Fermentation is the process by which grape “must” (a fancy winemaking term for unfermented grapes or juice) transforms into wine. During fermentation, yeast—our microbiological friends—convert grape sugars into alcohol. There’s a lot more than just alcohol production going on, though. Fermentation drives complex chemical reactions that affect the flavor, aroma, and even color of the finished wine.

Not Just Alcohol

At its simplest, fermentation is often described as the conversion of one molecule of glucose into two molecules each of ethanol (or ethyl alcohol) and carbon dioxide: C6H12O6 → 2C2H5OH + 2CO2. While that’s arguably the most important result, yeast are complex organisms that perform a wide array of biochemical processes in a fermenting wine.

Some of the compounds produced or affected by fermentation include:

  • Esters: Esters are aromatic compounds that contribute delicate fruity, citrusy, or floral aromas to a young wine. They exist in must as precursors that are bound to sugar molecules. As the yeast consume the sugar, the esters are liberated and become volatile.
  • Tannins: Present naturally in grape skins and seeds, tannins are antioxidant polyphenols that give wines dryness, astringency, and mouthfeel. The alcohol produced during fermentation enhances tannin extraction, while fermentation byproducts react with tannins, altering their structure, and, in turn, their perceived levels of astringency and bitterness.
  • Acetaldehyde: Created by yeast in the penultimate step on the pathway to ethanol, low levels of acetaldehyde can enhance fruity aromas in wine. In high concentrations, acetaldehyde may yield unwanted bruised apple–like aromas and flavors. Acetaldehyde’s ability to catalyze tannin polymerization plays an instrumental role in the stabilization of red wine structure and mouthfeel.
  • Anthocyanins: This highly reactive family of compounds in red grape skins gives red wine its color and antioxidative properties. These compounds polymerize in the presence of acetaldehyde to form a vast array of stable color components.
  • Sulfites: As with ethanol, yeast produce sulfites during fermentation to fend off competition from other microorganisms. These natural sulfites can somewhat protect the wine from microbial spoilage and premature oxidation, but their levels are typically bolstered by winemakers after fermentation.
  • Amino acids: Unfermented grape juice, or must, is rich in nitrogen-containing amino acids. Yeast consume most of these amino acids during fermentation, using the nitrogen to construct proteins and amino acids necessary to live and reproduce. Amino acids are the most important family of compounds in yeast nutrition and health.

From Many to One

The sweet, nutrient-rich must is an ideal medium for growing diverse species of yeast during the fermentation process. Naturally present yeast may include the familiar Saccharomyces, found in bread and beer, as well as more exotic genera such as CandidaKloeckera, and Hansenula. As a result, the beginning of fermentation involves a lot of biodiversity, with many different types of yeast competing for resources. If allowed to ferment, each type of yeast leaves behind its own particular signature of flavor and aroma compounds.

These naturally occurring yeasts contribute a subtle, nuanced complexity to the finished wine that reflects the unique character of the Rogue Valley and our vineyard in particular.

Since not all yeasts are suitable for making wine, many wineries employ sulfites to suppress the activity of wild yeasts before fermentation, followed by inoculation with a commercially developed, cultured strain of Saccharomyces yeast. While this usually yields a predictable fermentation dominated by one particular variety of yeast, it doesn’t leave a lot of room for the natural microbiological diversity of the vineyard to shine through.

At Franifer, we prefer to take a gentler approach, encouraging the growth of desirable yeast found in our vineyard. These naturally occurring yeasts contribute a subtle, nuanced complexity to the finished wine that reflects the unique character of the Rogue Valley and our vineyard in particular.

As fermentation progresses, some species of yeast begin to rapidly convert the natural sugars present in the must into carbon dioxide and ethanol, or ethyl alcohol. Produced as a defense mechanism, few species of yeast can tolerate even moderate levels of ethanol. At around 4–5% ABV, or alcohol by volume, many of the yeast species present at the start of fermentation—like the Candida mentioned earlier—die off. As ethanol levels continue to rise, one strain—Saccharomyces—emerges as the victor of this fierce competition and embarks on fermenting the wine to dryness.

Leave a Reply

Your email address will not be published. Required fields are marked *

You must be 18 years old to visit this site.

Please verify your age

- -