noun From Latin fermentare ("to leaven"), related to fervere ("to boil") — referencing the visual appearance of CO₂ bubbling through the liquid

Fermentation /ˌfɜːmenˈteɪʃən/

The biochemical process by which yeast converts sugars in grape juice into alcohol (ethanol) and carbon dioxide. Alcoholic fermentation is the fundamental transformation that turns grape juice into wine.

The four phases of alcoholic fermentation

Lag phase

Yeast adapts to the sugar-rich environment. Little visible activity. Lasts a few hours to two days.

Exponential growth

Yeast population explodes. Sugar consumption accelerates rapidly. CO₂ bubbles vigorously — the "boiling" effect that gave fermentation its name.

Stationary phase

Yeast population stabilises. Alcohol accumulates, sugar decreases. The majority of alcohol is produced in this phase.

Decline / completion

Sugar is nearly exhausted (dry wine) or the winemaker stops fermentation by chilling or adding sulphites (sweet wine). Yeast cells die and fall as lees.

Fermentation vessels

The vessel in which fermentation takes place affects heat retention, oxygen exposure and ultimately wine character. Stainless steel tanks are easy to clean and temperature-control — they preserve primary fruit and are standard for aromatic whites. Concrete tanks and eggs offer neutral, slightly permeable fermentation with natural temperature stability. Open-top wooden vats (for red wine) allow more oxygen during maceration, extracted gently and traditionally. Small oak barrels (barriques) ferment wine directly in wood — the most oxygen-permeable option, common for premium Chardonnay. For how fermentation vessel choice shapes style, see the What oak does to wine guide.

Fermentation and wine sweetness

A wine is dry when fermentation has converted essentially all the grape sugar to alcohol. It is sweet when fermentation is stopped (by chilling, by adding sulphites, or by reaching alcohol levels toxic to yeast) before all sugar is consumed. The residual sugar remaining at the end of fermentation is what creates sweetness in the finished wine. In Germany's Prädikat system, harvest ripeness levels (and therefore potential sugar levels) are strictly defined — but what happens in the winery determines whether the wine ends up dry or sweet. For more on how sweetness interacts with acidity in wine, see the acidity glossary entry.

Primary fermentation, secondary fermentation and temperature control

Fermentation in winemaking occurs in two distinct forms. Primary fermentation (alcoholic fermentation) is the core process described on this page — yeast converting grape sugar into alcohol and CO₂. Secondary fermentation has two meanings: it can refer to malolactic fermentation (the bacterial conversion of malic acid to lactic acid, which softens acidity and adds buttery character), or to the second alcoholic fermentation used in traditional-method sparkling wine production (where wine is re-fermented inside the bottle with added sugar and yeast to create carbonation). Wine fermentation temperature is one of the most critical variables the winemaker controls: cool fermentation (10-15 °C) for whites preserves delicate fruit and floral aromas; warm fermentation (25-32 °C) for reds maximises colour, tannin and body extraction. The WSET winemaking curriculum and Jancis Robinson's Oxford Companion to Wine both cover fermentation science in extensive detail.

Biochemistry sources: OIV (oiv.int) — oenological practices and fermentation standards. Yeast science: WSET (winemaking curriculum). Fermentation research: UC Davis Viticulture & Enology, AWRI, Lallemand Wine (yeast strain reference library).

Questions

About fermentation.

What does yeast actually do during fermentation?

Yeast (typically Saccharomyces cerevisiae) metabolises glucose and fructose in grape juice through a series of enzymatic reactions, producing ethanol (alcohol), carbon dioxide (CO₂) and heat as by-products. The equation is: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂. Alongside ethanol, yeast produces hundreds of secondary compounds — glycerol (body and texture), esters (fruit aromas), higher alcohols, and diacetyl (butter character) — that contribute significantly to the wine's flavour. The biochemistry of wine fermentation is detailed in research from UC Davis Department of Viticulture and Enology and AWRI. For how fermentation fits into the complete winemaking process, see the How wine is made guide.

What is the difference between wild yeast and commercial yeast fermentation?

Commercial yeast fermentations use selected, cultivated strains of Saccharomyces cerevisiae, chosen for reliability, predictable flavour contribution and tolerance to alcohol. Wild (or native) fermentation relies on naturally occurring yeasts present on the grape skins and in the winery environment — mostly non-Saccharomyces species early on, transitioning to Saccharomyces as alcohol levels rise. Wild fermentations can produce greater complexity and terroir expression, but are slower, more unpredictable and carry a higher risk of stuck fermentation or off-flavours. Most natural wine producers use wild yeast. See the natural wine article for how yeast choice relates to the natural wine movement.

What is a "stuck" fermentation?

A stuck fermentation is one that stops before all the sugar has been converted to alcohol, leaving residual sugar that the winemaker did not intend. It is caused by yeast stress — typically from high sugar concentration, high alcohol, nutrient deficiency or temperature extremes. A stuck fermentation is a serious problem: the remaining sugar and lack of active yeast make the wine vulnerable to bacterial spoilage. Winemakers restart stuck fermentations by adding a fresh yeast culture, nutrients and adjusting temperature.

How does fermentation temperature affect wine style?

Temperature is one of the most important variables a winemaker controls. Cold fermentation (10–14 °C for white wine) slows yeast activity, preserving delicate primary fruit and floral aromas — the method used for aromatic whites like Sauvignon Blanc, Pinot Grigio and Riesling. Warm fermentation (25–32 °C for red wine) enhances extraction of colour, tannin and body — typical for full-bodied reds. Very cold fermentation is also used for sparkling wine base wines, where freshness and acidity are priorities.

What is the best fermentation temperature for wine?

There is no single "best" temperature — the optimal range depends entirely on the style being made. White wine fermentation: 10–15 °C (cold fermentation) preserves fresh fruit and floral aromas; aromatic varieties like Sauvignon Blanc, Riesling and Pinot Grigio are fermented cool. Warmer fermentation (17–20 °C) is used for full-bodied whites where complexity and texture are the goal. Red wine fermentation: 25–32 °C extracts colour, tannin and body from the skins; lower temperatures (18–22 °C) produce lighter reds with more fruit. Very cold fermentation below 10 °C risks stuck fermentation (yeast becomes sluggish). Sparkling wine base wines ferment cold to preserve acidity and freshness. Temperature control equipment is standard in modern wineries.

What is the difference between primary and secondary fermentation in wine?

Primary fermentation is the main alcoholic fermentation where yeast converts grape sugar into ethanol and CO₂ — this is the transformation that turns grape juice into wine. It typically lasts one to three weeks. Secondary fermentation can mean one of two things depending on context: (1) Malolactic fermentation (MLF) — a bacterial conversion of sharp malic acid into softer lactic acid, which softens the wine's acidity and adds buttery, creamy character. MLF is standard in red wines and full-bodied whites. (2) In sparkling wine production (traditional method / méthode champenoise), secondary fermentation refers to the second alcoholic fermentation that takes place inside the sealed bottle — the added sugar and yeast produce CO₂ that cannot escape, dissolving into the wine as bubbles. Both types of secondary fermentation are optional, controlled by the winemaker. See the how wine is made guide for the full sequence of winemaking steps.

Is fermentation the same as malolactic fermentation?

No — these are two separate processes. Alcoholic fermentation (described on this page) converts sugar to alcohol via yeast. Malolactic fermentation (MLF) is a secondary bacterial process that converts sharp-tasting malic acid into softer lactic acid, reducing perceived acidity and adding a creamy, buttery texture. MLF is standard in red wines and most full-bodied whites (like Chardonnay). It is typically avoided in aromatic, high-acid whites (Riesling, Sauvignon Blanc, Champagne base wines) where the acidity is a feature. See the malolactic fermentation glossary entry for a full explanation.