Thank you Darla Hoffmann, Dustin Chabert, and Jeremy Eubanks for your input last week on Sustainable Viticulture. Take a look if you missed it!
This week: Co-fermentation
What is co-fermentation and what effect(s) does it have on the fermentation process and final wine?
Co-fermentation is the act of simultaneously fermenting two or more grape varieties during the winemaking process. Historically, this practice has been embraced in regions where mixed plantings or field blends are prevalent, like Chateauneuf-du-Pape. During the fermentation process, the addition of white grapes to the blend can soften tannins from the red grapes and also has the (counterintuitive) effect of deepening the color in the final wine due to certain chemical compounds within the phenolics of the white grapes that bond to the anthocyanin pigments in the red grapes, stabilizing and deepening the final color of the wine (most famous/discussed example of this that I've encountered is in Côte Rotie). There are also regions that mandate co-fermentation in keeping in line with traditional winemaking practice, like Weiner Gemischter Satz or Bourgogne Passe-Tout-Grains. In other ares, the practice has fallen out of favor as white varieties have become less prevalent and/or winemaking technology has improved, like Rioja and Chianti Classico. During this process, the winemaker may be seen to have less control over the final wine, as there is no opportunity to adjust the blend by vinifying different grape varieties separately, but there the effects of tannin-management and color stabilization cannot be achieved through blending.
To add to Dustin's response, the method of fermenting 2 or more grapes in one vessel started in the old world but made it's way to the new world as well, because of the notable, interesting and complex results. It is most common in field blends where different wine grapes all from the same vineyard are harvested at the same time. The grapes are easier to harvest together than to sort out individually, however they would seemingly display more characteristics of terroir being that the grapes grew side by side in the same vineyard. Introducing these grapes before fermentation provides different characteristics due in large part to the raw materials being combined. Blending already fermented wines will not be exposed to those same materials because the composition has already been altered. Co-fermentation promotes inter-mingling of these grapes early so the texture is much more harmonious. (maybe somewhat like 2 people meeting and forming their own opinion about each other, without the manipulation of others??) In other words, the grapes have a chance to co-exist and see how things turn out. After the initial meeting, it is much harder to make changes. There is no real way of knowing if a wine is co-fermented as there is no law requiring this to be listed on the bottle. However, some winemakers may choose to mention it on the label.