Carbonic Fermentation


Carbonic fermentation is a process that is different from typical fermentation and only used with a few types of wines.  The process involves adding yeast to a juice that has sugar that the yeast consumes producing ethanol (drinking alcohol) and CO2.  Carbonic fermentation creates wines that have fruity aromatics and are lighter in style compared to wines that have gone through the usual fermentation process.  Carbonic fermentation uses the whole grape clusters, including the stems and juice during the fermentation process. 

This process is unique and involves placing whole grape clusters with their stems carefully into a vat forming layers.  I am sure that everyone has picked up a grape cluster and realized how much weight, or force, it has. This force, once the clusters are layered, is being applied to the grapes on the bottom of the vat.  This causes some of the grapes at the bottom to be crushed due to the force exerted by the mass weight of the layers of grapes.  Carbon dioxide is injected into the vat to remove the oxygen.  This allows for fermentation to occur within the grape skin, delaying the activity of the yeast.  The process of fermentation occurring naturally within the grapes is the key difference from normal fermentation.  As the grape ferments within the skin, the internal pressure from carbon dioxide gas production due to fermentation and the weight of the grapes piled into the vat causes the grapes to burst and release their juice. This kind of fermentation process creates ethanol as well as fantastic fruit aromatics.

Carbonic fermentation is used extensively in the Gamay region of France in the making of Gamay Beaujolais which possess very fruity aromatics and light bodied wines.  The inclusion of stems imparts high tannins, however, in the process used in making the quick turnaround wines such as Beaujolais Nouveau in the Beaujolais tannin levels are not as high. 

Pinot Noirs are also not always crushed.  Fermenting with the stems is traditional in the Burgundy wines from France and has increased in 2008 and 2009 vintages.  The stems induce high tannins into the wines, providing complexity and aging benefits.  If the stems are unripe however, a green bell pepper aromatic which notably is due to the extraction of 2-methoxy-3-isopropylpyrazine.  If Pinot Noir grapes are de-stemmed by the vintner, they are maintained uncrushed to allow for a more aromatic wine and is known as a partial carbonic maceration process.  The Burgundians, due to not having de-stemmers, have always included the stems in their vinification process.  Reputable Domaines that still advocate whole clusters are Dujac and Domaine de la Romanee-Conti. Today, many have eliminated whole cluster fermentation or limit it to 10% to 30% of the clusters. New World vineyards that use as much as 100% whole cluster in selected wines are Ambullneo, Freestone Vineyards, Native 9, Tantara, and Windy Oaks among others.

The common denominator between carbonic fermentation or carbonic maceration is the production of high aromatics.  The addition of stems will increase the tannins.  So keep this in mind next time you enjoy this type of wine that has gone through this process and see what your palate detects. 

Rusty Sly