the dosage (filling dosage = sugar and yeast) to the bottle. During the second alcoholic fermentation of this wine, the sugar is converted into alcohol and carbon dioxide, which greatly increases the carbon dioxide pressure in the bottle. In addition, further esters and higher alcohols are produced, which have a positive effect on the flavour of the wine. Bottle fermentation is mandatory by law for champagne, but for other sparkling wines, the second fermentation can also take place in the tank using the méthode charmat. Bottle-fermented sparkling wines are declared on the label using EU-standardised designations. These are Méthode champenoise (only for Champagne), or Méthode classique (also Méthode traditionnelle), which is also permitted in the national language. If there is no indication on the label, it can be assumed that the sparkling wine was not produced according to the classic method.
Post-fermentation for still wine
Unwanted bottle fermentation or secondary fermentation in still wine can occur as alcoholic fermentation or malolactic fermentation (BSA). The prerequisite for this is sufficient residual sugar and the presence of bacteria or yeasts, which can multiply there due to the warm ambient temperature. The process is inhibited if the alcohol or sulphur content is high. This is usually caused by a lack of hygiene and/or a problem with filtration.
Alcoholic fermentation often produces a distinct must flavour, while malolactic fermentation produces lactic notes of milk or cheese. This manifests itself visibly through turbidity or haze. In extreme cases, the relatively high carbon dioxide content can also cause the bottle to explode. When opening the bottle, this is noticeable - unusual for a still wine - by a clear noise (a "pop"). In any case, secondary fermentation is a dreaded, unacceptable wine defect.
Further information
For information on the production of alcoholic beverages, see Champagne (sparkling wines), Distillation (distillates), Speciality wines, Spirits (types), Winemaking (wines and wine types) and Wine law (wine law issues).
Graphic: Sektkellerei J.Oppmann AG
