Carbonic maceration is a winemaking technique, associated with the French wine region of Beaujolais, in which whole grapes are fermented in a carbon dioxide rich environment prior to crushing. Conventional fermentation involves crushing the grapes to free the juice and pulp from the skin with yeasts used to convert sugar into alcohol.
Carbonic maceration ferments most of the juice while it is still inside the grape, although grapes at the bottom of the tank are crushed by gravity and undergo conventional fermentation. The resulting wine is fruity with very low tannins. It is ready to drink quickly but lacks the structure for long-term aging. In extreme cases, such as with Beaujolais Nouveau, the period between picking and bottling can be less than six weeks.

 

During carbonic maceration, carbon dioxide is pumped into a sealed container (i.e. stainless steel tank) filled with whole grapes. The carbon dioxide gas permeates through the grape skins and begins to stimulate fermentation. The entire process takes place inside each single, intact berry. The resulting wine is generally fruitier, bright in colour and less tannins than conventionally produced wines.
The Gamay grape lends itself well to the production of simple, fruity wines and Beaujolais winemakers have been able to create a unique identity based on this style of wine. Producers in other parts of France and in the New World have frequently utilized carbonic maceration with other grape varieties.
The process is almost always used in conjunction with red wine production since some of the flavours compounds produced by volatile phenols tend to form undesirable flavours with white wine grapes.
Semi-carbonic maceration is the technique where grapes are put through a short period of carbonic maceration, followed by conventional yeast fermentations. This is the process used in the production of Beaujolais Nouveau wines.
An alternative name for carbonic maceration is 'whole grape fermentation' which is quite distinct from the process known as 'whole bunch fermentation'.