Chemistry:Carbonic maceration

Beaujolais Nouveau made by carbonic maceration

Carbonic maceration is a winemaking technique, often associated with the French wine region of Beaujolais, in which whole grapes are fermented in a carbon dioxide rich environment prior to crushing. Conventional alcoholic fermentation involves crushing the grapes to free the juice and pulp from the skin with yeast serving to convert sugar into ethanol. Carbonic maceration ferments most of the juice while it is still inside the grape, although grapes at the bottom of the vessel 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 the most extreme case, such as with Beaujolais nouveau, the period between picking and bottling can be less than six weeks.

During carbonic maceration, an anaerobic environment is created by pumping carbon dioxide into a sealed container filled with whole grape clusters. The carbon dioxide gas permeates through the grape skins and begins to stimulate fermentation at an intracellular level. The entire process takes place inside each single, intact berry. Ethanol is produced as a by-product of this process but studies have shown that other unique chemical reactions take place that have a distinctive effect on the wine.

History

A Beaujolais wine.

The process of carbonic maceration occurs naturally in a partial state without deliberate intervention and has occurred in some form throughout history. If grapes are stored in a closed container, the force of gravity will crush the grapes on the bottom, releasing grape juice. Ambient yeasts present on the grape skins will interact with the sugars in the grape juice to start conventional ethanol fermentation. Carbon dioxide is released as a by product and, being denser than oxygen, will push out the oxygen through any permeable surface (such as slight gaps between wood planks) creating a mostly anaerobic environment for the uncrushed grape clusters to go through carbonic maceration. Some of the earliest documented studies on the process were conducted by the French scientist Louis Pasteur who noted in 1872 that grapes contained in an oxygen rich environment prior to crushing and fermentation produced wines of different flavors than grapes produced in a carbon dioxide rich environment. This was because the fermentation process had already started within the individual grape clusters prior to the introduction of yeasts during conventional fermentation.

Wine production

In carbonic maceration, fermentation takes place inside each individual grape berry.

The use of carbonic maceration is closely associated with the production of Beaujolais nouveau wine from the Gamay grape in the Beaujolais region and some wines in the Rioja Alavesa and Jumilla areas of Spain. This 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 distinctive style.

Producers in other parts of France and in the New World have frequently utilized carbonic maceration for their own Gamay production, or with other grape varieties. Winemakers in the Languedoc and Rhône wine regions will sometimes employ the technique on coarse and tannic grapes like Carignan, especially if they are to be blended with other varieties. In California, the process is sometimes used with Valdiguié grapes, producing what is labeled a "Nouveau" wine.

The process is almost always used in conjunction with red wine production since some of the flavor compounds produced by volatile phenols tend to form undesirable flavors with white wine grape varieties.

Other techniques

Semi-carbonic maceration is the winemaking 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. To an extent, most wines were historically treated to some form of semi or partial carbonic fermentation (as noted in history section above) with the amount dependent on the shape and size of the vessel that the grapes were stored in prior to crushing. The deeper the vessel, the greater the proportion of grapes that could be exposed to an anaerobic environment caused by the release of carbon dioxide from the crushed grapes on the bottom.

An alternative name for carbonic maceration is "whole grape fermentation", which is distinct from the process known as "whole bunch fermentation" which is common in the Burgundy wine production of Pinot noir. With "whole bunch fermentation", entire clusters of grapes (including stems) are fermented before being crushed. This creates a large "cap" of grape skins with pathways created by the stems that allows juice to flow more evenly through caps, increasing the levels of skin contact or maceration.