Troubleshooting - Slow or Stalled Yeast Fermentation

Fermentations that move too slowly—or stop altogether—can usually be traced back to one of a few core issues. Fixing these problems starts with identifying what went wrong.

1. Unhealthy or Stressed Yeast

If yeast is old, underfed, or damaged, it may struggle to ferment properly. Common causes:

• Yeast stored too long before use
• Shipping-related stress (e.g. temperature spikes)
• Lack of nutrients in wort

Fix: Add a yeast nutrient like Yeast Lightning Nutrient and rouse the yeast. This gives cells the fuel they need to finish the job.

2. Wort Variability

Fermentation problems can also come from changes in wort composition, such as:

• Base malt differences (e.g. lower FAN or enzymes)
• Too much or too little trub carried over into the fermentor
• Changes in brewhouse setup or process

Note: A higher than expected final gravity can also be caused by low wort fermentability—not just yeast problems. This can happen due to:

• Under-conversion during mash
• High mash temperature
• Inactive or degraded enzymes in the malt

Fix: Run a forced fermentation test to compare actual yeast performance vs. wort potential.

3. Temperature Swings

Inconsistent fermentation temperature is a top cause of sluggish or stalled fermentations.

At Homebrew Scale:

• Ambient room temps can swing a lot day to night
• Cold crashes or drafts can shock the yeast

At Pro Scale:

• Cone too cold? Premature yeast flocculation is common if cone glycol jackets over-cool the yeast bed
• Too low setpoint? Cold wort + aggressive cooling = yeast dormancy
• Bad piping design? First-in-first-out glycol systems lead to temp swings across tanks

Fix:

• Try restricting glycol flow to the cone
• Start lager fermentations cooler than the setpoint or cool in steps (1ºC at a time)
• Use a last-in-first-out (LIFO) glycol loop design if building a new system

4. Yeast-Inhibiting Ingredients

Some ingredients can slow or stall yeast—especially when used during active fermentation:

• High amounts of citrus or ginger
• Vanilla extract or other adjuncts solubilized in ethanol

Fix: Add these after fermentation, during conditioning or packaging.

How to Restart a Stuck Fermentation

If your beer stalls, try this:

Step 1: Add Nutrients & Rouse the Yeast

• Homebrew: Swirl or gently shake the vessel
• Pro: Rouse with CO₂ or nitrogen through the bottom port
• Use Yeast Lightning Nutrient for the best results
• Don’t add oxygen after 50% attenuation—it risks oxidation

Step 2: Pitch a Strong Dry Yeast

• Use a robust, alcohol-tolerant strain like House Ale
• Dry yeast doesn’t need oxygen and can finish the job
• Works even in high-gravity beers like Triple IPAs and Imperial Stouts

Troubleshooter: Stuck or Stalled Fermentation

Use this flow to diagnose what's going wrong and how to fix it:

Was the fermentation slow to start?

→ Yes

→ Check yeast age and wort oxygenation.

• Was yeast older than 7 days?
  • → Add nutrient + consider fresh pitch next time
• No wort oxygenation?
  • → Add dry yeast now. Oxygenate next batch.

Did gravity stop dropping before target FG?

→ Yes

→ Stuck fermentation.

• Add Yeast Lightning, rouse the tank
• If no activity in 12–24 hours → pitch dry yeast
• Also check fermentability:
  • Run a forced fermentation test
  • Compare with brewhouse specs or past batches
  • High FG may be wort-related, not yeast-related

What was the yeast generation or crop age?

• >5 generations or >7 days old?

  → Likely loss of vitality → Pitch fresh yeast next batch

• <3 generations and <7 days?

  → Nutrient or temperature issue likely

How was the wort treated?

• No nutrient added? → Add now
• Trub-heavy? → Try reducing carryover in next brew
• High OG (>1.080)? → Yeast may need more oxygen or nutrient next time

Was fermentation temperature consistent?

• Swings of 3°C or more?

  → Check glycol setpoints, restrict cone cooling

• Cone cooler than tank body?

  → Yeast may have flocculated early

  → Warm slightly, rouse, or pitch dry yeast

Using specialty ingredients?

• Added citrus, ginger, or extract during fermentation?

  → These can stall yeast → Try adding after fermentation next time