What "stuck" actually means
A slow fermentation and a stuck one are not the same thing. A slow fermentation keeps dropping in density every day, just more gradually than expected. A stuck fermentation is different: density stops moving before most of the sugar has been consumed, and stays there — the must is still sweet and the yeast has effectively stopped working.
The most common cause: not enough assimilable nitrogen (YAN)
Yeast doesn't just need sugar — it needs nitrogen to build new cells and the proteins it uses to ferment. That available nitrogen is measured as YAN (yeast assimilable nitrogen), which combines ammoniacal nitrogen (NH₄⁺) with the free amino acids in the must.
The industry reference is clear: below 150 mg N/L of total YAN, the risk of a stuck fermentation rises sharply. There's also a critical window: most nitrogen demand happens in the first third of fermentation, while the yeast population is still growing. If YAN is already below 120 mg N/L during those early days, the stress is far more severe than a late-stage deficit — that's where most batches are actually lost.
A related but distinct problem: if ammoniacal nitrogen specifically (NH₄⁺) drops below 60 mg/L, the risk isn't that fermentation stalls — it's that yeast starts producing H₂S (rotten egg smell) as it searches for nitrogen through a different metabolic pathway. This can happen with no stuck-fermentation risk at all, and vice versa.
Other real causes (not just nitrogen)
- Temperature far from the strain's optimum: too cold or too hot both slow yeast activity — every strain has a range where it performs best, and moving away from it in either direction costs momentum.
- Alcohol approaching the strain's tolerance ceiling: in very ripe musts (high starting sugar), the alcohol produced can approach the strain's tolerance limit before all the sugar is gone — activity fades progressively as that ceiling gets closer, not all at once.
- A weak or poorly acclimatized yeast strain: a strain chosen without considering the must's profile (starting sugar, working temperature) starts at a disadvantage.
- Not enough oxygen early on: yeast needs a bit of oxygen at the start to synthesize sterols it later needs to tolerate alcohol — without that, it weakens prematurely.
Catching it early
The tool is the same as always: a hydrometer, every day, at the same time. What matters isn't the isolated number — it's the trend. A slow fermentation keeps dropping, even if only a little each day. One that's about to get stuck starts flattening out — the daily drop gets smaller and smaller until it's practically zero, while a meaningful amount of sugar is still unconsumed.
How to restart it
It depends on when you catch it:
- Caught early (still in the first third, with a living yeast population): adding a nitrogen source (diammonium phosphate or an organic nutrient) is usually enough to restart it.
- If temperature is the issue: correct it gradually toward the strain's optimum, without sudden temperature shocks that stress the yeast further.
- If it's truly stuck (yeast population has collapsed): adding nitrogen alone, late, doesn't restart anything — that requires re-inoculating with a fresh, alcohol-tolerant strain, gradually acclimatized to the must's temperature and alcohol level before adding it.
Why this is exactly what LagarLab lets you practice: the fermentation engine simulates this mechanism for real — with the same YAN thresholds any winery works with (150 mg N/L general risk, 120 mg N/L critical early risk, 60 mg/L H₂S risk). You can trigger a stuck fermentation on purpose, see exactly why it happened, and practice fixing it — without risking a real batch.
What if you did it yourself, with real data?
In LagarLab you can trigger a stuck fermentation on purpose and practice restarting it — using the same YAN thresholds a real winery works with, without risking a batch.
Vinify for free: Agrelo (red)No sign-up. Prefer a white? Try Chablis.
Frequently Asked Questions
What's the real YAN threshold that risks a stuck fermentation?
The general reference is 150 mg N/L of total YAN (ammoniacal nitrogen + amino acids). Below that, stuck-fermentation risk rises sharply. If that deficit happens during the first third of fermentation, the critical threshold is stricter: 120 mg N/L.
Is a slow fermentation the same thing as a stuck one?
No. A slow fermentation keeps dropping in density every day, just more gradually. A stuck one stops dropping almost entirely before the sugar has been consumed, and stays there.
Can you restart a fully stuck fermentation just by adding nitrogen?
Only if it's caught early and the yeast population is still alive. If it has already collapsed, adding nitrogen alone isn't enough — you need to re-inoculate with a fresh, alcohol-tolerant strain, gradually acclimatized before adding it.
What is H2S risk and how does it relate to stuck fermentation?
It's a related but distinct issue: if ammoniacal nitrogen (NH4+) specifically drops below 60 mg/L, yeast can produce H2S (rotten egg smell) as it looks for nitrogen through a different metabolic pathway — without that necessarily meaning the fermentation is at risk of getting stuck.
Why can temperature cause a stuck fermentation?
Every yeast strain has an optimal temperature range. Moving away from that range, whether too cold or too hot, reduces fermentation activity — and combined with another stress factor (low nitrogen, high alcohol), it significantly raises the risk of the fermentation stopping altogether.
How do you prevent a stuck fermentation?
Most stuck fermentations are prevented before they start. Make sure the must has at least 150 mg N/L of assimilable nitrogen (YAN), and add nutrients early, during the first third of fermentation, when yeast takes up most of its nitrogen. Keep the temperature inside the right range for the wine style and the yeast strain, choose a strain whose alcohol tolerance is above the wine's potential alcohol, and avoid an acid shock: a very low pH (below 3.0) or too much molecular SO₂ (above 0.8 mg/L) can stall the yeast.