Fermentation

Alcoholic fermentation is the defining transformation in winemaking. Yeast (primarily Saccharomyces cerevisiae) consumes glucose and fructose in grape juice, producing ethanol and CO₂ as primary by-products, plus hundreds of secondary compounds — esters, acids, and higher alcohols — that contribute to the wine's aroma and flavour.

Fermentation generates heat, which must be managed. For white wines, low fermentation temperatures (12–18°C) preserve delicate aromatics. For red wines, higher temperatures (25–32°C) promote extraction and complex flavour development. In very cold conditions, fermentation can stop prematurely (stuck fermentation).

Fermentation is considered complete when all (or most) sugars have been consumed, leaving a dry wine. If the winemaker wants residual sugar (e.g. off-dry or sweet styles), fermentation can be halted early by chilling, filtration, or adding alcohol.

A second type of fermentation — malolactic fermentation (MLF) — is carried out by bacteria, not yeast, converting sharp malic acid into softer lactic acid. MLF commonly follows alcoholic fermentation in red winemaking and many white wines.