What fermentation actually is
Alcoholic fermentation is the process by which yeast consumes the sugar in grapes (glucose and fructose) and converts it into two things: ethanol and carbon dioxide. It's the same chemical reaction in every winery in the world, though the final result varies enormously depending on the grape, the temperature, and the winemaker's choices.
Step 1 — From grape to must
After harvest, the grapes are destemmed (separated from the green stalk) and gently crushed. What's left is the must: juice, pulp, skins, and seeds. In red wines, the must ferments together with the skins — that's where the color comes from. In white wines, the grapes are pressed first, and only the juice ferments, without the skins.
Step 2 — Yeast gets to work
The yeast can be the kind that naturally occurs on the grape skin, or a selected strain added by the winemaker for more control over the outcome. Its job is simple but constant: it eats sugar, and as a result releases alcohol, CO₂, and heat.
Why the tank "boils" without fire: the bubbling you see in a fermenting tank isn't external heat — it's CO₂ escaping as the yeast works. It's literally the must breathing.
Step 3 — Temperature calls the shots
This is where the winemaker really decides the wine's style — and how long fermentation takes:
- Red wines: ferment warmer, between 77°F and 86°F (25–30°C), to extract more color and tannin from the skins. The warmth also speeds up yeast activity, so reds tend to finish toward the shorter end of the range.
- White wines: ferment cooler, between 54°F and 64°F (12–18°C), to preserve the fresh, fruity aromas that heat would destroy. The cold slows the yeast down on purpose, which is why whites tend to take longer.
- Rosé wines: ferment at an in-between temperature, roughly 54°F to 68°F (12–20°C) — cool enough to protect the wine's delicate fruit, after a short initial skin contact (a few hours to a couple of days) before the juice is drawn off to ferment on its own.
Put temperature and style together, and a complete alcoholic fermentation usually takes between 5 and 15 days. Warm, fast-fermenting reds tend to land near the lower end; whites and rosés fermenting cold to protect their aromas take longer. Yeast strain and how much sugar the grapes started with shift that window too — more sugar means more work (and more days) for the yeast before it runs out of food.
Step 4 — Malolactic fermentation (only in some wines)
Many red wines — and some whites, like certain Chardonnays — go through a second fermentation, this time bacterial rather than yeast-driven. Bacteria from the genus Oenococcus convert malic acid (the same sharp acid found in a green apple) into lactic acid, which is softer (like the acid in milk). The result is a wine with less sharp acidity and a rounder texture. This process can take weeks or months, and usually happens in barrel.
How you know it's done
The winemaker measures the density of the must with a hydrometer throughout the process. At the start, the must is dense because of the dissolved sugar; as the yeast consumes it, the density drops. Once it reaches a stable value close to that of water, there's no more fermentable sugar left — fermentation is complete.
What if you did it yourself, with real data?
In LagarLab you simulate a real fermentation: choose the grape, control the temperature, and watch how each decision changes the final wine.
Vinify for free: Agrelo (red)No sign-up. Prefer a white? Try Chablis.
Frequently asked questions
What exactly are the bubbles you see in the tank during fermentation?
The wine isn't boiling — that's CO2 escaping. Yeasts consume the sugar in the grape and release alcohol and carbon dioxide, and that gas is what bubbles up.
Why do reds and whites ferment at different temperatures?
Reds ferment warmer, between 25°C and 30°C, because the heat helps extract color and tannin from the skins. Whites ferment cooler, between 12°C and 18°C, to preserve the fruity aromas that heat destroys.
How long does alcoholic fermentation take?
Between 5 and 15 days, depending on temperature, yeast, and the style of wine being made.
Do all wines go through malolactic fermentation?
No, it's optional. It's a second, bacterial fermentation that converts malic acid into lactic acid, giving a softer texture; the winemaker decides whether to use it or block it depending on the style they're after.
How does the winemaker know fermentation is done?
By measuring the must's density with a hydrometer. Density drops as yeasts consume the sugar, and it stabilizes near water's density once there's no more fermentable sugar left.
What does wine fermentation produce?
Mainly two things: ethanol and carbon dioxide, which the yeast makes from the grape sugars. As a rule of thumb, each degree Brix of sugar the yeast consumes turns into a little over half a percent of alcohol, so a must at 24 °Brix ends up around 13.5% ABV. Fermentation also releases heat, which is why tanks need cooling, plus smaller amounts of glycerol and aroma compounds that shape the wine's body and smell.