Rusty Sly on Carbonic Fermentation
During our last meeting of Grape of the Night (GOTN) in September 2012 the group tasted and learned about a wine called Gamay Beaujolais. One of the points that I commented on was about a different fermentation process used in the Gamay region of France called carbonic fermentation. This process creates wines that have fruity aromatics and are light in style compared to wines that have gone through the usual fermentation process; and involves fermentation of whole grape clusters including the stems.
A similar process is used in wines from the Republic of Georgia where the grapes, stems, etc are placed into amphoras and crushed. The difference with carbonic fermentation is that the grapes are not crushed. Allowing the stems to be included in the fermentation process creates a wine that is high in tannins and sometimes green bell pepper aromatics.
One grape that is not always crushed is pinot noir. This allows for a more aromatic wine based on what is termed a partial carbonic maceration process. Fermenting with the stems is traditional in Burgundy wines from France and has increased in 2008 and 2009 vintages. Since this technique increases the tannin level, it is probably the reason that French Burgundy wines age well beyond their American counterparts.
The process of carbonic fermentation is very unique and involves placing the whole grape clusters with their stems carefully into a vat forming layers of grape clusters. I am sure that everyone that has picked up a grape cluster at your local market can realize how much weight or force is being applied to the grapes on the bottom of the vat. This causes some of the grapes at the bottom to be crushed just due to the weight. Next, carbon dioxide is added into the vat to remove the oxygen. This allows for fermentation to occur within the grape skin delaying the activity of the yeast. This is the key difference from normal fermentation. The fermentation occurs naturally within the grapes. As the grape ferments within the skin, the internal pressure from carbon dioxide gas production along with 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.
This process is used extensively in the Beaujolais region where very fruity aromatics in light bodied wines are produced. The stems do add high tannins however, the process used in the Beaujolais region are geared around quick turnaround wines. This is very evident in Beaujolais Nouveau wines.