What is wine fermentation?
Quick answer
Fermentation is the biochemical process in which yeast converts the grape's natural sugars (glucose and fructose) into ethanol (alcohol) and carbon dioxide. It is the essential step that turns grape juice into wine.
Detailed answer
Alcoholic fermentation is the heart of winemaking. The simplified chemistry is: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ — sugar becomes ethanol and carbon dioxide. In practical terms, each gram of sugar per litre produces roughly 0.6% alcohol by volume. A must with 200 g/L of sugar will yield a wine around 12% ABV.
The yeast doing the work can be indigenous (wild yeast naturally present on grape skins and in the cellar) or selected (commercial strains chosen for reliability and specific aromatic properties). Natural wines rely exclusively on indigenous yeast, embracing the unpredictability that comes with it.
Fermentation typically lasts one to four weeks under controlled temperatures. For whites and rosés, cooler fermentation (12–18°C) preserves fresh, fruity aromas. For reds, warmer fermentation (25–30°C) promotes extraction of tannins and colour from the skins.
Most red wines — and some whites like Chardonnay — then undergo a second fermentation called malolactic fermentation (MLF). Lactic bacteria convert sharp malic acid (think green apple) into softer lactic acid (think yoghurt), rounding out the wine. Crisp whites such as Riesling and Sauvignon Blanc typically skip MLF to retain their bright acidity.
Beyond alcohol, fermentation produces hundreds of secondary compounds — esters, aldehydes, organic acids — that build the aromatic complexity you experience in a finished wine.