What Is Fermentation?

At its most fundamental level, alcoholic fermentation is the process by which yeast consumes sugars in grape juice and converts them into ethanol (alcohol) and carbon dioxide (CO₂). The basic equation looks like this:

Sugar + Yeast → Alcohol + Carbon Dioxide + Heat

It sounds simple — and in nature, it is. Wild yeasts on grape skins have been doing this job for millennia. But for consistent, quality wine at home, you want to understand and manage the process rather than leave it entirely to chance.

The Two Stages of Fermentation

Primary Fermentation

This is the vigorous, active phase. It typically lasts 5 to 14 days, during which:

  • Yeast populations grow rapidly
  • Sugars are converted to alcohol
  • CO₂ bubbles vigorously through your airlock
  • The must (crushed grapes or juice) froths and foams

You'll know primary fermentation is active when your airlock bubbles regularly and the liquid has a distinct yeasty smell. A hydrometer reading will show your specific gravity dropping steadily from its starting point (typically around 1.080–1.100) toward 1.020 or below.

Secondary Fermentation (Malolactic Fermentation)

Separate from alcoholic fermentation, malolactic fermentation (MLF) involves bacteria converting sharp malic acid into softer lactic acid. This is common in red wines and some whites (like Chardonnay) and gives a buttery, rounder mouthfeel. MLF can happen naturally or be induced with commercial bacteria cultures.

Key Variables to Control

Temperature

Temperature is the single most critical variable. The ideal range is:

  • Red wines: 22–28°C (72–82°F)
  • White wines: 12–18°C (54–64°F) for a slower, more aromatic fermentation

Too hot, and yeast becomes stressed, producing off-flavours and potentially dying. Too cold, and fermentation stalls or never starts.

Yeast Selection

Commercial yeast strains are reliable, predictable, and selected for specific wine styles. Popular choices for home winemakers include:

  • EC-1118 (Champagne yeast) — robust, high alcohol tolerance, neutral flavour
  • 71B — softens acidity, good for young fruity reds and rosés
  • RC212 — highlights fruit character in red wines, especially Pinot Noir

Nutrients and Sulfite Levels

Yeast needs more than just sugar. Adding yeast nutrients (DAP — diammonium phosphate) helps maintain healthy fermentation, especially in low-nutrient juice. A small addition of potassium metabisulfite (Campden tablets) before pitching yeast helps eliminate wild yeasts and bacteria without harming a commercial starter added 24 hours later.

Monitoring Fermentation Progress

A hydrometer is your most essential tool. Take regular specific gravity readings:

  1. Record the original gravity (OG) before fermentation starts
  2. Take readings every 2–3 days during active fermentation
  3. Fermentation is complete when specific gravity reaches 0.995–1.000 and holds stable over two consecutive days

The difference between OG and final gravity (FG) lets you calculate your approximate alcohol by volume (ABV).

Common Fermentation Problems

Problem Likely Cause Solution
Fermentation won't start Too cold, dead yeast, or too much sulfite Warm the must; re-pitch fresh yeast
Fermentation stalls mid-way Nutrient deficiency or temperature swing Add yeast nutrient; stabilise temperature
Vinegar smell Acetobacter bacteria (oxygen exposure) Keep vessels topped up; seal properly
Rotten egg (H₂S) smell Yeast stress from low nutrients Rack wine off sediment; add copper sulfate if severe

Moving Forward

Once primary fermentation is complete, it's time to rack (transfer) your wine off the gross lees — the layer of dead yeast and grape solids — and move into the secondary vessel for clarification and aging. Master fermentation, and you've mastered the core of winemaking.