Wine Education 411: What is Fermentation?

A: Fermentation is the conversation of a simple sugar to alcohol (ethanol), carbon dioxide (those bubbles you find in soda) and heat with the addition of a [wine] yeast, Saccharomyces cerevisiae, to the juice.  Generally speaking, this is how fresh grapes turn into wine.  

Sugar + Yeast --> Alcohol + CO2 + Heat

50 L Stainless Steel Varying Capacity Fermentation Tank
(Photo by author)

Keep in mind that wine is a transitory product, meaning that wine falls halfway between fresh grape juice and vinegar.  If allowed to "ferment fully" and "spoil," one would have vinegar!

Juice<------------> Wine <-------------> Vinegar

Although yeasts of various types exist naturally on the grapes, many winemakers will ferment the juice using commercial yeast strains.  This allows for better control of the fermentation and consistency of the wine product on an annual basis.  That being said, it has become "trendy" for winemakers to make wines using native yeast strains.  For a wine of this style look for "native," "natural," or "spontaneous" fermentation on the wine label or tasting notes.  These wines are typically of a higher price as they are a stylistic and difficult wine to produce.

Red Juice getting ready for inoculation

(Photo by author)

White juice coming off the press

(Photo by author)

Although carbon dioxide (CO2) is produced during fermentation, most of this is blown off as the fermentation progresses and dies.  However, some light, aromatic whites like Austrian Gruner Veltliner may contain a small amount of carbon dioxide to give the wine a "spritsy" effect and aromatic freshness.  I know that we've seen this in a previous blog post that featured Galen Glen's Gruner Veltliner.

Sparkling wines or Champagnes from France, however, contain a retained amount of carbon dioxide to ensure that you see bubbles in the glass, and feel the bubbles on the palate.

Additional effects of fermentation include aroma (smell) and flavor (things you taste on the palate like fruity, oaky, floral characteristics) are determined by grape variety, grape growing conditions, yeast selection, and processing decisions.  Theoretically (and in practice), one can take the same grapes, grown in the same lot in the same way, bring them into the winery and change some things during and after fermentation (like yeast selection, barrel choices, fermentation temperature, etc.) and come out with 2 very different wines!  [This is the part of wine that I, personally, study for my "day job."]  

Tasting subtle differences in finished or almost-finished wines
(Photo by author)

A good example of this is the Sophia Blanc de Blanc label by Francis Ford Coppola Winery in Sonoma, CA.  The Sophia Blanc de Blanc is processed in 2 wines - the base wine is the same up until the bottling/packaging choices:

Sophia Blanc de Blanc in the Bottle...

(Photo from Google Images)

• Option 1 features Sophia Blanc de Blanc in the bottle in which a second fermentation takes place in the bottle to incorporate biscuit, bready flavors (like traditional Champagne) in addition to its fruity base aroma/flavor, and carbonation in the final product.

...versus Sophia Blanc de Blanc in miniature cans

(Photo from Google Images)

• Option 2 features the same Sophia Blanc de Blanc base wine, but the wine is put into miniature soda cans.  Carbonation is manually put into the can much like soda is carbonated.  Therefore, the final product is fruitier, heavily "bubbled," and appears sweeter than the bottled Sophia Blanc de Blanc.  The result is 2 very different wines!