Stuck Fermentation: Prevention, Diagnosis, and Restart

A stuck fermentation has stopped before the sugar you meant to ferment has gone, and will not restart on its own. It clusters where you would expect: dried-grape and late-harvest musts, very ripe warm-climate reds, and any ferment that got hot. Almost every stuck ferment was preventable, and the prevention is cheap. The rescue is genuinely difficult, and the wine is at real risk while it sits there.

Stuck, Sluggish, or Finished?

Finished. The yeast consumed the fermentable sugar. A hydrometer reads at or slightly below 1.000 — often 0.990–0.995 in a dry wine, because alcohol is less dense than water. See reading a hydrometer.

Sluggish. Still working, just slowly — gravity falling by small amounts over days rather than hours. Cool, high-sugar and late-stage ferments are all legitimately slow; Amarone and passito ferments can run for weeks. Do not panic at slow progress; panic at stopped progress.

Stuck. Gravity has not moved for several days, sugar remains, and the surface has gone flat and quiet.

The only way to tell these apart is a written record. Log gravity and temperature at the same time each day: a flat line across three or four days with sugar remaining is your diagnosis.

The Five Real Causes

1. Osmotic pressure — the sugar was too high to start with

The classic appassimento, passito, Vinsanto, aszú and ice-wine failure. A must at 28–30 °Bx or beyond is hostile before fermentation begins: the sugar concentration draws water out of the yeast cell, so the yeast spends energy simply surviving. Osmotically stressed yeast ferment slowly and then quit, often well below the alcohol the strain is rated for. Dried-grape musts need a strain chosen for osmotic tolerance as well as alcohol tolerance — decided before you crush.

2. Alcohol above the strain's tolerance

Strains carry a manufacturer alcohol tolerance rating — commonly 14% for aromatic strains, up to 18% for high-tolerance workhorses like EC-1118 and K1V-1116. A must at 26–27 °Bx will comfortably exceed 14% and stall a 14%-rated strain. Run your starting Brix through the ABV calculator before harvest — see choosing your yeast. Those ratings are achieved under favourable conditions and fall as conditions worsen, particularly as temperature rises.

3. Nutrient starvation

Nitrogen-starved yeast produce hydrogen sulphide, ferment sluggishly, and give up early. High-sugar musts need more nitrogen than ordinary musts, and drying fruit does not concentrate assimilable nitrogen the way it concentrates sugar. Do not add all your nutrient at pitch — split it across the first third of the ferment. See nutrients and the YAN calculator.

4. Temperature

Heat plus alcohol is what actually kills a ferment. A hot ferment looks impressively vigorous for two days and then stops dead, because accumulating alcohol is far more toxic to yeast at elevated temperature. Most wine strains struggle once fermentation reaches the low 30s °C, and warm-climate reds self-heat there without help. Chilled well below the strain's stated range, a ferment also slows to a halt — see fermentation temperature. A cool, slow ferment is far more likely to finish than a hot, fast one.

5. Inhibitors and pH extremes

Too much SO₂ at must preparation, added too close to pitching, suppresses the yeast you just paid for — and how suppressive a given free SO₂ figure is depends on pH, because the molecular fraction does the work. Let it dissipate before inoculating; see understanding SO₂. High-pH musts from hot sites compound everything: the pH that makes a wine hard to protect microbially also means a stall becomes a spoilage event fast.

Prevention, In Order of Value

1. Decide on numbers before harvest, not on a hunch during it — see reading grape chemistry and harvest timing.
2. Pick a strain rated above your projected final alcohol, with margin.
3. Rehydrate properly, per the sachet, with a rehydration nutrient — the sterols it supplies buy real stress tolerance later.
4. Stage nutrients, aerate early rather than late, keep the ferment cool, and log gravity and temperature daily.

Restarting a Stuck Ferment

The instinct — sprinkle more yeast on top — reliably fails. Dry yeast dropped into wine at 14–15% alcohol dies before it establishes. What works is acclimatisation: build a healthy population in a friendly environment, then gradually teach it to tolerate the stuck wine.

1. Fix the cause first. Warm a cold ferment gently into range; cool a hot one; check free SO₂. Do not dump nitrogen into a stuck wine — nitrogen the yeast cannot use is food for spoilage organisms.
2. Rack off gross lees into a clean vessel. Dead and dying yeast release compounds that inhibit fresh yeast.
3. Build a starter with a high-tolerance restart strain — EC-1118 and K1V-1116 are the conventional choices — rehydrated per the manufacturer's instructions with a rehydration nutrient.
4. Step-feed it. Once the starter ferments actively, add a small volume of stuck wine and wait for vigorous activity to resume. Then roughly double the starter volume with stuck wine, wait again, and repeat. Each doubling raises the alcohol and sugar the population is adapted to.
5. Only when the starter is comfortably fermenting a substantial fraction of stuck wine do you add it to the batch. Rushing this is the usual reason a restart fails twice.

Yeast manufacturers publish detailed restart protocols for their own strains; follow one exactly rather than approximating it. Success depends heavily on the alcohol already reached and how long the wine sat.

The Danger of Leaving It

Never leave a fermentation stuck and unattended. A quiet vessel with residual sugar, no CO₂ blanket and little SO₂ is ideal habitat for acetic acid bacteria and for Brettanomyces, which will happily consume sugars Saccharomyces left behind. Volatile acidity climbs, and unlike a stall it does not come back — see volatile acidity.

If you decide not to restart, decide actively: rack clean, chill, add SO₂ appropriate to the pH, and treat it as a sweet wine needing stabilisation before bottling. See back-sweetening and bottling.

When Stopping Is the Point

In sweet-wine styles the stall is often the goal. Tokaji Aszú, Vinsanto, passito and many botrytis wines finish where the yeast gives out, leaving substantial residual sugar balanced by very high acidity. Hence the transferable rule for sweet wine: start with the highest-acid fruit you have — acid, not sugar, is what stops a wine tasting cloying. A stall there is only a failure if it stops at the wrong sweetness, which returns you to the same discipline: know your starting sugar, project where the alcohol lands, and record gravity daily.