In red wine production, the maceration process was traditionally done in large vats where the fermentation process would begin (in the picture this is noted by the presence of carbon dioxide bubbles). Secondary fermentation would take place when the wine is transferred to a second container such as a carboy or oak barrel.
Secondary fermentation is a process commonly associated with winemaking,[1] which entails a second period of fermentation in a different vessel than the one used to start the fermentation process. An example of this would be starting fermentation in a carboy or stainless steel tank and then moving it over to oak barrels. Rather than being a separate, second fermentation, this is most often one single fermentation period that is conducted in multiple vessels. However, the term does also apply to procedures that could be described as a second and distinct fermentation period.[1]
In wine production
With sparkling wines, the by product of secondary fermentation is the containment of the carbon dioxide bubbles which makes the wine "sparkling" as well as dead yeast cells known as lees(visible in the picture) that must be removed in a process known as disgorgement that happens prior to corking.
In sparkling wine production, the secondary fermentation often takes places in the wine bottle that the wine will be sold in. This is most commonly known as the méthode champenoise or "Champagne method" after the region most noted for sparkling wine production. When the base wine (or cuvee) has been produced from single grapevarietals or a blend, the wine is bottled with a mixture of yeast and fresh sugar known as the "liqueur de tirage". This secondary fermentation, also known as bottle fermentation, is the process that makes the wine "bubbly" due to the containment of carbon dioxide which is normally released as a by product in regular fermentation.
In still wine production, particularly of red wines[2] and some white wines like Chardonnay, the secondary fermentation process can also usher in the use of malolactic fermentation (or MLF) where the hard, green apple-like malic acid is converted into softer, butter-like lactic acid. In the case of a stuck fermentation, a winemaker may wish to transfer the must to a second vessel and use stronger, more aggressive yeasts with high fermentation temperatures to re-initiate the fermentation process. A similar process known as governo has been used by Tuscan winemakers since the 14th century with the isolation after harvest of a batch of grapes that can be added later to the wine to help prevent (or recover from) a stuck fermentation.[1]
