3.1 Fermentation & Winemaking by Wine Colour

Fermentation & Winemaking by Wine Colour

Every wine, whatever its color or style, starts from the same basic transformation: grape juice becoming an alcoholic beverage through fermentation. What separates a crisp white from a tannic red, or a bone-dry wine from a lusciously sweet one, is not a different chemical process but a series of deliberate choices a winemaker makes around that process. Understanding these choices is central to WSET Level 2, because knowing how a wine is made explains why it tastes the way it does.

Alcoholic Fermentation: The Engine of Winemaking

Alcoholic fermentation is the chemical reaction at the heart of every wine. Yeast (a single-celled microorganism naturally present on grape skins and in the winery, or added by the winemaker as a cultured strain) consumes the natural sugar in grape juice and converts it into:

• Ethanol (alcohol)
• Carbon dioxide (CO2), which escapes as gas from a still wine

Fermentation also releases heat, so winemakers closely monitor and control temperature; left unchecked, fermentation can run too hot, killing the yeast early or driving off delicate aromas. Yeast continues consuming sugar until either it runs out (a dry wine) or the alcohol level rises high enough to kill the yeast itself, typically around 15% abv for most strains.

The Basic Stages of Winemaking

While the exact order and detail vary by wine color, most wines pass through the same broad sequence of stages:

1. Harvesting - picking grapes at the desired ripeness
2. Crushing and destemming - breaking the grape skins and typically removing the stems to release the juice, now called must
3. Fermentation - yeast converts sugar to alcohol
4. Pressing - separating the liquid wine from the solid grape skins, seeds, and stems (the pomace)
5. Maturation/storage - the wine rests and develops before bottling
6. Blending - combining different batches, vessels, or varieties for balance and consistency
7. Packaging - bottling (or other packaging) for sale

The key difference between white and red winemaking is when pressing happens relative to fermentation, and that single difference explains most of the flavor and structural gap between the two colors.

Making White Wine

For white wine, grapes are pressed before fermentation. The skins are separated from the juice almost immediately after crushing, so the juice ferments largely on its own, without prolonged contact with skins, seeds, or stems. Because those solid parts are where most of a grape's color pigment and tannin live, white wine ends up pale in color and low (or absent) in tannin, no matter whether the grapes themselves were white- or black-skinned. White wine fermentation is typically carried out at cool temperatures (around 12-22°C) in stainless steel tanks, which helps preserve the delicate, fresh fruit and floral aromas that define most white wine styles. Because white wines lack the structural backbone that tannin provides, they instead rely heavily on acidity to give them freshness, balance, and grip.

Making Red Wine

Red wine takes the opposite approach: black grapes are crushed and then fermented with their skins (and frequently their seeds) still in the mix, a process called maceration. As fermentation proceeds, the alcohol produced acts as a solvent, pulling color pigments (anthocyanins), tannins, and flavor compounds out of the skins and into the developing wine. This is why red wine is red - the color comes almost entirely from skin contact, not from the grape's flesh, which is usually clear regardless of skin color.

During red fermentation, solid skins rise to the surface, pushed up by escaping CO2, and form a floating layer called the cap. Left alone, the cap would dry out and risk spoilage, so winemakers manage it actively, either by punching down (physically pushing the cap back into the liquid) or pumping over (drawing juice from the bottom of the tank and spraying it back over the cap). Both techniques maximize extraction of color, tannin, and flavor while keeping the cap moist and protected. Red fermentation also typically runs warmer than white fermentation (roughly 20-32°C), since higher temperatures aid extraction from the skins. After fermentation, the wine is pressed to separate it from the skins; winemakers may extend this skin contact after fermentation finishes (extended/post-fermentation maceration) to pull out further tannin and help stabilize the wine's color.

Making Rosé Wine

Rosé sits between white and red winemaking, and the most common method used to produce it is short maceration. Black grapes are crushed, and the skins are left in brief contact with the juice - typically a few hours up to about two days - just long enough to bleed some color and flavor into the juice without extracting the full tannin and depth of a red wine. The skins are then pressed off, and the lightly-tinted juice is fermented on its own, much like a white wine, usually at cool temperatures to preserve freshness. A second, less common method is blending - mixing a small proportion of finished red wine into white wine. Blending is permitted for Champagne but banned for still rosé wine in the EU and most other major producing countries, since it is seen as a shortcut that quality rosé producers avoid.

Making Sweet Wine

Sweetness in a finished wine comes from residual sugar - natural grape sugar that was never converted to alcohol. There are three main ways winemakers achieve this:

• Stopping fermentation early, before all the sugar has converted, by rapidly chilling the wine, filtering out the yeast, or fortifying with spirit (which raises the alcohol level high enough to kill the yeast)
• Adding back unfermented, sweet grape juice to an already-fermented dry base wine, a technique sometimes called süssreserve
• Starting with grapes that have exceptionally high natural sugar levels - from late harvest, botrytis (noble rot) affected fruit, or frozen grapes - so that even a complete, natural fermentation leaves substantial sugar behind, or the rising alcohol level kills the yeast before all the sugar is consumed

Each method produces a different style of sweetness, but all rely on the same underlying principle: preventing yeast from finishing the job of converting every gram of sugar into alcohol.