Fermentation is at the heart of winemaking. It’s the process that transforms simple grape juice into the complex, aromatic, and alcoholic beverage we know as wine. Whether you're a beginner or brushing up on your wine knowledge, understanding how fermentation works is essential to appreciating what’s in your glass.

What is fermentation in wine?

Wine fermentation is the natural process in which yeast consumes sugar in grape juice and converts it into alcohol and carbon dioxide (CO₂). This transformation not only gives wine its alcohol content but also produces a range of flavour compounds that define each wine’s character.

The role of yeast and sugar

• Sugar: Grapes naturally contain sugars, mainly glucose and fructose. The riper the grapes, the more sugar—and the higher potential alcohol.

• Yeast: Yeast is a microorganism that initiates fermentation. It can be:

  • Natural (wild) – found on grape skins or in the winery environment.

  • Cultured (added by the winemaker) – selected strains that offer control and consistency.

When yeast is introduced to grape must (the freshly crushed grape juice with skins and seeds), it begins feeding on the sugar, converting it into:

• Ethanol (alcohol)

• Carbon dioxide (which escapes unless the wine is being made sparkling)

• Heat

• Aromatic compounds (which influence the wine’s aroma and flavor)

Alcohol, CO₂, and flavour development

While alcohol is the obvious result of fermentation, it’s only one part of the picture. Fermentation also develops hundreds of volatile compounds that contribute to wine’s flavor profile, such as:

• Esters (fruit aromas)

• Aldehydes (nutty or oxidized notes)

• Higher alcohols (which affect mouthfeel)

• Sulfur compounds (sometimes desired, sometimes not)

Fermentation can take anywhere from a few days to a few weeks depending on temperature, yeast activity, and winemaking goals.

Primary vs. secondary fermentation

Wine fermentation isn’t always a one-step process. In fact, many wines go through two distinct fermentations:

Primary fermentation

• The main alcoholic fermentation where sugar is turned into alcohol.

• Usually occurs in stainless steel, oak, concrete, or clay vessels.

• Lasts anywhere from 5 days (for some reds) to a few weeks (for many whites).

Secondary fermentation

This can refer to two different processes depending on the wine:

1. Malolactic fermentation (MLF):

   • A bacterial process that converts sharp malic acid into softer lactic acid.

   • Common in red wines and some fuller-bodied whites.

   • Adds creaminess, body, and buttery flavours.

2. Sparkling wine fermentation:

   • In traditional sparkling wine production, a second fermentation happens in the bottle.

   • This traps CO₂, creating the bubbles.

The Swirl perspective: Understanding wine from the inside out

At Swirl, we believe wine becomes more enjoyable when you understand what goes into it. Fermentation is where the magic begins. Whether you prefer crisp whites, bold reds, or natural wines, the fermentation method impacts the flavors in every sip. Use the Swirl app to explore wines made with different fermentation techniques and find the ones that match your taste.