Stuck Fermentation — Causes and Fixes

A stuck fermentation is when your wine stops fermenting before it should — the SG is still high, but the airlock has gone quiet and there's no activity. It's one of the most common problems in home winemaking, and almost always fixable.

How to Confirm Fermentation is Stuck

First, confirm it's actually stuck. Take an SG reading. If it's above 1.010 and there's been no change for more than 3–4 days, your fermentation is likely stuck. If you're at 0.998 and the airlock has stopped, it's probably just finished normally.

Common Causes

1. Temperature Too Low

This is the most common cause. Yeast becomes sluggish below 15°C and stops entirely below around 10°C. In British winters, an unheated garage or shed can easily cause a stuck fermentation overnight.

Fix: Move your demijohn somewhere warmer — ideally 18–24°C. Activity should resume within 24–48 hours. A seedling heat mat is an excellent investment.

2. Temperature Too High

Above 30°C and you start killing yeast cells. Above 35°C, fermentation will stop entirely.

Fix: Move somewhere cooler immediately. Allow the must to cool to 20°C, then pitch a fresh yeast sachet.

3. Nutrient Deficiency

Fruit wines often lack the nutrients yeast needs to complete fermentation. This is especially common with flower wines (elderflower, dandelion) where there's very little natural yeast nutrient.

Fix: Add a teaspoon of yeast nutrient per 4.5 litres. Stir gently and fermentation should resume within 24 hours.

4. Yeast Added Too Soon After Campden

If you added your wine yeast less than 24 hours after adding Campden tablets, the residual SO₂ may have killed it.

Fix: Wait another 24 hours (totalling 48 hours from the Campden tablets) then pitch a fresh yeast sachet.

5. High Starting Gravity

If your starting SG was above 1.110, there's a lot of sugar for the yeast to get through. Some yeasts give up at high alcohol levels.

Fix: Use a high-alcohol yeast (EC-1118 or similar), or restart with a fresh pitch of EC-1118, which is very robust.

6. pH Too Low

Very acidic musts can inhibit yeast. This is common with citrus wines or when too much acid blend has been added.

Fix: Add precipitated chalk (calcium carbonate) at 2g per litre to reduce acidity, then repitch fresh yeast.

Restarting a Stuck Fermentation

• 1.Warm the must to 22°C
• 2.Add a teaspoon of yeast nutrient
• 3.Make a yeast starter: dissolve a sachet of EC-1118 yeast in 200ml of warm water (35–40°C) with a teaspoon of sugar. Wait 20 minutes until it's actively bubbling.
• 4.Gradually add small amounts of your stuck must to the starter over 30 minutes to acclimatise the yeast
• 5.Add the starter to your demijohn
• 6.Fermentation should restart within 48 hours

When All Else Fails

Sometimes a wine simply won't ferment dry. If your SG is below 1.020 and you're struggling to restart it, you may have to accept a slightly sweeter wine. Stabilise with Campden tablets and potassium sorbate, then bottle.

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