How Wine Ferments: A Step-by-Step Guide

Nothing magical happens inside a tank — it's chemistry. Here's what yeast does between the moment the must goes in and the wine comes out, explained plainly.

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.

Step 3 — Temperature calls the shots

This is where the winemaker really decides the wine's style:

• Red wines: ferment warmer, between 77°F and 86°F (25–30°C), to extract more color and tannin from the skins.
• White wines: ferment cooler, between 54°F and 64°F (12–18°C), to preserve the fresh, fruity aromas that heat would destroy.

A complete alcoholic fermentation usually takes between 5 and 15 days, though it can run longer depending on temperature and the initial sugar level.

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.