The first time it happens you assume the hydrometer is wrong. You pull a second sample, dry it off, spin it gently, and squint. Still 1.020. Still not moving. The airlock is quiet. The cap is sitting flat. Yesterday it was at 1.022, the day before that 1.024, and you can already feel that something has gone sideways.
A stuck fermentation is not a failure. It's a signal that one of four things has gone wrong, and the fix is almost always within reach if you catch it in time. The trick is catching it — most home winemakers don't notice a stall until day five or six, by which point the colony is in worse shape and the must is more vulnerable to spoilage organisms.
What "stuck" actually means
A fermentation is considered stuck when specific gravity reads the same value across three consecutive daily measurements — and that value is meaningfully above your expected dry endpoint (usually 0.995 ± 0.002 for table wines).
The "three days" part matters. Healthy fermentations slow down naturally as they approach dry — a 24-hour pause at 1.000 isn't a stall, it's a yeast colony running out of easy sugar. But three days static at 1.020 means the colony has stopped working before the job is done.
If your batch shows 0.000 SG/day for three consecutive days, you'll see a "Stalled" badge on the cellar wall and Vinea AI will surface a suggested first step. No need to math it out yourself — log the SG and we'll do the rest.
The four usual suspects
In our cellar dataset of two hundred-plus home batches, every stuck primary we've diagnosed has fallen into one of four buckets:
- Temperature out of range. Most strains stall below 60°F or above 85°F. Check both your liquid temp and ambient — a cool basement floor can pull a carboy down by 4°F overnight, and the inside of the must is often colder than the room.
- Yeast nutrient depleted. If you didn't stage your DAP / Fermaid O additions across the first 72 hours, you may have starved the colony before it finished the sugar. Single-shot nutrient at pitch is a common cause of mid-ferment stalls.
- Excessive starting Brix. Above 26°Bx (~1.110 SG), most home strains will tap out before fully attenuating. This is osmotic stress — the must is too sugary for the yeast to keep working in.
- Wrong strain for the job. A delicate Champagne yeast won't push through a heavy red the way EC-1118 will. If you pitched something tepid into a 25°Bx Cab, the strain is the suspect.
The diagnosis flow
Before you reach for a yeast packet, run through this in order. Most stalls resolve at step 1 or 2 without needing to re-pitch — and re-pitching into an unfixed problem just kills another colony.
| Step | Check | Target |
|---|---|---|
| 1. Temperature | Liquid + ambient | 65–75°F |
| 2. Nutrients | YAN remaining | ≥ 150 ppm |
| 3. pH | Must pH | 3.2 – 3.6 |
| 4. Gravity | Current SG | varies |
| 5. Sulfite | Free SO₂ | < 30 ppm |
The restart protocol
Once you've identified the cause and corrected it (warmed the must, added nutrient, whatever it was), the restart looks like this:
- Warm the must gently to 70°F over 6–8 hours — a fermentation belt on its lowest setting works well. Don't rush this; thermal shock is real.
- Add a balanced rehydration nutrient like Go-Ferm at the rate on the packet, dissolved in warm water (104°F).
- Build a starter in a sanitized jar: 1 cup of must + ½ cup warm water + the rehydrated yeast. Cover loosely.
- When the starter is visibly active (foam ring, ~30 minutes), step it up with another cup of must, wait 30 minutes, repeat once more. You're acclimating the colony to your specific must.
- Pitch into the main vessel and stir gently to suspend the lees. Keep the temperature steady at 70°F for the next 48 hours.
If your starter is more than 18°F off the temperature of your must when you pitch, the temperature shock can kill the colony before it has a chance to take hold. Always temper the starter up to within 10°F of the must before combining.
When to walk away
Not every stuck fermentation is worth saving. If you've passed 1.020 with a heavily oxidized must, or if your free SO₂ is climbing because of acetobacter, the wine you'd produce isn't the wine you wanted to make. Cut the loss, sanitize the equipment thoroughly, and start again with what you learned.
The cellar isn't a place where everything works. It's a place where you keep good notes and try the next thing. Vinea exists to make those notes easier to keep — so the next stalled primary teaches you something instead of frustrating you.
// What Vinea AI returns when it spots a stall
{
"verdict": "stalled",
"confidence": 0.78,
"reasoning": "SG flat at 1.020 for 3 days. Last temp 62°F (low for D254). No nutrient additions logged after pitch.",
"recommendations": [
{ "action": "Warm to 70°F over 6 hours", "priority": "now" },
{ "action": "Add 25–30 g of Fermaid O", "priority": "now" },
{ "action": "Recheck SG in 24 hours", "priority": "soon" }
]
}