How Does Wine Fermentation Work? (Padigan)

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.
  • TanninsPresent 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 One to Many

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.

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.

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