Wine · Terminology

Fermentation

Yeast converting grape sugar into alcohol and carbon dioxide — the reaction that makes wine wine.

Fermentation is the conversion of grape sugar into ethanol and carbon dioxide by yeast — chiefly Saccharomyces cerevisiae. Heat is released as a by-product, which is why fermenting tanks need cooling. In round numbers, 17 grams of sugar per litre yields about 1% alcohol.

It is the reaction that defines the drink, but the winemaker's job is mostly about controlling it: which yeast, at what temperature, in which vessel, and whether to let it run to dryness or stop it early and leave residual sugar.

In practice

  • Temperature sets the style: cool ferments (12–16 °C) preserve delicate aromatics in whites; warm ones (22–32 °C) extract colour and tannin in reds.
  • Cultured yeast gives predictability and a chosen aroma profile; ambient (wild) yeast gives complexity and risk.
  • Vessels: stainless steel, concrete egg, oak barrel or vat, amphora — each with its own oxygen and temperature behaviour.
  • A stuck fermentation — yeast dying before the sugar runs out — is the classic winery emergency.

Where you'll meet it

  • Every winemaking guide, from red to white to sparkling, where a second fermentation creates the bubbles.
  • Any discussion of alcohol levels, which are decided here.

The winemaker's dials

Decision Effect
Yeast strain Aroma profile, alcohol tolerance, speed, reliability
Temperature Aromatic retention vs. extraction; ester formation in whites
Vessel Oxygen exposure, thermal mass, oak character
Cap management (reds) Punch-down, pump-over, délestage — how much is extracted
Nutrition Nitrogen additions to prevent a stuck ferment
Stopping early Residual sugar, via chilling, filtering, sulfur or fortification

The other fermentations

  • Malolactic conversion — bacterial, not yeast; softens malic acid into lactic. Standard in reds, optional in whites.
  • Second fermentation in sparkling wine, which traps CO₂ in the bottle or tank.
  • Carbonic maceration, where intact berries ferment intracellularly before yeast takes over — the Nouveau style.

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