There are few moments more stressful for a home winemaker than checking on your bubbling, active fermentation only to find that it has gone completely silent, long before it should have. This is a "stuck fermentation"—when the yeast die off or go dormant before they have converted all the sugar in the must into alcohol.

A stuck fermentation is a serious problem. The leftover sugar leaves the wine sweet, unbalanced, and, worst of all, vulnerable to spoilage bacteria that can quickly ruin the entire batch. Fortunately, with a clear understanding of the causes, a stuck fermentation is often fixable, and even more importantly, preventable. This guide will walk you through the causes, solutions, and best practices to keep your fermentation healthy from start to finish.

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For a complete overview of fermentation science, be sure to download our Complete 70-Page Guide to Red Winemaking.

How to Know if Your Fermentation is Stuck

The most reliable sign of a stuck fermentation is your hydrometer reading.

• The Telltale Sign: You take a reading one day, and then another a day or two later, and the Brix or Specific Gravity reading has not moved.
• The Target: A fermentation is considered "stuck" if it stops before the wine is fully dry (a hydrometer reading of -1.5° to -2.0° Brix).

While a lack of bubbling in the airlock can be a clue, it's not a foolproof indicator. The only way to know for sure is with your hydrometer.

Top 4 Causes of a Stuck Fermentation

Yeast are living organisms, and they will stop working if their environment becomes inhospitable. Here are the four most common culprits:

• Temperature Extremes: Must that gets too hot (> 90°F) can kill yeast, while must that gets too cold (< 60°F) can make them go dormant.
• Poor Nutrition: A lack of essential vitamins, minerals, and nitrogen is a leading cause of stress. This often happens when winemakers use only DAP instead of a complete nutrient like Fermaid-K.
• High Starting Sugar: Musts with a starting Brix above 25° create a high-alcohol environment that can become toxic to weaker yeast strains.
• Improper Yeast Hydration: Yeast needs to be properly rehydrated in warm water with a supplement like Go-Ferm to ensure a healthy and robust cell population.

How to Fix a Stuck Fermentation: A Step-by-Step Plan

Restarting a stuck fermentation can be tricky because you are trying to introduce new yeast into a hostile environment. Here’s a prioritized action plan:

Step 1: Check and Adjust Temperature

Before you do anything else, check the temperature of the must. If it's below 65°F, gently warm the fermenter. Sometimes, this is all that's needed to get the yeast working again.

Step 2: Try a Nutrient Addition

If the temperature is fine, the yeast may have simply run out of food. Try adding a half-dose of a complete yeast nutrient like Fermaid-K. Wait 24 hours to see if fermentation restarts.

Step 3: Prepare a "Rescue" Yeast Starter (The Surefire Method)

If warming and nutrients don't work, you'll need to re-pitch with a strong, healthy yeast colony.

• Choose a Robust Yeast: Select a hardy yeast strain like Lalvin EC-1118, the industry standard for restarting stuck fermentations.
• Make a Starter: Rehydrate your new yeast according to the package directions, using Go-Ferm.
• Slowly Acclimate: Once active, add an equal amount of your stuck wine to the starter. Wait for bubbling. Repeat this process, doubling the volume with stuck wine each time, until you have a large, vigorous starter.
• Pitch the Starter: Once your starter is very active, pitch the entire volume into your stuck wine and stir gently.

Prevention is the Best Cure

The best way to deal with a stuck fermentation is to avoid it in the first place. By following good winemaking practices—properly hydrating your yeast with Go-Ferm, using a complete nutrient like Fermaid-K, managing your temperature, and ensuring your must is balanced—you create an ideal environment for your yeast to thrive and complete their journey from grape to great wine.

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