Understanding Fermentation Curve Patterns
Tracking your fermentation curve helps you catch problems early and keep your yeast working efficiently. A well-managed fermentation follows a predictable pattern. If the curve looks off, it’s time to troubleshoot.
Note: Lager fermentations (cold) typically take twice as long as ales at standard fermentation temperatures.
What a Normal Fermentation Curve Looks Like
A healthy fermentation usually follows these stages:
| Stage | Timeframe (Ales) | Timeframe (Lagers) | What Happens |
|---|---|---|---|
| Lag Phase | 0–12 hours | 12–24 hours | Yeast absorbs oxygen, builds reserves, and prepares to divide. Gravity stays the same. |
| Exponential Growth | 12–72 hours | 24–96 hours | Yeast rapidly multiplies, fermentation speeds up, gravity drops quickly, CO₂ production is high. |
| Slowing Phase | 3–5 days | 6–10 days | Sugar levels decrease, yeast activity slows, gravity approaches final target. |
| Final Gravity (FG) Reached | 5–14 days | 10–21 days | Fermentation completes, yeast starts flocculating (settling), CO₂ production stops. |
| Conditioning | Varies | Varies | Yeast cleans up off-flavors (diacetyl, sulfur), beer matures. |
A typical gravity curve starts flat, then drops steeply during active fermentation, and levels off at final gravity.
Signs of Stalled or Sluggish Fermentation
If the curve isn’t following the normal pattern, here’s what to watch for:
| Issue | Signs |
|---|---|
| Stuck Fermentation | Gravity stops dropping before target FG, no activity in airlock or blow-off tube. |
| Slow Fermentation | Gravity is decreasing, but much slower than expected. |
| Extended Lag Phase | No gravity change after 12+ hours for ales, 24+ hours for lagers. |
| Incomplete Attenuation | Gravity stabilizes too high, beer tastes overly sweet. |
When to Intervene
If fermentation stalls or moves too slowly, take action based on the cause:
| Possible Cause | Fix |
|---|---|
| Low Pitch Rate | Add more yeast (if early enough). |
| Under-Oxygenation | Aerate wort before fermentation next time. |
| Temperature Too Low | Raise temp to the yeast strain’s optimal range. |
| Nutrient Deficiency | Add yeast nutrient (if early in fermentation). |
| High Gravity Stress | Use a high-gravity yeast strain or oxygenate more next time. |
If gravity hasn’t moved in 24+ hours for ales or 48+ hours for lagers, ramp up temperature slightly and gently rouse the yeast to restart activity.
Key Takeaways
- A normal curve has a steep gravity drop early, then slows before reaching final gravity.
- Lagers take nearly twice as long as ales to complete each fermentation stage.
- If the curve stalls or drags, check yeast health, temperature, and oxygen levels.
- Small adjustments early can prevent stuck fermentations and improve beer quality.