A stuck fermentation happens when yeast stop converting sugar to alcohol before the job is done, and restarting one requires diagnosing why it stalled before throwing more yeast at the problem. The two most common culprits are low nutrients and rising alcohol levels, but temperature swings, excessive acidity, and even residual pesticides on fruit can shut yeast down. The good news: most stuck fermentations can be rescued with the right sequence of steps.
Stuck vs. Sluggish: Know What You’re Dealing With
Before you intervene, confirm the fermentation is actually stuck and not just slow. The best method is to take specific gravity or Brix readings twice a day and plot them on a simple graph. A sluggish fermentation still shows a downward trend, just at a frustrating pace. A truly stuck fermentation shows no measurable change over 24 to 48 hours despite sugar still remaining in the must or wort.
This distinction matters because the fixes are different. A sluggish ferment often responds to a small temperature bump or a nutrient addition. A fully stuck ferment typically needs a complete yeast restart, which is a more involved process with a higher failure rate if done carelessly.
Identify the Cause First
Restarting without addressing the root cause is the single most common mistake. You’ll pitch fresh yeast into the same hostile environment that killed the first batch, and the new yeast will stall too. Work through these possibilities:
- Nutrient deficiency. Yeast assimilable nitrogen (YAN) is the nutrient most likely to run out. Grapes from hot, dry growing seasons are especially prone to low YAN levels. As alcohol rises during fermentation, it becomes harder for yeast to absorb amino acids, so a borderline nutrient level at the start can become critically low by mid-ferment.
- Temperature. Most wine yeasts work best between 59°F and 77°F (15–25°C). If your fermentation space dropped below that range overnight or spiked above it during a hot afternoon, yeast activity can slow dramatically or stop entirely.
- High acidity. A pH below about 3.0 creates a punishing environment for yeast. If your must or juice tests very acidic, that’s likely contributing to the stall.
- Alcohol toxicity. Once alcohol reaches 13–14% in a standard fermentation, many yeast strains start struggling. If you’re making a high-gravity wine or mead, this is a prime suspect.
- Improper yeast hydration. If you pitched dry yeast directly into must without rehydrating it first, a significant percentage of those cells may have died on contact with the sugar solution, leaving you with a much smaller yeast population than you planned for.
Fix the Environment Before Adding Yeast
Add Yeast Hulls to Remove Toxins
During fermentation, yeast produce medium-chain fatty acids and other compounds that accumulate and become toxic to living yeast cells. Yeast hulls (sometimes called “ghost cells”) act like tiny sponges that adsorb these toxins and pull them out of solution. Add yeast hulls at a rate of about 25 g/hL (2 lb per 1,000 gallons) and let them sit for 24 to 48 hours. Then rack the liquid off the hulls and add a second, smaller dose of about 12.5 g/hL. This two-stage approach cleans up the environment so fresh yeast have a fighting chance.
Adjust Temperature
Bring the liquid to the lower end of your yeast strain’s ideal range, typically around 65–70°F (18–21°C) for most wine fermentations. Warmer temperatures speed yeast metabolism but also increase alcohol toxicity stress. Starting the restart on the cooler side gives the new yeast time to acclimate before ramping up activity.
Correct High Acidity
If your pH is below 3.0, consider raising it with potassium bicarbonate. It’s the gentlest of the common deacidification agents and produces less foaming than alternatives. As a rough guide, each 1 g/L of potassium bicarbonate raises pH by about 0.2 units and lowers titratable acidity by roughly 1.1 g/L. Dissolve it in a small amount of water or wine and add it slowly with stirring for at least 30 minutes. A target pH between 3.3 and 3.5 is a comfortable zone for most yeast. Make small additions and test between each one rather than overshooting.
Boost Nutrients
If low nitrogen was the likely cause, you have two main options. Diammonium phosphate (DAP) is a cheap, fast source of inorganic nitrogen, but yeast metabolize it quickly and it doesn’t provide the vitamins and micronutrients that support long-term cell health. Organic nutrient blends made from inactivated yeast provide a slower, more complete nitrogen source along with lipids, vitamins, and minerals. A common dosing protocol for organic nutrients is a split addition: 20 g/hL at the start and another 20 g/hL partway through fermentation. For a restart, adding the full dose upfront is typical since you’re trying to give the new yeast everything they need immediately.
The Restart Protocol
Once the environment is cleaned up, you’re ready to pitch new yeast. This is not as simple as sprinkling a packet on top. You need to gradually acclimate the fresh yeast to the alcohol and sugar levels already present in your stuck batch.
Choose a yeast strain bred for high alcohol tolerance and vigorous fermentation. EC-1118 (Prise de Mousse) is the classic restart workhorse, capable of fermenting in alcohol levels that would kill most other strains. Uvaferm 43 is another strong option. Use about 1 gram per gallon of your total batch volume.
Rehydrate the yeast in a specialized rehydration nutrient mixed with warm water (around 104°F / 40°C). Let it sit for 20 minutes. Then begin a stepwise acclimation: add a small volume of the stuck wine to the yeast starter, wait for signs of activity (bubbling, foaming), then add more. The goal is to slowly introduce the yeast to the alcohol concentration rather than shocking them with it all at once. Each addition should roughly double the volume of the starter. Over the course of several hours, you’ll build the starter up until it’s fermenting actively, then add it to the full batch.
Some winemakers blend equal parts stuck wine with fresh, unsulfited juice to dilute the alcohol and give the yeast an easier sugar source to start on. This approach has a higher success rate than pitching directly into the full-strength stuck wine, especially when alcohol is above 13%.
Monitoring After the Restart
Take Brix or specific gravity readings twice daily and graph them. You should see measurable movement within 24 to 48 hours. If the gravity hasn’t budged after two days, the restart has likely failed, and you’ll need to reassess whether you’ve actually fixed the underlying cause.
Keep the temperature stable. Even small fluctuations during a restart can derail the fragile new yeast population. Avoid the temptation to warm the batch up to speed things along. Patience matters more here than velocity.
A successful restart often ferments more slowly than a healthy primary fermentation. That’s normal. The yeast are working in a more stressful environment, and a slow, steady finish is far better than a fast start that stalls again at a higher alcohol level.
When Restarting Won’t Work
Some fermentations can’t be saved. If the must has been stuck for weeks, bacterial contamination may have produced enough acetic acid (vinegar) or lactic acid to make the environment unrecoverable. If you detect a sharp vinegar smell or the pH has shifted significantly since the stall began, the batch may be compromised beyond what fresh yeast can overcome.
Extremely high residual sugar combined with alcohol above 15% is another scenario where even the hardiest restart strains struggle. In these cases, your practical options are to blend the stuck wine with a dry wine to balance the residual sweetness, stabilize it and bottle it as a deliberately sweet wine, or accept the loss.