Primary Fermentation

Primary fermentation is where the magic happens—sugar becomes alcohol, and grape juice transforms into wine. Understanding this process helps you manage it effectively and troubleshoot when things go wrong.

What Happens During Primary Fermentation

Yeast consume sugar and produce alcohol and carbon dioxide as byproducts. It sounds simple, but the biochemistry is remarkably complex. Hundreds of flavor compounds form during this process, influenced by yeast strain, temperature, nutrients, and oxygen exposure.

For red wines, primary fermentation also extracts color, tannins, and flavor from grape skins. This is why reds ferment “on the skins” while whites are typically pressed before fermentation.

The Fermentation Timeline

Days 1-2 (Lag Phase): Yeast multiply rapidly but fermentation is not yet visible. The must may look dormant. This is normal—the yeast are building population.

Days 2-7 (Active Fermentation): Vigorous bubbling, foam production, and heat generation. This is peak fermentation activity. For reds, punch down or pump over the cap of skins 2-3 times daily.

Days 7-14 (Declining Fermentation): Activity slows as sugar depletes. Bubbling becomes less frequent. Monitor specific gravity closely.

Day 10-14+: Primary fermentation completes when specific gravity drops below 1.010 (for reds being pressed) or 0.995 (for fermentation to dryness).

Temperature Management

Temperature profoundly affects fermentation character:

Cool fermentation (55-65°F): Slower, preserves fruit aromatics, preferred for white wines and delicate reds.

Moderate fermentation (65-75°F): Good balance of speed and flavor development. Standard for most reds.

Warm fermentation (75-85°F): Faster extraction but risks producing harsh tannins and cooked flavors. Usually avoided.

Fermentation generates heat. A must that starts at 70°F can climb to 85°F+ during peak fermentation. Monitor and cool if necessary.

Managing the Cap (Red Wines)

The “cap” is the layer of grape skins that floats on fermenting red wine. It must stay wet to extract color and prevent acetification.

Punch down: Manually push the cap under the liquid 2-3 times daily. Traditional and effective.

Pump over: Pump liquid from the bottom over the cap. More gentle extraction than punch down.

Submerged cap: Keep the cap submerged with a screen or weighted device. Low maintenance but less extraction.

When to Press

For red wines, pressing timing affects style:

• Early press (SG 1.010-1.020): Lighter color, softer tannins, earlier drinking
• Extended maceration (after fermentation): Deeper color, more tannic, needs aging

Most home winemakers press when primary fermentation slows (around SG 1.010) and transfer to secondary for finishing.

Signs of Healthy Fermentation

• Steady bubbling through airlock
• Declining specific gravity readings
• Fermentation temperature in appropriate range
• Fresh, yeasty smell (not sulfurous or vinegar)

Troubleshooting Primary Fermentation

No activity after 48 hours: Temperature too cold, yeast pitch insufficient, or must lacks nutrients. Warm it up, add more yeast, or add yeast nutrient.

Fermentation stopped early: Could be stuck fermentation (address immediately) or fermentation to dryness (verify with hydrometer).

Sulfur smell: Stressed yeast. Add yeast nutrient, ensure temperature is appropriate, aerate gently.