Primary vs. Secondary Fermentation: What's the Difference?
If you've read about winemaking, you've likely encountered references to "primary fermentation" and "secondary fermentation." These terms describe two distinct phases in the winemaking process, each with its own characteristics, purposes, and management requirements.
Understanding the difference between these two fermentation phases is essential for any winemaker. It helps you know what to expect, when to take action, and how to achieve the style of wine you want. Let's dive in.
Primary Fermentation: The Main Event
Primary fermentation is the primary (pun intended) conversion of grape sugars into alcohol. This is what most people think of when they think about fermentation—yeast consuming sugar and producing ethanol and carbon dioxide.
What Happens During Primary Fermentation
When you add yeast to your grape juice (or "must"), the yeast begin consuming the sugars (glucose and fructose) present in the juice. Through the biochemical process of glycolysis and subsequent fermentation, these sugars are converted into:
- Ethanol (alcohol) - the main product
- Carbon dioxide - escapes as gas, causing bubbling
- Heat - fermentation generates warmth
- Various flavor compounds - esters, higher alcohols, and other molecules
How Long Does Primary Fermentation Last?
Primary fermentation typically lasts 5-14 days, depending on several factors:
- Temperature: Warmer fermentation = faster fermentation
- Sugar content: More sugar = longer fermentation
- Yeast strain: Different strains work at different speeds
- Nutrient availability: Well-fed yeast ferment more efficiently
Signs of Active Primary Fermentation
You can tell primary fermentation is underway when you observe:
- Bubbling: COâ‚‚ escaping through the airlock (often visible as vigorous bubbling)
- Foam: A layer of foam (called "krausen") forms on the surface
- Cap formation: For red wines, grape skins rise to form a "cap"
- Warmth: The fermenter feels warm to the touch
- Yeasty aroma: A pleasant, bread-like smell
🔬 Why This Works: The Yeast Colony Cycle
During primary fermentation, yeast populations go through distinct phases:
- Adaptation (Lag phase): Yeast cells recover from rehydration and prepare for growth
- Exponential growth: Yeast reproduce rapidly, doubling in population every few hours
- Stationary phase: As alcohol accumulates and nutrients deplete, yeast growth slows
- Decline: Eventually, alcohol levels become toxic and yeast cells die off
This entire cycle typically completes within two weeks, though some fermentations can drag on longer if conditions aren't optimal.
Secondary Fermentation: The Gentle Transformation
After primary fermentation, many wines undergo a second fermentation—though this one is very different from the first. Secondary fermentation is typically carried out not by yeast, but by bacteria.
What Is Malolactic Fermentation?
The most common form of secondary fermentation is malolactic fermentation (MLF), where specific bacteria (primarily Oenococcus oeni) convert sharp malic acid into softer lactic acid.
This process:
- Reduces the wine's total acidity
- Creates a smoother, rounder mouthfeel
- Adds "buttery" or "creamy" notes (from diacetyl, an MLF byproduct)
- Increases microbial stability
How Long Does Secondary Fermentation Last?
Malolactic fermentation is a much slower process than alcoholic fermentation, typically taking 2-6 weeks to complete. Some wines may take several months, especially if the process is carried out slowly in cool conditions.
Almost always: Most red wines, Chardonnay
Sometimes: Viognier, Marsanne, Roussanne
Rarely or never: Riesling, Sauvignon Blanc, Pinot Grigio, most aromatic whites
Signs of Malolactic Fermentation
MLF is much less dramatic than alcoholic fermentation. Signs include:
- Slight decrease in total acidity (measurable with testing)
- Buttery or creamy aromas developing
- Wine appears slightly hazy
- Slower or no bubble activity (MLF produces very little COâ‚‚)
Comparing Primary and Secondary Fermentation
| Characteristic | Primary Fermentation | Secondary (MLF) |
|---|---|---|
| Primary actor | Yeast (Saccharomyces) | Bacteria (Oenococcus) |
| Main process | Sugar → Alcohol | Malic acid → Lactic acid |
| Duration | 5-14 days | 2-6 weeks |
| Visibly active? | Yes (bubbling, foam) | No (very subtle) |
| Temperature | 55-75°F (12-24°C) | 65-75°F (18-24°C) |
| Key outcome | Alcohol production | Acidity softening |
When to Induce or Prevent MLF
As a winemaker, you have control over whether malolactic fermentation occurs. This is an important stylistic decision.
When to Induce MLF
Encourage MLF when you want:
- Softer, rounder mouthfeel
- Buttery complexity
- More body in the wine
- Reduced acidity (in high-acid wines)
- Greater microbial stability
When to Prevent MLF
Prevent MLF when you want:
- Crisp, bright acidity
- Fresh, fruity character
- Aromatic intensity
- Long-term aging potential (MLF can reduce age-worthiness)
🔬 Why MLF Changes Acidity
Malic acid is a dicarboxylic acid—it has two acidic hydrogen atoms. Lactic acid is a monocarboxylic acid—it has only one. When bacteria convert malic acid to lactic acid, they're effectively removing one "acid unit" from each molecule.
This matters because while pH might not change dramatically, the total amount of acid in the wine (called titratable acidity or TA) decreases noticeably. The wine tastes less sharp and more rounded on the palate.
The diacetyl that gives MLF wines their buttery character is a natural byproduct of the bacterial metabolism. Some winemakers actually add diacetyl directly to simulate the buttery character without undergoing MLF!
Managing Both Fermentations
Primary Fermentation Best Practices
- Maintain consistent temperature (use a thermometer)
- Punch down red wine caps 2-3 times daily
- Monitor specific gravity daily
- Ensure proper nutrition for yeast
- Keep the airlock clear and functioning
Secondary Fermentation Best Practices
- Wait until primary fermentation is complete (0° Brix)
- Ensure wine is racked off gross lees
- Maintain temperatures in the 65-75°F range
- Add MLF bacteria culture if inducing
- Test for malic acid completion
- Sulfite to stop MLF when desired level is reached
What If Secondary Fermentation Doesn't Happen?
Some wines simply won't undergo MLF naturally, even when you want them to. This can happen due to:
- Too low pH (below 3.2)
- Too high alcohol (above 14%)
- Too low temperature (below 60°F/15°C)
- High sulfite levels
- Lack of proper nutrients
If you want MLF but it's not occurring, you can:
- Warm the wine slightly
- Add a commercial MLF bacteria culture
- Reduce sulfite additions
- Ensure proper pH levels
Conclusion
Primary and secondary fermentation are two distinct but equally important phases in winemaking. Primary fermentation—the conversion of sugar to alcohol by yeast—is where the wine gets its character and alcohol content. Secondary fermentation (malolactic fermentation)—the conversion of malic acid to lactic acid by bacteria—is where that character gets refined and softened.
Understanding when each occurs, how to manage them, and how to make stylistic decisions about them gives you enormous control over the final wine. Master these two processes, and you're well on your way to making consistently excellent wines.
Now that you understand fermentation phases, learn about the most critical factor in winemaking: Sanitation in Winemaking: The One Thing That Matters Most